[netbsd-mini2440.git] / crypto / external / bsd / netpgp / dist / ref / draft-ietf-openpgp-rfc2440bis-12.txt
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1 Network Working Group Jon Callas
2 Category: INTERNET-DRAFT PGP Corporation
3 draft-ietf-openpgp-rfc2440bis-12.txt
4 Expires May 2005 Lutz Donnerhacke
5 November 2004
7 Obsoletes: 1991, 2440 Hal Finney
8 Network Associates
10 Rodney Thayer
12 OpenPGP Message Format
13 draft-ietf-openpgp-rfc2440bis-12.txt
16 Copyright 2004 by The Internet Society. All Rights Reserved.
18 Status of this Memo
20 This document is an Internet-Draft and is in full conformance with
21 all provisions of Section 10 of RFC2026.
23 Internet-Drafts are working documents of the Internet Engineering
24 Task Force (IETF), its areas, and its working groups. Note that
25 other groups may also distribute working documents as
26 Internet-Drafts.
28 Internet-Drafts are draft documents valid for a maximum of six
29 months and may be updated, replaced, or obsoleted by other documents
30 at any time. It is inappropriate to use Internet-Drafts as
31 reference material or to cite them other than as "work in progress."
33 The list of current Internet-Drafts can be accessed at
34 http://www.ietf.org/ietf/1id-abstracts.txt
36 The list of Internet-Draft Shadow Directories can be accessed at
37 http://www.ietf.org/shadow.html.
39 IPR Claim Notice
41 By submitting this Internet-Draft, any applicable patent or other
42 IPR claims of which we are aware have been disclosed in accordance
43 with RFC 3668.
45 IESG Note
47 This document defines many tag values, yet it doesn't describe a
48 mechanism for adding new tags (for new features). Traditionally the
49 Internet Assigned Numbers Authority (IANA) handles the allocation of
50 new values for future expansion and RFCs usually define the
51 procedure to be used by the IANA. However there are subtle (and not
52 so subtle) interactions that may occur in this protocol between new
53 features and existing features which result in a significant
54 reduction in over all security. Therefore this document does not
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59 define an extension procedure. Instead requests to define new tag
60 values (say for new encryption algorithms for example) should be
61 forwarded to the IESG Security Area Directors for consideration or
62 forwarding to the appropriate IETF Working Group for consideration.
64 Abstract
66 This document is maintained in order to publish all necessary
67 information needed to develop interoperable applications based on
68 the OpenPGP format. It is not a step-by-step cookbook for writing an
69 application. It describes only the format and methods needed to
70 read, check, generate, and write conforming packets crossing any
71 network. It does not deal with storage and implementation questions.
72 It does, however, discuss implementation issues necessary to avoid
73 security flaws.
75 OpenPGP software uses a combination of strong public-key and
76 symmetric cryptography to provide security services for electronic
77 communications and data storage. These services include
78 confidentiality, key management, authentication, and digital
79 signatures. This document specifies the message formats used in
80 OpenPGP.
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115 Table of Contents
117 Status of this Memo 1
118 IPR Claim Notice 1
119 IESG Note 1
120 Abstract 2
121 Table of Contents 3
122 1. Introduction 6
123 1.1. Terms 6
124 2. General functions 6
125 2.1. Confidentiality via Encryption 7
126 2.2. Authentication via Digital signature 7
127 2.3. Compression 8
128 2.4. Conversion to Radix-64 8
129 2.5. Signature-Only Applications 8
130 3. Data Element Formats 9
131 3.1. Scalar numbers 9
132 3.2. Multiprecision Integers 9
133 3.3. Key IDs 9
134 3.4. Text 10
135 3.5. Time fields 10
136 3.6. Keyrings 10
137 3.7. String-to-key (S2K) specifiers 10
138 3.7.1. String-to-key (S2K) specifier types 10
139 3.7.1.1. Simple S2K 10
140 3.7.1.2. Salted S2K 11
141 3.7.1.3. Iterated and Salted S2K 11
142 3.7.2. String-to-key usage 12
143 3.7.2.1. Secret key encryption 12
144 3.7.2.2. Symmetric-key message encryption 13
145 4. Packet Syntax 13
146 4.1. Overview 13
147 4.2. Packet Headers 13
148 4.2.1. Old-Format Packet Lengths 14
149 4.2.2. New-Format Packet Lengths 14
150 4.2.2.1. One-Octet Lengths 15
151 4.2.2.2. Two-Octet Lengths 15
152 4.2.2.3. Five-Octet Lengths 15
153 4.2.2.4. Partial Body Lengths 15
154 4.2.3. Packet Length Examples 16
155 4.3. Packet Tags 16
156 5. Packet Types 17
157 5.1. Public-Key Encrypted Session Key Packets (Tag 1) 17
158 5.2. Signature Packet (Tag 2) 18
159 5.2.1. Signature Types 18
160 5.2.2. Version 3 Signature Packet Format 20
161 5.2.3. Version 4 Signature Packet Format 23
162 5.2.3.1. Signature Subpacket Specification 23
163 5.2.3.2. Signature Subpacket Types 25
164 5.2.3.3. Notes on Self-Signatures 25
165 5.2.3.4. Signature creation time 26
166 5.2.3.5. Issuer 26
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171 5.2.3.6. Key expiration time 27
172 5.2.3.7. Preferred symmetric algorithms 27
173 5.2.3.8. Preferred hash algorithms 27
174 5.2.3.9. Preferred compression algorithms 27
175 5.2.3.10.Signature expiration time 27
176 5.2.3.11.Exportable Certification 28
177 5.2.3.12.Revocable 28
178 5.2.3.13.Trust signature 28
179 5.2.3.14.Regular expression 29
180 5.2.3.15.Revocation key 29
181 5.2.3.16.Notation Data 29
182 5.2.3.17.Key server preferences 30
183 5.2.3.18.Preferred key server 30
184 5.2.3.19.Primary User ID 31
185 5.2.3.20.Policy URL 31
186 5.2.3.21.Key Flags 31
187 5.2.3.22.Signer's User ID 32
188 5.2.3.23.Reason for Revocation 32
189 5.2.3.24.Features 33
190 5.2.3.25.Signature Target 34
191 5.2.3.26.Embedded Signature 34
192 5.2.4. Computing Signatures 34
193 5.2.4.1. Subpacket Hints 35
194 5.3. Symmetric-Key Encrypted Session Key Packets (Tag 3) 36
195 5.4. One-Pass Signature Packets (Tag 4) 36
196 5.5. Key Material Packet 37
197 5.5.1. Key Packet Variants 37
198 5.5.1.1. Public Key Packet (Tag 6) 37
199 5.5.1.2. Public Subkey Packet (Tag 14) 37
200 5.5.1.3. Secret Key Packet (Tag 5) 38
201 5.5.1.4. Secret Subkey Packet (Tag 7) 38
202 5.5.2. Public Key Packet Formats 38
203 5.5.3. Secret Key Packet Formats 39
204 5.6. Compressed Data Packet (Tag 8) 41
205 5.7. Symmetrically Encrypted Data Packet (Tag 9) 42
206 5.8. Marker Packet (Obsolete Literal Packet) (Tag 10) 43
207 5.9. Literal Data Packet (Tag 11) 43
208 5.10. Trust Packet (Tag 12) 44
209 5.11. User ID Packet (Tag 13) 44
210 5.12. User Attribute Packet (Tag 17) 44
211 5.12.1. The Image Attribute Subpacket 45
212 5.13. Sym. Encrypted Integrity Protected Data Packet (Tag 18) 45
213 5.14. Modification Detection Code Packet (Tag 19) 47
214 6. Radix-64 Conversions 48
215 6.1. An Implementation of the CRC-24 in "C" 48
216 6.2. Forming ASCII Armor 49
217 6.3. Encoding Binary in Radix-64 51
218 6.4. Decoding Radix-64 52
219 6.5. Examples of Radix-64 53
220 6.6. Example of an ASCII Armored Message 53
221 7. Cleartext signature framework 53
222 7.1. Dash-Escaped Text 54
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227 8. Regular Expressions 55
228 9. Constants 55
229 9.1. Public Key Algorithms 55
230 9.2. Symmetric Key Algorithms 56
231 9.3. Compression Algorithms 56
232 9.4. Hash Algorithms 57
233 10. Packet Composition 57
234 10.1. Transferable Public Keys 57
235 10.2. OpenPGP Messages 59
236 10.3. Detached Signatures 59
237 11. Enhanced Key Formats 59
238 11.1. Key Structures 59
239 11.2. Key IDs and Fingerprints 60
240 12. Notes on Algorithms 61
241 12.1. Symmetric Algorithm Preferences 61
242 12.2. Other Algorithm Preferences 62
243 12.2.1. Compression Preferences 62
244 12.2.2. Hash Algorithm Preferences 63
245 12.3. Plaintext 63
246 12.4. RSA 63
247 12.5. DSA 63
248 12.6. Elgamal 63
249 12.7. Reserved Algorithm Numbers 64
250 12.8. OpenPGP CFB mode 64
251 13. Security Considerations 65
252 14. Implementation Nits 67
253 15. Authors and Working Group Chair 68
254 16. References (Normative) 69
255 17. References (Non-Normative) 71
256 18. Full Copyright Statement 71
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283 1. Introduction
285 This document provides information on the message-exchange packet
286 formats used by OpenPGP to provide encryption, decryption, signing,
287 and key management functions. It is a revision of RFC2440, "OpenPGP
288 Message Format", which itself replaces RFC 1991, "PGP Message
289 Exchange Formats."
291 1.1. Terms
293 * OpenPGP - This is a definition for security software that uses
294 PGP 5.x as a basis, formalized in RFC 2440 and this document.
296 * PGP - Pretty Good Privacy. PGP is a family of software systems
297 developed by Philip R. Zimmermann from which OpenPGP is based.
299 * PGP 2.6.x - This version of PGP has many variants, hence the
300 term PGP 2.6.x. It used only RSA, MD5, and IDEA for its
301 cryptographic transforms. An informational RFC, RFC1991, was
302 written describing this version of PGP.
304 * PGP 5.x - This version of PGP is formerly known as "PGP 3" in
305 the community and also in the predecessor of this document,
306 RFC1991. It has new formats and corrects a number of problems in
307 the PGP 2.6.x design. It is referred to here as PGP 5.x because
308 that software was the first release of the "PGP 3" code base.
310 * GPG - GNU Privacy Guard, also called GnuPG. GPG is an OpenPGP
311 implementation that avoids all encumbered algorithms.
312 Consequently, early versions of GPG did not include RSA public
313 keys. GPG may or may not have (depending on version) support for
314 IDEA or other encumbered algorithms.
316 "PGP", "Pretty Good", and "Pretty Good Privacy" are trademarks of
317 PGP Corporation and are used with permission.
319 This document uses the terms "MUST", "SHOULD", and "MAY" as defined
320 in RFC2119, along with the negated forms of those terms.
322 2. General functions
324 OpenPGP provides data integrity services for messages and data files
325 by using these core technologies:
327 - digital signatures
329 - encryption
331 - compression
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339 - radix-64 conversion
341 In addition, OpenPGP provides key management and certificate
342 services, but many of these are beyond the scope of this document.
344 2.1. Confidentiality via Encryption
346 OpenPGP combines symmetric-key encryption and public key encryption
347 to provide confidentiality. When made confidential, first the object
348 is encrypted using a symmetric encryption algorithm. Each symmetric
349 key is used only once, for a single object. A new "session key" is
350 generated as a random number for each object (sometimes referred to
351 as a session). Since it is used only once, the session key is bound
352 to the message and transmitted with it. To protect the key, it is
353 encrypted with the receiver's public key. The sequence is as
354 follows:
356 1. The sender creates a message.
358 2. The sending OpenPGP generates a random number to be used as a
359 session key for this message only.
361 3. The session key is encrypted using each recipient's public key.
362 These "encrypted session keys" start the message.
364 4. The sending OpenPGP encrypts the message using the session key,
365 which forms the remainder of the message. Note that the message
366 is also usually compressed.
368 5. The receiving OpenPGP decrypts the session key using the
369 recipient's private key.
371 6. The receiving OpenPGP decrypts the message using the session
372 key. If the message was compressed, it will be decompressed.
374 With symmetric-key encryption, an object may be encrypted with a
375 symmetric key derived from a passphrase (or other shared secret), or
376 a two-stage mechanism similar to the public-key method described
377 above in which a session key is itself encrypted with a symmetric
378 algorithm keyed from a shared secret.
380 Both digital signature and confidentiality services may be applied
381 to the same message. First, a signature is generated for the message
382 and attached to the message. Then, the message plus signature is
383 encrypted using a symmetric session key. Finally, the session key is
384 encrypted using public-key encryption and prefixed to the encrypted
385 block.
387 2.2. Authentication via Digital signature
389 The digital signature uses a hash code or message digest algorithm,
390 and a public-key signature algorithm. The sequence is as follows:
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395 1. The sender creates a message.
397 2. The sending software generates a hash code of the message.
399 3. The sending software generates a signature from the hash code
400 using the sender's private key.
402 4. The binary signature is attached to the message.
404 5. The receiving software keeps a copy of the message signature.
406 6. The receiving software generates a new hash code for the
407 received message and verifies it using the message's signature.
408 If the verification is successful, the message is accepted as
409 authentic.
411 2.3. Compression
413 OpenPGP implementations SHOULD compress the message after applying
414 the signature but before encryption.
416 If an implementation does not implement compression, its authors
417 should be aware that most PGP messages in the world are compressed.
418 Thus, it may even be wise for a space-constrained implementation to
419 implement decompression, but not compression.
421 Furthermore, compression has the added side-effect that some types
422 of attacks can be thwarted by the fact that slightly altered,
423 compressed data rarely uncompresses without severe errors. This is
424 hardly rigorous, but it is operationally useful. These attacks can
425 be rigorously prevented by implementing and using Modification
426 Detection Codes as described in sections following.
428 2.4. Conversion to Radix-64
430 OpenPGP's underlying native representation for encrypted messages,
431 signature certificates, and keys is a stream of arbitrary octets.
432 Some systems only permit the use of blocks consisting of seven-bit,
433 printable text. For transporting OpenPGP's native raw binary octets
434 through channels that are not safe to raw binary data, a printable
435 encoding of these binary octets is needed. OpenPGP provides the
436 service of converting the raw 8-bit binary octet stream to a stream
437 of printable ASCII characters, called Radix-64 encoding or ASCII
438 Armor.
440 Implementations SHOULD provide Radix-64 conversions.
442 2.5. Signature-Only Applications
444 OpenPGP is designed for applications that use both encryption and
445 signatures, but there are a number of problems that are solved by a
446 signature-only implementation. Although this specification requires
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451 both encryption and signatures, it is reasonable for there to be
452 subset implementations that are non-conformant only in that they
453 omit encryption.
455 3. Data Element Formats
457 This section describes the data elements used by OpenPGP.
459 3.1. Scalar numbers
461 Scalar numbers are unsigned, and are always stored in big-endian
462 format. Using n[k] to refer to the kth octet being interpreted, the
463 value of a two-octet scalar is ((n[0] << 8) + n[1]). The value of a
464 four-octet scalar is ((n[0] << 24) + (n[1] << 16) + (n[2] << 8) +
465 n[3]).
467 3.2. Multiprecision Integers
469 Multiprecision Integers (also called MPIs) are unsigned integers
470 used to hold large integers such as the ones used in cryptographic
471 calculations.
473 An MPI consists of two pieces: a two-octet scalar that is the length
474 of the MPI in bits followed by a string of octets that contain the
475 actual integer.
477 These octets form a big-endian number; a big-endian number can be
478 made into an MPI by prefixing it with the appropriate length.
480 Examples:
482 (all numbers are in hexadecimal)
484 The string of octets [00 01 01] forms an MPI with the value 1. The
485 string [00 09 01 FF] forms an MPI with the value of 511.
487 Additional rules:
489 The size of an MPI is ((MPI.length + 7) / 8) + 2 octets.
491 The length field of an MPI describes the length starting from its
492 most significant non-zero bit. Thus, the MPI [00 02 01] is not
493 formed correctly. It should be [00 01 01].
495 Also note that when an MPI is encrypted, the length refers to the
496 plaintext MPI. It may be ill-formed in its ciphertext.
498 3.3. Key IDs
500 A Key ID is an eight-octet scalar that identifies a key.
501 Implementations SHOULD NOT assume that Key IDs are unique. The
502 section, "Enhanced Key Formats" below describes how Key IDs are
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507 formed.
509 3.4. Text
511 Unless otherwise specified, the character set for text is the UTF-8
512 [RFC2279] encoding of Unicode [ISO10646].
514 3.5. Time fields
516 A time field is an unsigned four-octet number containing the number
517 of seconds elapsed since midnight, 1 January 1970 UTC.
519 3.6. Keyrings
521 A keyring is a collection of one or more keys in a file or database.
522 Traditionally, a keyring is simply a sequential list of keys, but
523 may be any suitable database. It is beyond the scope of this
524 standard to discuss the details of keyrings or other databases.
526 3.7. String-to-key (S2K) specifiers
528 String-to-key (S2K) specifiers are used to convert passphrase
529 strings into symmetric-key encryption/decryption keys. They are
530 used in two places, currently: to encrypt the secret part of private
531 keys in the private keyring, and to convert passphrases to
532 encryption keys for symmetrically encrypted messages.
534 3.7.1. String-to-key (S2K) specifier types
536 There are three types of S2K specifiers currently supported, and
537 some reserved values:
539 ID S2K Type
540 -- --- ----
541 0 Simple S2K
542 1 Salted S2K
543 2 Illegal value
544 3 Iterated and Salted S2K
545 100 to 110 Private/Experimental S2K
547 These are described as follows:
549 3.7.1.1. Simple S2K
551 This directly hashes the string to produce the key data. See below
552 for how this hashing is done.
554 Octet 0: 0x00
555 Octet 1: hash algorithm
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563 Simple S2K hashes the passphrase to produce the session key. The
564 manner in which this is done depends on the size of the session key
565 (which will depend on the cipher used) and the size of the hash
566 algorithm's output. If the hash size is greater than the session key
567 size, the high-order (leftmost) octets of the hash are used as the
568 key.
570 If the hash size is less than the key size, multiple instances of
571 the hash context are created -- enough to produce the required key
572 data. These instances are preloaded with 0, 1, 2, ... octets of
573 zeros (that is to say, the first instance has no preloading, the
574 second gets preloaded with 1 octet of zero, the third is preloaded
575 with two octets of zeros, and so forth).
