[netbsd-mini2440.git] / crypto / external / bsd / netpgp / dist / ref / draft-ietf-openpgp-rfc2440bis-15.txt
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1 Network Working Group Jon Callas
2 Category: INTERNET-DRAFT PGP Corporation
3 draft-ietf-openpgp-rfc2440bis-15.txt
4 Expires April 2006 Lutz Donnerhacke
5 October 2005
7 Obsoletes: 1991, 2440 Hal Finney
8 PGP Corporation
10 Rodney Thayer
12 OpenPGP Message Format
13 draft-ietf-openpgp-rfc2440bis-15.txt
16 Copyright (C) The Internet Society (2005).
18 IPR Claim Notice
20 By submitting this Internet-Draft, each author represents that any
21 applicable patent or other IPR claims of which he or she is aware
22 have been or will be disclosed, and any of which he or she becomes
23 aware will be disclosed, in accordance with Section 6 of BCP 79.
25 The IETF takes no position regarding the validity or scope of any
26 Intellectual Property Rights or other rights that might be claimed
27 to pertain to the implementation or use of the technology described
28 in this document or the extent to which any license under such
29 rights might or might not be available; nor does it represent that
30 it has made any independent effort to identify any such rights.
31 Information on the procedures with respect to rights in RFC
32 documents can be found in BCP 78 and BCP 79.
34 Copies of IPR disclosures made to the IETF Secretariat and any
35 assurances of licenses to be made available, or the result of an
36 attempt made to obtain a general license or permission for the use
37 of such proprietary rights by implementers or users of this
38 specification can be obtained from the IETF on-line IPR repository
39 at http://www.ietf.org/ipr.
41 The IETF invites any interested party to bring to its attention any
42 copyrights, patents or patent applications, or other proprietary
43 rights that may cover technology that may be required to implement
44 this standard. Please address the information to the IETF at
45 ietf-ipr@ietf.org.
47 Status of this Memo
49 Internet-Drafts are working documents of the Internet Engineering
50 Task Force (IETF), its areas, and its working groups. Note that
51 other groups may also distribute working documents as
52 Internet-Drafts.
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59 Internet-Drafts are draft documents valid for a maximum of six
60 months and may be updated, replaced, or obsoleted by other documents
61 at any time. It is inappropriate to use Internet-Drafts as
62 reference material or to cite them other than as "work in progress."
64 The list of current Internet-Drafts can be accessed at
65 http://www.ietf.org/ietf/1id-abstracts.txt
67 The list of Internet-Draft Shadow Directories can be accessed at
68 http://www.ietf.org/shadow.html.
70 IANA Considerations
72 This document defines many tag values, yet it doesn't describe a
73 mechanism for adding new tags (for new features). Traditionally the
74 Internet Assigned Numbers Authority (IANA) handles the allocation of
75 new values for future expansion and RFCs usually define the
76 procedure to be used by the IANA. However there are subtle (and not
77 so subtle) interactions that may occur in this protocol between new
78 features and existing features which result in a significant
79 reduction in over all security. Therefore this document does not
80 define an extension procedure. Instead requests to define new tag
81 values (say for new encryption algorithms for example) should be
82 forwarded to the IESG Security Area Directors for consideration or
83 forwarding to the appropriate IETF Working Group for consideration.
85 Abstract
87 This document is maintained in order to publish all necessary
88 information needed to develop interoperable applications based on
89 the OpenPGP format. It is not a step-by-step cookbook for writing an
90 application. It describes only the format and methods needed to
91 read, check, generate, and write conforming packets crossing any
92 network. It does not deal with storage and implementation questions.
93 It does, however, discuss implementation issues necessary to avoid
94 security flaws.
96 OpenPGP software uses a combination of strong public-key and
97 symmetric cryptography to provide security services for electronic
98 communications and data storage. These services include
99 confidentiality, key management, authentication, and digital
100 signatures. This document specifies the message formats used in
101 OpenPGP.
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115 Table of Contents
117 IPR Claim Notice 1
118 Status of this Memo 1
119 IANA Considerations 2
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 21
161 5.2.3. Version 4 Signature Packet Format 23
162 5.2.3.1. Signature Subpacket Specification 24
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 27
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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 URI 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) 37
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) 38
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 40
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) 46
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" 49
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 54
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 56
230 9.2. Symmetric Key Algorithms 56
231 9.3. Compression Algorithms 57
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 60
238 11.1. Key Structures 60
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 64
249 12.7. Reserved Algorithm Numbers 64
250 12.8. OpenPGP CFB mode 64
251 13. Security Considerations 65
252 14. Implementation Nits 68
253 15. Authors' Addresses 69
254 16. References (Normative) 70
255 17. References (Non-Normative) 72
256 18. Full Copyright Statement 72
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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 RFC 2440, "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, RFC 1991, 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, RFC
306 1991. 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 RFC 2119, 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 OpenPGP messages in the world are
418 compressed. Thus, it may even be wise for a space-constrained
419 implementation to 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 Unused bits of an MPI MUST be zero.
497 Also note that when an MPI is encrypted, the length refers to the
498 plaintext MPI. It may be ill-formed in its ciphertext.
500 3.3. Key IDs
502 A Key ID is an eight-octet scalar that identifies a key.
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507 Implementations SHOULD NOT assume that Key IDs are unique. The
508 section, "Enhanced Key Formats" below describes how Key IDs are
509 formed.
511 3.4. Text
513 Unless otherwise specified, the character set for text is the UTF-8
514 [RFC2279] encoding of Unicode [ISO10646].
516 3.5. Time fields
518 A time field is an unsigned four-octet number containing the number
519 of seconds elapsed since midnight, 1 January 1970 UTC.
521 3.6. Keyrings
523 A keyring is a collection of one or more keys in a file or database.
524 Traditionally, a keyring is simply a sequential list of keys, but
525 may be any suitable database. It is beyond the scope of this
526 standard to discuss the details of keyrings or other databases.
528 3.7. String-to-key (S2K) specifiers
530 String-to-key (S2K) specifiers are used to convert passphrase
531 strings into symmetric-key encryption/decryption keys. They are
532 used in two places, currently: to encrypt the secret part of private
533 keys in the private keyring, and to convert passphrases to
534 encryption keys for symmetrically encrypted messages.
536 3.7.1. String-to-key (S2K) specifier types
538 There are three types of S2K specifiers currently supported, and
539 some reserved values:
541 ID S2K Type
542 -- --- ----
543 0 Simple S2K
544 1 Salted S2K
545 2 Illegal value
546 3 Iterated and Salted S2K
547 100 to 110 Private/Experimental S2K
549 These are described as follows:
551 3.7.1.1. Simple S2K
553 This directly hashes the string to produce the key data. See below
554 for how this hashing is done.
556 Octet 0: 0x00
557 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 packet
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 -- packet tag
745 Bits 1-0 - length-type
747 New format packets contain:
749 Bits 5-0 -- packet 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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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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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. See
999 section 5.2.4, "Computing Signatures," for detailed information on
1000 how to compute and verify signatures of each type.
1002 These meanings are:
1004 0x00: Signature of a binary document.
1005 This means the signer owns it, created it, or certifies that it
1006 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.
1027 0x11: Persona certification of a User ID and Public Key packet.
1028 The issuer of this certification has not done any verification
1029 of the claim that the owner of this key is the User ID
1030 specified.
1032 0x12: Casual certification of a User ID and Public Key packet.
1033 The issuer of this certification has done some casual
1034 verification of the claim of identity.
1036 0x13: Positive certification of a User ID and Public Key packet.
1037 The issuer of this certification has done substantial
1038 verification of the claim of identity.
1040 Please note that the vagueness of these certification claims is
1041 not a flaw, but a feature of the system. Because OpenPGP places
1042 final authority for validity upon the receiver of a
1043 certification, it may be that one authority's casual
1044 certification might be more rigorous than some other authority's
1045 positive certification. These classifications allow a
1046 certification authority to issue fine-grained claims.
1048 Most OpenPGP implementations make their "key signatures" as 0x10
1049 certifications. Some implementations can issue 0x11-0x13
1050 certifications, but few differentiate between the types.
1052 0x18: Subkey Binding Signature
1053 This signature is a statement by the top-level signing key that
1054 indicates that it owns the subkey. This signature is calculated
1055 directly on the subkey itself, not on any User ID or other
1056 packets. A signature that binds a signing subkey SHOULD have an
1057 embedded signature subpacket in this binding signature which
1058 contains a 0x19 signature made by the signing subkey on the
1059 primary key.
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1067 0x19 Primary Key Binding Signature
1068 This signature is a statement by a signing subkey, indicating
1069 that it is owned by the primary key. This signature is
1070 calculated directly on the primary key itself, and not on any
1071 User ID or other packets.
1073 0x1F: Signature directly on a key
1074 This signature is calculated directly on a key. It binds the
1075 information in the signature subpackets to the key, and is
1076 appropriate to be used for subpackets that provide information
1077 about the key, such as the revocation key subpacket. It is also
1078 appropriate for statements that non-self certifiers want to make
1079 about the key itself, rather than the binding between a key and
1080 a name.
1082 0x20: Key revocation signature
1083 The signature is calculated directly on the key being revoked.
1084 A revoked key is not to be used. Only revocation signatures by
1085 the key being revoked, or by an authorized revocation key,
1086 should be considered valid revocation signatures.
1088 0x28: Subkey revocation signature
1089 The signature is calculated directly on the subkey being
1090 revoked. A revoked subkey is not to be used. Only revocation
1091 signatures by the top-level signature key that is bound to this
1092 subkey, or by an authorized revocation key, should be considered
1093 valid revocation signatures.
1095 0x30: Certification revocation signature
1096 This signature revokes an earlier User ID certification
1097 signature (signature class 0x10 through 0x13) or direct-key
1098 signature (0x1F). It should be issued by the same key that
1099 issued the revoked signature or an authorized revocation key.
1100 The signature is computed over the same data as the certificate
1101 that it revokes, and should have a later creation date than that
1102 certificate.
1104 0x40: Timestamp signature.
1105 This signature is only meaningful for the timestamp contained in
1106 it.
1108 0x50: Third-Party Confirmation signature.
1109 This signature is a signature over some other OpenPGP signature
1110 packet(s). It is analogous to a notary seal on the signed data.
1111 A third-party signature SHOULD include Signature Target
1112 subpacket(s) to give easy identification. Note that we really do
1113 mean SHOULD. There are plausible uses for this (such as a blind
1114 party that only sees the signature, not the key nor source
1115 document) that cannot include a target subpacket.
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1123 5.2.2. Version 3 Signature Packet Format
1125 The body of a version 3 Signature Packet contains:
1127 - One-octet version number (3).
1129 - One-octet length of following hashed material. MUST be 5.
1131 - One-octet signature type.
1133 - Four-octet creation time.
1135 - Eight-octet key ID of signer.
1137 - One-octet public key algorithm.
1139 - One-octet hash algorithm.
1141 - Two-octet field holding left 16 bits of signed hash value.
1143 - One or more multiprecision integers comprising the signature.
1144 This portion is algorithm specific, as described below.
1146 The concatenation of the data to be signed, the signature type and
1147 creation time from the signature packet (5 additional octets) is
1148 hashed. The resulting hash value is used in the signature algorithm.
1149 The high 16 bits (first two octets) of the hash are included in the
1150 signature packet to provide a quick test to reject some invalid
1151 signatures.
1153 Algorithm Specific Fields for RSA signatures:
1155 - multiprecision integer (MPI) of RSA signature value m**d mod n.
1157 Algorithm Specific Fields for DSA signatures:
1159 - MPI of DSA value r.
1161 - MPI of DSA value s.
1163 The signature calculation is based on a hash of the signed data, as
1164 described above. The details of the calculation are different for
1165 DSA signature than for RSA signatures.
1167 The hash h is PKCS-1 padded exactly the same way as for the above
1168 described RSA signatures.
1170 With RSA signatures, the hash value is encoded as described in
1171 PKCS-1 section 9.2.1 encoded using PKCS-1 encoding type
1172 EMSA-PKCS1-v1_5 [RFC2437]. This requires inserting the hash value
1173 as an octet string into an ASN.1 structure. The object identifier
1174 for the type of hash being used is included in the structure. The
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1179 hexadecimal representations for the currently defined hash
1180 algorithms are:
1182 - MD5: 0x2A, 0x86, 0x48, 0x86, 0xF7, 0x0D, 0x02, 0x05
1184 - RIPEMD-160: 0x2B, 0x24, 0x03, 0x02, 0x01
1186 - SHA-1: 0x2B, 0x0E, 0x03, 0x02, 0x1A
1188 - SHA256: 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x01
1190 - SHA384: 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x02
1192 - SHA512: 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x03
1194 The ASN.1 OIDs are:
1196 - MD5: 1.2.840.113549.2.5
1198 - RIPEMD-160: 1.3.36.3.2.1
1200 - SHA-1: 1.3.14.3.2.26
1202 - SHA256: 2.16.840.1.101.3.4.2.1
1204 - SHA384: 2.16.840.1.101.3.4.2.2
1206 - SHA512: 2.16.840.1.101.3.4.2.3
1208 The full hash prefixes for these are:
1210 MD5: 0x30, 0x20, 0x30, 0x0C, 0x06, 0x08, 0x2A, 0x86,
1211 0x48, 0x86, 0xF7, 0x0D, 0x02, 0x05, 0x05, 0x00,
1212 0x04, 0x10
1214 RIPEMD-160: 0x30, 0x21, 0x30, 0x09, 0x06, 0x05, 0x2B, 0x24,
1215 0x03, 0x02, 0x01, 0x05, 0x00, 0x04, 0x14
1217 SHA-1: 0x30, 0x21, 0x30, 0x09, 0x06, 0x05, 0x2b, 0x0E,
1218 0x03, 0x02, 0x1A, 0x05, 0x00, 0x04, 0x14
1220 SHA256: 0x30, 0x31, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86,
1221 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x01, 0x05,
1222 0x00, 0x04, 0x20
1224 SHA384: 0x30, 0x41, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86,
1225 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x02, 0x05,
1226 0x00, 0x04, 0x30
1228 SHA512: 0x30, 0x51, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86,
1229 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x03, 0x05,
1230 0x00, 0x04, 0x40
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1235 DSA signatures MUST use hashes with a size of 160 bits, to match q,
1236 the size of the group generated by the DSA key's generator value.
1237 The hash function result is treated as a 160 bit number and used
1238 directly in the DSA signature algorithm.
1240 5.2.3. Version 4 Signature Packet Format
1242 The body of a version 4 Signature Packet contains:
1244 - One-octet version number (4).
1246 - One-octet signature type.
1248 - One-octet public key algorithm.
1250 - One-octet hash algorithm.
1252 - Two-octet scalar octet count for following hashed subpacket
1253 data. Note that this is the length in octets of all of the
1254 hashed subpackets; a pointer incremented by this number will
1255 skip over the hashed subpackets.
1257 - Hashed subpacket data set. (zero or more subpackets)
1259 - Two-octet scalar octet count for the following unhashed
1260 subpacket data. Note that this is the length in octets of all of
1261 the unhashed subpackets; a pointer incremented by this number
1262 will skip over the unhashed subpackets.
