draft-ietf-openpgp-formats-00.txt   draft-ietf-openpgp-formats-01.txt 
Network Working Group Jon Callas Network Working Group Jon Callas
Category: INTERNET-DRAFT Pretty Good Privacy Category: INTERNET-DRAFT Network Associates
draft-ietf-openpgp-formats-00.txt Lutz Donnerhacke draft-ietf-openpgp-formats-01.txt Lutz Donnerhacke
Expires May 1998 IN-Root-CA Individual Network e.V. Expires Aug 1998 IN-Root-CA Individual Network e.V.
November 1997 Hal Finney March 1997 Hal Finney
Pretty Good Privacy Network Associates
Rodney Thayer Rodney Thayer
Sable Technology Sable Technology
OP Formats - OpenPGP Message Format OP Formats - OpenPGP Message Format
draft-ietf-openpgp-formats-00.txt draft-ietf-openpgp-formats-01.txt
Copyright 1997 by The Internet Society. All Rights Reserved. Copyright 1998 by The Internet Society. All Rights Reserved.
Status of this Memo Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working This document is an Internet-Draft. Internet-Drafts are working
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Abstract Abstract
This document is maintained in order to publish all necessary This document is maintained in order to publish all necessary
information needed to develop interoperable applications based on the information needed to develop interoperable applications based on the
OP format. It is not a step-by-step cookbook for writing an OP format. It is not a step-by-step cookbook for writing an
application, it describes only the format and methods needed to read, application, it describes only the format and methods needed to read,
check, generate and write conforming packets crossing any network. It check, generate and write conforming packets crossing any network. It
does not deal with storing and implementation questions albeit it is does not deal with storing and implementation questions albeit it is
necessary to avoid security flaws. necessary to avoid security flaws.
OP (Open-PGP) software uses a combination of strong public-key and Open-PGP software uses a combination of strong public-key and
conventional cryptography to provide security services for electronic conventional cryptography to provide security services for electronic
communications and data storage. These services include communications and data storage. These services include
confidentiality, key management, authentication and digital signatures. confidentiality, key management, authentication and digital signatures.
This document specifies the message formats used in OP. This document specifies the message formats used in OP.
Table of Contents Table of Contents
1. Introduction 1. Introduction
1.1 Terms 1.1 Terms
2. General functions 2. General functions
2.1 Confidentiality 2.1 Confidentiality via Encryption
2.2 Digital signature 2.2 Authentication via Digital signature
2.3 Compression 2.3 Compression
2.4 Conversion to Radix-64 2.4 Conversion to Radix-64
2.4.1 Forming ASCII Armor 3. Data Element Formats
2.4.2 Encoding Binary in Radix-64
2.4.3 Decoding Radix-64
2.4.4 Examples of Radix-64
2.5 Example of an ASCII Armored Message
2.6 Cleartext signature framework
3.0 Data Element Formats
3.1 Scalar numbers 3.1 Scalar numbers
3.2 Multi-Precision Integers 3.2 Multi-Precision Integers
3.3 Counted Strings 3.3 Key IDs
3.4 Time fields 3.4 Text
3.5 String-to-key (S2K) specifiers 3.5 Time fields
3.5.1 String-to-key (S2k) specifier types 3.6 String-to-key (S2K) specifiers
3.5.1.1 Simple S2K 3.6.1 String-to-key (S2k) specifier types
3.5.1.2 Salted S2K 3.6.1.1 Simple S2K
3.5.1.3 Iterated and Salted S2K 3.6.1.2 Salted S2K
3.5.2 String-to-key usage 3.6.1.3 Iterated and Salted S2K
3.5.2.1 Secret key encryption 3.6.2 String-to-key usage
3.5.2.2 Conventional message encryption 3.6.2.1 Secret key encryption
3.5.3 String-to-key algorithms 3.6.2.2 Conventional message encryption
3.5.3.1 Simple S2K algorithm 3.6.3 String-to-key algorithms
3.5.3.2 Salted S2K algorithm 3.6.3.1 Simple S2K algorithm
3.5.3.3 Iterated-Salted S2K algorithm 3.6.3.2 Salted S2K algorithm
4.0 Packet Syntax 3.6.3.3 Iterated-Salted S2K algorithm
4. Packet Syntax
4.1 Overview 4.1 Overview
4.2 Packet Headers 4.2 Packet Headers
4.3 Packet Tags 4.3 Packet Tags
5.0 Packet Types 5. Packet Types
5.1 Encrypted Session Key Packets (Tag 1) 5.1 Public-Key Encrypted Session Key Packets (Tag 1)
5.2 Signature Packet (Tag 2) 5.2 Signature Packet (Tag 2)
5.2.1 Version 3 Signature Packet Format 5.2.1 Version 3 Signature Packet Format
5.2.2 Version 4 Signature Packet Format 5.2.2 Version 4 Signature Packet Format
5.2.2.1 Signature Subpacket Specification 5.2.2.1 Signature Subpacket Specification
5.2.2.2 Signature Subpacket Types 5.2.2.2 Signature Subpacket Types
5.2.3 Signature Types 5.2.3 Signature Types
5.3 Conventional Encrypted Session-Key Packets (Tag 3) 5.2.4 Computing Signatures
5.3 Symmetric-Key Encrypted Session-Key Packets (Tag 3)
5.4 One-Pass Signature Packets (Tag 4) 5.4 One-Pass Signature Packets (Tag 4)
5.5 Key Material Packet 5.5 Key Material Packet
5.5.1 Key Packet Variants 5.5.1 Key Packet Variants
5.5.1.1 Public Key Packet (Tag 6) 5.5.1.1 Public Key Packet (Tag 6)
5.5.1.2 Public Subkey Packet (Tag 14) 5.5.1.2 Public Subkey Packet (Tag 14)
5.5.1.3 Secret Key Packet (Tag 5) 5.5.1.3 Secret Key Packet (Tag 5)
5.5.1.4 Secret Subkey Packet (Tag 7) 5.5.1.4 Secret Subkey Packet (Tag 7)
5.5.2 Public Key Packet Formats 5.5.2 Public Key Packet Formats
5.5.3 Secret Key Packet Formats 5.5.3 Secret Key Packet Formats
5.6 Compressed Data Packet (Tag 8) 5.6 Compressed Data Packet (Tag 8)
5.7 Symmetrically Encrypted Data Packet (Tag 9) 5.7 Symmetrically Encrypted Data Packet (Tag 9)
5.8 Marker Packet (Obsolete Literal Packet) (Tag 10) 5.8 Marker Packet (Obsolete Literal Packet) (Tag 10)
5.9 Literal Data Packet (Tag 11) 5.9 Literal Data Packet (Tag 11)
5.10 Trust Packet (Tag 12) 5.10 Trust Packet (Tag 12)
5.11 User ID Packet (Tag 13) 5.11 User ID Packet (Tag 13)
5.12 Comment Packet (Tag 16) 6. Radix-64 Conversions
6. Constants 6.1 An Implementation of the CRC-24 in "C"
6.1 Public Key Algorithms 6.2 Forming ASCII Armor
6.2 Symmetric Key Algorithms 6.3 Encoding Binary in Radix-64
6.3 Compression Algorithms 6.4 Decoding Radix-64
6.4 Hash Algorithms 6.5 Examples of Radix-64
7. Packet Composition 6.6 Example of an ASCII Armored Message
7.1 Transferable Public Keys 7. Cleartext signature framework
7.2 OP Messages 8. Regular expressions
8. Enhanced Key Formats 9. Constants
8.1 Key Structures 9.1 Public Key Algorithms
8.4 V4 Key IDs and Fingerprints 9.2 Symmetric Key Algorithms
9. Security Considerations 9.3 Compression Algorithms
10. Authors and Working Group Chair 9.4 Hash Algorithms
11. References 10. Packet Composition
12. Full Copyright Statement 10.1 Transferable Public Keys
10.2 OP Messages
11. Enhanced Key Formats
11.1 Key Structures
11.2 V4 Key IDs and Fingerprints
12. Security Considerations
13. Authors and Working Group Chair
14. References
15. Full Copyright Statement
1. Introduction 1. Introduction
This document provides information on the message-exchange packet This document provides information on the message-exchange packet
formats used by OP to provide encryption, decryption, signing, key formats used by OP to provide encryption, decryption, signing, key
management and functions. It builds on the foundation provided RFC management and functions. It builds on the foundation provided RFC
1991 "PGP Message Exchange Formats" [1]. 1991 "PGP Message Exchange Formats."
