1 2 3 4 5 6 7Network Working Group J. Callas 8Request for Comments: 2440 Network Associates 9Category: Standards Track L. Donnerhacke 10 IN-Root-CA Individual Network e.V. 11 H. Finney 12 Network Associates 13 R. Thayer 14 EIS Corporation 15 November 1998 16 17 18 OpenPGP Message Format 19 20Status of this Memo 21 22 This document specifies an Internet standards track protocol for the 23 Internet community, and requests discussion and suggestions for 24 improvements. Please refer to the current edition of the "Internet 25 Official Protocol Standards" (STD 1) for the standardization state 26 and status of this protocol. Distribution of this memo is unlimited. 27 28Copyright Notice 29 30 Copyright (C) The Internet Society (1998). All Rights Reserved. 31 32IESG Note 33 34 This document defines many tag values, yet it doesn't describe a 35 mechanism for adding new tags (for new features). Traditionally the 36 Internet Assigned Numbers Authority (IANA) handles the allocation of 37 new values for future expansion and RFCs usually define the procedure 38 to be used by the IANA. However, there are subtle (and not so 39 subtle) interactions that may occur in this protocol between new 40 features and existing features which result in a significant 41 reduction in over all security. Therefore, this document does not 42 define an extension procedure. Instead requests to define new tag 43 values (say for new encryption algorithms for example) should be 44 forwarded to the IESG Security Area Directors for consideration or 45 forwarding to the appropriate IETF Working Group for consideration. 46 47Abstract 48 49 This document is maintained in order to publish all necessary 50 information needed to develop interoperable applications based on the 51 OpenPGP format. It is not a step-by-step cookbook for writing an 52 application. It describes only the format and methods needed to read, 53 check, generate, and write conforming packets crossing any network. 54 It does not deal with storage and implementation questions. It does, 55 56 57 58Callas, et. al. Standards Track [Page 1] 59 60RFC 2440 OpenPGP Message Format November 1998 61 62 63 however, discuss implementation issues necessary to avoid security 64 flaws. 65 66 Open-PGP software uses a combination of strong public-key and 67 symmetric cryptography to provide security services for electronic 68 communications and data storage. These services include 69 confidentiality, key management, authentication, and digital 70 signatures. This document specifies the message formats used in 71 OpenPGP. 72 73Table of Contents 74 75 Status of this Memo 1 76 IESG Note 1 77 Abstract 1 78 Table of Contents 2 79 1. Introduction 4 80 1.1. Terms 5 81 2. General functions 5 82 2.1. Confidentiality via Encryption 5 83 2.2. Authentication via Digital signature 6 84 2.3. Compression 7 85 2.4. Conversion to Radix-64 7 86 2.5. Signature-Only Applications 7 87 3. Data Element Formats 7 88 3.1. Scalar numbers 8 89 3.2. Multi-Precision Integers 8 90 3.3. Key IDs 8 91 3.4. Text 8 92 3.5. Time fields 9 93 3.6. String-to-key (S2K) specifiers 9 94 3.6.1. String-to-key (S2k) specifier types 9 95 3.6.1.1. Simple S2K 9 96 3.6.1.2. Salted S2K 10 97 3.6.1.3. Iterated and Salted S2K 10 98 3.6.2. String-to-key usage 11 99 3.6.2.1. Secret key encryption 11 100 3.6.2.2. Symmetric-key message encryption 11 101 4. Packet Syntax 12 102 4.1. Overview 12 103 4.2. Packet Headers 12 104 4.2.1. Old-Format Packet Lengths 13 105 4.2.2. New-Format Packet Lengths 13 106 4.2.2.1. One-Octet Lengths 14 107 4.2.2.2. Two-Octet Lengths 14 108 4.2.2.3. Five-Octet Lengths 14 109 4.2.2.4. Partial Body Lengths 14 110 4.2.3. Packet Length Examples 14 111 112 113 114Callas, et. al. Standards Track [Page 2] 115 116RFC 2440 OpenPGP Message Format November 1998 117 118 119 4.3. Packet Tags 15 120 5. Packet Types 16 121 5.1. Public-Key Encrypted Session Key Packets (Tag 1) 16 122 5.2. Signature Packet (Tag 2) 17 123 5.2.1. Signature Types 17 124 5.2.2. Version 3 Signature Packet Format 19 125 5.2.3. Version 4 Signature Packet Format 21 126 5.2.3.1. Signature Subpacket Specification 22 127 5.2.3.2. Signature Subpacket Types 24 128 5.2.3.3. Signature creation time 25 129 5.2.3.4. Issuer 25 130 5.2.3.5. Key expiration time 25 131 5.2.3.6. Preferred symmetric algorithms 25 132 5.2.3.7. Preferred hash algorithms 25 133 5.2.3.8. Preferred compression algorithms 26 134 5.2.3.9. Signature expiration time 26 135 5.2.3.10.Exportable Certification 26 136 5.2.3.11.Revocable 27 137 5.2.3.12.Trust signature 27 138 5.2.3.13.Regular expression 27 139 5.2.3.14.Revocation key 27 140 5.2.3.15.Notation Data 28 141 5.2.3.16.Key server preferences 28 142 5.2.3.17.Preferred key server 29 143 5.2.3.18.Primary user id 29 144 5.2.3.19.Policy URL 29 145 5.2.3.20.Key Flags 29 146 5.2.3.21.Signer's User ID 30 147 5.2.3.22.Reason for Revocation 30 148 5.2.4. Computing Signatures 31 149 5.2.4.1. Subpacket Hints 32 150 5.3. Symmetric-Key Encrypted Session-Key Packets (Tag 3) 32 151 5.4. One-Pass Signature Packets (Tag 4) 33 152 5.5. Key Material Packet 34 153 5.5.1. Key Packet Variants 34 154 5.5.1.1. Public Key Packet (Tag 6) 34 155 5.5.1.2. Public Subkey Packet (Tag 14) 34 156 5.5.1.3. Secret Key Packet (Tag 5) 35 157 5.5.1.4. Secret Subkey Packet (Tag 7) 35 158 5.5.2. Public Key Packet Formats 35 159 5.5.3. Secret Key Packet Formats 37 160 5.6. Compressed Data Packet (Tag 8) 38 161 5.7. Symmetrically Encrypted Data Packet (Tag 9) 39 162 5.8. Marker Packet (Obsolete Literal Packet) (Tag 10) 39 163 5.9. Literal Data Packet (Tag 11) 40 164 5.10. Trust Packet (Tag 12) 40 165 5.11. User ID Packet (Tag 13) 41 166 6. Radix-64 Conversions 41 167 168 169 170Callas, et. al. Standards Track [Page 3] 171 172RFC 2440 OpenPGP Message Format November 1998 173 174 175 6.1. An Implementation of the CRC-24 in "C" 42 176 6.2. Forming ASCII Armor 42 177 6.3. Encoding Binary in Radix-64 44 178 6.4. Decoding Radix-64 46 179 6.5. Examples of Radix-64 46 180 6.6. Example of an ASCII Armored Message 47 181 7. Cleartext signature framework 47 182 7.1. Dash-Escaped Text 47 183 8. Regular Expressions 48 184 9. Constants 49 185 9.1. Public Key Algorithms 49 186 9.2. Symmetric Key Algorithms 49 187 9.3. Compression Algorithms 50 188 9.4. Hash Algorithms 50 189 10. Packet Composition 50 190 10.1. Transferable Public Keys 50 191 10.2. OpenPGP Messages 52 192 10.3. Detached Signatures 52 193 11. Enhanced Key Formats 52 194 11.1. Key Structures 52 195 11.2. Key IDs and Fingerprints 53 196 12. Notes on Algorithms 54 197 12.1. Symmetric Algorithm Preferences 54 198 12.2. Other Algorithm Preferences 55 199 12.2.1. Compression Preferences 56 200 12.2.2. Hash Algorithm Preferences 56 201 12.3. Plaintext 56 202 12.4. RSA 56 203 12.5. Elgamal 57 204 12.6. DSA 58 205 12.7. Reserved Algorithm Numbers 58 206 12.8. OpenPGP CFB mode 58 207 13. Security Considerations 59 208 14. Implementation Nits 60 209 15. Authors and Working Group Chair 62 210 16. References 63 211 17. Full Copyright Statement 65 212 2131. Introduction 214 215 This document provides information on the message-exchange packet 216 formats used by OpenPGP to provide encryption, decryption, signing, 217 and key management functions. It builds on the foundation provided in 218 RFC 1991 "PGP Message Exchange Formats." 219 220 221 222 223 224 225 226Callas, et. al. Standards Track [Page 4] 227 228RFC 2440 OpenPGP Message Format November 1998 229 230 2311.1. Terms 232 233 * OpenPGP - This is a definition for security software that uses 234 PGP 5.x as a basis. 235 236 * PGP - Pretty Good Privacy. PGP is a family of software systems 237 developed by Philip R. Zimmermann from which OpenPGP is based. 238 239 * PGP 2.6.x - This version of PGP has many variants, hence the term 240 PGP 2.6.x. It used only RSA, MD5, and IDEA for its cryptographic 241 transforms. An informational RFC, RFC 1991, was written 242 describing this version of PGP. 243 244 * PGP 5.x - This version of PGP is formerly known as "PGP 3" in the 245 community and also in the predecessor of this document, RFC 1991. 246 It has new formats and corrects a number of problems in the PGP 247 2.6.x design. It is referred to here as PGP 5.x because that 248 software was the first release of the "PGP 3" code base. 249 250 "PGP", "Pretty Good", and "Pretty Good Privacy" are trademarks of 251 Network Associates, Inc. and are used with permission. 252 253 This document uses the terms "MUST", "SHOULD", and "MAY" as defined 254 in RFC 2119, along with the negated forms of those terms. 255 2562. General functions 257 258 OpenPGP provides data integrity services for messages and data files 259 by using these core technologies: 260 261 - digital signatures 262 263 - encryption 264 265 - compression 266 267 - radix-64 conversion 268 269 In addition, OpenPGP provides key management and certificate 270 services, but many of these are beyond the scope of this document. 271 2722.1. Confidentiality via Encryption 273 274 OpenPGP uses two encryption methods to provide confidentiality: 275 symmetric-key encryption and public key encryption. With public-key 276 encryption, the object is encrypted using a symmetric encryption 277 algorithm. Each symmetric key is used only once. A new "session key" 278 is generated as a random number for each message. Since it is used 279 280 281 282Callas, et. al. Standards Track [Page 5] 283 284RFC 2440 OpenPGP Message Format November 1998 285 286 287 only once, the session key is bound to the message and transmitted 288 with it. To protect the key, it is encrypted with the receiver's 289 public key. The sequence is as follows: 290 291 1. The sender creates a message. 292 293 2. The sending OpenPGP generates a random number to be used as a 294 session key for this message only. 295 296 3. The session key is encrypted using each recipient's public key. 297 These "encrypted session keys" start the message. 298 299 4. The sending OpenPGP encrypts the message using the session key, 300 which forms the remainder of the message. Note that the message 301 is also usually compressed. 302 303 5. The receiving OpenPGP decrypts the session key using the 304 recipient's private key. 305 306 6. The receiving OpenPGP decrypts the message using the session key. 307 If the message was compressed, it will be decompressed. 308 309 With symmetric-key encryption, an object may be encrypted with a 310 symmetric key derived from a passphrase (or other shared secret), or 311 a two-stage mechanism similar to the public-key method described 312 above in which a session key is itself encrypted with a symmetric 313 algorithm keyed from a shared secret. 314 315 Both digital signature and confidentiality services may be applied to 316 the same message. First, a signature is generated for the message and 317 attached to the message. Then, the message plus signature is 318 encrypted using a symmetric session key. Finally, the session key is 319 encrypted using public-key encryption and prefixed to the encrypted 320 block. 321 3222.2. Authentication via Digital signature 323 324 The digital signature uses a hash code or message digest algorithm, 325 and a public-key signature algorithm. The sequence is as follows: 326 327 1. The sender creates a message. 328 329 2. The sending software generates a hash code of the message. 330 331 3. The sending software generates a signature from the hash code 332 using the sender's private key. 333 334 4. The binary signature is attached to the message. 335 336 337 338Callas, et. al. Standards Track [Page 6] 339 340RFC 2440 OpenPGP Message Format November 1998 341 342 343 5. The receiving software keeps a copy of the message signature. 344 345 6. The receiving software generates a new hash code for the 346 received message and verifies it using the message's signature. 347 If the verification is successful, the message is accepted as 348 authentic. 349 3502.3. Compression 351 352 OpenPGP implementations MAY compress the message after applying the 353 signature but before encryption. 354 3552.4. Conversion to Radix-64 356 357 OpenPGP's underlying native representation for encrypted messages, 358 signature certificates, and keys is a stream of arbitrary octets. 359 Some systems only permit the use of blocks consisting of seven-bit, 360 printable text. For transporting OpenPGP's native raw binary octets 361 through channels that are not safe to raw binary data, a printable 362 encoding of these binary octets is needed. OpenPGP provides the 363 service of converting the raw 8-bit binary octet stream to a stream 364 of printable ASCII characters, called Radix-64 encoding or ASCII 365 Armor. 366 367 Implementations SHOULD provide Radix-64 conversions. 368 369 Note that many applications, particularly messaging applications, 370 will want more advanced features as described in the OpenPGP-MIME 371 document, RFC 2015. An application that implements OpenPGP for 372 messaging SHOULD implement OpenPGP-MIME. 373 3742.5. Signature-Only Applications 375 376 OpenPGP is designed for applications that use both encryption and 377 signatures, but there are a number of problems that are solved by a 378 signature-only implementation. Although this specification requires 379 both encryption and signatures, it is reasonable for there to be 380 subset implementations that are non-comformant only in that they omit 381 encryption. 382 3833. Data Element Formats 384 385 This section describes the data elements used by OpenPGP. 386 387 388 389 390 391 392 393 394Callas, et. al. Standards Track [Page 7] 395 396RFC 2440 OpenPGP Message Format November 1998 397 398 3993.1. Scalar numbers 400 401 Scalar numbers are unsigned, and are always stored in big-endian 402 format. Using n[k] to refer to the kth octet being interpreted, the 403 value of a two-octet scalar is ((n[0] << 8) + n[1]). The value of a 404 four-octet scalar is ((n[0] << 24) + (n[1] << 16) + (n[2] << 8) + 405 n[3]). 406 4073.2. Multi-Precision Integers 408 409 Multi-Precision Integers (also called MPIs) are unsigned integers 410 used to hold large integers such as the ones used in cryptographic 411 calculations. 412 413 An MPI consists of two pieces: a two-octet scalar that is the length 414 of the MPI in bits followed by a string of octets that contain the 415 actual integer. 416 417 These octets form a big-endian number; a big-endian number can be 418 made into an MPI by prefixing it with the appropriate length. 419 420 Examples: 421 422 (all numbers are in hexadecimal) 423 424 The string of octets [00 01 01] forms an MPI with the value 1. The 425 string [00 09 01 FF] forms an MPI with the value of 511. 426 427 Additional rules: 428 429 The size of an MPI is ((MPI.length + 7) / 8) + 2 octets. 430 431 The length field of an MPI describes the length starting from its 432 most significant non-zero bit. Thus, the MPI [00 02 01] is not formed 433 correctly. It should be [00 01 01]. 434 4353.3. Key IDs 436 437 A Key ID is an eight-octet scalar that identifies a key. 438 Implementations SHOULD NOT assume that Key IDs are unique. The 439 section, "Enhanced Key Formats" below describes how Key IDs are 440 formed. 441 4423.4. Text 443 444 The default character set for text is the UTF-8 [RFC2279] encoding of 445 Unicode [ISO10646]. 446 447 448 449 450Callas, et. al. Standards Track [Page 8] 451 452RFC 2440 OpenPGP Message Format November 1998 453 454 4553.5. Time fields 456 457 A time field is an unsigned four-octet number containing the number 458 of seconds elapsed since midnight, 1 January 1970 UTC. 459 4603.6. String-to-key (S2K) specifiers 461 462 String-to-key (S2K) specifiers are used to convert passphrase strings 463 into symmetric-key encryption/decryption keys. They are used in two 464 places, currently: to encrypt the secret part of private keys in the 465 private keyring, and to convert passphrases to encryption keys for 466 symmetrically encrypted messages. 467 4683.6.1. String-to-key (S2k) specifier types 469 470 There are three types of S2K specifiers currently supported, as 471 follows: 472 4733.6.1.1. Simple S2K 474 475 This directly hashes the string to produce the key data. See below 476 for how this hashing is done. 477 478 Octet 0: 0x00 479 Octet 1: hash algorithm 480 481 Simple S2K hashes the passphrase to produce the session key. The 482 manner in which this is done depends on the size of the session key 483 (which will depend on the cipher used) and the size of the hash 484 algorithm's output. If the hash size is greater than or equal to the 485 session key size, the high-order (leftmost) octets of the hash are 486 used as the key. 487 488 If the hash size is less than the key size, multiple instances of the 489 hash context are created -- enough to produce the required key data. 490 These instances are preloaded with 0, 1, 2, ... octets of zeros (that 491 is to say, the first instance has no preloading, the second gets 492 preloaded with 1 octet of zero, the third is preloaded with two 493 octets of zeros, and so forth). 