[netbsd-mini2440.git] / crypto / external / bsd / netpgp / dist / ref / draft-ietf-openpgp-rfc2440bis-22.txt
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1 Network Working Group Jon Callas
2 Internet-Draft PGP Corporation
3 Intended status: Standards Track
4 Expires October 2007 Lutz Donnerhacke
5 Apr 2007
7 Obsoletes: 1991, 2440 Hal Finney
8 PGP Corporation
10 David Shaw
12 Rodney Thayer
14 OpenPGP Message Format
15 draft-ietf-openpgp-rfc2440bis-22
18 Status of this Memo
20 By submitting this Internet-Draft, each author represents that any
21 applicable patent or other IPR claims of which he or she is aware
22 have been or will be disclosed, and any of which he or she becomes
23 aware will be disclosed, in accordance with Section 6 of BCP 79.
25 Internet-Drafts are working documents of the Internet Engineering
26 Task Force (IETF), its areas, and its working groups. Note that
27 other groups may also distribute working documents as
28 Internet-Drafts.
30 Internet-Drafts are draft documents valid for a maximum of six
31 months and may be updated, replaced, or obsoleted by other documents
32 at any time. It is inappropriate to use Internet-Drafts as reference
33 material or to cite them other than as "work in progress."
35 The list of current Internet-Drafts can be accessed at
36 http://www.ietf.org/1id-abstracts.html
38 The list of Internet-Draft Shadow Directories can be accessed at
39 http://www.ietf.org/shadow.html
41 Copyright Notice
43 Copyright (C) The IETF Trust (2007).
45 Abstract
47 This document is maintained in order to publish all necessary
48 information needed to develop interoperable applications based on
49 the OpenPGP format. It is not a step-by-step cookbook for writing an
50 application. It describes only the format and methods needed to
51 read, check, generate, and write conforming packets crossing any
52 network. It does not deal with storage and implementation questions.
53 It does, however, discuss implementation issues necessary to avoid
54 security flaws.
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59 OpenPGP software uses a combination of strong public-key and
60 symmetric cryptography to provide security services for electronic
61 communications and data storage. These services include
62 confidentiality, key management, authentication, and digital
63 signatures. This document specifies the message formats used in
64 OpenPGP.
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115 Table of Contents
117 Status of this Memo 1
118 Copyright Notice 1
119 Abstract 1
120 Table of Contents 3
121 1. Introduction 7
122 1.1. Terms 7
123 2. General functions 7
124 2.1. Confidentiality via Encryption 8
125 2.2. Authentication via Digital signature 9
126 2.3. Compression 9
127 2.4. Conversion to Radix-64 9
128 2.5. Signature-Only Applications 10
129 3. Data Element Formats 10
130 3.1. Scalar numbers 10
131 3.2. Multiprecision Integers 10
132 3.3. Key IDs 11
133 3.4. Text 11
134 3.5. Time fields 11
135 3.6. Keyrings 11
136 3.7. String-to-key (S2K) specifiers 11
137 3.7.1. String-to-key (S2K) specifier types 11
138 3.7.1.1. Simple S2K 12
139 3.7.1.2. Salted S2K 12
140 3.7.1.3. Iterated and Salted S2K 12
141 3.7.2. String-to-key usage 13
142 3.7.2.1. Secret key encryption 13
143 3.7.2.2. Symmetric-key message encryption 14
144 4. Packet Syntax 14
145 4.1. Overview 14
146 4.2. Packet Headers 14
147 4.2.1. Old-Format Packet Lengths 15
148 4.2.2. New-Format Packet Lengths 15
149 4.2.2.1. One-Octet Lengths 16
150 4.2.2.2. Two-Octet Lengths 16
151 4.2.2.3. Five-Octet Lengths 16
152 4.2.2.4. Partial Body Lengths 16
153 4.2.3. Packet Length Examples 17
154 4.3. Packet Tags 17
155 5. Packet Types 18
156 5.1. Public-Key Encrypted Session Key Packets (Tag 1) 18
157 5.2. Signature Packet (Tag 2) 19
158 5.2.1. Signature Types 20
159 5.2.2. Version 3 Signature Packet Format 22
160 5.2.3. Version 4 Signature Packet Format 24
161 5.2.3.1. Signature Subpacket Specification 25
162 5.2.3.2. Signature Subpacket Types 27
163 5.2.3.3. Notes on Self-Signatures 27
164 5.2.3.4. Signature creation time 28
165 5.2.3.5. Issuer 28
166 5.2.3.6. Key expiration time 28
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171 5.2.3.7. Preferred symmetric algorithms 28
172 5.2.3.8. Preferred hash algorithms 29
173 5.2.3.9. Preferred compression algorithms 29
174 5.2.3.10.Signature expiration time 29
175 5.2.3.11.Exportable Certification 29
176 5.2.3.12.Revocable 30
177 5.2.3.13.Trust signature 30
178 5.2.3.14.Regular expression 30
179 5.2.3.15.Revocation key 31
180 5.2.3.16.Notation Data 31
181 5.2.3.17.Key server preferences 32
182 5.2.3.18.Preferred key server 32
183 5.2.3.19.Primary User ID 32
184 5.2.3.20.Policy URI 33
185 5.2.3.21.Key Flags 33
186 5.2.3.22.Signer's User ID 34
187 5.2.3.23.Reason for Revocation 34
188 5.2.3.24.Features 35
189 5.2.3.25.Signature Target 35
190 5.2.3.26.Embedded Signature 36
191 5.2.4. Computing Signatures 36
192 5.2.4.1. Subpacket Hints 37
193 5.3. Symmetric-Key Encrypted Session Key Packets (Tag 3) 37
194 5.4. One-Pass Signature Packets (Tag 4) 38
195 5.5. Key Material Packet 39
196 5.5.1. Key Packet Variants 39
197 5.5.1.1. Public Key Packet (Tag 6) 39
198 5.5.1.2. Public Subkey Packet (Tag 14) 39
199 5.5.1.3. Secret Key Packet (Tag 5) 39
200 5.5.1.4. Secret Subkey Packet (Tag 7) 40
201 5.5.2. Public Key Packet Formats 40
202 5.5.3. Secret Key Packet Formats 41
203 5.6. Compressed Data Packet (Tag 8) 43
204 5.7. Symmetrically Encrypted Data Packet (Tag 9) 44
205 5.8. Marker Packet (Obsolete Literal Packet) (Tag 10) 44
206 5.9. Literal Data Packet (Tag 11) 45
207 5.10. Trust Packet (Tag 12) 46
208 5.11. User ID Packet (Tag 13) 46
209 5.12. User Attribute Packet (Tag 17) 46
210 5.12.1. The Image Attribute Subpacket 47
211 5.13. Sym. Encrypted Integrity Protected Data Packet (Tag 18) 47
212 5.14. Modification Detection Code Packet (Tag 19) 50
213 6. Radix-64 Conversions 51
214 6.1. An Implementation of the CRC-24 in "C" 51
215 6.2. Forming ASCII Armor 52
216 6.3. Encoding Binary in Radix-64 54
217 6.4. Decoding Radix-64 55
218 6.5. Examples of Radix-64 56
219 6.6. Example of an ASCII Armored Message 56
220 7. Cleartext signature framework 56
221 7.1. Dash-Escaped Text 57
222 8. Regular Expressions 58
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227 9. Constants 58
228 9.1. Public Key Algorithms 59
229 9.2. Symmetric Key Algorithms 59
230 9.3. Compression Algorithms 60
231 9.4. Hash Algorithms 60
232 10. IANA Considerations 60
233 10.1. New String-to-Key specifier types 60
234 10.2. New Packets 61
235 10.2.1. User Attribute Types 61
236 10.2.1.1.Image Format Subpacket Types 61
237 10.2.2. New Signature Subpackets 61
238 10.2.2.1.Signature Notation Data Subpackets 61
239 10.2.2.2.Key Server Preference Extensions 62
240 10.2.2.3.Key Flags Extensions 62
241 10.2.2.4.Reason For Revocation Extensions 62
242 10.2.2.5.Implementation Features 62
243 10.2.3. New Packet Versions 62
244 10.3. New Algorithms 63
245 10.3.1. Public Key Algorithms 63
246 10.3.2. Symmetric Key Algorithms 63
247 10.3.3. Hash Algorithms 63
248 10.3.4. Compression Algorithms 64
249 11. Packet Composition 64
250 11.1. Transferable Public Keys 64
251 11.2. Transferable Secret Keys 65
252 11.3. OpenPGP Messages 65
253 11.4. Detached Signatures 66
254 12. Enhanced Key Formats 66
255 12.1. Key Structures 66
256 12.2. Key IDs and Fingerprints 67
257 13. Notes on Algorithms 68
258 13.1. PKCS#1 Encoding In OpenPGP 68
259 13.1.1. EME-PKCS1-v1_5-ENCODE 69
260 13.1.2. EME-PKCS1-v1_5-DECODE 69
261 13.1.3. EMSA-PKCS1-v1_5 70
262 13.2. Symmetric Algorithm Preferences 71
263 13.3. Other Algorithm Preferences 71
264 13.3.1. Compression Preferences 71
265 13.3.2. Hash Algorithm Preferences 72
266 13.4. Plaintext 72
267 13.5. RSA 72
268 13.6. DSA 73
269 13.7. Elgamal 73
270 13.8. Reserved Algorithm Numbers 73
271 13.9. OpenPGP CFB mode 74
272 13.10. Private or Experimental Parameters 75
273 13.11. Extension of the MDC System 75
274 13.12. Meta-Considerations for Expansion 76
275 14. Security Considerations 76
276 15. Implementation Nits 79
277 16. Authors' Addresses 80
278 17. References (Normative) 81
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283 18. References (Informative) 83
284 19. Full Copyright Statement 84
285 20. Intellectual Property 84
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339 1. Introduction
341 This document provides information on the message-exchange packet
342 formats used by OpenPGP to provide encryption, decryption, signing,
343 and key management functions. It is a revision of RFC 2440, "OpenPGP
344 Message Format", which itself replaces RFC 1991, "PGP Message
345 Exchange Formats." [RFC1991] [RFC2440]
347 1.1. Terms
349 * OpenPGP - This is a definition for security software that uses
350 PGP 5.x as a basis, formalized in RFC 2440 and this document.
352 * PGP - Pretty Good Privacy. PGP is a family of software systems
353 developed by Philip R. Zimmermann from which OpenPGP is based.
355 * PGP 2.6.x - This version of PGP has many variants, hence the
356 term PGP 2.6.x. It used only RSA, MD5, and IDEA for its
357 cryptographic transforms. An informational RFC, RFC 1991, was
358 written describing this version of PGP.
360 * PGP 5.x - This version of PGP is formerly known as "PGP 3" in
361 the community and also in the predecessor of this document, RFC
362 1991. It has new formats and corrects a number of problems in
363 the PGP 2.6.x design. It is referred to here as PGP 5.x because
364 that software was the first release of the "PGP 3" code base.
366 * GnuPG - GNU Privacy Guard, also called GPG. GnuPG is an OpenPGP
367 implementation that avoids all encumbered algorithms.
368 Consequently, early versions of GnuPG did not include RSA public
369 keys. GnuPG may or may not have (depending on version) support
370 for IDEA or other encumbered algorithms.
372 "PGP", "Pretty Good", and "Pretty Good Privacy" are trademarks of
373 PGP Corporation and are used with permission. The term "OpenPGP"
374 refers to the protocol described in this and related documents.
376 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
377 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
378 document are to be interpreted as described in RFC 2119.
380 The key words "PRIVATE USE", "HIERARCHICAL ALLOCATION", "FIRST COME
381 FIRST SERVED", "EXPERT REVIEW", "SPECIFICATION REQUIRED", "IESG
382 APPROVAL", "IETF CONSENSUS", and "STANDARDS ACTION" that appear in
383 this document when used to describe namespace allocation are to be
384 interpreted as described in RFC 2434.
386 2. General functions
388 OpenPGP provides data integrity services for messages and data files
389 by using these core technologies:
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395 - digital signatures
397 - encryption
399 - compression
401 - radix-64 conversion
403 In addition, OpenPGP provides key management and certificate
404 services, but many of these are beyond the scope of this document.
406 2.1. Confidentiality via Encryption
408 OpenPGP combines symmetric-key encryption and public key encryption
409 to provide confidentiality. When made confidential, first the object
410 is encrypted using a symmetric encryption algorithm. Each symmetric
411 key is used only once, for a single object. A new "session key" is
412 generated as a random number for each object (sometimes referred to
413 as a session). Since it is used only once, the session key is bound
414 to the message and transmitted with it. To protect the key, it is
415 encrypted with the receiver's public key. The sequence is as
416 follows:
418 1. The sender creates a message.
420 2. The sending OpenPGP generates a random number to be used as a
421 session key for this message only.
423 3. The session key is encrypted using each recipient's public key.
424 These "encrypted session keys" start the message.
426 4. The sending OpenPGP encrypts the message using the session key,
427 which forms the remainder of the message. Note that the message
428 is also usually compressed.
430 5. The receiving OpenPGP decrypts the session key using the
431 recipient's private key.
433 6. The receiving OpenPGP decrypts the message using the session
434 key. If the message was compressed, it will be decompressed.
436 With symmetric-key encryption, an object may be encrypted with a
437 symmetric key derived from a passphrase (or other shared secret), or
438 a two-stage mechanism similar to the public-key method described
439 above in which a session key is itself encrypted with a symmetric
440 algorithm keyed from a shared secret.
442 Both digital signature and confidentiality services may be applied
443 to the same message. First, a signature is generated for the message
444 and attached to the message. Then, the message plus signature is
445 encrypted using a symmetric session key. Finally, the session key is
446 encrypted using public-key encryption and prefixed to the encrypted
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451 block.
453 2.2. Authentication via Digital signature
455 The digital signature uses a hash code or message digest algorithm,
456 and a public-key signature algorithm. The sequence is as follows:
458 1. The sender creates a message.
460 2. The sending software generates a hash code of the message.
462 3. The sending software generates a signature from the hash code
463 using the sender's private key.
465 4. The binary signature is attached to the message.
467 5. The receiving software keeps a copy of the message signature.
469 6. The receiving software generates a new hash code for the
470 received message and verifies it using the message's signature.
471 If the verification is successful, the message is accepted as
472 authentic.
474 2.3. Compression
476 OpenPGP implementations SHOULD compress the message after applying
477 the signature but before encryption.
479 If an implementation does not implement compression, its authors
480 should be aware that most OpenPGP messages in the world are
481 compressed. Thus, it may even be wise for a space-constrained
482 implementation to implement decompression, but not compression.
484 Furthermore, compression has the added side-effect that some types
485 of attacks can be thwarted by the fact that slightly altered,
486 compressed data rarely uncompresses without severe errors. This is
487 hardly rigorous, but it is operationally useful. These attacks can
488 be rigorously prevented by implementing and using Modification
489 Detection Codes as described in sections following.
491 2.4. Conversion to Radix-64
493 OpenPGP's underlying native representation for encrypted messages,
494 signature certificates, and keys is a stream of arbitrary octets.
495 Some systems only permit the use of blocks consisting of seven-bit,
496 printable text. For transporting OpenPGP's native raw binary octets
497 through channels that are not safe to raw binary data, a printable
498 encoding of these binary octets is needed. OpenPGP provides the
499 service of converting the raw 8-bit binary octet stream to a stream
500 of printable ASCII characters, called Radix-64 encoding or ASCII
501 Armor.
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507 Implementations SHOULD provide Radix-64 conversions.
509 2.5. Signature-Only Applications
511 OpenPGP is designed for applications that use both encryption and
512 signatures, but there are a number of problems that are solved by a
513 signature-only implementation. Although this specification requires
514 both encryption and signatures, it is reasonable for there to be
515 subset implementations that are non-conformant only in that they
516 omit encryption.
518 3. Data Element Formats
520 This section describes the data elements used by OpenPGP.
522 3.1. Scalar numbers
524 Scalar numbers are unsigned, and are always stored in big-endian
525 format. Using n[k] to refer to the kth octet being interpreted, the
526 value of a two-octet scalar is ((n[0] << 8) + n[1]). The value of a
527 four-octet scalar is ((n[0] << 24) + (n[1] << 16) + (n[2] << 8) +
528 n[3]).
530 3.2. Multiprecision Integers
532 Multiprecision Integers (also called MPIs) are unsigned integers
533 used to hold large integers such as the ones used in cryptographic
534 calculations.
536 An MPI consists of two pieces: a two-octet scalar that is the length
537 of the MPI in bits followed by a string of octets that contain the
538 actual integer.
540 These octets form a big-endian number; a big-endian number can be
541 made into an MPI by prefixing it with the appropriate length.
543 Examples:
545 (all numbers are in hexadecimal)
547 The string of octets [00 01 01] forms an MPI with the value 1. The
548 string [00 09 01 FF] forms an MPI with the value of 511.
550 Additional rules:
552 The size of an MPI is ((MPI.length + 7) / 8) + 2 octets.
554 The length field of an MPI describes the length starting from its
555 most significant non-zero bit. Thus, the MPI [00 02 01] is not
556 formed correctly. It should be [00 01 01].
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563 Unused bits of an MPI MUST be zero.
565 Also note that when an MPI is encrypted, the length refers to the
566 plaintext MPI. It may be ill-formed in its ciphertext.
568 3.3. Key IDs
570 A Key ID is an eight-octet scalar that identifies a key.
571 Implementations SHOULD NOT assume that Key IDs are unique. The
572 section, "Enhanced Key Formats" below describes how Key IDs are
573 formed.
575 3.4. Text
577 Unless otherwise specified, the character set for text is the UTF-8
578 [RFC3629] encoding of Unicode [ISO10646].
580 3.5. Time fields
582 A time field is an unsigned four-octet number containing the number
583 of seconds elapsed since midnight, 1 January 1970 UTC.
585 3.6. Keyrings
587 A keyring is a collection of one or more keys in a file or database.
588 Traditionally, a keyring is simply a sequential list of keys, but
589 may be any suitable database. It is beyond the scope of this
590 standard to discuss the details of keyrings or other databases.
592 3.7. String-to-key (S2K) specifiers
594 String-to-key (S2K) specifiers are used to convert passphrase
595 strings into symmetric-key encryption/decryption keys. They are used
596 in two places, currently: to encrypt the secret part of private keys
597 in the private keyring, and to convert passphrases to encryption
598 keys for symmetrically encrypted messages.
600 3.7.1. String-to-key (S2K) specifier types
602 There are three types of S2K specifiers currently supported, and
603 some reserved values:
605 ID S2K Type
606 -- --- ----
607 0 Simple S2K
608 1 Salted S2K
609 2 Reserved value
610 3 Iterated and Salted S2K
611 100 to 110 Private/Experimental S2K
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619 These are described as follows:
621 3.7.1.1. Simple S2K
623 This directly hashes the string to produce the key data. See below
624 for how this hashing is done.
626 Octet 0: 0x00
627 Octet 1: hash algorithm
629 Simple S2K hashes the passphrase to produce the session key. The
630 manner in which this is done depends on the size of the session key
631 (which will depend on the cipher used) and the size of the hash
632 algorithm's output. If the hash size is greater than the session key
633 size, the high-order (leftmost) octets of the hash are used as the
634 key.
636 If the hash size is less than the key size, multiple instances of
637 the hash context are created -- enough to produce the required key
638 data. These instances are preloaded with 0, 1, 2, ... octets of
639 zeros (that is to say, the first instance has no preloading, the
640 second gets preloaded with 1 octet of zero, the third is preloaded
641 with two octets of zeros, and so forth).
643 As the data is hashed, it is given independently to each hash
644 context. Since the contexts have been initialized differently, they
645 will each produce different hash output. Once the passphrase is
646 hashed, the output data from the multiple hashes is concatenated,
647 first hash leftmost, to produce the key data, with any excess octets
648 on the right discarded.
650 3.7.1.2. Salted S2K
652 This includes a "salt" value in the S2K specifier -- some arbitrary
653 data -- that gets hashed along with the passphrase string, to help
654 prevent dictionary attacks.
656 Octet 0: 0x01
657 Octet 1: hash algorithm
658 Octets 2-9: 8-octet salt value
660 Salted S2K is exactly like Simple S2K, except that the input to the
661 hash function(s) consists of the 8 octets of salt from the S2K
662 specifier, followed by the passphrase.
664 3.7.1.3. Iterated and Salted S2K
666 This includes both a salt and an octet count. The salt is combined
667 with the passphrase and the resulting value is hashed repeatedly.
668 This further increases the amount of work an attacker must do to try
669 dictionary attacks.
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675 Octet 0: 0x03
676 Octet 1: hash algorithm
677 Octets 2-9: 8-octet salt value
678 Octet 10: count, a one-octet, coded value
680 The count is coded into a one-octet number using the following
681 formula:
683 #define EXPBIAS 6
684 count = ((Int32)16 + (c & 15)) << ((c >> 4) + EXPBIAS);
686 The above formula is in C, where "Int32" is a type for a 32-bit
687 integer, and the variable "c" is the coded count, Octet 10.
689 Iterated-Salted S2K hashes the passphrase and salt data multiple
690 times. The total number of octets to be hashed is specified in the
691 encoded count in the S2K specifier. Note that the resulting count
692 value is an octet count of how many octets will be hashed, not an
693 iteration count.
695 Initially, one or more hash contexts are set up as with the other
696 S2K algorithms, depending on how many octets of key data are needed.
697 Then the salt, followed by the passphrase data is repeatedly hashed
698 until the number of octets specified by the octet count has been
699 hashed. The one exception is that if the octet count is less than
700 the size of the salt plus passphrase, the full salt plus passphrase
701 will be hashed even though that is greater than the octet count.