1.1 Terms 1.1 Terms
OP - OpenPGP. This is a definition for security software that uses PGP OP - OpenPGP. This is a definition for security software that uses PGP
5.x as a basis. 5.x as a basis.
PGP - Pretty Good Privacy. PGP is a family of software systems PGP - Pretty Good Privacy. PGP is a family of software systems
developed by Philip R. Zimmermann from which OP is based. developed by Philip R. Zimmermann from which OP is based.
PGP 2.6.x - This version of PGP has many variants, hence the term PGP PGP 2.6.x - This version of PGP has many variants, hence the term PGP
2.6.x. It used only RSA and IDEA for its cryptography. 2.6.x. It used only RSA and IDEA for its cryptography.
PGP 5.x - This version of PGP is formerly known as "PGP 3" in the PGP 5.x - This version of PGP is formerly known as "PGP 3" in the
community and also in the predecessor of this document, RFC1991. It community and also in the predecessor of this document, RFC1991. It
has new formats and corrects a number of problems in the PGP 2.6.x. It has new formats and corrects a number of problems in the PGP 2.6.x. It
is referred to here as PGP 5.x because that software was the first is referred to here as PGP 5.x because that software was the first
release of the "PGP 3" code base. release of the "PGP 3" code base.
"PGP", "Pretty Good", and "Pretty Good Privacy" are trademarks of "PGP", "Pretty Good", and "Pretty Good Privacy" are trademarks of
Pretty Good Privacy, Inc. Network Associates, Inc.
2. General functions 2. General functions
OP provides data integrity services for messages and data files by OP provides data integrity services for messages and data files by
using these core technologies: using these core technologies:
-digital signature -encryption -compression -radix-64 conversion -digital signature
-encryption
-compression
-radix-64 conversion
In addition, OP provides key management and certificate services. In addition, OP provides key management and certificate services.
2.1 Confidentiality via Encryption 2.1 Confidentiality via Encryption
OP offers two encryption options to provide confidentiality: OP offers two encryption options to provide confidentiality:
conventional (symmetric-key) encryption and public key encryption. conventional (symmetric-key) encryption and public key encryption.
With public-key encryption, the message is actually encrypted using a With public-key encryption, the message is actually encrypted using a
conventional encryption algorithm. In this mode, each conventional key conventional encryption algorithm. In this mode, each conventional key
is used only once. That is, a new key is generated as a random number is used only once. That is, a new key is generated as a random number
skipping to change at page 5, line 16 skipping to change at page 5, line 30
2.3 Compression 2.3 Compression
OP implementations MAY compress the message after applying the OP implementations MAY compress the message after applying the
signature but before encryption. signature but before encryption.
2.4 Conversion to Radix-64 2.4 Conversion to Radix-64
OP's underlying native representation for encrypted messages, signature OP's underlying native representation for encrypted messages, signature
certificates, and keys is a stream of arbitrary octets. Some systems certificates, and keys is a stream of arbitrary octets. Some systems
only permit the use of blocks consisting of seven-bit, printable text. only permit the use of blocks consisting of seven-bit, printable text.
For transporting OP's native raw binary octets through email channels, For transporting OP's native raw binary octets through channels that
a printable encoding of these binary octets is needed. OP provides the are not safe to raw binary data, a printable encoding of these binary
service of converting the raw 8-bit binary octet stream to a stream of octets is needed. OP provides the service of converting the raw 8-bit
printable ASCII characters, called Radix-64 encoding or ASCII Armor. binary octet stream to a stream of printable ASCII characters, called
Radix-64 encoding or ASCII Armor.
In principle, any printable encoding scheme that met the requirements
of the email channel would suffice, since it would not change the
underlying binary bit streams of the native OP data structures. The OP
standard specifies one such printable encoding scheme to ensure
interoperability.
OP's Radix-64 encoding is composed of two parts: a base64 encoding of
the binary data, and a checksum. The base64 encoding is identical to
the MIME base64 content-transfer-encoding [RFC 2045, Section 6.8]. An
OP implementation MAY use ASCII Armor to protect the raw binary data.
The checksum is a 24-bit CRC converted to four characters of radix-64
encoding by the same MIME base64 transformation, preceded by an equals
sign (=). The CRC is computed by using the generator 0x864CFB and an
initialization of 0xB704CE. The accumulation is done on the data
before it is converted to radix-64, rather than on the converted data.
(For more information on CRC functions, see chapter 19 of [CAMPBELL].)
{{Editor's note: This is old text, dating back to RFC 1991. I have
never liked the glib way the CRC has been dismissed, but I also know
that this is no place to start a discussion of CRC theory. Should we
construct a sample implementation in C and put it in an appendix? --
jdcc}}
The checksum with its leading equal sign MAY appear on the first line
after the Base64 encoded data.
Rationale for CRC-24: The size of 24 bits fits evenly into printable
base64. The nonzero initialization can detect more errors than a zero
initialization.
2.4.1 Forming ASCII Armor
When OP encodes data into ASCII Armor, it puts specific headers around
the data, so OP can reconstruct the data later. OP informs the user
what kind of data is encoded in the ASCII armor through the use of the
headers.
Concatenating the following data creates ASCII Armor:
- An Armor Header Line, appropriate for the type of data - Armor
Headers - A blank (zero-length, or containing only whitespace) line -
The ASCII-Armored data - An Armor Checksum - The Armor Tail, which
depends on the Armor Header Line.