494 495 As the data is hashed, it is given independently to each hash 496 context. Since the contexts have been initialized differently, they 497 will each produce different hash output. Once the passphrase is 498 hashed, the output data from the multiple hashes is concatenated, 499 first hash leftmost, to produce the key data, with any excess octets 500 on the right discarded. 501 502 503 504 505 506Callas, et. al. Standards Track [Page 9] 507 508RFC 2440 OpenPGP Message Format November 1998 509 510 5113.6.1.2. Salted S2K 512 513 This includes a "salt" value in the S2K specifier -- some arbitrary 514 data -- that gets hashed along with the passphrase string, to help 515 prevent dictionary attacks. 516 517 Octet 0: 0x01 518 Octet 1: hash algorithm 519 Octets 2-9: 8-octet salt value 520 521 Salted S2K is exactly like Simple S2K, except that the input to the 522 hash function(s) consists of the 8 octets of salt from the S2K 523 specifier, followed by the passphrase. 524 5253.6.1.3. Iterated and Salted S2K 526 527 This includes both a salt and an octet count. The salt is combined 528 with the passphrase and the resulting value is hashed repeatedly. 529 This further increases the amount of work an attacker must do to try 530 dictionary attacks. 531 532 Octet 0: 0x03 533 Octet 1: hash algorithm 534 Octets 2-9: 8-octet salt value 535 Octet 10: count, a one-octet, coded value 536 537 The count is coded into a one-octet number using the following 538 formula: 539 540 #define EXPBIAS 6 541 count = ((Int32)16 + (c & 15)) << ((c >> 4) + EXPBIAS); 542 543 The above formula is in C, where "Int32" is a type for a 32-bit 544 integer, and the variable "c" is the coded count, Octet 10. 545 546 Iterated-Salted S2K hashes the passphrase and salt data multiple 547 times. The total number of octets to be hashed is specified in the 548 encoded count in the S2K specifier. Note that the resulting count 549 value is an octet count of how many octets will be hashed, not an 550 iteration count. 551 552 Initially, one or more hash contexts are set up as with the other S2K 553 algorithms, depending on how many octets of key data are needed. 554 Then the salt, followed by the passphrase data is repeatedly hashed 555 until the number of octets specified by the octet count has been 556 hashed. The one exception is that if the octet count is less than 557 the size of the salt plus passphrase, the full salt plus passphrase 558 will be hashed even though that is greater than the octet count. 559 560 561 562Callas, et. al. Standards Track [Page 10] 563 564RFC 2440 OpenPGP Message Format November 1998 565 566 567 After the hashing is done the data is unloaded from the hash 568 context(s) as with the other S2K algorithms. 569 5703.6.2. String-to-key usage 571 572 Implementations SHOULD use salted or iterated-and-salted S2K 573 specifiers, as simple S2K specifiers are more vulnerable to 574 dictionary attacks. 575 5763.6.2.1. Secret key encryption 577 578 An S2K specifier can be stored in the secret keyring to specify how 579 to convert the passphrase to a key that unlocks the secret data. 580 Older versions of PGP just stored a cipher algorithm octet preceding 581 the secret data or a zero to indicate that the secret data was 582 unencrypted. The MD5 hash function was always used to convert the 583 passphrase to a key for the specified cipher algorithm. 584 585 For compatibility, when an S2K specifier is used, the special value 586 255 is stored in the position where the hash algorithm octet would 587 have been in the old data structure. This is then followed 588 immediately by a one-octet algorithm identifier, and then by the S2K 589 specifier as encoded above. 590 591 Therefore, preceding the secret data there will be one of these 592 possibilities: 593 594 0: secret data is unencrypted (no pass phrase) 595 255: followed by algorithm octet and S2K specifier 596 Cipher alg: use Simple S2K algorithm using MD5 hash 597 598 This last possibility, the cipher algorithm number with an implicit 599 use of MD5 and IDEA, is provided for backward compatibility; it MAY 600 be understood, but SHOULD NOT be generated, and is deprecated. 601 602 These are followed by an 8-octet Initial Vector for the decryption of 603 the secret values, if they are encrypted, and then the secret key 604 values themselves. 605 6063.6.2.2. Symmetric-key message encryption 607 608 OpenPGP can create a Symmetric-key Encrypted Session Key (ESK) packet 609 at the front of a message. This is used to allow S2K specifiers to 610 be used for the passphrase conversion or to create messages with a 611 mix of symmetric-key ESKs and public-key ESKs. This allows a message 612 to be decrypted either with a passphrase or a public key. 613 614 615 616 617 618Callas, et. al. Standards Track [Page 11] 619 620RFC 2440 OpenPGP Message Format November 1998 621 622 623 PGP 2.X always used IDEA with Simple string-to-key conversion when 624 encrypting a message with a symmetric algorithm. This is deprecated, 625 but MAY be used for backward-compatibility. 626 6274. Packet Syntax 628 629 This section describes the packets used by OpenPGP. 630 6314.1. Overview 632 633 An OpenPGP message is constructed from a number of records that are 634 traditionally called packets. A packet is a chunk of data that has a 635 tag specifying its meaning. An OpenPGP message, keyring, certificate, 636 and so forth consists of a number of packets. Some of those packets 637 may contain other OpenPGP packets (for example, a compressed data 638 packet, when uncompressed, contains OpenPGP packets). 639 640 Each packet consists of a packet header, followed by the packet body. 641 The packet header is of variable length. 642 6434.2. Packet Headers 644 645 The first octet of the packet header is called the "Packet Tag." It 646 determines the format of the header and denotes the packet contents. 647 The remainder of the packet header is the length of the packet. 648 649 Note that the most significant bit is the left-most bit, called bit 650 7. A mask for this bit is 0x80 in hexadecimal. 651 652 +---------------+ 653 PTag |7 6 5 4 3 2 1 0| 654 +---------------+ 655 Bit 7 -- Always one 656 Bit 6 -- New packet format if set 657 658 PGP 2.6.x only uses old format packets. Thus, software that 659 interoperates with those versions of PGP must only use old format 660 packets. If interoperability is not an issue, either format may be 661 used. Note that old format packets have four bits of content tags, 662 and new format packets have six; some features cannot be used and 663 still be backward-compatible. 664 665 Old format packets contain: 666 667 Bits 5-2 -- content tag 668 Bits 1-0 - length-type 669 670 671 672 673 674Callas, et. al. Standards Track [Page 12] 675 676RFC 2440 OpenPGP Message Format November 1998 677 678 679 New format packets contain: 680 681 Bits 5-0 -- content tag 682 6834.2.1. Old-Format Packet Lengths 684 685 The meaning of the length-type in old-format packets is: 686 687 0 - The packet has a one-octet length. The header is 2 octets long. 688 689 1 - The packet has a two-octet length. The header is 3 octets long. 690 691 2 - The packet has a four-octet length. The header is 5 octets long. 692 693 3 - The packet is of indeterminate length. The header is 1 octet 694 long, and the implementation must determine how long the packet 695 is. If the packet is in a file, this means that the packet 696 extends until the end of the file. In general, an implementation 697 SHOULD NOT use indeterminate length packets except where the end 698 of the data will be clear from the context, and even then it is 699 better to use a definite length, or a new-format header. The 700 new-format headers described below have a mechanism for precisely 701 encoding data of indeterminate length. 702 7034.2.2. New-Format Packet Lengths 704 705 New format packets have four possible ways of encoding length: 706 707 1. A one-octet Body Length header encodes packet lengths of up to 708 191 octets. 709 710 2. A two-octet Body Length header encodes packet lengths of 192 to 711 8383 octets. 712 713 3. A five-octet Body Length header encodes packet lengths of up to 714 4,294,967,295 (0xFFFFFFFF) octets in length. (This actually 715 encodes a four-octet scalar number.) 716 717 4. When the length of the packet body is not known in advance by the 718 issuer, Partial Body Length headers encode a packet of 719 indeterminate length, effectively making it a stream. 720 721 722 723 724 725 726 727 728 729 730Callas, et. al. Standards Track [Page 13] 731 732RFC 2440 OpenPGP Message Format November 1998 733 734 7354.2.2.1. One-Octet Lengths 736 737 A one-octet Body Length header encodes a length of from 0 to 191 738 octets. This type of length header is recognized because the one 739 octet value is less than 192. The body length is equal to: 740 741 bodyLen = 1st_octet; 742 7434.2.2.2. Two-Octet Lengths 744 745 A two-octet Body Length header encodes a length of from 192 to 8383 746 octets. It is recognized because its first octet is in the range 192 747 to 223. The body length is equal to: 748 749 bodyLen = ((1st_octet - 192) << 8) + (2nd_octet) + 192 750 7514.2.2.3. Five-Octet Lengths 752 753 A five-octet Body Length header consists of a single octet holding 754 the value 255, followed by a four-octet scalar. The body length is 755 equal to: 756 757 bodyLen = (2nd_octet << 24) | (3rd_octet << 16) | 758 (4th_octet << 8) | 5th_octet 759 7604.2.2.4. Partial Body Lengths 761 762 A Partial Body Length header is one octet long and encodes the length 763 of only part of the data packet. This length is a power of 2, from 1 764 to 1,073,741,824 (2 to the 30th power). It is recognized by its one 765 octet value that is greater than or equal to 224, and less than 255. 766 The partial body length is equal to: 767 768 partialBodyLen = 1 << (1st_octet & 0x1f); 769 770 Each Partial Body Length header is followed by a portion of the 771 packet body data. The Partial Body Length header specifies this 772 portion's length. Another length header (of one of the three types -- 773 one octet, two-octet, or partial) follows that portion. The last 774 length header in the packet MUST NOT be a partial Body Length header. 775 Partial Body Length headers may only be used for the non-final parts 776 of the packet. 777 7784.2.3. Packet Length Examples 779 780 These examples show ways that new-format packets might encode the 781 packet lengths. 782 783 784 785 786Callas, et. al. Standards Track [Page 14] 787 788RFC 2440 OpenPGP Message Format November 1998 789 790 791 A packet with length 100 may have its length encoded in one octet: 792 0x64. This is followed by 100 octets of data. 793 794 A packet with length 1723 may have its length coded in two octets: 795 0xC5, 0xFB. This header is followed by the 1723 octets of data. 796 797 A packet with length 100000 may have its length encoded in five 798 octets: 0xFF, 0x00, 0x01, 0x86, 0xA0. 799 800 It might also be encoded in the following octet stream: 0xEF, first 801 32768 octets of data; 0xE1, next two octets of data; 0xE0, next one 802 octet of data; 0xF0, next 65536 octets of data; 0xC5, 0xDD, last 1693 803 octets of data. This is just one possible encoding, and many 804 variations are possible on the size of the Partial Body Length 805 headers, as long as a regular Body Length header encodes the last 806 portion of the data. Note also that the last Body Length header can 807 be a zero-length header. 808 809 An implementation MAY use Partial Body Lengths for data packets, be 810 they literal, compressed, or encrypted. The first partial length MUST 811 be at least 512 octets long. Partial Body Lengths MUST NOT be used 812 for any other packet types. 813 814 Please note that in all of these explanations, the total length of 815 the packet is the length of the header(s) plus the length of the 816 body. 817 8184.3. Packet Tags 819 820 The packet tag denotes what type of packet the body holds. Note that 821 old format headers can only have tags less than 16, whereas new 822 format headers can have tags as great as 63. The defined tags (in 823 decimal) are: 824 825 0 -- Reserved - a packet tag must not have this value 826 1 -- Public-Key Encrypted Session Key Packet 827 2 -- Signature Packet 828 3 -- Symmetric-Key Encrypted Session Key Packet 829 4 -- One-Pass Signature Packet 830 5 -- Secret Key Packet 831 6 -- Public Key Packet 832 7 -- Secret Subkey Packet 833 8 -- Compressed Data Packet 834 9 -- Symmetrically Encrypted Data Packet 835 10 -- Marker Packet 836 11 -- Literal Data Packet 837 12 -- Trust Packet 838 839 840 841 842Callas, et. al. Standards Track [Page 15] 843 844RFC 2440 OpenPGP Message Format November 1998 845 846 847 13 -- User ID Packet 848 14 -- Public Subkey Packet 849 60 to 63 -- Private or Experimental Values 850 8515. Packet Types 852 8535.1. Public-Key Encrypted Session Key Packets (Tag 1) 854 855 A Public-Key Encrypted Session Key packet holds the session key used 856 to encrypt a message. Zero or more Encrypted Session Key packets 857 (either Public-Key or Symmetric-Key) may precede a Symmetrically 858 Encrypted Data Packet, which holds an encrypted message. The message 859 is encrypted with the session key, and the session key is itself 860 encrypted and stored in the Encrypted Session Key packet(s). The 861 Symmetrically Encrypted Data Packet is preceded by one Public-Key 862 Encrypted Session Key packet for each OpenPGP key to which the 863 message is encrypted. The recipient of the message finds a session 864 key that is encrypted to their public key, decrypts the session key, 865 and then uses the session key to decrypt the message. 866 867 The body of this packet consists of: 868 869 - A one-octet number giving the version number of the packet type. 870 The currently defined value for packet version is 3. An 871 implementation should accept, but not generate a version of 2, 872 which is equivalent to V3 in all other respects. 873 874 - An eight-octet number that gives the key ID of the public key 875 that the session key is encrypted to. 876 877 - A one-octet number giving the public key algorithm used. 878 879 - A string of octets that is the encrypted session key. This string 880 takes up the remainder of the packet, and its contents are 881 dependent on the public key algorithm used. 882 883 Algorithm Specific Fields for RSA encryption 884 885 - multiprecision integer (MPI) of RSA encrypted value m**e mod n. 886 887 Algorithm Specific Fields for Elgamal encryption: 888 889 - MPI of Elgamal (Diffie-Hellman) value g**k mod p. 890 891 - MPI of Elgamal (Diffie-Hellman) value m * y**k mod p. 892 893 894 895 896 897 898Callas, et. al. Standards Track [Page 16] 899 900RFC 2440 OpenPGP Message Format November 1998 901 902 903 The value "m" in the above formulas is derived from the session key 904 as follows. First the session key is prefixed with a one-octet 905 algorithm identifier that specifies the symmetric encryption 906 algorithm used to encrypt the following Symmetrically Encrypted Data 907 Packet. Then a two-octet checksum is appended which is equal to the 908 sum of the preceding session key octets, not including the algorithm 909 identifier, modulo 65536. This value is then padded as described in 910 PKCS-1 block type 02 [RFC2313] to form the "m" value used in the 911 formulas above. 912 913 Note that when an implementation forms several PKESKs with one 914 session key, forming a message that can be decrypted by several keys, 915 the implementation MUST make new PKCS-1 padding for each key. 916 917 An implementation MAY accept or use a Key ID of zero as a "wild card" 918 or "speculative" Key ID. In this case, the receiving implementation 919 would try all available private keys, checking for a valid decrypted 920 session key. This format helps reduce traffic analysis of messages. 921 9225.2. Signature Packet (Tag 2) 923 924 A signature packet describes a binding between some public key and 925 some data. The most common signatures are a signature of a file or a 926 block of text, and a signature that is a certification of a user ID. 927 928 Two versions of signature packets are defined. Version 3 provides 929 basic signature information, while version 4 provides an expandable 930 format with subpackets that can specify more information about the 931 signature. PGP 2.6.x only accepts version 3 signatures. 932 933 Implementations MUST accept V3 signatures. Implementations SHOULD 934 generate V4 signatures. Implementations MAY generate a V3 signature 935 that can be verified by PGP 2.6.x. 936 937 Note that if an implementation is creating an encrypted and signed 938 message that is encrypted to a V3 key, it is reasonable to create a 939 V3 signature. 940 9415.2.1. Signature Types 942 943 There are a number of possible meanings for a signature, which are 944 specified in a signature type octet in any given signature. These 945 meanings are: 946 947 0x00: Signature of a binary document. 