702 After the hashing is done the data is unloaded from the hash
703 context(s) as with the other S2K algorithms.
705 3.7.2. String-to-key usage
707 Implementations SHOULD use salted or iterated-and-salted S2K
708 specifiers, as simple S2K specifiers are more vulnerable to
709 dictionary attacks.
711 3.7.2.1. Secret key encryption
713 An S2K specifier can be stored in the secret keyring to specify how
714 to convert the passphrase to a key that unlocks the secret data.
715 Older versions of PGP just stored a cipher algorithm octet preceding
716 the secret data or a zero to indicate that the secret data was
717 unencrypted. The MD5 hash function was always used to convert the
718 passphrase to a key for the specified cipher algorithm.
720 For compatibility, when an S2K specifier is used, the special value
721 254 or 255 is stored in the position where the hash algorithm octet
722 would have been in the old data structure. This is then followed
723 immediately by a one-octet algorithm identifier, and then by the S2K
724 specifier as encoded above.
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731 Therefore, preceding the secret data there will be one of these
732 possibilities:
734 0: secret data is unencrypted (no passphrase)
735 255 or 254: followed by algorithm octet and S2K specifier
736 Cipher alg: use Simple S2K algorithm using MD5 hash
738 This last possibility, the cipher algorithm number with an implicit
739 use of MD5 and IDEA, is provided for backward compatibility; it MAY
740 be understood, but SHOULD NOT be generated, and is deprecated.
742 These are followed by an Initial Vector of the same length as the
743 block size of the cipher for the decryption of the secret values, if
744 they are encrypted, and then the secret key values themselves.
746 3.7.2.2. Symmetric-key message encryption
748 OpenPGP can create a Symmetric-key Encrypted Session Key (ESK)
749 packet at the front of a message. This is used to allow S2K
750 specifiers to be used for the passphrase conversion or to create
751 messages with a mix of symmetric-key ESKs and public-key ESKs. This
752 allows a message to be decrypted either with a passphrase or a
753 public key pair.
755 PGP 2.X always used IDEA with Simple string-to-key conversion when
756 encrypting a message with a symmetric algorithm. This is deprecated,
757 but MAY be used for backward-compatibility.
759 4. Packet Syntax
761 This section describes the packets used by OpenPGP.
763 4.1. Overview
765 An OpenPGP message is constructed from a number of records that are
766 traditionally called packets. A packet is a chunk of data that has a
767 tag specifying its meaning. An OpenPGP message, keyring,
768 certificate, and so forth consists of a number of packets. Some of
769 those packets may contain other OpenPGP packets (for example, a
770 compressed data packet, when uncompressed, contains OpenPGP
771 packets).
773 Each packet consists of a packet header, followed by the packet
774 body. The packet header is of variable length.
776 4.2. Packet Headers
778 The first octet of the packet header is called the "Packet Tag." It
779 determines the format of the header and denotes the packet contents.
780 The remainder of the packet header is the length of the packet.
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787 Note that the most significant bit is the left-most bit, called bit
788 7. A mask for this bit is 0x80 in hexadecimal.
790 +---------------+
791 PTag |7 6 5 4 3 2 1 0|
792 +---------------+
793 Bit 7 -- Always one
794 Bit 6 -- New packet format if set
796 PGP 2.6.x only uses old format packets. Thus, software that
797 interoperates with those versions of PGP must only use old format
798 packets. If interoperability is not an issue, the new packet format
799 is RECOMMENDED. Note that old format packets have four bits of
800 packet tags, and new format packets have six; some features cannot
801 be used and still be backward-compatible.
803 Also note that packets with a tag greater than or equal to 16 MUST
804 use new format packets. The old format packets can only express tags
805 less than or equal to 15.
807 Old format packets contain:
809 Bits 5-2 -- packet tag
810 Bits 1-0 - length-type
812 New format packets contain:
814 Bits 5-0 -- packet tag
816 4.2.1. Old-Format Packet Lengths
818 The meaning of the length-type in old-format packets is:
820 0 - The packet has a one-octet length. The header is 2 octets long.
822 1 - The packet has a two-octet length. The header is 3 octets long.
824 2 - The packet has a four-octet length. The header is 5 octets long.
826 3 - The packet is of indeterminate length. The header is 1 octet
827 long, and the implementation must determine how long the packet
828 is. If the packet is in a file, this means that the packet
829 extends until the end of the file. In general, an implementation
830 SHOULD NOT use indeterminate length packets except where the end
831 of the data will be clear from the context, and even then it is
832 better to use a definite length, or a new-format header. The
833 new-format headers described below have a mechanism for
834 precisely encoding data of indeterminate length.
836 4.2.2. New-Format Packet Lengths
838 New format packets have four possible ways of encoding length:
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843 1. A one-octet Body Length header encodes packet lengths of up to
844 191 octets.
846 2. A two-octet Body Length header encodes packet lengths of 192 to
847 8383 octets.
849 3. A five-octet Body Length header encodes packet lengths of up to
850 4,294,967,295 (0xFFFFFFFF) octets in length. (This actually
851 encodes a four-octet scalar number.)
853 4. When the length of the packet body is not known in advance by
854 the issuer, Partial Body Length headers encode a packet of
855 indeterminate length, effectively making it a stream.
857 4.2.2.1. One-Octet Lengths
859 A one-octet Body Length header encodes a length of from 0 to 191
860 octets. This type of length header is recognized because the one
861 octet value is less than 192. The body length is equal to:
863 bodyLen = 1st_octet;
865 4.2.2.2. Two-Octet Lengths
867 A two-octet Body Length header encodes a length of from 192 to 8383
868 octets. It is recognized because its first octet is in the range 192
869 to 223. The body length is equal to:
871 bodyLen = ((1st_octet - 192) << 8) + (2nd_octet) + 192
873 4.2.2.3. Five-Octet Lengths
875 A five-octet Body Length header consists of a single octet holding
876 the value 255, followed by a four-octet scalar. The body length is
877 equal to:
879 bodyLen = (2nd_octet << 24) | (3rd_octet << 16) |
880 (4th_octet << 8) | 5th_octet
882 This basic set of one, two, and five-octet lengths is also used
883 internally to some packets.
885 4.2.2.4. Partial Body Lengths
887 A Partial Body Length header is one octet long and encodes the
888 length of only part of the data packet. This length is a power of 2,
889 from 1 to 1,073,741,824 (2 to the 30th power). It is recognized by
890 its one octet value that is greater than or equal to 224, and less
891 than 255. The partial body length is equal to:
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899 partialBodyLen = 1 << (1st_octet & 0x1f);
901 Each Partial Body Length header is followed by a portion of the
902 packet body data. The Partial Body Length header specifies this
903 portion's length. Another length header (one octet, two-octet,
904 five-octet, or partial) follows that portion. The last length header
905 in the packet MUST NOT be a partial Body Length header. Partial Body
906 Length headers may only be used for the non-final parts of the
907 packet.
909 Note also that the last Body Length header can be a zero-length
910 header.
912 An implementation MAY use Partial Body Lengths for data packets, be
913 they literal, compressed, or encrypted. The first partial length
914 MUST be at least 512 octets long. Partial Body Lengths MUST NOT be
915 used for any other packet types.
917 4.2.3. Packet Length Examples
919 These examples show ways that new-format packets might encode the
920 packet lengths.
922 A packet with length 100 may have its length encoded in one octet:
923 0x64. This is followed by 100 octets of data.
925 A packet with length 1723 may have its length coded in two octets:
926 0xC5, 0xFB. This header is followed by the 1723 octets of data.
928 A packet with length 100000 may have its length encoded in five
929 octets: 0xFF, 0x00, 0x01, 0x86, 0xA0.
931 It might also be encoded in the following octet stream: 0xEF, first
932 32768 octets of data; 0xE1, next two octets of data; 0xE0, next one
933 octet of data; 0xF0, next 65536 octets of data; 0xC5, 0xDD, last
934 1693 octets of data. This is just one possible encoding, and many
935 variations are possible on the size of the Partial Body Length
936 headers, as long as a regular Body Length header encodes the last
937 portion of the data.
939 Please note that in all of these explanations, the total length of
940 the packet is the length of the header(s) plus the length of the
941 body.
943 4.3. Packet Tags
945 The packet tag denotes what type of packet the body holds. Note that
946 old format headers can only have tags less than 16, whereas new
947 format headers can have tags as great as 63. The defined tags (in
948 decimal) are:
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953 \fINTERNET-DRAFT OpenPGP Message Format Apr 24, 2007
955 0 -- Reserved - a packet tag MUST NOT have this value
956 1 -- Public-Key Encrypted Session Key Packet
957 2 -- Signature Packet
958 3 -- Symmetric-Key Encrypted Session Key Packet
959 4 -- One-Pass Signature Packet
960 5 -- Secret Key Packet
961 6 -- Public Key Packet
962 7 -- Secret Subkey Packet
963 8 -- Compressed Data Packet
964 9 -- Symmetrically Encrypted Data Packet
965 10 -- Marker Packet
966 11 -- Literal Data Packet
967 12 -- Trust Packet
968 13 -- User ID Packet
969 14 -- Public Subkey Packet
970 17 -- User Attribute Packet
971 18 -- Sym. Encrypted and Integrity Protected Data Packet
972 19 -- Modification Detection Code Packet
973 60 to 63 -- Private or Experimental Values
975 5. Packet Types
977 5.1. Public-Key Encrypted Session Key Packets (Tag 1)
979 A Public-Key Encrypted Session Key packet holds the session key used
980 to encrypt a message. Zero or more Public-Key Encrypted Session Key
981 packets and/or Symmetric-Key Encrypted Session Key packets may
982 precede a Symmetrically Encrypted Data Packet, which holds an
983 encrypted message. The message is encrypted with the session key,
984 and the session key is itself encrypted and stored in the Encrypted
985 Session Key packet(s). The Symmetrically Encrypted Data Packet is
986 preceded by one Public-Key Encrypted Session Key packet for each
987 OpenPGP key to which the message is encrypted. The recipient of the
988 message finds a session key that is encrypted to their public key,
989 decrypts the session key, and then uses the session key to decrypt
990 the message.
992 The body of this packet consists of:
994 - A one-octet number giving the version number of the packet type.
995 The currently defined value for packet version is 3.
997 - An eight-octet number that gives the key ID of the public key
998 that the session key is encrypted to. If the session key is
999 encrypted to a subkey then the key ID of this subkey is used
1000 here instead of the key ID of the primary key.
1002 - A one-octet number giving the public key algorithm used.
1004 - A string of octets that is the encrypted session key. This
1005 string takes up the remainder of the packet, and its contents
1006 are dependent on the public key algorithm used.
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1011 Algorithm Specific Fields for RSA encryption
1013 - multiprecision integer (MPI) of RSA encrypted value m**e mod n.
1015 Algorithm Specific Fields for Elgamal encryption:
1017 - MPI of Elgamal (Diffie-Hellman) value g**k mod p.
1019 - MPI of Elgamal (Diffie-Hellman) value m * y**k mod p.
1021 The value "m" in the above formulas is derived from the session key
1022 as follows. First the session key is prefixed with a one-octet
1023 algorithm identifier that specifies the symmetric encryption
1024 algorithm used to encrypt the following Symmetrically Encrypted Data
1025 Packet. Then a two-octet checksum is appended which is equal to the
1026 sum of the preceding session key octets, not including the algorithm
1027 identifier, modulo 65536. This value is then encoded as described in
1028 PKCS#1 block encoding EME-PKCS1-v1_5 in Section 12.1 of RFC 3447 to
1029 form the "m" value used in the formulas above. See Section 13.1 of
1030 this document for notes on OpenPGP's use of PKCS#1.
1032 Note that when an implementation forms several PKESKs with one
1033 session key, forming a message that can be decrypted by several
1034 keys, the implementation MUST make a new PKCS#1 encoding for each
1035 key.
1037 An implementation MAY accept or use a Key ID of zero as a "wild
1038 card" or "speculative" Key ID. In this case, the receiving
1039 implementation would try all available private keys, checking for a
1040 valid decrypted session key. This format helps reduce traffic
1041 analysis of messages.
1043 5.2. Signature Packet (Tag 2)
1045 A signature packet describes a binding between some public key and
1046 some data. The most common signatures are a signature of a file or a
1047 block of text, and a signature that is a certification of a User ID.
1049 Two versions of signature packets are defined. Version 3 provides
1050 basic signature information, while version 4 provides an expandable
1051 format with subpackets that can specify more information about the
1052 signature. PGP 2.6.x only accepts version 3 signatures.
1054 Implementations SHOULD accept V3 signatures. Implementations SHOULD
1055 generate V4 signatures.
1057 Note that if an implementation is creating an encrypted and signed
1058 message that is encrypted to a V3 key, it is reasonable to create a
1059 V3 signature.
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1067 5.2.1. Signature Types
1069 There are a number of possible meanings for a signature, which are
1070 indicated in a signature type octet in any given signature. Please
1071 note that the vagueness of these meanings is not a flaw, but a
1072 feature of the system. Because OpenPGP places final authority for
1073 validity upon the receiver of a signature, it may be that one
1074 signer's casual act might be more rigorous than some other
1075 authority's positive act. See section 5.2.4, "Computing Signatures,"
1076 for detailed information on how to compute and verify signatures of
1077 each type.
1079 These meanings are:
1081 0x00: Signature of a binary document.
1082 This means the signer owns it, created it, or certifies that it
1083 has not been modified.
1085 0x01: Signature of a canonical text document.
1086 This means the signer owns it, created it, or certifies that it
1087 has not been modified. The signature is calculated over the text
1088 data with its line endings converted to <CR><LF>.
1090 0x02: Standalone signature.
1091 This signature is a signature of only its own subpacket
1092 contents. It is calculated identically to a signature over a
1093 zero-length binary document. Note that it doesn't make sense to
1094 have a V3 standalone signature.
1096 0x10: Generic certification of a User ID and Public Key packet.
1097 The issuer of this certification does not make any particular
1098 assertion as to how well the certifier has checked that the
1099 owner of the key is in fact the person described by the User ID.
1101 0x11: Persona certification of a User ID and Public Key packet.
1102 The issuer of this certification has not done any verification
1103 of the claim that the owner of this key is the User ID
1104 specified.
1106 0x12: Casual certification of a User ID and Public Key packet.
1107 The issuer of this certification has done some casual
1108 verification of the claim of identity.
1110 0x13: Positive certification of a User ID and Public Key packet.
1111 The issuer of this certification has done substantial
1112 verification of the claim of identity.
1114 Most OpenPGP implementations make their "key signatures" as 0x10
1115 certifications. Some implementations can issue 0x11-0x13
1116 certifications, but few differentiate between the types.
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1123 0x18: Subkey Binding Signature
1124 This signature is a statement by the top-level signing key that
1125 indicates that it owns the subkey. This signature is calculated
1126 directly on the primary key and subkey, and not on any User ID
1127 or other packets. A signature that binds a signing subkey MUST
1128 have an embedded signature subpacket in this binding signature
1129 which contains a 0x19 signature made by the signing subkey on
1130 the primary key and subkey.
1132 0x19 Primary Key Binding Signature
1133 This signature is a statement by a signing subkey, indicating
1134 that it is owned by the primary key and subkey. This signature
1135 is calculated the same way as a 0x18 signature: directly on the
1136 primary key and subkey, and not on any User ID or other packets.
1138 0x1F: Signature directly on a key
1139 This signature is calculated directly on a key. It binds the
1140 information in the signature subpackets to the key, and is
1141 appropriate to be used for subpackets that provide information
1142 about the key, such as the revocation key subpacket. It is also
1143 appropriate for statements that non-self certifiers want to make
1144 about the key itself, rather than the binding between a key and
1145 a name.
1147 0x20: Key revocation signature
1148 The signature is calculated directly on the key being revoked. A
1149 revoked key is not to be used. Only revocation signatures by the
1150 key being revoked, or by an authorized revocation key, should be
1151 considered valid revocation signatures.
1153 0x28: Subkey revocation signature
1154 The signature is calculated directly on the subkey being
1155 revoked. A revoked subkey is not to be used. Only revocation
1156 signatures by the top-level signature key that is bound to this
1157 subkey, or by an authorized revocation key, should be considered
1158 valid revocation signatures.
1160 0x30: Certification revocation signature
1161 This signature revokes an earlier User ID certification
1162 signature (signature class 0x10 through 0x13) or direct-key
1163 signature (0x1F). It should be issued by the same key that
1164 issued the revoked signature or an authorized revocation key.
1165 The signature is computed over the same data as the certificate
1166 that it revokes, and should have a later creation date than that
1167 certificate.
1169 0x40: Timestamp signature.
1170 This signature is only meaningful for the timestamp contained in
1171 it.
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1179 0x50: Third-Party Confirmation signature.
1180 This signature is a signature over some other OpenPGP signature
1181 packet(s). It is analogous to a notary seal on the signed data.
1182 A third-party signature SHOULD include Signature Target
1183 subpacket(s) to give easy identification. Note that we really do
1184 mean SHOULD. There are plausible uses for this (such as a blind
1185 party that only sees the signature, not the key nor source
1186 document) that cannot include a target subpacket.
1188 5.2.2. Version 3 Signature Packet Format
1190 The body of a version 3 Signature Packet contains:
1192 - One-octet version number (3).
1194 - One-octet length of following hashed material. MUST be 5.
1196 - One-octet signature type.
1198 - Four-octet creation time.
1200 - Eight-octet key ID of signer.
1202 - One-octet public key algorithm.
1204 - One-octet hash algorithm.
1206 - Two-octet field holding left 16 bits of signed hash value.
1208 - One or more multiprecision integers comprising the signature.
1209 This portion is algorithm specific, as described below.
1211 The concatenation of the data to be signed, the signature type and
1212 creation time from the signature packet (5 additional octets) is
1213 hashed. The resulting hash value is used in the signature algorithm.
1214 The high 16 bits (first two octets) of the hash are included in the
1215 signature packet to provide a quick test to reject some invalid
1216 signatures.
1218 Algorithm Specific Fields for RSA signatures:
1220 - multiprecision integer (MPI) of RSA signature value m**d mod n.
1222 Algorithm Specific Fields for DSA signatures:
1224 - MPI of DSA value r.
1226 - MPI of DSA value s.
1228 The signature calculation is based on a hash of the signed data, as
1229 described above. The details of the calculation are different for
1230 DSA signatures than for RSA signatures.
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1235 With RSA signatures, the hash value is encoded as described in
1236 PKCS#1 section 9.2.1 of RFC 3447 encoded using PKCS#1 encoding type
1237 EMSA-PKCS1-v1_5 as described in section 12.1 of RFC 3447. This
1238 requires inserting the hash value as an octet string into an ASN.1
1239 structure. The object identifier for the type of hash being used is
1240 included in the structure. The hexadecimal representations for the
1241 currently defined hash algorithms are:
1243 - MD5: 0x2A, 0x86, 0x48, 0x86, 0xF7, 0x0D, 0x02, 0x05
1245 - RIPEMD-160: 0x2B, 0x24, 0x03, 0x02, 0x01
1247 - SHA-1: 0x2B, 0x0E, 0x03, 0x02, 0x1A
1249 - SHA224: 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x04
1251 - SHA256: 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x01
1253 - SHA384: 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x02
1255 - SHA512: 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x03
1257 The ASN.1 OIDs are:
1259 - MD5: 1.2.840.113549.2.5
1261 - RIPEMD-160: 1.3.36.3.2.1
1263 - SHA-1: 1.3.14.3.2.26
1265 - SHA224: 2.16.840.1.101.3.4.2.4
1267 - SHA256: 2.16.840.1.101.3.4.2.1
1269 - SHA384: 2.16.840.1.101.3.4.2.2
1271 - SHA512: 2.16.840.1.101.3.4.2.3
1273 The full hash prefixes for these are:
1275 MD5: 0x30, 0x20, 0x30, 0x0C, 0x06, 0x08, 0x2A, 0x86,
1276 0x48, 0x86, 0xF7, 0x0D, 0x02, 0x05, 0x05, 0x00,
1277 0x04, 0x10
1279 RIPEMD-160: 0x30, 0x21, 0x30, 0x09, 0x06, 0x05, 0x2B, 0x24,
1280 0x03, 0x02, 0x01, 0x05, 0x00, 0x04, 0x14
1282 SHA-1: 0x30, 0x21, 0x30, 0x09, 0x06, 0x05, 0x2b, 0x0E,
1283 0x03, 0x02, 0x1A, 0x05, 0x00, 0x04, 0x14
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1291 SHA224: 0x30, 0x31, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86,
1292 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x04, 0x05,
1293 0x00, 0x04, 0x1C
1295 SHA256: 0x30, 0x31, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86,
1296 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x01, 0x05,
1297 0x00, 0x04, 0x20
1299 SHA384: 0x30, 0x41, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86,
1300 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x02, 0x05,
1301 0x00, 0x04, 0x30
1303 SHA512: 0x30, 0x51, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86,
1304 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x03, 0x05,
1305 0x00, 0x04, 0x40
1307 DSA signatures MUST use hashes that are equal in size to the number
1308 of bits of q, the group generated by the DSA key's generator value.
1309 If the output size of the chosen hash is larger than the number of
1310 bits of q, the hash result is truncated to fit by taking the number
1311 of leftmost bits equal to the number of bits of q. This (possibly
1312 truncated) hash function result is treated as a number and used
1313 directly in the DSA signature algorithm.
1315 5.2.3. Version 4 Signature Packet Format
1317 The body of a version 4 Signature Packet contains:
1319 - One-octet version number (4).