An Armor Header Line consists of the appropriate header line text
surrounded by five (5) dashes ('-', 0x2D) on either side of the header
line text. The header line text is chosen based upon the type of data
that is being encoded in Armor, and how it is being encoded. Header
line texts include the following strings:
BEGIN PGP MESSAGE used for signed, encrypted, or compressed files
BEGIN PGP PUBLIC KEY BLOCK used for armoring public keys
BEGIN PGP PRIVATE KEY BLOCK used for armoring private keys
BEGIN PGP MESSAGE, PART X/Y used for multi-part messages, where the
armor is split amongst Y parts, and this is the Xth part out of Y.
BEGIN PGP MESSAGE, PART X used for multi-part messages, where this is
the Xth part of an unspecified number of parts. Requires the MESSAGE-ID
Armor Header to be used.
BEGIN PGP SIGNATURE used for detached signatures, OP/MIME signatures,
and signatures following clearsigned messages
The Armor Headers are pairs of strings that can give the user or the
receiving OP message block some information about how to decode or use
the message. The Armor Headers are a part of the armor, not a part of
the message, and hence are not protected by any signatures applied to
the message.
The format of an Armor Header is that of a key-value pair. A colon
(':' 0x38) and a single space (0x20) separate the key and value. OP
should consider improperly formatted Armor Headers to be corruption of
the ASCII Armor. Unknown keys should be reported to the user, but OP
should continue to process the message. Currently defined Armor Header
Keys include "Version" and "Comment", which define the OP Version used
to encode the message and a user-defined comment.
The "MessageID" Armor Header specifies a 32-character string of
printable characters. The string must be the same for all parts of a
multi-part message that uses the "PART X" Armor Header. MessageID
strings should be chosen with enough internal randomness that no two
messages would have the same MessageID string.
The MessageID should not appear unless it is in a multi-part message.
If it appears at all, it should be computed from the message in a
deterministic fashion, rather than contain a purely random value. This
is to allow anyone to determine that the MessageID cannot serve as a
covert means of leaking cryptographic key information.
{{Editor's note: This needs to be cleaned up, with a table of the
defined headers. Also, the MessageID description is too vague about
how random the id has to be.}}
The Armor Tail Line is composed in the same manner as the Armor Header
Line, except the string "BEGIN" is replaced by the string "END."
2.4.2 Encoding Binary in Radix-64
The encoding process represents 24-bit groups of input bits as output
strings of 4 encoded characters. Proceeding from left to right, a
24-bit input group is formed by concatenating three 8-bit input groups.
These 24 bits are then treated as four concatenated 6-bit groups, each
of which is translated into a single digit in the Radix-64 alphabet.
When encoding a bit stream with the Radix-64 encoding, the bit stream
must be presumed to be ordered with the most-significant-bit first.
That is, the first bit in the stream will be the high-order bit in the
first 8-bit byte, and the eighth bit will be the low-order bit in the
first 8-bit byte, and so on.
+--first octet--+-second octet--+--third octet--+
|7 6 5 4 3 2 1 0|7 6 5 4 3 2 1 0|7 6 5 4 3 2 1 0|
+-----------+---+-------+-------+---+-----------+
|5 4 3 2 1 0|5 4 3 2 1 0|5 4 3 2 1 0|5 4 3 2 1 0|
+--1.index--+--2.index--+--3.index--+--4.index--+
Each 6-bit group is used as an index into an array of 64 printable
characters from the table below. The character referenced by the index
is placed in the output string.
Value Encoding Value Encoding Value Encoding Value Encoding
0 A 17 R 34 i 51 z
1 B 18 S 35 j 52 0
2 C 19 T 36 k 53 1
3 D 20 U 37 l 54 2
4 E 21 V 38 m 55 3
5 F 22 W 39 n 56 4
6 G 23 X 40 o 57 5
7 H 24 Y 41 p 58 6
8 I 25 Z 42 q 59 7
9 J 26 a 43 r 60 8
10 K 27 b 44 s 61 9
11 L 28 c 45 t 62 +
12 M 29 d 46 u 63 /
13 N 30 e 47 v
14 O 31 f 48 w (pad) =
15 P 32 g 49 x
16 Q 33 h 50 y
The encoded output stream must be represented in lines of no more than
76 characters each.
Special processing is performed if fewer than 24 bits are available at
the end of the data being encoded. There are three possibilities:
- The last data group has 24 bits (3 octets). No special processing is
needed.
- The last data group has 16 bits (2 octets). The first two 6-bit
groups are processed as above. The third (incomplete) data group has
two zero-value bits added to it, and is processed as above. A pad
character (=) is added to the output.
- The last data group has 8 bits (1 octet). The first 6-bit group is
processed as above. The second (incomplete) data group has four
zero-value bits added to it, and is processed as above. Two pad
characters (=) are added to the output.
2.4.3 Decoding Radix-64
Any characters outside of the base64 alphabet are ignored in Radix-64
data. Decoding software must ignore all line breaks or other
characters not found in the table above.
In Radix-64 data, characters other than those in the table, line
breaks, and other white space probably indicate a transmission error,
about which a warning message or even a message rejection might be
appropriate under some circumstances.
Because it is used only for padding at the end of the data, the
occurrence of any "=" characters may be taken as evidence that the end
of the data has been reached (without truncation in transit). No such
assurance is possible, however, when the number of octets transmitted
was a multiple of three and no "=" characters are present.
2.4.4 Examples of Radix-64
Input data: 0x14fb9c03d97e
Hex: 1 4 f b 9 c | 0 3 d 9 7 e
8-bit: 00010100 11111011 10011100 | 00000011 11011001 11111110
6-bit: 000101 001111 101110 011100 | 000000 111101 100111 111110
Decimal: 5 15 46 28 0 61 37 63
Output: F P u c A 9 l /
Input data: 0x14fb9c03d9
Hex: 1 4 f b 9 c | 0 3 d 9
8-bit: 00010100 11111011 10011100 | 00000011 11011001
pad with 00
6-bit: 000101 001111 101110 011100 | 000000 111101 100100
Decimal: 5 15 46 28 0 61 36
pad with =
Output: F P u c A 9 k =
Input data: 0x14fb9c03
Hex: 1 4 f b 9 c | 0 3
8-bit: 00010100 11111011 10011100 | 00000011
pad with 0000
6-bit: 000101 001111 101110 011100 | 000000 110000
Decimal: 5 15 46 28 0 48
pad with = =
Output: F P u c A w = =
2.5 Example of an ASCII Armored Message
-----BEGIN PGP MESSAGE-----
Version: OP V0.0
owFbx8DAYFTCWlySkpkHZDKEFCXmFedmFhdn5ucpZKdWFiv4hgaHKPj5hygUpSbn
l6UWpabo8XIBAA==
=3m1o
-----END PGP MESSAGE-----
Note that this example is indented by two spaces.
2.6 Cleartext signature framework
Sometimes it is necessary to sign a textual octet stream without ASCII
armoring the stream itself, so the signed text is still readable
without special software. In order to bind a signature to such a
cleartext, this framework is used. (Note that RFC 2015 defines another
way to clear sign messages for environments that support MIME.)