577 As the data is hashed, it is given independently to each hash
578 context. Since the contexts have been initialized differently, they
579 will each produce different hash output. Once the passphrase is
580 hashed, the output data from the multiple hashes is concatenated,
581 first hash leftmost, to produce the key data, with any excess octets
582 on the right discarded.
584 3.7.1.2. Salted S2K
586 This includes a "salt" value in the S2K specifier -- some arbitrary
587 data -- that gets hashed along with the passphrase string, to help
588 prevent dictionary attacks.
590 Octet 0: 0x01
591 Octet 1: hash algorithm
592 Octets 2-9: 8-octet salt value
594 Salted S2K is exactly like Simple S2K, except that the input to the
595 hash function(s) consists of the 8 octets of salt from the S2K
596 specifier, followed by the passphrase.
598 3.7.1.3. Iterated and Salted S2K
600 This includes both a salt and an octet count. The salt is combined
601 with the passphrase and the resulting value is hashed repeatedly.
602 This further increases the amount of work an attacker must do to try
603 dictionary attacks.
605 Octet 0: 0x03
606 Octet 1: hash algorithm
607 Octets 2-9: 8-octet salt value
608 Octet 10: count, a one-octet, coded value
610 The count is coded into a one-octet number using the following
611 formula:
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619 #define EXPBIAS 6
620 count = ((Int32)16 + (c & 15)) << ((c >> 4) + EXPBIAS);
622 The above formula is in C, where "Int32" is a type for a 32-bit
623 integer, and the variable "c" is the coded count, Octet 10.
625 Iterated-Salted S2K hashes the passphrase and salt data multiple
626 times. The total number of octets to be hashed is specified in the
627 encoded count in the S2K specifier. Note that the resulting count
628 value is an octet count of how many octets will be hashed, not an
629 iteration count.
631 Initially, one or more hash contexts are set up as with the other
632 S2K algorithms, depending on how many octets of key data are needed.
633 Then the salt, followed by the passphrase data is repeatedly hashed
634 until the number of octets specified by the octet count has been
635 hashed. The one exception is that if the octet count is less than
636 the size of the salt plus passphrase, the full salt plus passphrase
637 will be hashed even though that is greater than the octet count.
638 After the hashing is done the data is unloaded from the hash
639 context(s) as with the other S2K algorithms.
641 3.7.2. String-to-key usage
643 Implementations SHOULD use salted or iterated-and-salted S2K
644 specifiers, as simple S2K specifiers are more vulnerable to
645 dictionary attacks.
647 3.7.2.1. Secret key encryption
649 An S2K specifier can be stored in the secret keyring to specify how
650 to convert the passphrase to a key that unlocks the secret data.
651 Older versions of PGP just stored a cipher algorithm octet preceding
652 the secret data or a zero to indicate that the secret data was
653 unencrypted. The MD5 hash function was always used to convert the
654 passphrase to a key for the specified cipher algorithm.
656 For compatibility, when an S2K specifier is used, the special value
657 255 is stored in the position where the hash algorithm octet would
658 have been in the old data structure. This is then followed
659 immediately by a one-octet algorithm identifier, and then by the S2K
660 specifier as encoded above.
662 Therefore, preceding the secret data there will be one of these
663 possibilities:
665 0: secret data is unencrypted (no pass phrase)
666 255 or 254: followed by algorithm octet and S2K specifier
667 Cipher alg: use Simple S2K algorithm using MD5 hash
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675 This last possibility, the cipher algorithm number with an implicit
676 use of MD5 and IDEA, is provided for backward compatibility; it MAY
677 be understood, but SHOULD NOT be generated, and is deprecated.
679 These are followed by an Initial Vector of the same length as the
680 block size of the cipher for the decryption of the secret values, if
681 they are encrypted, and then the secret key values themselves.
683 3.7.2.2. Symmetric-key message encryption
685 OpenPGP can create a Symmetric-key Encrypted Session Key (ESK)
686 packet at the front of a message. This is used to allow S2K
687 specifiers to be used for the passphrase conversion or to create
688 messages with a mix of symmetric-key ESKs and public-key ESKs. This
689 allows a message to be decrypted either with a passphrase or a
690 public key pair.
692 PGP 2.X always used IDEA with Simple string-to-key conversion when
693 encrypting a message with a symmetric algorithm. This is deprecated,
694 but MAY be used for backward-compatibility.
696 4. Packet Syntax
698 This section describes the packets used by OpenPGP.
700 4.1. Overview
702 An OpenPGP message is constructed from a number of records that are
703 traditionally called packets. A packet is a chunk of data that has a
704 tag specifying its meaning. An OpenPGP message, keyring,
705 certificate, and so forth consists of a number of packets. Some of
706 those packets may contain other OpenPGP packets (for example, a
707 compressed data packet, when uncompressed, contains OpenPGP
708 packets).
710 Each packet consists of a packet header, followed by the packet
711 body. The packet header is of variable length.
713 4.2. Packet Headers
715 The first octet of the packet header is called the "Packet Tag." It
716 determines the format of the header and denotes the packet contents.
717 The remainder of the packet header is the length of the packet.
719 Note that the most significant bit is the left-most bit, called bit
720 7. A mask for this bit is 0x80 in hexadecimal.
722 +---------------+
723 PTag |7 6 5 4 3 2 1 0|
724 +---------------+
725 Bit 7 -- Always one
726 Bit 6 -- New packet format if set
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731 PGP 2.6.x only uses old format packets. Thus, software that
732 interoperates with those versions of PGP must only use old format
733 packets. If interoperability is not an issue, the new packet format
734 is preferred. Note that old format packets have four bits of content
735 tags, and new format packets have six; some features cannot be used
736 and still be backward-compatible.
738 Also note that packets with a tag greater than or equal to 16 MUST
739 use new format packets. The old format packets can only express tags
740 less than or equal to 15.
742 Old format packets contain:
744 Bits 5-2 -- content tag
745 Bits 1-0 - length-type
747 New format packets contain:
749 Bits 5-0 -- content tag
751 4.2.1. Old-Format Packet Lengths
753 The meaning of the length-type in old-format packets is:
755 0 - The packet has a one-octet length. The header is 2 octets long.
757 1 - The packet has a two-octet length. The header is 3 octets long.
759 2 - The packet has a four-octet length. The header is 5 octets long.
761 3 - The packet is of indeterminate length. The header is 1 octet
762 long, and the implementation must determine how long the packet
763 is. If the packet is in a file, this means that the packet
764 extends until the end of the file. In general, an implementation
765 SHOULD NOT use indeterminate length packets except where the end
766 of the data will be clear from the context, and even then it is
767 better to use a definite length, or a new-format header. The
768 new-format headers described below have a mechanism for
769 precisely encoding data of indeterminate length.
771 4.2.2. New-Format Packet Lengths
773 New format packets have four possible ways of encoding length:
775 1. A one-octet Body Length header encodes packet lengths of up to
776 191 octets.
778 2. A two-octet Body Length header encodes packet lengths of 192 to
779 8383 octets.
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785 INTERNET-DRAFT OpenPGP Message Format Nov 23, 2004
787 3. A five-octet Body Length header encodes packet lengths of up to
788 4,294,967,295 (0xFFFFFFFF) octets in length. (This actually
789 encodes a four-octet scalar number.)
791 4. When the length of the packet body is not known in advance by
792 the issuer, Partial Body Length headers encode a packet of
793 indeterminate length, effectively making it a stream.
795 4.2.2.1. One-Octet Lengths
797 A one-octet Body Length header encodes a length of from 0 to 191
798 octets. This type of length header is recognized because the one
799 octet value is less than 192. The body length is equal to:
801 bodyLen = 1st_octet;
803 4.2.2.2. Two-Octet Lengths
805 A two-octet Body Length header encodes a length of from 192 to 8383
806 octets. It is recognized because its first octet is in the range
807 192 to 223. The body length is equal to:
809 bodyLen = ((1st_octet - 192) << 8) + (2nd_octet) + 192
811 4.2.2.3. Five-Octet Lengths
813 A five-octet Body Length header consists of a single octet holding
814 the value 255, followed by a four-octet scalar. The body length is
815 equal to:
817 bodyLen = (2nd_octet << 24) | (3rd_octet << 16) |
818 (4th_octet << 8) | 5th_octet
820 This basic set of one, two, and five-octet lengths is also used
821 internally to some packets.
823 4.2.2.4. Partial Body Lengths
825 A Partial Body Length header is one octet long and encodes the
826 length of only part of the data packet. This length is a power of 2,
827 from 1 to 1,073,741,824 (2 to the 30th power). It is recognized by
828 its one octet value that is greater than or equal to 224, and less
829 than 255. The partial body length is equal to:
831 partialBodyLen = 1 << (1st_octet & 0x1f);
833 Each Partial Body Length header is followed by a portion of the
834 packet body data. The Partial Body Length header specifies this
835 portion's length. Another length header (one octet, two-octet,
836 five-octet, or partial) follows that portion. The last length header
837 in the packet MUST NOT be a partial Body Length header. Partial
838 Body Length headers may only be used for the non-final parts of the
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841 INTERNET-DRAFT OpenPGP Message Format Nov 23, 2004
843 packet.
845 It might also be encoded in the following octet stream: 0xEF, first
846 32768 octets of data; 0xE1, next two octets of data; 0xE0, next one
847 octet of data; 0xF0, next 65536 octets of data; 0xC5, 0xDD, last
848 1693 octets of data. This is just one possible encoding, and many
849 variations are possible on the size of the Partial Body Length
850 headers, as long as a regular Body Length header encodes the last
851 portion of the data.
853 Note also that the last Body Length header can be a zero-length
854 header.
856 An implementation MAY use Partial Body Lengths for data packets, be
857 they literal, compressed, or encrypted. The first partial length
858 MUST be at least 512 octets long. Partial Body Lengths MUST NOT be
859 used for any other packet types.
861 4.2.3. Packet Length Examples
863 These examples show ways that new-format packets might encode the
864 packet lengths.
866 A packet with length 100 may have its length encoded in one octet:
867 0x64. This is followed by 100 octets of data.
869 A packet with length 1723 may have its length coded in two octets:
870 0xC5, 0xFB. This header is followed by the 1723 octets of data.
872 A packet with length 100000 may have its length encoded in five
873 octets: 0xFF, 0x00, 0x01, 0x86, 0xA0.
875 Please note that in all of these explanations, the total length of
876 the packet is the length of the header(s) plus the length of the
877 body.
879 4.3. Packet Tags
881 The packet tag denotes what type of packet the body holds. Note that
882 old format headers can only have tags less than 16, whereas new
883 format headers can have tags as great as 63. The defined tags (in
884 decimal) are:
886 0 -- Reserved - a packet tag must not have this value
887 1 -- Public-Key Encrypted Session Key Packet
888 2 -- Signature Packet
889 3 -- Symmetric-Key Encrypted Session Key Packet
890 4 -- One-Pass Signature Packet
891 5 -- Secret Key Packet
892 6 -- Public Key Packet
893 7 -- Secret Subkey Packet
894 8 -- Compressed Data Packet
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899 9 -- Symmetrically Encrypted Data Packet
900 10 -- Marker Packet
901 11 -- Literal Data Packet
902 12 -- Trust Packet
903 13 -- User ID Packet
904 14 -- Public Subkey Packet
905 17 -- User Attribute Packet
906 18 -- Sym. Encrypted and Integrity Protected Data Packet
907 19 -- Modification Detection Code Packet
908 60 to 63 -- Private or Experimental Values
910 5. Packet Types
912 5.1. Public-Key Encrypted Session Key Packets (Tag 1)
914 A Public-Key Encrypted Session Key packet holds the session key used
915 to encrypt a message. Zero or more Encrypted Session Key packets
916 (either Public-Key or Symmetric-Key) may precede a Symmetrically
917 Encrypted Data Packet, which holds an encrypted message. The
918 message is encrypted with the session key, and the session key is
919 itself encrypted and stored in the Encrypted Session Key packet(s).
920 The Symmetrically Encrypted Data Packet is preceded by one
921 Public-Key Encrypted Session Key packet for each OpenPGP key to
922 which the message is encrypted. The recipient of the message finds
923 a session key that is encrypted to their public key, decrypts the
924 session key, and then uses the session key to decrypt the message.
926 The body of this packet consists of:
928 - A one-octet number giving the version number of the packet type.
929 The currently defined value for packet version is 3.
931 - An eight-octet number that gives the key ID of the public key
932 that the session key is encrypted to. If the session key is
933 encrypted to a subkey then the key ID of this subkey is used
934 here instead of the key ID of the primary key.
936 - A one-octet number giving the public key algorithm used.
938 - A string of octets that is the encrypted session key. This
939 string takes up the remainder of the packet, and its contents
940 are dependent on the public key algorithm used.
942 Algorithm Specific Fields for RSA encryption
944 - multiprecision integer (MPI) of RSA encrypted value m**e mod n.
946 Algorithm Specific Fields for Elgamal encryption:
948 - MPI of Elgamal (Diffie-Hellman) value g**k mod p.
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955 - MPI of Elgamal (Diffie-Hellman) value m * y**k mod p.
957 The value "m" in the above formulas is derived from the session key
958 as follows. First the session key is prefixed with a one-octet
959 algorithm identifier that specifies the symmetric encryption
960 algorithm used to encrypt the following Symmetrically Encrypted Data
961 Packet. Then a two-octet checksum is appended which is equal to the
962 sum of the preceding session key octets, not including the algorithm
963 identifier, modulo 65536. This value is then encoded as described
964 in PKCS-1 block encoding EME-PKCS1-v1_5 [RFC2437] to form the "m"
965 value used in the formulas above.
967 Note that when an implementation forms several PKESKs with one
968 session key, forming a message that can be decrypted by several
969 keys, the implementation MUST make new PKCS-1 encoding for each key.
971 An implementation MAY accept or use a Key ID of zero as a "wild
972 card" or "speculative" Key ID. In this case, the receiving
973 implementation would try all available private keys, checking for a
974 valid decrypted session key. This format helps reduce traffic
975 analysis of messages.
977 5.2. Signature Packet (Tag 2)
979 A signature packet describes a binding between some public key and
980 some data. The most common signatures are a signature of a file or a
981 block of text, and a signature that is a certification of a User ID.
983 Two versions of signature packets are defined. Version 3 provides
984 basic signature information, while version 4 provides an expandable
985 format with subpackets that can specify more information about the
986 signature. PGP 2.6.x only accepts version 3 signatures.
988 Implementations SHOULD accept V3 signatures. Implementations SHOULD
989 generate V4 signatures.
991 Note that if an implementation is creating an encrypted and signed
992 message that is encrypted to a V3 key, it is reasonable to create a
993 V3 signature.
995 5.2.1. Signature Types
997 There are a number of possible meanings for a signature, which are
998 specified in a signature type octet in any given signature. These
999 meanings are:
1001 0x00: Signature of a binary document.
1002 This means the signer owns it, created it, or certifies that it
1003 has not been modified.
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1011 0x01: Signature of a canonical text document.
1012 This means the signer owns it, created it, or certifies that it
1013 has not been modified. The signature is calculated over the
1014 text data with its line endings converted to <CR><LF>.
1016 0x02: Standalone signature.
1017 This signature is a signature of only its own subpacket
1018 contents. It is calculated identically to a signature over a
1019 zero-length binary document. Note that it doesn't make sense to
1020 have a V3 standalone signature.
1022 0x10: Generic certification of a User ID and Public Key packet.
1023 The issuer of this certification does not make any particular
1024 assertion as to how well the certifier has checked that the
1025 owner of the key is in fact the person described by the User ID.
1026 Note that all PGP "key signatures" are this type of
1027 certification.
1029 0x11: Persona certification of a User ID and Public Key packet.
1030 The issuer of this certification has not done any verification
1031 of the claim that the owner of this key is the User ID
1032 specified.
1034 0x12: Casual certification of a User ID and Public Key packet.
1035 The issuer of this certification has done some casual
1036 verification of the claim of identity.
1038 0x13: Positive certification of a User ID and Public Key packet.
1039 The issuer of this certification has done substantial
1040 verification of the claim of identity.
1042 Please note that the vagueness of these certification claims is
1043 not a flaw, but a feature of the system. Because PGP places
1044 final authority for validity upon the receiver of a
1045 certification, it may be that one authority's casual
1046 certification might be more rigorous than some other authority's
1047 positive certification. These classifications allow a
1048 certification authority to issue fine-grained claims.
1050 0x18: Subkey Binding Signature
1051 This signature is a statement by the top-level signing key that
1052 indicates that it owns the subkey. This signature is calculated
1053 directly on the subkey itself, not on any User ID or other
1054 packets. A signature that binds a signing subkey also has an
1055 embedded signature subpacket in this binding signature which
1056 contains a 0x19 signature made by the signing subkey on the
1057 primary key.
1059 0x19 Primary Key Binding Signature
1060 This signature is a statement by a signing subkey, indicating
1061 that it is owned by the primary key. This signature is
1062 calculated directly on the primary key itself, and not on any
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1067 User ID or other packets.
1069 0x1F: Signature directly on a key
1070 This signature is calculated directly on a key. It binds the
1071 information in the signature subpackets to the key, and is
1072 appropriate to be used for subpackets that provide information
1073 about the key, such as the revocation key subpacket. It is also
1074 appropriate for statements that non-self certifiers want to make
1075 about the key itself, rather than the binding between a key and
1076 a name.
1078 0x20: Key revocation signature
1079 The signature is calculated directly on the key being revoked.
1080 A revoked key is not to be used. Only revocation signatures by
1081 the key being revoked, or by an authorized revocation key,
1082 should be considered valid revocation signatures.
1084 0x28: Subkey revocation signature
1085 The signature is calculated directly on the subkey being
1086 revoked. A revoked subkey is not to be used. Only revocation
1087 signatures by the top-level signature key that is bound to this
1088 subkey, or by an authorized revocation key, should be considered
1089 valid revocation signatures.
1091 0x30: Certification revocation signature
1092 This signature revokes an earlier User ID certification
1093 signature (signature class 0x10 through 0x13) or direct-key
1094 signature (0x1F). It should be issued by the same key that
1095 issued the revoked signature or an authorized revocation key.
1096 The signature should have a later creation date than the
1097 signature it revokes.
1099 0x40: Timestamp signature.
1100 This signature is only meaningful for the timestamp contained in
1101 it.
1103 0x50: Third-Party Confirmation signature.
1104 This signature is a signature over some other OpenPGP signature
1105 packet(s). It is analogous to a notary seal on the signed data.