1321 - One-octet signature type.
1323 - One-octet public key algorithm.
1325 - One-octet hash algorithm.
1327 - Two-octet scalar octet count for following hashed subpacket
1328 data. Note that this is the length in octets of all of the
1329 hashed subpackets; a pointer incremented by this number will
1330 skip over the hashed subpackets.
1332 - Hashed subpacket data set. (zero or more subpackets)
1334 - Two-octet scalar octet count for the following unhashed
1335 subpacket data. Note that this is the length in octets of all of
1336 the unhashed subpackets; a pointer incremented by this number
1337 will skip over the unhashed subpackets.
1339 - Unhashed subpacket data set. (zero or more subpackets)
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1347 - Two-octet field holding the left 16 bits of the signed hash
1348 value.
1350 - One or more multiprecision integers comprising the signature.
1351 This portion is algorithm specific, as described above.
1353 The concatenation of the data being signed and the signature data
1354 from the version number through the hashed subpacket data
1355 (inclusive) is hashed. The resulting hash value is what is signed.
1356 The left 16 bits of the hash are included in the signature packet to
1357 provide a quick test to reject some invalid signatures.
1359 There are two fields consisting of signature subpackets. The first
1360 field is hashed with the rest of the signature data, while the
1361 second is unhashed. The second set of subpackets is not
1362 cryptographically protected by the signature and should include only
1363 advisory information.
1365 The algorithms for converting the hash function result to a
1366 signature are described in a section below.
1368 5.2.3.1. Signature Subpacket Specification
1370 A subpacket data set consists of zero or more signature subpackets.
1371 In signature packets the subpacket data set is preceded by a
1372 two-octet scalar count of the length in octets of all the
1373 subpackets. A pointer incremented by this number will skip over the
1374 subpacket data set.
1376 Each subpacket consists of a subpacket header and a body. The header
1377 consists of:
1379 - the subpacket length (1, 2, or 5 octets)
1381 - the subpacket type (1 octet)
1383 and is followed by the subpacket specific data.
1385 The length includes the type octet but not this length. Its format
1386 is similar to the "new" format packet header lengths, but cannot
1387 have partial body lengths. That is:
1389 if the 1st octet < 192, then
1390 lengthOfLength = 1
1391 subpacketLen = 1st_octet
1393 if the 1st octet >= 192 and < 255, then
1394 lengthOfLength = 2
1395 subpacketLen = ((1st_octet - 192) << 8) + (2nd_octet) + 192
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1403 if the 1st octet = 255, then
1404 lengthOfLength = 5
1405 subpacket length = [four-octet scalar starting at 2nd_octet]
1407 The value of the subpacket type octet may be:
1409 0 = reserved
1410 1 = reserved
1411 2 = signature creation time
1412 3 = signature expiration time
1413 4 = exportable certification
1414 5 = trust signature
1415 6 = regular expression
1416 7 = revocable
1417 8 = reserved
1418 9 = key expiration time
1419 10 = placeholder for backward compatibility
1420 11 = preferred symmetric algorithms
1421 12 = revocation key
1422 13 = reserved
1423 14 = reserved
1424 15 = reserved
1425 16 = issuer key ID
1426 17 = reserved
1427 18 = reserved
1428 19 = reserved
1429 20 = notation data
1430 21 = preferred hash algorithms
1431 22 = preferred compression algorithms
1432 23 = key server preferences
1433 24 = preferred key server
1434 25 = primary User ID
1435 26 = policy URI
1436 27 = key flags
1437 28 = signer's User ID
1438 29 = reason for revocation
1439 30 = features
1440 31 = signature target
1441 32 = embedded signature
1443 100 to 110 = private or experimental
1445 An implementation SHOULD ignore any subpacket of a type that it does
1446 not recognize.
1448 Bit 7 of the subpacket type is the "critical" bit. If set, it
1449 denotes that the subpacket is one that is critical for the evaluator
1450 of the signature to recognize. If a subpacket is encountered that is
1451 marked critical but is unknown to the evaluating software, the
1452 evaluator SHOULD consider the signature to be in error.
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1459 An evaluator may "recognize" a subpacket, but not implement it. The
1460 purpose of the critical bit is to allow the signer to tell an
1461 evaluator that it would prefer a new, unknown feature to generate an
1462 error than be ignored.
1464 Implementations SHOULD implement "preferences" and the "reason for
1465 revocation" subpackets. Note, however, that if an implementation
1466 chooses not to implement some of the preferences, it is required to
1467 behave in a polite manner to respect the wishes of those users who
1468 do implement these preferences.
1470 5.2.3.2. Signature Subpacket Types
1472 A number of subpackets are currently defined. Some subpackets apply
1473 to the signature itself and some are attributes of the key.
1474 Subpackets that are found on a self-signature are placed on a
1475 certification made by the key itself. Note that a key may have more
1476 than one User ID, and thus may have more than one self-signature,
1477 and differing subpackets.
1479 A subpacket may be found either in the hashed or unhashed subpacket
1480 sections of a signature. If a subpacket is not hashed, then the
1481 information in it cannot be considered definitive because it is not
1482 part of the signature proper.
1484 5.2.3.3. Notes on Self-Signatures
1486 A self-signature is a binding signature made by the key the
1487 signature refers to. There are three types of self-signatures, the
1488 certification signatures (types 0x10-0x13), the direct-key signature
1489 (type 0x1f), and the subkey binding signature (type 0x18). For
1490 certification self-signatures, each User ID may have a
1491 self-signature, and thus different subpackets in those
1492 self-signatures. For subkey binding signatures, each subkey in fact
1493 has a self-signature. Subpackets that appear in a certification
1494 self-signature apply to the username, and subpackets that appear in
1495 the subkey self-signature apply to the subkey. Lastly, subpackets on
1496 the direct-key signature apply to the entire key.
1498 Implementing software should interpret a self-signature's preference
1499 subpackets as narrowly as possible. For example, suppose a key has
1500 two usernames, Alice and Bob. Suppose that Alice prefers the
1501 symmetric algorithm CAST5, and Bob prefers IDEA or TripleDES. If the
1502 software locates this key via Alice's name, then the preferred
1503 algorithm is CAST5, if software locates the key via Bob's name, then
1504 the preferred algorithm is IDEA. If the key is located by key ID,
1505 the algorithm of the primary User ID of the key provides the
1506 preferred symmetric algorithm.
1508 Revoking a self-signature or allowing it to expire has a semantic
1509 meaning that varies with the signature type. Revoking the
1510 self-signature on a User ID effectively retires that user name. The
1512 Callas, et al. Expires Oct 24, 2007 [Page 27]
1513 \fINTERNET-DRAFT OpenPGP Message Format Apr 24, 2007
1515 self-signature is a statement, "My name X is tied to my signing key
1516 K" and is corroborated by other users' certifications. If another
1517 user revokes their certification, they are effectively saying that
1518 they no longer believe that name and that key are tied together.
1519 Similarly, if the user themselves revokes their self-signature, it
1520 means the user no longer goes by that name, no longer has that email
1521 address, etc. Revoking a binding signature effectively retires that
1522 subkey. Revoking a direct-key signature cancels that signature.
1523 Please see the "Reason for Revocation" subpacket below for more
1524 relevant detail.
1526 Since a self-signature contains important information about the
1527 key's use, an implementation SHOULD allow the user to rewrite the
1528 self-signature, and important information in it, such as preferences
1529 and key expiration.
1531 It is good practice to verify that a self-signature imported into an
1532 implementation doesn't advertise features that the implementation
1533 doesn't support, rewriting the signature as appropriate.
1535 An implementation that encounters multiple self-signatures on the
1536 same object may resolve the ambiguity in any way it sees fit, but it
1537 is RECOMMENDED that priority be given to the most recent
1538 self-signature.
1540 5.2.3.4. Signature creation time
1542 (4 octet time field)
1544 The time the signature was made.
1546 MUST be present in the hashed area.
1548 5.2.3.5. Issuer
1550 (8 octet key ID)
1552 The OpenPGP key ID of the key issuing the signature.
1554 5.2.3.6. Key expiration time
1556 (4 octet time field)
1558 The validity period of the key. This is the number of seconds after
1559 the key creation time that the key expires. If this is not present
1560 or has a value of zero, the key never expires. This is found only on
1561 a self-signature.
1563 5.2.3.7. Preferred symmetric algorithms
1565 (array of one-octet values)
1568 Callas, et al. Expires Oct 24, 2007 [Page 28]
1569 \fINTERNET-DRAFT OpenPGP Message Format Apr 24, 2007
1571 Symmetric algorithm numbers that indicate which algorithms the key
1572 holder prefers to use. The subpacket body is an ordered list of
1573 octets with the most preferred listed first. It is assumed that only
1574 algorithms listed are supported by the recipient's software.
1575 Algorithm numbers are in section 9. This is only found on a
1576 self-signature.
1578 5.2.3.8. Preferred hash algorithms
1580 (array of one-octet values)
1582 Message digest algorithm numbers that indicate which algorithms the
1583 key holder prefers to receive. Like the preferred symmetric
1584 algorithms, the list is ordered. Algorithm numbers are in section 9.
1585 This is only found on a self-signature.
1587 5.2.3.9. Preferred compression algorithms
1589 (array of one-octet values)
1591 Compression algorithm numbers that indicate which algorithms the key
1592 holder prefers to use. Like the preferred symmetric algorithms, the
1593 list is ordered. Algorithm numbers are in section 9. If this
1594 subpacket is not included, ZIP is preferred. A zero denotes that
1595 uncompressed data is preferred; the key holder's software might have
1596 no compression software in that implementation. This is only found
1597 on a self-signature.
1599 5.2.3.10. Signature expiration time
1601 (4 octet time field)
1603 The validity period of the signature. This is the number of seconds
1604 after the signature creation time that the signature expires. If
1605 this is not present or has a value of zero, it never expires.
1607 5.2.3.11. Exportable Certification
1609 (1 octet of exportability, 0 for not, 1 for exportable)
1611 This subpacket denotes whether a certification signature is
1612 "exportable," to be used by other users than the signature's issuer.
1613 The packet body contains a Boolean flag indicating whether the
1614 signature is exportable. If this packet is not present, the
1615 certification is exportable; it is equivalent to a flag containing a
1616 1.
1618 Non-exportable, or "local," certifications are signatures made by a
1619 user to mark a key as valid within that user's implementation only.
1620 Thus, when an implementation prepares a user's copy of a key for
1621 transport to another user (this is the process of "exporting" the
1622 key), any local certification signatures are deleted from the key.
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1625 \fINTERNET-DRAFT OpenPGP Message Format Apr 24, 2007
1627 The receiver of a transported key "imports" it, and likewise trims
1628 any local certifications. In normal operation, there won't be any,
1629 assuming the import is performed on an exported key. However, there
1630 are instances where this can reasonably happen. For example, if an
1631 implementation allows keys to be imported from a key database in
1632 addition to an exported key, then this situation can arise.
1634 Some implementations do not represent the interest of a single user
1635 (for example, a key server). Such implementations always trim local
1636 certifications from any key they handle.
1638 5.2.3.12. Revocable
1640 (1 octet of revocability, 0 for not, 1 for revocable)
1642 Signature's revocability status. The packet body contains a Boolean
1643 flag indicating whether the signature is revocable. Signatures that
1644 are not revocable have any later revocation signatures ignored. They
1645 represent a commitment by the signer that he cannot revoke his
1646 signature for the life of his key. If this packet is not present,
1647 the signature is revocable.
1649 5.2.3.13. Trust signature
1651 (1 octet "level" (depth), 1 octet of trust amount)
1653 Signer asserts that the key is not only valid, but also trustworthy,
1654 at the specified level. Level 0 has the same meaning as an ordinary
1655 validity signature. Level 1 means that the signed key is asserted to
1656 be a valid trusted introducer, with the 2nd octet of the body
1657 specifying the degree of trust. Level 2 means that the signed key is
1658 asserted to be trusted to issue level 1 trust signatures, i.e. that
1659 it is a "meta introducer". Generally, a level n trust signature
1660 asserts that a key is trusted to issue level n-1 trust signatures.
1661 The trust amount is in a range from 0-255, interpreted such that
1662 values less than 120 indicate partial trust and values of 120 or
1663 greater indicate complete trust. Implementations SHOULD emit values
1664 of 60 for partial trust and 120 for complete trust.
1666 5.2.3.14. Regular expression
1668 (null-terminated regular expression)
1670 Used in conjunction with trust signature packets (of level > 0) to
1671 limit the scope of trust that is extended. Only signatures by the
1672 target key on User IDs that match the regular expression in the body
1673 of this packet have trust extended by the trust signature subpacket.
1674 The regular expression uses the same syntax as the Henry Spencer's
1675 "almost public domain" regular expression package. A description of
1676 the syntax is found in a section below.
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1683 5.2.3.15. Revocation key
1685 (1 octet of class, 1 octet of PK algorithm ID, 20 octets of
1686 fingerprint)
1688 Authorizes the specified key to issue revocation signatures for this
1689 key. Class octet must have bit 0x80 set. If the bit 0x40 is set,
1690 then this means that the revocation information is sensitive. Other
1691 bits are for future expansion to other kinds of authorizations. This
1692 is found on a self-signature.
1694 If the "sensitive" flag is set, the keyholder feels this subpacket
1695 contains private trust information that describes a real-world
1696 sensitive relationship. If this flag is set, implementations SHOULD
1697 NOT export this signature to other users except in cases where the
1698 data needs to be available: when the signature is being sent to the
1699 designated revoker, or when it is accompanied by a revocation
1700 signature from that revoker. Note that it may be appropriate to
1701 isolate this subpacket within a separate signature so that it is not
1702 combined with other subpackets that need to be exported.
1704 5.2.3.16. Notation Data
1706 (4 octets of flags, 2 octets of name length (M),
1707 2 octets of value length (N),
1708 M octets of name data,
1709 N octets of value data)
1711 This subpacket describes a "notation" on the signature that the
1712 issuer wishes to make. The notation has a name and a value, each of
1713 which are strings of octets. There may be more than one notation in
1714 a signature. Notations can be used for any extension the issuer of
1715 the signature cares to make. The "flags" field holds four octets of
1716 flags.
1718 All undefined flags MUST be zero. Defined flags are:
1720 First octet: 0x80 = human-readable. This note value is text.
1721 Other octets: none.
1723 Notation names are arbitrary strings encoded in UTF-8. They reside
1724 two name spaces: The IETF name space and the user name space.
1726 The IETF name space is registered with IANA. These names MUST NOT
1727 contain the "@" character (0x40). This this is a tag for the user
1728 name space.
1730 Names in the user name space consist of a UTF-8 string tag followed
1731 by "@" followed by a DNS domain name. Note that the tag MUST NOT
1732 contain an "@" character. For example, the "sample" tag used by
1733 Example Corporation could be "sample@example.com".
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1739 Names in a user space are owned and controlled by the owners of that
1740 domain. Obviously, it's of bad form to create a new name in a DNS
1741 space that you don't own.
1743 Since the user name space is in the form of an email address,
1744 implementers MAY wish to arrange for that address to reach a person
1745 who can be consulted about the use of the named tag. Note that due
1746 to UTF-8 encoding, not all valid user space name tags are valid
1747 email addresses.
1749 If there is a critical notation, the criticality applies to that
1750 specific notation and not to notations in general.
1752 5.2.3.17. Key server preferences
1754 (N octets of flags)
1756 This is a list of one-bit flags that indicate preferences that the
1757 key holder has about how the key is handled on a key server. All
1758 undefined flags MUST be zero.
1760 First octet: 0x80 = No-modify
1761 the key holder requests that this key only be modified or
1762 updated by the key holder or an administrator of the key server.
1764 This is found only on a self-signature.
1766 5.2.3.18. Preferred key server
1768 (String)
1770 This is a URI of a key server that the key holder prefers be used
1771 for updates. Note that keys with multiple User IDs can have a
1772 preferred key server for each User ID. Note also that since this is
1773 a URI, the key server can actually be a copy of the key retrieved by
1774 ftp, http, finger, etc.
1776 5.2.3.19. Primary User ID
1778 (1 octet, Boolean)
1780 This is a flag in a User ID's self signature that states whether
1781 this User ID is the main User ID for this key. It is reasonable for
1782 an implementation to resolve ambiguities in preferences, etc. by
1783 referring to the primary User ID. If this flag is absent, its value
1784 is zero. If more than one User ID in a key is marked as primary, the
1785 implementation may resolve the ambiguity in any way it sees fit, but
1786 it is RECOMMENDED that priority be given to the User ID with the
1787 most recent self-signature.
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1795 When appearing on a self-signature on a User ID packet, this
1796 subpacket applies only to User ID packets. When appearing on a
1797 self-signature on a User Attribute packet, this subpacket applies
1798 only to User Attribute packets. That is to say, there are two
1799 different and independent "primaries" - one for User IDs, and one
1800 for User Attributes.
1802 5.2.3.20. Policy URI
1804 (String)
1806 This subpacket contains a URI of a document that describes the
1807 policy that the signature was issued under.
1809 5.2.3.21. Key Flags
1811 (N octets of flags)
1813 This subpacket contains a list of binary flags that hold information
1814 about a key. It is a string of octets, and an implementation MUST
1815 NOT assume a fixed size. This is so it can grow over time. If a list
1816 is shorter than an implementation expects, the unstated flags are
1817 considered to be zero. The defined flags are:
1819 First octet:
1821 0x01 - This key may be used to certify other keys.
1823 0x02 - This key may be used to sign data.
1825 0x04 - This key may be used to encrypt communications.
1827 0x08 - This key may be used to encrypt storage.
1829 0x10 - The private component of this key may have been split by
1830 a secret-sharing mechanism.
1832 0x20 - This key may be used for authentication.
1834 0x80 - The private component of this key may be in the
1835 possession of more than one person.
1837 Usage notes:
1839 The flags in this packet may appear in self-signatures or in
1840 certification signatures. They mean different things depending on
1841 who is making the statement -- for example, a certification
1842 signature that has the "sign data" flag is stating that the
1843 certification is for that use. On the other hand, the
1844 "communications encryption" flag in a self-signature is stating a
1845 preference that a given key be used for communications. Note
1846 however, that it is a thorny issue to determine what is
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1851 "communications" and what is "storage." This decision is left wholly
1852 up to the implementation; the authors of this document do not claim
1853 any special wisdom on the issue, and realize that accepted opinion
1854 may change.
1856 The "split key" (0x10) and "group key" (0x80) flags are placed on a
1857 self-signature only; they are meaningless on a certification
1858 signature. They SHOULD be placed only on a direct-key signature
1859 (type 0x1f) or a subkey signature (type 0x18), one that refers to
1860 the key the flag applies to.
1862 5.2.3.22. Signer's User ID
1864 (String)
1866 This subpacket allows a keyholder to state which User ID is
1867 responsible for the signing. Many keyholders use a single key for
1868 different purposes, such as business communications as well as
1869 personal communications. This subpacket allows such a keyholder to
1870 state which of their roles is making a signature.
1872 This subpacket is not appropriate to use to refer to a User
1873 Attribute packet.
1875 5.2.3.23. Reason for Revocation
1877 (1 octet of revocation code, N octets of reason string)
1879 This subpacket is used only in key revocation and certification
1880 revocation signatures. It describes the reason why the key or
1881 certificate was revoked.
1883 The first octet contains a machine-readable code that denotes the
1884 reason for the revocation:
1886 0 - No reason specified (key revocations or cert revocations)
1887 1 - Key is superseded (key revocations)
1888 2 - Key material has been compromised (key revocations)
1889 3 - Key is retired and no longer used (key revocations)
1890 32 - User ID information is no longer valid (cert revocations)
1892 Following the revocation code is a string of octets which gives
1893 information about the reason for revocation in human-readable form
1894 (UTF-8). The string may be null, that is, of zero length. The length
1895 of the subpacket is the length of the reason string plus one.
1897 An implementation SHOULD implement this subpacket, include it in all
1898 revocation signatures, and interpret revocations appropriately.
1899 There are important semantic differences between the reasons, and
1900 there are thus important reasons for revoking signatures.
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1907 If a key has been revoked because of a compromise, all signatures
1908 created by that key are suspect. However, if it was merely
1909 superseded or retired, old signatures are still valid. If the
1910 revoked signature is the self-signature for certifying a User ID, a
1911 revocation denotes that that user name is no longer in use. Such a
1912 revocation SHOULD include an 0x20 code.
1914 Note that any signature may be revoked, including a certification on
1915 some other person's key. There are many good reasons for revoking a
1916 certification signature, such as the case where the keyholder leaves
1917 the employ of a business with an email address. A revoked
1918 certification is no longer a part of validity calculations.
1920 5.2.3.24. Features
1922 (N octets of flags)
1924 The features subpacket denotes which advanced OpenPGP features a
1925 user's implementation supports. This is so that as features are
1926 added to OpenPGP that cannot be backwards-compatible, a user can
1927 state that they can use that feature. The flags are single bits that
1928 indicate that a given feature is supported.
1930 This subpacket is similar to a preferences subpacket, and only
1931 appears in a self-signature.
1933 An implementation SHOULD NOT use a feature listed when sending to a
1934 user who does not state that they can use it.
1936 Defined features are:
1938 First octet:
1940 0x01 - Modification Detection (packets 18 and 19)
1942 If an implementation implements any of the defined features, it
1943 SHOULD implement the features subpacket, too.
1945 An implementation may freely infer features from other suitable
1946 implementation-dependent mechanisms.
1948 5.2.3.25. Signature Target
1950 (1 octet PK algorithm, 1 octet hash algorithm, N octets hash)
1952 This subpacket identifies a specific target signature that a
1953 signature refers to. For revocation signatures, this subpacket
1954 provides explicit designation of which signature is being revoked.