The cleartext signed message consists of:
- The cleartext header '-----BEGIN PGP SIGNED MESSAGE-----' on a
single line,
- Zero or more "Hash" Armor Headers (3.1.2.4),
- Exactly one empty line not included into the message digest,
- The dash-escaped cleartext that is included into the message digest,
- The ASCII armored signature(s) including the Armor Header and Armor
Tail Lines.
If the "Hash" armor header is given, the specified message digest
algorithm is used for the signature. If this header is missing, SHA-1
is assumed. If more than one message digest is used in the signature,
the "Hash" armor header contains a comma-delimited list of used message
digests. As an abbreviation, the "Hash" armor header may be placed on
the cleartext header line, inserting a comma after the word 'MESSAGE',
as follows:
'-----BEGIN PGP SIGNED MESSAGE, Hash: MD5, SHA1'.
{{Editor's note: Should the above armor header line stay or go?
There's no reason that the "Hash:" armor header can't have multiple
hashes in it. I think anything that reduces parsing complexity is a
Good Thing. --jdcc}}
Current message digest names are:
- "SHA1"
- "MD5"
- "RIPEMD160"
Dash escaped cleartext is the ordinary cleartext where every line
starting with a dash '-' (0x2D) is prepended by the sequence dash '-'
(0x2D) and space ' ' (0x20). This prevents the parser from recognizing
armor headers of the cleartext itself. The message digest is computed
using the cleartext itself, not the dash escaped form.
As with binary signatures on text documents (see below), the cleartext Implementations SHOULD provide Radix-64 conversions.
signature is calculated on the text using canonical <CR><LF> line
endings. The line ending (i.e. the <CR><LF>) before the '-----BEGIN
PGP SIGNATURE-----' line that terminates the signed text is not
considered part of the signed text.
Also, any trailing whitespace (spaces, and tabs, 0x09) at the end of Note that many applications, particularly messaging applications, will
any line is ignored when the cleartext signature is calculated. want more advanced features as described in the OpenPGP-MIME document,
RFC2015. An application that implements OP for messaging SHOULD also
implement OpenPGP-MIME.
3. Data Element Formats 3. Data Element Formats
This section describes the data elements used by OP. This section describes the data elements used by OP.
3.1 Scalar numbers 3.1 Scalar numbers
Scalar numbers are unsigned, and are always stored in big-endian Scalar numbers are unsigned, and are always stored in big-endian
format. Using n[k] to refer to the kth octet being interpreted, the format. Using n[k] to refer to the kth octet being interpreted, the
value of a two-octet scalar is ((n[0] << 8) + n[1]). The value of a value of a two-octet scalar is ((n[0] << 8) + n[1]). The value of a
skipping to change at page 11, line 16 skipping to change at page 6, line 33
string [00 09 01 FF] forms an MPI with the value of 511. string [00 09 01 FF] forms an MPI with the value of 511.
Additional rules: Additional rules:
The size of an MPI is ((MPI.length + 7) / 8) + 2. The size of an MPI is ((MPI.length + 7) / 8) + 2.
The length field of an MPI describes the length starting from its most The length field of an MPI describes the length starting from its most
significant non-zero bit. Thus, the MPI [00 02 01] is not formed significant non-zero bit. Thus, the MPI [00 02 01] is not formed
correctly. It should be [00 01 01]. correctly. It should be [00 01 01].
3.3 Counted Strings 3.3 Key IDs
A counted string consists of a length and then N octets of string data. A Key ID is an eight-octet number that identifies a key.
Its default character set is UTF-8 [RFC2044] encoding of Unicode Implementations SHOULD NOT assume that Key IDs are unique. The
[ISO10646]. section, "Enhanced Key Formats" below describes how Key IDs are formed.
3.4 Time fields 3.4 Text
The default character set for text is the UTF-8 [RFC2044] encoding of
Unicode [ISO10646].
3.5 Time fields
A time field is an unsigned four-octet number containing the number of A time field is an unsigned four-octet number containing the number of
seconds elapsed since midnight, 1 January 1970 UTC. seconds elapsed since midnight, 1 January 1970 UTC.
3.5 String-to-key (S2K) specifiers 3.6 String-to-key (S2K) specifiers
String-to-key (S2K) specifiers are used to convert passphrase strings String-to-key (S2K) specifiers are used to convert passphrase strings
into conventional encryption/decryption keys. They are used in two into conventional encryption/decryption keys. They are used in two
places, currently: to encrypt the secret part of private keys in the places, currently: to encrypt the secret part of private keys in the
private keyring, and to convert passphrases to encryption keys for private keyring, and to convert passphrases to encryption keys for
conventionally encrypted messages. conventionally encrypted messages.
3.5.1 String-to-key (S2k) specifier types 3.6.1 String-to-key (S2k) specifier types
There are three types of S2K specifiers currently supported, as There are three types of S2K specifiers currently supported, as
follows: follows:
3.5.1.1 Simple S2K 3.6.1.1 Simple S2K
This directly hashes the string to produce the key data. See below for This directly hashes the string to produce the key data. See below for
how this hashing is done. how this hashing is done.
Octet 0: 0x00 Octet 0: 0x00
Octet 1: hash algorithm Octet 1: hash algorithm
3.5.1.2 Salted S2K 3.6.1.2 Salted S2K
This includes a "salt" value in the S2K specifier -- some arbitrary This includes a "salt" value in the S2K specifier -- some arbitrary
data -- that gets hashed along with the passphrase string, to help data -- that gets hashed along with the passphrase string, to help
prevent dictionary attacks. prevent dictionary attacks.
Octet 0: 0x01 Octet 0: 0x01
Octet 1: hash algorithm Octet 1: hash algorithm
Octets 2-9: 8-octet salt value Octets 2-9: 8-octet salt value
3.5.1.3 Iterated and Salted S2K 3.6.1.3 Iterated and Salted S2K
This includes both a salt and an octet count. The salt is combined This includes both a salt and an octet count. The salt is combined
with the passphrase and the resulting value is hashed repeatedly. This with the passphrase and the resulting value is hashed repeatedly. This
further increases the amount of work an attacker must do to try further increases the amount of work an attacker must do to try
dictionary attacks. dictionary attacks.
Octet 0: 0x03 Octet 0: 0x04
Octet 1: hash algorithm Octet 1: hash algorithm
Octets 2-9: 8-octet salt value Octets 2-9: 8-octet salt value
Octet 10: count, in special format (described below) Octets 10-13: count, a four-octet, unsigned value
3.5.2 String-to-key usage Note that the value 0x03 for octet 0 of a S2K specifier is reserved; it
denotes an obsolete form of the Interated and Salted S2K.
Implementations MUST implement simple S2K and salted S2K specifiers. 3.6.2 String-to-key usage
Implementations MAY implement iterated and salted S2K specifiers.
Implementations SHOULD use salted S2K specifiers, as simple S2K
specifiers are more vulnerable to dictionary attacks.