1264 - Unhashed subpacket data set. (zero or more subpackets)
1266 - Two-octet field holding the left 16 bits of the signed hash
1267 value.
1269 - One or more multiprecision integers comprising the signature.
1270 This portion is algorithm specific, as described above.
1272 The concatenation of the data being signed and the signature data
1273 from the version number through the hashed subpacket data
1274 (inclusive) is hashed. The resulting hash value is what is signed.
1275 The left 16 bits of the hash are included in the signature packet to
1276 provide a quick test to reject some invalid signatures.
1278 There are two fields consisting of signature subpackets. The first
1279 field is hashed with the rest of the signature data, while the
1280 second is unhashed. The second set of subpackets is not
1281 cryptographically protected by the signature and should include only
1282 advisory information.
1284 The algorithms for converting the hash function result to a
1285 signature are described in a section below.
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1291 5.2.3.1. Signature Subpacket Specification
1293 A subpacket data set consists of zero or more signature subpackets.
1294 In signature packets the subpacket data set is preceded by a
1295 two-octet scalar count of the length in octets of all the
1296 subpackets. A pointer incremented by this number will skip over the
1297 subpacket data set.
1299 Each subpacket consists of a subpacket header and a body. The
1300 header consists of:
1302 - the subpacket length (1, 2, or 5 octets)
1304 - the subpacket type (1 octet)
1306 and is followed by the subpacket specific data.
1308 The length includes the type octet but not this length. Its format
1309 is similar to the "new" format packet header lengths, but cannot
1310 have partial body lengths. That is:
1312 if the 1st octet < 192, then
1313 lengthOfLength = 1
1314 subpacketLen = 1st_octet
1316 if the 1st octet >= 192 and < 255, then
1317 lengthOfLength = 2
1318 subpacketLen = ((1st_octet - 192) << 8) + (2nd_octet) + 192
1320 if the 1st octet = 255, then
1321 lengthOfLength = 5
1322 subpacket length = [four-octet scalar starting at 2nd_octet]
1324 The value of the subpacket type octet may be:
1326 2 = signature creation time
1327 3 = signature expiration time
1328 4 = exportable certification
1329 5 = trust signature
1330 6 = regular expression
1331 7 = revocable
1332 9 = key expiration time
1333 10 = placeholder for backward compatibility
1334 11 = preferred symmetric algorithms
1335 12 = revocation key
1336 16 = issuer key ID
1337 20 = notation data
1338 21 = preferred hash algorithms
1339 22 = preferred compression algorithms
1340 23 = key server preferences
1341 24 = preferred key server
1342 25 = primary User ID
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1347 26 = policy URI
1348 27 = key flags
1349 28 = signer's User ID
1350 29 = reason for revocation
1351 30 = features
1352 31 = signature target
1353 32 = embedded signature
1355 100 to 110 = internal or user-defined
1357 An implementation SHOULD ignore any subpacket of a type that it does
1358 not recognize.
1360 Bit 7 of the subpacket type is the "critical" bit. If set, it
1361 denotes that the subpacket is one that is critical for the evaluator
1362 of the signature to recognize. If a subpacket is encountered that
1363 is marked critical but is unknown to the evaluating software, the
1364 evaluator SHOULD consider the signature to be in error.
1366 An evaluator may "recognize" a subpacket, but not implement it. The
1367 purpose of the critical bit is to allow the signer to tell an
1368 evaluator that it would prefer a new, unknown feature to generate an
1369 error than be ignored.
1371 Implementations SHOULD implement "preferences" and the "reason for
1372 revocation" subpackets. Note, however, that if an implementation
1373 chooses not to implement some of the preferences, it is required to
1374 behave in a polite manner to respect the wishes of those users who
1375 do implement these preferences.
1377 5.2.3.2. Signature Subpacket Types
1379 A number of subpackets are currently defined. Some subpackets apply
1380 to the signature itself and some are attributes of the key.
1381 Subpackets that are found on a self-signature are placed on a
1382 certification made by the key itself. Note that a key may have more
1383 than one User ID, and thus may have more than one self-signature,
1384 and differing subpackets.
1386 A subpacket may be found either in the hashed or unhashed subpacket
1387 sections of a signature. If a subpacket is not hashed, then the
1388 information in it cannot be considered definitive because it is not
1389 part of the signature proper.
1391 5.2.3.3. Notes on Self-Signatures
1393 A self-signature is a binding signature made by the key the
1394 signature refers to. There are three types of self-signatures, the
1395 certification signatures (types 0x10-0x13), the direct-key signature
1396 (type 0x1f), and the subkey binding signature (type 0x18). For
1397 certification self-signatures, each User ID may have a
1398 self-signature, and thus different subpackets in those
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1403 self-signatures. For subkey binding signatures, each subkey in fact
1404 has a self-signature. Subpackets that appear in a certification
1405 self-signature apply to the username, and subpackets that appear in
1406 the subkey self-signature apply to the subkey. Lastly, subpackets on
1407 the direct-key signature apply to the entire key.
1409 Implementing software should interpret a self-signature's preference
1410 subpackets as narrowly as possible. For example, suppose a key has
1411 two usernames, Alice and Bob. Suppose that Alice prefers the
1412 symmetric algorithm CAST5, and Bob prefers IDEA or TripleDES. If the
1413 software locates this key via Alice's name, then the preferred
1414 algorithm is CAST5, if software locates the key via Bob's name, then
1415 the preferred algorithm is IDEA. If the key is located by key ID,
1416 the algorithm of the primary User ID of the key provides the default
1417 symmetric algorithm.
1419 Revoking a self-signature or allowing it to expire has a semantic
1420 meaning that varies with the signature type. Revoking the
1421 self-signature on a User ID effectively retires that user name. The
1422 self-signature is a statement, "My name X is tied to my signing key
1423 K" and is corroborated by other users' certifications. If another
1424 user revokes their certification, they are effectively saying that
1425 they no longer believe that name and that key are tied together.
1426 Similarly, if the user themselves revokes their self-signature, it
1427 means the user no longer goes by that name, no longer has that email
1428 address, etc. Revoking a binding signature effectively retires that
1429 subkey. Revoking a direct-key signature cancels that signature.
1430 Please see the "Reason for Revocation" subpacket below for more
1431 relevant detail.
1433 Since a self-signature contains important information about the
1434 key's use, an implementation SHOULD allow the user to rewrite the
1435 self-signature, and important information in it, such as preferences
1436 and key expiration.
1438 It is good practice to verify that a self-signature imported into an
1439 implementation doesn't advertise features that the implementation
1440 doesn't support, rewriting the signature as appropriate.
1442 An implementation that encounters multiple self-signatures on the
1443 same object may resolve the ambiguity in any way it sees fit, but it
1444 is RECOMMENDED that priority be given to the most recent
1445 self-signature.
1447 5.2.3.4. Signature creation time
1449 (4 octet time field)
1451 The time the signature was made.
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1459 MUST be present in the hashed area.
1461 5.2.3.5. Issuer
1463 (8 octet key ID)
1465 The OpenPGP key ID of the key issuing the signature.
1467 5.2.3.6. Key expiration time
1469 (4 octet time field)
1471 The validity period of the key. This is the number of seconds after
1472 the key creation time that the key expires. If this is not present
1473 or has a value of zero, the key never expires. This is found only on
1474 a self-signature.
1476 5.2.3.7. Preferred symmetric algorithms
1478 (array of one-octet values)
1480 Symmetric algorithm numbers that indicate which algorithms the key
1481 holder prefers to use. The subpacket body is an ordered list of
1482 octets with the most preferred listed first. It is assumed that only
1483 algorithms listed are supported by the recipient's software.
1484 Algorithm numbers in section 9. This is only found on a
1485 self-signature.
1487 5.2.3.8. Preferred hash algorithms
1489 (array of one-octet values)
1491 Message digest algorithm numbers that indicate which algorithms the
1492 key holder prefers to receive. Like the preferred symmetric
1493 algorithms, the list is ordered. Algorithm numbers are in section 9.
1494 This is only found on a self-signature.
1496 5.2.3.9. Preferred compression algorithms
1498 (array of one-octet values)
1500 Compression algorithm numbers that indicate which algorithms the key
1501 holder prefers to use. Like the preferred symmetric algorithms, the
1502 list is ordered. Algorithm numbers are in section 9. If this
1503 subpacket is not included, ZIP is preferred. A zero denotes that
1504 uncompressed data is preferred; the key holder's software might have
1505 no compression software in that implementation. This is only found
1506 on a self-signature.
1508 5.2.3.10. Signature expiration time
1510 (4 octet time field)
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1515 The validity period of the signature. This is the number of seconds
1516 after the signature creation time that the signature expires. If
1517 this is not present or has a value of zero, it never expires.
1519 5.2.3.11. Exportable Certification
1521 (1 octet of exportability, 0 for not, 1 for exportable)
1523 This subpacket denotes whether a certification signature is
1524 "exportable," to be used by other users than the signature's issuer.
1525 The packet body contains a Boolean flag indicating whether the
1526 signature is exportable. If this packet is not present, the
1527 certification is exportable; it is equivalent to a flag containing a
1528 1.
1530 Non-exportable, or "local," certifications are signatures made by a
1531 user to mark a key as valid within that user's implementation only.
1532 Thus, when an implementation prepares a user's copy of a key for
1533 transport to another user (this is the process of "exporting" the
1534 key), any local certification signatures are deleted from the key.
1536 The receiver of a transported key "imports" it, and likewise trims
1537 any local certifications. In normal operation, there won't be any,
1538 assuming the import is performed on an exported key. However, there
1539 are instances where this can reasonably happen. For example, if an
1540 implementation allows keys to be imported from a key database in
1541 addition to an exported key, then this situation can arise.
1543 Some implementations do not represent the interest of a single user
1544 (for example, a key server). Such implementations always trim local
1545 certifications from any key they handle.
1547 5.2.3.12. Revocable
1549 (1 octet of revocability, 0 for not, 1 for revocable)
1551 Signature's revocability status. Packet body contains a Boolean
1552 flag indicating whether the signature is revocable. Signatures that
1553 are not revocable have any later revocation signatures ignored.
1554 They represent a commitment by the signer that he cannot revoke his
1555 signature for the life of his key. If this packet is not present,
1556 the signature is revocable.
1558 5.2.3.13. Trust signature
1560 (1 octet "level" (depth), 1 octet of trust amount)
1562 Signer asserts that the key is not only valid, but also trustworthy,
1563 at the specified level. Level 0 has the same meaning as an ordinary
1564 validity signature. Level 1 means that the signed key is asserted
1565 to be a valid trusted introducer, with the 2nd octet of the body
1566 specifying the degree of trust. Level 2 means that the signed key is
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1571 asserted to be trusted to issue level 1 trust signatures, i.e. that
1572 it is a "meta introducer". Generally, a level n trust signature
1573 asserts that a key is trusted to issue level n-1 trust signatures.
1574 The trust amount is in a range from 0-255, interpreted such that
1575 values less than 120 indicate partial trust and values of 120 or
1576 greater indicate complete trust. Implementations SHOULD emit values
1577 of 60 for partial trust and 120 for complete trust.
1579 5.2.3.14. Regular expression
1581 (null-terminated regular expression)
1583 Used in conjunction with trust signature packets (of level > 0) to
1584 limit the scope of trust that is extended. Only signatures by the
1585 target key on User IDs that match the regular expression in the body
1586 of this packet have trust extended by the trust signature subpacket.
1587 The regular expression uses the same syntax as the Henry Spencer's
1588 "almost public domain" regular expression package. A description of
1589 the syntax is found in a section below.
1591 5.2.3.15. Revocation key
1593 (1 octet of class, 1 octet of algid, 20 octets of fingerprint)
1595 Authorizes the specified key to issue revocation signatures for this
1596 key. Class octet must have bit 0x80 set. If the bit 0x40 is set,
1597 then this means that the revocation information is sensitive. Other
1598 bits are for future expansion to other kinds of authorizations. This
1599 is found on a self-signature.
1601 If the "sensitive" flag is set, the keyholder feels this subpacket
1602 contains private trust information that describes a real-world
1603 sensitive relationship. If this flag is set, implementations SHOULD
1604 NOT export this signature to other users except in cases where the
1605 data needs to be available: when the signature is being sent to the
1606 designated revoker, or when it is accompanied by a revocation
1607 signature from that revoker. Note that it may be appropriate to
1608 isolate this subpacket within a separate signature so that it is not
1609 combined with other subpackets that need to be exported.
1611 5.2.3.16. Notation Data
1613 (4 octets of flags, 2 octets of name length (M),
1614 2 octets of value length (N),
1615 M octets of name data,
1616 N octets of value data)
1618 This subpacket describes a "notation" on the signature that the
1619 issuer wishes to make. The notation has a name and a value, each of
1620 which are strings of octets. There may be more than one notation in
1621 a signature. Notations can be used for any extension the issuer of
1622 the signature cares to make. The "flags" field holds four octets of
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1627 flags.
1629 All undefined flags MUST be zero. Defined flags are:
1631 First octet: 0x80 = human-readable. This note value is text, a
1632 note from one person to another, and need
1633 not have meaning to software.
1634 Other octets: none.
1636 Notation names are arbitrary strings encoded in UTF-8. They reside
1637 two name spaces: The IETF name space and the user name space.
1639 The IETF name space is registered with IANA. These names MUST NOT
1640 contain the "@" character (0x40). This this is a tag for the user
1641 name space.
1643 Names in the user name space consist of a UTF-8 string tag followed
1644 by "@" followed by a DNS domain name. Note that the tag MUST NOT
1645 contain an "@" character. For example, the "sample" tag used by
1646 Example Corporation could be "sample@example.com".
1648 Names in a user space are owned and controlled by the owners of that
1649 domain. Obviously, it's of bad form to create a new name in a DNS
1650 space that you don't own.
1652 Since the user name space is in the form of an email address,
1653 implementers MAY wish to arrange for that address to reach a person
1654 who can be consulted about the use of the named tag. Note that due
1655 to UTF-8 encoding, not all valid user space name tags are valid
1656 email addresses.
1658 If there is a critical notation, the criticality applies to that
1659 specific notation and not to notations in general.
1661 5.2.3.17. Key server preferences
1663 (N octets of flags)
1665 This is a list of one-bit flags that indicate preferences that the
1666 key holder has about how the key is handled on a key server. All
1667 undefined flags MUST be zero.
1669 First octet: 0x80 = No-modify
1670 the key holder requests that this key only be modified or
1671 updated by the key holder or an administrator of the key server.
1673 This is found only on a self-signature.
1675 5.2.3.18. Preferred key server
1677 (String)
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1683 This is a URI of a key server that the key holder prefers be used
1684 for updates. Note that keys with multiple User IDs can have a
1685 preferred key server for each User ID. Note also that since this is
1686 a URI, the key server can actually be a copy of the key retrieved by
1687 ftp, http, finger, etc.