1955 For a third-party or timestamp signature, this designates what
1956 signature is signed. All arguments are an identifier of that target
1957 signature.
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1961 \fINTERNET-DRAFT OpenPGP Message Format Apr 24, 2007
1963 The N octets of hash data MUST be the size of the hash of the
1964 signature. For example, a target signature with a SHA-1 hash MUST
1965 have 20 octets of hash data.
1967 5.2.3.26. Embedded Signature
1969 (1 signature packet body)
1971 This subpacket contains a complete signature packet body as
1972 specified in section 5.2 above. It is useful when one signature
1973 needs to refer to, or be incorporated in, another signature.
1975 5.2.4. Computing Signatures
1977 All signatures are formed by producing a hash over the signature
1978 data, and then using the resulting hash in the signature algorithm.
1980 For binary document signatures (type 0x00), the document data is
1981 hashed directly. For text document signatures (type 0x01), the
1982 document is canonicalized by converting line endings to <CR><LF>,
1983 and the resulting data is hashed.
1985 When a signature is made over a key, the hash data starts with the
1986 octet 0x99, followed by a two-octet length of the key, and then body
1987 of the key packet. (Note that this is an old-style packet header for
1988 a key packet with two-octet length.) A subkey binding signature
1989 (type 0x18) or primary key binding signature (type 0x19) then hashes
1990 the subkey using the same format as the main key (also using 0x99 as
1991 the first octet). Key revocation signatures (types 0x20 and 0x28)
1992 hash only the key being revoked.
1994 A certification signature (type 0x10 through 0x13) hashes the User
1995 ID being bound to the key into the hash context after the above
1996 data. A V3 certification hashes the contents of the User ID or
1997 attribute packet packet, without any header. A V4 certification
1998 hashes the constant 0xb4 for User ID certifications or the constant
1999 0xd1 for User Attribute certifications, followed by a four-octet
2000 number giving the length of the User ID or User Attribute data, and
2001 then the User ID or User Attribute data.
2003 When a signature is made over a signature packet (type 0x50), the
2004 hash data starts with the octet 0x88, followed by the four-octet
2005 length of the signature, and then the body of the signature packet.
2006 (Note that this is an old-style packet header for a signature packet
2007 with the length-of-length set to zero). The unhashed subpacket data
2008 of the signature packet being hashed is not included in the hash and
2009 the unhashed subpacket data length value is set to zero.
2011 Once the data body is hashed, then a trailer is hashed. A V3
2012 signature hashes five octets of the packet body, starting from the
2013 signature type field. This data is the signature type, followed by
2014 the four-octet signature time. A V4 signature hashes the packet body
2016 Callas, et al. Expires Oct 24, 2007 [Page 36]
2017 \fINTERNET-DRAFT OpenPGP Message Format Apr 24, 2007
2019 starting from its first field, the version number, through the end
2020 of the hashed subpacket data. Thus, the fields hashed are the
2021 signature version, the signature type, the public key algorithm, the
2022 hash algorithm, the hashed subpacket length, and the hashed
2023 subpacket body.
2025 V4 signatures also hash in a final trailer of six octets: the
2026 version of the signature packet, i.e. 0x04; 0xFF; a four-octet,
2027 big-endian number that is the length of the hashed data from the
2028 signature packet (note that this number does not include these final
2029 six octets.
2031 After all this has been hashed in a single hash context the
2032 resulting hash field is used in the signature algorithm, and placed
2033 at the end of the signature packet.
2035 5.2.4.1. Subpacket Hints
2037 It is certainly possible for a signature to contain conflicting
2038 information in subpackets. For example, a signature may contain
2039 multiple copies of a preference or multiple expiration times. In
2040 most cases, an implementation SHOULD use the last subpacket in the
2041 signature, but MAY use any conflict resolution scheme that makes
2042 more sense. Please note that we are intentionally leaving conflict
2043 resolution to the implementer; most conflicts are simply syntax
2044 errors, and the wishy-washy language here allows a receiver to be
2045 generous in what they accept, while putting pressure on a creator to
2046 be stingy in what they generate.
2048 Some apparent conflicts may actually make sense -- for example,
2049 suppose a keyholder has an V3 key and a V4 key that share the same
2050 RSA key material. Either of these keys can verify a signature
2051 created by the other, and it may be reasonable for a signature to
2052 contain an issuer subpacket for each key, as a way of explicitly
2053 tying those keys to the signature.
2055 5.3. Symmetric-Key Encrypted Session Key Packets (Tag 3)
2057 The Symmetric-Key Encrypted Session Key packet holds the
2058 symmetric-key encryption of a session key used to encrypt a message.
2059 Zero or more Public-Key Encrypted Session Key packets and/or
2060 Symmetric-Key Encrypted Session Key packets may precede a
2061 Symmetrically Encrypted Data Packet that holds an encrypted message.
2062 The message is encrypted with a session key, and the session key is
2063 itself encrypted and stored in the Encrypted Session Key packet or
2064 the Symmetric-Key Encrypted Session Key packet.
2066 If the Symmetrically Encrypted Data Packet is preceded by one or
2067 more Symmetric-Key Encrypted Session Key packets, each specifies a
2068 passphrase that may be used to decrypt the message. This allows a
2069 message to be encrypted to a number of public keys, and also to one
2070 or more passphrases. This packet type is new, and is not generated
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2073 \fINTERNET-DRAFT OpenPGP Message Format Apr 24, 2007
2075 by PGP 2.x or PGP 5.0.
2077 The body of this packet consists of:
2079 - A one-octet version number. The only currently defined version
2080 is 4.
2082 - A one-octet number describing the symmetric algorithm used.
2084 - A string-to-key (S2K) specifier, length as defined above.
2086 - Optionally, the encrypted session key itself, which is decrypted
2087 with the string-to-key object.
2089 If the encrypted session key is not present (which can be detected
2090 on the basis of packet length and S2K specifier size), then the S2K
2091 algorithm applied to the passphrase produces the session key for
2092 decrypting the file, using the symmetric cipher algorithm from the
2093 Symmetric-Key Encrypted Session Key packet.
2095 If the encrypted session key is present, the result of applying the
2096 S2K algorithm to the passphrase is used to decrypt just that
2097 encrypted session key field, using CFB mode with an IV of all zeros.
2098 The decryption result consists of a one-octet algorithm identifier
2099 that specifies the symmetric-key encryption algorithm used to
2100 encrypt the following Symmetrically Encrypted Data Packet, followed
2101 by the session key octets themselves.
2103 Note: because an all-zero IV is used for this decryption, the S2K
2104 specifier MUST use a salt value, either a Salted S2K or an
2105 Iterated-Salted S2K. The salt value will insure that the decryption
2106 key is not repeated even if the passphrase is reused.
2108 5.4. One-Pass Signature Packets (Tag 4)
2110 The One-Pass Signature packet precedes the signed data and contains
2111 enough information to allow the receiver to begin calculating any
2112 hashes needed to verify the signature. It allows the Signature
2113 Packet to be placed at the end of the message, so that the signer
2114 can compute the entire signed message in one pass.
2116 A One-Pass Signature does not interoperate with PGP 2.6.x or
2117 earlier.
2119 The body of this packet consists of:
2121 - A one-octet version number. The current version is 3.
2123 - A one-octet signature type. Signature types are described in
2124 section 5.2.1.
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2131 - A one-octet number describing the hash algorithm used.
2133 - A one-octet number describing the public key algorithm used.
2135 - An eight-octet number holding the key ID of the signing key.
2137 - A one-octet number holding a flag showing whether the signature
2138 is nested. A zero value indicates that the next packet is
2139 another One-Pass Signature packet that describes another
2140 signature to be applied to the same message data.
2142 Note that if a message contains more than one one-pass signature,
2143 then the signature packets bracket the message; that is, the first
2144 signature packet after the message corresponds to the last one-pass
2145 packet and the final signature packet corresponds to the first
2146 one-pass packet.
2148 5.5. Key Material Packet
2150 A key material packet contains all the information about a public or
2151 private key. There are four variants of this packet type, and two
2152 major versions. Consequently, this section is complex.
2154 5.5.1. Key Packet Variants
2156 5.5.1.1. Public Key Packet (Tag 6)
2158 A Public Key packet starts a series of packets that forms an OpenPGP
2159 key (sometimes called an OpenPGP certificate).
2161 5.5.1.2. Public Subkey Packet (Tag 14)
2163 A Public Subkey packet (tag 14) has exactly the same format as a
2164 Public Key packet, but denotes a subkey. One or more subkeys may be
2165 associated with a top-level key. By convention, the top-level key
2166 provides signature services, and the subkeys provide encryption
2167 services.
2169 Note: in PGP 2.6.x, tag 14 was intended to indicate a comment
2170 packet. This tag was selected for reuse because no previous version
2171 of PGP ever emitted comment packets but they did properly ignore
2172 them. Public Subkey packets are ignored by PGP 2.6.x and do not
2173 cause it to fail, providing a limited degree of backward
2174 compatibility.
2176 5.5.1.3. Secret Key Packet (Tag 5)
2178 A Secret Key packet contains all the information that is found in a
2179 Public Key packet, including the public key material, but also
2180 includes the secret key material after all the public key fields.
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2187 5.5.1.4. Secret Subkey Packet (Tag 7)
2189 A Secret Subkey packet (tag 7) is the subkey analog of the Secret
2190 Key packet, and has exactly the same format.
2192 5.5.2. Public Key Packet Formats
2194 There are two versions of key-material packets. Version 3 packets
2195 were first generated by PGP 2.6. Version 4 keys first appeared in
2196 PGP 5.0, and are the preferred key version for OpenPGP.
2198 OpenPGP implementations MUST create keys with version 4 format. V3
2199 keys are deprecated; an implementation MUST NOT generate a V3 key,
2200 but MAY accept it.
2202 A version 3 public key or public subkey packet contains:
2204 - A one-octet version number (3).
2206 - A four-octet number denoting the time that the key was created.
2208 - A two-octet number denoting the time in days that this key is
2209 valid. If this number is zero, then it does not expire.
2211 - A one-octet number denoting the public key algorithm of this key
2213 - A series of multiprecision integers comprising the key material:
2215 - a multiprecision integer (MPI) of RSA public modulus n;
2217 - an MPI of RSA public encryption exponent e.
2219 V3 keys are deprecated. They contain three weaknesses in them.
2220 First, it is relatively easy to construct a V3 key that has the same
2221 key ID as any other key because the key ID is simply the low 64 bits
2222 of the public modulus. Secondly, because the fingerprint of a V3 key
2223 hashes the key material, but not its length, there is an increased
2224 opportunity for fingerprint collisions. Third, there are weaknesses
2225 in the MD5 hash algorithm that make developers prefer other
2226 algorithms. See below for a fuller discussion of key IDs and
2227 fingerprints.
2229 V2 keys are identical to the deprecated V3 keys except for the
2230 version number. An implementation MUST NOT generate them and MAY
2231 accept or reject them as it sees fit.
2233 The version 4 format is similar to the version 3 format except for
2234 the absence of a validity period. This has been moved to the
2235 signature packet. In addition, fingerprints of version 4 keys are
2236 calculated differently from version 3 keys, as described in section
2237 "Enhanced Key Formats."
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2243 A version 4 packet contains:
2245 - A one-octet version number (4).
2247 - A four-octet number denoting the time that the key was created.
2249 - A one-octet number denoting the public key algorithm of this key
2251 - A series of multiprecision integers comprising the key material.
2252 This algorithm-specific portion is:
2254 Algorithm Specific Fields for RSA public keys:
2256 - multiprecision integer (MPI) of RSA public modulus n;
2258 - MPI of RSA public encryption exponent e.
2260 Algorithm Specific Fields for DSA public keys:
2262 - MPI of DSA prime p;
2264 - MPI of DSA group order q (q is a prime divisor of p-1);
2266 - MPI of DSA group generator g;
2268 - MPI of DSA public key value y (= g**x mod p where x is
2269 secret).
2271 Algorithm Specific Fields for Elgamal public keys:
2273 - MPI of Elgamal prime p;
2275 - MPI of Elgamal group generator g;
2277 - MPI of Elgamal public key value y (= g**x mod p where x is
2278 secret).
2280 5.5.3. Secret Key Packet Formats
2282 The Secret Key and Secret Subkey packets contain all the data of the
2283 Public Key and Public Subkey packets, with additional
2284 algorithm-specific secret key data appended, usually in encrypted
2285 form.
2287 The packet contains:
2289 - A Public Key or Public Subkey packet, as described above
2291 - One octet indicating string-to-key usage conventions. Zero
2292 indicates that the secret key data is not encrypted. 255 or 254
2293 indicates that a string-to-key specifier is being given. Any
2294 other value is a symmetric-key encryption algorithm identifier.
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2299 - [Optional] If string-to-key usage octet was 255 or 254, a
2300 one-octet symmetric encryption algorithm.
2302 - [Optional] If string-to-key usage octet was 255 or 254, a
2303 string-to-key specifier. The length of the string-to-key
2304 specifier is implied by its type, as described above.
2306 - [Optional] If secret data is encrypted (string-to-key usage
2307 octet not zero), an Initial Vector (IV) of the same length as
2308 the cipher's block size.
2310 - Plain or encrypted multiprecision integers comprising the secret
2311 key data. These algorithm-specific fields are as described
2312 below.
2314 - If the string-to-key usage octet is zero or 255, then a
2315 two-octet checksum of the plaintext of the algorithm-specific
2316 portion (sum of all octets, mod 65536). If the string-to-key
2317 usage octet was 254, then a 20-octet SHA-1 hash of the plaintext
2318 of the algorithm-specific portion. This checksum or hash is
2319 encrypted together with the algorithm-specific fields (if
2320 string-to-key usage octet is not zero). Note that for all other
2321 values, a two-octet checksum is required.
2323 Algorithm Specific Fields for RSA secret keys:
2325 - multiprecision integer (MPI) of RSA secret exponent d.
2327 - MPI of RSA secret prime value p.
2329 - MPI of RSA secret prime value q (p < q).
2331 - MPI of u, the multiplicative inverse of p, mod q.
2333 Algorithm Specific Fields for DSA secret keys:
2335 - MPI of DSA secret exponent x.
2337 Algorithm Specific Fields for Elgamal secret keys:
2339 - MPI of Elgamal secret exponent x.
2341 Secret MPI values can be encrypted using a passphrase. If a
2342 string-to-key specifier is given, that describes the algorithm for
2343 converting the passphrase to a key, else a simple MD5 hash of the
2344 passphrase is used. Implementations MUST use a string-to-key
2345 specifier; the simple hash is for backward compatibility and is
2346 deprecated, though implementations MAY continue to use existing
2347 private keys in the old format. The cipher for encrypting the MPIs
2348 is specified in the secret key packet.
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2353 \fINTERNET-DRAFT OpenPGP Message Format Apr 24, 2007
2355 Encryption/decryption of the secret data is done in CFB mode using
2356 the key created from the passphrase and the Initial Vector from the
2357 packet. A different mode is used with V3 keys (which are only RSA)
2358 than with other key formats. With V3 keys, the MPI bit count prefix
2359 (i.e., the first two octets) is not encrypted. Only the MPI
2360 non-prefix data is encrypted. Furthermore, the CFB state is
2361 resynchronized at the beginning of each new MPI value, so that the
2362 CFB block boundary is aligned with the start of the MPI data.
2364 With V4 keys, a simpler method is used. All secret MPI values are
2365 encrypted in CFB mode, including the MPI bitcount prefix.
2367 The two-octet checksum that follows the algorithm-specific portion
2368 is the algebraic sum, mod 65536, of the plaintext of all the
2369 algorithm-specific octets (including MPI prefix and data). With V3
2370 keys, the checksum is stored in the clear. With V4 keys, the
2371 checksum is encrypted like the algorithm-specific data. This value
2372 is used to check that the passphrase was correct. However, this
2373 checksum is deprecated; an implementation SHOULD NOT use it, but
2374 should rather use the SHA-1 hash denoted with a usage octet of 254.
2375 The reason for this is that there are some attacks that involve
2376 undetectably modifying the secret key.
2378 5.6. Compressed Data Packet (Tag 8)
2380 The Compressed Data packet contains compressed data. Typically, this
2381 packet is found as the contents of an encrypted packet, or following
2382 a Signature or One-Pass Signature packet, and contains a literal
2383 data packet.
2385 The body of this packet consists of:
2387 - One octet that gives the algorithm used to compress the packet.
2389 - The remainder of the packet is compressed data.
2391 A Compressed Data Packet's body contains an block that compresses
2392 some set of packets. See section "Packet Composition" for details on
2393 how messages are formed.
2395 ZIP-compressed packets are compressed with raw RFC 1951 DEFLATE
2396 blocks. Note that PGP V2.6 uses 13 bits of compression. If an
2397 implementation uses more bits of compression, PGP V2.6 cannot
2398 decompress it.
2400 ZLIB-compressed packets are compressed with RFC 1950 ZLIB-style
2401 blocks.
2403 BZip2-compressed packets are compressed using the BZip2 [BZ2]
2404 algorithm.
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2411 5.7. Symmetrically Encrypted Data Packet (Tag 9)
2413 The Symmetrically Encrypted Data packet contains data encrypted with
2414 a symmetric-key algorithm. When it has been decrypted, it contains
2415 other packets (usually a literal data packet or compressed data
2416 packet, but in theory other Symmetrically Encrypted Data Packets or
2417 sequences of packets that form whole OpenPGP messages).
2419 The body of this packet consists of:
2421 - Encrypted data, the output of the selected symmetric-key cipher
2422 operating in OpenPGP's variant of Cipher Feedback (CFB) mode.
2424 The symmetric cipher used may be specified in an Public-Key or
2425 Symmetric-Key Encrypted Session Key packet that precedes the
2426 Symmetrically Encrypted Data Packet. In that case, the cipher
2427 algorithm octet is prefixed to the session key before it is
2428 encrypted. If no packets of these types precede the encrypted data,
2429 the IDEA algorithm is used with the session key calculated as the
2430 MD5 hash of the passphrase, though this use is deprecated.
2432 The data is encrypted in CFB mode, with a CFB shift size equal to
2433 the cipher's block size. The Initial Vector (IV) is specified as all
2434 zeros. Instead of using an IV, OpenPGP prefixes a string of length
2435 equal to the block size of the cipher plus two to the data before it
2436 is encrypted. The first block-size octets (for example, 8 octets for
2437 a 64-bit block length) are random, and the following two octets are
2438 copies of the last two octets of the IV. For example, in an 8 octet
2439 block, octet 9 is a repeat of octet 7, and octet 10 is a repeat of
2440 octet 8. In a cipher of length 16, octet 17 is a repeat of octet 15
2441 and octet 18 is a repeat of octet 16. As a pedantic clarification,
2442 in both these examples, we consider the first octet to be numbered
2443 1.
2445 After encrypting the first block-size-plus-two octets, the CFB state
2446 is resynchronized. The last block-size octets of ciphertext are
2447 passed through the cipher and the block boundary is reset.
2449 The repetition of 16 bits in the random data prefixed to the message
2450 allows the receiver to immediately check whether the session key is
2451 incorrect. See the Security Considerations section for hints on the
2452 proper use of this "quick check."
2454 5.8. Marker Packet (Obsolete Literal Packet) (Tag 10)
2456 An experimental version of PGP used this packet as the Literal
2457 packet, but no released version of PGP generated Literal packets
2458 with this tag. With PGP 5.x, this packet has been re-assigned and is
2459 reserved for use as the Marker packet.
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2467 The body of this packet consists of:
2469 - The three octets 0x50, 0x47, 0x50 (which spell "PGP" in UTF-8).
2471 Such a packet MUST be ignored when received. It may be placed at the
2472 beginning of a message that uses features not available in PGP 2.6.x
2473 in order to cause that version to report that newer software is
2474 necessary to process the message.
2476 5.9. Literal Data Packet (Tag 11)
2478 A Literal Data packet contains the body of a message; data that is
2479 not to be further interpreted.
2481 The body of this packet consists of:
2483 - A one-octet field that describes how the data is formatted.
2485 If it is a 'b' (0x62), then the literal packet contains binary data.
2486 If it is a 't' (0x74), then it contains text data, and thus may need
2487 line ends converted to local form, or other text-mode changes. The
2488 tag 'u' (0x75) means the same as 't', but also indicates that
2489 implementation believes that the literal data contains UTF-8 text.
2491 Early versions of PGP also defined a value of 'l' as a 'local' mode
2492 for machine-local conversions. RFC 1991 incorrectly stated this
2493 local mode flag as '1' (ASCII numeral one). Both of these local
2494 modes are deprecated.
2496 - File name as a string (one-octet length, followed by a file
2497 name). This may be a zero-length string. Commonly, if the source
2498 of the encrypted data is a file, this will be the name of the
2499 encrypted file. An implementation MAY consider the file name in
2500 the literal packet to be a more authoritative name than the
2501 actual file name.
2503 If the special name "_CONSOLE" is used, the message is considered to
2504 be "for your eyes only". This advises that the message data is
2505 unusually sensitive, and the receiving program should process it
2506 more carefully, perhaps avoiding storing the received data to disk,
2507 for example.
2509 - A four-octet number that indicates a date associated with the
2510 literal data. Commonly, the date might be the modification date
2511 of a file, or the time the packet was created, or a zero that
2512 indicates no specific time.
2514 - The remainder of the packet is literal data.
2516 Text data is stored with <CR><LF> text endings (i.e. network-normal
2517 line endings). These should be converted to native line endings by
2518 the receiving software.