3.5.2.1 Secret key encryption Implementations SHOULD use salted or iterated-and-salted S2K
specifiers, as simple S2K specifiers are more vulnerable to dictionary
attacks.
3.6.2.1 Secret key encryption
An S2K specifier can be stored in the secret keyring to specify how to An S2K specifier can be stored in the secret keyring to specify how to
convert the passphrase to a key that unlocks the secret data. Older convert the passphrase to a key that unlocks the secret data. Older
versions of PGP just stored a cipher algorithm octet preceding the versions of PGP just stored a cipher algorithm octet preceding the
secret data or a zero to indicate that the secret data was unencrypted. secret data or a zero to indicate that the secret data was unencrypted.
The MD5 hash function was always used to convert the passphrase to a The MD5 hash function was always used to convert the passphrase to a
key for the specified cipher algorithm. key for the specified cipher algorithm.
For compatibility, when an S2K specifier is used, the special value 255 For compatibility, when an S2K specifier is used, the special value 255
is stored in the position where the hash algorithm octet would have is stored in the position where the hash algorithm octet would have
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Cipher alg use Simple S2K algorithm using MD5 hash Cipher alg use Simple S2K algorithm using MD5 hash
This last possibility, the cipher algorithm number with an implicit use This last possibility, the cipher algorithm number with an implicit use
of MD5 is provided for backward compatibility; it should be understood, of MD5 is provided for backward compatibility; it should be understood,
but not generated. but not generated.
These are followed by an 8-octet Initial Vector for the decryption of These are followed by an 8-octet Initial Vector for the decryption of
the secret values, if they are encrypted, and then the secret key the secret values, if they are encrypted, and then the secret key
values themselves. values themselves.
3.5.2.2 Conventional message encryption 3.6.2.2 Conventional message encryption
PGP 2.X always used IDEA with Simple string-to-key conversion when PGP 2.X always used IDEA with Simple string-to-key conversion when
conventionally encrypting a message. PGP 5 can create a Conventional conventionally encrypting a message. PGP 5 can create a Conventional
Encrypted Session Key packet at the front of a message. This can be Encrypted Session Key packet at the front of a message. This can be
used to allow S2K specifiers to be used for the passphrase conversion, used to allow S2K specifiers to be used for the passphrase conversion,
to allow other ciphers than IDEA to be used, or to create messages with to allow other ciphers than IDEA to be used, or to create messages with
a mix of conventional ESKs and public key ESKs. This allows a message a mix of conventional ESKs and public key ESKs. This allows a message
to be decrypted either with a passphrase or a public key. to be decrypted either with a passphrase or a public key.
3.5.3 String-to-key algorithms 3.6.3 String-to-key algorithms
3.5.3.1 Simple S2K algorithm 3.6.3.1 Simple S2K algorithm
Simple S2K hashes the passphrase to produce the session key. The Simple S2K hashes the passphrase to produce the session key. The
manner in which this is done depends on the size of the session key manner in which this is done depends on the size of the session key
(which will depend on the cipher used) and the size of the hash (which will depend on the cipher used) and the size of the hash
algorithm's output. If the hash size is greater than or equal to the algorithm's output. If the hash size is greater than or equal to the
session key size, the leftmost octets of the hash are used as the key. session key size, the leftmost octets of the hash are used as the key.
If the hash size is less than the key size, multiple instances of the If the hash size is less than the key size, multiple instances of the
hash context are created -- enough to produce the required key data. hash context are created -- enough to produce the required key data.
These instances are preloaded with 0, 1, 2, ... octets of zeros (that These instances are preloaded with 0, 1, 2, ... octets of zeros (that
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hash context are created -- enough to produce the required key data. hash context are created -- enough to produce the required key data.
These instances are preloaded with 0, 1, 2, ... octets of zeros (that These instances are preloaded with 0, 1, 2, ... octets of zeros (that
is to say, the first instance has no preloading, the second gets is to say, the first instance has no preloading, the second gets
preloaded with 1 octet of zero, the third is preloaded with two octets preloaded with 1 octet of zero, the third is preloaded with two octets
of zeros, and so forth). of zeros, and so forth).
As the data is hashed, it is given independently to each hash context. As the data is hashed, it is given independently to each hash context.
Since the contexts have been initialized differently, they will each Since the contexts have been initialized differently, they will each
produce different hash output. Once the passphrase is hashed, the produce different hash output. Once the passphrase is hashed, the
output data from the multiple hashes is concatenated, first hash output data from the multiple hashes is concatenated, first hash
leftmost, to produce the key data, with any excess octets on the right leftmost, to produce the key data, with any excess octets on the right
discarded. discarded.
3.5.3.2 Salted S2K algorithm 3.6.3.2 Salted S2K algorithm
Salted S2K is exactly like Simple S2K, except that the input to the Salted S2K is exactly like Simple S2K, except that the input to the
hash function(s) consists of the 8 octets of salt from the S2K hash function(s) consists of the 8 octets of salt from the S2K
specifier, followed by the passphrase. specifier, followed by the passphrase.
3.5.3.3 Iterated-Salted S2K algorithm 3.6.3.3 Iterated-Salted S2K algorithm
{{Editor's note: This is very complex, with bizarre things like an
8-bit floating point format. Should we just drop it? --jdcc}}
Iterated-Salted S2K hashes the passphrase and salt data multiple times. Iterated-Salted S2K hashes the passphrase and salt data multiple times.
The total number of octets to be hashed is encoded in the count octet The total number of octets to be hashed is specified in the four-octet
that follows the salt in the S2K specifier. The count value is stored count in the S2K specifier. Note that the resulting count value is an
as a normalized floating-point value with 4 bits of exponent and 4 bits octet count of how many octets will be hashed, not an iteration count.
of mantissa. The formula to convert from the count octet to a count of
the number of octets to be hashed is as follows, letting the high 4
bits of the count octet be CEXP and the low four bits be CMANT:
count of octets to be hashed = (16 + CMANT) << (CEXP + 6)
This allows encoding hash counts as low as 16 << 6 or 1024 (using an
octet value of 0), and as high as 31 << 21 or 65011712 (using an octet
value of 0xff). Note that the resulting count value is an octet count
of how many octets will be hashed, not an iteration count.
Initially, one or more hash contexts are set up as with the other S2K Initially, one or more hash contexts are set up as with the other S2K
algorithms, depending on how many octets of key data are needed. Then algorithms, depending on how many octets of key data are needed. Then
the salt, followed by the passphrase data is repeatedly hashed until the salt, followed by the passphrase data is repeatedly hashed until
the number of octets specified by the octet count has been hashed. The the number of octets specified by the octet count has been hashed. The
one exception is that if the octet count is less than the size of the one exception is that if the octet count is less than the size of the
salt plus passphrase, the full salt plus passphrase will be hashed even salt plus passphrase, the full salt plus passphrase will be hashed even
though that is greater than the octet count. After the hashing is done though that is greater than the octet count. After the hashing is done
the data is unloaded from the hash context(s) as with the other S2K the data is unloaded from the hash context(s) as with the other S2K
algorithms. algorithms.