948 Typically, this means the signer owns it, created it, or 949 certifies that it has not been modified. 950 951 952 953 954Callas, et. al. Standards Track [Page 17] 955 956RFC 2440 OpenPGP Message Format November 1998 957 958 959 0x01: Signature of a canonical text document. 960 Typically, this means the signer owns it, created it, or 961 certifies that it has not been modified. The signature is 962 calculated over the text data with its line endings converted 963 to <CR><LF> and trailing blanks removed. 964 965 0x02: Standalone signature. 966 This signature is a signature of only its own subpacket 967 contents. It is calculated identically to a signature over a 968 zero-length binary document. Note that it doesn't make sense to 969 have a V3 standalone signature. 970 971 0x10: Generic certification of a User ID and Public Key packet. 972 The issuer of this certification does not make any particular 973 assertion as to how well the certifier has checked that the 974 owner of the key is in fact the person described by the user 975 ID. Note that all PGP "key signatures" are this type of 976 certification. 977 978 0x11: Persona certification of a User ID and Public Key packet. 979 The issuer of this certification has not done any verification 980 of the claim that the owner of this key is the user ID 981 specified. 982 983 0x12: Casual certification of a User ID and Public Key packet. 984 The issuer of this certification has done some casual 985 verification of the claim of identity. 986 987 0x13: Positive certification of a User ID and Public Key packet. 988 The issuer of this certification has done substantial 989 verification of the claim of identity. 990 991 Please note that the vagueness of these certification claims is 992 not a flaw, but a feature of the system. Because PGP places 993 final authority for validity upon the receiver of a 994 certification, it may be that one authority's casual 995 certification might be more rigorous than some other 996 authority's positive certification. These classifications allow 997 a certification authority to issue fine-grained claims. 998 999 0x18: Subkey Binding Signature 1000 This signature is a statement by the top-level signing key 1001 indicates that it owns the subkey. This signature is calculated 1002 directly on the subkey itself, not on any User ID or other 1003 packets. 1004 1005 1006 1007 1008 1009 1010Callas, et. al. Standards Track [Page 18] 1011 1012RFC 2440 OpenPGP Message Format November 1998 1013 1014 1015 0x1F: Signature directly on a key 1016 This signature is calculated directly on a key. It binds the 1017 information in the signature subpackets to the key, and is 1018 appropriate to be used for subpackets that provide information 1019 about the key, such as the revocation key subpacket. It is also 1020 appropriate for statements that non-self certifiers want to 1021 make about the key itself, rather than the binding between a 1022 key and a name. 1023 1024 0x20: Key revocation signature 1025 The signature is calculated directly on the key being revoked. 1026 A revoked key is not to be used. Only revocation signatures by 1027 the key being revoked, or by an authorized revocation key, 1028 should be considered valid revocation signatures. 1029 1030 0x28: Subkey revocation signature 1031 The signature is calculated directly on the subkey being 1032 revoked. A revoked subkey is not to be used. Only revocation 1033 signatures by the top-level signature key that is bound to this 1034 subkey, or by an authorized revocation key, should be 1035 considered valid revocation signatures. 1036 1037 0x30: Certification revocation signature 1038 This signature revokes an earlier user ID certification 1039 signature (signature class 0x10 through 0x13). It should be 1040 issued by the same key that issued the revoked signature or an 1041 authorized revocation key The signature should have a later 1042 creation date than the signature it revokes. 1043 1044 0x40: Timestamp signature. 1045 This signature is only meaningful for the timestamp contained 1046 in it. 1047 10485.2.2. Version 3 Signature Packet Format 1049 1050 The body of a version 3 Signature Packet contains: 1051 1052 - One-octet version number (3). 1053 1054 - One-octet length of following hashed material. MUST be 5. 1055 1056 - One-octet signature type. 1057 1058 - Four-octet creation time. 1059 1060 - Eight-octet key ID of signer. 1061 1062 - One-octet public key algorithm. 1063 1064 1065 1066Callas, et. al. Standards Track [Page 19] 1067 1068RFC 2440 OpenPGP Message Format November 1998 1069 1070 1071 - One-octet hash algorithm. 1072 1073 - Two-octet field holding left 16 bits of signed hash value. 1074 1075 - One or more multi-precision integers comprising the signature. 1076 This portion is algorithm specific, as described below. 1077 1078 The data being signed is hashed, and then the signature type and 1079 creation time from the signature packet are hashed (5 additional 1080 octets). The resulting hash value is used in the signature 1081 algorithm. The high 16 bits (first two octets) of the hash are 1082 included in the signature packet to provide a quick test to reject 1083 some invalid signatures. 1084 1085 Algorithm Specific Fields for RSA signatures: 1086 1087 - multiprecision integer (MPI) of RSA signature value m**d. 1088 1089 Algorithm Specific Fields for DSA signatures: 1090 1091 - MPI of DSA value r. 1092 1093 - MPI of DSA value s. 1094 1095 The signature calculation is based on a hash of the signed data, as 1096 described above. The details of the calculation are different for 1097 DSA signature than for RSA signatures. 1098 1099 With RSA signatures, the hash value is encoded as described in PKCS-1 1100 section 10.1.2, "Data encoding", producing an ASN.1 value of type 1101 DigestInfo, and then padded using PKCS-1 block type 01 [RFC2313]. 1102 This requires inserting the hash value as an octet string into an 1103 ASN.1 structure. The object identifier for the type of hash being 1104 used is included in the structure. The hexadecimal representations 1105 for the currently defined hash algorithms are: 1106 1107 - MD2: 0x2A, 0x86, 0x48, 0x86, 0xF7, 0x0D, 0x02, 0x02 1108 1109 - MD5: 0x2A, 0x86, 0x48, 0x86, 0xF7, 0x0D, 0x02, 0x05 1110 1111 - RIPEMD-160: 0x2B, 0x24, 0x03, 0x02, 0x01 1112 1113 - SHA-1: 0x2B, 0x0E, 0x03, 0x02, 0x1A 1114 1115 1116 1117 1118 1119 1120 1121 1122Callas, et. al. Standards Track [Page 20] 1123 1124RFC 2440 OpenPGP Message Format November 1998 1125 1126 1127 The ASN.1 OIDs are: 1128 1129 - MD2: 1.2.840.113549.2.2 1130 1131 - MD5: 1.2.840.113549.2.5 1132 1133 - RIPEMD-160: 1.3.36.3.2.1 1134 1135 - SHA-1: 1.3.14.3.2.26 1136 1137 The full hash prefixes for these are: 1138 1139 MD2: 0x30, 0x20, 0x30, 0x0C, 0x06, 0x08, 0x2A, 0x86, 1140 0x48, 0x86, 0xF7, 0x0D, 0x02, 0x02, 0x05, 0x00, 1141 0x04, 0x10 1142 1143 MD5: 0x30, 0x20, 0x30, 0x0C, 0x06, 0x08, 0x2A, 0x86, 1144 0x48, 0x86, 0xF7, 0x0D, 0x02, 0x05, 0x05, 0x00, 1145 0x04, 0x10 1146 1147 RIPEMD-160: 0x30, 0x21, 0x30, 0x09, 0x06, 0x05, 0x2B, 0x24, 1148 0x03, 0x02, 0x01, 0x05, 0x00, 0x04, 0x14 1149 1150 SHA-1: 0x30, 0x21, 0x30, 0x09, 0x06, 0x05, 0x2b, 0x0E, 1151 0x03, 0x02, 0x1A, 0x05, 0x00, 0x04, 0x14 1152 1153 DSA signatures MUST use hashes with a size of 160 bits, to match q, 1154 the size of the group generated by the DSA key's generator value. 1155 The hash function result is treated as a 160 bit number and used 1156 directly in the DSA signature algorithm. 1157 11585.2.3. Version 4 Signature Packet Format 1159 1160 The body of a version 4 Signature Packet contains: 1161 1162 - One-octet version number (4). 1163 1164 - One-octet signature type. 1165 1166 - One-octet public key algorithm. 1167 1168 - One-octet hash algorithm. 1169 1170 - Two-octet scalar octet count for following hashed subpacket 1171 data. Note that this is the length in octets of all of the hashed 1172 subpackets; a pointer incremented by this number will skip over 1173 the hashed subpackets. 1174 1175 1176 1177 1178Callas, et. al. Standards Track [Page 21] 1179 1180RFC 2440 OpenPGP Message Format November 1998 1181 1182 1183 - Hashed subpacket data. (zero or more subpackets) 1184 1185 - Two-octet scalar octet count for following unhashed subpacket 1186 data. Note that this is the length in octets of all of the 1187 unhashed subpackets; a pointer incremented by this number will 1188 skip over the unhashed subpackets. 1189 1190 - Unhashed subpacket data. (zero or more subpackets) 1191 1192 - Two-octet field holding left 16 bits of signed hash value. 1193 1194 - One or more multi-precision integers comprising the signature. 1195 This portion is algorithm specific, as described above. 1196 1197 The data being signed is hashed, and then the signature data from the 1198 version number through the hashed subpacket data (inclusive) is 1199 hashed. The resulting hash value is what is signed. The left 16 bits 1200 of the hash are included in the signature packet to provide a quick 1201 test to reject some invalid signatures. 1202 1203 There are two fields consisting of signature subpackets. The first 1204 field is hashed with the rest of the signature data, while the second 1205 is unhashed. The second set of subpackets is not cryptographically 1206 protected by the signature and should include only advisory 1207 information. 1208 1209 The algorithms for converting the hash function result to a signature 1210 are described in a section below. 1211 12125.2.3.1. Signature Subpacket Specification 1213 1214 The subpacket fields consist of zero or more signature subpackets. 1215 Each set of subpackets is preceded by a two-octet scalar count of the 1216 length of the set of subpackets. 1217 1218 Each subpacket consists of a subpacket header and a body. The header 1219 consists of: 1220 1221 - the subpacket length (1, 2, or 5 octets) 1222 1223 - the subpacket type (1 octet) 1224 1225 and is followed by the subpacket specific data. 1226 1227 The length includes the type octet but not this length. Its format is 1228 similar to the "new" format packet header lengths, but cannot have 1229 partial body lengths. That is: 1230 1231 1232 1233 1234Callas, et. al. Standards Track [Page 22] 1235 1236RFC 2440 OpenPGP Message Format November 1998 1237 1238 1239 if the 1st octet < 192, then 1240 lengthOfLength = 1 1241 subpacketLen = 1st_octet 1242 1243 if the 1st octet >= 192 and < 255, then 1244 lengthOfLength = 2 1245 subpacketLen = ((1st_octet - 192) << 8) + (2nd_octet) + 192 1246 1247 if the 1st octet = 255, then 1248 lengthOfLength = 5 1249 subpacket length = [four-octet scalar starting at 2nd_octet] 1250 1251 The value of the subpacket type octet may be: 1252 1253 2 = signature creation time 1254 3 = signature expiration time 1255 4 = exportable certification 1256 5 = trust signature 1257 6 = regular expression 1258 7 = revocable 1259 9 = key expiration time 1260 10 = placeholder for backward compatibility 1261 11 = preferred symmetric algorithms 1262 12 = revocation key 1263 16 = issuer key ID 1264 20 = notation data 1265 21 = preferred hash algorithms 1266 22 = preferred compression algorithms 1267 23 = key server preferences 1268 24 = preferred key server 1269 25 = primary user id 1270 26 = policy URL 1271 27 = key flags 1272 28 = signer's user id 1273 29 = reason for revocation 1274 100 to 110 = internal or user-defined 1275 1276 An implementation SHOULD ignore any subpacket of a type that it does 1277 not recognize. 1278 1279 Bit 7 of the subpacket type is the "critical" bit. If set, it 1280 denotes that the subpacket is one that is critical for the evaluator 1281 of the signature to recognize. If a subpacket is encountered that is 1282 marked critical but is unknown to the evaluating software, the 1283 evaluator SHOULD consider the signature to be in error. 1284 1285 1286 1287 1288 1289 1290Callas, et. al. Standards Track [Page 23] 1291 1292RFC 2440 OpenPGP Message Format November 1998 1293 1294 1295 An evaluator may "recognize" a subpacket, but not implement it. The 1296 purpose of the critical bit is to allow the signer to tell an 1297 evaluator that it would prefer a new, unknown feature to generate an 1298 error than be ignored. 1299 1300 Implementations SHOULD implement "preferences". 1301 13025.2.3.2. Signature Subpacket Types 1303 1304 A number of subpackets are currently defined. Some subpackets apply 1305 to the signature itself and some are attributes of the key. 1306 Subpackets that are found on a self-signature are placed on a user id 1307 certification made by the key itself. Note that a key may have more 1308 than one user id, and thus may have more than one self-signature, and 1309 differing subpackets. 1310 1311 A self-signature is a binding signature made by the key the signature 1312 refers to. There are three types of self-signatures, the 1313 certification signatures (types 0x10-0x13), the direct-key signature 1314 (type 0x1f), and the subkey binding signature (type 0x18). For 1315 certification self-signatures, each user ID may have a self- 1316 signature, and thus different subpackets in those self-signatures. 1317 For subkey binding signatures, each subkey in fact has a self- 1318 signature. Subpackets that appear in a certification self-signature 1319 apply to the username, and subpackets that appear in the subkey 1320 self-signature apply to the subkey. Lastly, subpackets on the direct 1321 key signature apply to the entire key. 1322 1323 Implementing software should interpret a self-signature's preference 1324 subpackets as narrowly as possible. For example, suppose a key has 1325 two usernames, Alice and Bob. Suppose that Alice prefers the 1326 symmetric algorithm CAST5, and Bob prefers IDEA or Triple-DES. If the 1327 software locates this key via Alice's name, then the preferred 1328 algorithm is CAST5, if software locates the key via Bob's name, then 1329 the preferred algorithm is IDEA. If the key is located by key id, 1330 then algorithm of the default user id of the key provides the default 1331 symmetric algorithm. 1332 1333 A subpacket may be found either in the hashed or unhashed subpacket 1334 sections of a signature. If a subpacket is not hashed, then the 1335 information in it cannot be considered definitive because it is not 1336 part of the signature proper. 1337 1338 1339 1340 1341 1342 1343 1344 1345 1346Callas, et. al. Standards Track [Page 24] 1347 1348RFC 2440 OpenPGP Message Format November 1998 1349 1350 13515.2.3.3. Signature creation time 1352 1353 (4 octet time field) 1354 1355 The time the signature was made. 1356 1357 MUST be present in the hashed area. 1358 13595.2.3.4. Issuer 1360 1361 (8 octet key ID) 1362 1363 The OpenPGP key ID of the key issuing the signature. 1364 13655.2.3.5. Key expiration time 1366 1367 (4 octet time field) 1368 1369 The validity period of the key. This is the number of seconds after 1370 the key creation time that the key expires. If this is not present 1371 or has a value of zero, the key never expires. This is found only on 1372 a self-signature. 1373 13745.2.3.6. Preferred symmetric algorithms 1375 1376 (sequence of one-octet values) 1377 1378 Symmetric algorithm numbers that indicate which algorithms the key 1379 holder prefers to use. The subpacket body is an ordered list of 1380 octets with the most preferred listed first. It is assumed that only 1381 algorithms listed are supported by the recipient's software. 1382 Algorithm numbers in section 9. This is only found on a self- 1383 signature. 1384 13855.2.3.7. Preferred hash algorithms 1386 1387 (array of one-octet values) 1388 1389 Message digest algorithm numbers that indicate which algorithms the 1390 key holder prefers to receive. Like the preferred symmetric 1391 algorithms, the list is ordered. Algorithm numbers are in section 6. 1392 This is only found on a self-signature. 1393 1394 1395 1396 1397 1398 1399 1400 1401 1402Callas, et. al. Standards Track [Page 25] 1403 1404RFC 2440 OpenPGP Message Format November 1998 1405 1406 14075.2.3.8. Preferred compression algorithms 1408 1409 (array of one-octet values) 1410 1411 Compression algorithm numbers that indicate which algorithms the key 1412 holder prefers to use. Like the preferred symmetric algorithms, the 1413 list is ordered. Algorithm numbers are in section 6. If this 1414 subpacket is not included, ZIP is preferred. A zero denotes that 1415 uncompressed data is preferred; the key holder's software might have 1416 no compression software in that implementation. This is only found on 1417 a self-signature. 1418 14195.2.3.9. Signature expiration time 1420 1421 (4 octet time field) 1422 1423 The validity period of the signature. This is the number of seconds 1424 after the signature creation time that the signature expires. If this 1425 is not present or has a value of zero, it never expires. 1426 14275.2.3.10. Exportable Certification 1428 1429 (1 octet of exportability, 0 for not, 1 for exportable) 1430 1431 This subpacket denotes whether a certification signature is 1432 "exportable", to be used by other users than the signature's issuer. 