1689 5.2.3.19. Primary User ID
1691 (1 octet, Boolean)
1693 This is a flag in a User ID's self signature that states whether
1694 this User ID is the main User ID for this key. It is reasonable for
1695 an implementation to resolve ambiguities in preferences, etc. by
1696 referring to the primary User ID. If this flag is absent, its value
1697 is zero. If more than one User ID in a key is marked as primary, the
1698 implementation may resolve the ambiguity in any way it sees fit, but
1699 it is RECOMMENDED that priority be given to the User ID with the
1700 most recent self-signature.
1702 When appearing on a self-signature on a User ID packet, this
1703 subpacket applies only to User ID packets. When appearing on a
1704 self-signature on a User Attribute packet, this subpacket applies
1705 only to User Attribute packets. That is to say, there are two
1706 different and independent "primaries" - one for User IDs, and one
1707 for User Attributes.
1709 5.2.3.20. Policy URI
1711 (String)
1713 This subpacket contains a URI of a document that describes the
1714 policy that the signature was issued under.
1716 5.2.3.21. Key Flags
1718 (N octets of flags)
1720 This subpacket contains a list of binary flags that hold information
1721 about a key. It is a string of octets, and an implementation MUST
1722 NOT assume a fixed size. This is so it can grow over time. If a list
1723 is shorter than an implementation expects, the unstated flags are
1724 considered to be zero. The defined flags are:
1726 First octet:
1728 0x01 - This key may be used to certify other keys.
1730 0x02 - This key may be used to sign data.
1732 0x04 - This key may be used to encrypt communications.
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1739 0x08 - This key may be used to encrypt storage.
1741 0x10 - The private component of this key may have been split by
1742 a secret-sharing mechanism.
1744 0x20 - This key may be used for authentication.
1746 0x80 - The private component of this key may be in the
1747 possession of more than one person.
1749 Usage notes:
1751 The flags in this packet may appear in self-signatures or in
1752 certification signatures. They mean different things depending on
1753 who is making the statement -- for example, a certification
1754 signature that has the "sign data" flag is stating that the
1755 certification is for that use. On the other hand, the
1756 "communications encryption" flag in a self-signature is stating a
1757 preference that a given key be used for communications. Note
1758 however, that it is a thorny issue to determine what is
1759 "communications" and what is "storage." This decision is left wholly
1760 up to the implementation; the authors of this document do not claim
1761 any special wisdom on the issue, and realize that accepted opinion
1762 may change.
1764 The "split key" (0x10) and "group key" (0x80) flags are placed on a
1765 self-signature only; they are meaningless on a certification
1766 signature. They SHOULD be placed only on a direct-key signature
1767 (type 0x1f) or a subkey signature (type 0x18), one that refers to
1768 the key the flag applies to.
1770 5.2.3.22. Signer's User ID
1772 (String)
1774 This subpacket allows a keyholder to state which User ID is
1775 responsible for the signing. Many keyholders use a single key for
1776 different purposes, such as business communications as well as
1777 personal communications. This subpacket allows such a keyholder to
1778 state which of their roles is making a signature.
1780 This subpacket is not appropriate to use to refer to a User
1781 Attribute packet.
1783 5.2.3.23. Reason for Revocation
1785 (1 octet of revocation code, N octets of reason string)
1787 This subpacket is used only in key revocation and certification
1788 revocation signatures. It describes the reason why the key or
1789 certificate was revoked.
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1795 The first octet contains a machine-readable code that denotes the
1796 reason for the revocation:
1798 0x00 - No reason specified (key revocations or cert revocations)
1799 0x01 - Key is superseded (key revocations)
1800 0x02 - Key material has been compromised (key revocations)
1801 0x03 - Key is retired and no longer used (key revocations)
1802 0x20 - User ID information is no longer valid (cert revocations)
1804 Following the revocation code is a string of octets which gives
1805 information about the reason for revocation in human-readable form
1806 (UTF-8). The string may be null, that is, of zero length. The length
1807 of the subpacket is the length of the reason string plus one.
1809 An implementation SHOULD implement this subpacket, include it in all
1810 revocation signatures, and interpret revocations appropriately.
1811 There are important semantic differences between the reasons, and
1812 there are thus important reasons for revoking signatures.
1814 If a key has been revoked because of a compromise, all signatures
1815 created by that key are suspect. However, if it was merely
1816 superseded or retired, old signatures are still valid. If the
1817 revoked signature is the self-signature for certifying a User ID, a
1818 revocation denotes that that user name is no longer in use. Such a
1819 revocation SHOULD include an 0x20 subpacket.
1821 Note that any signature may be revoked, including a certification on
1822 some other person's key. There are many good reasons for revoking a
1823 certification signature, such as the case where the keyholder leaves
1824 the employ of a business with an email address. A revoked
1825 certification is no longer a part of validity calculations.
1827 5.2.3.24. Features
1829 (N octets of flags)
1831 The features subpacket denotes which advanced OpenPGP features a
1832 user's implementation supports. This is so that as features are
1833 added to OpenPGP that cannot be backwards-compatible, a user can
1834 state that they can use that feature. The flags are single bits that
1835 indicate that a given feature is supported.
1837 This subpacket is similar to a preferences subpacket, and only
1838 appears in a self-signature.
1840 An implementation SHOULD NOT use a feature listed when sending to a
1841 user who does not state that they can use it.
1843 Defined features are:
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1851 First octet:
1853 0x01 - Modification Detection (packets 18 and 19)
1855 If an implementation implements any of the defined features, it
1856 SHOULD implement the features subpacket, too.
1858 An implementation may freely infer features from other suitable
1859 implementation-dependent mechanisms.
1861 5.2.3.25. Signature Target
1863 (1 octet PK algorithm, 1 octet hash algorithm, N octets hash)
1865 This subpacket identifies a specific target signature that a
1866 signature refers to. For revocation signatures, this subpacket
1867 provides explicit designation of which signature is being revoked.
1868 For a third-party or timestamp signature, this designates what
1869 signature is signed. All arguments are an identifier of that target
1870 signature.
1872 The N octets of hash data MUST be the size of the hash of the
1873 signature. For example, a target signature with a SHA-1 hash MUST
1874 have 20 octets of hash data.
1876 5.2.3.26. Embedded Signature
1878 (1 signature packet body)
1880 This subpacket contains a complete signature packet body as
1881 specified in section 5.2 above. It is useful when one signature
1882 needs to refer to, or be incorporated in, another signature.
1884 5.2.4. Computing Signatures
1886 All signatures are formed by producing a hash over the signature
1887 data, and then using the resulting hash in the signature algorithm.
1889 For binary document signatures (type 0x00), the document data is
1890 hashed directly. For text document signatures (type 0x01), the
1891 document is canonicalized by converting line endings to <CR><LF>,
1892 and the resulting data is hashed.
1894 When a signature is made over a key, the hash data starts with the
1895 octet 0x99, followed by a two-octet length of the key, and then body
1896 of the key packet. (Note that this is an old-style packet header for
1897 a key packet with two-octet length.) A subkey binding signature
1898 (type 0x18) or primary key binding signature (type 0x19) then hashes
1899 the subkey using the same format as the main key (also using 0x99 as
1900 the first octet). Key revocation signatures (types 0x20 and 0x28)
1901 hash only the key being revoked.
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1907 A certification signature (type 0x10 through 0x13) hashes the User
1908 ID being bound to the key into the hash context after the above
1909 data. A V3 certification hashes the contents of the User ID or
1910 attribute packet packet, without any header. A V4 certification
1911 hashes the constant 0xb4 for User ID certifications or the constant
1912 0xd1 for User Attribute certifications, followed by a four-octet
1913 number giving the length of the User ID or User Attribute data, and
1914 then the User ID or User Attribute data.
1916 When a signature is made over a signature packet (type 0x50), the
1917 hash data starts with the octet 0x88, followed by the four-octet
1918 length of the signature, and then the body of the signature packet.
1919 (Note that this is an old-style packet header for a signature packet
1920 with the length-of-length set to zero). The unhashed subpacket data
1921 of the signature packet being hashed is not included in the hash and
1922 the unhashed subpacket data length value is set to zero.
1924 Once the data body is hashed, then a trailer is hashed. A V3
1925 signature hashes five octets of the packet body, starting from the
1926 signature type field. This data is the signature type, followed by
1927 the four-octet signature time. A V4 signature hashes the packet body
1928 starting from its first field, the version number, through the end
1929 of the hashed subpacket data. Thus, the fields hashed are the
1930 signature version, the signature type, the public key algorithm, the
1931 hash algorithm, the hashed subpacket length, and the hashed
1932 subpacket body.
1934 V4 signatures also hash in a final trailer of six octets: the
1935 version of the signature packet, i.e. 0x04; 0xFF; a four-octet,
1936 big-endian number that is the length of the hashed data from the
1937 signature packet (note that this number does not include these final
1938 six octets.
1940 After all this has been hashed in a single hash context the
1941 resulting hash field is used in the signature algorithm, and placed
1942 at the end of the signature packet.
1944 5.2.4.1. Subpacket Hints
1946 It is certainly possible for a signature to contain conflicting
1947 information in subpackets. For example, a signature may contain
1948 multiple copies of a preference or multiple expiration times. In
1949 most cases, an implementation SHOULD use the last subpacket in the
1950 signature, but MAY use any conflict resolution scheme that makes
1951 more sense. Please note that we are intentionally leaving conflict
1952 resolution to the implementer; most conflicts are simply syntax
1953 errors, and the wishy-washy language here allows a receiver to be
1954 generous in what they accept, while putting pressure on a creator to
1955 be stingy in what they generate.
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1963 Some apparent conflicts may actually make sense -- for example,
1964 suppose a keyholder has an V3 key and a V4 key that share the same
1965 RSA key material. Either of these keys can verify a signature
1966 created by the other, and it may be reasonable for a signature to
1967 contain an issuer subpacket for each key, as a way of explicitly
1968 tying those keys to the signature.
1970 5.3. Symmetric-Key Encrypted Session Key Packets (Tag 3)
1972 The Symmetric-Key Encrypted Session Key packet holds the
1973 symmetric-key encryption of a session key used to encrypt a message.
1974 Zero or more Encrypted Session Key packets and/or Symmetric-Key
1975 Encrypted Session Key packets may precede a Symmetrically Encrypted
1976 Data Packet that holds an encrypted message. The message is
1977 encrypted with a session key, and the session key is itself
1978 encrypted and stored in the Encrypted Session Key packet or the
1979 Symmetric-Key Encrypted Session Key packet.
1981 If the Symmetrically Encrypted Data Packet is preceded by one or
1982 more Symmetric-Key Encrypted Session Key packets, each specifies a
1983 passphrase that may be used to decrypt the message. This allows a
1984 message to be encrypted to a number of public keys, and also to one
1985 or more pass phrases. This packet type is new, and is not generated
1986 by PGP 2.x or PGP 5.0.
1988 The body of this packet consists of:
1990 - A one-octet version number. The only currently defined version
1991 is 4.
1993 - A one-octet number describing the symmetric algorithm used.
1995 - A string-to-key (S2K) specifier, length as defined above.
1997 - Optionally, the encrypted session key itself, which is decrypted
1998 with the string-to-key object.
2000 If the encrypted session key is not present (which can be detected
2001 on the basis of packet length and S2K specifier size), then the S2K
2002 algorithm applied to the passphrase produces the session key for
2003 decrypting the file, using the symmetric cipher algorithm from the
2004 Symmetric-Key Encrypted Session Key packet.
2006 If the encrypted session key is present, the result of applying the
2007 S2K algorithm to the passphrase is used to decrypt just that
2008 encrypted session key field, using CFB mode with an IV of all zeros.
2009 The decryption result consists of a one-octet algorithm identifier
2010 that specifies the symmetric-key encryption algorithm used to
2011 encrypt the following Symmetrically Encrypted Data Packet, followed
2012 by the session key octets themselves.
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2019 Note: because an all-zero IV is used for this decryption, the S2K
2020 specifier MUST use a salt value, either a Salted S2K or an
2021 Iterated-Salted S2K. The salt value will insure that the decryption
2022 key is not repeated even if the passphrase is reused.
2024 5.4. One-Pass Signature Packets (Tag 4)
2026 The One-Pass Signature packet precedes the signed data and contains
2027 enough information to allow the receiver to begin calculating any
2028 hashes needed to verify the signature. It allows the Signature
2029 Packet to be placed at the end of the message, so that the signer
2030 can compute the entire signed message in one pass.
2032 A One-Pass Signature does not interoperate with PGP 2.6.x or
2033 earlier.
2035 The body of this packet consists of:
2037 - A one-octet version number. The current version is 3.
2039 - A one-octet signature type. Signature types are described in
2040 section 5.2.1.
2042 - A one-octet number describing the hash algorithm used.
2044 - A one-octet number describing the public key algorithm used.
2046 - An eight-octet number holding the key ID of the signing key.
2048 - A one-octet number holding a flag showing whether the signature
2049 is nested. A zero value indicates that the next packet is
2050 another One-Pass Signature packet that describes another
2051 signature to be applied to the same message data.
2053 Note that if a message contains more than one one-pass signature,
2054 then the signature packets bracket the message; that is, the first
2055 signature packet after the message corresponds to the last one-pass
2056 packet and the final signature packet corresponds to the first
2057 one-pass packet.
2059 5.5. Key Material Packet
2061 A key material packet contains all the information about a public or
2062 private key. There are four variants of this packet type, and two
2063 major versions. Consequently, this section is complex.
2065 5.5.1. Key Packet Variants
2067 5.5.1.1. Public Key Packet (Tag 6)
2069 A Public Key packet starts a series of packets that forms an OpenPGP
2070 key (sometimes called an OpenPGP certificate).
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2075 5.5.1.2. Public Subkey Packet (Tag 14)
2077 A Public Subkey packet (tag 14) has exactly the same format as a
2078 Public Key packet, but denotes a subkey. One or more subkeys may be
2079 associated with a top-level key. By convention, the top-level key
2080 provides signature services, and the subkeys provide encryption
2081 services.
2083 Note: in PGP 2.6.x, tag 14 was intended to indicate a comment
2084 packet. This tag was selected for reuse because no previous version
2085 of PGP ever emitted comment packets but they did properly ignore
2086 them. Public Subkey packets are ignored by PGP 2.6.x and do not
2087 cause it to fail, providing a limited degree of backward
2088 compatibility.
2090 5.5.1.3. Secret Key Packet (Tag 5)
2092 A Secret Key packet contains all the information that is found in a
2093 Public Key packet, including the public key material, but also
2094 includes the secret key material after all the public key fields.
2096 5.5.1.4. Secret Subkey Packet (Tag 7)
2098 A Secret Subkey packet (tag 7) is the subkey analog of the Secret
2099 Key packet, and has exactly the same format.
2101 5.5.2. Public Key Packet Formats
2103 There are two versions of key-material packets. Version 3 packets
2104 were first generated by PGP 2.6. Version 4 keys first appeared in
2105 PGP 5.0, and are the preferred key version for OpenPGP.
2107 OpenPGP implementations SHOULD create keys with version 4 format. V3
2108 keys are deprecated; an implementation SHOULD NOT generate a V3 key,
2109 but MAY accept it. An implementation MUST NOT create a V3 key with a
2110 public key algorithm other than RSA.
2112 A version 3 public key or public subkey packet contains:
2114 - A one-octet version number (3).