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+---------------+ +---------------+
PTag |7 6 5 4 3 2 1 0| PTag |7 6 5 4 3 2 1 0|
+---------------+ +---------------+
Bit 7 -- Always one Bit 7 -- Always one
Bit 6 -- New packet format if set Bit 6 -- New packet format if set
PGP 2.6.X only uses old format packets. Thus, software that PGP 2.6.X only uses old format packets. Thus, software that
interoperates with those versions of PGP must only use old format interoperates with those versions of PGP must only use old format
packets. If interoperability is not an issue, either format may be packets. If interoperability is not an issue, either format may be
used. used. Note that old format packets have four bits of content tags, and
new format packets have six; some features cannot be used and still be
backwards-compatible.
Old format packets contain: Old format packets contain:
Bits 5-2 -- content tag Bits 5-2 -- content tag
Bits 1-0 - length-type Bits 1-0 - length-type
New format packets contain: New format packets contain:
Bits 5-0 -- content tag Bits 5-0 -- content tag
The meaning of the length-type in old-format packets is: The meaning of the length-type in old-format packets is:
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packet is the length of the header(s) plus the length of the body. packet is the length of the header(s) plus the length of the body.
4.3 Packet Tags 4.3 Packet Tags
The packet tag denotes what type of packet the body holds. Note that The packet tag denotes what type of packet the body holds. Note that
old format packets can only have tags less than 16, whereas new format old format packets can only have tags less than 16, whereas new format
packets can have tags as great as 63. The defined tags (in decimal) packets can have tags as great as 63. The defined tags (in decimal)
are: are:
0 -- Reserved. A packet must not have a tag with this value. 0 -- Reserved. A packet must not have a tag with this value.
1 -- Encrypted Session Key Packet 1 -- Public-Key Encrypted Session Key Packet
2 -- Signature Packet 2 -- Signature Packet
3 -- Conventionally Encrypted Session Key Packet
3 -- Symmetric-Key Encrypted Session Key Packet
4 -- One-Pass Signature Packet 4 -- One-Pass Signature Packet
5 -- Secret Key Packet 5 -- Secret Key Packet
6 -- Public Key Packet 6 -- Public Key Packet
7 -- Secret Subkey Packet 7 -- Secret Subkey Packet
8 -- Compressed Data Packet 8 -- Compressed Data Packet
9 -- Symmetrically Encrypted Data Packet 9 -- Symmetrically Encrypted Data Packet
10 -- Marker Packet 10 -- Marker Packet
11 -- Literal Data Packet 11 -- Literal Data Packet
12 -- Trust Packet 12 -- Trust Packet
13 -- Name Packet 13 -- Name Packet
14 -- Subkey Packet 14 -- Subkey Packet
15 -- Reserved 15 -- Reserved
16 -- Comment Packet
60 to 63 -- Private or Experimental Values 60 to 63 -- Private or Experimental Values
5. Packet Types 5. Packet Types
5.1 Encrypted Session Key Packets (Tag 1) 5.1 Public-Key Encrypted Session Key Packets (Tag 1)
An Encrypted Session Key packet holds the key used to encrypt a message A Public-Key Encrypted Session Key packet holds the key used to encrypt
that is itself encrypted with a public key. Zero or more Encrypted a message that is itself encrypted with a public key. Zero or more
Session Key packets and/or Conventional Encrypted Session Key packets Encrypted Session Key packets and/or Conventional Encrypted Session Key
may precede a Symmetrically Encrypted Data Packet, which holds an packets may precede a Symmetrically Encrypted Data Packet, which holds
encrypted message. The message is encrypted with a session key, and an encrypted message. The message is encrypted with a session key, and
the session key is itself encrypted and stored in the Encrypted Session the session key is itself encrypted and stored in the Encrypted Session
Key packet or the Conventional Encrypted Session Key packet. The Key packet(s). The Symmetrically Encrypted Data Packet is preceded by
Symmetrically Encrypted Data Packet is preceded by one Encrypted one Public-Key Encrypted Session Key packet for each OP key to which
Session Key packet for each OP key to which the message is encrypted. the message is encrypted. The recipient of the message finds a session
The recipient of the message finds a session key that is encrypted to key that is encrypted to their public key, decrypts the session key,
their public key, decrypts the session key, and then uses the session and then uses the session key to decrypt the message.
key to decrypt the message.
The body of this packet consists of: The body of this packet consists of:
- A one-octet number giving the version number of the packet type. - A one-octet number giving the version number of the packet type.
The currently defined value for packet version is 3. An The currently defined value for packet version is 3. An
implementation should accept, but not generate a version of 2, implementation should accept, but not generate a version of 2,
which is equivalent to V3 in all other respects. which is equivalent to V3 in all other respects.
- An eight-octet number that gives the key ID of the public key that - An eight-octet number that gives the key ID of the public key that
the session key is encrypted to. the session key is encrypted to.
- A one-octet number giving the public key algorithm used. - A one-octet number giving the public key algorithm used.
- A string of octets that is the encrypted session key. This - A string of octets that is the encrypted session key. This
string takes up the remainder of the packet, and its contents are string takes up the remainder of the packet, and its contents are
dependent on the public key algorithm used. dependent on the public key algorithm used.
Algorithm Specific Fields for RSA encryption Algorithm Specific Fields for RSA encryption
- multiprecision integer (MPI) of RSA encrypted value m**e. - multiprecision integer (MPI) of RSA encrypted value m**e mod n.
Algorithm Specific Fields for Elgamal encryption: Algorithm Specific Fields for Elgamal encryption:
- MPI of DSA value g**k. - MPI of DSA value g**k mod p.
- MPI of DSA value m * y**k. - MPI of DSA value m * y**k mod p.
The encrypted value "m" in the above formulas is derived from the The encrypted value "m" in the above formulas is derived from the
session key as follows. First the session key is prepended with a session key as follows. First the session key is prepended with a
one-octet algorithm identifier that specifies the conventional one-octet algorithm identifier that specifies the conventional
encryption algorithm used to encrypt the following Symmetrically encryption algorithm used to encrypt the following Symmetrically
Encrypted Data Packet. Then a two-octet checksum is appended which is Encrypted Data Packet. Then a two-octet checksum is appended which is
equal to the sum of the preceding octets, including the algorithm equal to the sum of the preceding octets, including the algorithm
identifier and session key, modulo 65536. This value is then padded as identifier and session key, modulo 65536. This value is then padded as
described in PKCS-1 block type 02 [PKCS1] to form the "m" value used in described in PKCS-1 block type 02 [PKCS1] to form the "m" value used in
the formulas above. the formulas above.
An implementation MAY use a Key ID of zero as a "wild card" or
"speculative" Key ID. In this case, the implementation would try all
available private keys, checking for a valid decrypted session key.
This format helps reduce traffic analysis of messages.
5.2 Signature Packet (Tag 2) 5.2 Signature Packet (Tag 2)
A signature packet describes a binding between some public key and some A signature packet describes a binding between some public key and some
data. The most common signatures are a signature of a file or a block data. The most common signatures are a signature of a file or a block
of text, and a signature that is a certification of a user ID. of text, and a signature that is a certification of a user ID.