1106 A third-party signature SHOULD include Signature Target
1107 subpacket(s) to give easy identification. Note that we really do
1108 mean SHOULD. There are plausible uses for this (such as a blind
1109 party that only sees the signature, not the key nor source
1110 document) that cannot include a target subpacket.
1112 5.2.2. Version 3 Signature Packet Format
1114 The body of a version 3 Signature Packet contains:
1116 - One-octet version number (3).
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1123 - One-octet length of following hashed material. MUST be 5.
1125 - One-octet signature type.
1127 - Four-octet creation time.
1129 - Eight-octet key ID of signer.
1131 - One-octet public key algorithm.
1133 - One-octet hash algorithm.
1135 - Two-octet field holding left 16 bits of signed hash value.
1137 - One or more multiprecision integers comprising the signature.
1138 This portion is algorithm specific, as described below.
1140 The data being signed is hashed, and then the signature type and
1141 creation time from the signature packet are hashed (5 additional
1142 octets). The resulting hash value is used in the signature
1143 algorithm. The high 16 bits (first two octets) of the hash are
1144 included in the signature packet to provide a quick test to reject
1145 some invalid signatures.
1147 Algorithm Specific Fields for RSA signatures:
1149 - multiprecision integer (MPI) of RSA signature value m**d mod n.
1151 Algorithm Specific Fields for DSA signatures:
1153 - MPI of DSA value r.
1155 - MPI of DSA value s.
1157 The signature calculation is based on a hash of the signed data, as
1158 described above. The details of the calculation are different for
1159 DSA signature than for RSA signatures.
1161 The hash h is PKCS-1 padded exactly the same way as for the above
1162 described RSA signatures.
1164 With RSA signatures, the hash value is encoded as described in
1165 PKCS-1 section 9.2.1 encoded using PKCS-1 encoding type
1166 EMSA-PKCS1-v1_5 [RFC2437]. This requires inserting the hash value
1167 as an octet string into an ASN.1 structure. The object identifier
1168 for the type of hash being used is included in the structure. The
1169 hexadecimal representations for the currently defined hash
1170 algorithms are:
1172 - MD5: 0x2A, 0x86, 0x48, 0x86, 0xF7, 0x0D, 0x02, 0x05
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1177 INTERNET-DRAFT OpenPGP Message Format Nov 23, 2004
1179 - RIPEMD-160: 0x2B, 0x24, 0x03, 0x02, 0x01
1181 - SHA-1: 0x2B, 0x0E, 0x03, 0x02, 0x1A
1183 - SHA256: 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x01
1185 - SHA384: 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x02
1187 - SHA512: 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x03
1189 The ASN.1 OIDs are:
1191 - MD5: 1.2.840.113549.2.5
1193 - RIPEMD-160: 1.3.36.3.2.1
1195 - SHA-1: 1.3.14.3.2.26
1197 - SHA256: 2.16.840.1.101.3.4.2.1
1199 - SHA384: 2.16.840.1.101.3.4.2.2
1201 - SHA512: 2.16.840.1.101.3.4.2.3
1203 The full hash prefixes for these are:
1205 MD5: 0x30, 0x20, 0x30, 0x0C, 0x06, 0x08, 0x2A, 0x86,
1206 0x48, 0x86, 0xF7, 0x0D, 0x02, 0x05, 0x05, 0x00,
1207 0x04, 0x10
1209 RIPEMD-160: 0x30, 0x21, 0x30, 0x09, 0x06, 0x05, 0x2B, 0x24,
1210 0x03, 0x02, 0x01, 0x05, 0x00, 0x04, 0x14
1212 SHA-1: 0x30, 0x21, 0x30, 0x09, 0x06, 0x05, 0x2b, 0x0E,
1213 0x03, 0x02, 0x1A, 0x05, 0x00, 0x04, 0x14
1215 SHA256: 0x30, 0x31, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86,
1216 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x01, 0x05,
1217 0x00, 0x04, 0x20
1219 SHA384: 0x30, 0x41, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86,
1220 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x02, 0x05,
1221 0x00, 0x04, 0x30
1223 SHA512: 0x30, 0x51, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86,
1224 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x03, 0x05,
1225 0x00, 0x04, 0x40
1227 DSA signatures MUST use hashes with a size of 160 bits, to match q,
1228 the size of the group generated by the DSA key's generator value.
1229 The hash function result is treated as a 160 bit number and used
1230 directly in the DSA signature algorithm.
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1235 5.2.3. Version 4 Signature Packet Format
1237 The body of a version 4 Signature Packet contains:
1239 - One-octet version number (4).
1241 - One-octet signature type.
1243 - One-octet public key algorithm.
1245 - One-octet hash algorithm.
1247 - Two-octet scalar octet count for following hashed subpacket
1248 data. Note that this is the length in octets of all of the
1249 hashed subpackets; a pointer incremented by this number will
1250 skip over the hashed subpackets.
1252 - Hashed subpacket data. (zero or more subpackets)
1254 - Two-octet scalar octet count for following unhashed subpacket
1255 data. Note that this is the length in octets of all of the
1256 unhashed subpackets; a pointer incremented by this number will
1257 skip over the unhashed subpackets.
1259 - Unhashed subpacket data. (zero or more subpackets)
1261 - Two-octet field holding left 16 bits of signed hash value.
1263 - One or more multiprecision integers comprising the signature.
1264 This portion is algorithm specific, as described above.
1266 The data being signed is hashed, and then the signature data from
1267 the version number through the hashed subpacket data (inclusive) is
1268 hashed. The resulting hash value is what is signed. The left 16
1269 bits of the hash are included in the signature packet to provide a
1270 quick test to reject some invalid signatures.
1272 There are two fields consisting of signature subpackets. The first
1273 field is hashed with the rest of the signature data, while the
1274 second is unhashed. The second set of subpackets is not
1275 cryptographically protected by the signature and should include only
1276 advisory information.
1278 The algorithms for converting the hash function result to a
1279 signature are described in a section below.
1281 5.2.3.1. Signature Subpacket Specification
1283 The subpacket fields consist of zero or more signature subpackets.
1284 Each set of subpackets is preceded by a two-octet scalar count of
1285 the length of the set of subpackets.
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1291 Each subpacket consists of a subpacket header and a body. The
1292 header consists of:
1294 - the subpacket length (1, 2, or 5 octets)
1296 - the subpacket type (1 octet)
1298 and is followed by the subpacket specific data.
1300 The length includes the type octet but not this length. Its format
1301 is similar to the "new" format packet header lengths, but cannot
1302 have partial body lengths. That is:
1304 if the 1st octet < 192, then
1305 lengthOfLength = 1
1306 subpacketLen = 1st_octet
1308 if the 1st octet >= 192 and < 255, then
1309 lengthOfLength = 2
1310 subpacketLen = ((1st_octet - 192) << 8) + (2nd_octet) + 192
1312 if the 1st octet = 255, then
1313 lengthOfLength = 5
1314 subpacket length = [four-octet scalar starting at 2nd_octet]
1316 The value of the subpacket type octet may be:
1318 2 = signature creation time
1319 3 = signature expiration time
1320 4 = exportable certification
1321 5 = trust signature
1322 6 = regular expression
1323 7 = revocable
1324 9 = key expiration time
1325 10 = placeholder for backward compatibility
1326 11 = preferred symmetric algorithms
1327 12 = revocation key
1328 16 = issuer key ID
1329 20 = notation data
1330 21 = preferred hash algorithms
1331 22 = preferred compression algorithms
1332 23 = key server preferences
1333 24 = preferred key server
1334 25 = primary User ID
1335 26 = policy URL
1336 27 = key flags
1337 28 = signer's User ID
1338 29 = reason for revocation
1339 30 = features
1340 31 = signature target
1341 32 = embedded signature
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1345 INTERNET-DRAFT OpenPGP Message Format Nov 23, 2004
1347 100 to 110 = internal or user-defined
1349 An implementation SHOULD ignore any subpacket of a type that it does
1350 not recognize.
1352 Bit 7 of the subpacket type is the "critical" bit. If set, it
1353 denotes that the subpacket is one that is critical for the evaluator
1354 of the signature to recognize. If a subpacket is encountered that
1355 is marked critical but is unknown to the evaluating software, the
1356 evaluator SHOULD consider the signature to be in error.
1358 An evaluator may "recognize" a subpacket, but not implement it. The
1359 purpose of the critical bit is to allow the signer to tell an
1360 evaluator that it would prefer a new, unknown feature to generate an
1361 error than be ignored.
1363 Implementations SHOULD implement "preferences" and the "reason for
1364 revocation" subpackets. Note, however, that if an implementation
1365 chooses not to implement some of the preferences, it is required to
1366 behave in a polite manner to respect the wishes of those users who
1367 do implement these preferences.
1369 5.2.3.2. Signature Subpacket Types
1371 A number of subpackets are currently defined. Some subpackets apply
1372 to the signature itself and some are attributes of the key.
1373 Subpackets that are found on a self-signature are placed on a
1374 certification made by the key itself. Note that a key may have more
1375 than one User ID, and thus may have more than one self-signature,
1376 and differing subpackets.
1378 A subpacket may be found either in the hashed or unhashed subpacket
1379 sections of a signature. If a subpacket is not hashed, then the
1380 information in it cannot be considered definitive because it is not
1381 part of the signature proper.
1383 5.2.3.3. Notes on Self-Signatures
1385 A self-signature is a binding signature made by the key the
1386 signature refers to. There are three types of self-signatures, the
1387 certification signatures (types 0x10-0x13), the direct-key signature
1388 (type 0x1f), and the subkey binding signature (type 0x18). For
1389 certification self-signatures, each User ID may have a
1390 self-signature, and thus different subpackets in those
1391 self-signatures. For subkey binding signatures, each subkey in fact
1392 has a self-signature. Subpackets that appear in a certification
1393 self-signature apply to the username, and subpackets that appear in
1394 the subkey self-signature apply to the subkey. Lastly, subpackets on
1395 the direct-key signature apply to the entire key.
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1401 INTERNET-DRAFT OpenPGP Message Format Nov 23, 2004
1403 Implementing software should interpret a self-signature's preference
1404 subpackets as narrowly as possible. For example, suppose a key has
1405 two usernames, Alice and Bob. Suppose that Alice prefers the
1406 symmetric algorithm CAST5, and Bob prefers IDEA or TripleDES. If the
1407 software locates this key via Alice's name, then the preferred
1408 algorithm is CAST5, if software locates the key via Bob's name, then
1409 the preferred algorithm is IDEA. If the key is located by key ID,
1410 the algorithm of the primary User ID of the key provides the default
1411 symmetric algorithm.
1413 Revoking a self-signature or allowing it to expire has a semantic
1414 meaning that varies with the signature type. Revoking the
1415 self-signature on a User ID effectively retires that user name. The
1416 self-signature is a statement, "My name X is tied to my signing key
1417 K" and is corroborated by other users' certifications. If another
1418 user revokes their certification, they are effectively saying that
1419 they no longer believe that name and that key are tied together.
1420 Similarly, if the user themselves revokes their self-signature, it
1421 means the user no longer goes by that name, no longer has that email
1422 address, etc. Revoking a binding signature effectively retires that
1423 subkey. Revoking a direct-key signature cancels that signature.
1424 Please see the "Reason for Revocation" subpacket below for more
1425 relevant detail.
1427 Since a self-signature contains important information about the
1428 key's use, an implementation SHOULD allow the user to rewrite the
1429 self-signature, and important information in it, such as preferences
1430 and key expiration.
1432 It is good practice to verify that a self-signature imported into an
1433 implementation doesn't advertise features that the implementation
1434 doesn't support, rewriting the signature as appropriate.
1436 An implementation that encounters multiple self-signatures on the
1437 same object may resolve the ambiguity in any way it sees fit, but it
1438 is RECOMMENDED that priority be given to the most recent
1439 self-signature.
1441 5.2.3.4. Signature creation time
1443 (4 octet time field)
1445 The time the signature was made.
1447 MUST be present in the hashed area.
1449 5.2.3.5. Issuer
1451 (8 octet key ID)
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1459 The OpenPGP key ID of the key issuing the signature.
1461 5.2.3.6. Key expiration time
1463 (4 octet time field)
1465 The validity period of the key. This is the number of seconds after
1466 the key creation time that the key expires. If this is not present
1467 or has a value of zero, the key never expires. This is found only on
1468 a self-signature.
1470 5.2.3.7. Preferred symmetric algorithms
1472 (sequence of one-octet values)
1474 Symmetric algorithm numbers that indicate which algorithms the key
1475 holder prefers to use. The subpacket body is an ordered list of
1476 octets with the most preferred listed first. It is assumed that only
1477 algorithms listed are supported by the recipient's software.
1478 Algorithm numbers in section 9. This is only found on a
1479 self-signature.
1481 5.2.3.8. Preferred hash algorithms
1483 (array of one-octet values)
1485 Message digest algorithm numbers that indicate which algorithms the
1486 key holder prefers to receive. Like the preferred symmetric
1487 algorithms, the list is ordered. Algorithm numbers are in section 6.
1488 This is only found on a self-signature.
1490 5.2.3.9. Preferred compression algorithms
1492 (array of one-octet values)
1494 Compression algorithm numbers that indicate which algorithms the key
1495 holder prefers to use. Like the preferred symmetric algorithms, the
1496 list is ordered. Algorithm numbers are in section 6. If this
1497 subpacket is not included, ZIP is preferred. A zero denotes that
1498 uncompressed data is preferred; the key holder's software might have
1499 no compression software in that implementation. This is only found
1500 on a self-signature.
1502 5.2.3.10. Signature expiration time
1504 (4 octet time field)
1506 The validity period of the signature. This is the number of seconds
1507 after the signature creation time that the signature expires. If
1508 this is not present or has a value of zero, it never expires.
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1515 5.2.3.11. Exportable Certification
1517 (1 octet of exportability, 0 for not, 1 for exportable)
1519 This subpacket denotes whether a certification signature is
1520 "exportable," to be used by other users than the signature's issuer.
1521 The packet body contains a Boolean flag indicating whether the
1522 signature is exportable. If this packet is not present, the
1523 certification is exportable; it is equivalent to a flag containing a
1524 1.
1526 Non-exportable, or "local," certifications are signatures made by a
1527 user to mark a key as valid within that user's implementation only.
1528 Thus, when an implementation prepares a user's copy of a key for
1529 transport to another user (this is the process of "exporting" the
1530 key), any local certification signatures are deleted from the key.
1532 The receiver of a transported key "imports" it, and likewise trims
1533 any local certifications. In normal operation, there won't be any,
1534 assuming the import is performed on an exported key. However, there
1535 are instances where this can reasonably happen. For example, if an
1536 implementation allows keys to be imported from a key database in
1537 addition to an exported key, then this situation can arise.
1539 Some implementations do not represent the interest of a single user
1540 (for example, a key server). Such implementations always trim local
1541 certifications from any key they handle.
1543 5.2.3.12. Revocable
1545 (1 octet of revocability, 0 for not, 1 for revocable)
1547 Signature's revocability status. Packet body contains a Boolean
1548 flag indicating whether the signature is revocable. Signatures that
1549 are not revocable have any later revocation signatures ignored.
1550 They represent a commitment by the signer that he cannot revoke his
1551 signature for the life of his key. If this packet is not present,
1552 the signature is revocable.
1554 5.2.3.13. Trust signature
1556 (1 octet "level" (depth), 1 octet of trust amount)
1558 Signer asserts that the key is not only valid, but also trustworthy,
1559 at the specified level. Level 0 has the same meaning as an ordinary
1560 validity signature. Level 1 means that the signed key is asserted
1561 to be a valid trusted introducer, with the 2nd octet of the body
1562 specifying the degree of trust. Level 2 means that the signed key is
1563 asserted to be trusted to issue level 1 trust signatures, i.e. that
1564 it is a "meta introducer". Generally, a level n trust signature
1565 asserts that a key is trusted to issue level n-1 trust signatures.
1566 The trust amount is in a range from 0-255, interpreted such that
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1571 values less than 120 indicate partial trust and values of 120 or
1572 greater indicate complete trust. Implementations SHOULD emit values
1573 of 60 for partial trust and 120 for complete trust.
1575 5.2.3.14. Regular expression
1577 (null-terminated regular expression)
1579 Used in conjunction with trust signature packets (of level > 0) to
1580 limit the scope of trust that is extended. Only signatures by the
1581 target key on User IDs that match the regular expression in the body
1582 of this packet have trust extended by the trust signature subpacket.
1583 The regular expression uses the same syntax as the Henry Spencer's
1584 "almost public domain" regular expression package. A description of
1585 the syntax is found in a section below.
1587 5.2.3.15. Revocation key
1589 (1 octet of class, 1 octet of algid, 20 octets of fingerprint)
1591 Authorizes the specified key to issue revocation signatures for this
1592 key. Class octet must have bit 0x80 set. If the bit 0x40 is set,
1593 then this means that the revocation information is sensitive. Other
1594 bits are for future expansion to other kinds of authorizations. This
1595 is found on a self-signature.
1597 If the "sensitive" flag is set, the keyholder feels this subpacket
1598 contains private trust information that describes a real-world
1599 sensitive relationship. If this flag is set, implementations SHOULD
1600 NOT export this signature to other users except in cases where the
1601 data needs to be available: when the signature is being sent to the
1602 designated revoker, or when it is accompanied by a revocation
1603 signature from that revoker. Note that it may be appropriate to
1604 isolate this subpacket within a separate signature so that it is not
1605 combined with other subpackets that need to be exported.
1607 5.2.3.16. Notation Data
1609 (4 octets of flags, 2 octets of name length (M),
1610 2 octets of value length (N),
1611 M octets of name data,
1612 N octets of value data)
1614 This subpacket describes a "notation" on the signature that the
1615 issuer wishes to make. The notation has a name and a value, each of
1616 which are strings of octets. There may be more than one notation in
1617 a signature. Notations can be used for any extension the issuer of
1618 the signature cares to make. The "flags" field holds four octets of
1619 flags.
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1627 All undefined flags MUST be zero. Defined flags are:
1629 First octet: 0x80 = human-readable. This note value is text, a
1630 note from one person to another, and need
1631 not have meaning to software.
1632 Other octets: none.
1634 Notation names are arbitrary strings encoded in UTF-8. They reside
1635 two name spaces: The IETF name space and the user name space.
1637 The IETF name space is registered with IANA. These names MUST NOT
1638 contain the "@" character (0x40) is this is a tag for the user name
1639 space.