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2523 5.10. Trust Packet (Tag 12)
2525 The Trust packet is used only within keyrings and is not normally
2526 exported. Trust packets contain data that record the user's
2527 specifications of which key holders are trustworthy introducers,
2528 along with other information that implementing software uses for
2529 trust information. The format of trust packets is defined by a given
2530 implementation.
2532 Trust packets SHOULD NOT be emitted to output streams that are
2533 transferred to other users, and they SHOULD be ignored on any input
2534 other than local keyring files.
2536 5.11. User ID Packet (Tag 13)
2538 A User ID packet consists of UTF-8 text that is intended to
2539 represent the name and email address of the key holder. By
2540 convention, it includes an RFC 2822 mail name-addr, but there are no
2541 restrictions on its content. The packet length in the header
2542 specifies the length of the User ID.
2544 5.12. User Attribute Packet (Tag 17)
2546 The User Attribute packet is a variation of the User ID packet. It
2547 is capable of storing more types of data than the User ID packet
2548 which is limited to text. Like the User ID packet, a User Attribute
2549 packet may be certified by the key owner ("self-signed") or any
2550 other key owner who cares to certify it. Except as noted, a User
2551 Attribute packet may be used anywhere that a User ID packet may be
2552 used.
2554 While User Attribute packets are not a required part of the OpenPGP
2555 standard, implementations SHOULD provide at least enough
2556 compatibility to properly handle a certification signature on the
2557 User Attribute packet. A simple way to do this is by treating the
2558 User Attribute packet as a User ID packet with opaque contents, but
2559 an implementation may use any method desired.
2561 The User Attribute packet is made up of one or more attribute
2562 subpackets. Each subpacket consists of a subpacket header and a
2563 body. The header consists of:
2565 - the subpacket length (1, 2, or 5 octets)
2567 - the subpacket type (1 octet)
2569 and is followed by the subpacket specific data.
2571 The only currently defined subpacket type is 1, signifying an image.
2572 An implementation SHOULD ignore any subpacket of a type that it does
2573 not recognize. Subpacket types 100 through 110 are reserved for
2574 private or experimental use.
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2579 5.12.1. The Image Attribute Subpacket
2581 The image attribute subpacket is used to encode an image, presumably
2582 (but not required to be) that of the key owner.
2584 The image attribute subpacket begins with an image header. The first
2585 two octets of the image header contain the length of the image
2586 header. Note that unlike other multi-octet numerical values in this
2587 document, due to an historical accident this value is encoded as a
2588 little-endian number. The image header length is followed by a
2589 single octet for the image header version. The only currently
2590 defined version of the image header is 1, which is a 16 octet image
2591 header. The first three octets of a version 1 image header are thus
2592 0x10 0x00 0x01.
2594 The fourth octet of a version 1 image header designates the encoding
2595 format of the image. The only currently defined encoding format is
2596 the value 1 to indicate JPEG. Image format types 100 through 110 are
2597 reserved for private or experimental use. The rest of the version 1
2598 image header is made up of 12 reserved octets, all of which MUST be
2599 set to 0.
2601 The rest of the image subpacket contains the image itself. As the
2602 only currently defined image type is JPEG, the image is encoded in
2603 the JPEG File Interchange Format (JFIF), a standard file format for
2604 JPEG images. [JFIF]
2606 An implementation MAY try and determine the type of an image by
2607 examination of the image data if it is unable to handle a particular
2608 version of the image header or if a specified encoding format value
2609 is not recognized.
2611 5.13. Sym. Encrypted Integrity Protected Data Packet (Tag 18)
2613 The Symmetrically Encrypted Integrity Protected Data Packet is a
2614 variant of the Symmetrically Encrypted Data Packet. It is a new
2615 feature created for OpenPGP that addresses the problem of detecting
2616 a modification to encrypted data. It is used in combination with a
2617 Modification Detection Code Packet.
2619 There is a corresponding feature in the features signature subpacket
2620 that denotes that an implementation can properly use this packet
2621 type. An implementation MUST support decrypting these packets and
2622 SHOULD prefer generating them to the older Symmetrically Encrypted
2623 Data Packet when possible. Since this data packet protects against
2624 modification attacks, this standard encourages its proliferation.
2625 While blanket adoption of this data packet would create
2626 interoperability problems, rapid adoption is nevertheless important.
2627 An implementation SHOULD specifically denote support for this
2628 packet, but it MAY infer it from other mechanisms.
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2633 \fINTERNET-DRAFT OpenPGP Message Format Apr 24, 2007
2635 For example, an implementation might infer from the use of a cipher
2636 such as AES or Twofish that a user supports this feature. It might
2637 place in the unhashed portion of another user's key signature a
2638 features subpacket. It might also present a user with an opportunity
2639 to regenerate their own self-signature with a features subpacket.
2641 This packet contains data encrypted with a symmetric-key algorithm
2642 and protected against modification by the SHA-1 hash algorithm. When
2643 it has been decrypted, it will typically contain other packets
2644 (often a literal data packet or compressed data packet). The last
2645 decrypted packet in this packet's payload MUST be a Modification
2646 Detection Code packet.
2648 The body of this packet consists of:
2650 - A one-octet version number. The only currently defined value is
2651 1.
2653 - Encrypted data, the output of the selected symmetric-key cipher
2654 operating in Cipher Feedback mode with shift amount equal to the
2655 block size of the cipher (CFB-n where n is the block size).
2657 The symmetric cipher used MUST be specified in a Public-Key or
2658 Symmetric-Key Encrypted Session Key packet that precedes the
2659 Symmetrically Encrypted Data Packet. In either case, the cipher
2660 algorithm octet is prefixed to the session key before it is
2661 encrypted.
2663 The data is encrypted in CFB mode, with a CFB shift size equal to
2664 the cipher's block size. The Initial Vector (IV) is specified as all
2665 zeros. Instead of using an IV, OpenPGP prefixes an octet string to
2666 the data before it is encrypted. The length of the octet string
2667 equals the block size of the cipher in octets, plus two. The first
2668 octets in the group, of length equal to the block size of the
2669 cipher, are random; the last two octets are each copies of their 2nd
2670 preceding octet. For example, with a cipher whose block size is 128
2671 bits or 16 octets, the prefix data will contain 16 random octets,
2672 then two more octets, which are copies of the 15th and 16th octets,
2673 respectively. Unlike the Symmetrically Encrypted Data Packet, no
2674 special CFB resynchronization is done after encrypting this prefix
2675 data. See OpenPGP CFB Mode below for more details.
2677 The repetition of 16 bits in the random data prefixed to the message
2678 allows the receiver to immediately check whether the session key is
2679 incorrect.
2681 The plaintext of the data to be encrypted is passed through the
2682 SHA-1 hash function, and the result of the hash is appended to the
2683 plaintext in a Modification Detection Code packet. The input to the
2684 hash function includes the prefix data described above; it includes
2685 all of the plaintext, and then also includes two octets of values
2686 0xD3, 0x14. These represent the encoding of a Modification Detection
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2689 \fINTERNET-DRAFT OpenPGP Message Format Apr 24, 2007
2691 Code packet tag and length field of 20 octets.
2693 The resulting hash value is stored in a Modification Detection Code
2694 packet which MUST use the two octet encoding just given to represent
2695 its tag and length field. The body of the MDC packet is the 20 octet
2696 output of the SHA-1 hash.
2698 The Modification Detection Code packet is appended to the plaintext
2699 and encrypted along with the plaintext using the same CFB context.
2701 During decryption, the plaintext data should be hashed with SHA-1,
2702 including the prefix data as well as the packet tag and length field
2703 of the Modification Detection Code packet. The body of the MDC
2704 packet, upon decryption, is compared with the result of the SHA-1
2705 hash.
2707 Any failure of the MDC indicates that the message has been modified
2708 and MUST be treated as a security problem. Failures include a
2709 difference in the hash values, but also the absence of an MDC
2710 packet, or an MDC packet in any position other than the end of the
2711 plaintext. Any failure SHOULD be reported to the user.
2713 Note: future designs of new versions of this packet should consider
2714 rollback attacks since it will be possible for an attacker to change
2715 the version back to 1.
2717 NON-NORMATIVE EXPLANATION
2719 The MDC system, as packets 18 and 19 are called, were created to
2720 provide an integrity mechanism that is less strong than a
2721 signature, yet stronger than bare CFB encryption.
2723 It is a limitation of CFB encryption that damage to the
2724 ciphertext will corrupt the affected cipher blocks and the block
2725 following. Additionally, if data is removed from the end of a
2726 CFB-encrypted block, that removal is undetectable. (Note also
2727 that CBC mode has a similar limitation, but data removed from
2728 the front of the block is undetectable.)
2730 The obvious way to protect or authenticate an encrypted block is
2731 to digitally sign it. However, many people do not wish to
2732 habitually sign data, for a large number of reasons beyond the
2733 scope of this document. Suffice it to say that many people
2734 consider properties such as deniability to be as valuable as
2735 integrity.
2737 OpenPGP addresses this desire to have more security than raw
2738 encryption and yet preserve deniability with the MDC system. An
2739 MDC is intentionally not a MAC. Its name was not selected by
2740 accident. It is analogous to a checksum.
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2747 Despite the fact that it is a relatively modest system, it has
2748 proved itself in the real world. It is an effective defense to
2749 several attacks that have surfaced since it has been created. It
2750 has met its modest goals admirably.
2752 Consequently, because it is a modest security system, it has
2753 modest requirements on the hash function(s) it employs. It does
2754 not rely on a hash function being collision-free, it relies on a
2755 hash function being one-way. If a forger, Frank, wishes to send
2756 Alice a (digitally) unsigned message that says, "I've always
2757 secretly loved you, signed Bob" it is far easier for him to
2758 construct a new message than it is to modify anything
2759 intercepted from Bob. (Note also that if Bob wishes to
2760 communicate secretly with Alice, but without authentication nor
2761 identification and with a threat model that includes forgers, he
2762 has a problem that transcends mere cryptography.)
2764 Note also that unlike nearly every other OpenPGP subsystem,
2765 there are no parameters in the MDC system. It hard-defines SHA-1
2766 as its hash function. This is not an accident. It is an
2767 intentional choice to avoid downgrade and cross-grade attacks
2768 while making a simple, fast system. (A downgrade attack would be
2769 an attack that replaced SHA-256 with SHA-1, for example. A
2770 cross-grade attack would replace SHA-1 with another 160-bit
2771 hash, such as RIPE-MD/160, for example.)
2773 However, given the present state of hash function cryptanalysis
2774 and cryptography, it may be desirable to upgrade the MDC system
2775 to a new hash function. See section 10.5 in the IANA
2776 considerations for guidance.
2778 5.14. Modification Detection Code Packet (Tag 19)
2780 The Modification Detection Code packet contains a SHA-1 hash of
2781 plaintext data which is used to detect message modification. It is
2782 only used with a Symmetrically Encrypted Integrity Protected Data
2783 packet. The Modification Detection Code packet MUST be the last
2784 packet in the plaintext data which is encrypted in the Symmetrically
2785 Encrypted Integrity Protected Data packet, and MUST appear in no
2786 other place.
2788 A Modification Detection Code packet MUST have a length of 20
2789 octets.
2791 The body of this packet consists of:
2793 - A 20-octet SHA-1 hash of the preceding plaintext data of the
2794 Symmetrically Encrypted Integrity Protected Data packet,
2795 including prefix data, the tag octet, and length octet of the
2796 Modification Detection Code packet.
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2803 Note that the Modification Detection Code packet MUST always use a
2804 new-format encoding of the packet tag, and a one-octet encoding of
2805 the packet length. The reason for this is that the hashing rules for
2806 modification detection include a one-octet tag and one-octet length
2807 in the data hash. While this is a bit restrictive, it reduces
2808 complexity.
2810 6. Radix-64 Conversions
2812 As stated in the introduction, OpenPGP's underlying native
2813 representation for objects is a stream of arbitrary octets, and some
2814 systems desire these objects to be immune to damage caused by
2815 character set translation, data conversions, etc.
2817 In principle, any printable encoding scheme that met the
2818 requirements of the unsafe channel would suffice, since it would not
2819 change the underlying binary bit streams of the native OpenPGP data
2820 structures. The OpenPGP standard specifies one such printable
2821 encoding scheme to ensure interoperability.
2823 OpenPGP's Radix-64 encoding is composed of two parts: a base64
2824 encoding of the binary data, and a checksum. The base64 encoding is
2825 identical to the MIME base64 content-transfer-encoding [RFC2045].
2827 The checksum is a 24-bit CRC converted to four characters of
2828 radix-64 encoding by the same MIME base64 transformation, preceded
2829 by an equals sign (=). The CRC is computed by using the generator
2830 0x864CFB and an initialization of 0xB704CE. The accumulation is done
2831 on the data before it is converted to radix-64, rather than on the
2832 converted data. A sample implementation of this algorithm is in the
2833 next section.
2835 The checksum with its leading equal sign MAY appear on the first
2836 line after the Base64 encoded data.
2838 Rationale for CRC-24: The size of 24 bits fits evenly into printable
2839 base64. The nonzero initialization can detect more errors than a
2840 zero initialization.
2842 6.1. An Implementation of the CRC-24 in "C"
2844 #define CRC24_INIT 0xb704ceL
2845 #define CRC24_POLY 0x1864cfbL
2847 typedef long crc24;
2848 crc24 crc_octets(unsigned char *octets, size_t len)
2849 {
2850 crc24 crc = CRC24_INIT;
2851 int i;
2856 Callas, et al. Expires Oct 24, 2007 [Page 51]
2857 \fINTERNET-DRAFT OpenPGP Message Format Apr 24, 2007
2859 while (len--) {
2860 crc ^= (*octets++) << 16;
2861 for (i = 0; i < 8; i++) {
2862 crc <<= 1;
2863 if (crc & 0x1000000)
2864 crc ^= CRC24_POLY;
2865 }
2866 }
2867 return crc & 0xffffffL;
2868 }
2870 6.2. Forming ASCII Armor
2872 When OpenPGP encodes data into ASCII Armor, it puts specific headers
2873 around the Radix-64 encoded data, so OpenPGP can reconstruct the
2874 data later. An OpenPGP implementation MAY use ASCII armor to protect
2875 raw binary data. OpenPGP informs the user what kind of data is
2876 encoded in the ASCII armor through the use of the headers.
2878 Concatenating the following data creates ASCII Armor:
2880 - An Armor Header Line, appropriate for the type of data
2882 - Armor Headers
2884 - A blank (zero-length, or containing only whitespace) line
2886 - The ASCII-Armored data
2888 - An Armor Checksum
2890 - The Armor Tail, which depends on the Armor Header Line.
2892 An Armor Header Line consists of the appropriate header line text
2893 surrounded by five (5) dashes ('-', 0x2D) on either side of the
2894 header line text. The header line text is chosen based upon the type
2895 of data that is being encoded in Armor, and how it is being encoded.
2896 Header line texts include the following strings:
2898 BEGIN PGP MESSAGE
2899 Used for signed, encrypted, or compressed files.
2901 BEGIN PGP PUBLIC KEY BLOCK
2902 Used for armoring public keys
2904 BEGIN PGP PRIVATE KEY BLOCK
2905 Used for armoring private keys
2907 BEGIN PGP MESSAGE, PART X/Y
2908 Used for multi-part messages, where the armor is split amongst Y
2909 parts, and this is the Xth part out of Y.
2912 Callas, et al. Expires Oct 24, 2007 [Page 52]
2913 \fINTERNET-DRAFT OpenPGP Message Format Apr 24, 2007
2915 BEGIN PGP MESSAGE, PART X
2916 Used for multi-part messages, where this is the Xth part of an
2917 unspecified number of parts. Requires the MESSAGE-ID Armor
2918 Header to be used.
2920 BEGIN PGP SIGNATURE
2921 Used for detached signatures, OpenPGP/MIME signatures, and
2922 cleartext signatures. Note that PGP 2.x uses BEGIN PGP MESSAGE
2923 for detached signatures.
2925 Note that all these Armor Header Lines are to consist of a complete
2926 line. That is to say, there is always a line ending preceding the
2927 starting five dashes, and following the ending five dashes. The
2928 header lines, therefore, MUST start at the beginning of a line, and
2929 MUST NOT have text other than whitespace following them on the same
2930 line. These line endings are considered a part of the Armor Header
2931 Line for the purposes of determining the content they delimit. This
2932 is particularly important when computing a cleartext signature (see
2933 below).
2935 The Armor Headers are pairs of strings that can give the user or the
2936 receiving OpenPGP implementation some information about how to
2937 decode or use the message. The Armor Headers are a part of the
2938 armor, not a part of the message, and hence are not protected by any
2939 signatures applied to the message.
2941 The format of an Armor Header is that of a key-value pair. A colon
2942 (':' 0x38) and a single space (0x20) separate the key and value.
2943 OpenPGP should consider improperly formatted Armor Headers to be
2944 corruption of the ASCII Armor. Unknown keys should be reported to
2945 the user, but OpenPGP should continue to process the message.
2947 Note that some transport methods are sensitive to line length. While
2948 there is a limit of 76 characters for the Radix-64 data (section
2949 6.3), there is no limit to the length of Armor Headers. Care should
2950 be taken that the Armor Headers are short enough to survive
2951 transport. One way to do this is to repeat an Armor Header key
2952 multiple times with different values for each so that no one line is
2953 overly long.
2955 Currently defined Armor Header Keys are:
2957 - "Version", that states the OpenPGP implementation and version
2958 used to encode the message.
2960 - "Comment", a user-defined comment. OpenPGP defines all text to
2961 be in UTF-8. A comment may be any UTF-8 string. However, the
2962 whole point of armoring is to provide seven-bit-clean data.
2963 Consequently, if a comment has characters that are outside the
2964 US-ASCII range of UTF, they may very well not survive transport.
2968 Callas, et al. Expires Oct 24, 2007 [Page 53]
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2971 - "MessageID", a 32-character string of printable characters. The
2972 string must be the same for all parts of a multi-part message
2973 that uses the "PART X" Armor Header. MessageID strings should be
2974 unique enough that the recipient of the mail can associate all
2975 the parts of a message with each other. A good checksum or
2976 cryptographic hash function is sufficient.
2978 The MessageID SHOULD NOT appear unless it is in a multi-part
2979 message. If it appears at all, it MUST be computed from the
2980 finished (encrypted, signed, etc.) message in a deterministic
2981 fashion, rather than contain a purely random value. This is to
2982 allow the legitimate recipient to determine that the MessageID
2983 cannot serve as a covert means of leaking cryptographic key
2984 information.
2986 - "Hash", a comma-separated list of hash algorithms used in this
2987 message. This is used only in cleartext signed messages.
2989 - "Charset", a description of the character set that the plaintext
2990 is in. Please note that OpenPGP defines text to be in UTF-8. An
2991 implementation will get best results by translating into and out
2992 of UTF-8. However, there are many instances where this is easier
2993 said than done. Also, there are communities of users who have no
2994 need for UTF-8 because they are all happy with a character set
2995 like ISO Latin-5 or a Japanese character set. In such instances,
2996 an implementation MAY override the UTF-8 default by using this
2997 header key. An implementation MAY implement this key and any
2998 translations it cares to; an implementation MAY ignore it and
2999 assume all text is UTF-8.
3001 The Armor Tail Line is composed in the same manner as the Armor
3002 Header Line, except the string "BEGIN" is replaced by the string
3003 "END".
3005 6.3. Encoding Binary in Radix-64
3007 The encoding process represents 24-bit groups of input bits as
3008 output strings of 4 encoded characters. Proceeding from left to
3009 right, a 24-bit input group is formed by concatenating three 8-bit
3010 input groups. These 24 bits are then treated as four concatenated
3011 6-bit groups, each of which is translated into a single digit in the
3012 Radix-64 alphabet. When encoding a bit stream with the Radix-64
3013 encoding, the bit stream must be presumed to be ordered with the
3014 most-significant-bit first. That is, the first bit in the stream
3015 will be the high-order bit in the first 8-bit octet, and the eighth
3016 bit will be the low-order bit in the first 8-bit octet, and so on.
3018 +--first octet--+-second octet--+--third octet--+
3019 |7 6 5 4 3 2 1 0|7 6 5 4 3 2 1 0|7 6 5 4 3 2 1 0|
3020 +-----------+---+-------+-------+---+-----------+
3021 |5 4 3 2 1 0|5 4 3 2 1 0|5 4 3 2 1 0|5 4 3 2 1 0|
3022 +--1.index--+--2.index--+--3.index--+--4.index--+
3024 Callas, et al. Expires Oct 24, 2007 [Page 54]
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3027 Each 6-bit group is used as an index into an array of 64 printable
3028 characters from the table below. The character referenced by the
3029 index is placed in the output string.
3031 Value Encoding Value Encoding Value Encoding Value Encoding
3032 0 A 17 R 34 i 51 z
3033 1 B 18 S 35 j 52 0
3034 2 C 19 T 36 k 53 1
3035 3 D 20 U 37 l 54 2
3036 4 E 21 V 38 m 55 3
3037 5 F 22 W 39 n 56 4
3038 6 G 23 X 40 o 57 5
3039 7 H 24 Y 41 p 58 6
3040 8 I 25 Z 42 q 59 7
3041 9 J 26 a 43 r 60 8
3042 10 K 27 b 44 s 61 9
3043 11 L 28 c 45 t 62 +
3044 12 M 29 d 46 u 63 /
3045 13 N 30 e 47 v
3046 14 O 31 f 48 w (pad) =
3047 15 P 32 g 49 x
3048 16 Q 33 h 50 y
3050 The encoded output stream must be represented in lines of no more
3051 than 76 characters each.