1433 The packet body contains a boolean flag indicating whether the 1434 signature is exportable. If this packet is not present, the 1435 certification is exportable; it is equivalent to a flag containing a 1436 1. 1437 1438 Non-exportable, or "local", certifications are signatures made by a 1439 user to mark a key as valid within that user's implementation only. 1440 Thus, when an implementation prepares a user's copy of a key for 1441 transport to another user (this is the process of "exporting" the 1442 key), any local certification signatures are deleted from the key. 1443 1444 The receiver of a transported key "imports" it, and likewise trims 1445 any local certifications. In normal operation, there won't be any, 1446 assuming the import is performed on an exported key. However, there 1447 are instances where this can reasonably happen. For example, if an 1448 implementation allows keys to be imported from a key database in 1449 addition to an exported key, then this situation can arise. 1450 1451 Some implementations do not represent the interest of a single user 1452 (for example, a key server). Such implementations always trim local 1453 certifications from any key they handle. 1454 1455 1456 1457 1458Callas, et. al. Standards Track [Page 26] 1459 1460RFC 2440 OpenPGP Message Format November 1998 1461 1462 14635.2.3.11. Revocable 1464 1465 (1 octet of revocability, 0 for not, 1 for revocable) 1466 1467 Signature's revocability status. Packet body contains a boolean flag 1468 indicating whether the signature is revocable. Signatures that are 1469 not revocable have any later revocation signatures ignored. They 1470 represent a commitment by the signer that he cannot revoke his 1471 signature for the life of his key. If this packet is not present, 1472 the signature is revocable. 1473 14745.2.3.12. Trust signature 1475 1476 (1 octet "level" (depth), 1 octet of trust amount) 1477 1478 Signer asserts that the key is not only valid, but also trustworthy, 1479 at the specified level. Level 0 has the same meaning as an ordinary 1480 validity signature. Level 1 means that the signed key is asserted to 1481 be a valid trusted introducer, with the 2nd octet of the body 1482 specifying the degree of trust. Level 2 means that the signed key is 1483 asserted to be trusted to issue level 1 trust signatures, i.e. that 1484 it is a "meta introducer". Generally, a level n trust signature 1485 asserts that a key is trusted to issue level n-1 trust signatures. 1486 The trust amount is in a range from 0-255, interpreted such that 1487 values less than 120 indicate partial trust and values of 120 or 1488 greater indicate complete trust. Implementations SHOULD emit values 1489 of 60 for partial trust and 120 for complete trust. 1490 14915.2.3.13. Regular expression 1492 1493 (null-terminated regular expression) 1494 1495 Used in conjunction with trust signature packets (of level > 0) to 1496 limit the scope of trust that is extended. Only signatures by the 1497 target key on user IDs that match the regular expression in the body 1498 of this packet have trust extended by the trust signature subpacket. 1499 The regular expression uses the same syntax as the Henry Spencer's 1500 "almost public domain" regular expression package. A description of 1501 the syntax is found in a section below. 1502 15035.2.3.14. Revocation key 1504 1505 (1 octet of class, 1 octet of algid, 20 octets of fingerprint) 1506 1507 Authorizes the specified key to issue revocation signatures for this 1508 key. Class octet must have bit 0x80 set. If the bit 0x40 is set, 1509 then this means that the revocation information is sensitive. Other 1510 bits are for future expansion to other kinds of authorizations. This 1511 1512 1513 1514Callas, et. al. Standards Track [Page 27] 1515 1516RFC 2440 OpenPGP Message Format November 1998 1517 1518 1519 is found on a self-signature. 1520 1521 If the "sensitive" flag is set, the keyholder feels this subpacket 1522 contains private trust information that describes a real-world 1523 sensitive relationship. If this flag is set, implementations SHOULD 1524 NOT export this signature to other users except in cases where the 1525 data needs to be available: when the signature is being sent to the 1526 designated revoker, or when it is accompanied by a revocation 1527 signature from that revoker. Note that it may be appropriate to 1528 isolate this subpacket within a separate signature so that it is not 1529 combined with other subpackets that need to be exported. 1530 15315.2.3.15. Notation Data 1532 1533 (4 octets of flags, 2 octets of name length (M), 1534 2 octets of value length (N), 1535 M octets of name data, 1536 N octets of value data) 1537 1538 This subpacket describes a "notation" on the signature that the 1539 issuer wishes to make. The notation has a name and a value, each of 1540 which are strings of octets. There may be more than one notation in a 1541 signature. Notations can be used for any extension the issuer of the 1542 signature cares to make. The "flags" field holds four octets of 1543 flags. 1544 1545 All undefined flags MUST be zero. Defined flags are: 1546 1547 First octet: 0x80 = human-readable. This note is text, a note 1548 from one person to another, and has no 1549 meaning to software. 1550 Other octets: none. 1551 15525.2.3.16. Key server preferences 1553 1554 (N octets of flags) 1555 1556 This is a list of flags that indicate preferences that the key holder 1557 has about how the key is handled on a key server. All undefined flags 1558 MUST be zero. 1559 1560 First octet: 0x80 = No-modify 1561 the key holder requests that this key only be modified or updated 1562 by the key holder or an administrator of the key server. 1563 1564 This is found only on a self-signature. 1565 1566 1567 1568 1569 1570Callas, et. al. Standards Track [Page 28] 1571 1572RFC 2440 OpenPGP Message Format November 1998 1573 1574 15755.2.3.17. Preferred key server 1576 1577 (String) 1578 1579 This is a URL of a key server that the key holder prefers be used for 1580 updates. Note that keys with multiple user ids can have a preferred 1581 key server for each user id. Note also that since this is a URL, the 1582 key server can actually be a copy of the key retrieved by ftp, http, 1583 finger, etc. 1584 15855.2.3.18. Primary user id 1586 1587 (1 octet, boolean) 1588 1589 This is a flag in a user id's self signature that states whether this 1590 user id is the main user id for this key. It is reasonable for an 1591 implementation to resolve ambiguities in preferences, etc. by 1592 referring to the primary user id. If this flag is absent, its value 1593 is zero. If more than one user id in a key is marked as primary, the 1594 implementation may resolve the ambiguity in any way it sees fit. 1595 15965.2.3.19. Policy URL 1597 1598 (String) 1599 1600 This subpacket contains a URL of a document that describes the policy 1601 that the signature was issued under. 1602 16035.2.3.20. Key Flags 1604 1605 (Octet string) 1606 1607 This subpacket contains a list of binary flags that hold information 1608 about a key. It is a string of octets, and an implementation MUST NOT 1609 assume a fixed size. This is so it can grow over time. If a list is 1610 shorter than an implementation expects, the unstated flags are 1611 considered to be zero. The defined flags are: 1612 1613 First octet: 1614 1615 0x01 - This key may be used to certify other keys. 1616 1617 0x02 - This key may be used to sign data. 1618 1619 0x04 - This key may be used to encrypt communications. 1620 1621 0x08 - This key may be used to encrypt storage. 1622 1623 1624 1625 1626Callas, et. al. Standards Track [Page 29] 1627 1628RFC 2440 OpenPGP Message Format November 1998 1629 1630 1631 0x10 - The private component of this key may have been split by a 1632 secret-sharing mechanism. 1633 1634 0x80 - The private component of this key may be in the possession 1635 of more than one person. 1636 1637 Usage notes: 1638 1639 The flags in this packet may appear in self-signatures or in 1640 certification signatures. They mean different things depending on who 1641 is making the statement -- for example, a certification signature 1642 that has the "sign data" flag is stating that the certification is 1643 for that use. On the other hand, the "communications encryption" flag 1644 in a self-signature is stating a preference that a given key be used 1645 for communications. Note however, that it is a thorny issue to 1646 determine what is "communications" and what is "storage." This 1647 decision is left wholly up to the implementation; the authors of this 1648 document do not claim any special wisdom on the issue, and realize 1649 that accepted opinion may change. 1650 1651 The "split key" (0x10) and "group key" (0x80) flags are placed on a 1652 self-signature only; they are meaningless on a certification 1653 signature. They SHOULD be placed only on a direct-key signature (type 1654 0x1f) or a subkey signature (type 0x18), one that refers to the key 1655 the flag applies to. 1656 16575.2.3.21. Signer's User ID 1658 1659 This subpacket allows a keyholder to state which user id is 1660 responsible for the signing. Many keyholders use a single key for 1661 different purposes, such as business communications as well as 1662 personal communications. This subpacket allows such a keyholder to 1663 state which of their roles is making a signature. 1664 16655.2.3.22. Reason for Revocation 1666 1667 (1 octet of revocation code, N octets of reason string) 1668 1669 This subpacket is used only in key revocation and certification 1670 revocation signatures. It describes the reason why the key or 1671 certificate was revoked. 1672 1673 The first octet contains a machine-readable code that denotes the 1674 reason for the revocation: 1675 1676 1677 1678 1679 1680 1681 1682Callas, et. al. Standards Track [Page 30] 1683 1684RFC 2440 OpenPGP Message Format November 1998 1685 1686 1687 0x00 - No reason specified (key revocations or cert revocations) 1688 0x01 - Key is superceded (key revocations) 1689 0x02 - Key material has been compromised (key revocations) 1690 0x03 - Key is no longer used (key revocations) 1691 0x20 - User id information is no longer valid (cert revocations) 1692 1693 Following the revocation code is a string of octets which gives 1694 information about the reason for revocation in human-readable form 1695 (UTF-8). The string may be null, that is, of zero length. The length 1696 of the subpacket is the length of the reason string plus one. 1697 16985.2.4. Computing Signatures 1699 1700 All signatures are formed by producing a hash over the signature 1701 data, and then using the resulting hash in the signature algorithm. 1702 1703 The signature data is simple to compute for document signatures 1704 (types 0x00 and 0x01), for which the document itself is the data. 1705 For standalone signatures, this is a null string. 1706 1707 When a signature is made over a key, the hash data starts with the 1708 octet 0x99, followed by a two-octet length of the key, and then body 1709 of the key packet. (Note that this is an old-style packet header for 1710 a key packet with two-octet length.) A subkey signature (type 0x18) 1711 then hashes the subkey, using the same format as the main key. Key 1712 revocation signatures (types 0x20 and 0x28) hash only the key being 1713 revoked. 1714 1715 A certification signature (type 0x10 through 0x13) hashes the user id 1716 being bound to the key into the hash context after the above data. A 1717 V3 certification hashes the contents of the name packet, without any 1718 header. A V4 certification hashes the constant 0xb4 (which is an 1719 old-style packet header with the length-of-length set to zero), a 1720 four-octet number giving the length of the username, and then the 1721 username data. 1722 1723 Once the data body is hashed, then a trailer is hashed. A V3 1724 signature hashes five octets of the packet body, starting from the 1725 signature type field. This data is the signature type, followed by 1726 the four-octet signature time. A V4 signature hashes the packet body 1727 starting from its first field, the version number, through the end of 1728 the hashed subpacket data. Thus, the fields hashed are the signature 1729 version, the signature type, the public key algorithm, the hash 1730 algorithm, the hashed subpacket length, and the hashed subpacket 1731 body. 1732 1733 1734 1735 1736 1737 1738Callas, et. al. Standards Track [Page 31] 1739 1740RFC 2440 OpenPGP Message Format November 1998 1741 1742 1743 V4 signatures also hash in a final trailer of six octets: the version 1744 of the signature packet, i.e. 0x04; 0xFF; a four-octet, big-endian 1745 number that is the length of the hashed data from the signature 1746 packet (note that this number does not include these final six 1747 octets. 1748 1749 After all this has been hashed, the resulting hash field is used in 1750 the signature algorithm, and placed at the end of the signature 1751 packet. 1752 17535.2.4.1. Subpacket Hints 1754 1755 An implementation SHOULD put the two mandatory subpackets, creation 1756 time and issuer, as the first subpackets in the subpacket list, 1757 simply to make it easier for the implementer to find them. 1758 1759 It is certainly possible for a signature to contain conflicting 1760 information in subpackets. For example, a signature may contain 1761 multiple copies of a preference or multiple expiration times. In most 1762 cases, an implementation SHOULD use the last subpacket in the 1763 signature, but MAY use any conflict resolution scheme that makes more 1764 sense. Please note that we are intentionally leaving conflict 1765 resolution to the implementer; most conflicts are simply syntax 1766 errors, and the wishy-washy language here allows a receiver to be 1767 generous in what they accept, while putting pressure on a creator to 1768 be stingy in what they generate. 1769 1770 Some apparent conflicts may actually make sense -- for example, 1771 suppose a keyholder has an V3 key and a V4 key that share the same 1772 RSA key material. Either of these keys can verify a signature created 1773 by the other, and it may be reasonable for a signature to contain an 1774 issuer subpacket for each key, as a way of explicitly tying those 1775 keys to the signature. 1776 17775.3. Symmetric-Key Encrypted Session-Key Packets (Tag 3) 1778 1779 The Symmetric-Key Encrypted Session Key packet holds the symmetric- 1780 key encryption of a session key used to encrypt a message. Zero or 1781 more Encrypted Session Key packets and/or Symmetric-Key Encrypted 1782 Session Key packets may precede a Symmetrically Encrypted Data Packet 1783 that holds an encrypted message. The message is encrypted with a 1784 session key, and the session key is itself encrypted and stored in 1785 the Encrypted Session Key packet or the Symmetric-Key Encrypted 1786 Session Key packet. 1787 1788 If the Symmetrically Encrypted Data Packet is preceded by one or more 1789 Symmetric-Key Encrypted Session Key packets, each specifies a 1790 passphrase that may be used to decrypt the message. This allows a 1791 1792 1793 1794Callas, et. al. Standards Track [Page 32] 1795 1796RFC 2440 OpenPGP Message Format November 1998 1797 1798 1799 message to be encrypted to a number of public keys, and also to one 1800 or more pass phrases. This packet type is new, and is not generated 1801 by PGP 2.x or PGP 5.0. 1802 1803 The body of this packet consists of: 1804 1805 - A one-octet version number. The only currently defined version 1806 is 4. 1807 1808 - A one-octet number describing the symmetric algorithm used. 1809 1810 - A string-to-key (S2K) specifier, length as defined above. 1811 1812 - Optionally, the encrypted session key itself, which is decrypted 1813 with the string-to-key object. 1814 1815 If the encrypted session key is not present (which can be detected on 1816 the basis of packet length and S2K specifier size), then the S2K 1817 algorithm applied to the passphrase produces the session key for 1818 decrypting the file, using the symmetric cipher algorithm from the 1819 Symmetric-Key Encrypted Session Key packet. 1820 1821 If the encrypted session key is present, the result of applying the 1822 S2K algorithm to the passphrase is used to decrypt just that 1823 encrypted session key field, using CFB mode with an IV of all zeros. 1824 The decryption result consists of a one-octet algorithm identifier 1825 that specifies the symmetric-key encryption algorithm used to encrypt 1826 the following Symmetrically Encrypted Data Packet, followed by the 1827 session key octets themselves. 1828 1829 Note: because an all-zero IV is used for this decryption, the S2K 1830 specifier MUST use a salt value, either a Salted S2K or an Iterated- 1831 Salted S2K. The salt value will insure that the decryption key is 1832 not repeated even if the passphrase is reused. 