1641 Names in the user name space consist of a UTF-8 string tag followed
1642 by "@" followed by a DNS domain name. Note that the tag MUST NOT
1643 contain an "@" character. For example, the "sample" tag used by
1644 Example Corporation could be "sample@example.com".
1646 Names in a user space are owned and controlled by the owners of that
1647 domain. Obviously, it's of bad form to create a new name in a DNS
1648 space that you don't own.
1650 Since the user name space is in the form of an email address,
1651 implementers MAY wish to arrange for that address to reach a person
1652 who can be consulted about the use of the named tag. Note that due
1653 to UTF-8 encoding, not all valid user space name tags are valid
1654 email addresses.
1656 5.2.3.17. Key server preferences
1658 (N octets of flags)
1660 This is a list of one-bit flags that indicate preferences that the
1661 key holder has about how the key is handled on a key server. All
1662 undefined flags MUST be zero.
1664 First octet: 0x80 = No-modify
1665 the key holder requests that this key only be modified or
1666 updated by the key holder or an administrator of the key server.
1668 This is found only on a self-signature.
1670 5.2.3.18. Preferred key server
1672 (String)
1674 This is a URL of a key server that the key holder prefers be used
1675 for updates. Note that keys with multiple User IDs can have a
1676 preferred key server for each User ID. Note also that since this is
1677 a URL, the key server can actually be a copy of the key retrieved by
1678 ftp, http, finger, etc.
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1683 5.2.3.19. Primary User ID
1685 (1 octet, Boolean)
1687 This is a flag in a User ID's self signature that states whether
1688 this User ID is the main User ID for this key. It is reasonable for
1689 an implementation to resolve ambiguities in preferences, etc. by
1690 referring to the primary User ID. If this flag is absent, its value
1691 is zero. If more than one User ID in a key is marked as primary, the
1692 implementation may resolve the ambiguity in any way it sees fit, but
1693 it is RECOMMENDED that priority be given to the User ID with the
1694 most recent self-signature.
1696 When appearing on a self-signature on a User ID packet, this
1697 subpacket applies only to User ID packets. When appearing on a
1698 self-signature on a User Attribute packet, this subpacket applies
1699 only to User Attribute packets. That is to say, there are two
1700 different and independent "primaries" - one for User IDs, and one
1701 for User Attributes.
1703 5.2.3.20. Policy URL
1705 (String)
1707 This subpacket contains a URL of a document that describes the
1708 policy that the signature was issued under.
1710 5.2.3.21. Key Flags
1712 (N octets of flags)
1714 This subpacket contains a list of binary flags that hold information
1715 about a key. It is a string of octets, and an implementation MUST
1716 NOT assume a fixed size. This is so it can grow over time. If a list
1717 is shorter than an implementation expects, the unstated flags are
1718 considered to be zero. The defined flags are:
1720 First octet:
1722 0x01 - This key may be used to certify other keys.
1724 0x02 - This key may be used to sign data.
1726 0x04 - This key may be used to encrypt communications.
1728 0x08 - This key may be used to encrypt storage.
1730 0x10 - The private component of this key may have been split by
1731 a secret-sharing mechanism.
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1739 0x20 - This key may be used for authentication.
1741 0x80 - The private component of this key may be in the
1742 possession of more than one person.
1744 Usage notes:
1746 The flags in this packet may appear in self-signatures or in
1747 certification signatures. They mean different things depending on
1748 who is making the statement -- for example, a certification
1749 signature that has the "sign data" flag is stating that the
1750 certification is for that use. On the other hand, the
1751 "communications encryption" flag in a self-signature is stating a
1752 preference that a given key be used for communications. Note
1753 however, that it is a thorny issue to determine what is
1754 "communications" and what is "storage." This decision is left wholly
1755 up to the implementation; the authors of this document do not claim
1756 any special wisdom on the issue, and realize that accepted opinion
1757 may change.
1759 The "split key" (0x10) and "group key" (0x80) flags are placed on a
1760 self-signature only; they are meaningless on a certification
1761 signature. They SHOULD be placed only on a direct-key signature
1762 (type 0x1f) or a subkey signature (type 0x18), one that refers to
1763 the key the flag applies to.
1765 5.2.3.22. Signer's User ID
1767 (String)
1769 This subpacket allows a keyholder to state which User ID is
1770 responsible for the signing. Many keyholders use a single key for
1771 different purposes, such as business communications as well as
1772 personal communications. This subpacket allows such a keyholder to
1773 state which of their roles is making a signature.
1775 This subpacket is not appropriate to use to refer to a User
1776 Attribute packet.
1778 5.2.3.23. Reason for Revocation
1780 (1 octet of revocation code, N octets of reason string)
1782 This subpacket is used only in key revocation and certification
1783 revocation signatures. It describes the reason why the key or
1784 certificate was revoked.
1786 The first octet contains a machine-readable code that denotes the
1787 reason for the revocation:
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1795 0x00 - No reason specified (key revocations or cert revocations)
1796 0x01 - Key is superceded (key revocations)
1797 0x02 - Key material has been compromised (key revocations)
1798 0x03 - Key is retired and no longer used (key revocations)
1799 0x20 - User ID information is no longer valid (cert revocations)
1801 Following the revocation code is a string of octets which gives
1802 information about the reason for revocation in human-readable form
1803 (UTF-8). The string may be null, that is, of zero length. The length
1804 of the subpacket is the length of the reason string plus one.
1806 An implementation SHOULD implement this subpacket, include it in all
1807 revocation signatures, and interpret revocations appropriately.
1808 There are important semantic differences between the reasons, and
1809 there are thus important reasons for revoking signatures.
1811 If a key has been revoked because of a compromise, all signatures
1812 created by that key are suspect. However, if it was merely
1813 superceded or retired, old signatures are still valid. If the
1814 revoked signature is the self-signature for certifying a User ID, a
1815 revocation denotes that that user name is no longer in use. Such a
1816 revocation SHOULD include an 0x20 subpacket.
1818 Note that any signature may be revoked, including a certification on
1819 some other person's key. There are many good reasons for revoking a
1820 certification signature, such as the case where the keyholder leaves
1821 the employ of a business with an email address. A revoked
1822 certification is no longer a part of validity calculations.
1824 5.2.3.24. Features
1826 (N octets of flags)
1828 The features subpacket denotes which advanced OpenPGP features a
1829 user's implementation supports. This is so that as features are
1830 added to OpenPGP that cannot be backwards-compatible, a user can
1831 state that they can use that feature. The flags are single bits that
1832 indicate that a given feature is supported.
1834 This subpacket is similar to a preferences subpacket, and only
1835 appears in a self-signature.
1837 An implementation SHOULD NOT use a feature listed when sending to a
1838 user who does not state that they can use it.
1840 Defined features are:
1842 First octet:
1844 0x01 - Modification Detection (packets 18 and 19)
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1851 If an implementation implements any of the defined features, it
1852 SHOULD implement the features subpacket, too.
1854 An implementation may freely infer features from other suitable
1855 implementation-dependent mechanisms.
1857 5.2.3.25. Signature Target
1859 (1 octet PK algorithm, 1 octet hash algorithm, N octets hash)
1861 This subpacket identifies a specific target signature that a
1862 signature refers to. For revocation signatures, this subpacket
1863 provides explicit designation of which signature is being revoked.
1864 For a third-party or timestamp signature, this designates what
1865 signature is signed. All arguments are an identifier of that target
1866 signature.
1868 The N octets of hash data MUST be the size of the hash of the
1869 signature. For example, a target signature with a SHA-1 hash MUST
1870 have 20 octets of hash data.
1872 5.2.3.26. Embedded Signature
1874 (1 signature packet body)
1876 This subpacket contains a complete signature packet body as
1877 specified in section 5.2 above. It is useful when one signature
1878 needs to refer to, or be incorporated in, another signature.
1880 5.2.4. Computing Signatures
1882 All signatures are formed by producing a hash over the signature
1883 data, and then using the resulting hash in the signature algorithm.
1885 The signature data is simple to compute for document signatures
1886 (types 0x00 and 0x01), for which the document itself is the data.
1887 For standalone signatures, this is a null string.
1889 When a signature is made over a key, the hash data starts with the
1890 octet 0x99, followed by a two-octet length of the key, and then body
1891 of the key packet. (Note that this is an old-style packet header for
1892 a key packet with two-octet length.) A subkey binding signature
1893 (type 0x18) or primary key binding signature (type 0x19) then hashes
1894 the subkey using the same format as the main key (also using 0x99 as
1895 the first octet). Key revocation signatures (types 0x20 and 0x28)
1896 hash only the key being revoked.
1898 When a signature is made over a signature packet, the hash data
1899 starts with the octet 0x88, followed by the four-octet length of the
1900 signature, and then the body of the signature packet. The unhashed
1901 subpacket data of the signature packet being hashed is not included
1902 in the hash and the unhashed subpacket data length value is set to
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1907 zero. (Note that this is an old-style packet header for a signature
1908 packet with the length-of-length set to zero).
1910 A certification signature (type 0x10 through 0x13) hashes the User
1911 ID being bound to the key into the hash context after the above
1912 data. A V3 certification hashes the contents of the User ID or
1913 attribute packet packet, without any header. A V4 certification
1914 hashes the constant 0xb4 for User ID certifications or the constant
1915 0xd1 for User Attribute certifications, followed by a four-octet
1916 number giving the length of the User ID or User Attribute data, and
1917 then the User ID or User Attribute data.
1919 Once the data body is hashed, then a trailer is hashed. A V3
1920 signature hashes five octets of the packet body, starting from the
1921 signature type field. This data is the signature type, followed by
1922 the four-octet signature time. A V4 signature hashes the packet body
1923 starting from its first field, the version number, through the end
1924 of the hashed subpacket data. Thus, the fields hashed are the
1925 signature version, the signature type, the public key algorithm, the
1926 hash algorithm, the hashed subpacket length, and the hashed
1927 subpacket body.
1929 V4 signatures also hash in a final trailer of six octets: the
1930 version of the signature packet, i.e. 0x04; 0xFF; a four-octet,
1931 big-endian number that is the length of the hashed data from the
1932 signature packet (note that this number does not include these final
1933 six octets.
1935 After all this has been hashed in a single hash context the
1936 resulting hash field is used in the signature algorithm, and placed
1937 at the end of the signature packet.
1939 5.2.4.1. Subpacket Hints
1941 It is certainly possible for a signature to contain conflicting
1942 information in subpackets. For example, a signature may contain
1943 multiple copies of a preference or multiple expiration times. In
1944 most cases, an implementation SHOULD use the last subpacket in the
1945 signature, but MAY use any conflict resolution scheme that makes
1946 more sense. Please note that we are intentionally leaving conflict
1947 resolution to the implementer; most conflicts are simply syntax
1948 errors, and the wishy-washy language here allows a receiver to be
1949 generous in what they accept, while putting pressure on a creator to
1950 be stingy in what they generate.
1952 Some apparent conflicts may actually make sense -- for example,
1953 suppose a keyholder has an V3 key and a V4 key that share the same
1954 RSA key material. Either of these keys can verify a signature
1955 created by the other, and it may be reasonable for a signature to
1956 contain an issuer subpacket for each key, as a way of explicitly
1957 tying those keys to the signature.
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1963 5.3. Symmetric-Key Encrypted Session Key Packets (Tag 3)
1965 The Symmetric-Key Encrypted Session Key packet holds the
1966 symmetric-key encryption of a session key used to encrypt a message.
1967 Zero or more Encrypted Session Key packets and/or Symmetric-Key
1968 Encrypted Session Key packets may precede a Symmetrically Encrypted
1969 Data Packet that holds an encrypted message. The message is
1970 encrypted with a session key, and the session key is itself
1971 encrypted and stored in the Encrypted Session Key packet or the
1972 Symmetric-Key Encrypted Session Key packet.
1974 If the Symmetrically Encrypted Data Packet is preceded by one or
1975 more Symmetric-Key Encrypted Session Key packets, each specifies a
1976 passphrase that may be used to decrypt the message. This allows a
1977 message to be encrypted to a number of public keys, and also to one
1978 or more pass phrases. This packet type is new, and is not generated
1979 by PGP 2.x or PGP 5.0.
1981 The body of this packet consists of:
1983 - A one-octet version number. The only currently defined version
1984 is 4.
1986 - A one-octet number describing the symmetric algorithm used.
1988 - A string-to-key (S2K) specifier, length as defined above.
1990 - Optionally, the encrypted session key itself, which is decrypted
1991 with the string-to-key object.
1993 If the encrypted session key is not present (which can be detected
1994 on the basis of packet length and S2K specifier size), then the S2K
1995 algorithm applied to the passphrase produces the session key for
1996 decrypting the file, using the symmetric cipher algorithm from the
1997 Symmetric-Key Encrypted Session Key packet.
1999 If the encrypted session key is present, the result of applying the
2000 S2K algorithm to the passphrase is used to decrypt just that
2001 encrypted session key field, using CFB mode with an IV of all zeros.
2002 The decryption result consists of a one-octet algorithm identifier
2003 that specifies the symmetric-key encryption algorithm used to
2004 encrypt the following Symmetrically Encrypted Data Packet, followed
2005 by the session key octets themselves.
2007 Note: because an all-zero IV is used for this decryption, the S2K
2008 specifier MUST use a salt value, either a Salted S2K or an
2009 Iterated-Salted S2K. The salt value will insure that the decryption
2010 key is not repeated even if the passphrase is reused.
2012 5.4. One-Pass Signature Packets (Tag 4)
2014 The One-Pass Signature packet precedes the signed data and contains
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2017 INTERNET-DRAFT OpenPGP Message Format Nov 23, 2004
2019 enough information to allow the receiver to begin calculating any
2020 hashes needed to verify the signature. It allows the Signature
2021 Packet to be placed at the end of the message, so that the signer
2022 can compute the entire signed message in one pass.
2024 A One-Pass Signature does not interoperate with PGP 2.6.x or
2025 earlier.
2027 The body of this packet consists of:
2029 - A one-octet version number. The current version is 3.
2031 - A one-octet signature type. Signature types are described in
2032 section 5.2.1.
2034 - A one-octet number describing the hash algorithm used.
2036 - A one-octet number describing the public key algorithm used.
2038 - An eight-octet number holding the key ID of the signing key.
2040 - A one-octet number holding a flag showing whether the signature
2041 is nested. A zero value indicates that the next packet is
2042 another One-Pass Signature packet that describes another
2043 signature to be applied to the same message data.
2045 Note that if a message contains more than one one-pass signature,
2046 then the signature packets bracket the message; that is, the first
2047 signature packet after the message corresponds to the last one-pass
2048 packet and the final signature packet corresponds to the first
2049 one-pass packet.
2051 5.5. Key Material Packet
2053 A key material packet contains all the information about a public or
2054 private key. There are four variants of this packet type, and two
2055 major versions. Consequently, this section is complex.
2057 5.5.1. Key Packet Variants
2059 5.5.1.1. Public Key Packet (Tag 6)
2061 A Public Key packet starts a series of packets that forms an OpenPGP
2062 key (sometimes called an OpenPGP certificate).
2064 5.5.1.2. Public Subkey Packet (Tag 14)
2066 A Public Subkey packet (tag 14) has exactly the same format as a
2067 Public Key packet, but denotes a subkey. One or more subkeys may be
2068 associated with a top-level key. By convention, the top-level key
2069 provides signature services, and the subkeys provide encryption
2070 services.
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2075 Note: in PGP 2.6.x, tag 14 was intended to indicate a comment
2076 packet. This tag was selected for reuse because no previous version
2077 of PGP ever emitted comment packets but they did properly ignore
2078 them. Public Subkey packets are ignored by PGP 2.6.x and do not
2079 cause it to fail, providing a limited degree of backward
2080 compatibility.
2082 5.5.1.3. Secret Key Packet (Tag 5)
2084 A Secret Key packet contains all the information that is found in a
2085 Public Key packet, including the public key material, but also
2086 includes the secret key material after all the public key fields.
2088 5.5.1.4. Secret Subkey Packet (Tag 7)
2090 A Secret Subkey packet (tag 7) is the subkey analog of the Secret
2091 Key packet, and has exactly the same format.
2093 5.5.2. Public Key Packet Formats
2095 There are two versions of key-material packets. Version 3 packets
2096 were first generated by PGP 2.6. Version 4 keys first appeared in
2097 PGP 5.0, and are the preferred key version for OpenPGP.
2099 OpenPGP implementations SHOULD create keys with version 4 format. V3
2100 keys are deprecated; an implementation SHOULD NOT generate a V3 key,
2101 but MAY accept it. An implementation MUST NOT create a V3 key with a
2102 public key algorithm other than RSA.
2104 A version 3 public key or public subkey packet contains:
2106 - A one-octet version number (3).
2108 - A four-octet number denoting the time that the key was created.
2110 - A two-octet number denoting the time in days that this key is
2111 valid. If this number is zero, then it does not expire.
2113 - A one-octet number denoting the public key algorithm of this key
2115 - A series of multiprecision integers comprising the key material:
2117 - a multiprecision integer (MPI) of RSA public modulus n;
2119 - an MPI of RSA public encryption exponent e.
2121 V3 keys are deprecated. They contain three weaknesses in them.
2122 First, it is relatively easy to construct a V3 key that has the same
2123 key ID as any other key because the key ID is simply the low 64 bits
2124 of the public modulus. Secondly, because the fingerprint of a V3 key
2125 hashes the key material, but not its length, there is an increased
2126 opportunity for fingerprint collisions. Third, there are minor
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2131 weaknesses in the MD5 hash algorithm that make developers prefer
2132 other algorithms. See below for a fuller discussion of key IDs and
2133 fingerprints.
2135 The version 4 format is similar to the version 3 format except for
2136 the absence of a validity period. This has been moved to the
2137 signature packet. In addition, fingerprints of version 4 keys are
2138 calculated differently from version 3 keys, as described in section
2139 "Enhanced Key Formats."
2141 A version 4 packet contains:
2143 - A one-octet version number (4).
2145 - A four-octet number denoting the time that the key was created.
2147 - A one-octet number denoting the public key algorithm of this key
2149 - A series of multiprecision integers comprising the key material.
2150 This algorithm-specific portion is:
2152 Algorithm Specific Fields for RSA public keys:
2154 - multiprecision integer (MPI) of RSA public modulus n;
2156 - MPI of RSA public encryption exponent e.
2158 Algorithm Specific Fields for DSA public keys:
2160 - MPI of DSA prime p;
2162 - MPI of DSA group order q (q is a prime divisor of p-1);
2164 - MPI of DSA group generator g;
2166 - MPI of DSA public key value y (= g**x mod p where x is
2167 secret).