Two versions of signature packets are defined. Version 3 provides Two versions of signature packets are defined. Version 3 provides
basic signature information, while version 4 provides an expandable basic signature information, while version 4 provides an expandable
format with subpackets that can specify more information about the format with subpackets that can specify more information about the
signature. PGP 2.6.X only accepts version 3 signatures. signature. PGP 2.6.X only accepts version 3 signatures.
Implementations MUST accept V3 signatures. Implementations SHOULD Implementations MUST accept V3 signatures. Implementations SHOULD
generate V4 signatures, unless there is a need to generate a signature generate V4 signatures, unless there is a need to generate a signature
that can be verified by PGP 2.6.x. that can be verified by old implementations.
Note that if an implementation is creating an encrypted and signed
message that is encrypted to a V3 key, it is reasonable to create a V3
signature.
5.2.1 Version 3 Signature Packet Format 5.2.1 Version 3 Signature Packet Format
A version 3 Signature packet contains: A version 3 Signature packet contains:
- One-octet version number (3). - One-octet version number (3).
- One-octet length of following hashed material. MUST be 5. - One-octet length of following hashed material. MUST be 5.
- One-octet signature type. - One-octet signature type.
- Four-octet creation time. - Four-octet creation time.
- Eight-octet key ID of signer. - Eight-octet key ID of signer.
- One-octet public key algorithm. - One-octet public key algorithm.
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signature than for RSA signatures. signature than for RSA signatures.
With RSA signatures, the hash value is encoded as described in PKCS-1 With RSA signatures, the hash value is encoded as described in PKCS-1
section 10.1.2, "Data encoding", producing an ASN.1 value of type section 10.1.2, "Data encoding", producing an ASN.1 value of type
DigestInfo, and then padded using PKCS-1 block type 01 [PKCS1]. This DigestInfo, and then padded using PKCS-1 block type 01 [PKCS1]. This
requires inserting the hash value as an octet string into an ASN.1 requires inserting the hash value as an octet string into an ASN.1
structure. The object identifier for the type of hash being used is structure. The object identifier for the type of hash being used is
included in the structure. The hexadecimal representations for the included in the structure. The hexadecimal representations for the
currently defined hash algorithms are: currently defined hash algorithms are:
- SHA-1: 0x2b, 0x0e, 0x03, 0x02, 0x1a
- MD5: 0x2a, 0x86, 0x48, 0x86, 0xf7, 0x0d, 0x02, 0x05 - MD5: 0x2a, 0x86, 0x48, 0x86, 0xf7, 0x0d, 0x02, 0x05
- SHA-1: 0x2b, 0x0e, 0x03, 0x02, 0x1a
- RIPEMD-160: 0x2b, 0x24, 0x03, 0x02, 0x01 - RIPEMD-160: 0x2b, 0x24, 0x03, 0x02, 0x01
The ASN.1 OIDs are: The ASN.1 OIDs are:
- MD5: 1.2.840.113549.2.5 - MD5: 1.2.840.113549.2.5
- SHA-1: 1.3.14.3.2.26 - SHA-1: 1.3.14.3.2.26
- RIPEMD160: 1.3.36.3.2.1 - RIPEMD160: 1.3.36.3.2.1
DSA signatures SHOULD use hashes with a size of 160 bits, to match q, DSA signatures SHOULD use hashes with a size of 160 bits, to match q,
the size of the group generated by the DSA key's generator value. The the size of the group generated by the DSA key's generator value. The
hash function result is treated as a 160 bit number and used directly hash function result is treated as a 160 bit number and used directly
in the DSA signature algorithm. in the DSA signature algorithm.
5.2.2 Version 4 Signature Packet Format 5.2.2 Version 4 Signature Packet Format
A version 4 Signature packet contains: A version 4 Signature packet contains:
- One-octet version number (4). - One-octet version number (4).
- One-octet signature type. - One-octet signature type.
- One-octet public key algorithm. - One-octet public key algorithm.
- One-octet hash algorithm. - One-octet hash algorithm.
- Two-octet octet count for following hashed subpacket data. - Two-octet octet count for following hashed subpacket data.
- Hashed subpacket data. - Hashed subpacket data. (zero or more subpackets)
- Two-octet octet count for following unhashed subpacket data. - Two-octet octet count for following unhashed subpacket data.
- Unhashed subpacket data. - Unhashed subpacket data. (zero or more subpackets)
- Two-octet field holding left 16 bits of signed hash value. - Two-octet field holding left 16 bits of signed hash value.
- One or more multi-precision integers comprising the signature. - One or more multi-precision integers comprising the signature.
This portion is algorithm specific, as described above. This portion is algorithm specific, as described above.
The data being signed is hashed, and then the signature data from the The data being signed is hashed, and then the signature data from the
version number through the hashed subpacket data is hashed. The version number through the hashed subpacket data is hashed. The
resulting hash value is what is signed. The left 16 bits of the hash resulting hash value is what is signed. The left 16 bits of the hash
are included in the signature packet to provide a quick test to reject are included in the signature packet to provide a quick test to reject
some invalid signatures. some invalid signatures.
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of the set of subpackets. of the set of subpackets.
Each subpacket consists of a subpacket header and a body. The header Each subpacket consists of a subpacket header and a body. The header
consists of: consists of:
- subpacket length (1 or 2 octets): - subpacket length (1 or 2 octets):
Length includes the type octet but not this length, Length includes the type octet but not this length,
1st octet < 192, then length is octet value 1st octet < 192, then length is octet value
1st octet >= 192, then length is 2 octets and equal to 1st octet >= 192, then length is 2 octets and equal to
(1st octet - 192) * 256 + (2nd octet) + 192 (1st octet - 192) * 256 + (2nd octet) + 192
- subpacket type (1 octet): - subpacket type (1 octet):
If bit 7 is set, subpacket understanding is critical, If bit 7 is set, subpacket understanding is critical,
2 = signature creation time, 2 = signature creation time,
3 = signature expiration time, 3 = signature expiration time,
4 = exportable, 4 = exportable,
5 = trust signature, 5 = trust signature,
6 = regular expression, 6 = regular expression,
7 = revocable, 7 = revocable,
9 = key expiration time, 9 = key expiration time,
10 = additional recipient request, 10 = placeholder for backwards compatibility
11 = preferred symmetric algorithms, 11 = preferred symmetric algorithms,
12 = revocation key, 12 = revocation key,
16 = issuer key ID, 16 = issuer key ID,
20 = notation data, 20 = notation data,
21 = preferred hash algorithms, 21 = preferred hash algorithms,
22 = preferred compression algorithms, 22 = preferred compression algorithms,
23 = key server preferences, 23 = key server preferences,
24 = preferred key server 24 = preferred key server,
25 = primary user id,
26 = policy URL,
27 = key flags, 28 = Signer's user id
- subpacket specific data: - subpacket specific data:
Bit 7 of the subpacket type is the "critical" bit. If set, it implies An implementation SHOULD ignore any subpacket that it does not
that it is critical that the subpacket be one which is understood by recognize.
the software. If a subpacket is encountered which is marked critical
but the software does not understand, the handling depends on the Bit 7 of the subpacket type is the "critical" bit. If set, it denotes
relationship between the issuing key and the key that is signed. If that the subpacket is one which is critical that the evaluator of the
the signature is a valid self-signature (for which the issuer is the signature recognize. If a subpacket is encountered which is marked
key that is being signed, either directly or via a username binding), critical but is unknown to the evaluating software, the evaluator
then the key should not be used. In other cases, the signature SHOULD consider the signature to be in error.
containing the critical subpacket should be ignored.