1833 18345.4. One-Pass Signature Packets (Tag 4) 1835 1836 The One-Pass Signature packet precedes the signed data and contains 1837 enough information to allow the receiver to begin calculating any 1838 hashes needed to verify the signature. It allows the Signature 1839 Packet to be placed at the end of the message, so that the signer can 1840 compute the entire signed message in one pass. 1841 1842 A One-Pass Signature does not interoperate with PGP 2.6.x or earlier. 1843 1844 The body of this packet consists of: 1845 1846 1847 1848 1849 1850Callas, et. al. Standards Track [Page 33] 1851 1852RFC 2440 OpenPGP Message Format November 1998 1853 1854 1855 - A one-octet version number. The current version is 3. 1856 1857 - A one-octet signature type. Signature types are described in 1858 section 5.2.1. 1859 1860 - A one-octet number describing the hash algorithm used. 1861 1862 - A one-octet number describing the public key algorithm used. 1863 1864 - An eight-octet number holding the key ID of the signing key. 1865 1866 - A one-octet number holding a flag showing whether the signature 1867 is nested. A zero value indicates that the next packet is 1868 another One-Pass Signature packet that describes another 1869 signature to be applied to the same message data. 1870 1871 Note that if a message contains more than one one-pass signature, 1872 then the signature packets bracket the message; that is, the first 1873 signature packet after the message corresponds to the last one-pass 1874 packet and the final signature packet corresponds to the first one- 1875 pass packet. 1876 18775.5. Key Material Packet 1878 1879 A key material packet contains all the information about a public or 1880 private key. There are four variants of this packet type, and two 1881 major versions. Consequently, this section is complex. 1882 18835.5.1. Key Packet Variants 1884 18855.5.1.1. Public Key Packet (Tag 6) 1886 1887 A Public Key packet starts a series of packets that forms an OpenPGP 1888 key (sometimes called an OpenPGP certificate). 1889 18905.5.1.2. Public Subkey Packet (Tag 14) 1891 1892 A Public Subkey packet (tag 14) has exactly the same format as a 1893 Public Key packet, but denotes a subkey. One or more subkeys may be 1894 associated with a top-level key. By convention, the top-level key 1895 provides signature services, and the subkeys provide encryption 1896 services. 1897 1898 Note: in PGP 2.6.x, tag 14 was intended to indicate a comment packet. 1899 This tag was selected for reuse because no previous version of PGP 1900 ever emitted comment packets but they did properly ignore them. 1901 Public Subkey packets are ignored by PGP 2.6.x and do not cause it to 1902 fail, providing a limited degree of backward compatibility. 1903 1904 1905 1906Callas, et. al. Standards Track [Page 34] 1907 1908RFC 2440 OpenPGP Message Format November 1998 1909 1910 19115.5.1.3. Secret Key Packet (Tag 5) 1912 1913 A Secret Key packet contains all the information that is found in a 1914 Public Key packet, including the public key material, but also 1915 includes the secret key material after all the public key fields. 1916 19175.5.1.4. Secret Subkey Packet (Tag 7) 1918 1919 A Secret Subkey packet (tag 7) is the subkey analog of the Secret Key 1920 packet, and has exactly the same format. 1921 19225.5.2. Public Key Packet Formats 1923 1924 There are two versions of key-material packets. Version 3 packets 1925 were first generated by PGP 2.6. Version 2 packets are identical in 1926 format to Version 3 packets, but are generated by PGP 2.5 or before. 1927 V2 packets are deprecated and they MUST NOT be generated. PGP 5.0 1928 introduced version 4 packets, with new fields and semantics. PGP 1929 2.6.x will not accept key-material packets with versions greater than 1930 3. 1931 1932 OpenPGP implementations SHOULD create keys with version 4 format. An 1933 implementation MAY generate a V3 key to ensure interoperability with 1934 old software; note, however, that V4 keys correct some security 1935 deficiencies in V3 keys. These deficiencies are described below. An 1936 implementation MUST NOT create a V3 key with a public key algorithm 1937 other than RSA. 1938 1939 A version 3 public key or public subkey packet contains: 1940 1941 - A one-octet version number (3). 1942 1943 - A four-octet number denoting the time that the key was created. 1944 1945 - A two-octet number denoting the time in days that this key is 1946 valid. If this number is zero, then it does not expire. 1947 1948 - A one-octet number denoting the public key algorithm of this key 1949 1950 - A series of multi-precision integers comprising the key 1951 material: 1952 1953 - a multiprecision integer (MPI) of RSA public modulus n; 1954 1955 - an MPI of RSA public encryption exponent e. 1956 1957 1958 1959 1960 1961 1962Callas, et. al. Standards Track [Page 35] 1963 1964RFC 2440 OpenPGP Message Format November 1998 1965 1966 1967 V3 keys SHOULD only be used for backward compatibility because of 1968 three weaknesses in them. First, it is relatively easy to construct a 1969 V3 key that has the same key ID as any other key because the key ID 1970 is simply the low 64 bits of the public modulus. Secondly, because 1971 the fingerprint of a V3 key hashes the key material, but not its 1972 length, which increases the opportunity for fingerprint collisions. 1973 Third, there are minor weaknesses in the MD5 hash algorithm that make 1974 developers prefer other algorithms. See below for a fuller discussion 1975 of key IDs and fingerprints. 1976 1977 The version 4 format is similar to the version 3 format except for 1978 the absence of a validity period. This has been moved to the 1979 signature packet. In addition, fingerprints of version 4 keys are 1980 calculated differently from version 3 keys, as described in section 1981 "Enhanced Key Formats." 1982 1983 A version 4 packet contains: 1984 1985 - A one-octet version number (4). 1986 1987 - A four-octet number denoting the time that the key was created. 1988 1989 - A one-octet number denoting the public key algorithm of this key 1990 1991 - A series of multi-precision integers comprising the key 1992 material. This algorithm-specific portion is: 1993 1994 Algorithm Specific Fields for RSA public keys: 1995 1996 - multiprecision integer (MPI) of RSA public modulus n; 1997 1998 - MPI of RSA public encryption exponent e. 1999 2000 Algorithm Specific Fields for DSA public keys: 2001 2002 - MPI of DSA prime p; 2003 2004 - MPI of DSA group order q (q is a prime divisor of p-1); 2005 2006 - MPI of DSA group generator g; 2007 2008 - MPI of DSA public key value y (= g**x where x is secret). 2009 2010 Algorithm Specific Fields for Elgamal public keys: 2011 2012 - MPI of Elgamal prime p; 2013 2014 - MPI of Elgamal group generator g; 2015 2016 2017 2018Callas, et. al. Standards Track [Page 36] 2019 2020RFC 2440 OpenPGP Message Format November 1998 2021 2022 2023 - MPI of Elgamal public key value y (= g**x where x is 2024 secret). 2025 20265.5.3. Secret Key Packet Formats 2027 2028 The Secret Key and Secret Subkey packets contain all the data of the 2029 Public Key and Public Subkey packets, with additional algorithm- 2030 specific secret key data appended, in encrypted form. 2031 2032 The packet contains: 2033 2034 - A Public Key or Public Subkey packet, as described above 2035 2036 - One octet indicating string-to-key usage conventions. 0 2037 indicates that the secret key data is not encrypted. 255 2038 indicates that a string-to-key specifier is being given. Any 2039 other value is a symmetric-key encryption algorithm specifier. 2040 2041 - [Optional] If string-to-key usage octet was 255, a one-octet 2042 symmetric encryption algorithm. 2043 2044 - [Optional] If string-to-key usage octet was 255, a string-to-key 2045 specifier. The length of the string-to-key specifier is implied 2046 by its type, as described above. 2047 2048 - [Optional] If secret data is encrypted, eight-octet Initial 2049 Vector (IV). 2050 2051 - Encrypted multi-precision integers comprising the secret key 2052 data. These algorithm-specific fields are as described below. 2053 2054 - Two-octet checksum of the plaintext of the algorithm-specific 2055 portion (sum of all octets, mod 65536). 2056 2057 Algorithm Specific Fields for RSA secret keys: 2058 2059 - multiprecision integer (MPI) of RSA secret exponent d. 2060 2061 - MPI of RSA secret prime value p. 2062 2063 - MPI of RSA secret prime value q (p < q). 2064 2065 - MPI of u, the multiplicative inverse of p, mod q. 2066 2067 Algorithm Specific Fields for DSA secret keys: 2068 2069 - MPI of DSA secret exponent x. 2070 2071 2072 2073 2074Callas, et. al. Standards Track [Page 37] 2075 2076RFC 2440 OpenPGP Message Format November 1998 2077 2078 2079 Algorithm Specific Fields for Elgamal secret keys: 2080 2081 - MPI of Elgamal secret exponent x. 2082 2083 Secret MPI values can be encrypted using a passphrase. If a string- 2084 to-key specifier is given, that describes the algorithm for 2085 converting the passphrase to a key, else a simple MD5 hash of the 2086 passphrase is used. Implementations SHOULD use a string-to-key 2087 specifier; the simple hash is for backward compatibility. The cipher 2088 for encrypting the MPIs is specified in the secret key packet. 2089 2090 Encryption/decryption of the secret data is done in CFB mode using 2091 the key created from the passphrase and the Initial Vector from the 2092 packet. A different mode is used with V3 keys (which are only RSA) 2093 than with other key formats. With V3 keys, the MPI bit count prefix 2094 (i.e., the first two octets) is not encrypted. Only the MPI non- 2095 prefix data is encrypted. Furthermore, the CFB state is 2096 resynchronized at the beginning of each new MPI value, so that the 2097 CFB block boundary is aligned with the start of the MPI data. 2098 2099 With V4 keys, a simpler method is used. All secret MPI values are 2100 encrypted in CFB mode, including the MPI bitcount prefix. 2101 2102 The 16-bit checksum that follows the algorithm-specific portion is 2103 the algebraic sum, mod 65536, of the plaintext of all the algorithm- 2104 specific octets (including MPI prefix and data). With V3 keys, the 2105 checksum is stored in the clear. With V4 keys, the checksum is 2106 encrypted like the algorithm-specific data. This value is used to 2107 check that the passphrase was correct. 2108 21095.6. Compressed Data Packet (Tag 8) 2110 2111 The Compressed Data packet contains compressed data. Typically, this 2112 packet is found as the contents of an encrypted packet, or following 2113 a Signature or One-Pass Signature packet, and contains literal data 2114 packets. 2115 2116 The body of this packet consists of: 2117 2118 - One octet that gives the algorithm used to compress the packet. 2119 2120 - The remainder of the packet is compressed data. 2121 2122 A Compressed Data Packet's body contains an block that compresses 2123 some set of packets. See section "Packet Composition" for details on 2124 how messages are formed. 2125 2126 2127 2128 2129 2130Callas, et. al. Standards Track [Page 38] 2131 2132RFC 2440 OpenPGP Message Format November 1998 2133 2134 2135 ZIP-compressed packets are compressed with raw RFC 1951 DEFLATE 2136 blocks. Note that PGP V2.6 uses 13 bits of compression. If an 2137 implementation uses more bits of compression, PGP V2.6 cannot 2138 decompress it. 2139 2140 ZLIB-compressed packets are compressed with RFC 1950 ZLIB-style 2141 blocks. 2142 21435.7. Symmetrically Encrypted Data Packet (Tag 9) 2144 2145 The Symmetrically Encrypted Data packet contains data encrypted with 2146 a symmetric-key algorithm. When it has been decrypted, it will 2147 typically contain other packets (often literal data packets or 2148 compressed data packets). 2149 2150 The body of this packet consists of: 2151 2152 - Encrypted data, the output of the selected symmetric-key cipher 2153 operating in PGP's variant of Cipher Feedback (CFB) mode. 2154 2155 The symmetric cipher used may be specified in an Public-Key or 2156 Symmetric-Key Encrypted Session Key packet that precedes the 2157 Symmetrically Encrypted Data Packet. In that case, the cipher 2158 algorithm octet is prefixed to the session key before it is 2159 encrypted. If no packets of these types precede the encrypted data, 2160 the IDEA algorithm is used with the session key calculated as the MD5 2161 hash of the passphrase. 2162 2163 The data is encrypted in CFB mode, with a CFB shift size equal to the 2164 cipher's block size. The Initial Vector (IV) is specified as all 2165 zeros. Instead of using an IV, OpenPGP prefixes a 10-octet string to 2166 the data before it is encrypted. The first eight octets are random, 2167 and the 9th and 10th octets are copies of the 7th and 8th octets, 2168 respectively. After encrypting the first 10 octets, the CFB state is 2169 resynchronized if the cipher block size is 8 octets or less. The 2170 last 8 octets of ciphertext are passed through the cipher and the 2171 block boundary is reset. 2172 2173 The repetition of 16 bits in the 80 bits of random data prefixed to 2174 the message allows the receiver to immediately check whether the 2175 session key is incorrect. 2176 21775.8. Marker Packet (Obsolete Literal Packet) (Tag 10) 2178 2179 An experimental version of PGP used this packet as the Literal 2180 packet, but no released version of PGP generated Literal packets with 2181 this tag. With PGP 5.x, this packet has been re-assigned and is 2182 reserved for use as the Marker packet. 2183 2184 2185 2186Callas, et. al. Standards Track [Page 39] 2187 2188RFC 2440 OpenPGP Message Format November 1998 2189 2190 2191 The body of this packet consists of: 2192 2193 - The three octets 0x50, 0x47, 0x50 (which spell "PGP" in UTF-8). 2194 2195 Such a packet MUST be ignored when received. It may be placed at the 2196 beginning of a message that uses features not available in PGP 2.6.x 2197 in order to cause that version to report that newer software is 2198 necessary to process the message. 2199 22005.9. Literal Data Packet (Tag 11) 2201 2202 A Literal Data packet contains the body of a message; data that is 2203 not to be further interpreted. 2204 2205 The body of this packet consists of: 2206 2207 - A one-octet field that describes how the data is formatted. 2208 2209 If it is a 'b' (0x62), then the literal packet contains binary data. 2210 If it is a 't' (0x74), then it contains text data, and thus may need 2211 line ends converted to local form, or other text-mode changes. RFC 2212 1991 also defined a value of 'l' as a 'local' mode for machine-local 2213 conversions. This use is now deprecated. 2214 2215 - File name as a string (one-octet length, followed by file name), 2216 if the encrypted data should be saved as a file. 2217 2218 If the special name "_CONSOLE" is used, the message is considered to 2219 be "for your eyes only". This advises that the message data is 2220 unusually sensitive, and the receiving program should process it more 2221 carefully, perhaps avoiding storing the received data to disk, for 2222 example. 2223 2224 - A four-octet number that indicates the modification date of the 2225 file, or the creation time of the packet, or a zero that 2226 indicates the present time. 2227 2228 - The remainder of the packet is literal data. 2229 2230 Text data is stored with <CR><LF> text endings (i.e. network-normal 2231 line endings). These should be converted to native line endings by 2232 the receiving software. 2233 22345.10. Trust Packet (Tag 12) 2235 2236 The Trust packet is used only within keyrings and is not normally 2237 exported. Trust packets contain data that record the user's 2238 specifications of which key holders are trustworthy introducers, 2239 2240 2241 2242Callas, et. al. Standards Track [Page 40] 2243 2244RFC 2440 OpenPGP Message Format November 1998 2245 2246 2247 along with other information that implementing software uses for 2248 trust information. 2249 2250 Trust packets SHOULD NOT be emitted to output streams that are 2251 transferred to other users, and they SHOULD be ignored on any input 2252 other than local keyring files. 2253 22545.11. User ID Packet (Tag 13) 2255 2256 A User ID packet consists of data that is intended to represent the 2257 name and email address of the key holder. By convention, it includes 2258 an RFC 822 mail name, but there are no restrictions on its content. 2259 The packet length in the header specifies the length of the user id. 2260 If it is text, it is encoded in UTF-8. 2261 22626. Radix-64 Conversions 2263 2264 As stated in the introduction, OpenPGP's underlying native 2265 representation for objects is a stream of arbitrary octets, and some 2266 systems desire these objects to be immune to damage caused by 2267 character set translation, data conversions, etc. 2268 2269 In principle, any printable encoding scheme that met the requirements 2270 of the unsafe channel would suffice, since it would not change the 2271 underlying binary bit streams of the native OpenPGP data structures. 