2116 - A four-octet number denoting the time that the key was created.
2118 - A two-octet number denoting the time in days that this key is
2119 valid. If this number is zero, then it does not expire.
2121 - A one-octet number denoting the public key algorithm of this key
2123 - A series of multiprecision integers comprising the key material:
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2131 - a multiprecision integer (MPI) of RSA public modulus n;
2133 - an MPI of RSA public encryption exponent e.
2135 V3 keys are deprecated. They contain three weaknesses in them.
2136 First, it is relatively easy to construct a V3 key that has the same
2137 key ID as any other key because the key ID is simply the low 64 bits
2138 of the public modulus. Secondly, because the fingerprint of a V3 key
2139 hashes the key material, but not its length, there is an increased
2140 opportunity for fingerprint collisions. Third, there are minor
2141 weaknesses in the MD5 hash algorithm that make developers prefer
2142 other algorithms. See below for a fuller discussion of key IDs and
2143 fingerprints.
2145 V2 keys are identical to the deprecated V3 keys except for the
2146 version number. An implementation MUST NOT generate them and may
2147 accept or reject them as it sees fit.
2149 The version 4 format is similar to the version 3 format except for
2150 the absence of a validity period. This has been moved to the
2151 signature packet. In addition, fingerprints of version 4 keys are
2152 calculated differently from version 3 keys, as described in section
2153 "Enhanced Key Formats."
2155 A version 4 packet contains:
2157 - A one-octet version number (4).
2159 - A four-octet number denoting the time that the key was created.
2161 - A one-octet number denoting the public key algorithm of this key
2163 - A series of multiprecision integers comprising the key material.
2164 This algorithm-specific portion is:
2166 Algorithm Specific Fields for RSA public keys:
2168 - multiprecision integer (MPI) of RSA public modulus n;
2170 - MPI of RSA public encryption exponent e.
2172 Algorithm Specific Fields for DSA public keys:
2174 - MPI of DSA prime p;
2176 - MPI of DSA group order q (q is a prime divisor of p-1);
2178 - MPI of DSA group generator g;
2180 - MPI of DSA public key value y (= g**x mod p where x is
2181 secret).
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2187 Algorithm Specific Fields for Elgamal public keys:
2189 - MPI of Elgamal prime p;
2191 - MPI of Elgamal group generator g;
2193 - MPI of Elgamal public key value y (= g**x mod p where x is
2194 secret).
2196 5.5.3. Secret Key Packet Formats
2198 The Secret Key and Secret Subkey packets contain all the data of the
2199 Public Key and Public Subkey packets, with additional
2200 algorithm-specific secret key data appended, usually in encrypted
2201 form.
2203 The packet contains:
2205 - A Public Key or Public Subkey packet, as described above
2207 - One octet indicating string-to-key usage conventions. Zero
2208 indicates that the secret key data is not encrypted. 255 or 254
2209 indicates that a string-to-key specifier is being given. Any
2210 other value is a symmetric-key encryption algorithm identifier.
2212 - [Optional] If string-to-key usage octet was 255 or 254, a
2213 one-octet symmetric encryption algorithm.
2215 - [Optional] If string-to-key usage octet was 255 or 254, a
2216 string-to-key specifier. The length of the string-to-key
2217 specifier is implied by its type, as described above.
2219 - [Optional] If secret data is encrypted (string-to-key usage
2220 octet not zero), an Initial Vector (IV) of the same length as
2221 the cipher's block size.
2223 - Plain or encrypted multiprecision integers comprising the secret
2224 key data. These algorithm-specific fields are as described
2225 below.
2227 - If the string-to-key usage octet is zero or 255, then a
2228 two-octet checksum of the plaintext of the algorithm-specific
2229 portion (sum of all octets, mod 65536). If the string-to-key
2230 usage octet was 254, then a 20-octet SHA-1 hash of the plaintext
2231 of the algorithm-specific portion. This checksum or hash is
2232 encrypted together with the algorithm-specific fields (if
2233 string-to-key usage octet is not zero). Note that for all other
2234 values, a two-octet checksum is required.
2236 Algorithm Specific Fields for RSA secret keys:
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2243 - multiprecision integer (MPI) of RSA secret exponent d.
2245 - MPI of RSA secret prime value p.
2247 - MPI of RSA secret prime value q (p < q).
2249 - MPI of u, the multiplicative inverse of p, mod q.
2251 Algorithm Specific Fields for DSA secret keys:
2253 - MPI of DSA secret exponent x.
2255 Algorithm Specific Fields for Elgamal secret keys:
2257 - MPI of Elgamal secret exponent x.
2259 Secret MPI values can be encrypted using a passphrase. If a
2260 string-to-key specifier is given, that describes the algorithm for
2261 converting the passphrase to a key, else a simple MD5 hash of the
2262 passphrase is used. Implementations MUST use a string-to-key
2263 specifier; the simple hash is for backward compatibility and is
2264 deprecated, though implementations MAY continue to use existing
2265 private keys in the old format. The cipher for encrypting the MPIs
2266 is specified in the secret key packet.
2268 Encryption/decryption of the secret data is done in CFB mode using
2269 the key created from the passphrase and the Initial Vector from the
2270 packet. A different mode is used with V3 keys (which are only RSA)
2271 than with other key formats. With V3 keys, the MPI bit count prefix
2272 (i.e., the first two octets) is not encrypted. Only the MPI
2273 non-prefix data is encrypted. Furthermore, the CFB state is
2274 resynchronized at the beginning of each new MPI value, so that the
2275 CFB block boundary is aligned with the start of the MPI data.
2277 With V4 keys, a simpler method is used. All secret MPI values are
2278 encrypted in CFB mode, including the MPI bitcount prefix.
2280 The two-octet checksum that follows the algorithm-specific portion
2281 is the algebraic sum, mod 65536, of the plaintext of all the
2282 algorithm-specific octets (including MPI prefix and data). With V3
2283 keys, the checksum is stored in the clear. With V4 keys, the
2284 checksum is encrypted like the algorithm-specific data. This value
2285 is used to check that the passphrase was correct. However, this
2286 checksum is deprecated; an implementation SHOULD NOT use it, but
2287 should rather use the SHA-1 hash denoted with a usage octet of 254.
2288 The reason for this is that there are some attacks on the private
2289 key that can undetectably modify the secret key. Using a SHA-1 hash
2290 prevents this.
2292 5.6. Compressed Data Packet (Tag 8)
2294 The Compressed Data packet contains compressed data. Typically, this
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2297 \fINTERNET-DRAFT OpenPGP Message Format Oct 11, 2005
2299 packet is found as the contents of an encrypted packet, or following
2300 a Signature or One-Pass Signature packet, and contains a literal
2301 data packet.
2303 The body of this packet consists of:
2305 - One octet that gives the algorithm used to compress the packet.
2307 - The remainder of the packet is compressed data.
2309 A Compressed Data Packet's body contains an block that compresses
2310 some set of packets. See section "Packet Composition" for details on
2311 how messages are formed.
2313 ZIP-compressed packets are compressed with raw RFC 1951 DEFLATE
2314 blocks. Note that PGP V2.6 uses 13 bits of compression. If an
2315 implementation uses more bits of compression, PGP V2.6 cannot
2316 decompress it.
2318 ZLIB-compressed packets are compressed with RFC 1950 ZLIB-style
2319 blocks.
2321 5.7. Symmetrically Encrypted Data Packet (Tag 9)
2323 The Symmetrically Encrypted Data packet contains data encrypted with
2324 a symmetric-key algorithm. When it has been decrypted, it contains
2325 other packets (usually a literal data packet or compressed data
2326 packet, but in theory other Symmetrically Encrypted Data Packets or
2327 sequences of packets that form whole OpenPGP messages).
2329 The body of this packet consists of:
2331 - Encrypted data, the output of the selected symmetric-key cipher
2332 operating in OpenPGP's variant of Cipher Feedback (CFB) mode.
2334 The symmetric cipher used may be specified in an Public-Key or
2335 Symmetric-Key Encrypted Session Key packet that precedes the
2336 Symmetrically Encrypted Data Packet. In that case, the cipher
2337 algorithm octet is prefixed to the session key before it is
2338 encrypted. If no packets of these types precede the encrypted data,
2339 the IDEA algorithm is used with the session key calculated as the
2340 MD5 hash of the passphrase, though this use is deprecated.
2342 The data is encrypted in CFB mode, with a CFB shift size equal to
2343 the cipher's block size. The Initial Vector (IV) is specified as
2344 all zeros. Instead of using an IV, OpenPGP prefixes a string of
2345 length equal to the block size of the cipher plus two to the data
2346 before it is encrypted. The first block-size octets (for example, 8
2347 octets for a 64-bit block length) are random, and the following two
2348 octets are copies of the last two octets of the IV. For example, in
2349 an 8 octet block, octet 9 is a repeat of octet 7, and octet 10 is a
2350 repeat of octet 8. In a cipher of length 16, octet 17 is a repeat of
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2355 octet 15 and octet 18 is a repeat of octet 16. As a pedantic
2356 clarification, in both these examples, we consider the first octet
2357 to be numbered 1.
2359 After encrypting the first block-size-plus-two octets, the CFB state
2360 is resynchronized. The last block-size octets of ciphertext are
2361 passed through the cipher and the block boundary is reset.
2363 The repetition of 16 bits in the random data prefixed to the message
2364 allows the receiver to immediately check whether the session key is
2365 incorrect. See the Security Considerations section for hints on the
2366 proper use of this "quick check."
2368 5.8. Marker Packet (Obsolete Literal Packet) (Tag 10)
2370 An experimental version of PGP used this packet as the Literal
2371 packet, but no released version of PGP generated Literal packets
2372 with this tag. With PGP 5.x, this packet has been re-assigned and is
2373 reserved for use as the Marker packet.
2375 The body of this packet consists of:
2377 - The three octets 0x50, 0x47, 0x50 (which spell "PGP" in UTF-8).
2379 Such a packet MUST be ignored when received. It may be placed at
2380 the beginning of a message that uses features not available in PGP
2381 2.6.x in order to cause that version to report that newer software
2382 is necessary to process the message.
2384 5.9. Literal Data Packet (Tag 11)
2386 A Literal Data packet contains the body of a message; data that is
2387 not to be further interpreted.
2389 The body of this packet consists of:
2391 - A one-octet field that describes how the data is formatted.
2393 If it is a 'b' (0x62), then the literal packet contains binary data.
2394 If it is a 't' (0x74), then it contains text data, and thus may need
2395 line ends converted to local form, or other text-mode changes. The
2396 tag 'u' (0x75) means the same as 't', but also indicates that
2397 implementation believes that the literal data contains UTF-8 text.
2399 Early versions of PGP also defined a value of 'l' as a 'local' mode
2400 for machine-local conversions. RFC 1991 incorrectly stated this
2401 local mode flag as '1' (ASCII numeral one). Both of these local
2402 modes are deprecated.
2404 - File name as a string (one-octet length, followed by a file
2405 name). This may be a zero-length string. Commonly, if the source
2406 of the encrypted data is a file, this will be the name of the
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2411 encrypted file. An implementation MAY consider the file name in
2412 the literal packet to be a more authoritative name than the
2413 actual file name.
2415 If the special name "_CONSOLE" is used, the message is considered to
2416 be "for your eyes only". This advises that the message data is
2417 unusually sensitive, and the receiving program should process it
2418 more carefully, perhaps avoiding storing the received data to disk,
2419 for example.
2421 - A four-octet number that indicates a date associated with the
2422 literal data. Commonly, the date might be the modification date
2423 of a file, or the time the packet was created, or a zero that
2424 indicates no specific time.
2426 - The remainder of the packet is literal data.
2428 Text data is stored with <CR><LF> text endings (i.e. network-normal
2429 line endings). These should be converted to native line endings by
2430 the receiving software.
2432 5.10. Trust Packet (Tag 12)
2434 The Trust packet is used only within keyrings and is not normally
2435 exported. Trust packets contain data that record the user's
2436 specifications of which key holders are trustworthy introducers,
2437 along with other information that implementing software uses for
2438 trust information. The format of trust packets is defined by a given
2439 implementation.
2441 Trust packets SHOULD NOT be emitted to output streams that are
2442 transferred to other users, and they SHOULD be ignored on any input
2443 other than local keyring files.
2445 5.11. User ID Packet (Tag 13)
2447 A User ID packet consists of UTF-8 text that is intended to
2448 represent the name and email address of the key holder. By
2449 convention, it includes an RFC 2822 mail name, but there are no
2450 restrictions on its content. The packet length in the header
2451 specifies the length of the User ID.
2453 5.12. User Attribute Packet (Tag 17)
2455 The User Attribute packet is a variation of the User ID packet. It
2456 is capable of storing more types of data than the User ID packet
2457 which is limited to text. Like the User ID packet, a User Attribute
2458 packet may be certified by the key owner ("self-signed") or any
2459 other key owner who cares to certify it. Except as noted, a User
2460 Attribute packet may be used anywhere that a User ID packet may be
2461 used.
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2467 While User Attribute packets are not a required part of the OpenPGP
2468 standard, implementations SHOULD provide at least enough
2469 compatibility to properly handle a certification signature on the
2470 User Attribute packet. A simple way to do this is by treating the
2471 User Attribute packet as a User ID packet with opaque contents, but
2472 an implementation may use any method desired.
2474 The User Attribute packet is made up of one or more attribute
2475 subpackets. Each subpacket consists of a subpacket header and a
2476 body. The header consists of:
2478 - the subpacket length (1, 2, or 5 octets)
2480 - the subpacket type (1 octet)
2482 and is followed by the subpacket specific data.
2484 The only currently defined subpacket type is 1, signifying an image.
2485 An implementation SHOULD ignore any subpacket of a type that it does
2486 not recognize. Subpacket types 100 through 110 are reserved for
2487 private or experimental use.
2489 5.12.1. The Image Attribute Subpacket
2491 The image attribute subpacket is used to encode an image, presumably
2492 (but not required to be) that of the key owner.
2494 The image attribute subpacket begins with an image header. The
2495 first two octets of the image header contain the length of the image
2496 header. Note that unlike other multi-octet numerical values in this
2497 document, due to an historical accident this value is encoded as a
2498 little-endian number. The image header length is followed by a
2499 single octet for the image header version. The only currently
2500 defined version of the image header is 1, which is a 16 octet image
2501 header. The first three octets of a version 1 image header are thus
2502 0x10 0x00 0x01.
2504 The fourth octet of a version 1 image header designates the encoding
2505 format of the image. The only currently defined encoding format is
2506 the value 1 to indicate JPEG. Image format types 100 through 110
2507 are reserved for private or experimental use. The rest of the
2508 version 1 image header is made up of 12 reserved octets, all of
2509 which MUST be set to 0.
2511 The rest of the image subpacket contains the image itself. As the
2512 only currently defined image type is JPEG, the image is encoded in
2513 the JPEG File Interchange Format (JFIF), a standard file format for
2514 JPEG images. [JFIF]
2516 An implementation MAY try and determine the type of an image by
2517 examination of the image data if it is unable to handle a particular
2518 version of the image header or if a specified encoding format value
2520 Callas, et al. Expires Apr 11, 2006 [Page 45]
2521 \fINTERNET-DRAFT OpenPGP Message Format Oct 11, 2005
2523 is not recognized.