2169 Algorithm Specific Fields for Elgamal public keys:
2171 - MPI of Elgamal prime p;
2173 - MPI of Elgamal group generator g;
2175 - MPI of Elgamal public key value y (= g**x mod p where x is
2176 secret).
2178 5.5.3. Secret Key Packet Formats
2180 The Secret Key and Secret Subkey packets contain all the data of the
2181 Public Key and Public Subkey packets, with additional
2182 algorithm-specific secret key data appended, usually in encrypted
2184 Callas, et al. Expires May 23, 2005 [Page 39]
2185 INTERNET-DRAFT OpenPGP Message Format Nov 23, 2004
2187 form.
2189 The packet contains:
2191 - A Public Key or Public Subkey packet, as described above
2193 - One octet indicating string-to-key usage conventions. Zero
2194 indicates that the secret key data is not encrypted. 255 or 254
2195 indicates that a string-to-key specifier is being given. Any
2196 other value is a symmetric-key encryption algorithm identifier.
2198 - [Optional] If string-to-key usage octet was 255 or 254, a
2199 one-octet symmetric encryption algorithm.
2201 - [Optional] If string-to-key usage octet was 255 or 254, a
2202 string-to-key specifier. The length of the string-to-key
2203 specifier is implied by its type, as described above.
2205 - [Optional] If secret data is encrypted (string-to-key usage
2206 octet not zero), an Initial Vector (IV) of the same length as
2207 the cipher's block size.
2209 - Plain or encrypted multiprecision integers comprising the secret
2210 key data. These algorithm-specific fields are as described
2211 below.
2213 - If the string-to-key usage octet is zero or 255, then a
2214 two-octet checksum of the plaintext of the algorithm-specific
2215 portion (sum of all octets, mod 65536). If the string-to-key
2216 usage octet was 254, then a 20-octet SHA-1 hash of the plaintext
2217 of the algorithm-specific portion. This checksum or hash is
2218 encrypted together with the algorithm-specific fields (if
2219 string-to-key usage octet is not zero). Note that for all other
2220 values, a two-octet checksum is required.
2222 Algorithm Specific Fields for RSA secret keys:
2224 - multiprecision integer (MPI) of RSA secret exponent d.
2226 - MPI of RSA secret prime value p.
2228 - MPI of RSA secret prime value q (p < q).
2230 - MPI of u, the multiplicative inverse of p, mod q.
2232 Algorithm Specific Fields for DSA secret keys:
2234 - MPI of DSA secret exponent x.
2236 Algorithm Specific Fields for Elgamal secret keys:
2240 Callas, et al. Expires May 23, 2005 [Page 40]
2241 INTERNET-DRAFT OpenPGP Message Format Nov 23, 2004
2243 - MPI of Elgamal secret exponent x.
2245 Secret MPI values can be encrypted using a passphrase. If a
2246 string-to-key specifier is given, that describes the algorithm for
2247 converting the passphrase to a key, else a simple MD5 hash of the
2248 passphrase is used. Implementations MUST use a string-to-key
2249 specifier; the simple hash is for backward compatibility and is
2250 deprecated, though implementations MAY continue to use existing
2251 private keys in the old format. The cipher for encrypting the MPIs
2252 is specified in the secret key packet.
2254 Encryption/decryption of the secret data is done in CFB mode using
2255 the key created from the passphrase and the Initial Vector from the
2256 packet. A different mode is used with V3 keys (which are only RSA)
2257 than with other key formats. With V3 keys, the MPI bit count prefix
2258 (i.e., the first two octets) is not encrypted. Only the MPI
2259 non-prefix data is encrypted. Furthermore, the CFB state is
2260 resynchronized at the beginning of each new MPI value, so that the
2261 CFB block boundary is aligned with the start of the MPI data.
2263 With V4 keys, a simpler method is used. All secret MPI values are
2264 encrypted in CFB mode, including the MPI bitcount prefix.
2266 The two-octet checksum that follows the algorithm-specific portion
2267 is the algebraic sum, mod 65536, of the plaintext of all the
2268 algorithm-specific octets (including MPI prefix and data). With V3
2269 keys, the checksum is stored in the clear. With V4 keys, the
2270 checksum is encrypted like the algorithm-specific data. This value
2271 is used to check that the passphrase was correct. However, this
2272 checksum is deprecated; an implementation SHOULD NOT use it, but
2273 should rather use the SHA-1 hash denoted with a usage octet of 254.
2274 The reason for this is that there are some attacks on the private
2275 key that can undetectably modify the secret key. Using a SHA-1 hash
2276 prevents this.
2278 5.6. Compressed Data Packet (Tag 8)
2280 The Compressed Data packet contains compressed data. Typically, this
2281 packet is found as the contents of an encrypted packet, or following
2282 a Signature or One-Pass Signature packet, and contains literal data
2283 packets.
2285 The body of this packet consists of:
2287 - One octet that gives the algorithm used to compress the packet.
2289 - The remainder of the packet is compressed data.
2291 A Compressed Data Packet's body contains an block that compresses
2292 some set of packets. See section "Packet Composition" for details on
2293 how messages are formed.
2296 Callas, et al. Expires May 23, 2005 [Page 41]
2297 INTERNET-DRAFT OpenPGP Message Format Nov 23, 2004
2299 ZIP-compressed packets are compressed with raw RFC1951 DEFLATE
2300 blocks. Note that PGP V2.6 uses 13 bits of compression. If an
2301 implementation uses more bits of compression, PGP V2.6 cannot
2302 decompress it.
2304 ZLIB-compressed packets are compressed with RFC1950 ZLIB-style
2305 blocks.
2307 5.7. Symmetrically Encrypted Data Packet (Tag 9)
2309 The Symmetrically Encrypted Data packet contains data encrypted with
2310 a symmetric-key algorithm. When it has been decrypted, it contains
2311 other packets (usually literal data packets or compressed data
2312 packets, but in theory other Symmetrically Encrypted Data Packets or
2313 sequences of packets that form whole OpenPGP messages).
2315 The body of this packet consists of:
2317 - Encrypted data, the output of the selected symmetric-key cipher
2318 operating in PGP's variant of Cipher Feedback (CFB) mode.
2320 The symmetric cipher used may be specified in an Public-Key or
2321 Symmetric-Key Encrypted Session Key packet that precedes the
2322 Symmetrically Encrypted Data Packet. In that case, the cipher
2323 algorithm octet is prefixed to the session key before it is
2324 encrypted. If no packets of these types precede the encrypted data,
2325 the IDEA algorithm is used with the session key calculated as the
2326 MD5 hash of the passphrase, though this use is deprecated.
2328 The data is encrypted in CFB mode, with a CFB shift size equal to
2329 the cipher's block size. The Initial Vector (IV) is specified as
2330 all zeros. Instead of using an IV, OpenPGP prefixes a string of
2331 length equal to the block size of the cipher plus two to the data
2332 before it is encrypted. The first block-size octets (for example, 8
2333 octets for a 64-bit block length) are random, and the following two
2334 octets are copies of the last two octets of the IV. For example, in
2335 an 8 octet block, octet 9 is a repeat of octet 7, and octet 10 is a
2336 repeat of octet 8. In a cipher of length 16, octet 17 is a repeat of
2337 octet 15 and octet 18 is a repeat of octet 16. As a pedantic
2338 clarification, in both these examples, we consider the first octet
2339 to be numbered 1.
2341 After encrypting the first block-size-plus-two octets, the CFB state
2342 is resynchronized. The last block-size octets of ciphertext are
2343 passed through the cipher and the block boundary is reset.
2345 The repetition of 16 bits in the random data prefixed to the message
2346 allows the receiver to immediately check whether the session key is
2347 incorrect.
2352 Callas, et al. Expires May 23, 2005 [Page 42]
2353 INTERNET-DRAFT OpenPGP Message Format Nov 23, 2004
2355 5.8. Marker Packet (Obsolete Literal Packet) (Tag 10)
2357 An experimental version of PGP used this packet as the Literal
2358 packet, but no released version of PGP generated Literal packets
2359 with this tag. With PGP 5.x, this packet has been re-assigned and is
2360 reserved for use as the Marker packet.
2362 The body of this packet consists of:
2364 - The three octets 0x50, 0x47, 0x50 (which spell "PGP" in UTF-8).
2366 Such a packet MUST be ignored when received. It may be placed at
2367 the beginning of a message that uses features not available in PGP
2368 2.6.x in order to cause that version to report that newer software
2369 is necessary to process the message.
2371 5.9. Literal Data Packet (Tag 11)
2373 A Literal Data packet contains the body of a message; data that is
2374 not to be further interpreted.
2376 The body of this packet consists of:
2378 - A one-octet field that describes how the data is formatted.
2380 If it is a 'b' (0x62), then the literal packet contains binary data.
2381 If it is a 't' (0x74), then it contains text data, and thus may need
2382 line ends converted to local form, or other text-mode changes. The
2383 tag 'u' (0x75) means the same as 't', but also indicates that
2384 implementation believes that the literal data contains UTF-8 text.
2386 Early versions of PGP also defined a value of 'l' as a 'local' mode
2387 for machine-local conversions. RFC 1991 incorrectly stated this
2388 local mode flag as '1' (ASCII numeral one). Both of these local
2389 modes are deprecated.
2391 - File name as a string (one-octet length, followed by file name),
2392 if the encrypted data should be saved as a file.
2394 If the special name "_CONSOLE" is used, the message is considered to
2395 be "for your eyes only". This advises that the message data is
2396 unusually sensitive, and the receiving program should process it
2397 more carefully, perhaps avoiding storing the received data to disk,
2398 for example.
2400 - A four-octet number that indicates the modification date of the
2401 file, or the creation time of the packet, or a zero that
2402 indicates the present time.
2404 - The remainder of the packet is literal data.
2408 Callas, et al. Expires May 23, 2005 [Page 43]
2409 INTERNET-DRAFT OpenPGP Message Format Nov 23, 2004
2411 Text data is stored with <CR><LF> text endings (i.e. network-normal
2412 line endings). These should be converted to native line endings by
2413 the receiving software.
2415 5.10. Trust Packet (Tag 12)
2417 The Trust packet is used only within keyrings and is not normally
2418 exported. Trust packets contain data that record the user's
2419 specifications of which key holders are trustworthy introducers,
2420 along with other information that implementing software uses for
2421 trust information. The format of trust packets is defined by a given
2422 implementation.
2424 Trust packets SHOULD NOT be emitted to output streams that are
2425 transferred to other users, and they SHOULD be ignored on any input
2426 other than local keyring files.
2428 5.11. User ID Packet (Tag 13)
2430 A User ID packet consists of UTF-8 text that is intended to
2431 represent the name and email address of the key holder. By
2432 convention, it includes an RFC822 mail name, but there are no
2433 restrictions on its content. The packet length in the header
2434 specifies the length of the User ID.
2436 5.12. User Attribute Packet (Tag 17)
2438 The User Attribute packet is a variation of the User ID packet. It
2439 is capable of storing more types of data than the User ID packet
2440 which is limited to text. Like the User ID packet, a User Attribute
2441 packet may be certified by the key owner ("self-signed") or any
2442 other key owner who cares to certify it. Except as noted, a User
2443 Attribute packet may be used anywhere that a User ID packet may be
2444 used.
2446 While User Attribute packets are not a required part of the OpenPGP
2447 standard, implementations SHOULD provide at least enough
2448 compatibility to properly handle a certification signature on the
2449 User Attribute packet. A simple way to do this is by treating the
2450 User Attribute packet as a User ID packet with opaque contents, but
2451 an implementation may use any method desired.
2453 The User Attribute packet is made up of one or more attribute
2454 subpackets. Each subpacket consists of a subpacket header and a
2455 body. The header consists of:
2457 - the subpacket length (1, 2, or 5 octets)
2459 - the subpacket type (1 octet)
2464 Callas, et al. Expires May 23, 2005 [Page 44]
2465 INTERNET-DRAFT OpenPGP Message Format Nov 23, 2004
2467 and is followed by the subpacket specific data.
2469 The only currently defined subpacket type is 1, signifying an image.
2470 An implementation SHOULD ignore any subpacket of a type that it does
2471 not recognize. Subpacket types 100 through 110 are reserved for
2472 private or experimental use.
2474 5.12.1. The Image Attribute Subpacket
2476 The image attribute subpacket is used to encode an image, presumably
2477 (but not required to be) that of the key owner.
2479 The image attribute subpacket begins with an image header. The
2480 first two octets of the image header contain the length of the image
2481 header. Note that unlike other multi-octet numerical values in this
2482 document, due to an historical accident this value is encoded as a
2483 little-endian number. The image header length is followed by a
2484 single octet for the image header version. The only currently
2485 defined version of the image header is 1, which is a 16 octet image
2486 header. The first three octets of a version 1 image header are thus
2487 0x10 0x00 0x01.
2489 The fourth octet of a version 1 image header designates the encoding
2490 format of the image. The only currently defined encoding format is
2491 the value 1 to indicate JPEG. Image format types 100 through 110
2492 are reserved for private or experimental use. The rest of the
2493 version 1 image header is made up of 12 reserved octets, all of
2494 which MUST be set to 0.
2496 The rest of the image subpacket contains the image itself. As the
2497 only currently defined image type is JPEG, the image is encoded in
2498 the JPEG File Interchange Format (JFIF), a standard file format for
2499 JPEG images. [JFIF]
2501 An implementation MAY try and determine the type of an image by
2502 examination of the image data if it is unable to handle a particular
2503 version of the image header or if a specified encoding format value
2504 is not recognized.
2506 5.13. Sym. Encrypted Integrity Protected Data Packet (Tag 18)
2508 The Symmetrically Encrypted Integrity Protected Data Packet is a
2509 variant of the Symmetrically Encrypted Data Packet. It is a new
2510 feature created for OpenPGP that addresses the problem of detecting
2511 a modification to encrypted data. It is used in combination with a
2512 Modification Detection Code Packet.
2514 There is a corresponding feature in the features signature subpacket
2515 that denotes that an implementation can properly use this packet
2516 type. An implementation MUST support decrypting these packets and
2517 SHOULD prefer generating them to the older Symmetrically Encrypted
2518 Data Packet when possible. Since this data packet protects against
2520 Callas, et al. Expires May 23, 2005 [Page 45]
2521 INTERNET-DRAFT OpenPGP Message Format Nov 23, 2004
2523 modification attacks, this standard encourages its proliferation.
2524 While blanket adoption of this data packet would create
2525 interoperability problems, rapid adoption is nevertheless important.
2526 An implementation SHOULD specifically denote support for this
2527 packet, but it MAY infer it from other mechanisms.
2529 For example, an implementation might infer from the use of a cipher
2530 such as AES or Twofish that a user supports this feature. It might
2531 place in the unhashed portion of another user's key signature a
2532 features subpacket. It might also present a user with an opportunity
2533 to regenerate their own self-signature with a features subpacket.
2535 This packet contains data encrypted with a symmetric-key algorithm
2536 and protected against modification by the SHA-1 hash algorithm. When
2537 it has been decrypted, it will typically contain other packets
2538 (often literal data packets or compressed data packets). The last
2539 decrypted packet in this packet's payload MUST be a Modification
2540 Detection Code packet.
2542 The body of this packet consists of:
2544 - A one-octet version number. The only currently defined value is
2545 1.
2547 - Encrypted data, the output of the selected symmetric-key cipher
2548 operating in Cipher Feedback mode with shift amount equal to the
2549 block size of the cipher (CFB-n where n is the block size).
2551 The symmetric cipher used MUST be specified in a Public-Key or
2552 Symmetric-Key Encrypted Session Key packet that precedes the
2553 Symmetrically Encrypted Data Packet. In either case, the cipher
2554 algorithm octet is prefixed to the session key before it is
2555 encrypted.
2557 The data is encrypted in CFB mode, with a CFB shift size equal to
2558 the cipher's block size. The Initial Vector (IV) is specified as
2559 all zeros. Instead of using an IV, OpenPGP prefixes an octet string
2560 to the data before it is encrypted. The length of the octet string
2561 equals the block size of the cipher in octets, plus two. The first
2562 octets in the group, of length equal to the block size of the
2563 cipher, are random; the last two octets are each copies of their 2nd
2564 preceding octet. For example, with a cipher whose block size is 128
2565 bits or 16 octets, the prefix data will contain 16 random octets,
2566 then two more octets, which are copies of the 15th and 16th octets,
2567 respectively. Unlike the Symmetrically Encrypted Data Packet, no
2568 special CFB resynchronization is done after encrypting this prefix
2569 data. See OpenPGP CFB Mode below for more details.
2571 The repetition of 16 bits in the random data prefixed to the message
2572 allows the receiver to immediately check whether the session key is
2573 incorrect.
2576 Callas, et al. Expires May 23, 2005 [Page 46]
2577 INTERNET-DRAFT OpenPGP Message Format Nov 23, 2004
2579 The plaintext of the data to be encrypted is passed through the
2580 SHA-1 hash function, and the result of the hash is appended to the
2581 plaintext in a Modification Detection Code packet. The input to the
2582 hash function includes the prefix data described above; it includes
2583 all of the plaintext, and then also includes two octets of values
2584 0xD3, 0x14. These represent the encoding of a Modification
2585 Detection Code packet tag and length field of 20 octets.
2587 The resulting hash value is stored in a Modification Detection Code
2588 packet which MUST use the two octet encoding just given to represent
2589 its tag and length field. The body of the MDC packet is the 20
2590 octet output of the SHA-1 hash.
2592 The Modification Detection Code packet is appended to the plaintext
2593 and encrypted along with the plaintext using the same CFB context.
2595 During decryption, the plaintext data should be hashed with SHA-1,
2596 including the prefix data as well as the packet tag and length field
2597 of the Modification Detection Code packet. The body of the MDC
2598 packet, upon decryption, is compared with the result of the SHA-1
2599 hash.
2601 Any failure of the MDC indicates that the message has been modified
2602 and MUST be treated as a security problem. Failures include a
2603 difference in the hash values, but also the absence of an MDC
2604 packet, or an MDC packet in any position other than the end of the
2605 plaintext. Any failure SHOULD be reported to the user.
2607 Note: future designs of new versions of this packet should consider
2608 rollback attacks since it will be possible for an attacker to change
2609 the version back to 1.
2611 5.14. Modification Detection Code Packet (Tag 19)
2613 The Modification Detection Code packet contains a SHA-1 hash of
2614 plaintext data which is used to detect message modification. It is
2615 only used with a Symmetrically Encrypted Integrity Protected Data
2616 packet. The Modification Detection Code packet MUST be the last
2617 packet in the plaintext data which is encrypted in the Symmetrically
2618 Encrypted Integrity Protected Data packet, and MUST appear in no
2619 other place.