2272 The OpenPGP standard specifies one such printable encoding scheme to 2273 ensure interoperability. 2274 2275 OpenPGP's Radix-64 encoding is composed of two parts: a base64 2276 encoding of the binary data, and a checksum. The base64 encoding is 2277 identical to the MIME base64 content-transfer-encoding [RFC2231, 2278 Section 6.8]. An OpenPGP implementation MAY use ASCII Armor to 2279 protect the raw binary data. 2280 2281 The checksum is a 24-bit CRC converted to four characters of radix-64 2282 encoding by the same MIME base64 transformation, preceded by an 2283 equals sign (=). The CRC is computed by using the generator 0x864CFB 2284 and an initialization of 0xB704CE. The accumulation is done on the 2285 data before it is converted to radix-64, rather than on the converted 2286 data. A sample implementation of this algorithm is in the next 2287 section. 2288 2289 The checksum with its leading equal sign MAY appear on the first line 2290 after the Base64 encoded data. 2291 2292 Rationale for CRC-24: The size of 24 bits fits evenly into printable 2293 base64. The nonzero initialization can detect more errors than a 2294 zero initialization. 2295 2296 2297 2298Callas, et. al. Standards Track [Page 41] 2299 2300RFC 2440 OpenPGP Message Format November 1998 2301 2302 23036.1. An Implementation of the CRC-24 in "C" 2304 2305 #define CRC24_INIT 0xb704ceL 2306 #define CRC24_POLY 0x1864cfbL 2307 2308 typedef long crc24; 2309 crc24 crc_octets(unsigned char *octets, size_t len) 2310 { 2311 crc24 crc = CRC24_INIT; 2312 int i; 2313 2314 while (len--) { 2315 crc ^= (*octets++) << 16; 2316 for (i = 0; i < 8; i++) { 2317 crc <<= 1; 2318 if (crc & 0x1000000) 2319 crc ^= CRC24_POLY; 2320 } 2321 } 2322 return crc & 0xffffffL; 2323 } 2324 23256.2. Forming ASCII Armor 2326 2327 When OpenPGP encodes data into ASCII Armor, it puts specific headers 2328 around the data, so OpenPGP can reconstruct the data later. OpenPGP 2329 informs the user what kind of data is encoded in the ASCII armor 2330 through the use of the headers. 2331 2332 Concatenating the following data creates ASCII Armor: 2333 2334 - An Armor Header Line, appropriate for the type of data 2335 2336 - Armor Headers 2337 2338 - A blank (zero-length, or containing only whitespace) line 2339 2340 - The ASCII-Armored data 2341 2342 - An Armor Checksum 2343 2344 - The Armor Tail, which depends on the Armor Header Line. 2345 2346 An Armor Header Line consists of the appropriate header line text 2347 surrounded by five (5) dashes ('-', 0x2D) on either side of the 2348 header line text. The header line text is chosen based upon the type 2349 of data that is being encoded in Armor, and how it is being encoded. 2350 Header line texts include the following strings: 2351 2352 2353 2354Callas, et. al. Standards Track [Page 42] 2355 2356RFC 2440 OpenPGP Message Format November 1998 2357 2358 2359 BEGIN PGP MESSAGE 2360 Used for signed, encrypted, or compressed files. 2361 2362 BEGIN PGP PUBLIC KEY BLOCK 2363 Used for armoring public keys 2364 2365 BEGIN PGP PRIVATE KEY BLOCK 2366 Used for armoring private keys 2367 2368 BEGIN PGP MESSAGE, PART X/Y 2369 Used for multi-part messages, where the armor is split amongst Y 2370 parts, and this is the Xth part out of Y. 2371 2372 BEGIN PGP MESSAGE, PART X 2373 Used for multi-part messages, where this is the Xth part of an 2374 unspecified number of parts. Requires the MESSAGE-ID Armor Header 2375 to be used. 2376 2377 BEGIN PGP SIGNATURE 2378 Used for detached signatures, OpenPGP/MIME signatures, and 2379 natures following clearsigned messages. Note that PGP 2.x s BEGIN 2380 PGP MESSAGE for detached signatures. 2381 2382 The Armor Headers are pairs of strings that can give the user or the 2383 receiving OpenPGP implementation some information about how to decode 2384 or use the message. The Armor Headers are a part of the armor, not a 2385 part of the message, and hence are not protected by any signatures 2386 applied to the message. 2387 2388 The format of an Armor Header is that of a key-value pair. A colon 2389 (':' 0x38) and a single space (0x20) separate the key and value. 2390 OpenPGP should consider improperly formatted Armor Headers to be 2391 corruption of the ASCII Armor. Unknown keys should be reported to 2392 the user, but OpenPGP should continue to process the message. 2393 2394 Currently defined Armor Header Keys are: 2395 2396 - "Version", that states the OpenPGP Version used to encode the 2397 message. 2398 2399 - "Comment", a user-defined comment. 2400 2401 - "MessageID", a 32-character string of printable characters. The 2402 string must be the same for all parts of a multi-part message 2403 that uses the "PART X" Armor Header. MessageID strings should be 2404 2405 2406 2407 2408 2409 2410Callas, et. al. Standards Track [Page 43] 2411 2412RFC 2440 OpenPGP Message Format November 1998 2413 2414 2415 unique enough that the recipient of the mail can associate all 2416 the parts of a message with each other. A good checksum or 2417 cryptographic hash function is sufficient. 2418 2419 - "Hash", a comma-separated list of hash algorithms used in this 2420 message. This is used only in clear-signed messages. 2421 2422 - "Charset", a description of the character set that the plaintext 2423 is in. Please note that OpenPGP defines text to be in UTF-8 by 2424 default. An implementation will get best results by translating 2425 into and out of UTF-8. However, there are many instances where 2426 this is easier said than done. Also, there are communities of 2427 users who have no need for UTF-8 because they are all happy with 2428 a character set like ISO Latin-5 or a Japanese character set. In 2429 such instances, an implementation MAY override the UTF-8 default 2430 by using this header key. An implementation MAY implement this 2431 key and any translations it cares to; an implementation MAY 2432 ignore it and assume all text is UTF-8. 2433 2434 The MessageID SHOULD NOT appear unless it is in a multi-part 2435 message. If it appears at all, it MUST be computed from the 2436 finished (encrypted, signed, etc.) message in a deterministic 2437 fashion, rather than contain a purely random value. This is to 2438 allow the legitimate recipient to determine that the MessageID 2439 cannot serve as a covert means of leaking cryptographic key 2440 information. 2441 2442 The Armor Tail Line is composed in the same manner as the Armor 2443 Header Line, except the string "BEGIN" is replaced by the string 2444 "END." 2445 24466.3. Encoding Binary in Radix-64 2447 2448 The encoding process represents 24-bit groups of input bits as output 2449 strings of 4 encoded characters. Proceeding from left to right, a 2450 24-bit input group is formed by concatenating three 8-bit input 2451 groups. These 24 bits are then treated as four concatenated 6-bit 2452 groups, each of which is translated into a single digit in the 2453 Radix-64 alphabet. When encoding a bit stream with the Radix-64 2454 encoding, the bit stream must be presumed to be ordered with the 2455 most-significant-bit first. That is, the first bit in the stream will 2456 be the high-order bit in the first 8-bit octet, and the eighth bit 2457 will be the low-order bit in the first 8-bit octet, and so on. 2458 2459 2460 2461 2462 2463 2464 2465 2466Callas, et. al. Standards Track [Page 44] 2467 2468RFC 2440 OpenPGP Message Format November 1998 2469 2470 2471 +--first octet--+-second octet--+--third octet--+ 2472 |7 6 5 4 3 2 1 0|7 6 5 4 3 2 1 0|7 6 5 4 3 2 1 0| 2473 +-----------+---+-------+-------+---+-----------+ 2474 |5 4 3 2 1 0|5 4 3 2 1 0|5 4 3 2 1 0|5 4 3 2 1 0| 2475 +--1.index--+--2.index--+--3.index--+--4.index--+ 2476 2477 Each 6-bit group is used as an index into an array of 64 printable 2478 characters from the table below. The character referenced by the 2479 index is placed in the output string. 2480 2481 Value Encoding Value Encoding Value Encoding Value Encoding 2482 0 A 17 R 34 i 51 z 2483 1 B 18 S 35 j 52 0 2484 2 C 19 T 36 k 53 1 2485 3 D 20 U 37 l 54 2 2486 4 E 21 V 38 m 55 3 2487 5 F 22 W 39 n 56 4 2488 6 G 23 X 40 o 57 5 2489 7 H 24 Y 41 p 58 6 2490 8 I 25 Z 42 q 59 7 2491 9 J 26 a 43 r 60 8 2492 10 K 27 b 44 s 61 9 2493 11 L 28 c 45 t 62 + 2494 12 M 29 d 46 u 63 / 2495 13 N 30 e 47 v 2496 14 O 31 f 48 w (pad) = 2497 15 P 32 g 49 x 2498 16 Q 33 h 50 y 2499 2500 The encoded output stream must be represented in lines of no more 2501 than 76 characters each. 2502 2503 Special processing is performed if fewer than 24 bits are available 2504 at the end of the data being encoded. There are three possibilities: 2505 2506 1. The last data group has 24 bits (3 octets). No special 2507 processing is needed. 2508 2509 2. The last data group has 16 bits (2 octets). The first two 6-bit 2510 groups are processed as above. The third (incomplete) data group 2511 has two zero-value bits added to it, and is processed as above. 2512 A pad character (=) is added to the output. 2513 2514 3. The last data group has 8 bits (1 octet). The first 6-bit group 2515 is processed as above. The second (incomplete) data group has 2516 four zero-value bits added to it, and is processed as above. Two 2517 pad characters (=) are added to the output. 2518 2519 2520 2521 2522Callas, et. al. Standards Track [Page 45] 2523 2524RFC 2440 OpenPGP Message Format November 1998 2525 2526 25276.4. Decoding Radix-64 2528 2529 Any characters outside of the base64 alphabet are ignored in Radix-64 2530 data. Decoding software must ignore all line breaks or other 2531 characters not found in the table above. 2532 2533 In Radix-64 data, characters other than those in the table, line 2534 breaks, and other white space probably indicate a transmission error, 2535 about which a warning message or even a message rejection might be 2536 appropriate under some circumstances. 2537 2538 Because it is used only for padding at the end of the data, the 2539 occurrence of any "=" characters may be taken as evidence that the 2540 end of the data has been reached (without truncation in transit). No 2541 such assurance is possible, however, when the number of octets 2542 transmitted was a multiple of three and no "=" characters are 2543 present. 2544 25456.5. Examples of Radix-64 2546 2547 Input data: 0x14fb9c03d97e 2548 Hex: 1 4 f b 9 c | 0 3 d 9 7 e 2549 8-bit: 00010100 11111011 10011100 | 00000011 11011001 2550 11111110 2551 6-bit: 000101 001111 101110 011100 | 000000 111101 100111 2552 111110 2553 Decimal: 5 15 46 28 0 61 37 62 2554 Output: F P u c A 9 l + 2555 2556 Input data: 0x14fb9c03d9 2557 Hex: 1 4 f b 9 c | 0 3 d 9 2558 8-bit: 00010100 11111011 10011100 | 00000011 11011001 2559 pad with 00 2560 6-bit: 000101 001111 101110 011100 | 000000 111101 100100 2561 Decimal: 5 15 46 28 0 61 36 2562 pad with = 2563 Output: F P u c A 9 k = 2564 2565 Input data: 0x14fb9c03 2566 Hex: 1 4 f b 9 c | 0 3 2567 8-bit: 00010100 11111011 10011100 | 00000011 2568 pad with 0000 2569 6-bit: 000101 001111 101110 011100 | 000000 110000 2570 Decimal: 5 15 46 28 0 48 2571 pad with = = 2572 Output: F P u c A w = = 2573 2574 2575 2576 2577 2578Callas, et. al. Standards Track [Page 46] 2579 2580RFC 2440 OpenPGP Message Format November 1998 2581 2582 25836.6. Example of an ASCII Armored Message 2584 2585 2586 -----BEGIN PGP MESSAGE----- 2587 Version: OpenPrivacy 0.99 2588 2589 yDgBO22WxBHv7O8X7O/jygAEzol56iUKiXmV+XmpCtmpqQUKiQrFqclFqUDBovzS 2590 vBSFjNSiVHsuAA== 2591 =njUN 2592 -----END PGP MESSAGE----- 2593 2594 Note that this example is indented by two spaces. 2595 25967. Cleartext signature framework 2597 2598 It is desirable to sign a textual octet stream without ASCII armoring 2599 the stream itself, so the signed text is still readable without 2600 special software. In order to bind a signature to such a cleartext, 2601 this framework is used. (Note that RFC 2015 defines another way to 2602 clear sign messages for environments that support MIME.) 2603 2604 The cleartext signed message consists of: 2605 2606 - The cleartext header '-----BEGIN PGP SIGNED MESSAGE-----' on a 2607 single line, 2608 2609 - One or more "Hash" Armor Headers, 2610 2611 - Exactly one empty line not included into the message digest, 2612 2613 - The dash-escaped cleartext that is included into the message 2614 digest, 2615 2616 - The ASCII armored signature(s) including the '-----BEGIN PGP 2617 SIGNATURE-----' Armor Header and Armor Tail Lines. 2618 2619 If the "Hash" armor header is given, the specified message digest 2620 algorithm is used for the signature. If there are no such headers, 2621 MD5 is used, an implementation MAY omit them for V2.x compatibility. 2622 If more than one message digest is used in the signature, the "Hash" 2623 armor header contains a comma-delimited list of used message digests. 2624 2625 Current message digest names are described below with the algorithm 2626 IDs. 2627 26287.1. Dash-Escaped Text 2629 2630 The cleartext content of the message must also be dash-escaped. 2631 2632 2633 2634Callas, et. al. Standards Track [Page 47] 2635 2636RFC 2440 OpenPGP Message Format November 1998 2637 2638 2639 Dash escaped cleartext is the ordinary cleartext where every line 2640 starting with a dash '-' (0x2D) is prefixed by the sequence dash '-' 2641 (0x2D) and space ' ' (0x20). This prevents the parser from 2642 recognizing armor headers of the cleartext itself. The message digest 2643 is computed using the cleartext itself, not the dash escaped form. 2644 2645 As with binary signatures on text documents, a cleartext signature is 2646 calculated on the text using canonical <CR><LF> line endings. The 2647 line ending (i.e. the <CR><LF>) before the '-----BEGIN PGP 2648 SIGNATURE-----' line that terminates the signed text is not 2649 considered part of the signed text. 2650 2651 Also, any trailing whitespace (spaces, and tabs, 0x09) at the end of 2652 any line is ignored when the cleartext signature is calculated. 2653 26548. Regular Expressions 2655 2656 A regular expression is zero or more branches, separated by '|'. It 2657 matches anything that matches one of the branches. 2658 2659 A branch is zero or more pieces, concatenated. It matches a match for 2660 the first, followed by a match for the second, etc. 2661 2662 A piece is an atom possibly followed by '*', '+', or '?'. An atom 2663 followed by '*' matches a sequence of 0 or more matches of the atom. 2664 An atom followed by '+' matches a sequence of 1 or more matches of 2665 the atom. An atom followed by '?' matches a match of the atom, or the 2666 null string. 2667 2668 An atom is a regular expression in parentheses (matching a match for 2669 the regular expression), a range (see below), '.' (matching any 2670 single character), '^' (matching the null string at the beginning of 2671 the input string), '$' (matching the null string at the end of the 2672 input string), a '\' followed by a single character (matching that 2673 character), or a single character with no other significance 2674 (matching that character). 2675 2676 A range is a sequence of characters enclosed in '[]'. It normally 2677 matches any single character from the sequence. If the sequence 2678 begins with '^', it matches any single character not from the rest of 2679 the sequence. If two characters in the sequence are separated by '-', 2680 this is shorthand for the full list of ASCII characters between them 2681 (e.g. '[0-9]' matches any decimal digit). To include a literal ']' in 2682 the sequence, make it the first character (following a possible '^'). 2683 To include a literal '-', make it the first or last character. 2684 2685 2686 2687 2688 2689 2690Callas, et. al. Standards Track [Page 48] 2691 2692RFC 2440 OpenPGP Message Format November 1998 2693 2694 26959. Constants 2696 2697 This section describes the constants used in OpenPGP. 2698 2699 Note that these tables are not exhaustive lists; an implementation 2700 MAY implement an algorithm not on these lists. 2701 2702 See the section "Notes on Algorithms" below for more discussion of 2703 the algorithms. 2704 27059.1. Public Key Algorithms 2706 2707 ID Algorithm 2708 -- --------- 2709 1 - RSA (Encrypt or Sign) 2710 2 - RSA Encrypt-Only 2711 3 - RSA Sign-Only 2712 16 - Elgamal (Encrypt-Only), see [ELGAMAL] 2713 17 - DSA (Digital Signature Standard) 2714 18 - Reserved for Elliptic Curve 2715 19 - Reserved for ECDSA 2716 20 - Elgamal (Encrypt or Sign) 2717 2718 2719 2720 2721 2722 21 - Reserved for Diffie-Hellman (X9.42, 2723 as defined for IETF-S/MIME) 2724 100 to 110 - Private/Experimental algorithm. 2725 2726 Implementations MUST implement DSA for signatures, and Elgamal for 2727 encryption. Implementations SHOULD implement RSA keys. 2728 Implementations MAY implement any other algorithm. 