2525 5.13. Sym. Encrypted Integrity Protected Data Packet (Tag 18)
2527 The Symmetrically Encrypted Integrity Protected Data Packet is a
2528 variant of the Symmetrically Encrypted Data Packet. It is a new
2529 feature created for OpenPGP that addresses the problem of detecting
2530 a modification to encrypted data. It is used in combination with a
2531 Modification Detection Code Packet.
2533 There is a corresponding feature in the features signature subpacket
2534 that denotes that an implementation can properly use this packet
2535 type. An implementation MUST support decrypting these packets and
2536 SHOULD prefer generating them to the older Symmetrically Encrypted
2537 Data Packet when possible. Since this data packet protects against
2538 modification attacks, this standard encourages its proliferation.
2539 While blanket adoption of this data packet would create
2540 interoperability problems, rapid adoption is nevertheless important.
2541 An implementation SHOULD specifically denote support for this
2542 packet, but it MAY infer it from other mechanisms.
2544 For example, an implementation might infer from the use of a cipher
2545 such as AES or Twofish that a user supports this feature. It might
2546 place in the unhashed portion of another user's key signature a
2547 features subpacket. It might also present a user with an opportunity
2548 to regenerate their own self-signature with a features subpacket.
2550 This packet contains data encrypted with a symmetric-key algorithm
2551 and protected against modification by the SHA-1 hash algorithm. When
2552 it has been decrypted, it will typically contain other packets
2553 (often literal data packets or compressed data packets). The last
2554 decrypted packet in this packet's payload MUST be a Modification
2555 Detection Code packet.
2557 The body of this packet consists of:
2559 - A one-octet version number. The only currently defined value is
2560 1.
2562 - Encrypted data, the output of the selected symmetric-key cipher
2563 operating in Cipher Feedback mode with shift amount equal to the
2564 block size of the cipher (CFB-n where n is the block size).
2566 The symmetric cipher used MUST be specified in a Public-Key or
2567 Symmetric-Key Encrypted Session Key packet that precedes the
2568 Symmetrically Encrypted Data Packet. In either case, the cipher
2569 algorithm octet is prefixed to the session key before it is
2570 encrypted.
2572 The data is encrypted in CFB mode, with a CFB shift size equal to
2573 the cipher's block size. The Initial Vector (IV) is specified as
2574 all zeros. Instead of using an IV, OpenPGP prefixes an octet string
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2577 \fINTERNET-DRAFT OpenPGP Message Format Oct 11, 2005
2579 to the data before it is encrypted. The length of the octet string
2580 equals the block size of the cipher in octets, plus two. The first
2581 octets in the group, of length equal to the block size of the
2582 cipher, are random; the last two octets are each copies of their 2nd
2583 preceding octet. For example, with a cipher whose block size is 128
2584 bits or 16 octets, the prefix data will contain 16 random octets,
2585 then two more octets, which are copies of the 15th and 16th octets,
2586 respectively. Unlike the Symmetrically Encrypted Data Packet, no
2587 special CFB resynchronization is done after encrypting this prefix
2588 data. See OpenPGP CFB Mode below for more details.
2590 The repetition of 16 bits in the random data prefixed to the message
2591 allows the receiver to immediately check whether the session key is
2592 incorrect.
2594 The plaintext of the data to be encrypted is passed through the
2595 SHA-1 hash function, and the result of the hash is appended to the
2596 plaintext in a Modification Detection Code packet. The input to the
2597 hash function includes the prefix data described above; it includes
2598 all of the plaintext, and then also includes two octets of values
2599 0xD3, 0x14. These represent the encoding of a Modification
2600 Detection Code packet tag and length field of 20 octets.
2602 The resulting hash value is stored in a Modification Detection Code
2603 packet which MUST use the two octet encoding just given to represent
2604 its tag and length field. The body of the MDC packet is the 20
2605 octet output of the SHA-1 hash.
2607 The Modification Detection Code packet is appended to the plaintext
2608 and encrypted along with the plaintext using the same CFB context.
2610 During decryption, the plaintext data should be hashed with SHA-1,
2611 including the prefix data as well as the packet tag and length field
2612 of the Modification Detection Code packet. The body of the MDC
2613 packet, upon decryption, is compared with the result of the SHA-1
2614 hash.
2616 Any failure of the MDC indicates that the message has been modified
2617 and MUST be treated as a security problem. Failures include a
2618 difference in the hash values, but also the absence of an MDC
2619 packet, or an MDC packet in any position other than the end of the
2620 plaintext. Any failure SHOULD be reported to the user.
2622 Note: future designs of new versions of this packet should consider
2623 rollback attacks since it will be possible for an attacker to change
2624 the version back to 1.
2626 5.14. Modification Detection Code Packet (Tag 19)
2628 The Modification Detection Code packet contains a SHA-1 hash of
2629 plaintext data which is used to detect message modification. It is
2630 only used with a Symmetrically Encrypted Integrity Protected Data
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2633 \fINTERNET-DRAFT OpenPGP Message Format Oct 11, 2005
2635 packet. The Modification Detection Code packet MUST be the last
2636 packet in the plaintext data which is encrypted in the Symmetrically
2637 Encrypted Integrity Protected Data packet, and MUST appear in no
2638 other place.
2640 A Modification Detection Code packet MUST have a length of 20
2641 octets.
2643 The body of this packet consists of:
2645 - A 20-octet SHA-1 hash of the preceding plaintext data of the
2646 Symmetrically Encrypted Integrity Protected Data packet,
2647 including prefix data, the tag octet, and length octet of the
2648 Modification Detection Code packet.
2650 Note that the Modification Detection Code packet MUST always use a
2651 new-format encoding of the packet tag, and a one-octet encoding of
2652 the packet length. The reason for this is that the hashing rules for
2653 modification detection include a one-octet tag and one-octet length
2654 in the data hash. While this is a bit restrictive, it reduces
2655 complexity.
2657 6. Radix-64 Conversions
2659 As stated in the introduction, OpenPGP's underlying native
2660 representation for objects is a stream of arbitrary octets, and some
2661 systems desire these objects to be immune to damage caused by
2662 character set translation, data conversions, etc.
2664 In principle, any printable encoding scheme that met the
2665 requirements of the unsafe channel would suffice, since it would not
2666 change the underlying binary bit streams of the native OpenPGP data
2667 structures. The OpenPGP standard specifies one such printable
2668 encoding scheme to ensure interoperability.
2670 OpenPGP's Radix-64 encoding is composed of two parts: a base64
2671 encoding of the binary data, and a checksum. The base64 encoding is
2672 identical to the MIME base64 content-transfer-encoding [RFC2045].
2674 The checksum is a 24-bit CRC converted to four characters of
2675 radix-64 encoding by the same MIME base64 transformation, preceded
2676 by an equals sign (=). The CRC is computed by using the generator
2677 0x864CFB and an initialization of 0xB704CE. The accumulation is
2678 done on the data before it is converted to radix-64, rather than on
2679 the converted data. A sample implementation of this algorithm is in
2680 the next section.
2682 The checksum with its leading equal sign MAY appear on the first
2683 line after the Base64 encoded data.
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2691 Rationale for CRC-24: The size of 24 bits fits evenly into printable
2692 base64. The nonzero initialization can detect more errors than a
2693 zero initialization.
2695 6.1. An Implementation of the CRC-24 in "C"
2697 #define CRC24_INIT 0xb704ceL
2698 #define CRC24_POLY 0x1864cfbL
2700 typedef long crc24;
2701 crc24 crc_octets(unsigned char *octets, size_t len)
2702 {
2703 crc24 crc = CRC24_INIT;
2704 int i;
2706 while (len--) {
2707 crc ^= (*octets++) << 16;
2708 for (i = 0; i < 8; i++) {
2709 crc <<= 1;
2710 if (crc & 0x1000000)
2711 crc ^= CRC24_POLY;
2712 }
2713 }
2714 return crc & 0xffffffL;
2715 }
2717 6.2. Forming ASCII Armor
2719 When OpenPGP encodes data into ASCII Armor, it puts specific headers
2720 around the Radix-64 encoded data, so OpenPGP can reconstruct the
2721 data later. An OpenPGP implementation MAY use ASCII armor to protect
2722 raw binary data. OpenPGP informs the user what kind of data is
2723 encoded in the ASCII armor through the use of the headers.
2725 Concatenating the following data creates ASCII Armor:
2727 - An Armor Header Line, appropriate for the type of data
2729 - Armor Headers
2731 - A blank (zero-length, or containing only whitespace) line
2733 - The ASCII-Armored data
2735 - An Armor Checksum
2737 - The Armor Tail, which depends on the Armor Header Line.
2739 An Armor Header Line consists of the appropriate header line text
2740 surrounded by five (5) dashes ('-', 0x2D) on either side of the
2741 header line text. The header line text is chosen based upon the
2742 type of data that is being encoded in Armor, and how it is being
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2747 encoded. Header line texts include the following strings:
2749 BEGIN PGP MESSAGE
2750 Used for signed, encrypted, or compressed files.
2752 BEGIN PGP PUBLIC KEY BLOCK
2753 Used for armoring public keys
2755 BEGIN PGP PRIVATE KEY BLOCK
2756 Used for armoring private keys
2758 BEGIN PGP MESSAGE, PART X/Y
2759 Used for multi-part messages, where the armor is split amongst Y
2760 parts, and this is the Xth part out of Y.
2762 BEGIN PGP MESSAGE, PART X
2763 Used for multi-part messages, where this is the Xth part of an
2764 unspecified number of parts. Requires the MESSAGE-ID Armor
2765 Header to be used.
2767 BEGIN PGP SIGNATURE
2768 Used for detached signatures, OpenPGP/MIME signatures, and
2769 cleartext signatures. Note that PGP 2.x uses BEGIN PGP MESSAGE
2770 for detached signatures.
2772 Note that all these Armor Header Lines are to consist of a complete
2773 line. That is to say, there is always a line ending preceding the
2774 starting five dashes, and following the ending five dashes. The
2775 header lines, therefore, MUST start at the beginning of a line, and
2776 MUST NOT have text following them on the same line. These line
2777 endings are considered a part of the Armor Header Line for the
2778 purposes of determining the content they delimit. This is
2779 particularly important when computing a cleartext signature (see
2780 below).
2782 The Armor Headers are pairs of strings that can give the user or the
2783 receiving OpenPGP implementation some information about how to
2784 decode or use the message. The Armor Headers are a part of the
2785 armor, not a part of the message, and hence are not protected by any
2786 signatures applied to the message.
2788 The format of an Armor Header is that of a key-value pair. A colon
2789 (':' 0x38) and a single space (0x20) separate the key and value.
2790 OpenPGP should consider improperly formatted Armor Headers to be
2791 corruption of the ASCII Armor. Unknown keys should be reported to
2792 the user, but OpenPGP should continue to process the message.
2794 Currently defined Armor Header Keys are:
2796 - "Version", that states the OpenPGP implementation and version
2797 used to encode the message.
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2803 - "Comment", a user-defined comment. OpenPGP defines all text to
2804 be in UTF-8. A comment may be any UTF-8 string. However, the
2805 whole point of armoring is to provide seven-bit-clean data.
2806 Consequently, if a comment has characters that are outside the
2807 US-ASCII range of UTF, they may very well not survive transport.
2809 - "MessageID", a 32-character string of printable characters. The
2810 string must be the same for all parts of a multi-part message
2811 that uses the "PART X" Armor Header. MessageID strings should
2812 be unique enough that the recipient of the mail can associate
2813 all the parts of a message with each other. A good checksum or
2814 cryptographic hash function is sufficient.
2816 The MessageID SHOULD NOT appear unless it is in a multi-part
2817 message. If it appears at all, it MUST be computed from the
2818 finished (encrypted, signed, etc.) message in a deterministic
2819 fashion, rather than contain a purely random value. This is to
2820 allow the legitimate recipient to determine that the MessageID
2821 cannot serve as a covert means of leaking cryptographic key
2822 information.
2824 - "Hash", a comma-separated list of hash algorithms used in this
2825 message. This is used only in cleartext signed messages.
2827 - "Charset", a description of the character set that the plaintext
2828 is in. Please note that OpenPGP defines text to be in UTF-8. An
2829 implementation will get best results by translating into and out
2830 of UTF-8. However, there are many instances where this is easier
2831 said than done. Also, there are communities of users who have no
2832 need for UTF-8 because they are all happy with a character set
2833 like ISO Latin-5 or a Japanese character set. In such instances,
2834 an implementation MAY override the UTF-8 default by using this
2835 header key. An implementation MAY implement this key and any
2836 translations it cares to; an implementation MAY ignore it and
2837 assume all text is UTF-8.
2839 The Armor Tail Line is composed in the same manner as the Armor
2840 Header Line, except the string "BEGIN" is replaced by the string
2841 "END".
2843 6.3. Encoding Binary in Radix-64
2845 The encoding process represents 24-bit groups of input bits as
2846 output strings of 4 encoded characters. Proceeding from left to
2847 right, a 24-bit input group is formed by concatenating three 8-bit
2848 input groups. These 24 bits are then treated as four concatenated
2849 6-bit groups, each of which is translated into a single digit in the
2850 Radix-64 alphabet. When encoding a bit stream with the Radix-64
2851 encoding, the bit stream must be presumed to be ordered with the
2852 most-significant-bit first. That is, the first bit in the stream
2853 will be the high-order bit in the first 8-bit octet, and the eighth
2854 bit will be the low-order bit in the first 8-bit octet, and so on.
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2857 \fINTERNET-DRAFT OpenPGP Message Format Oct 11, 2005
2859 +--first octet--+-second octet--+--third octet--+
2860 |7 6 5 4 3 2 1 0|7 6 5 4 3 2 1 0|7 6 5 4 3 2 1 0|
2861 +-----------+---+-------+-------+---+-----------+
2862 |5 4 3 2 1 0|5 4 3 2 1 0|5 4 3 2 1 0|5 4 3 2 1 0|
2863 +--1.index--+--2.index--+--3.index--+--4.index--+
2865 Each 6-bit group is used as an index into an array of 64 printable
2866 characters from the table below. The character referenced by the
2867 index is placed in the output string.
2869 Value Encoding Value Encoding Value Encoding Value Encoding
2870 0 A 17 R 34 i 51 z
2871 1 B 18 S 35 j 52 0
2872 2 C 19 T 36 k 53 1
2873 3 D 20 U 37 l 54 2
2874 4 E 21 V 38 m 55 3
2875 5 F 22 W 39 n 56 4
2876 6 G 23 X 40 o 57 5
2877 7 H 24 Y 41 p 58 6
2878 8 I 25 Z 42 q 59 7
2879 9 J 26 a 43 r 60 8
2880 10 K 27 b 44 s 61 9
2881 11 L 28 c 45 t 62 +
2882 12 M 29 d 46 u 63 /
2883 13 N 30 e 47 v
2884 14 O 31 f 48 w (pad) =
2885 15 P 32 g 49 x
2886 16 Q 33 h 50 y
2888 The encoded output stream must be represented in lines of no more
2889 than 76 characters each.