3053 Special processing is performed if fewer than 24 bits are available
3054 at the end of the data being encoded. There are three possibilities:
3056 1. The last data group has 24 bits (3 octets). No special
3057 processing is needed.
3059 2. The last data group has 16 bits (2 octets). The first two 6-bit
3060 groups are processed as above. The third (incomplete) data group
3061 has two zero-value bits added to it, and is processed as above.
3062 A pad character (=) is added to the output.
3064 3. The last data group has 8 bits (1 octet). The first 6-bit group
3065 is processed as above. The second (incomplete) data group has
3066 four zero-value bits added to it, and is processed as above. Two
3067 pad characters (=) are added to the output.
3069 6.4. Decoding Radix-64
3071 In Radix-64 data, characters other than those in the table, line
3072 breaks, and other white space probably indicate a transmission
3073 error, about which a warning message or even a message rejection
3074 might be appropriate under some circumstances. Decoding software
3075 must ignore all white space.
3080 Callas, et al. Expires Oct 24, 2007 [Page 55]
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3083 Because it is used only for padding at the end of the data, the
3084 occurrence of any "=" characters may be taken as evidence that the
3085 end of the data has been reached (without truncation in transit). No
3086 such assurance is possible, however, when the number of octets
3087 transmitted was a multiple of three and no "=" characters are
3088 present.
3090 6.5. Examples of Radix-64
3092 Input data: 0x14fb9c03d97e
3093 Hex: 1 4 f b 9 c | 0 3 d 9 7 e
3094 8-bit: 00010100 11111011 10011100 | 00000011 11011001 11111110
3095 6-bit: 000101 001111 101110 011100 | 000000 111101 100111 111110
3096 Decimal: 5 15 46 28 0 61 37 62
3097 Output: F P u c A 9 l +
3098 Input data: 0x14fb9c03d9
3099 Hex: 1 4 f b 9 c | 0 3 d 9
3100 8-bit: 00010100 11111011 10011100 | 00000011 11011001
3101 pad with 00
3102 6-bit: 000101 001111 101110 011100 | 000000 111101 100100
3103 Decimal: 5 15 46 28 0 61 36
3104 pad with =
3105 Output: F P u c A 9 k =
3106 Input data: 0x14fb9c03
3107 Hex: 1 4 f b 9 c | 0 3
3108 8-bit: 00010100 11111011 10011100 | 00000011
3109 pad with 0000
3110 6-bit: 000101 001111 101110 011100 | 000000 110000
3111 Decimal: 5 15 46 28 0 48
3112 pad with = =
3113 Output: F P u c A w = =
3115 6.6. Example of an ASCII Armored Message
3117 -----BEGIN PGP MESSAGE-----
3118 Version: OpenPrivacy 0.99
3120 yDgBO22WxBHv7O8X7O/jygAEzol56iUKiXmV+XmpCtmpqQUKiQrFqclFqUDBovzS
3121 vBSFjNSiVHsuAA==
3122 =njUN
3123 -----END PGP MESSAGE-----
3125 Note that this example has extra indenting; an actual armored
3126 message would have no leading whitespace.
3128 7. Cleartext signature framework
3130 It is desirable to be able to sign a textual octet stream without
3131 ASCII armoring the stream itself, so the signed text is still
3132 readable without special software. In order to bind a signature to
3133 such a cleartext, this framework is used. (Note that this framework
3134 is not intended to be reversible. RFC 3156 defines another way to
3136 Callas, et al. Expires Oct 24, 2007 [Page 56]
3137 \fINTERNET-DRAFT OpenPGP Message Format Apr 24, 2007
3139 sign cleartext messages for environments that support MIME.)
3141 The cleartext signed message consists of:
3143 - The cleartext header '-----BEGIN PGP SIGNED MESSAGE-----' on a
3144 single line,
3146 - One or more "Hash" Armor Headers,
3148 - Exactly one empty line not included into the message digest,
3150 - The dash-escaped cleartext that is included into the message
3151 digest,
3153 - The ASCII armored signature(s) including the '-----BEGIN PGP
3154 SIGNATURE-----' Armor Header and Armor Tail Lines.
3156 If the "Hash" armor header is given, the specified message digest
3157 algorithm(s) are used for the signature. If there are no such
3158 headers, MD5 is used. If MD5 is the only hash used, then an
3159 implementation MAY omit this header for improved V2.x compatibility.
3160 If more than one message digest is used in the signature, the "Hash"
3161 armor header contains a comma-delimited list of used message
3162 digests.
3164 Current message digest names are described below with the algorithm
3165 IDs.
3167 An implementation SHOULD add a line break after the cleartext, but
3168 MAY omit it if the cleartext ends with a line break. This is for
3169 visual clarity.
3171 7.1. Dash-Escaped Text
3173 The cleartext content of the message must also be dash-escaped.
3175 Dash escaped cleartext is the ordinary cleartext where every line
3176 starting with a dash '-' (0x2D) is prefixed by the sequence dash '-'
3177 (0x2D) and space ' ' (0x20). This prevents the parser from
3178 recognizing armor headers of the cleartext itself. An implementation
3179 MAY dash escape any line, SHOULD dash escape lines commencing "From"
3180 followed by a space, and MUST dash escape any line commencing in a
3181 dash. The message digest is computed using the cleartext itself, not
3182 the dash escaped form.
3184 As with binary signatures on text documents, a cleartext signature
3185 is calculated on the text using canonical <CR><LF> line endings. The
3186 line ending (i.e. the <CR><LF>) before the '-----BEGIN PGP
3187 SIGNATURE-----' line that terminates the signed text is not
3188 considered part of the signed text.
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3195 When reversing dash-escaping, an implementation MUST strip the
3196 string "- " if it occurs at the beginning of a line, and SHOULD warn
3197 on "-" and any character other than a space at the beginning of a
3198 line.
3200 Also, any trailing whitespace -- spaces (0x20) and tabs (0x09) -- at
3201 the end of any line is removed when the cleartext signature is
3202 generated.
3204 8. Regular Expressions
3206 A regular expression is zero or more branches, separated by '|'. It
3207 matches anything that matches one of the branches.
3209 A branch is zero or more pieces, concatenated. It matches a match
3210 for the first, followed by a match for the second, etc.
3212 A piece is an atom possibly followed by '*', '+', or '?'. An atom
3213 followed by '*' matches a sequence of 0 or more matches of the atom.
3214 An atom followed by '+' matches a sequence of 1 or more matches of
3215 the atom. An atom followed by '?' matches a match of the atom, or
3216 the null string.
3218 An atom is a regular expression in parentheses (matching a match for
3219 the regular expression), a range (see below), '.' (matching any
3220 single character), '^' (matching the null string at the beginning of
3221 the input string), '$' (matching the null string at the end of the
3222 input string), a '\' followed by a single character (matching that
3223 character), or a single character with no other significance
3224 (matching that character).
3226 A range is a sequence of characters enclosed in '[]'. It normally
3227 matches any single character from the sequence. If the sequence
3228 begins with '^', it matches any single character not from the rest
3229 of the sequence. If two characters in the sequence are separated by
3230 '-', this is shorthand for the full list of ASCII characters between
3231 them (e.g. '[0-9]' matches any decimal digit). To include a literal
3232 ']' in the sequence, make it the first character (following a
3233 possible '^'). To include a literal '-', make it the first or last
3234 character.
3236 9. Constants
3238 This section describes the constants used in OpenPGP.
3240 Note that these tables are not exhaustive lists; an implementation
3241 MAY implement an algorithm not on these lists, so long as the
3242 algorithm number(s) are chosen from the private or experimental
3243 algorithm range.
3248 Callas, et al. Expires Oct 24, 2007 [Page 58]
3249 \fINTERNET-DRAFT OpenPGP Message Format Apr 24, 2007
3251 See the section "Notes on Algorithms" below for more discussion of
3252 the algorithms.
3254 9.1. Public Key Algorithms
3256 ID Algorithm
3257 -- ---------
3258 1 - RSA (Encrypt or Sign) [HAC]
3259 2 - RSA Encrypt-Only [HAC]
3260 3 - RSA Sign-Only [HAC]
3261 16 - Elgamal (Encrypt-Only), see [ELGAMAL] [HAC]
3262 17 - DSA (Digital Signature Algorithm) [FIPS186] [HAC]
3263 18 - Reserved for Elliptic Curve
3264 19 - Reserved for ECDSA
3265 20 - Reserved (formerly Elgamal Encrypt or Sign)
3266 21 - Reserved for Diffie-Hellman (X9.42,
3267 as defined for IETF-S/MIME)
3268 100 to 110 - Private/Experimental algorithm.
3270 Implementations MUST implement DSA for signatures, and Elgamal for
3271 encryption. Implementations SHOULD implement RSA keys (1). RSA
3272 Encrypt-Only (2) and RSA Sign-Only are deprecated and SHOULD NOT be
3273 generated, but may be interpreted. See Section 13.5. See Section
3274 13.8 for notes on Elliptic Curve (18), ECDSA (19), Elgamal Encrypt
3275 or Sign (20), and X9.42 (21). Implementations MAY implement any
3276 other algorithm.
3278 9.2. Symmetric Key Algorithms
3280 ID Algorithm
3281 -- ---------
3282 0 - Plaintext or unencrypted data
3283 1 - IDEA [IDEA]
3284 2 - TripleDES (DES-EDE, [SCHNEIER] [HAC] -
3285 168 bit key derived from 192)
3286 3 - CAST5 (128 bit key, as per RFC 2144)
3287 4 - Blowfish (128 bit key, 16 rounds) [BLOWFISH]
3288 5 - Reserved
3289 6 - Reserved
3290 7 - AES with 128-bit key [AES]
3291 8 - AES with 192-bit key
3292 9 - AES with 256-bit key
3293 10 - Twofish with 256-bit key [TWOFISH]
3294 100 to 110 - Private/Experimental algorithm.
3296 Implementations MUST implement TripleDES. Implementations SHOULD
3297 implement AES-128 and CAST5. Implementations that interoperate with
3298 PGP 2.6 or earlier need to support IDEA, as that is the only
3299 symmetric cipher those versions use. Implementations MAY implement
3300 any other algorithm.
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3307 9.3. Compression Algorithms
3309 ID Algorithm
3310 -- ---------
3311 0 - Uncompressed
3312 1 - ZIP [RFC 1951]
3313 2 - ZLIB [RFC 1950]
3314 3 - BZip2 [BZ2]
3315 100 to 110 - Private/Experimental algorithm.
3317 Implementations MUST implement uncompressed data. Implementations
3318 SHOULD implement ZIP. Implementations MAY implement any other
3319 algorithm.
3321 9.4. Hash Algorithms
3323 ID Algorithm Text Name
3324 -- --------- ---- ----
3325 1 - MD5 [HAC] "MD5"
3326 2 - SHA-1 [FIPS180] "SHA1"
3327 3 - RIPE-MD/160 [HAC] "RIPEMD160"
3328 4 - Reserved
3329 5 - Reserved
3330 6 - Reserved
3331 7 - Reserved
3332 8 - SHA256 [FIPS180] "SHA256"
3333 9 - SHA384 [FIPS180] "SHA384"
3334 10 - SHA512 [FIPS180] "SHA512"
3335 11 - SHA224 [FIPS180] "SHA224"
3336 100 to 110 - Private/Experimental algorithm.
3338 Implementations MUST implement SHA-1. Implementations MAY implement
3339 other algorithms. MD5 is deprecated.
3341 10. IANA Considerations
3343 OpenPGP is highly parameterized and consequently there are a number
3344 of considerations for allocating parameters for extensions. This
3345 section describes how IANA should look at extensions to the protocol
3346 as described in this document.
3348 10.1. New String-to-Key specifier types
3350 OpenPGP S2K specifiers contain a mechanism for new algorithms to
3351 turn a string into a key. This specification creates a registry of
3352 S2K specifier types. The registry includes the S2K type, the name of
3353 the S2K and a reference to the defining specification. The initial
3354 values for this registry can be found in 3.7.1. Adding a new S2K
3355 specifier MUST be done through the IETF CONSENSUS method, as
3356 described in [RFC2434].
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3363 10.2. New Packets
3365 Major new features of OpenPGP are defined though new packet types.
3366 This specification creates a registry of packet types. The registry
3367 includes the packet type, the name of the packet and a reference to
3368 the defining specification. The initial values for this registry can
3369 be found in 4.3. Adding a new packet type MUST be done through the
3370 IETF CONSENSUS method, as described in [RFC2434].
3372 10.2.1. User Attribute Types
3374 The User Attribute packet permits an extensible mechanism for other
3375 types of certificate identification. This specification creates a
3376 registry of User Attribute types. The registry includes the User
3377 Attribute type, the name of the User Attribute and a reference to
3378 the defining specification. The initial values for this registry can
3379 be found in 5.12. Adding a new User Attribute type MUST be done
3380 through the IETF CONSENSUS method, as described in [RFC2434].
3382 10.2.1.1. Image Format Subpacket Types
3384 Within User Attribute packets, there is an extensible mechanism for
3385 other types of image-based user attributes. This specification
3386 creates a registry of Image Attribute subpacket types. The registry
3387 includes the Image Attribute subpacket type, the name of the Image
3388 Attribute subpacket and a reference to the defining specification.
3389 The initial values for this registry can be found in 5.12.1. Adding
3390 a new Image Attribute subpacket type MUST be done through the IETF
3391 CONSENSUS method, as described in [RFC2434].
3393 10.2.2. New Signature Subpackets
3395 OpenPGP signatures contain a mechanism for signed (or unsigned) data
3396 to be added to them for a variety of purposes in the signature
3397 subpackets as discussed in section 5.2.3.1. This specification
3398 creates a registry of signature subpacket types. The registry
3399 includes the signature subpacket type, the name of the subpacket and
3400 a reference to the defining specification. The initial values for
3401 this registry can be found in 5.2.3.1. Adding a new signature
3402 subpacket MUST be done through the IETF CONSENSUS method, as
3403 described in [RFC2434].
3405 10.2.2.1. Signature Notation Data Subpackets
3407 OpenPGP signatures further contain a mechanism for extensions in
3408 signatures. These are the Notation Data subpackets, which contain a
3409 key/value pair. Notations contain a user space which is completely
3410 unmanaged and an IETF space.
3412 This specification creates a registry of Signature Notation Data
3413 types. The registry includes the Signature Notation Data type, the
3414 name of the Signature Notation Data, its allowed values, and a
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3419 reference to the defining specification. The initial values for this
3420 registry can be found in 5.2.3.16. Adding a new Signature Notation
3421 Data subpacket MUST be done through the EXPERT REVIEW method, as
3422 described in [RFC2434].
3424 10.2.2.2. Key Server Preference Extensions
3426 OpenPGP signatures contain a mechanism for preferences to be
3427 specified about key servers. This specification creates a registry
3428 of key server preferences. The registry includes the key server
3429 preference, the name of the preference and a reference to the
3430 defining specification. The initial values for this registry can be
3431 found in 5.2.3.17. Adding a new key server preference MUST be done
3432 through the IETF CONSENSUS method, as described in [RFC2434].
3434 10.2.2.3. Key Flags Extensions
3436 OpenPGP signatures contain a mechanism for flags to be specified
3437 about key usage. This specification creates a registry of key usage
3438 flags. The registry includes the key flags value, the name of the
3439 flag and a reference to the defining specification. The initial
3440 values for this registry can be found in 5.2.3.21. Adding a new key
3441 usage flag MUST be done through the IETF CONSENSUS method, as
3442 described in [RFC2434].
3444 10.2.2.4. Reason For Revocation Extensions
3446 OpenPGP signatures contain a mechanism for flags to be specified
3447 about why a key was revoked. This specification creates a registry
3448 of reason-for-revocation flags. The registry includes the
3449 reason-for-revocation flags value, the name of the flag and a
3450 reference to the defining specification. The initial values for this
3451 registry can be found in 5.2.3.23. Adding a new feature flag MUST be
3452 done through the IETF CONSENSUS method, as described in [RFC2434].
3454 10.2.2.5. Implementation Features
3456 OpenPGP signatures contain a mechanism for flags to be specified
3457 stating which optional features an implementation supports. This
3458 specification creates a registry of feature-implementation flags.
3459 The registry includes the feature-implementation flags value, the
3460 name of the flag and a reference to the defining specification. The
3461 initial values for this registry can be found in 5.2.3.24. Adding a
3462 new feature-implementation flag MUST be done through the IETF
3463 CONSENSUS method, as described in [RFC2434].
3465 Also see section 10.6 for more information about when feature flags
3466 are needed.
3468 10.2.3. New Packet Versions
3470 The core OpenPGP packets all have version numbers, and can be
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3475 revised by introducing a new version of an existing packet. This
3476 specification creates a registry of packet types. The registry
3477 includes the packet type, the number of the version and a reference
3478 to the defining specification. The initial values for this registry
3479 can be found in 5. Adding a new packet version MUST be done through
3480 the IETF CONSENSUS method, as described in [RFC2434].
3482 10.3. New Algorithms
3484 Chapter 9 lists the core algorithms that OpenPGP uses. Adding in a
3485 new algorithm is usually simple. For example, adding in a new
3486 symmetric cipher usually would not need anything more than
3487 allocating a constant for that cipher. If that cipher had other than
3488 a 64-bit or 128-bit block size, there might need to be additional
3489 documentation describing how OpenPGP-CFB mode would be adjusted.
3490 Similarly, when DSA was expanded from a maximum of 1024-bit public
3491 keys to 3072-bit public keys, the revision of FIPS 186 contained
3492 enough information itself to allow implementation. Changes to this
3493 document were emphasis more than required.
3495 10.3.1. Public Key Algorithms
3497 OpenPGP specifies a number of public key algorithms. This
3498 specification creates a registry of public key algorithm
3499 identifiers. The registry includes the algorithm name, its key sizes
3500 and parameters, and a reference to the defining specification. The
3501 initial values for this registry can be found in section 9. Adding a
3502 new public key algorithm MUST be done through the IETF CONSENSUS
3503 method, as described in [RFC2434].
3505 10.3.2. Symmetric Key Algorithms
3507 OpenPGP specifies a number of symmetric key algorithms. This
3508 specification creates a registry of symmetric key algorithm
3509 identifiers. The registry includes the algorithm name, its key sizes
3510 and block size, and a reference to the defining specification. The
3511 initial values for this registry can be found in section 9. Adding a
3512 new symmetric key algorithm MUST be done through the IETF CONSENSUS
3513 method, as described in [RFC2434].
3515 10.3.3. Hash Algorithms
3517 OpenPGP specifies a number of hash algorithms. This specification
3518 creates a registry of hash algorithm identifiers. The registry
3519 includes the algorithm name, a text representation of that name, its
3520 block size, an OID hash prefix, and a reference to the defining
3521 specification. The initial values for this registry can be found in
3522 section 9 for the algorithm identifiers and text names, and section
3523 5.2.2 for the OIDs and expanded signature prefixes. Adding a new
3524 hash algorithm MUST be done through the IETF CONSENSUS method, as
3525 described in [RFC2434].
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3531 10.3.4. Compression Algorithms
3533 OpenPGP specifies a number of compression algorithms. This
3534 specification creates a registry of compression algorithm
3535 identifiers. The registry includes the algorithm name, and a
3536 reference to the defining specification. The initial values for this
3537 registry can be found in section 9.3. Adding a new compression key
3538 algorithm MUST be done through the IETF CONSENSUS method, as
3539 described in [RFC2434].
3541 11. Packet Composition
3543 OpenPGP packets are assembled into sequences in order to create
3544 messages and to transfer keys. Not all possible packet sequences are
3545 meaningful and correct. This section describes the rules for how
3546 packets should be placed into sequences.
3548 11.1. Transferable Public Keys
3550 OpenPGP users may transfer public keys. The essential elements of a
3551 transferable public key are:
3553 - One Public Key packet
3555 - Zero or more revocation signatures
3557 - One or more User ID packets
3559 - After each User ID packet, zero or more signature packets
3560 (certifications)
3562 - Zero or more User Attribute packets
3564 - After each User Attribute packet, zero or more signature packets
3565 (certifications)
3567 - Zero or more Subkey packets
3569 - After each Subkey packet, one signature packet, plus optionally
3570 a revocation.
3572 The Public Key packet occurs first. Each of the following User ID
3573 packets provides the identity of the owner of this public key. If
3574 there are multiple User ID packets, this corresponds to multiple
3575 means of identifying the same unique individual user; for example, a
3576 user may have more than one email address, and construct a User ID
3577 for each one.
3579 Immediately following each User ID packet, there are zero or more
3580 signature packets. Each signature packet is calculated on the
3581 immediately preceding User ID packet and the initial Public Key
3582 packet. The signature serves to certify the corresponding public key
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3587 and User ID. In effect, the signer is testifying to his or her
3588 belief that this public key belongs to the user identified by this
3589 User ID.
3591 Within the same section as the User ID packets, there are zero or
3592 more User Attribute packets. Like the User ID packets, a User
3593 Attribute packet is followed by zero or more signature packets
3594 calculated on the immediately preceding User Attribute packet and
3595 the initial Public Key packet.
3597 User Attribute packets and User ID packets may be freely intermixed
3598 in this section, so long as the signatures that follow them are
3599 maintained on the proper User Attribute or User ID packet.
3601 After the User ID or Attribute packets there may be zero or more
3602 Subkey packets. In general, subkeys are provided in cases where the
3603 top-level public key is a signature-only key. However, any V4 key
3604 may have subkeys, and the subkeys may be encryption-only keys,
3605 signature-only keys, or general-purpose keys. V3 keys MUST NOT have
3606 subkeys.