2729 27309.2. Symmetric Key Algorithms 2731 2732 ID Algorithm 2733 -- --------- 2734 0 - Plaintext or unencrypted data 2735 1 - IDEA [IDEA] 2736 2 - Triple-DES (DES-EDE, as per spec - 2737 168 bit key derived from 192) 2738 3 - CAST5 (128 bit key, as per RFC 2144) 2739 4 - Blowfish (128 bit key, 16 rounds) [BLOWFISH] 2740 5 - SAFER-SK128 (13 rounds) [SAFER] 2741 6 - Reserved for DES/SK 2742 7 - Reserved for AES with 128-bit key 2743 2744 2745 2746Callas, et. al. Standards Track [Page 49] 2747 2748RFC 2440 OpenPGP Message Format November 1998 2749 2750 2751 8 - Reserved for AES with 192-bit key 2752 9 - Reserved for AES with 256-bit key 2753 100 to 110 - Private/Experimental algorithm. 2754 2755 Implementations MUST implement Triple-DES. Implementations SHOULD 2756 implement IDEA and CAST5.Implementations MAY implement any other 2757 algorithm. 2758 27599.3. Compression Algorithms 2760 2761 ID Algorithm 2762 -- --------- 2763 0 - Uncompressed 2764 1 - ZIP (RFC 1951) 2765 2 - ZLIB (RFC 1950) 2766 100 to 110 - Private/Experimental algorithm. 2767 2768 Implementations MUST implement uncompressed data. Implementations 2769 SHOULD implement ZIP. Implementations MAY implement ZLIB. 2770 27719.4. Hash Algorithms 2772 2773 ID Algorithm Text Name 2774 -- --------- ---- ---- 2775 1 - MD5 "MD5" 2776 2 - SHA-1 "SHA1" 2777 3 - RIPE-MD/160 "RIPEMD160" 2778 4 - Reserved for double-width SHA (experimental) 2779 5 - MD2 "MD2" 2780 6 - Reserved for TIGER/192 "TIGER192" 2781 7 - Reserved for HAVAL (5 pass, 160-bit) 2782 "HAVAL-5-160" 2783 100 to 110 - Private/Experimental algorithm. 2784 2785 Implementations MUST implement SHA-1. Implementations SHOULD 2786 implement MD5. 2787 278810. Packet Composition 2789 2790 OpenPGP packets are assembled into sequences in order to create 2791 messages and to transfer keys. Not all possible packet sequences are 2792 meaningful and correct. This describes the rules for how packets 2793 should be placed into sequences. 2794 279510.1. Transferable Public Keys 2796 2797 OpenPGP users may transfer public keys. The essential elements of a 2798 transferable public key are: 2799 2800 2801 2802Callas, et. al. Standards Track [Page 50] 2803 2804RFC 2440 OpenPGP Message Format November 1998 2805 2806 2807 - One Public Key packet 2808 2809 - Zero or more revocation signatures 2810 2811 - One or more User ID packets 2812 2813 - After each User ID packet, zero or more signature packets 2814 (certifications) 2815 2816 - Zero or more Subkey packets 2817 2818 - After each Subkey packet, one signature packet, optionally a 2819 revocation. 2820 2821 The Public Key packet occurs first. Each of the following User ID 2822 packets provides the identity of the owner of this public key. If 2823 there are multiple User ID packets, this corresponds to multiple 2824 means of identifying the same unique individual user; for example, a 2825 user may have more than one email address, and construct a User ID 2826 for each one. 2827 2828 Immediately following each User ID packet, there are zero or more 2829 signature packets. Each signature packet is calculated on the 2830 immediately preceding User ID packet and the initial Public Key 2831 packet. The signature serves to certify the corresponding public key 2832 and user ID. In effect, the signer is testifying to his or her 2833 belief that this public key belongs to the user identified by this 2834 user ID. 2835 2836 After the User ID packets there may be one or more Subkey packets. 2837 In general, subkeys are provided in cases where the top-level public 2838 key is a signature-only key. However, any V4 key may have subkeys, 2839 and the subkeys may be encryption-only keys, signature-only keys, or 2840 general-purpose keys. 2841 2842 Each Subkey packet must be followed by one Signature packet, which 2843 should be a subkey binding signature issued by the top level key. 2844 2845 Subkey and Key packets may each be followed by a revocation Signature 2846 packet to indicate that the key is revoked. Revocation signatures 2847 are only accepted if they are issued by the key itself, or by a key 2848 that is authorized to issue revocations via a revocation key 2849 subpacket in a self-signature by the top level key. 2850 2851 Transferable public key packet sequences may be concatenated to allow 2852 transferring multiple public keys in one operation. 2853 2854 2855 2856 2857 2858Callas, et. al. Standards Track [Page 51] 2859 2860RFC 2440 OpenPGP Message Format November 1998 2861 2862 286310.2. OpenPGP Messages 2864 2865 An OpenPGP message is a packet or sequence of packets that 2866 corresponds to the following grammatical rules (comma represents 2867 sequential composition, and vertical bar separates alternatives): 2868 2869 OpenPGP Message :- Encrypted Message | Signed Message | 2870 Compressed Message | Literal Message. 2871 2872 Compressed Message :- Compressed Data Packet. 2873 2874 Literal Message :- Literal Data Packet. 2875 2876 ESK :- Public Key Encrypted Session Key Packet | 2877 Symmetric-Key Encrypted Session Key Packet. 2878 2879 ESK Sequence :- ESK | ESK Sequence, ESK. 2880 2881 Encrypted Message :- Symmetrically Encrypted Data Packet | 2882 ESK Sequence, Symmetrically Encrypted Data Packet. 2883 2884 One-Pass Signed Message :- One-Pass Signature Packet, 2885 OpenPGP Message, Corresponding Signature Packet. 2886 2887 Signed Message :- Signature Packet, OpenPGP Message | 2888 One-Pass Signed Message. 2889 2890 In addition, decrypting a Symmetrically Encrypted Data packet and 2891 2892 decompressing a Compressed Data packet must yield a valid OpenPGP 2893 Message. 2894 289510.3. Detached Signatures 2896 2897 Some OpenPGP applications use so-called "detached signatures." For 2898 example, a program bundle may contain a file, and with it a second 2899 file that is a detached signature of the first file. These detached 2900 signatures are simply a signature packet stored separately from the 2901 data that they are a signature of. 2902 290311. Enhanced Key Formats 2904 290511.1. Key Structures 2906 2907 The format of an OpenPGP V3 key is as follows. Entries in square 2908 brackets are optional and ellipses indicate repetition. 2909 2910 2911 2912 2913 2914Callas, et. al. Standards Track [Page 52] 2915 2916RFC 2440 OpenPGP Message Format November 1998 2917 2918 2919 RSA Public Key 2920 [Revocation Self Signature] 2921 User ID [Signature ...] 2922 [User ID [Signature ...] ...] 2923 2924 Each signature certifies the RSA public key and the preceding user 2925 ID. The RSA public key can have many user IDs and each user ID can 2926 have many signatures. 2927 2928 The format of an OpenPGP V4 key that uses two public keys is similar 2929 except that the other keys are added to the end as 'subkeys' of the 2930 primary key. 2931 2932 Primary-Key 2933 [Revocation Self Signature] 2934 [Direct Key Self Signature...] 2935 User ID [Signature ...] 2936 [User ID [Signature ...] ...] 2937 [[Subkey [Binding-Signature-Revocation] 2938 Primary-Key-Binding-Signature] ...] 2939 2940 A subkey always has a single signature after it that is issued using 2941 the primary key to tie the two keys together. This binding signature 2942 may be in either V3 or V4 format, but V4 is preferred, of course. 2943 2944 In the above diagram, if the binding signature of a subkey has been 2945 revoked, the revoked binding signature may be removed, leaving only 2946 one signature. 2947 2948 In a key that has a main key and subkeys, the primary key MUST be a 2949 key capable of signing. The subkeys may be keys of any other type. 2950 There may be other constructions of V4 keys, too. For example, there 2951 may be a single-key RSA key in V4 format, a DSA primary key with an 2952 RSA encryption key, or RSA primary key with an Elgamal subkey, etc. 2953 2954 It is also possible to have a signature-only subkey. This permits a 2955 primary key that collects certifications (key signatures) but is used 2956 only used for certifying subkeys that are used for encryption and 2957 signatures. 2958 295911.2. Key IDs and Fingerprints 2960 2961 For a V3 key, the eight-octet key ID consists of the low 64 bits of 2962 the public modulus of the RSA key. 2963 2964 The fingerprint of a V3 key is formed by hashing the body (but not 2965 the two-octet length) of the MPIs that form the key material (public 2966 modulus n, followed by exponent e) with MD5. 2967 2968 2969 2970Callas, et. al. Standards Track [Page 53] 2971 2972RFC 2440 OpenPGP Message Format November 1998 2973 2974 2975 A V4 fingerprint is the 160-bit SHA-1 hash of the one-octet Packet 2976 Tag, followed by the two-octet packet length, followed by the entire 2977 Public Key packet starting with the version field. The key ID is the 2978 low order 64 bits of the fingerprint. Here are the fields of the 2979 hash material, with the example of a DSA key: 2980 2981 a.1) 0x99 (1 octet) 2982 2983 a.2) high order length octet of (b)-(f) (1 octet) 2984 2985 a.3) low order length octet of (b)-(f) (1 octet) 2986 2987 b) version number = 4 (1 octet); 2988 2989 c) time stamp of key creation (4 octets); 2990 2991 d) algorithm (1 octet): 17 = DSA (example); 2992 2993 e) Algorithm specific fields. 2994 2995 Algorithm Specific Fields for DSA keys (example): 2996 2997 e.1) MPI of DSA prime p; 2998 2999 e.2) MPI of DSA group order q (q is a prime divisor of p-1); 3000 3001 e.3) MPI of DSA group generator g; 3002 3003 e.4) MPI of DSA public key value y (= g**x where x is secret). 3004 3005 Note that it is possible for there to be collisions of key IDs -- two 3006 different keys with the same key ID. Note that there is a much 3007 smaller, but still non-zero probability that two different keys have 3008 the same fingerprint. 3009 3010 Also note that if V3 and V4 format keys share the same RSA key 3011 material, they will have different key ids as well as different 3012 fingerprints. 3013 301412. Notes on Algorithms 3015 301612.1. Symmetric Algorithm Preferences 3017 3018 The symmetric algorithm preference is an ordered list of algorithms 3019 that the keyholder accepts. Since it is found on a self-signature, it 3020 is possible that a keyholder may have different preferences. For 3021 example, Alice may have TripleDES only specified for "alice@work.com" 3022 but CAST5, Blowfish, and TripleDES specified for "alice@home.org". 3023 3024 3025 3026Callas, et. al. Standards Track [Page 54] 3027 3028RFC 2440 OpenPGP Message Format November 1998 3029 3030 3031 Note that it is also possible for preferences to be in a subkey's 3032 binding signature. 3033 3034 Since TripleDES is the MUST-implement algorithm, if it is not 3035 explicitly in the list, it is tacitly at the end. However, it is good 3036 form to place it there explicitly. Note also that if an 3037 implementation does not implement the preference, then it is 3038 implicitly a TripleDES-only implementation. 3039 3040 An implementation MUST not use a symmetric algorithm that is not in 3041 the recipient's preference list. When encrypting to more than one 3042 recipient, the implementation finds a suitable algorithm by taking 3043 the intersection of the preferences of the recipients. Note that the 3044 MUST-implement algorithm, TripleDES, ensures that the intersection is 3045 not null. The implementation may use any mechanism to pick an 3046 algorithm in the intersection. 3047 3048 If an implementation can decrypt a message that a keyholder doesn't 3049 have in their preferences, the implementation SHOULD decrypt the 3050 message anyway, but MUST warn the keyholder than protocol has been 3051 violated. (For example, suppose that Alice, above, has software that 3052 implements all algorithms in this specification. Nonetheless, she 3053 prefers subsets for work or home. If she is sent a message encrypted 3054 with IDEA, which is not in her preferences, the software warns her 3055 that someone sent her an IDEA-encrypted message, but it would ideally 3056 decrypt it anyway.) 3057 3058 An implementation that is striving for backward compatibility MAY 3059 consider a V3 key with a V3 self-signature to be an implicit 3060 preference for IDEA, and no ability to do TripleDES. This is 3061 technically non-compliant, but an implementation MAY violate the 3062 above rule in this case only and use IDEA to encrypt the message, 3063 provided that the message creator is warned. Ideally, though, the 3064 implementation would follow the rule by actually generating two 3065 messages, because it is possible that the OpenPGP user's 3066 implementation does not have IDEA, and thus could not read the 3067 message. Consequently, an implementation MAY, but SHOULD NOT use IDEA 3068 in an algorithm conflict with a V3 key. 3069 307012.2. Other Algorithm Preferences 3071 3072 Other algorithm preferences work similarly to the symmetric algorithm 3073 preference, in that they specify which algorithms the keyholder 3074 accepts. There are two interesting cases that other comments need to 3075 be made about, though, the compression preferences and the hash 3076 preferences. 3077 3078 3079 3080 3081 3082Callas, et. al. Standards Track [Page 55] 3083 3084RFC 2440 OpenPGP Message Format November 1998 3085 3086 308712.2.1. Compression Preferences 3088 3089 Compression has been an integral part of PGP since its first days. 3090 OpenPGP and all previous versions of PGP have offered compression. 3091 And in this specification, the default is for messages to be 3092 compressed, although an implementation is not required to do so. 3093 Consequently, the compression preference gives a way for a keyholder 3094 to request that messages not be compressed, presumably because they 3095 are using a minimal implementation that does not include compression. 3096 Additionally, this gives a keyholder a way to state that it can 3097 support alternate algorithms. 3098 3099 Like the algorithm preferences, an implementation MUST NOT use an 3100 algorithm that is not in the preference vector. If the preferences 3101 are not present, then they are assumed to be [ZIP(1), 3102 UNCOMPRESSED(0)]. 3103 310412.2.2. Hash Algorithm Preferences 3105 3106 Typically, the choice of a hash algorithm is something the signer 3107 does, rather than the verifier, because a signer does not typically 3108 know who is going to be verifying the signature. This preference, 3109 though, allows a protocol based upon digital signatures ease in 3110 negotiation. 3111 3112 Thus, if Alice is authenticating herself to Bob with a signature, it 3113 makes sense for her to use a hash algorithm that Bob's software uses. 3114 This preference allows Bob to state in his key which algorithms Alice 3115 may use. 3116 311712.3. Plaintext 3118 3119 Algorithm 0, "plaintext", may only be used to denote secret keys that 3120 are stored in the clear. Implementations must not use plaintext in 3121 Symmetrically Encrypted Data Packets; they must use Literal Data 3122 Packets to encode unencrypted or literal data. 3123 312412.4. RSA 3125 3126 There are algorithm types for RSA-signature-only, and RSA-encrypt- 3127 only keys. These types are deprecated. The "key flags" subpacket in a 3128 signature is a much better way to express the same idea, and 3129 generalizes it to all algorithms. An implementation SHOULD NOT create 3130 such a key, but MAY interpret it. 3131 3132 An implementation SHOULD NOT implement RSA keys of size less than 768 3133 bits. 3134 3135 3136 3137 3138Callas, et. al. Standards Track [Page 56] 3139 3140RFC 2440 OpenPGP Message Format November 1998 3141 3142 3143 It is permissible for an implementation to support RSA merely for 3144 backward compatibility; for example, such an implementation would 3145 support V3 keys with IDEA symmetric cryptography. Note that this is 3146 an exception to the other MUST-implement rules. An implementation 3147 that supports RSA in V4 keys MUST implement the MUST-implement 3148 features. 3149 315012.5. Elgamal 3151 3152 If an Elgamal key is to be used for both signing and encryption, 3153 extra care must be taken in creating the key. 3154 3155 An ElGamal key consists of a generator g, a prime modulus p, a secret 3156 exponent x, and a public value y = g^x mod p. 3157 3158 The generator and prime must be chosen so that solving the discrete 3159 log problem is intractable. The group g should generate the 3160 multiplicative group mod p-1 or a large subgroup of it, and the order 3161 of g should have at least one large prime factor. A good choice is 3162 to use a "strong" Sophie-Germain prime in choosing p, so that both p 3163 and (p-1)/2 are primes. In fact, this choice is so good that 3164 implementors SHOULD do it, as it avoids a small subgroup attack. 3165 3166 In addition, a result of Bleichenbacher [BLEICHENBACHER] shows that 3167 if the generator g has only small prime factors, and if g divides the 3168 order of the group it generates, then signatures can be forged. In 3169 particular, choosing g=2 is a bad choice if the group order may be 3170 even. On the other hand, a generator of 2 is a fine choice for an 3171 encryption-only key, as this will make the encryption faster. 