2621 A Modification Detection Code packet MUST have a length of 20
2622 octets.
2624 The body of this packet consists of:
2626 - A 20-octet SHA-1 hash of the preceding plaintext data of the
2627 Symmetrically Encrypted Integrity Protected Data packet,
2628 including prefix data, the tag octet, and length octet of the
2629 Modification Detection Code packet.
2632 Callas, et al. Expires May 23, 2005 [Page 47]
2633 INTERNET-DRAFT OpenPGP Message Format Nov 23, 2004
2635 Note that the Modification Detection Code packet MUST always use a
2636 new-format encoding of the packet tag, and a one-octet encoding of
2637 the packet length. The reason for this is that the hashing rules for
2638 modification detection include a one-octet tag and one-octet length
2639 in the data hash. While this is a bit restrictive, it reduces
2640 complexity.
2642 6. Radix-64 Conversions
2644 As stated in the introduction, OpenPGP's underlying native
2645 representation for objects is a stream of arbitrary octets, and some
2646 systems desire these objects to be immune to damage caused by
2647 character set translation, data conversions, etc.
2649 In principle, any printable encoding scheme that met the
2650 requirements of the unsafe channel would suffice, since it would not
2651 change the underlying binary bit streams of the native OpenPGP data
2652 structures. The OpenPGP standard specifies one such printable
2653 encoding scheme to ensure interoperability.
2655 OpenPGP's Radix-64 encoding is composed of two parts: a base64
2656 encoding of the binary data, and a checksum. The base64 encoding is
2657 identical to the MIME base64 content-transfer-encoding [RFC 2045].
2659 The checksum is a 24-bit CRC converted to four characters of
2660 radix-64 encoding by the same MIME base64 transformation, preceded
2661 by an equals sign (=). The CRC is computed by using the generator
2662 0x864CFB and an initialization of 0xB704CE. The accumulation is
2663 done on the data before it is converted to radix-64, rather than on
2664 the converted data. A sample implementation of this algorithm is in
2665 the next section.
2667 The checksum with its leading equal sign MAY appear on the first
2668 line after the Base64 encoded data.
2670 Rationale for CRC-24: The size of 24 bits fits evenly into printable
2671 base64. The nonzero initialization can detect more errors than a
2672 zero initialization.
2674 6.1. An Implementation of the CRC-24 in "C"
2676 #define CRC24_INIT 0xb704ceL
2677 #define CRC24_POLY 0x1864cfbL
2679 typedef long crc24;
2680 crc24 crc_octets(unsigned char *octets, size_t len)
2681 {
2682 crc24 crc = CRC24_INIT;
2683 int i;
2688 Callas, et al. Expires May 23, 2005 [Page 48]
2689 INTERNET-DRAFT OpenPGP Message Format Nov 23, 2004
2691 while (len--) {
2692 crc ^= (*octets++) << 16;
2693 for (i = 0; i < 8; i++) {
2694 crc <<= 1;
2695 if (crc & 0x1000000)
2696 crc ^= CRC24_POLY;
2697 }
2698 }
2699 return crc & 0xffffffL;
2700 }
2702 6.2. Forming ASCII Armor
2704 When OpenPGP encodes data into ASCII Armor, it puts specific headers
2705 around the Radix-64 encoded data, so OpenPGP can reconstruct the
2706 data later. An OpenPGP implementation MAY use ASCII armor to protect
2707 raw binary data. OpenPGP informs the user what kind of data is
2708 encoded in the ASCII armor through the use of the headers.
2710 Concatenating the following data creates ASCII Armor:
2712 - An Armor Header Line, appropriate for the type of data
2714 - Armor Headers
2716 - A blank (zero-length, or containing only whitespace) line
2718 - The ASCII-Armored data
2720 - An Armor Checksum
2722 - The Armor Tail, which depends on the Armor Header Line.
2724 An Armor Header Line consists of the appropriate header line text
2725 surrounded by five (5) dashes ('-', 0x2D) on either side of the
2726 header line text. The header line text is chosen based upon the
2727 type of data that is being encoded in Armor, and how it is being
2728 encoded. Header line texts include the following strings:
2730 BEGIN PGP MESSAGE
2731 Used for signed, encrypted, or compressed files.
2733 BEGIN PGP PUBLIC KEY BLOCK
2734 Used for armoring public keys
2736 BEGIN PGP PRIVATE KEY BLOCK
2737 Used for armoring private keys
2739 BEGIN PGP MESSAGE, PART X/Y
2740 Used for multi-part messages, where the armor is split amongst Y
2741 parts, and this is the Xth part out of Y.
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2745 INTERNET-DRAFT OpenPGP Message Format Nov 23, 2004
2747 BEGIN PGP MESSAGE, PART X
2748 Used for multi-part messages, where this is the Xth part of an
2749 unspecified number of parts. Requires the MESSAGE-ID Armor
2750 Header to be used.
2752 BEGIN PGP SIGNATURE
2753 Used for detached signatures, OpenPGP/MIME signatures, and
2754 cleartext signatures. Note that PGP 2.x uses BEGIN PGP MESSAGE
2755 for detached signatures.
2757 Note that all these Armor Header Lines are to consist of a complete
2758 line. That is to say, there is always a line ending preceding the
2759 starting five dashes, and following the ending five dashes. The
2760 header lines, therefore, MUST start at the beginning of a line, and
2761 MUST NOT have text following them on the same line. These line
2762 endings are considered a part of the Armor Header Line for the
2763 purposes of determining the content they delimit. This is
2764 particularly important when computing a cleartext signature (see
2765 below).
2767 The Armor Headers are pairs of strings that can give the user or the
2768 receiving OpenPGP implementation some information about how to
2769 decode or use the message. The Armor Headers are a part of the
2770 armor, not a part of the message, and hence are not protected by any
2771 signatures applied to the message.
2773 The format of an Armor Header is that of a key-value pair. A colon
2774 (':' 0x38) and a single space (0x20) separate the key and value.
2775 OpenPGP should consider improperly formatted Armor Headers to be
2776 corruption of the ASCII Armor. Unknown keys should be reported to
2777 the user, but OpenPGP should continue to process the message.
2779 Currently defined Armor Header Keys are:
2781 - "Version", that states the OpenPGP implementation and version
2782 used to encode the message.
2784 - "Comment", a user-defined comment. OpenPGP defines all text to
2785 be in UTF-8. A comment may be any UTF-8 string. However, the
2786 whole point of armoring is to provide seven-bit-clean data.
2787 Consequently, if a comment has characters that are outside the
2788 US-ASCII range of UTF, they may very well not survive transport.
2790 - "MessageID", a 32-character string of printable characters. The
2791 string must be the same for all parts of a multi-part message
2792 that uses the "PART X" Armor Header. MessageID strings should
2793 be unique enough that the recipient of the mail can associate
2794 all the parts of a message with each other. A good checksum or
2795 cryptographic hash function is sufficient.
2800 Callas, et al. Expires May 23, 2005 [Page 50]
2801 INTERNET-DRAFT OpenPGP Message Format Nov 23, 2004
2803 The MessageID SHOULD NOT appear unless it is in a multi-part
2804 message. If it appears at all, it MUST be computed from the
2805 finished (encrypted, signed, etc.) message in a deterministic
2806 fashion, rather than contain a purely random value. This is to
2807 allow the legitimate recipient to determine that the MessageID
2808 cannot serve as a covert means of leaking cryptographic key
2809 information.
2811 - "Hash", a comma-separated list of hash algorithms used in this
2812 message. This is used only in cleartext signed messages.
2814 - "Charset", a description of the character set that the plaintext
2815 is in. Please note that OpenPGP defines text to be in UTF-8. An
2816 implementation will get best results by translating into and out
2817 of UTF-8. However, there are many instances where this is easier
2818 said than done. Also, there are communities of users who have no
2819 need for UTF-8 because they are all happy with a character set
2820 like ISO Latin-5 or a Japanese character set. In such instances,
2821 an implementation MAY override the UTF-8 default by using this
2822 header key. An implementation MAY implement this key and any
2823 translations it cares to; an implementation MAY ignore it and
2824 assume all text is UTF-8.
2826 The Armor Tail Line is composed in the same manner as the Armor
2827 Header Line, except the string "BEGIN" is replaced by the string
2828 "END".
2830 6.3. Encoding Binary in Radix-64
2832 The encoding process represents 24-bit groups of input bits as
2833 output strings of 4 encoded characters. Proceeding from left to
2834 right, a 24-bit input group is formed by concatenating three 8-bit
2835 input groups. These 24 bits are then treated as four concatenated
2836 6-bit groups, each of which is translated into a single digit in the
2837 Radix-64 alphabet. When encoding a bit stream with the Radix-64
2838 encoding, the bit stream must be presumed to be ordered with the
2839 most-significant-bit first. That is, the first bit in the stream
2840 will be the high-order bit in the first 8-bit octet, and the eighth
2841 bit will be the low-order bit in the first 8-bit octet, and so on.
2843 +--first octet--+-second octet--+--third octet--+
2844 |7 6 5 4 3 2 1 0|7 6 5 4 3 2 1 0|7 6 5 4 3 2 1 0|
2845 +-----------+---+-------+-------+---+-----------+
2846 |5 4 3 2 1 0|5 4 3 2 1 0|5 4 3 2 1 0|5 4 3 2 1 0|
2847 +--1.index--+--2.index--+--3.index--+--4.index--+
2849 Each 6-bit group is used as an index into an array of 64 printable
2850 characters from the table below. The character referenced by the
2851 index is placed in the output string.
2856 Callas, et al. Expires May 23, 2005 [Page 51]
2857 INTERNET-DRAFT OpenPGP Message Format Nov 23, 2004
2859 Value Encoding Value Encoding Value Encoding Value Encoding
2860 0 A 17 R 34 i 51 z
2861 1 B 18 S 35 j 52 0
2862 2 C 19 T 36 k 53 1
2863 3 D 20 U 37 l 54 2
2864 4 E 21 V 38 m 55 3
2865 5 F 22 W 39 n 56 4
2866 6 G 23 X 40 o 57 5
2867 7 H 24 Y 41 p 58 6
2868 8 I 25 Z 42 q 59 7
2869 9 J 26 a 43 r 60 8
2870 10 K 27 b 44 s 61 9
2871 11 L 28 c 45 t 62 +
2872 12 M 29 d 46 u 63 /
2873 13 N 30 e 47 v
2874 14 O 31 f 48 w (pad) =
2875 15 P 32 g 49 x
2876 16 Q 33 h 50 y
2878 The encoded output stream must be represented in lines of no more
2879 than 76 characters each.
2881 Special processing is performed if fewer than 24 bits are available
2882 at the end of the data being encoded. There are three possibilities:
2884 1. The last data group has 24 bits (3 octets). No special
2885 processing is needed.
2887 2. The last data group has 16 bits (2 octets). The first two 6-bit
2888 groups are processed as above. The third (incomplete) data group
2889 has two zero-value bits added to it, and is processed as above.
2890 A pad character (=) is added to the output.
2892 3. The last data group has 8 bits (1 octet). The first 6-bit group
2893 is processed as above. The second (incomplete) data group has
2894 four zero-value bits added to it, and is processed as above. Two
2895 pad characters (=) are added to the output.
2897 6.4. Decoding Radix-64
2899 Any characters outside of the base64 alphabet are ignored in
2900 Radix-64 data. Decoding software must ignore all line breaks or
2901 other characters not found in the table above.
2903 In Radix-64 data, characters other than those in the table, line
2904 breaks, and other white space probably indicate a transmission
2905 error, about which a warning message or even a message rejection
2906 might be appropriate under some circumstances.
2908 Because it is used only for padding at the end of the data, the
2909 occurrence of any "=" characters may be taken as evidence that the
2910 end of the data has been reached (without truncation in transit). No
2912 Callas, et al. Expires May 23, 2005 [Page 52]
2913 INTERNET-DRAFT OpenPGP Message Format Nov 23, 2004
2915 such assurance is possible, however, when the number of octets
2916 transmitted was a multiple of three and no "=" characters are
2917 present.
2919 6.5. Examples of Radix-64
2921 Input data: 0x14fb9c03d97e
2922 Hex: 1 4 f b 9 c | 0 3 d 9 7 e
2923 8-bit: 00010100 11111011 10011100 | 00000011 11011001
2924 11111110
2925 6-bit: 000101 001111 101110 011100 | 000000 111101 100111
2926 111110
2927 Decimal: 5 15 46 28 0 61 37 62
2928 Output: F P u c A 9 l +
2930 Input data: 0x14fb9c03d9
2931 Hex: 1 4 f b 9 c | 0 3 d 9
2932 8-bit: 00010100 11111011 10011100 | 00000011 11011001
2933 pad with 00
2934 6-bit: 000101 001111 101110 011100 | 000000 111101 100100
2935 Decimal: 5 15 46 28 0 61 36
2936 pad with =
2937 Output: F P u c A 9 k =
2939 Input data: 0x14fb9c03
2940 Hex: 1 4 f b 9 c | 0 3
2941 8-bit: 00010100 11111011 10011100 | 00000011
2942 pad with 0000
2943 6-bit: 000101 001111 101110 011100 | 000000 110000
2944 Decimal: 5 15 46 28 0 48
2945 pad with = =
2946 Output: F P u c A w = =
2948 6.6. Example of an ASCII Armored Message
2951 -----BEGIN PGP MESSAGE-----
2952 Version: OpenPrivacy 0.99
2953 yDgBO22WxBHv7O8X7O/jygAEzol56iUKiXmV+XmpCtmpqQUKiQrFqclFqUDBovzS
2954 vBSFjNSiVHsuAA==
2955 =njUN
2956 -----END PGP MESSAGE-----
2958 Note that this example is indented by two spaces.
2960 7. Cleartext signature framework
2962 It is desirable to sign a textual octet stream without ASCII
2963 armoring the stream itself, so the signed text is still readable
2964 without special software. In order to bind a signature to such a
2965 cleartext, this framework is used. (Note that RFC 3156 defines
2966 another way to sign cleartext messages for environments that support
2968 Callas, et al. Expires May 23, 2005 [Page 53]
2969 INTERNET-DRAFT OpenPGP Message Format Nov 23, 2004
2971 MIME.)
2973 The cleartext signed message consists of:
2975 - The cleartext header '-----BEGIN PGP SIGNED MESSAGE-----' on a
2976 single line,
2978 - One or more "Hash" Armor Headers,
2980 - Exactly one empty line not included into the message digest,
2982 - The dash-escaped cleartext that is included into the message
2983 digest,
2985 - The ASCII armored signature(s) including the '-----BEGIN PGP
2986 SIGNATURE-----' Armor Header and Armor Tail Lines.
2988 If the "Hash" armor header is given, the specified message digest
2989 algorithm(s) are used for the signature. If there are no such
2990 headers, MD5 is used. If MD5 is the only hash used, then an
2991 implementation MAY omit this header for improved V2.x compatibility.
2992 If more than one message digest is used in the signature, the "Hash"
2993 armor header contains a comma-delimited list of used message
2994 digests.
2996 Current message digest names are described below with the algorithm
2997 IDs.
2999 7.1. Dash-Escaped Text
3001 The cleartext content of the message must also be dash-escaped.
3003 Dash escaped cleartext is the ordinary cleartext where every line
3004 starting with a dash '-' (0x2D) is prefixed by the sequence dash '-'
3005 (0x2D) and space ' ' (0x20). This prevents the parser from
3006 recognizing armor headers of the cleartext itself. An implementation
3007 MAY dash escape any line, SHOULD dash escape lines commencing "From"
3008 followed by a space, and MUST dash escape any line commencing in a
3009 dash. The message digest is computed using the cleartext itself, not
3010 the dash escaped form.
3012 As with binary signatures on text documents, a cleartext signature
3013 is calculated on the text using canonical <CR><LF> line endings.
3014 The line ending (i.e. the <CR><LF>) before the '-----BEGIN PGP
3015 SIGNATURE-----' line that terminates the signed text is not
3016 considered part of the signed text.
3018 When reversing dash-escaping, an implementation MUST strip the
3019 string "- " if it occurs at the beginning of a line, and SHOULD warn
3020 on "-" and any character other than a space at the beginning of a
3021 line.
3024 Callas, et al. Expires May 23, 2005 [Page 54]
3025 INTERNET-DRAFT OpenPGP Message Format Nov 23, 2004
3027 Also, any trailing whitespace -- spaces (0x20) and tabs (0x09) -- at
3028 the end of any line is removed when the cleartext signature is
3029 generated.
3031 8. Regular Expressions
3033 A regular expression is zero or more branches, separated by '|'. It
3034 matches anything that matches one of the branches.
3036 A branch is zero or more pieces, concatenated. It matches a match
3037 for the first, followed by a match for the second, etc.
3039 A piece is an atom possibly followed by '*', '+', or '?'. An atom
3040 followed by '*' matches a sequence of 0 or more matches of the atom.
3041 An atom followed by '+' matches a sequence of 1 or more matches of
3042 the atom. An atom followed by '?' matches a match of the atom, or
3043 the null string.
3045 An atom is a regular expression in parentheses (matching a match for
3046 the regular expression), a range (see below), '.' (matching any
3047 single character), '^' (matching the null string at the beginning of
3048 the input string), '$' (matching the null string at the end of the
3049 input string), a '\' followed by a single character (matching that
3050 character), or a single character with no other significance
3051 (matching that character).
3053 A range is a sequence of characters enclosed in '[]'. It normally
3054 matches any single character from the sequence. If the sequence
3055 begins with '^', it matches any single character not from the rest
3056 of the sequence. If two characters in the sequence are separated by
3057 '-', this is shorthand for the full list of ASCII characters between
3058 them (e.g. '[0-9]' matches any decimal digit). To include a literal
3059 ']' in the sequence, make it the first character (following a
3060 possible '^'). To include a literal '-', make it the first or last
3061 character.
3063 9. Constants
3065 This section describes the constants used in OpenPGP.
3067 Note that these tables are not exhaustive lists; an implementation
3068 MAY implement an algorithm not on these lists, so long as the
3069 algorithm number(s) are chosen from the private or experimental
3070 algorithm range.
3072 See the section "Notes on Algorithms" below for more discussion of
3073 the algorithms.