2891 Special processing is performed if fewer than 24 bits are available
2892 at the end of the data being encoded. There are three possibilities:
2894 1. The last data group has 24 bits (3 octets). No special
2895 processing is needed.
2897 2. The last data group has 16 bits (2 octets). The first two 6-bit
2898 groups are processed as above. The third (incomplete) data group
2899 has two zero-value bits added to it, and is processed as above.
2900 A pad character (=) is added to the output.
2902 3. The last data group has 8 bits (1 octet). The first 6-bit group
2903 is processed as above. The second (incomplete) data group has
2904 four zero-value bits added to it, and is processed as above. Two
2905 pad characters (=) are added to the output.
2907 6.4. Decoding Radix-64
2909 Any characters outside of the base64 alphabet are ignored in
2910 Radix-64 data. Decoding software must ignore all line breaks or
2912 Callas, et al. Expires Apr 11, 2006 [Page 52]
2913 \fINTERNET-DRAFT OpenPGP Message Format Oct 11, 2005
2915 other characters not found in the table above.
2917 In Radix-64 data, characters other than those in the table, line
2918 breaks, and other white space probably indicate a transmission
2919 error, about which a warning message or even a message rejection
2920 might be appropriate under some circumstances.
2922 Because it is used only for padding at the end of the data, the
2923 occurrence of any "=" characters may be taken as evidence that the
2924 end of the data has been reached (without truncation in transit). No
2925 such assurance is possible, however, when the number of octets
2926 transmitted was a multiple of three and no "=" characters are
2927 present.
2929 6.5. Examples of Radix-64
2931 Input data: 0x14fb9c03d97e
2932 Hex: 1 4 f b 9 c | 0 3 d 9 7 e
2933 8-bit: 00010100 11111011 10011100 | 00000011 11011001
2934 11111110
2935 6-bit: 000101 001111 101110 011100 | 000000 111101 100111
2936 111110
2937 Decimal: 5 15 46 28 0 61 37 62
2938 Output: F P u c A 9 l +
2940 Input data: 0x14fb9c03d9
2941 Hex: 1 4 f b 9 c | 0 3 d 9
2942 8-bit: 00010100 11111011 10011100 | 00000011 11011001
2943 pad with 00
2944 6-bit: 000101 001111 101110 011100 | 000000 111101 100100
2945 Decimal: 5 15 46 28 0 61 36
2946 pad with =
2947 Output: F P u c A 9 k =
2949 Input data: 0x14fb9c03
2950 Hex: 1 4 f b 9 c | 0 3
2951 8-bit: 00010100 11111011 10011100 | 00000011
2952 pad with 0000
2953 6-bit: 000101 001111 101110 011100 | 000000 110000
2954 Decimal: 5 15 46 28 0 48
2955 pad with = =
2956 Output: F P u c A w = =
2958 6.6. Example of an ASCII Armored Message
2960 -----BEGIN PGP MESSAGE-----
2961 Version: OpenPrivacy 0.99
2963 yDgBO22WxBHv7O8X7O/jygAEzol56iUKiXmV+XmpCtmpqQUKiQrFqclFqUDBovzS
2964 vBSFjNSiVHsuAA==
2965 =njUN
2966 -----END PGP MESSAGE-----
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2971 Note that this example is indented by two spaces.
2973 7. Cleartext signature framework
2975 It is desirable to sign a textual octet stream without ASCII
2976 armoring the stream itself, so the signed text is still readable
2977 without special software. In order to bind a signature to such a
2978 cleartext, this framework is used. (Note that RFC 3156 defines
2979 another way to sign cleartext messages for environments that support
2980 MIME.)
2982 The cleartext signed message consists of:
2984 - The cleartext header '-----BEGIN PGP SIGNED MESSAGE-----' on a
2985 single line,
2987 - One or more "Hash" Armor Headers,
2989 - Exactly one empty line not included into the message digest,
2991 - The dash-escaped cleartext that is included into the message
2992 digest,
2994 - The ASCII armored signature(s) including the '-----BEGIN PGP
2995 SIGNATURE-----' Armor Header and Armor Tail Lines.
2997 If the "Hash" armor header is given, the specified message digest
2998 algorithm(s) are used for the signature. If there are no such
2999 headers, MD5 is used. If MD5 is the only hash used, then an
3000 implementation MAY omit this header for improved V2.x compatibility.
3001 If more than one message digest is used in the signature, the "Hash"
3002 armor header contains a comma-delimited list of used message
3003 digests.
3005 Current message digest names are described below with the algorithm
3006 IDs.
3008 7.1. Dash-Escaped Text
3010 The cleartext content of the message must also be dash-escaped.
3012 Dash escaped cleartext is the ordinary cleartext where every line
3013 starting with a dash '-' (0x2D) is prefixed by the sequence dash '-'
3014 (0x2D) and space ' ' (0x20). This prevents the parser from
3015 recognizing armor headers of the cleartext itself. An implementation
3016 MAY dash escape any line, SHOULD dash escape lines commencing "From"
3017 followed by a space, and MUST dash escape any line commencing in a
3018 dash. The message digest is computed using the cleartext itself, not
3019 the dash escaped form.
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3027 As with binary signatures on text documents, a cleartext signature
3028 is calculated on the text using canonical <CR><LF> line endings.
3029 The line ending (i.e. the <CR><LF>) before the '-----BEGIN PGP
3030 SIGNATURE-----' line that terminates the signed text is not
3031 considered part of the signed text.
3033 When reversing dash-escaping, an implementation MUST strip the
3034 string "- " if it occurs at the beginning of a line, and SHOULD warn
3035 on "-" and any character other than a space at the beginning of a
3036 line.
3038 Also, any trailing whitespace -- spaces (0x20) and tabs (0x09) -- at
3039 the end of any line is removed when the cleartext signature is
3040 generated.
3042 8. Regular Expressions
3044 A regular expression is zero or more branches, separated by '|'. It
3045 matches anything that matches one of the branches.
3047 A branch is zero or more pieces, concatenated. It matches a match
3048 for the first, followed by a match for the second, etc.
3050 A piece is an atom possibly followed by '*', '+', or '?'. An atom
3051 followed by '*' matches a sequence of 0 or more matches of the atom.
3052 An atom followed by '+' matches a sequence of 1 or more matches of
3053 the atom. An atom followed by '?' matches a match of the atom, or
3054 the null string.
3056 An atom is a regular expression in parentheses (matching a match for
3057 the regular expression), a range (see below), '.' (matching any
3058 single character), '^' (matching the null string at the beginning of
3059 the input string), '$' (matching the null string at the end of the
3060 input string), a '\' followed by a single character (matching that
3061 character), or a single character with no other significance
3062 (matching that character).
3064 A range is a sequence of characters enclosed in '[]'. It normally
3065 matches any single character from the sequence. If the sequence
3066 begins with '^', it matches any single character not from the rest
3067 of the sequence. If two characters in the sequence are separated by
3068 '-', this is shorthand for the full list of ASCII characters between
3069 them (e.g. '[0-9]' matches any decimal digit). To include a literal
3070 ']' in the sequence, make it the first character (following a
3071 possible '^'). To include a literal '-', make it the first or last
3072 character.
3074 9. Constants
3076 This section describes the constants used in OpenPGP.
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3083 Note that these tables are not exhaustive lists; an implementation
3084 MAY implement an algorithm not on these lists, so long as the
3085 algorithm number(s) are chosen from the private or experimental
3086 algorithm range.
3088 See the section "Notes on Algorithms" below for more discussion of
3089 the algorithms.
3091 9.1. Public Key Algorithms
3093 ID Algorithm
3094 -- ---------
3095 1 - RSA (Encrypt or Sign) [HAC]
3096 2 - RSA Encrypt-Only
3097 3 - RSA Sign-Only
3098 16 - Elgamal (Encrypt-Only), see [ELGAMAL] [HAC]
3099 17 - DSA (Digital Signature Algorithm) [FIPS186] [HAC]
3100 18 - Reserved for Elliptic Curve
3101 19 - Reserved for ECDSA
3102 20 - Reserved (formerly Elgamal Encrypt or Sign)
3103 21 - Reserved for Diffie-Hellman (X9.42,
3104 as defined for IETF-S/MIME)
3105 100 to 110 - Private/Experimental algorithm.
3107 Implementations MUST implement DSA for signatures, and Elgamal for
3108 encryption. Implementations SHOULD implement RSA keys.
3109 Implementations MAY implement any other algorithm.
3111 9.2. Symmetric Key Algorithms
3113 ID Algorithm
3114 -- ---------
3115 0 - Plaintext or unencrypted data
3116 1 - IDEA [IDEA]
3117 2 - TripleDES (DES-EDE, [SCHNEIER] [HAC] -
3118 168 bit key derived from 192)
3119 3 - CAST5 (128 bit key, as per RFC 2144)
3120 4 - Blowfish (128 bit key, 16 rounds) [BLOWFISH]
3121 5 - Reserved
3122 6 - Reserved
3123 7 - AES with 128-bit key [AES]
3124 8 - AES with 192-bit key
3125 9 - AES with 256-bit key
3126 10 - Twofish with 256-bit key [TWOFISH]
3127 100 to 110 - Private/Experimental algorithm.
3129 Implementations MUST implement TripleDES. Implementations SHOULD
3130 implement AES-128 and CAST5. Implementations that interoperate with
3131 PGP 2.6 or earlier need to support IDEA, as that is the only
3132 symmetric cipher those versions use. Implementations MAY implement
3133 any other algorithm.
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3139 9.3. Compression Algorithms
3141 ID Algorithm
3142 -- ---------
3143 0 - Uncompressed
3144 1 - ZIP (RFC 1951)
3145 2 - ZLIB (RFC 1950)
3146 3 - BZip2 [BZ2]
3147 100 to 110 - Private/Experimental algorithm.
3149 Implementations MUST implement uncompressed data. Implementations
3150 SHOULD implement ZIP. Implementations MAY implement any other
3151 algorithm.
3153 9.4. Hash Algorithms
3155 ID Algorithm Text Name
3156 -- --------- ---- ----
3157 1 - MD5 "MD5"
3158 2 - SHA-1 [FIPS180] "SHA1"
3159 3 - RIPE-MD/160 "RIPEMD160"
3160 4 - Reserved
3161 5 - Reserved
3162 6 - Reserved
3163 7 - Reserved
3164 8 - SHA256 [FIPS180] "SHA256"
3165 9 - SHA384 [FIPS180] "SHA384"
3166 10 - SHA512 [FIPS180] "SHA512"
3167 100 to 110 - Private/Experimental algorithm.
3169 Implementations MUST implement SHA-1. Implementations MAY implement
3170 other algorithms.
3172 10. Packet Composition
3174 OpenPGP packets are assembled into sequences in order to create
3175 messages and to transfer keys. Not all possible packet sequences
3176 are meaningful and correct. This section describes the rules for
3177 how packets should be placed into sequences.
3179 10.1. Transferable Public Keys
3181 OpenPGP users may transfer public keys. The essential elements of a
3182 transferable public key are:
3184 - One Public Key packet
3186 - Zero or more revocation signatures
3188 - One or more User ID packets
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3195 - After each User ID packet, zero or more signature packets
3196 (certifications)
3198 - Zero or more User Attribute packets
3200 - After each User Attribute packet, zero or more signature packets
3201 (certifications)
3203 - Zero or more Subkey packets
3205 - After each Subkey packet, one signature packet, plus optionally
3206 a revocation.
3208 The Public Key packet occurs first. Each of the following User ID
3209 packets provides the identity of the owner of this public key. If
3210 there are multiple User ID packets, this corresponds to multiple
3211 means of identifying the same unique individual user; for example, a
3212 user may have more than one email address, and construct a User ID
3213 for each one.
3215 Immediately following each User ID packet, there are zero or more
3216 signature packets. Each signature packet is calculated on the
3217 immediately preceding User ID packet and the initial Public Key
3218 packet. The signature serves to certify the corresponding public key
3219 and User ID. In effect, the signer is testifying to his or her
3220 belief that this public key belongs to the user identified by this
3221 User ID.
3223 Within the same section as the User ID packets, there are zero or
3224 more User Attribute packets. Like the User ID packets, a User
3225 Attribute packet is followed by zero or more signature packets
3226 calculated on the immediately preceding User Attribute packet and
3227 the initial Public Key packet.
3229 User Attribute packets and User ID packets may be freely intermixed
3230 in this section, so long as the signatures that follow them are
3231 maintained on the proper User Attribute or User ID packet.
3233 After the User ID or Attribute packets there may be one or more
3234 Subkey packets. In general, subkeys are provided in cases where the
3235 top-level public key is a signature-only key. However, any V4 key
3236 may have subkeys, and the subkeys may be encryption-only keys,
3237 signature-only keys, or general-purpose keys. V3 keys MUST NOT have
3238 subkeys.
3240 Each Subkey packet must be followed by one Signature packet, which
3241 should be a subkey binding signature issued by the top level key.
3242 For subkeys that can issue signatures, the subkey binding signature
3243 must contain an embedded signature subpacket with a primary key
3244 binding signature (0x19) issued by the subkey on the top level key.
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3251 Subkey and Key packets may each be followed by a revocation
3252 Signature packet to indicate that the key is revoked. Revocation
3253 signatures are only accepted if they are issued by the key itself,
3254 or by a key that is authorized to issue revocations via a revocation
3255 key subpacket in a self-signature by the top level key.
3257 Transferable public key packet sequences may be concatenated to
3258 allow transferring multiple public keys in one operation.
3260 10.2. OpenPGP Messages
3262 An OpenPGP message is a packet or sequence of packets that
3263 corresponds to the following grammatical rules (comma represents
3264 sequential composition, and vertical bar separates alternatives):
3266 OpenPGP Message :- Encrypted Message | Signed Message |
3267 Compressed Message | Literal Message.
3269 Compressed Message :- Compressed Data Packet.
3271 Literal Message :- Literal Data Packet.
3273 ESK :- Public Key Encrypted Session Key Packet |
3274 Symmetric-Key Encrypted Session Key Packet.
3276 ESK Sequence :- ESK | ESK Sequence, ESK.
3278 Encrypted Data :- Symmetrically Encrypted Data Packet |
3279 Symmetrically Encrypted Integrity Protected Data Packet
3281 Encrypted Message :- Encrypted Data | ESK Sequence, Encrypted Data.
3283 One-Pass Signed Message :- One-Pass Signature Packet,
3284 OpenPGP Message, Corresponding Signature Packet.
3286 Signed Message :- Signature Packet, OpenPGP Message |
3287 One-Pass Signed Message.
3289 In addition, decrypting a Symmetrically Encrypted Data Packet or a
3290 Symmetrically Encrypted Integrity Protected Data Packet as well as
3292 decompressing a Compressed Data packet must yield a valid OpenPGP
3293 Message.
3295 10.3. Detached Signatures
3297 Some OpenPGP applications use so-called "detached signatures." For
3298 example, a program bundle may contain a file, and with it a second
3299 file that is a detached signature of the first file. These detached
3300 signatures are simply a signature packet stored separately from the
3301 data that they are a signature of.