3608 Each Subkey packet MUST be followed by one Signature packet, which
3609 should be a subkey binding signature issued by the top level key.
3610 For subkeys that can issue signatures, the subkey binding signature
3611 MUST contain an embedded signature subpacket with a primary key
3612 binding signature (0x19) issued by the subkey on the top level key.
3614 Subkey and Key packets may each be followed by a revocation
3615 Signature packet to indicate that the key is revoked. Revocation
3616 signatures are only accepted if they are issued by the key itself,
3617 or by a key that is authorized to issue revocations via a revocation
3618 key subpacket in a self-signature by the top level key.
3620 Transferable public key packet sequences may be concatenated to
3621 allow transferring multiple public keys in one operation.
3623 11.2. Transferable Secret Keys
3625 OpenPGP users may transfer secret keys. The format of a transferable
3626 secret key is the same as a transferable public key except that
3627 secret key and secret subkey packets are used instead of the public
3628 key and public subkey packets. Implementations SHOULD include
3629 self-signatures on any user IDs and subkeys, as this allows for a
3630 complete public key to be automatically extracted from the
3631 transferable secret key. Implementations MAY choose to omit the
3632 self-signatures, especially if a transferable public key accompanies
3633 the transferable secret key.
3635 11.3. OpenPGP Messages
3637 An OpenPGP message is a packet or sequence of packets that
3638 corresponds to the following grammatical rules (comma represents
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3643 sequential composition, and vertical bar separates alternatives):
3645 OpenPGP Message :- Encrypted Message | Signed Message |
3646 Compressed Message | Literal Message.
3648 Compressed Message :- Compressed Data Packet.
3650 Literal Message :- Literal Data Packet.
3652 ESK :- Public Key Encrypted Session Key Packet |
3653 Symmetric-Key Encrypted Session Key Packet.
3655 ESK Sequence :- ESK | ESK Sequence, ESK.
3657 Encrypted Data :- Symmetrically Encrypted Data Packet |
3658 Symmetrically Encrypted Integrity Protected Data Packet
3660 Encrypted Message :- Encrypted Data | ESK Sequence, Encrypted Data.
3662 One-Pass Signed Message :- One-Pass Signature Packet,
3663 OpenPGP Message, Corresponding Signature Packet.
3665 Signed Message :- Signature Packet, OpenPGP Message |
3666 One-Pass Signed Message.
3668 In addition, decrypting a Symmetrically Encrypted Data Packet or a
3669 Symmetrically Encrypted Integrity Protected Data Packet as well as
3670 decompressing a Compressed Data packet must yield a valid OpenPGP
3671 Message.
3673 11.4. Detached Signatures
3675 Some OpenPGP applications use so-called "detached signatures." For
3676 example, a program bundle may contain a file, and with it a second
3677 file that is a detached signature of the first file. These detached
3678 signatures are simply a signature packet stored separately from the
3679 data that they are a signature of.
3681 12. Enhanced Key Formats
3683 12.1. Key Structures
3685 The format of an OpenPGP V3 key is as follows. Entries in square
3686 brackets are optional and ellipses indicate repetition.
3688 RSA Public Key
3689 [Revocation Self Signature]
3690 User ID [Signature ...]
3691 [User ID [Signature ...] ...]
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3699 Each signature certifies the RSA public key and the preceding User
3700 ID. The RSA public key can have many User IDs and each User ID can
3701 have many signatures. V3 keys are deprecated. Implementations MUST
3702 NOT generate new V3 keys, but MAY continue to use existing ones.
3704 The format of an OpenPGP V4 key that uses multiple public keys is
3705 similar except that the other keys are added to the end as "subkeys"
3706 of the primary key.
3708 Primary-Key
3709 [Revocation Self Signature]
3710 [Direct Key Signature...]
3711 User ID [Signature ...]
3712 [User ID [Signature ...] ...]
3713 [User Attribute [Signature ...] ...]
3714 [[Subkey [Binding-Signature-Revocation]
3715 Primary-Key-Binding-Signature] ...]
3717 A subkey always has a single signature after it that is issued using
3718 the primary key to tie the two keys together. This binding signature
3719 may be in either V3 or V4 format, but SHOULD be V4. Subkeys that can
3720 issue signatures MUST have a V4 binding signature due to the
3721 REQUIRED embedded primary key binding signature.
3723 In the above diagram, if the binding signature of a subkey has been
3724 revoked, the revoked key may be removed, leaving only one key.
3726 In a V4 key, the primary key MUST be a key capable of certification.
3727 The subkeys may be keys of any other type. There may be other
3728 constructions of V4 keys, too. For example, there may be a
3729 single-key RSA key in V4 format, a DSA primary key with an RSA
3730 encryption key, or RSA primary key with an Elgamal subkey, etc.
3732 It is also possible to have a signature-only subkey. This permits a
3733 primary key that collects certifications (key signatures) but is
3734 used only used for certifying subkeys that are used for encryption
3735 and signatures.
3737 12.2. Key IDs and Fingerprints
3739 For a V3 key, the eight-octet key ID consists of the low 64 bits of
3740 the public modulus of the RSA key.
3742 The fingerprint of a V3 key is formed by hashing the body (but not
3743 the two-octet length) of the MPIs that form the key material (public
3744 modulus n, followed by exponent e) with MD5. Note that both V3 keys
3745 and MD5 are deprecated.
3747 A V4 fingerprint is the 160-bit SHA-1 hash of the octet 0x99,
3748 followed by the two-octet packet length, followed by the entire
3749 Public Key packet starting with the version field. The key ID is the
3750 low order 64 bits of the fingerprint. Here are the fields of the
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3755 hash material, with the example of a DSA key:
3757 a.1) 0x99 (1 octet)
3759 a.2) high order length octet of (b)-(f) (1 octet)
3761 a.3) low order length octet of (b)-(f) (1 octet)
3763 b) version number = 4 (1 octet);
3765 c) time stamp of key creation (4 octets);
3767 d) algorithm (1 octet): 17 = DSA (example);
3769 e) Algorithm specific fields.
3771 Algorithm Specific Fields for DSA keys (example):
3773 e.1) MPI of DSA prime p;
3775 e.2) MPI of DSA group order q (q is a prime divisor of p-1);
3777 e.3) MPI of DSA group generator g;
3779 e.4) MPI of DSA public key value y (= g**x mod p where x is secret).
3781 Note that it is possible for there to be collisions of key IDs --
3782 two different keys with the same key ID. Note that there is a much
3783 smaller, but still non-zero probability that two different keys have
3784 the same fingerprint.
3786 Also note that if V3 and V4 format keys share the same RSA key
3787 material, they will have different key IDs as well as different
3788 fingerprints.
3790 Finally, the key ID and fingerprint of a subkey are calculated in
3791 the same way as for a primary key, including the 0x99 as the first
3792 octet (even though this is not a valid packet ID for a public
3793 subkey).
3795 13. Notes on Algorithms
3797 13.1. PKCS#1 Encoding In OpenPGP
3799 This standard makes use of the PKCS#1 functions EME-PKCS1-v1_5 and
3800 EMSA-PKCS1-v1_5. However, the calling conventions of these functions
3801 has changed in the past. To avoid potential confusion and
3802 interoperability problems, we are including local copies in this
3803 document, adapted from those in PKCS#1 v2.1 [RFC3447]. RFC-3447
3804 should be treated as the ultimate authority on PKCS#1 for OpenPGP.
3805 Nonetheless, we believe that there is value in having a
3806 self-contained document that avoids problems in the future with
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3811 needed changes in the conventions.
3813 13.1.1. EME-PKCS1-v1_5-ENCODE
3815 Input:
3817 k = the length in octets of the key modulus
3819 M = message to be encoded, an octet string of length mLen, where
3820 mLen <= k - 11
3822 Output:
3824 EM = encoded message, an octet string of length k
3826 Error: "message too long"
3828 1. Length checking: If mLen > k - 11, output "message too long" and
3829 stop.
3831 2. Generate an octet string PS of length k - mLen - 3 consisting of
3832 pseudo-randomly generated nonzero octets. The length of PS will
3833 be at least eight octets.
3835 3. Concatenate PS, the message M, and other padding to form an
3836 encoded message EM of length k octets as
3838 EM = 0x00 || 0x02 || PS || 0x00 || M.
3840 4. Output EM.
3842 13.1.2. EME-PKCS1-v1_5-DECODE
3844 Input:
3846 EM = encoded message, an octet string
3848 Output:
3850 M = message, an octet string
3852 Error: "decryption error"
3854 To decode an EME-PKCS1_v1_5 message, separate the encoded message EM
3855 into an octet string PS consisting of nonzero octets and a message M
3856 as
3858 EM = 0x00 || 0x02 || PS || 0x00 || M.
3860 If the first octet of EM does not have hexadecimal value 0x00, if
3861 the second octet of EM does not have hexadecimal value 0x02, if
3862 there is no octet with hexadecimal value 0x00 to separate PS from M,
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3867 or if the length of PS is less than 8 octets, output "decryption
3868 error" and stop. See also the security note in section 13 regarding
3869 differences in reporting between a decryption error and a padding
3870 error.
3872 13.1.3. EMSA-PKCS1-v1_5
3874 This encoding method is deterministic and only has an encoding
3875 operation.
3877 Option:
3879 Hash hash function (hLen denotes the length in octets of the hash
3880 function output)
3882 Input:
3884 M = message to be encoded
3886 mL = intended length in octets of the encoded message, at least tLen
3887 + 11, where tLen is the octet length of the DER encoding T of a
3888 certain value computed during the encoding operation
3890 Output:
3892 EM = encoded message, an octet string of length emLen
3894 Errors: "message too long"; "intended encoded message length too
3895 short"
3897 Steps:
3899 1. Apply the hash function to the message M to produce a hash value
3900 H:
3902 H = Hash(M).
3904 If the hash function outputs "message too long," output "message
3905 too long" and stop.
3907 2. Using the list in section 5.2.2, produce an ASN.1 DER value for
3908 the hash function used. Let T be the full hash prefix from
3909 section 5.2.2, and let tLen be the length in octets of T.
3911 3. If emLen < tLen + 11, output "intended encoded message length
3912 too short" and stop.
3914 4. Generate an octet string PS consisting of emLen - tLen - 3
3915 octets with hexadecimal value 0xff. The length of PS will be at
3916 least 8 octets.
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3923 5. Concatenate PS, the hash prefix T, and other padding to form the
3924 encoded message EM as
3926 EM = 0x00 || 0x01 || PS || 0x00 || T.
3928 6. Output EM.
3930 13.2. Symmetric Algorithm Preferences
3932 The symmetric algorithm preference is an ordered list of algorithms
3933 that the keyholder accepts. Since it is found on a self-signature,
3934 it is possible that a keyholder may have multiple, different
3935 preferences. For example, Alice may have TripleDES only specified
3936 for "alice@work.com" but CAST5, Blowfish, and TripleDES specified
3937 for "alice@home.org". Note that it is also possible for preferences
3938 to be in a subkey's binding signature.
3940 Since TripleDES is the MUST-implement algorithm, if it is not
3941 explicitly in the list, it is tacitly at the end. However, it is
3942 good form to place it there explicitly. Note also that if an
3943 implementation does not implement the preference, then it is
3944 implicitly a TripleDES-only implementation.
3946 An implementation MUST NOT use a symmetric algorithm that is not in
3947 the recipient's preference list. When encrypting to more than one
3948 recipient, the implementation finds a suitable algorithm by taking
3949 the intersection of the preferences of the recipients. Note that the
3950 MUST-implement algorithm, TripleDES, ensures that the intersection
3951 is not null. The implementation may use any mechanism to pick an
3952 algorithm in the intersection.
3954 If an implementation can decrypt a message that a keyholder doesn't
3955 have in their preferences, the implementation SHOULD decrypt the
3956 message anyway, but MUST warn the keyholder that the protocol has
3957 been violated. For example, suppose that Alice, above, has software
3958 that implements all algorithms in this specification. Nonetheless,
3959 she prefers subsets for work or home. If she is sent a message
3960 encrypted with IDEA, which is not in her preferences, the software
3961 warns her that someone sent her an IDEA-encrypted message, but it
3962 would ideally decrypt it anyway.
3964 13.3. Other Algorithm Preferences
3966 Other algorithm preferences work similarly to the symmetric
3967 algorithm preference, in that they specify which algorithms the
3968 keyholder accepts. There are two interesting cases that other
3969 comments need to be made about, though, the compression preferences
3970 and the hash preferences.
3972 13.3.1. Compression Preferences
3974 Compression has been an integral part of PGP since its first days.
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3979 OpenPGP and all previous versions of PGP have offered compression.
3980 In this specification, the default is for messages to be compressed,
3981 although an implementation is not required to do so. Consequently,
3982 the compression preference gives a way for a keyholder to request
3983 that messages not be compressed, presumably because they are using a
3984 minimal implementation that does not include compression.
3985 Additionally, this gives a keyholder a way to state that it can
3986 support alternate algorithms.
3988 Like the algorithm preferences, an implementation MUST NOT use an
3989 algorithm that is not in the preference vector. If the preferences
3990 are not present, then they are assumed to be [ZIP(1),
3991 UNCOMPRESSED(0)].
3993 Additionally, an implementation MUST implement this preference to
3994 the degree of recognizing when to send an uncompressed message. A
3995 robust implementation would satisfy this requirement by looking at
3996 the recipient's preference and acting accordingly. A minimal
3997 implementation can satisfy this requirement by never generating a
3998 compressed message, since all implementations can handle messages
3999 that have not been compressed.
4001 13.3.2. Hash Algorithm Preferences
4003 Typically, the choice of a hash algorithm is something the signer
4004 does, rather than the verifier, because a signer rarely knows who is
4005 going to be verifying the signature. This preference, though, allows
4006 a protocol based upon digital signatures ease in negotiation.
4008 Thus, if Alice is authenticating herself to Bob with a signature, it
4009 makes sense for her to use a hash algorithm that Bob's software
4010 uses. This preference allows Bob to state in his key which
4011 algorithms Alice may use.
4013 Since SHA1 is the MUST-implement hash algorithm, if it is not
4014 explicitly in the list, it is tacitly at the end. However, it is
4015 good form to place it there explicitly.
4017 13.4. Plaintext
4019 Algorithm 0, "plaintext," may only be used to denote secret keys
4020 that are stored in the clear. Implementations MUST NOT use plaintext
4021 in Symmetrically Encrypted Data Packets; they must use Literal Data
4022 Packets to encode unencrypted or literal data.
4024 13.5. RSA
4026 There are algorithm types for RSA Sign-Only, and RSA Encrypt-Only
4027 keys. These types are deprecated. The "key flags" subpacket in a
4028 signature is a much better way to express the same idea, and
4029 generalizes it to all algorithms. An implementation SHOULD NOT
4030 create such a key, but MAY interpret it.
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4035 An implementation SHOULD NOT implement RSA keys of size less than
4036 1024 bits.
4038 13.6. DSA
4040 An implementation SHOULD NOT implement DSA keys of size less than
4041 1024 bits. It MUST NOT implement a DSA key with a q size of less
4042 than 160 bits. DSA keys MUST also be a multiple of 64 bits, and the
4043 q size MUST be a multiple of 8 bits. The Digital Signature Standard
4044 (DSS) [FIPS186] specifies that DSA be used in one of the following
4045 ways:
4047 * 1024-bit key, 160-bit q, SHA-1, SHA-224, SHA-256, SHA-384 or
4048 SHA-512 hash
4050 * 2048-bit key, 224-bit q, SHA-224, SHA-256, SHA-384 or SHA-512
4051 hash
4053 * 2048-bit key, 256-bit q, SHA-256, SHA-384 or SHA-512 hash
4055 * 3072-bit key, 256-bit q, SHA-256, SHA-384 or SHA-512 hash
4057 The above key and q size pairs were chosen to best balance the
4058 strength of the key with the strength of the hash. Implementations
4059 SHOULD use one of the above key and q size pairs when generating DSA
4060 keys. If DSS compliance is desired, one of the specified SHA hashes
4061 must be used as well. [FIPS186] is the ultimate authority on DSS,
4062 and should be consulted for all questions of DSS compliance.
4064 Note that earlier versions of this standard only allowed a 160-bit q
4065 with no truncation allowed, so earlier implementations may not be
4066 able to handle signatures with a different q size or a truncated
4067 hash.
4069 13.7. Elgamal
4071 An implementation SHOULD NOT implement Elgamal keys of size less
4072 than 1024 bits.
4074 13.8. Reserved Algorithm Numbers
4076 A number of algorithm IDs have been reserved for algorithms that
4077 would be useful to use in an OpenPGP implementation, yet there are
4078 issues that prevent an implementer from actually implementing the
4079 algorithm. These are marked in the Public Algorithms section as
4080 "(reserved for)".
4082 The reserved public key algorithms, Elliptic Curve (18), ECDSA (19),
4083 and X9.42 (21) do not have the necessary parameters, parameter
4084 order, or semantics defined.
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4091 Previous versions of OpenPGP permitted Elgamal [ELGAMAL] signatures
4092 with a public key identifier of 20. These are no longer permitted.
4093 An implementation MUST NOT generate such keys. An implementation
4094 MUST NOT generate Elgamal signatures. See [BLEICHENBACHER].
4096 13.9. OpenPGP CFB mode
4098 OpenPGP does symmetric encryption using a variant of Cipher Feedback
4099 Mode (CFB mode). This section describes the procedure it uses in
4100 detail. This mode is what is used for Symmetrically Encrypted Data
4101 Packets; the mechanism used for encrypting secret key material is
4102 similar, but described in those sections above.
4104 In the description below, the value BS is the block size in octets
4105 of the cipher. Most ciphers have a block size of 8 octets. The AES
4106 and Twofish have a block size of 16 octets. Also note that the
4107 description below assumes that the IV and CFB arrays start with an
4108 index of 1 (unlike the C language, which assumes arrays start with a
4109 zero index).
4111 OpenPGP CFB mode uses an initialization vector (IV) of all zeros,
4112 and prefixes the plaintext with BS+2 octets of random data, such
4113 that octets BS+1 and BS+2 match octets BS-1 and BS. It does a CFB
4114 resynchronization after encrypting those BS+2 octets.
4116 Thus, for an algorithm that has a block size of 8 octets (64 bits),
4117 the IV is 10 octets long and octets 7 and 8 of the IV are the same
4118 as octets 9 and 10. For an algorithm with a block size of 16 octets
4119 (128 bits), the IV is 18 octets long, and octets 17 and 18 replicate
4120 octets 15 and 16. Those extra two octets are an easy check for a
4121 correct key.
4123 Step by step, here is the procedure:
4125 1. The feedback register (FR) is set to the IV, which is all zeros.
4127 2. FR is encrypted to produce FRE (FR Encrypted). This is the
4128 encryption of an all-zero value.
4130 3. FRE is xored with the first BS octets of random data prefixed to
4131 the plaintext to produce C[1] through C[BS], the first BS octets
4132 of ciphertext.
4134 4. FR is loaded with C[1] through C[BS].
4136 5. FR is encrypted to produce FRE, the encryption of the first BS
4137 octets of ciphertext.
4139 6. The left two octets of FRE get xored with the next two octets of
4140 data that were prefixed to the plaintext. This produces C[BS+1]
4141 and C[BS+2], the next two octets of ciphertext.
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4147 7. (The resynchronization step) FR is loaded with C[3] through
4148 C[BS+2].
4150 8. FR is encrypted to produce FRE.
4152 9. FRE is xored with the first BS octets of the given plaintext,
4153 now that we have finished encrypting the BS+2 octets of prefixed
4154 data. This produces C[BS+3] through C[BS+(BS+2)], the next BS
4155 octets of ciphertext.
4157 10. FR is loaded with C[BS+3] to C[BS + (BS+2)] (which is C11-C18
4158 for an 8-octet block).
4160 11. FR is encrypted to produce FRE.
4162 12. FRE is xored with the next BS octets of plaintext, to produce
4163 the next BS octets of ciphertext. These are loaded into FR and
4164 the process is repeated until the plaintext is used up.
4166 13.10. Private or Experimental Parameters
4168 S2K specifiers, Signature subpacket types, user attribute types,
4169 image format types, and algorithms described in Section 9 all
4170 reserve the range 100 to 110 for private and experimental use.
4171 Packet types reserve the range 60 to 63 for private and experimental
4172 use. These are intentionally managed with the PRIVATE USE method, as
4173 described in [RFC2434].
4175 However, implementations need to be careful with these and promote
4176 them to full IANA-managed parameters when they grow beyond the
4177 original, limited system.
4179 13.11. Extension of the MDC System
4181 As described in the non-normative explanation in section 5.13, the
4182 MDC system is uniquely unparameterized in OpenPGP, and that this was
4183 an intentional decision to avoid cross-grade attacks. If the MDC
4184 system is extended to a stronger hash function, there must be care
4185 given to avoiding downgrade and cross-grade attacks.
4187 One simple way to do this is to create new packets for a new MDC.
4188 For example, instead of the MDC system using packets 18 and 19, a
4189 new MDC could use 20 and 21. This has obvious drawbacks (it uses two
4190 packet numbers for each new hash function in a space that is limited
4191 to a maximum of 60).
4193 Another simple way to extend the MDC system is to create new
4194 versions of packet 18, and reflect this in packet 19. For example,
4195 suppose that V2 of packet 18 implicitly used SHA-256. This would
4196 require packet 19 to have a length of 32 octets. The change in the
4197 version in packet 18 and the size of packet 19 prevent a downgrade
4198 attack.