3075 9.1. Public Key Algorithms
3077 ID Algorithm
3078 -- ---------
3080 Callas, et al. Expires May 23, 2005 [Page 55]
3081 INTERNET-DRAFT OpenPGP Message Format Nov 23, 2004
3083 1 - RSA (Encrypt or Sign)
3084 2 - RSA Encrypt-Only
3085 3 - RSA Sign-Only
3086 16 - Elgamal (Encrypt-Only), see [ELGAMAL]
3087 17 - DSA (Digital Signature Algorithm) [SCHNEIER]
3088 18 - Reserved for Elliptic Curve
3089 19 - Reserved for ECDSA
3090 20 - Reserved (formerly Elgamal Encrypt or Sign)
3091 21 - Reserved for Diffie-Hellman (X9.42,
3092 as defined for IETF-S/MIME)
3093 100 to 110 - Private/Experimental algorithm.
3095 Implementations MUST implement DSA for signatures, and Elgamal for
3096 encryption. Implementations SHOULD implement RSA keys.
3097 Implementations MAY implement any other algorithm.
3099 9.2. Symmetric Key Algorithms
3101 ID Algorithm
3102 -- ---------
3103 0 - Plaintext or unencrypted data
3104 1 - IDEA [IDEA]
3105 2 - TripleDES (DES-EDE, [SCHNEIER] -
3106 168 bit key derived from 192)
3107 3 - CAST5 (128 bit key, as per RFC2144)
3108 4 - Blowfish (128 bit key, 16 rounds) [BLOWFISH]
3109 5 - Reserved
3110 6 - Reserved
3111 7 - AES with 128-bit key [AES]
3112 8 - AES with 192-bit key
3113 9 - AES with 256-bit key
3114 10 - Twofish with 256-bit key [TWOFISH]
3115 100 to 110 - Private/Experimental algorithm.
3117 Implementations MUST implement TripleDES. Implementations SHOULD
3118 implement AES-128 and CAST5. Implementations that interoperate with
3119 PGP 2.6 or earlier need to support IDEA, as that is the only
3120 symmetric cipher those versions use. Implementations MAY implement
3121 any other algorithm.
3123 9.3. Compression Algorithms
3125 ID Algorithm
3126 -- ---------
3127 0 - Uncompressed
3128 1 - ZIP (RFC1951)
3129 2 - ZLIB (RFC1950)
3130 3 - BZip2 [BZ2]
3131 100 to 110 - Private/Experimental algorithm.
3136 Callas, et al. Expires May 23, 2005 [Page 56]
3137 INTERNET-DRAFT OpenPGP Message Format Nov 23, 2004
3139 Implementations MUST implement uncompressed data. Implementations
3140 SHOULD implement ZIP. Implementations MAY implement any other
3141 algorithm.
3143 9.4. Hash Algorithms
3145 ID Algorithm Text Name
3146 -- --------- ---- ----
3147 1 - MD5 "MD5"
3148 2 - SHA-1 "SHA1"
3149 3 - RIPE-MD/160 "RIPEMD160"
3150 4 - Reserved
3151 5 - Reserved
3152 6 - Reserved
3153 7 - Reserved
3154 8 - SHA256 "SHA256"
3155 9 - SHA384 "SHA384"
3156 10 - SHA512 "SHA512"
3157 100 to 110 - Private/Experimental algorithm.
3159 Implementations MUST implement SHA-1. Implementations MAY implement
3160 other algorithms.
3162 10. Packet Composition
3164 OpenPGP packets are assembled into sequences in order to create
3165 messages and to transfer keys. Not all possible packet sequences
3166 are meaningful and correct. This section describes the rules for
3167 how packets should be placed into sequences.
3169 10.1. Transferable Public Keys
3171 OpenPGP users may transfer public keys. The essential elements of a
3172 transferable public key are:
3174 - One Public Key packet
3176 - Zero or more revocation signatures
3178 - One or more User ID packets
3180 - After each User ID packet, zero or more signature packets
3181 (certifications)
3183 - Zero or more User Attribute packets
3185 - After each User Attribute packet, zero or more signature packets
3186 (certifications)
3188 - Zero or more Subkey packets
3192 Callas, et al. Expires May 23, 2005 [Page 57]
3193 INTERNET-DRAFT OpenPGP Message Format Nov 23, 2004
3195 - After each Subkey packet, one signature packet, plus optionally
3196 a revocation.
3198 The Public Key packet occurs first. Each of the following User ID
3199 packets provides the identity of the owner of this public key. If
3200 there are multiple User ID packets, this corresponds to multiple
3201 means of identifying the same unique individual user; for example, a
3202 user may have more than one email address, and construct a User ID
3203 for each one.
3205 Immediately following each User ID packet, there are zero or more
3206 signature packets. Each signature packet is calculated on the
3207 immediately preceding User ID packet and the initial Public Key
3208 packet. The signature serves to certify the corresponding public key
3209 and User ID. In effect, the signer is testifying to his or her
3210 belief that this public key belongs to the user identified by this
3211 User ID.
3213 Within the same section as the User ID packets, there are zero or
3214 more User Attribute packets. Like the User ID packets, a User
3215 Attribute packet is followed by zero or more signature packets
3216 calculated on the immediately preceding User Attribute packet and
3217 the initial Public Key packet.
3219 User Attribute packets and User ID packets may be freely intermixed
3220 in this section, so long as the signatures that follow them are
3221 maintained on the proper User Attribute or User ID packet.
3223 After the User ID or Attribute packets there may be one or more
3224 Subkey packets. In general, subkeys are provided in cases where the
3225 top-level public key is a signature-only key. However, any V4 key
3226 may have subkeys, and the subkeys may be encryption-only keys,
3227 signature-only keys, or general-purpose keys. V3 keys MUST NOT have
3228 subkeys.
3230 Each Subkey packet must be followed by one Signature packet, which
3231 should be a subkey binding signature issued by the top level key.
3232 For subkeys that can issue signatures, the subkey binding signature
3233 must contain an embedded signature subpacket with a primary key
3234 binding signature (0x19) issued by the subkey on the top level key.
3236 Subkey and Key packets may each be followed by a revocation
3237 Signature packet to indicate that the key is revoked. Revocation
3238 signatures are only accepted if they are issued by the key itself,
3239 or by a key that is authorized to issue revocations via a revocation
3240 key subpacket in a self-signature by the top level key.
3242 Transferable public key packet sequences may be concatenated to
3243 allow transferring multiple public keys in one operation.
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3249 INTERNET-DRAFT OpenPGP Message Format Nov 23, 2004
3251 10.2. OpenPGP Messages
3253 An OpenPGP message is a packet or sequence of packets that
3254 corresponds to the following grammatical rules (comma represents
3255 sequential composition, and vertical bar separates alternatives):
3257 OpenPGP Message :- Encrypted Message | Signed Message |
3258 Compressed Message | Literal Message.
3260 Compressed Message :- Compressed Data Packet.
3262 Literal Message :- Literal Data Packet |
3263 Literal Message, Literal Data Packet.
3265 ESK :- Public Key Encrypted Session Key Packet |
3266 Symmetric-Key Encrypted Session Key Packet.
3268 ESK Sequence :- ESK | ESK Sequence, ESK.
3270 Encrypted Data :- Symmetrically Encrypted Data Packet |
3271 Symmetrically Encrypted Integrity Protected Data Packet
3273 Encrypted Message :- Encrypted Data | ESK Sequence, Encrypted Data.
3275 One-Pass Signed Message :- One-Pass Signature Packet,
3276 OpenPGP Message, Corresponding Signature Packet.
3278 Signed Message :- Signature Packet, OpenPGP Message |
3279 One-Pass Signed Message.
3281 In addition, decrypting a Symmetrically Encrypted Data Packet or a
3282 Symmetrically Encrypted Integrity Protected Data Packet as well as
3284 decompressing a Compressed Data packet must yield a valid OpenPGP
3285 Message.
3287 10.3. Detached Signatures
3289 Some OpenPGP applications use so-called "detached signatures." For
3290 example, a program bundle may contain a file, and with it a second
3291 file that is a detached signature of the first file. These detached
3292 signatures are simply a signature packet stored separately from the
3293 data that they are a signature of.
3295 11. Enhanced Key Formats
3297 11.1. Key Structures
3299 The format of an OpenPGP V3 key is as follows. Entries in square
3300 brackets are optional and ellipses indicate repetition.
3304 Callas, et al. Expires May 23, 2005 [Page 59]
3305 INTERNET-DRAFT OpenPGP Message Format Nov 23, 2004
3307 RSA Public Key
3308 [Revocation Self Signature]
3309 User ID [Signature ...]
3310 [User ID [Signature ...] ...]
3312 Each signature certifies the RSA public key and the preceding User
3313 ID. The RSA public key can have many User IDs and each User ID can
3314 have many signatures. V3 keys are deprecated. Implementations MUST
3315 NOT generate new V3 keys, but MAY continue to use existing ones.
3317 The format of an OpenPGP V4 key that uses multiple public keys is
3318 similar except that the other keys are added to the end as "subkeys"
3319 of the primary key.
3321 Primary-Key
3322 [Revocation Self Signature]
3323 [Direct Key Signature...]
3324 User ID [Signature ...]
3325 [User ID [Signature ...] ...]
3326 [User Attribute [Signature ...] ...]
3327 [[Subkey [Binding-Signature-Revocation]
3328 Primary-Key-Binding-Signature] ...]
3330 A subkey always has a single signature after it that is issued using
3331 the primary key to tie the two keys together. This binding
3332 signature may be in either V3 or V4 format, but SHOULD be V4.
3334 In the above diagram, if the binding signature of a subkey has been
3335 revoked, the revoked key may be removed, leaving only one key.
3337 In a V4 key, the primary key MUST be a key capable of certification.
3338 The subkeys may be keys of any other type. There may be other
3339 constructions of V4 keys, too. For example, there may be a
3340 single-key RSA key in V4 format, a DSA primary key with an RSA
3341 encryption key, or RSA primary key with an Elgamal subkey, etc.
3343 It is also possible to have a signature-only subkey. This permits a
3344 primary key that collects certifications (key signatures) but is
3345 used only used for certifying subkeys that are used for encryption
3346 and signatures.
3348 11.2. Key IDs and Fingerprints
3350 For a V3 key, the eight-octet key ID consists of the low 64 bits of
3351 the public modulus of the RSA key.
3353 The fingerprint of a V3 key is formed by hashing the body (but not
3354 the two-octet length) of the MPIs that form the key material (public
3355 modulus n, followed by exponent e) with MD5. Note that both V3 keys
3356 and MD5 are deprecated.
3360 Callas, et al. Expires May 23, 2005 [Page 60]
3361 INTERNET-DRAFT OpenPGP Message Format Nov 23, 2004
3363 A V4 fingerprint is the 160-bit SHA-1 hash of the octet 0x99,
3364 followed by the two-octet packet length, followed by the entire
3365 Public Key packet starting with the version field. The key ID is
3366 the low order 64 bits of the fingerprint. Here are the fields of
3367 the hash material, with the example of a DSA key:
3369 a.1) 0x99 (1 octet)
3371 a.2) high order length octet of (b)-(f) (1 octet)
3373 a.3) low order length octet of (b)-(f) (1 octet)
3375 b) version number = 4 (1 octet);
3377 c) time stamp of key creation (4 octets);
3379 d) algorithm (1 octet): 17 = DSA (example);
3381 e) Algorithm specific fields.
3383 Algorithm Specific Fields for DSA keys (example):
3385 e.1) MPI of DSA prime p;
3387 e.2) MPI of DSA group order q (q is a prime divisor of p-1);
3389 e.3) MPI of DSA group generator g;
3391 e.4) MPI of DSA public key value y (= g**x mod p where x is secret).
3393 Note that it is possible for there to be collisions of key IDs --
3394 two different keys with the same key ID. Note that there is a much
3395 smaller, but still non-zero probability that two different keys have
3396 the same fingerprint.
3398 Also note that if V3 and V4 format keys share the same RSA key
3399 material, they will have different key IDs as well as different
3400 fingerprints.
3402 Finally, the key ID and fingerprint of a subkey are calculated in
3403 the same way as for a primary key, including the 0x99 as the first
3404 octet (even though this is not a valid packet ID for a public
3405 subkey).
3407 12. Notes on Algorithms
3409 12.1. Symmetric Algorithm Preferences
3411 The symmetric algorithm preference is an ordered list of algorithms
3412 that the keyholder accepts. Since it is found on a self-signature,
3413 it is possible that a keyholder may have different preferences. For
3414 example, Alice may have TripleDES only specified for
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3417 INTERNET-DRAFT OpenPGP Message Format Nov 23, 2004
3419 "alice@work.com" but CAST5, Blowfish, and TripleDES specified for
3420 "alice@home.org". Note that it is also possible for preferences to
3421 be in a subkey's binding signature.
3423 Since TripleDES is the MUST-implement algorithm, if it is not
3424 explicitly in the list, it is tacitly at the end. However, it is
3425 good form to place it there explicitly. Note also that if an
3426 implementation does not implement the preference, then it is
3427 implicitly a TripleDES-only implementation.
3429 An implementation MUST NOT use a symmetric algorithm that is not in
3430 the recipient's preference list. When encrypting to more than one
3431 recipient, the implementation finds a suitable algorithm by taking
3432 the intersection of the preferences of the recipients. Note that the
3433 MUST-implement algorithm, TripleDES, ensures that the intersection
3434 is not null. The implementation may use any mechanism to pick an
3435 algorithm in the intersection.
3437 If an implementation can decrypt a message that a keyholder doesn't
3438 have in their preferences, the implementation SHOULD decrypt the
3439 message anyway, but MUST warn the keyholder that the protocol has
3440 been violated. (For example, suppose that Alice, above, has software
3441 that implements all algorithms in this specification. Nonetheless,
3442 she prefers subsets for work or home. If she is sent a message
3443 encrypted with IDEA, which is not in her preferences, the software
3444 warns her that someone sent her an IDEA-encrypted message, but it
3445 would ideally decrypt it anyway.)
3447 12.2. Other Algorithm Preferences
3449 Other algorithm preferences work similarly to the symmetric
3450 algorithm preference, in that they specify which algorithms the
3451 keyholder accepts. There are two interesting cases that other
3452 comments need to be made about, though, the compression preferences
3453 and the hash preferences.
3455 12.2.1. Compression Preferences
3457 Compression has been an integral part of PGP since its first days.
3458 OpenPGP and all previous versions of PGP have offered compression.
3459 In this specification, the default is for messages to be compressed,
3460 although an implementation is not required to do so. Consequently,
3461 the compression preference gives a way for a keyholder to request
3462 that messages not be compressed, presumably because they are using a
3463 minimal implementation that does not include compression.
3464 Additionally, this gives a keyholder a way to state that it can
3465 support alternate algorithms.
3467 Like the algorithm preferences, an implementation MUST NOT use an
3468 algorithm that is not in the preference vector. If the preferences
3469 are not present, then they are assumed to be [ZIP(1),
3470 UNCOMPRESSED(0)].
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3473 INTERNET-DRAFT OpenPGP Message Format Nov 23, 2004
3475 Additionally, an implementation MUST implement this preference to
3476 the degree of recognizing when to send an uncompressed message. A
3477 robust implementation would satisfy this requirement by looking at
3478 the recipient's preference and acting accordingly. A minimal
3479 implementation can satisfy this requirement by never generating a
3480 compressed message, since all implementations can handle messages
3481 that have not been compressed.
3483 12.2.2. Hash Algorithm Preferences
3485 Typically, the choice of a hash algorithm is something the signer
3486 does, rather than the verifier, because a signer rarely knows who is
3487 going to be verifying the signature. This preference, though, allows
3488 a protocol based upon digital signatures ease in negotiation.
3490 Thus, if Alice is authenticating herself to Bob with a signature, it
3491 makes sense for her to use a hash algorithm that Bob's software
3492 uses. This preference allows Bob to state in his key which
3493 algorithms Alice may use.
3495 Since SHA1 is the MUST-implement hash algorithm, if it is not
3496 explicitly in the list, it is tacitly at the end. However, it is
3497 good form to place it there explicitly.
3499 12.3. Plaintext
3501 Algorithm 0, "plaintext," may only be used to denote secret keys
3502 that are stored in the clear. Implementations MUST NOT use plaintext
3503 in Symmetrically Encrypted Data Packets; they must use Literal Data
3504 Packets to encode unencrypted or literal data.
3506 12.4. RSA
3508 There are algorithm types for RSA-signature-only, and
3509 RSA-encrypt-only keys. These types are deprecated. The "key flags"
3510 subpacket in a signature is a much better way to express the same
3511 idea, and generalizes it to all algorithms. An implementation SHOULD
3512 NOT create such a key, but MAY interpret it.
3514 An implementation SHOULD NOT implement RSA keys of size less than
3515 1024 bits.
3517 12.5. DSA
3519 An implementation SHOULD NOT implement DSA keys of size less than
3520 1024 bits. Note that present DSA is limited to a maximum of 1024 bit
3521 keys, which are recommended for long-term use. Also, DSA keys MUST
3522 be an even multiple of 64 bits long.
3524 12.6. Elgamal
3526 An implementation SHOULD NOT implement Elgamal keys of size less
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3529 INTERNET-DRAFT OpenPGP Message Format Nov 23, 2004
3531 than 1024 bits.
3533 12.7. Reserved Algorithm Numbers
3535 A number of algorithm IDs have been reserved for algorithms that
3536 would be useful to use in an OpenPGP implementation, yet there are
3537 issues that prevent an implementer from actually implementing the
3538 algorithm. These are marked in the Public Algorithms section as
3539 "(reserved for)".
3541 The reserved public key algorithms, Elliptic Curve (18), ECDSA (19),
3542 and X9.42 (21) do not have the necessary parameters, parameter
3543 order, or semantics defined.
3545 Previous versions of OpenPGP permitted Elgamal [ELGAMAL] signatures
3546 with a public key identifier of 20. These are no longer permitted.
3547 An implementation MUST NOT generate such keys. An implementation
3548 MUST NOT generate Elgamal signatures.
3550 12.8. OpenPGP CFB mode
3552 OpenPGP does symmetric encryption using a variant of Cipher Feedback
3553 Mode (CFB mode). This section describes the procedure it uses in
3554 detail. This mode is what is used for Symmetrically Encrypted Data
3555 Packets; the mechanism used for encrypting secret key material is
3556 similar, but described in those sections above.
3558 In the description below, the value BS is the block size in octets
3559 of the cipher. Most ciphers have a block size of 8 octets. The AES
3560 and Twofish have a block size of 16 octets. Also note that the
3561 description below assumes that the IV and CFB arrays start with an
3562 index of 1 (unlike the C language, which assumes arrays start with a
3563 zero index).