3172 3173 While verifying Elgamal signatures, note that it is important to test 3174 that r and s are less than p. If this test is not done then 3175 signatures can be trivially forged by using large r values of 3176 approximately twice the length of p. This attack is also discussed 3177 in the Bleichenbacher paper. 3178 3179 Details on safe use of Elgamal signatures may be found in [MENEZES], 3180 which discusses all the weaknesses described above. 3181 3182 If an implementation allows Elgamal signatures, then it MUST use the 3183 algorithm identifier 20 for an Elgamal public key that can sign. 3184 3185 An implementation SHOULD NOT implement Elgamal keys of size less than 3186 768 bits. For long-term security, Elgamal keys should be 1024 bits or 3187 longer. 3188 3189 3190 3191 3192 3193 3194Callas, et. al. Standards Track [Page 57] 3195 3196RFC 2440 OpenPGP Message Format November 1998 3197 3198 319912.6. DSA 3200 3201 An implementation SHOULD NOT implement DSA keys of size less than 768 3202 bits. Note that present DSA is limited to a maximum of 1024 bit keys, 3203 which are recommended for long-term use. 3204 320512.7. Reserved Algorithm Numbers 3206 3207 A number of algorithm IDs have been reserved for algorithms that 3208 would be useful to use in an OpenPGP implementation, yet there are 3209 issues that prevent an implementor from actually implementing the 3210 algorithm. These are marked in the Public Algorithms section as 3211 "(reserved for)". 3212 3213 The reserved public key algorithms, Elliptic Curve (18), ECDSA (19), 3214 and X9.42 (21) do not have the necessary parameters, parameter order, 3215 or semantics defined. 3216 3217 The reserved symmetric key algorithm, DES/SK (6), does not have 3218 semantics defined. 3219 3220 The reserved hash algorithms, TIGER192 (6), and HAVAL-5-160 (7), do 3221 not have OIDs. The reserved algorithm number 4, reserved for a 3222 double-width variant of SHA1, is not presently defined. 3223 3224 We have reserver three algorithm IDs for the US NIST's Advanced 3225 Encryption Standard. This algorithm will work with (at least) 128, 3226 192, and 256-bit keys. We expect that this algorithm will be selected 3227 from the candidate algorithms in the year 2000. 3228 322912.8. OpenPGP CFB mode 3230 3231 OpenPGP does symmetric encryption using a variant of Cipher Feedback 3232 Mode (CFB mode). This section describes the procedure it uses in 3233 detail. This mode is what is used for Symmetrically Encrypted Data 3234 Packets; the mechanism used for encrypting secret key material is 3235 similar, but described in those sections above. 3236 3237 OpenPGP CFB mode uses an initialization vector (IV) of all zeros, and 3238 prefixes the plaintext with ten octets of random data, such that 3239 octets 9 and 10 match octets 7 and 8. It does a CFB "resync" after 3240 encrypting those ten octets. 3241 3242 Note that for an algorithm that has a larger block size than 64 bits, 3243 the equivalent function will be done with that entire block. For 3244 example, a 16-octet block algorithm would operate on 16 octets, and 3245 then produce two octets of check, and then work on 16-octet blocks. 3246 3247 3248 3249 3250Callas, et. al. Standards Track [Page 58] 3251 3252RFC 2440 OpenPGP Message Format November 1998 3253 3254 3255 Step by step, here is the procedure: 3256 3257 1. The feedback register (FR) is set to the IV, which is all zeros. 3258 3259 2. FR is encrypted to produce FRE (FR Encrypted). This is the 3260 encryption of an all-zero value. 3261 3262 3. FRE is xored with the first 8 octets of random data prefixed to 3263 the plaintext to produce C1-C8, the first 8 octets of ciphertext. 3264 3265 4. FR is loaded with C1-C8. 3266 3267 5. FR is encrypted to produce FRE, the encryption of the first 8 3268 octets of ciphertext. 3269 3270 6. The left two octets of FRE get xored with the next two octets of 3271 data that were prefixed to the plaintext. This produces C9-C10, 3272 the next two octets of ciphertext. 3273 3274 7. (The resync step) FR is loaded with C3-C10. 3275 3276 8. FR is encrypted to produce FRE. 3277 3278 9. FRE is xored with the first 8 octets of the given plaintext, now 3279 that we have finished encrypting the 10 octets of prefixed data. 3280 This produces C11-C18, the next 8 octets of ciphertext. 3281 3282 10. FR is loaded with C11-C18 3283 3284 11. FR is encrypted to produce FRE. 3285 3286 12. FRE is xored with the next 8 octets of plaintext, to produce the 3287 next 8 octets of ciphertext. These are loaded into FR and the 3288 process is repeated until the plaintext is used up. 3289 329013. Security Considerations 3291 3292 As with any technology involving cryptography, you should check the 3293 current literature to determine if any algorithms used here have been 3294 found to be vulnerable to attack. 3295 3296 This specification uses Public Key Cryptography technologies. 3297 Possession of the private key portion of a public-private key pair is 3298 assumed to be controlled by the proper party or parties. 3299 3300 Certain operations in this specification involve the use of random 3301 numbers. An appropriate entropy source should be used to generate 3302 these numbers. See RFC 1750. 3303 3304 3305 3306Callas, et. al. Standards Track [Page 59] 3307 3308RFC 2440 OpenPGP Message Format November 1998 3309 3310 3311 The MD5 hash algorithm has been found to have weaknesses (pseudo- 3312 collisions in the compress function) that make some people deprecate 3313 its use. They consider the SHA-1 algorithm better. 3314 3315 Many security protocol designers think that it is a bad idea to use a 3316 single key for both privacy (encryption) and integrity (signatures). 3317 In fact, this was one of the motivating forces behind the V4 key 3318 format with separate signature and encryption keys. If you as an 3319 implementor promote dual-use keys, you should at least be aware of 3320 this controversy. 3321 3322 The DSA algorithm will work with any 160-bit hash, but it is 3323 sensitive to the quality of the hash algorithm, if the hash algorithm 3324 is broken, it can leak the secret key. The Digital Signature Standard 3325 (DSS) specifies that DSA be used with SHA-1. RIPEMD-160 is 3326 considered by many cryptographers to be as strong. An implementation 3327 should take care which hash algorithms are used with DSA, as a weak 3328 hash can not only allow a signature to be forged, but could leak the 3329 secret key. These same considerations about the quality of the hash 3330 algorithm apply to Elgamal signatures. 3331 3332 If you are building an authentication system, the recipient may 3333 specify a preferred signing algorithm. However, the signer would be 3334 foolish to use a weak algorithm simply because the recipient requests 3335 it. 3336 3337 Some of the encryption algorithms mentioned in this document have 3338 been analyzed less than others. For example, although CAST5 is 3339 presently considered strong, it has been analyzed less than Triple- 3340 DES. Other algorithms may have other controversies surrounding them. 3341 3342 Some technologies mentioned here may be subject to government control 3343 in some countries. 3344 334514. Implementation Nits 3346 3347 This section is a collection of comments to help an implementer, 3348 particularly with an eye to backward compatibility. Previous 3349 implementations of PGP are not OpenPGP-compliant. Often the 3350 differences are small, but small differences are frequently more 3351 vexing than large differences. Thus, this list of potential problems 3352 and gotchas for a developer who is trying to be backward-compatible. 3353 3354 * PGP 5.x does not accept V4 signatures for anything other than 3355 key material. 3356 3357 * PGP 5.x does not recognize the "five-octet" lengths in new-format 3358 headers or in signature subpacket lengths. 3359 3360 3361 3362Callas, et. al. Standards Track [Page 60] 3363 3364RFC 2440 OpenPGP Message Format November 1998 3365 3366 3367 * PGP 5.0 rejects an encrypted session key if the keylength differs 3368 from the S2K symmetric algorithm. This is a bug in its validation 3369 function. 3370 3371 * PGP 5.0 does not handle multiple one-pass signature headers and 3372 trailers. Signing one will compress the one-pass signed literal 3373 and prefix a V3 signature instead of doing a nested one-pass 3374 signature. 3375 3376 * When exporting a private key, PGP 2.x generates the header "BEGIN 3377 PGP SECRET KEY BLOCK" instead of "BEGIN PGP PRIVATE KEY BLOCK". 3378 All previous versions ignore the implied data type, and look 3379 directly at the packet data type. 3380 3381 * In a clear-signed signature, PGP 5.0 will figure out the correct 3382 hash algorithm if there is no "Hash:" header, but it will reject 3383 a mismatch between the header and the actual algorithm used. The 3384 "standard" (i.e. Zimmermann/Finney/et al.) version of PGP 2.x 3385 rejects the "Hash:" header and assumes MD5. There are a number of 3386 enhanced variants of PGP 2.6.x that have been modified for SHA-1 3387 signatures. 3388 3389 * PGP 5.0 can read an RSA key in V4 format, but can only recognize 3390 it with a V3 keyid, and can properly use only a V3 format RSA 3391 key. 3392 3393 * Neither PGP 5.x nor PGP 6.0 recognize Elgamal Encrypt and Sign 3394 keys. They only handle Elgamal Encrypt-only keys. 3395 3396 * There are many ways possible for two keys to have the same key 3397 material, but different fingerprints (and thus key ids). Perhaps 3398 the most interesting is an RSA key that has been "upgraded" to V4 3399 format, but since a V4 fingerprint is constructed by hashing the 3400 key creation time along with other things, two V4 keys created at 3401 different times, yet with the same key material will have 3402 different fingerprints. 3403 3404 * If an implementation is using zlib to interoperate with PGP 2.x, 3405 then the "windowBits" parameter should be set to -13. 3406 3407 3408 3409 3410 3411 3412 3413 3414 3415 3416 3417 3418Callas, et. al. Standards Track [Page 61] 3419 3420RFC 2440 OpenPGP Message Format November 1998 3421 3422 342315. Authors and Working Group Chair 3424 3425 The working group can be contacted via the current chair: 3426 3427 John W. Noerenberg, II 3428 Qualcomm, Inc 3429 6455 Lusk Blvd 3430 San Diego, CA 92131 USA 3431 3432 Phone: +1 619-658-3510 3433 EMail: jwn2@qualcomm.com 3434 3435 3436 The principal authors of this memo are: 3437 3438 Jon Callas 3439 Network Associates, Inc. 3440 3965 Freedom Circle 3441 Santa Clara, CA 95054, USA 3442 3443 Phone: +1 408-346-5860 3444 EMail: jon@pgp.com, jcallas@nai.com 3445 3446 3447 Lutz Donnerhacke 3448 IKS GmbH 3449 Wildenbruchstr. 15 3450 07745 Jena, Germany 3451 3452 Phone: +49-3641-675642 3453 EMail: lutz@iks-jena.de 3454 3455 3456 Hal Finney 3457 Network Associates, Inc. 3458 3965 Freedom Circle 3459 Santa Clara, CA 95054, USA 3460 3461 EMail: hal@pgp.com 3462 3463 3464 Rodney Thayer 3465 EIS Corporation 3466 Clearwater, FL 33767, USA 3467 3468 EMail: rodney@unitran.com 3469 3470 3471 3472 3473 3474Callas, et. al. Standards Track [Page 62] 3475 3476RFC 2440 OpenPGP Message Format November 1998 3477 3478 3479 This memo also draws on much previous work from a number of other 3480 authors who include: Derek Atkins, Charles Breed, Dave Del Torto, 3481 Marc Dyksterhouse, Gail Haspert, Gene Hoffman, Paul Hoffman, Raph 3482 Levien, Colin Plumb, Will Price, William Stallings, Mark Weaver, and 3483 Philip R. Zimmermann. 3484 348516. References 3486 3487 [BLEICHENBACHER] Bleichenbacher, Daniel, "Generating ElGamal 3488 signatures without knowing the secret key," 3489 Eurocrypt 96. Note that the version in the 3490 proceedings has an error. A revised version is 3491 available at the time of writing from 3492 <ftp://ftp.inf.ethz.ch/pub/publications/papers/ti/isc 3493 /ElGamal.ps> 3494 3495 [BLOWFISH] Schneier, B. "Description of a New Variable-Length 3496 Key, 64-Bit Block Cipher (Blowfish)" Fast Software 3497 Encryption, Cambridge Security Workshop Proceedings 3498 (December 1993), Springer-Verlag, 1994, pp191-204 3499 3500 <http://www.counterpane.com/bfsverlag.html> 3501 3502 [DONNERHACKE] Donnerhacke, L., et. al, "PGP263in - an improved 3503 international version of PGP", ftp://ftp.iks- 3504 jena.de/mitarb/lutz/crypt/software/pgp/ 3505 3506 [ELGAMAL] T. ElGamal, "A Public-Key Cryptosystem and a 3507 Signature Scheme Based on Discrete Logarithms," IEEE 3508 Transactions on Information Theory, v. IT-31, n. 4, 3509 1985, pp. 469-472. 3510 3511 [IDEA] Lai, X, "On the design and security of block 3512 ciphers", ETH Series in Information Processing, J.L. 3513 Massey (editor), Vol. 1, Hartung-Gorre Verlag 3514 Knostanz, Technische Hochschule (Zurich), 1992 3515 3516 [ISO-10646] ISO/IEC 10646-1:1993. International Standard -- 3517 Information technology -- Universal Multiple-Octet 3518 Coded Character Set (UCS) -- Part 1: Architecture 3519 and Basic Multilingual Plane. UTF-8 is described in 3520 Annex R, adopted but not yet published. UTF-16 is 3521 described in Annex Q, adopted but not yet published. 3522 3523 [MENEZES] Alfred Menezes, Paul van Oorschot, and Scott 3524 Vanstone, "Handbook of Applied Cryptography," CRC 3525 Press, 1996. 3526 3527 3528 3529 3530Callas, et. al. Standards Track [Page 63] 3531 3532RFC 2440 OpenPGP Message Format November 1998 3533 3534 3535 [RFC822] Crocker, D., "Standard for the format of ARPA 3536 Internet text messages", STD 11, RFC 822, August 3537 1982. 3538 3539 [RFC1423] Balenson, D., "Privacy Enhancement for Internet 3540 Electronic Mail: Part III: Algorithms, Modes, and 3541 Identifiers", RFC 1423, October 1993. 3542 3543 [RFC1641] Goldsmith, D. and M. Davis, "Using Unicode with 3544 MIME", RFC 1641, July 1994. 3545 3546 [RFC1750] Eastlake, D., Crocker, S. and J. Schiller, 3547 "Randomness Recommendations for Security", RFC 1750, 3548 December 1994. 3549 3550 [RFC1951] Deutsch, P., "DEFLATE Compressed Data Format 3551 Specification version 1.3.", RFC 1951, May 1996. 3552 3553 [RFC1983] Malkin, G., "Internet Users' Glossary", FYI 18, RFC 3554 1983, August 1996. 3555 3556 [RFC1991] Atkins, D., Stallings, W. and P. Zimmermann, "PGP 3557 Message Exchange Formats", RFC 1991, August 1996. 3558 3559 [RFC2015] Elkins, M., "MIME Security with Pretty Good Privacy 3560 (PGP)", RFC 2015, October 1996. 3561 3562 [RFC2231] Borenstein, N. and N. Freed, "Multipurpose Internet 3563 Mail Extensions (MIME) Part One: Format of Internet 3564 Message Bodies.", RFC 2231, November 1996. 3565 3566 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 3567 Requirement Level", BCP 14, RFC 2119, March 1997. 3568 3569 [RFC2144] Adams, C., "The CAST-128 Encryption Algorithm", RFC 3570 2144, May 1997. 3571 3572 [RFC2279] Yergeau., F., "UTF-8, a transformation format of 3573 Unicode and ISO 10646", RFC 2279, January 1998. 3574 3575 [RFC2313] Kaliski, B., "PKCS #1: RSA Encryption Standard 3576 version 1.5", RFC 2313, March 1998. 3577 3578 [SAFER] Massey, J.L. "SAFER K-64: One Year Later", B. 3579 Preneel, editor, Fast Software Encryption, Second 3580 International Workshop (LNCS 1008) pp212-241, 3581 Springer-Verlag 1995 3582 3583 3584 3585 3586Callas, et. al. Standards Track [Page 64] 3587 3588RFC 2440 OpenPGP Message Format November 1998 3589 3590 359117. Full Copyright Statement 3592 3593 Copyright (C) The Internet Society (1998). All Rights Reserved. 3594 3595 This document and translations of it may be copied and furnished to 3596 others, and derivative works that comment on or otherwise explain it 3597 or assist in its implementation may be prepared, copied, published 3598 and distributed, in whole or in part, without restriction of any 3599 kind, provided that the above copyright notice and this paragraph are 3600 included on all such copies and derivative works. However, this 3601 document itself may not be modified in any way, such as by removing 3602 the copyright notice or references to the Internet Society or other 3603 Internet organizations, except as needed for the purpose of 3604 developing Internet standards in which case the procedures for 3605 copyrights defined in the Internet Standards process must be 3606 followed, or as required to translate it into languages other than 3607 English. 3608 3609 The limited permissions granted above are perpetual and will not be 3610 revoked by the Internet Society or its successors or assigns. 3611 3612 This document and the information contained herein is provided on an 3613 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING 3614 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING 3615 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION 3616 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF 3617 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 3618 3619 3620 3621 3622 3623 3624 3625 3626 3627 3628 3629 3630 3631 3632 3633 3634 3635 3636 3637 3638 3639 3640 3641 3642Callas, et. al. Standards Track [Page 65] 3643 3644