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4203 There are two drawbacks to this latter approach. The first is that
4204 using the version number of a packet to carry algorithm information
4205 is not tidy from a protocol-design standpoint. it is possible that
4206 there might be several versions of the MDC system in common use, but
4207 this untidiness would reflect untidiness in cryptographic consensus
4208 about hash function security. The second is that different versions
4209 of packet 19 would have to have unique sizes. If there were two
4210 versions each with 256-bit hashes, they could not both have 32-octet
4211 packet 19s without admitting the chance of a cross-grade attack.
4213 Yet another, complex approach to extend the MDC system would be a
4214 hybrid of the two above -- create a new pair of MDC packets that are
4215 fully parameterized, and yet protected from downgrade and
4216 cross-grade.
4218 Any change to the MDC system MUST be done through the IETF CONSENSUS
4219 method, as described in [RFC2434].
4221 13.12. Meta-Considerations for Expansion
4223 If OpenPGP is extended in a way that is not backwards-compatible,
4224 meaning that old implementations will not gracefully handle their
4225 absence of a new feature, the extension proposal can be declared in
4226 the key holder's self-signature as part of the Features signature
4227 subpacket.
4229 We cannot state definitively what extensions will not be
4230 upwards-compatible, but typically new algorithms are
4231 upwards-compatible, but new packets are not.
4233 If an extension proposal does not update the Features system, it
4234 SHOULD include an explanation of why this is unnecessary. If the
4235 proposal contains neither an extension to the Features system nor an
4236 explanation of why such an extension is unnecessary, the proposal
4237 SHOULD be rejected.
4239 14. Security Considerations
4241 * As with any technology involving cryptography, you should check
4242 the current literature to determine if any algorithms used here
4243 have been found to be vulnerable to attack.
4245 * This specification uses Public Key Cryptography technologies. It
4246 is assumed that the private key portion of a public-private key
4247 pair is controlled and secured by the proper party or parties.
4249 * Certain operations in this specification involve the use of
4250 random numbers. An appropriate entropy source should be used to
4251 generate these numbers. See RFC 4086.
4256 Callas, et al. Expires Oct 24, 2007 [Page 76]
4257 \fINTERNET-DRAFT OpenPGP Message Format Apr 24, 2007
4259 * The MD5 hash algorithm has been found to have weaknesses, with
4260 collisions found in a number of cases. MD5 is deprecated for use
4261 in OpenPGP. Implementations MUST NOT generate new signatures
4262 using MD5 as a hash function. They MAY continue to consider old
4263 signatures that used MD5 as valid.
4265 * SHA-224 and SHA-384 require the same work as SHA-256 and SHA-512
4266 respectively. In general, there are few reasons to use them
4267 outside of DSS compatibility. You need a situation where one
4268 needs more security than smaller hashes, but does not want to
4269 have the full 256-bit or 512-bit data length.
4271 * Many security protocol designers think that it is a bad idea to
4272 use a single key for both privacy (encryption) and integrity
4273 (signatures). In fact, this was one of the motivating forces
4274 behind the V4 key format with separate signature and encryption
4275 keys. If you as an implementer promote dual-use keys, you should
4276 at least be aware of this controversy.
4278 * The DSA algorithm will work with any hash, but is sensitive to
4279 the quality of the hash algorithm. Verifiers should be aware
4280 that even if the signer used a strong hash, an attacker could
4281 have modified the signature to use a weak one. Only signatures
4282 using acceptably strong hash algorithms should be accepted as
4283 valid.
4285 * As OpenPGP combines many different asymmetric, symmetric, and
4286 hash algorithms, each with different measures of strength, care
4287 should be taken that the weakest element of an OpenPGP message
4288 is still sufficiently strong for the purpose at hand. While
4289 consensus about the the strength of a given algorithm may
4290 evolve, NIST Special Publication 800-57 [SP800-57] recommends
4291 the following list of equivalent strengths:
4293 Asymmetric | Hash | Symmetric
4294 key size | size | key size
4295 ------------+--------+-----------
4296 1024 160 80
4297 2048 224 112
4298 3072 256 128
4299 7680 384 192
4300 15360 512 256
4303 * There is a somewhat-related potential security problem in
4304 signatures. If an attacker can find a message that hashes to the
4305 same hash with a different algorithm, a bogus signature
4306 structure can be constructed that evaluates correctly.
4308 For example, suppose Alice DSA signs message M using hash
4309 algorithm H. Suppose that Mallet finds a message M' that has the
4310 same hash value as M with H'. Mallet can then construct a
4312 Callas, et al. Expires Oct 24, 2007 [Page 77]
4313 \fINTERNET-DRAFT OpenPGP Message Format Apr 24, 2007
4315 signature block that verifies as Alice's signature of M' with
4316 H'. However, this would also constitute a weakness in either H
4317 or H' or both. Should this ever occur, a revision will have to
4318 be made to this document to revise the allowed hash algorithms.
4320 * If you are building an authentication system, the recipient may
4321 specify a preferred signing algorithm. However, the signer would
4322 be foolish to use a weak algorithm simply because the recipient
4323 requests it.
4325 * Some of the encryption algorithms mentioned in this document
4326 have been analyzed less than others. For example, although CAST5
4327 is presently considered strong, it has been analyzed less than
4328 TripleDES. Other algorithms may have other controversies
4329 surrounding them.
4331 * In late summer 2002, Jallad, Katz, and Schneier published an
4332 interesting attack on the OpenPGP protocol and some of its
4333 implementations [JKS02]. In this attack, the attacker modifies a
4334 message and sends it to a user who then returns the erroneously
4335 decrypted message to the attacker. The attacker is thus using
4336 the user as a random oracle, and can often decrypt the message.
4338 Compressing data can ameliorate this attack. The incorrectly
4339 decrypted data nearly always decompresses in ways that defeats
4340 the attack. However, this is not a rigorous fix, and leaves open
4341 some small vulnerabilities. For example, if an implementation
4342 does not compress a message before encryption (perhaps because
4343 it knows it was already compressed), then that message is
4344 vulnerable. Because of this happenstance -- that modification
4345 attacks can be thwarted by decompression errors, an
4346 implementation SHOULD treat a decompression error as a security
4347 problem, not merely a data problem.
4349 This attack can be defeated by the use of Modification
4350 Detection, provided that the implementation does not let the
4351 user naively return the data to the attacker. An implementation
4352 MUST treat an MDC failure as a security problem, not merely a
4353 data problem.
4355 In either case, the implementation MAY allow the user access to
4356 the erroneous data, but MUST warn the user as to potential
4357 security problems should that data be returned to the sender.
4359 While this attack is somewhat obscure, requiring a special set
4360 of circumstances to create it, it is nonetheless quite serious
4361 as it permits someone to trick a user to decrypt a message.
4362 Consequently, it is important that:
4364 1. Implementers treat MDC errors and decompression failures as
4365 security problems.
4368 Callas, et al. Expires Oct 24, 2007 [Page 78]
4369 \fINTERNET-DRAFT OpenPGP Message Format Apr 24, 2007
4371 2. Implementers implement Modification Detection with all due
4372 speed and encourage its spread.
4374 3. Users migrate to implementations that support Modification
4375 Detection with all due speed.
4377 * PKCS#1 has been found to be vulnerable to attacks in which a
4378 system that reports errors in padding differently from errors in
4379 decryption becomes a random oracle that can leak the private key
4380 in mere millions of queries. Implementations must be aware of
4381 this attack and prevent it from happening. The simplest solution
4382 is report a single error code for all variants of decryption
4383 errors so as not to leak information to an attacker.
4385 * Some technologies mentioned here may be subject to government
4386 control in some countries.
4388 * In winter 2005, Serge Mister and Robert Zuccherato from Entrust
4389 released a paper describing a way that the "quick check" in
4390 OpenPGP CFB mode can be used with a random oracle to decrypt two
4391 octets of every cipher block [MZ05]. They recommend as
4392 prevention not using the quick check at all.
4394 Many implementers have taken this advice to heart for any data
4395 that is symmetrically encrypted and for which the session key is
4396 public-key encrypted. In this case, the quick check is not
4397 needed as the public key encryption of the session key should
4398 guarantee that it is the right session key. In other cases, the
4399 implementation should use the quick check with care.
4401 On the one hand, there is a danger to using it if there is a
4402 random oracle that can leak information to an attacker. In
4403 plainer language, there is a danger to using the quick check if
4404 timing information about the check can be exposed to an
4405 attacker, particularly via an automated service that allows
4406 rapidly repeated queries.
4408 On the other hand, it is inconvenient to the user to be informed
4409 that they typed in the wrong passphrase only after a petabyte of
4410 data is decrypted. There are many cases in cryptographic
4411 engineering where the implementer must use care and wisdom, and
4412 this is one.
4414 15. Implementation Nits
4416 This section is a collection of comments to help an implementer,
4417 particularly with an eye to backward compatibility. Previous
4418 implementations of PGP are not OpenPGP-compliant. Often the
4419 differences are small, but small differences are frequently more
4420 vexing than large differences. Thus, this is a non-comprehensive
4421 list of potential problems and gotchas for a developer who is trying
4422 to be backward-compatible.
4424 Callas, et al. Expires Oct 24, 2007 [Page 79]
4425 \fINTERNET-DRAFT OpenPGP Message Format Apr 24, 2007
4427 * The IDEA algorithm is patented, and yet it is required for PGP
4428 2.x interoperability. It is also the de-facto preferred
4429 algorithm for a V3 key with a V3 self-signature (or no
4430 self-signature).
4432 * When exporting a private key, PGP 2.x generates the header
4433 "BEGIN PGP SECRET KEY BLOCK" instead of "BEGIN PGP PRIVATE KEY
4434 BLOCK". All previous versions ignore the implied data type, and
4435 look directly at the packet data type.
4437 * PGP 2.0 through 2.5 generated V2 Public Key Packets. These are
4438 identical to the deprecated V3 keys except for the version
4439 number. An implementation MUST NOT generate them and may accept
4440 or reject them as it sees fit. Some older PGP versions generated
4441 V2 PKESK packets (Tag 1) as well. An implementation may accept
4442 or reject V2 PKESK packets as it sees fit, and MUST NOT generate
4443 them.
4445 * PGP 2.6.x will not accept key-material packets with versions
4446 greater than 3.
4448 * There are many ways possible for two keys to have the same key
4449 material, but different fingerprints (and thus key IDs). Perhaps
4450 the most interesting is an RSA key that has been "upgraded" to
4451 V4 format, but since a V4 fingerprint is constructed by hashing
4452 the key creation time along with other things, two V4 keys
4453 created at different times, yet with the same key material will
4454 have different fingerprints.
4456 * If an implementation is using zlib to interoperate with PGP 2.x,
4457 then the "windowBits" parameter should be set to -13.
4459 * The 0x19 back signatures were not required for signing subkeys
4460 until relatively recently. Consquently, there may be keys in the
4461 wild that do not have these back signatures. Implementing
4462 software may handle these keys as it sees fit.
4464 16. Authors' Addresses
4466 The working group can be contacted via the current chair:
4468 Derek Atkins
4469 IHTFP Consulting, Inc.
4470 6 Farragut Ave
4471 Somerville, MA 02144 USA
4472 Email: derek@ihtfp.com
4473 Tel: +1 617 623 3745
4475 The principal authors of this draft are:
4480 Callas, et al. Expires Oct 24, 2007 [Page 80]
4481 \fINTERNET-DRAFT OpenPGP Message Format Apr 24, 2007
4483 Jon Callas
4484 Email: jon@callas.org
4486 Lutz Donnerhacke
4487 IKS GmbH
4488 Wildenbruchstr. 15
4489 07745 Jena, Germany
4491 EMail: lutz@iks-jena.de
4493 Hal Finney
4494 Email: hal@finney.org
4496 David Shaw
4497 Email: dshaw@jabberwocky.com
4499 Rodney Thayer
4500 Email: rodney@canola-jones.com
4502 This memo also draws on much previous work from a number of other
4503 authors who include: Derek Atkins, Charles Breed, Dave Del Torto,
4504 Marc Dyksterhouse, Gail Haspert, Gene Hoffman, Paul Hoffman, Ben
4505 Laurie, Raph Levien, Colin Plumb, Will Price, David Shaw, William
4506 Stallings, Mark Weaver, and Philip R. Zimmermann.
4508 17. References (Normative)
4511 [AES] NIST, FIPS PUB 197, "Advanced Encryption Standard
4512 (AES)," November 2001.
4514 http://csrc.nist.gov/publications/fips/fips197/
4515 fips-197.{ps,pdf}
4517 [BLOWFISH] Schneier, B. "Description of a New Variable-Length
4518 Key, 64-Bit Block Cipher (Blowfish)" Fast Software
4519 Encryption, Cambridge Security Workshop Proceedings
4520 (December 1993), Springer-Verlag, 1994, pp191-204
4521 <http://www.counterpane.com/bfsverlag.html>
4523 [BZ2] J. Seward, jseward@acm.org, "The Bzip2 and libbzip2
4524 home page" <http://www.bzip.org/>
4526 [ELGAMAL] T. Elgamal, "A Public-Key Cryptosystem and a
4527 Signature Scheme Based on Discrete Logarithms,"
4528 IEEE Transactions on Information Theory, v. IT-31,
4529 n. 4, 1985, pp. 469-472.
4531 [FIPS180] Secure Hash Signature Standard (SHS) (FIPS PUB
4532 180-2).
4533 <http://csrc.nist.gov/publications/fips/
4534 fips180-2/fips180-2withchangenotice.pdf>
4536 Callas, et al. Expires Oct 24, 2007 [Page 81]
4537 \fINTERNET-DRAFT OpenPGP Message Format Apr 24, 2007
4539 [FIPS186] Digital Signature Standard (DSS) (FIPS PUB 186-2).
4540 <http://csrc.nist.gov/publications/fips/fips186-2/
4541 fips186-2-change1.pdf>
4542 FIPS 186-3 describes keys greater than 1024 bits.
4543 The latest draft is at:
4544 <http://csrc.nist.gov/publications/drafts/
4545 fips_186-3/Draft-FIPS-186-3%20_March2006.pdf>
4547 [HAC] Alfred Menezes, Paul van Oorschot, and Scott
4548 Vanstone, "Handbook of Applied Cryptography," CRC
4549 Press, 1996.
4550 <http://www.cacr.math.uwaterloo.ca/hac/>
4552 [IDEA] Lai, X, "On the design and security of block
4553 ciphers", ETH Series in Information Processing,
4554 J.L. Massey (editor), Vol. 1, Hartung-Gorre Verlag
4555 Knostanz, Technische Hochschule (Zurich), 1992
4557 [ISO10646] ISO/IEC 10646-1:1993. International Standard --
4558 Information technology -- Universal Multiple-Octet
4559 Coded Character Set (UCS) -- Part 1: Architecture
4560 and Basic Multilingual Plane.
4562 [JFIF] JPEG File Interchange Format (Version 1.02).
4563 Eric Hamilton, C-Cube Microsystems, Milpitas, CA,
4564 September 1, 1992.
4566 [RFC1991] Atkins, D., Stallings, W. and P. Zimmermann, "PGP
4567 Message Exchange Formats", RFC 1991, August 1996.
4569 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
4570 Requirement Level", BCP 14, RFC 2119, March 1997.
4571 [RFC2045] Borenstein, N. and N. Freed, "Multipurpose Internet
4572 Mail Extensions (MIME) Part One: Format of Internet
4573 Message Bodies.", RFC 2045, November 1996.
4575 [RFC2144] Adams, C., "The CAST-128 Encryption Algorithm", RFC
4576 2144, May 1997.
4578 [RFC2434] Narten, T. and H. Alvestrand, "Guidelines for
4579 Writing an IANA Considerations Section in RFCs",
4580 BCP 26, RFC 2434, October 1998.
4581 [RFC2822] Resnick, P., "Internet Message Format", RFC 2822.
4583 [RFC3156] M. Elkins, D. Del Torto, R. Levien, T. Roessler,
4584 "MIME Security with OpenPGP", RFC 3156,
4585 August 2001.
4587 [RFC3447] B. Kaliski and J. Staddon, "PKCS #1: RSA
4588 Cryptography Specifications Version 2.1",
4589 RFC 3447, February 2003.
4592 Callas, et al. Expires Oct 24, 2007 [Page 82]
4593 \fINTERNET-DRAFT OpenPGP Message Format Apr 24, 2007
4595 [RFC3629] Yergeau., F., "UTF-8, a transformation format of
4596 Unicode and ISO 10646", RFC 3629, November 2003.
4598 [RFC4086] Eastlake, D., Crocker, S. and J. Schiller,
4599 "Randomness Recommendations for Security", RFC
4600 4086, June 2005.
4602 [SCHNEIER] Schneier, B., "Applied Cryptography Second Edition:
4603 protocols, algorithms, and source code in C", 1996.
4605 [TWOFISH] B. Schneier, J. Kelsey, D. Whiting, D. Wagner, C.
4606 Hall, and N. Ferguson, "The Twofish Encryption
4607 Algorithm", John Wiley & Sons, 1999.
4610 18. References (Informative)
4613 [BLEICHENBACHER] Bleichenbacher, Daniel, "Generating Elgamal
4614 signatures without knowing the secret key,"
4615 Eurocrypt 96. Note that the version in the
4616 proceedings has an error. A revised version is
4617 available at the time of writing from
4618 <ftp://ftp.inf.ethz.ch/pub/publications/papers/ti
4619 /isc/ElGamal.ps>
4621 [JKS02] Kahil Jallad, Jonathan Katz, Bruce Schneier
4622 "Implementation of Chosen-Ciphertext Attacks
4623 against PGP and GnuPG"
4624 http://www.counterpane.com/pgp-attack.html
4626 [MAURER] Ueli Maurer, "Modelling a Public-Key
4627 Infrastructure", Proc. 1996 European Symposium on
4628 Research in Computer Security (ESORICS' 96),
4629 Lecture Notes in Computer Science, Springer-Verlag,
4630 vol. 1146, pp. 325-350, Sep 1996.
4632 [MZ05] Serge Mister, Robert Zuccherato, "An Attack on
4633 CFB Mode Encryption As Used By OpenPGP," IACR
4634 ePrint Archive: Report 2005/033, 8 Feb 2005
4635 http://eprint.iacr.org/2005/033
4637 [RFC1423] Balenson, D., "Privacy Enhancement for Internet
4638 Electronic Mail: Part III: Algorithms, Modes, and
4639 Identifiers", RFC 1423, October 1993.
4641 [RFC1951] Deutsch, P., "DEFLATE Compressed Data Format
4642 Specification version 1.3.", RFC 1951, May 1996.
4644 [RFC2440] Callas, J., Donnerhacke, L., Finney, H., and
4645 Thayer, R. "OpenPGP Message Format", RFC 2440,
4646 November, 1998.
4648 Callas, et al. Expires Oct 24, 2007 [Page 83]
4649 \fINTERNET-DRAFT OpenPGP Message Format Apr 24, 2007
4651 [SP800-57] NIST Special Publication 800-57, Recommendation on
4652 Key Management
4653 <http://csrc.nist.gov/publications/nistpubs/
4654 800-57/SP800-57-Part1.pdf>
4655 <http://csrc.nist.gov/publications/nistpubs/
4656 800-57/SP800-57-Part2.pdf>
4659 19. Full Copyright Statement
4661 Copyright (C) 2007 by The IETF Trust.
4663 This document is subject to the rights, licenses and restrictions
4664 contained in BCP 78, and except as set forth therein, the authors
4665 retain all their rights.
4667 This document and the information contained herein are provided on
4668 an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
4669 REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE
4670 IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL
4671 WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY
4672 WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE
4673 ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS
4674 FOR A PARTICULAR PURPOSE.
4676 This document and translations of it may be copied and furnished to
4677 others, and derivative works that comment on or otherwise explain it
4678 or assist in its implementation may be prepared, copied, published
4679 and distributed, in whole or in part, without restriction of any
4680 kind, provided that the above copyright notice and this paragraph
4681 are included on all such copies and derivative works. However, this
4682 document itself may not be modified in any way, such as by removing
4683 the copyright notice or references to the Internet Society or other
4684 Internet organizations, except as needed for the purpose of
4685 developing Internet standards in which case the procedures for
4686 copyrights defined in the Internet Standards process must be
4687 followed, or as required to translate it into languages other than
4688 English.
4690 The limited permissions granted above are perpetual and will not be
4691 revoked by the Internet Society or its successors or assigns.
4693 20. Intellectual Property
4695 The IETF takes no position regarding the validity or scope of any
4696 Intellectual Property Rights or other rights that might be claimed
4697 to pertain to the implementation or use of the technology described
4698 in this document or the extent to which any license under such
4699 rights might or might not be available; nor does it represent that
4700 it has made any independent effort to identify any such rights.
4701 Information on the procedures with respect to rights in RFC
4702 documents can be found in BCP 78 and BCP 79.
4704 Callas, et al. Expires Oct 24, 2007 [Page 84]
4705 \fINTERNET-DRAFT OpenPGP Message Format Apr 24, 2007
4707 Copies of IPR disclosures made to the IETF Secretariat and any
4708 assurances of licenses to be made available, or the result of an
4709 attempt made to obtain a general license or permission for the use
4710 of such proprietary rights by implementers or users of this
4711 specification can be obtained from the IETF on-line IPR repository
4712 at http://www.ietf.org/ipr.
4714 The IETF invites any interested party to bring to its attention any
4715 copyrights, patents or patent applications, or other proprietary
4716 rights that may cover technology that may be required to implement
4717 this standard. Please address the information to the IETF at
4718 ietf-ipr@ietf.org.
4760 Callas, et al. Expires Oct 24, 2007 [Page 85]