3565 OpenPGP CFB mode uses an initialization vector (IV) of all zeros,
3566 and prefixes the plaintext with BS+2 octets of random data, such
3567 that octets BS+1 and BS+2 match octets BS-1 and BS. It does a CFB
3568 "resync" after encrypting those BS+2 octets.
3570 Thus, for an algorithm that has a block size of 8 octets (64 bits),
3571 the IV is 10 octets long and octets 7 and 8 of the IV are the same
3572 as octets 9 and 10. For an algorithm with a block size of 16 octets
3573 (128 bits), the IV is 18 octets long, and octets 17 and 18 replicate
3574 octets 15 and 16. Those extra two octets are an easy check for a
3575 correct key.
3577 Step by step, here is the procedure:
3579 1. The feedback register (FR) is set to the IV, which is all zeros.
3584 Callas, et al. Expires May 23, 2005 [Page 64]
3585 INTERNET-DRAFT OpenPGP Message Format Nov 23, 2004
3587 2. FR is encrypted to produce FRE (FR Encrypted). This is the
3588 encryption of an all-zero value.
3590 3. FRE is xored with the first BS octets of random data prefixed to
3591 the plaintext to produce C[1] through C[BS], the first BS octets
3592 of ciphertext.
3594 4. FR is loaded with C[1] through C[BS].
3596 5. FR is encrypted to produce FRE, the encryption of the first BS
3597 octets of ciphertext.
3599 6. The left two octets of FRE get xored with the next two octets of
3600 data that were prefixed to the plaintext. This produces C[BS+1]
3601 and C[BS+2], the next two octets of ciphertext.
3603 7. (The resync step) FR is loaded with C[3] through C[BS+2].
3605 8. FR is encrypted to produce FRE.
3607 9. FRE is xored with the first BS octets of the given plaintext,
3608 now that we have finished encrypting the BS+2 octets of prefixed
3609 data. This produces C[BS+3] through C[BS+(BS+2)], the next BS
3610 octets of ciphertext.
3612 10. FR is loaded with C[BS+3] to C[BS + (BS+2)] (which is C11-C18
3613 for an 8-octet block).
3615 11. FR is encrypted to produce FRE.
3617 12. FRE is xored with the next BS octets of plaintext, to produce
3618 the next BS octets of ciphertext. These are loaded into FR and
3619 the process is repeated until the plaintext is used up.
3621 13. Security Considerations
3623 * As with any technology involving cryptography, you should check
3624 the current literature to determine if any algorithms used here
3625 have been found to be vulnerable to attack.
3627 * This specification uses Public Key Cryptography technologies. It
3628 is assumed that the private key portion of a public-private key
3629 pair is controlled and secured by the proper party or parties.
3631 * Certain operations in this specification involve the use of
3632 random numbers. An appropriate entropy source should be used to
3633 generate these numbers. See RFC 1750.
3635 * The MD5 hash algorithm has been found to have weaknesses, with
3636 collisions found in a number of cases. MD5 is deprecated for use
3637 in OpenPGP. Implementations MUST NOT generate new signatures
3638 using MD5 as a hash function. They MAY continue to consider old
3640 Callas, et al. Expires May 23, 2005 [Page 65]
3641 INTERNET-DRAFT OpenPGP Message Format Nov 23, 2004
3643 signatures that used MD5 as valid.
3645 * SHA384 requires the same work as SHA512. In general, there are
3646 few reasons to use it -- you need a situation where one needs
3647 more security than SHA256, but do not want to have the 512-bit
3648 data length.
3650 * Many security protocol designers think that it is a bad idea to
3651 use a single key for both privacy (encryption) and integrity
3652 (signatures). In fact, this was one of the motivating forces
3653 behind the V4 key format with separate signature and encryption
3654 keys. If you as an implementer promote dual-use keys, you should
3655 at least be aware of this controversy.
3657 * The DSA algorithm will work with any 160-bit hash, but it is
3658 sensitive to the quality of the hash algorithm, if the hash
3659 algorithm is broken, it can leak the secret key. The Digital
3660 Signature Standard (DSS) specifies that DSA be used with SHA-1.
3661 RIPEMD-160 is considered by many cryptographers to be as strong.
3662 An implementation should take care which hash algorithms are
3663 used with DSA, as a weak hash can not only allow a signature to
3664 be forged, but could leak the secret key.
3666 * There is a somewhat-related potential security problem in
3667 signatures. If an attacker can find a message that hashes to the
3668 same hash with a different algorithm, a bogus signature
3669 structure can be constructed that evaluates correctly.
3671 For example, suppose Alice DSA signs message M using hash
3672 algorithm H. Suppose that Mallet finds a message M' that has the
3673 same hash value as M with H'. Mallet can then construct a
3674 signature block that verifies as Alice's signature of M' with
3675 H'. However, this would also constitute a weakness in either H
3676 or H' or both. Should this ever occur, a revision will have to
3677 be made to this document to revise the allowed hash algorithms.
3679 * If you are building an authentication system, the recipient may
3680 specify a preferred signing algorithm. However, the signer would
3681 be foolish to use a weak algorithm simply because the recipient
3682 requests it.
3684 * Some of the encryption algorithms mentioned in this document
3685 have been analyzed less than others. For example, although
3686 CAST5 is presently considered strong, it has been analyzed less
3687 than TripleDES. Other algorithms may have other controversies
3688 surrounding them.
3690 * In late summer 2002, Jallad, Katz, and Schneier published an
3691 interesting attack on the OpenPGP protocol and some of its
3692 implementations [JKS02]. In this attack, the attacker modifies a
3693 message and sends it to a user who then returns the erroneously
3694 decrypted message to the attacker. The attacker is thus using
3696 Callas, et al. Expires May 23, 2005 [Page 66]
3697 INTERNET-DRAFT OpenPGP Message Format Nov 23, 2004
3699 the user as a random oracle, and can often decrypt the message.
3701 Compressing data can ameliorate this attack. The incorrectly
3702 decrypted data nearly always decompresses in ways that defeats
3703 the attack. However, this is not a rigorous fix, and leaves open
3704 some small vulnerabilities. For example, if an implementation
3705 does not compress a message before encryption (perhaps because
3706 it knows it was already compressed), then that message is
3707 vulnerable. Because of this happenstance -- that modification
3708 attacks can be thwarted by decompression errors, an
3709 implementation SHOULD treat a decompression error as a security
3710 problem, not merely a data problem.
3712 This attack can be defeated by the use of Modification
3713 Detection, provided that the implementation does not let the
3714 user naively return the data to the attacker. An implementation
3715 MUST treat an MDC failure as a security problem, not merely a
3716 data problem.
3718 In either case, the implementation MAY allow the user access to
3719 the erroneous data, but MUST warn the user as to potential
3720 security problems should that data be returned to the sender.
3722 While this attack is somewhat obscure, requiring a special set
3723 of circumstances to create it, it is nonetheless quite serious
3724 as it permits someone to trick a user to decrypt a message.
3725 Consequently, it is important that:
3727 1. Implementers treat MDC errors and decompression failures as
3728 security problems.
3730 2. Implementers implement Modification Detection with all due
3731 speed and encourage its spread.
3733 3. Users migrate to implementations that support Modification
3734 Detection with all due speed.
3736 * PKCS1 has been found to be vulnerable to attacks in which a
3737 system that reports errors in padding differently from errors in
3738 decryption becomes a random oracle that can leak the private key
3739 in mere millions of queries. Implementations must be aware of
3740 this attack and prevent it from happening. The simplest solution
3741 is report a single error code for all variants of decryption
3742 errors so as not to leak information to an attacker.
3744 * Some technologies mentioned here may be subject to government
3745 control in some countries.
3747 14. Implementation Nits
3749 This section is a collection of comments to help an implementer,
3750 particularly with an eye to backward compatibility. Previous
3752 Callas, et al. Expires May 23, 2005 [Page 67]
3753 INTERNET-DRAFT OpenPGP Message Format Nov 23, 2004
3755 implementations of PGP are not OpenPGP-compliant. Often the
3756 differences are small, but small differences are frequently more
3757 vexing than large differences. Thus, this is a non-comprehensive
3758 list of potential problems and gotchas for a developer who is trying
3759 to be backward-compatible.
3761 * The IDEA algorithm is patented, and yet it is required for PGP
3762 2.x interoperability. It is also the defacto preferred algorithm
3763 for a V3 key with a V3 self-signature (or no self-signature).
3765 * When exporting a private key, PGP 2.x generates the header
3766 "BEGIN PGP SECRET KEY BLOCK" instead of "BEGIN PGP PRIVATE KEY
3767 BLOCK". All previous versions ignore the implied data type, and
3768 look directly at the packet data type.
3770 * PGP 2.0 through 2.5 generated V2 Public Key Packets. These are
3771 identical to the deprecated V3 keys except for the version
3772 number. An implementation MUST NOT generate them and may accept
3773 or reject them as it sees fit. Similarly, these versions
3774 generated V2 PKESK packets (Tag 1). An implementation may accept
3775 or reject V2 PKESK packets as it sees fit, and MUST NOT generate
3776 them.
3778 * PGP 2.6.x will not accept key-material packets with versions
3779 greater than 3.
3781 * There are many ways possible for two keys to have the same key
3782 material, but different fingerprints (and thus key IDs). Perhaps
3783 the most interesting is an RSA key that has been "upgraded" to
3784 V4 format, but since a V4 fingerprint is constructed by hashing
3785 the key creation time along with other things, two V4 keys
3786 created at different times, yet with the same key material will
3787 have different fingerprints.
3789 * If an implementation is using zlib to interoperate with PGP 2.x,
3790 then the "windowBits" parameter should be set to -13.
3792 * PGP 2.6.X and 5.0 do not trim trailing whitespace from a
3793 "canonical text" signature. They only remove it from cleartext
3794 signatures. These signatures are not OpenPGP compliant --
3795 OpenPGP requires trimming the whitespace. If you wish to
3796 interoperate with PGP 2.6.X or PGP 5, you may wish to accept
3797 these non-compliant signatures.
3799 15. Authors and Working Group Chair
3801 The working group can be contacted via the current chair:
3803 Derek Atkins
3804 IHTFP Consulting, Inc.
3805 6 Farragut Ave
3806 Somerville, MA 02144 USA
3808 Callas, et al. Expires May 23, 2005 [Page 68]
3809 INTERNET-DRAFT OpenPGP Message Format Nov 23, 2004
3811 Email: derek@ihtfp.com
3812 Tel: +1 617 623 3745
3814 The principal authors of this draft are:
3816 Jon Callas
3818 Email: jon@callas.org
3819 Tel: +1 (408) 448-6801
3821 Lutz Donnerhacke
3822 IKS GmbH
3823 Wildenbruchstr. 15
3824 07745 Jena, Germany
3826 EMail: lutz@iks-jena.de
3827 Tel: +49-3641-675642
3829 Hal Finney
3830 Network Associates, Inc.
3831 3965 Freedom Circle
3832 Santa Clara, CA 95054, USA
3834 Email: hal@finney.org
3836 Rodney Thayer
3838 Email: rodney@tillerman.to
3840 This memo also draws on much previous work from a number of other
3841 authors who include: Derek Atkins, Charles Breed, Dave Del Torto,
3842 Marc Dyksterhouse, Gail Haspert, Gene Hoffman, Paul Hoffman, Raph
3843 Levien, Colin Plumb, Will Price, David Shaw, William Stallings, Mark
3844 Weaver, and Philip R. Zimmermann.
3846 16. References (Normative)
3849 [AES] Advanced Encryption Standards Questions and Answers
3850 <http://csrc.nist.gov/encryption/aes/round2/
3851 aesfact.html>
3853 <http://csrc.nist.gov/encryption/aes/round2/
3854 r2algs.html#Rijndael>
3856 [BLOWFISH] Schneier, B. "Description of a New Variable-Length
3857 Key, 64-Bit Block Cipher (Blowfish)" Fast Software
3858 Encryption, Cambridge Security Workshop Proceedings
3859 (December 1993), Springer-Verlag, 1994, pp191-204
3860 <http://www.counterpane.com/bfsverlag.html>
3864 Callas, et al. Expires May 23, 2005 [Page 69]
3865 INTERNET-DRAFT OpenPGP Message Format Nov 23, 2004
3867 [BZ2] J. Seward, jseward@acm.org, "The Bzip2 and libbzip2
3868 home page"
3869 <http://sources.redhat.com/bzip2/>
3870 [ELGAMAL] T. Elgamal, "A Public-Key Cryptosystem and a
3871 Signature Scheme Based on Discrete Logarithms,"
3872 IEEE Transactions on Information Theory, v. IT-31,
3873 n. 4, 1985, pp. 469-472.
3874 [IDEA] Lai, X, "On the design and security of block
3875 ciphers", ETH Series in Information Processing,
3876 J.L. Massey (editor), Vol. 1, Hartung-Gorre Verlag
3877 Knostanz, Technische Hochschule (Zurich), 1992
3878 [ISO10646] ISO/IEC 10646-1:1993. International Standard --
3879 Information technology -- Universal Multiple-Octet
3880 Coded Character Set (UCS) -- Part 1: Architecture
3881 and Basic Multilingual Plane.
3882 [JFIF] JPEG File Interchange Format (Version 1.02).
3883 Eric Hamilton, C-Cube Microsystems, Milpitas, CA,
3884 September 1, 1992.
3886 [MENEZES] Alfred Menezes, Paul van Oorschot, and Scott
3887 Vanstone, "Handbook of Applied Cryptography," CRC
3888 Press, 1996.
3889 [RFC822] Crocker, D., "Standard for the format of ARPA
3890 Internet text messages", STD 11, RFC 822, August
3891 1982.
3892 [RFC1423] Balenson, D., "Privacy Enhancement for Internet
3893 Electronic Mail: Part III: Algorithms, Modes, and
3894 Identifiers", RFC 1423, October 1993.
3895 [RFC1641] Goldsmith, D. and M. Davis, "Using Unicode with
3896 MIME", RFC 1641, July 1994.
3897 [RFC1750] Eastlake, D., Crocker, S. and J. Schiller,
3898 "Randomness Recommendations for Security", RFC
3899 1750, December 1994.
3900 [RFC1951] Deutsch, P., "DEFLATE Compressed Data Format
3901 Specification version 1.3.", RFC 1951, May 1996.
3902 [RFC1991] Atkins, D., Stallings, W. and P. Zimmermann, "PGP
3903 Message Exchange Formats", RFC 1991, August 1996.
3904 [RFC2045] Borenstein, N. and N. Freed, "Multipurpose Internet
3905 Mail Extensions (MIME) Part One: Format of Internet
3906 Message Bodies.", RFC 2045, November 1996.
3907 [RFC2144] Adams, C., "The CAST-128 Encryption Algorithm", RFC
3908 2144, May 1997.
3909 [RFC2279] Yergeau., F., "UTF-8, a transformation format of
3910 Unicode and ISO 10646", RFC 2279, January 1998.
3911 [RFC2437] B. Kaliski and J. Staddon, " PKCS #1: RSA
3912 Cryptography Specifications Version 2.0",
3913 RFC 2437, October 1998.
3914 [RFC3156] M. Elkins, D. Del Torto, R. Levien, T. Roessler,
3915 "MIME Security with OpenPGP", RFC 3156,
3916 August 2001.
3917 [SCHNEIER] Schneier, B., "Applied Cryptography Second Edition:
3918 protocols, algorithms, and source code in C", 1996.
3920 Callas, et al. Expires May 23, 2005 [Page 70]
3921 INTERNET-DRAFT OpenPGP Message Format Nov 23, 2004
3923 [TWOFISH] B. Schneier, J. Kelsey, D. Whiting, D. Wagner, C.
3924 Hall, and N. Ferguson, "The Twofish Encryption
3925 Algorithm", John Wiley & Sons, 1999.
3927 17. References (Non-Normative)
3930 [BLEICHENBACHER] Bleichenbacher, Daniel, "Generating Elgamal
3931 signatures without knowing the secret key,"
3932 Eurocrypt 96. Note that the version in the
3933 proceedings has an error. A revised version is
3934 available at the time of writing from
3935 <ftp://ftp.inf.ethz.ch/pub/publications/papers/ti
3936 /isc/ElGamal.ps>
3937 [DONNERHACKE] Donnerhacke, L., et. al, "PGP263in - an improved
3938 international version of PGP", ftp://ftp.iks-
3939 jena.de/mitarb/lutz/crypt/software/pgp/
3940 [JKS02] Kahil Jallad, Jonathan Katz, Bruce Schneier
3941 "Implementation of Chosen-Ciphertext Attacks
3942 against PGP and GnuPG"
3943 http://www.counterpane.com/pgp-attack.html
3945 [RFC1983] Malkin, G., "Internet Users' Glossary", FYI 18, RFC
3946 1983, August 1996.
3947 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
3948 Requirement Level", BCP 14, RFC 2119, March 1997.
3952 18. Full Copyright Statement
3954 Copyright 2004 by The Internet Society. All Rights Reserved.
3956 This document is subject to the rights, licenses and restrictions
3957 contained in BCP 78, and except as set forth therein, the authors
3958 retain all their rights.
3960 This document and the information contained herein are provided on
3961 an "AS IS" basis and the contributor, the organization he/she
3962 represents or is sponsored by (if any), the internet society and the
3963 internet engineering task force disclaim all warranties, express or
3964 implied, including but not limited to any warranty that the use of
3965 the information herein will not infringe any rights or any implied
3966 warranties of merchantability or fitness for a particular purpose.
3968 This document and translations of it may be copied and furnished to
3969 others, and derivative works that comment on or otherwise explain it
3970 or assist in its implementation may be prepared, copied, published
3971 and distributed, in whole or in part, without restriction of any
3972 kind, provided that the above copyright notice and this paragraph
3973 are included on all such copies and derivative works. However, this
3974 document itself may not be modified in any way, such as by removing
3976 Callas, et al. Expires May 23, 2005 [Page 71]
3977 INTERNET-DRAFT OpenPGP Message Format Nov 23, 2004
3979 the copyright notice or references to the Internet Society or other
3980 Internet organizations, except as needed for the purpose of
3981 developing Internet standards in which case the procedures for
3982 copyrights defined in the Internet Standards process must be
3983 followed, or as required to translate it into languages other than
3984 English.
3986 The limited permissions granted above are perpetual and will not be
3987 revoked by the Internet Society or its successors or assigns.
4032 Callas, et al. Expires May 23, 2005 [Page 72]