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3307 11. Enhanced Key Formats
3309 11.1. Key Structures
3311 The format of an OpenPGP V3 key is as follows. Entries in square
3312 brackets are optional and ellipses indicate repetition.
3314 RSA Public Key
3315 [Revocation Self Signature]
3316 User ID [Signature ...]
3317 [User ID [Signature ...] ...]
3319 Each signature certifies the RSA public key and the preceding User
3320 ID. The RSA public key can have many User IDs and each User ID can
3321 have many signatures. V3 keys are deprecated. Implementations MUST
3322 NOT generate new V3 keys, but MAY continue to use existing ones.
3324 The format of an OpenPGP V4 key that uses multiple public keys is
3325 similar except that the other keys are added to the end as "subkeys"
3326 of the primary key.
3328 Primary-Key
3329 [Revocation Self Signature]
3330 [Direct Key Signature...]
3331 User ID [Signature ...]
3332 [User ID [Signature ...] ...]
3333 [User Attribute [Signature ...] ...]
3334 [[Subkey [Binding-Signature-Revocation]
3335 Primary-Key-Binding-Signature] ...]
3337 A subkey always has a single signature after it that is issued using
3338 the primary key to tie the two keys together. This binding
3339 signature may be in either V3 or V4 format, but SHOULD be V4.
3341 In the above diagram, if the binding signature of a subkey has been
3342 revoked, the revoked key may be removed, leaving only one key.
3344 In a V4 key, the primary key MUST be a key capable of certification.
3345 The subkeys may be keys of any other type. There may be other
3346 constructions of V4 keys, too. For example, there may be a
3347 single-key RSA key in V4 format, a DSA primary key with an RSA
3348 encryption key, or RSA primary key with an Elgamal subkey, etc.
3350 It is also possible to have a signature-only subkey. This permits a
3351 primary key that collects certifications (key signatures) but is
3352 used only used for certifying subkeys that are used for encryption
3353 and signatures.
3355 11.2. Key IDs and Fingerprints
3357 For a V3 key, the eight-octet key ID consists of the low 64 bits of
3358 the public modulus of the RSA key.
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3363 The fingerprint of a V3 key is formed by hashing the body (but not
3364 the two-octet length) of the MPIs that form the key material (public
3365 modulus n, followed by exponent e) with MD5. Note that both V3 keys
3366 and MD5 are deprecated.
3368 A V4 fingerprint is the 160-bit SHA-1 hash of the octet 0x99,
3369 followed by the two-octet packet length, followed by the entire
3370 Public Key packet starting with the version field. The key ID is
3371 the low order 64 bits of the fingerprint. Here are the fields of
3372 the hash material, with the example of a DSA key:
3374 a.1) 0x99 (1 octet)
3376 a.2) high order length octet of (b)-(f) (1 octet)
3378 a.3) low order length octet of (b)-(f) (1 octet)
3380 b) version number = 4 (1 octet);
3382 c) time stamp of key creation (4 octets);
3384 d) algorithm (1 octet): 17 = DSA (example);
3386 e) Algorithm specific fields.
3388 Algorithm Specific Fields for DSA keys (example):
3390 e.1) MPI of DSA prime p;
3392 e.2) MPI of DSA group order q (q is a prime divisor of p-1);
3394 e.3) MPI of DSA group generator g;
3396 e.4) MPI of DSA public key value y (= g**x mod p where x is secret).
3398 Note that it is possible for there to be collisions of key IDs --
3399 two different keys with the same key ID. Note that there is a much
3400 smaller, but still non-zero probability that two different keys have
3401 the same fingerprint.
3403 Also note that if V3 and V4 format keys share the same RSA key
3404 material, they will have different key IDs as well as different
3405 fingerprints.
3407 Finally, the key ID and fingerprint of a subkey are calculated in
3408 the same way as for a primary key, including the 0x99 as the first
3409 octet (even though this is not a valid packet ID for a public
3410 subkey).
3412 12. Notes on Algorithms
3414 12.1. Symmetric Algorithm Preferences
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3420 The symmetric algorithm preference is an ordered list of algorithms
3421 that the keyholder accepts. Since it is found on a self-signature,
3422 it is possible that a keyholder may have different preferences. For
3423 example, Alice may have TripleDES only specified for
3424 "alice@work.com" but CAST5, Blowfish, and TripleDES specified for
3425 "alice@home.org". Note that it is also possible for preferences to
3426 be in a subkey's binding signature.
3428 Since TripleDES is the MUST-implement algorithm, if it is not
3429 explicitly in the list, it is tacitly at the end. However, it is
3430 good form to place it there explicitly. Note also that if an
3431 implementation does not implement the preference, then it is
3432 implicitly a TripleDES-only implementation.
3434 An implementation MUST NOT use a symmetric algorithm that is not in
3435 the recipient's preference list. When encrypting to more than one
3436 recipient, the implementation finds a suitable algorithm by taking
3437 the intersection of the preferences of the recipients. Note that the
3438 MUST-implement algorithm, TripleDES, ensures that the intersection
3439 is not null. The implementation may use any mechanism to pick an
3440 algorithm in the intersection.
3442 If an implementation can decrypt a message that a keyholder doesn't
3443 have in their preferences, the implementation SHOULD decrypt the
3444 message anyway, but MUST warn the keyholder that the protocol has
3445 been violated. (For example, suppose that Alice, above, has software
3446 that implements all algorithms in this specification. Nonetheless,
3447 she prefers subsets for work or home. If she is sent a message
3448 encrypted with IDEA, which is not in her preferences, the software
3449 warns her that someone sent her an IDEA-encrypted message, but it
3450 would ideally decrypt it anyway.)
3452 12.2. Other Algorithm Preferences
3454 Other algorithm preferences work similarly to the symmetric
3455 algorithm preference, in that they specify which algorithms the
3456 keyholder accepts. There are two interesting cases that other
3457 comments need to be made about, though, the compression preferences
3458 and the hash preferences.
3460 12.2.1. Compression Preferences
3462 Compression has been an integral part of PGP since its first days.
3463 OpenPGP and all previous versions of PGP have offered compression.
3464 In this specification, the default is for messages to be compressed,
3465 although an implementation is not required to do so. Consequently,
3466 the compression preference gives a way for a keyholder to request
3467 that messages not be compressed, presumably because they are using a
3468 minimal implementation that does not include compression.
3469 Additionally, this gives a keyholder a way to state that it can
3470 support alternate algorithms.
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3475 Like the algorithm preferences, an implementation MUST NOT use an
3476 algorithm that is not in the preference vector. If the preferences
3477 are not present, then they are assumed to be [ZIP(1),
3478 UNCOMPRESSED(0)].
3480 Additionally, an implementation MUST implement this preference to
3481 the degree of recognizing when to send an uncompressed message. A
3482 robust implementation would satisfy this requirement by looking at
3483 the recipient's preference and acting accordingly. A minimal
3484 implementation can satisfy this requirement by never generating a
3485 compressed message, since all implementations can handle messages
3486 that have not been compressed.
3488 12.2.2. Hash Algorithm Preferences
3490 Typically, the choice of a hash algorithm is something the signer
3491 does, rather than the verifier, because a signer rarely knows who is
3492 going to be verifying the signature. This preference, though, allows
3493 a protocol based upon digital signatures ease in negotiation.
3495 Thus, if Alice is authenticating herself to Bob with a signature, it
3496 makes sense for her to use a hash algorithm that Bob's software
3497 uses. This preference allows Bob to state in his key which
3498 algorithms Alice may use.
3500 Since SHA1 is the MUST-implement hash algorithm, if it is not
3501 explicitly in the list, it is tacitly at the end. However, it is
3502 good form to place it there explicitly.
3504 12.3. Plaintext
3506 Algorithm 0, "plaintext," may only be used to denote secret keys
3507 that are stored in the clear. Implementations MUST NOT use plaintext
3508 in Symmetrically Encrypted Data Packets; they must use Literal Data
3509 Packets to encode unencrypted or literal data.
3511 12.4. RSA
3513 There are algorithm types for RSA-signature-only, and
3514 RSA-encrypt-only keys. These types are deprecated. The "key flags"
3515 subpacket in a signature is a much better way to express the same
3516 idea, and generalizes it to all algorithms. An implementation SHOULD
3517 NOT create such a key, but MAY interpret it.
3519 An implementation SHOULD NOT implement RSA keys of size less than
3520 1024 bits.
3522 12.5. DSA
3524 An implementation SHOULD NOT implement DSA keys of size less than
3525 1024 bits. Note that present DSA is limited to a maximum of 1024 bit
3526 keys, which are recommended for long-term use. Also, DSA keys MUST
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3531 be an even multiple of 64 bits long.
3533 12.6. Elgamal
3535 An implementation SHOULD NOT implement Elgamal keys of size less
3536 than 1024 bits.
3538 12.7. Reserved Algorithm Numbers
3540 A number of algorithm IDs have been reserved for algorithms that
3541 would be useful to use in an OpenPGP implementation, yet there are
3542 issues that prevent an implementer from actually implementing the
3543 algorithm. These are marked in the Public Algorithms section as
3544 "(reserved for)".
3546 The reserved public key algorithms, Elliptic Curve (18), ECDSA (19),
3547 and X9.42 (21) do not have the necessary parameters, parameter
3548 order, or semantics defined.
3550 Previous versions of OpenPGP permitted Elgamal [ELGAMAL] signatures
3551 with a public key identifier of 20. These are no longer permitted.
3552 An implementation MUST NOT generate such keys. An implementation
3553 MUST NOT generate Elgamal signatures.
3555 12.8. OpenPGP CFB mode
3557 OpenPGP does symmetric encryption using a variant of Cipher Feedback
3558 Mode (CFB mode). This section describes the procedure it uses in
3559 detail. This mode is what is used for Symmetrically Encrypted Data
3560 Packets; the mechanism used for encrypting secret key material is
3561 similar, but described in those sections above.
3563 In the description below, the value BS is the block size in octets
3564 of the cipher. Most ciphers have a block size of 8 octets. The AES
3565 and Twofish have a block size of 16 octets. Also note that the
3566 description below assumes that the IV and CFB arrays start with an
3567 index of 1 (unlike the C language, which assumes arrays start with a
3568 zero index).
3570 OpenPGP CFB mode uses an initialization vector (IV) of all zeros,
3571 and prefixes the plaintext with BS+2 octets of random data, such
3572 that octets BS+1 and BS+2 match octets BS-1 and BS. It does a CFB
3573 "resync" after encrypting those BS+2 octets.
3575 Thus, for an algorithm that has a block size of 8 octets (64 bits),
3576 the IV is 10 octets long and octets 7 and 8 of the IV are the same
3577 as octets 9 and 10. For an algorithm with a block size of 16 octets
3578 (128 bits), the IV is 18 octets long, and octets 17 and 18 replicate
3579 octets 15 and 16. Those extra two octets are an easy check for a
3580 correct key.
3584 Callas, et al. Expires Apr 11, 2006 [Page 64]
3585 \fINTERNET-DRAFT OpenPGP Message Format Oct 11, 2005
3587 Step by step, here is the procedure:
3589 1. The feedback register (FR) is set to the IV, which is all zeros.
3591 2. FR is encrypted to produce FRE (FR Encrypted). This is the
3592 encryption of an all-zero value.
3594 3. FRE is xored with the first BS octets of random data prefixed to
3595 the plaintext to produce C[1] through C[BS], the first BS octets
3596 of ciphertext.
3598 4. FR is loaded with C[1] through C[BS].
3600 5. FR is encrypted to produce FRE, the encryption of the first BS
3601 octets of ciphertext.
3603 6. The left two octets of FRE get xored with the next two octets of
3604 data that were prefixed to the plaintext. This produces C[BS+1]
3605 and C[BS+2], the next two octets of ciphertext.
3607 7. (The resync step) FR is loaded with C[3] through C[BS+2].
3609 8. FR is encrypted to produce FRE.
3611 9. FRE is xored with the first BS octets of the given plaintext,
3612 now that we have finished encrypting the BS+2 octets of prefixed
3613 data. This produces C[BS+3] through C[BS+(BS+2)], the next BS
3614 octets of ciphertext.
3616 10. FR is loaded with C[BS+3] to C[BS + (BS+2)] (which is C11-C18
3617 for an 8-octet block).
3619 11. FR is encrypted to produce FRE.
3621 12. FRE is xored with the next BS octets of plaintext, to produce
3622 the next BS octets of ciphertext. These are loaded into FR and
3623 the process is repeated until the plaintext is used up.
3625 13. Security Considerations
3627 * As with any technology involving cryptography, you should check
3628 the current literature to determine if any algorithms used here
3629 have been found to be vulnerable to attack.
3631 * This specification uses Public Key Cryptography technologies. It
3632 is assumed that the private key portion of a public-private key
3633 pair is controlled and secured by the proper party or parties.
3635 * Certain operations in this specification involve the use of
3636 random numbers. An appropriate entropy source should be used to
3637 generate these numbers. See RFC 1750.
3640 Callas, et al. Expires Apr 11, 2006 [Page 65]
3641 \fINTERNET-DRAFT OpenPGP Message Format Oct 11, 2005
3643 * The MD5 hash algorithm has been found to have weaknesses, with
3644 collisions found in a number of cases. MD5 is deprecated for use
3645 in OpenPGP. Implementations MUST NOT generate new signatures
3646 using MD5 as a hash function. They MAY continue to consider old
3647 signatures that used MD5 as valid.
3649 * SHA384 requires the same work as SHA512. In general, there are
3650 few reasons to use it -- you need a situation where one needs
3651 more security than SHA256, but do not want to have the 512-bit
3652 data length.
3654 * Many security protocol designers think that it is a bad idea to
3655 use a single key for both privacy (encryption) and integrity
3656 (signatures). In fact, this was one of the motivating forces
3657 behind the V4 key format with separate signature and encryption
3658 keys. If you as an implementer promote dual-use keys, you should
3659 at least be aware of this controversy.
3661 * The DSA algorithm will work with any 160-bit hash, but it is
3662 sensitive to the quality of the hash algorithm, if the hash
3663 algorithm is broken, it can leak the secret key. The Digital
3664 Signature Standard (DSS) specifies that DSA be used with SHA-1.
3665 RIPEMD-160 is considered by many cryptographers to be as strong.
3666 An implementation should take care which hash algorithms are
3667 used with DSA, as a weak hash can not only allow a signature to
3668 be forged, but could leak the secret key.
3670 * There is a somewhat-related potential security problem in
3671 signatures. If an attacker can find a message that hashes to the
3672 same hash with a different algorithm, a bogus signature
3673 structure can be constructed that evaluates correctly.
3675 For example, suppose Alice DSA signs message M using hash
3676 algorithm H. Suppose that Mallet finds a message M' that has the
3677 same hash value as M with H'. Mallet can then construct a
3678 signature block that verifies as Alice's signature of M' with
3679 H'. However, this would also constitute a weakness in either H
3680 or H' or both. Should this ever occur, a revision will have to
3681 be made to this document to revise the allowed hash algorithms.
3683 * If you are building an authentication system, the recipient may
3684 specify a preferred signing algorithm. However, the signer would
3685 be foolish to use a weak algorithm simply because the recipient
3686 requests it.
3688 * Some of the encryption algorithms mentioned in this document
3689 have been analyzed less than others. For example, although
3690 CAST5 is presently considered strong, it has been analyzed less
3691 than TripleDES. Other algorithms may have other controversies
3692 surrounding them.
3696 Callas, et al. Expires Apr 11, 2006 [Page 66]
3697 \fINTERNET-DRAFT OpenPGP Message Format Oct 11, 2005
3699 * In late summer 2002, Jallad, Katz, and Schneier published an
3700 interesting attack on the OpenPGP protocol and some of its
3701 implementations [JKS02]. In this attack, the attacker modifies a
3702 message and sends it to a user who then returns the erroneously
3703 decrypted message to the attacker. The attacker is thus using
3704 the user as a random oracle, and can often decrypt the message.
3706 Compressing data can ameliorate this attack. The incorrectly
3707 decrypted data nearly always decompresses in ways that defeats
3708 the attack. However, this is not a rigorous fix, and leaves open
3709 some small vulnerabilities. For example, if an implementation
3710 does not compress a message before encryption (perhaps because
3711 it knows it was already compressed), then that message is
3712 vulnerable. Because of this happenstance -- that modification
3713 attacks can be thwarted by decompression errors, an
3714 implementation SHOULD treat a decompression error as a security
3715 problem, not merely a data problem.
3717 This attack can be defeated by the use of Modification
3718 Detection, provided that the implementation does not let the
3719 user naively return the data to the attacker. An implementation
3720 MUST treat an MDC failure as a security problem, not merely a
3721 data problem.
3723 In either case, the implementation MAY allow the user access to
3724 the erroneous data, but MUST warn the user as to potential
3725 security problems should that data be returned to the sender.
3727 While this attack is somewhat obscure, requiring a special set
3728 of circumstances to create it, it is nonetheless quite serious
3729 as it permits someone to trick a user to decrypt a message.
3730 Consequently, it is important that:
3732 1. Implementers treat MDC errors and decompression failures as
3733 security problems.
3735 2. Implementers implement Modification Detection with all due
3736 speed and encourage its spread.
3738 3. Users migrate to implementations that support Modification
3739 Detection with all due speed.
3741 * PKCS1 has been found to be vulnerable to attacks in which a
3742 system that reports errors in padding differently from errors in
3743 decryption becomes a random oracle that can leak the private key
3744 in mere millions of queries. Implementations must be aware of
3745 this attack and prevent it from happening. The simplest solution
3746 is report a single error code for all variants of decryption
3747 errors so as not to leak information to an attacker.
3752 Callas, et al. Expires Apr 11, 2006 [Page 67]
3753 \fINTERNET-DRAFT OpenPGP Message Format Oct 11, 2005
3755 * Some technologies mentioned here may be subject to government
3756 control in some countries.
3758 * In winter 2005, Serge Mister and Robert Zuccherato from Entrust
3759 released a paper describing a way that the "quick check" in
3760 OpenPGP CFB mode can be used with a random oracle to decrypt two
3761 octets of every cipher block [MZ05]. They recommend as
3762 prevention not using the quick check at all.
3764 Many implementers have taken this advice to heart for any data
3765 that is symmetrically encrypted and for which the session key is
3766 public-key encrypted. In this case, the quick check is not
3767 needed as the public key encryption of the session key should
3768 guarantee that it is the right session key. In other cases, the
3769 implementation should use the quick check with care.
3771 On the one hand, there is a danger to using it if there is a
3772 random oracle that can leak information to an attacker. In
3773 plainer language, there is a danger to using the quick check if
3774 timing information about the check can be exposed to an
3775 attacker, particularly via an automated service that allows
3776 rapidly repeated queries
3778 On the other hand, it is inconvenient to the user to be informed
3779 that they typed in the wrong passphrase only after a petabyte of
3780 data is decrypted. There are many cases in cryptographic
3781 engineering where the implementer must use care and wisdom, and
3782 this is one.
3784 14. Implementation Nits
3786 This section is a collection of comments to help an implementer,
3787 particularly with an eye to backward compatibility. Previous
3788 implementations of PGP are not OpenPGP-compliant. Often the
3789 differences are small, but small differences are frequently more
3790 vexing than large differences. Thus, this is a non-comprehensive
3791 list of potential problems and gotchas for a developer who is trying
3792 to be backward-compatible.
3794 * The IDEA algorithm is patented, and yet it is required for PGP
3795 2.x interoperability. It is also the defacto preferred algorithm
3796 for a V3 key with a V3 self-signature (or no self-signature).
3798 * When exporting a private key, PGP 2.x generates the header
3799 "BEGIN PGP SECRET KEY BLOCK" instead of "BEGIN PGP PRIVATE KEY
3800 BLOCK". All previous versions ignore the implied data type, and
3801 look directly at the packet data type.
3803 * PGP 2.0 through 2.5 generated V2 Public Key Packets. These are
3804 identical to the deprecated V3 keys except for the version
3805 number. An implementation MUST NOT generate them and may accept
3806 or reject them as it sees fit. Some older PGP versions generated
3808 Callas, et al. Expires Apr 11, 2006 [Page 68]
3809 \fINTERNET-DRAFT OpenPGP Message Format Oct 11, 2005
3811 V2 PKESK packets (Tag 1) as well. An implementation may accept
3812 or reject V2 PKESK packets as it sees fit, and MUST NOT generate
3813 them.
3815 * PGP 2.6.x will not accept key-material packets with versions
3816 greater than 3.
3818 * There are many ways possible for two keys to have the same key
3819 material, but different fingerprints (and thus key IDs). Perhaps
3820 the most interesting is an RSA key that has been "upgraded" to
3821 V4 format, but since a V4 fingerprint is constructed by hashing
3822 the key creation time along with other things, two V4 keys
3823 created at different times, yet with the same key material will
3824 have different fingerprints.
3826 * If an implementation is using zlib to interoperate with PGP 2.x,
3827 then the "windowBits" parameter should be set to -13.
3829 * PGP 2.6.X and 5.0 do not trim trailing whitespace from a
3830 "canonical text" signature. They only remove it from cleartext
3831 signatures. These signatures are not OpenPGP compliant --
3832 OpenPGP requires trimming the whitespace. If you wish to
3833 interoperate with PGP 2.6.X or PGP 5, you may wish to accept
3834 these non-compliant signatures.
3836 15. Authors' Addresses
3838 The working group can be contacted via the current chair:
3840 Derek Atkins
3841 IHTFP Consulting, Inc.
3842 6 Farragut Ave
3843 Somerville, MA 02144 USA
3844 Email: derek@ihtfp.com
3845 Tel: +1 617 623 3745
3847 The principal authors of this draft are:
3849 Jon Callas
3850 Email: jon@callas.org
3852 Lutz Donnerhacke
3853 IKS GmbH
3854 Wildenbruchstr. 15
3855 07745 Jena, Germany
3857 EMail: lutz@iks-jena.de
3859 Hal Finney
3860 Email: hal@finney.org
3864 Callas, et al. Expires Apr 11, 2006 [Page 69]
3865 \fINTERNET-DRAFT OpenPGP Message Format Oct 11, 2005
3867 Rodney Thayer
3868 Email: rodney@tillerman.to
3870 This memo also draws on much previous work from a number of other
3871 authors who include: Derek Atkins, Charles Breed, Dave Del Torto,
3872 Marc Dyksterhouse, Gail Haspert, Gene Hoffman, Paul Hoffman, Raph
3873 Levien, Colin Plumb, Will Price, David Shaw, William Stallings, Mark
3874 Weaver, and Philip R. Zimmermann.
3876 16. References (Normative)
3879 [AES] Advanced Encryption Standards Questions and Answers
3880 <http://csrc.nist.gov/encryption/aes/round2/
3881 aesfact.html>
3883 <http://csrc.nist.gov/encryption/aes/round2/
3884 r2algs.html#Rijndael>
3886 [BLOWFISH] Schneier, B. "Description of a New Variable-Length
3887 Key, 64-Bit Block Cipher (Blowfish)" Fast Software
3888 Encryption, Cambridge Security Workshop Proceedings
3889 (December 1993), Springer-Verlag, 1994, pp191-204
3890 <http://www.counterpane.com/bfsverlag.html>
3892 [BZ2] J. Seward, jseward@acm.org, "The Bzip2 and libbzip2
3893 home page"
3894 <http://sources.redhat.com/bzip2/>
3896 [ELGAMAL] T. Elgamal, "A Public-Key Cryptosystem and a
3897 Signature Scheme Based on Discrete Logarithms,"
3898 IEEE Transactions on Information Theory, v. IT-31,
3899 n. 4, 1985, pp. 469-472.
3901 [FIPS180] Secure Hash Signature Standard (SHS) (FIPS PUB
3902 180-2).
3903 <http://csrc.nist.gov/publications/fips/
3904 fips180-2/fips180-2withchangenotice.pdf>
3906 [FIPS186] Digital Signature Standard (DSS) (FIPS PUB 186-2).
3907 <http://csrc.nist.gov/publications/fips/fips186-2/
3908 fips186-2-change1.pdf>
3910 [HAC] Alfred Menezes, Paul van Oorschot, and Scott
3911 Vanstone, "Handbook of Applied Cryptography," CRC
3912 Press, 1996.
3913 <http://www.cacr.math.uwaterloo.ca/hac/>
3915 [IDEA] Lai, X, "On the design and security of block
3916 ciphers", ETH Series in Information Processing,
3917 J.L. Massey (editor), Vol. 1, Hartung-Gorre Verlag
3918 Knostanz, Technische Hochschule (Zurich), 1992
3920 Callas, et al. Expires Apr 11, 2006 [Page 70]
3921 \fINTERNET-DRAFT OpenPGP Message Format Oct 11, 2005
3923 [ISO10646] ISO/IEC 10646-1:1993. International Standard --
3924 Information technology -- Universal Multiple-Octet
3925 Coded Character Set (UCS) -- Part 1: Architecture
3926 and Basic Multilingual Plane.
3928 [JFIF] JPEG File Interchange Format (Version 1.02).
3929 Eric Hamilton, C-Cube Microsystems, Milpitas, CA,
3930 September 1, 1992.
3932 [RFC1423] Balenson, D., "Privacy Enhancement for Internet
3933 Electronic Mail: Part III: Algorithms, Modes, and
3934 Identifiers", RFC 1423, October 1993.
3936 [RFC1641] Goldsmith, D. and M. Davis, "Using Unicode with
3937 MIME", RFC 1641, July 1994.
3939 [RFC1750] Eastlake, D., Crocker, S. and J. Schiller,
3940 "Randomness Recommendations for Security", RFC
3941 1750, December 1994.
3943 [RFC1951] Deutsch, P., "DEFLATE Compressed Data Format
3944 Specification version 1.3.", RFC 1951, May 1996.
3946 [RFC1991] Atkins, D., Stallings, W. and P. Zimmermann, "PGP
3947 Message Exchange Formats", RFC 1991, August 1996.
3949 [RFC2045] Borenstein, N. and N. Freed, "Multipurpose Internet
3950 Mail Extensions (MIME) Part One: Format of Internet
3951 Message Bodies.", RFC 2045, November 1996.
3953 [RFC2144] Adams, C., "The CAST-128 Encryption Algorithm", RFC
3954 2144, May 1997.
3956 [RFC2279] Yergeau., F., "UTF-8, a transformation format of
3957 Unicode and ISO 10646", RFC 2279, January 1998.
3959 [RFC2437] B. Kaliski and J. Staddon, " PKCS #1: RSA
3960 Cryptography Specifications Version 2.0",
3961 RFC 2437, October 1998.
3963 [RFC2822] Resnick, P., "Internet Message Format", RFC 2822.
3965 [RFC3156] M. Elkins, D. Del Torto, R. Levien, T. Roessler,
3966 "MIME Security with OpenPGP", RFC 3156,
3967 August 2001.
3969 [SCHNEIER] Schneier, B., "Applied Cryptography Second Edition:
3970 protocols, algorithms, and source code in C", 1996.
3972 [TWOFISH] B. Schneier, J. Kelsey, D. Whiting, D. Wagner, C.
3973 Hall, and N. Ferguson, "The Twofish Encryption
3974 Algorithm", John Wiley & Sons, 1999.
3976 Callas, et al. Expires Apr 11, 2006 [Page 71]
3977 \fINTERNET-DRAFT OpenPGP Message Format Oct 11, 2005
3979 17. References (Non-Normative)
3982 [BLEICHENBACHER] Bleichenbacher, Daniel, "Generating Elgamal
3983 signatures without knowing the secret key,"
3984 Eurocrypt 96. Note that the version in the
3985 proceedings has an error. A revised version is
3986 available at the time of writing from
3987 <ftp://ftp.inf.ethz.ch/pub/publications/papers/ti
3988 /isc/ElGamal.ps>
3990 [DONNERHACKE] Donnerhacke, L., et. al, "PGP263in - an improved
3991 international version of PGP", ftp://ftp.iks-
3992 jena.de/mitarb/lutz/crypt/software/pgp/
3994 [JKS02] Kahil Jallad, Jonathan Katz, Bruce Schneier
3995 "Implementation of Chosen-Ciphertext Attacks
3996 against PGP and GnuPG"
3997 http://www.counterpane.com/pgp-attack.html
3999 [MZ05] Serge Mister, Robert Zuccherato, "An Attack on
4000 CFB Mode Encryption As Used By OpenPGP," IACR
4001 ePrint Archive: Report 2005/033, 8 Feb 2005
4002 http://eprint.iacr.org/2005/033
4004 [RFC1983] Malkin, G., "Internet Users' Glossary", FYI 18, RFC
4005 1983, August 1996.
4007 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
4008 Requirement Level", BCP 14, RFC 2119, March 1997.
4011 18. Full Copyright Statement
4013 Copyright (C) 2005 by The Internet Society.
4015 This document is subject to the rights, licenses and restrictions
4016 contained in BCP 78, and except as set forth therein, the authors
4017 retain all their rights.
4019 This document and the information contained herein are provided on
4020 an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
4021 REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE
4022 INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR
4023 IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
4024 THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
4025 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
4027 This document and translations of it may be copied and furnished to
4028 others, and derivative works that comment on or otherwise explain it
4029 or assist in its implementation may be prepared, copied, published
4030 and distributed, in whole or in part, without restriction of any
4032 Callas, et al. Expires Apr 11, 2006 [Page 72]
4033 \fINTERNET-DRAFT OpenPGP Message Format Oct 11, 2005
4035 kind, provided that the above copyright notice and this paragraph
4036 are included on all such copies and derivative works. However, this
4037 document itself may not be modified in any way, such as by removing
4038 the copyright notice or references to the Internet Society or other
4039 Internet organizations, except as needed for the purpose of
4040 developing Internet standards in which case the procedures for
4041 copyrights defined in the Internet Standards process must be
4042 followed, or as required to translate it into languages other than
4043 English.
4045 The limited permissions granted above are perpetual and will not be
4046 revoked by the Internet Society or its successors or assigns.
4088 Callas, et al. Expires Apr 11, 2006 [Page 73]