An evaluator may "recognize" a subpacket, but not implement it. The
purpose of the critical bit is to allow the signer to tell an evaluator
that it would prefer a new, unknown feature to generate an error than
be ignored.
5.2.2.2 Signature Subpacket Types 5.2.2.2 Signature Subpacket Types
Several types of subpackets are currently defined. Some subpackets Several types of subpackets are currently defined. Some subpackets
apply to the signature itself and some are attributes of the key. apply to the signature itself and some are attributes of the key.
Subpackets that are found on a self-signature are placed on a user name Subpackets that are found on a self-signature are placed on a user name
certification made by the key itself. Note that a key may have more certification made by the key itself. Note that a key may have more
than one user name, and thus may have more than one self-signature, and than one user name, and thus may have more than one self-signature, and
differing subpackets. differing subpackets.
A self-signature is a binding signature made by the key the signature
refers to. There are three types of self-signatures, the certification
signatures (types 0x10-0x13), the direct-key signature (type 0x1f), and
the subkey binding signature (type 0x18). For certification
self-signatures, username may have a self-signature, and thus different
subpackets in those self-signatures. For subkey binding signatures,
each subkey in fact has a self-signature. Subpackets that appear in a
certification self-signature apply to the username, and subpackets that
appear in the subkey self-signature apply to the subkey. Lastly,
subpackets on the direct key signature apply to the entire key.
Implementing software should interpret a self-signature's preference Implementing software should interpret a self-signature's preference
subpackets as narrowly as possible. For example, suppose a key has two subpackets as narrowly as possible. For example, suppose a key has two
usernames, Alice and Bob. Suppose that Alice prefers the symmetric usernames, Alice and Bob. Suppose that Alice prefers the symmetric
algorithm CAST5, and Bob prefers IDEA or Triple-DES. If the software algorithm CAST5, and Bob prefers IDEA or Triple-DES. If the software
locates this key via Alice's name, then the preferred algorithm is locates this key via Alice's name, then the preferred algorithm is
CAST5, if software locates the key via Bob's name, then the preferred CAST5, if software locates the key via Bob's name, then the preferred
algorithm is IDEA. If the key is located by key id, then algorithm of algorithm is IDEA. If the key is located by key id, then algorithm of
the default user name of the key provides the default symmetric the default user name of the key provides the default symmetric
algorithm. algorithm.
The descriptions below describe whether a subpacket is typically found A subpacket may be found either in the hashed or unhashed subpacket
in the hashed or unhashed subpacket sections. If a subpacket is not sections of a signature. If a subpacket is not hashed, then the
hashed, then it cannot be trusted. information in it cannot be considered definitive because it is not
part of the signature proper.
Signature creation time (4 octet time field) (Hashed) Subpacket types:
The time the signature was made. Always included with new signatures. Signature creation time (4 octet time field)
Issuer (8 octet key ID) (Non-hashed) The time the signature was made. Always included with new
signatures.
The OP key ID of the key issuing the signature. Issuer (8 octet key ID)
Key expiration time (4 octet time field) (Hashed) The OP key ID of the key issuing the signature.
The validity period of the key. This is the number of seconds after Key expiration time (4 octet time field)
the key creation time that the key expires. If this is not present or
has a value of zero, the key never expires. This is found only on a
self-signature.
Preferred symmetric algorithms (array of one-octet values) (Hashed) The validity period of the key. This is the number of seconds
after the key creation time that the key expires. If this is
not present or has a value of zero, the key never expires. This
is found only on a self-signature.
Symmetric algorithm numbers that indicate which algorithms the key Preferred symmetric algorithms (array of one-octet values)
holder prefers to use. This is an ordered list of octets with the most
preferred listed first. It should be assumed that only algorithms
listed are supported by the recipient's software. Algorithm numbers in
section 6. This is only found on a self-signature.
Preferred hash algorithms (array of one-octet values) (Hashed) Symmetric algorithm numbers that indicate which algorithms the
key holder prefers to use. This is an ordered list of octets
with the most preferred listed first. It should be assumed
that only algorithms listed are supported by the recipient's
software. Algorithm numbers in section 6. This is only found
on a self-signature.
Message digest algorithm numbers that indicate which algorithms the key Preferred hash algorithms (array of one-octet values)
holder prefers to receive. Like the preferred symmetric algorithms,
the list is ordered. Algorithm numbers are in section 6. This is only
found on a self-signature.
{{Editor's note: The above preference (hash algs) is controversial. I Message digest algorithm numbers that indicate which algorithms
included it in for symmetry, because if someone wants to build a the key holder prefers to receive. Like the preferred
minimal OP implementation, there needs to be a way to tell someone that symmetric algorithms, the list is ordered. Algorithm numbers
you won't be able to verify a signature unless it's made with some set are in section 6. This is only found on a self-signature.
of algorithms. It also permits to prefer DSA with RIPEMD-160, for
example. If you have an opinion, please state it.}}
Preferred compression algorithms (array of one-octet values) Preferred compression algorithms (array of one-octet values)
(Hashed)
Compression algorithm numbers that indicate which algorithms the key
holder prefers to use. Like the preferred symmetric algorithms, the
list is ordered. Algorithm numbers are in section 6. If this
subpacket is not included, ZIP is preferred. A zero denotes that no
compression is preferred; the key holder's software may not have
compression software. This is only found on a self-signature.
Signature expiration time (4 octet time field) (Hashed) Compression algorithm numbers that indicate which algorithms
the key holder prefers to use. Like the preferred symmetric
algorithms, the list is ordered. Algorithm numbers are in
section 6. If this subpacket is not included, ZIP is
preferred. A zero denotes that uncompressed data is preferred;
the key holder's software may not have compression software.
This is only found on a self-signature.
The validity period of the signature. This is the number of seconds Signature expiration time (4 octet time field)
after the signature creation time that the signature expires. If this The validity period of the signature. This is the number of
is not present or has a value of zero, it never expires. seconds after the signature creation time that the signature
expires. If this is not present or has a value of zero, it
never expires.
Exportable (1 octet of exportability, 0 for not, 1 for exportable) Exportable (1 octet of exportability, 0 for not, 1 for exportable)
(Hashed)
Signature's exportability status. Packet body contains a boolean flag Signature's exportability status. Packet body contains a
indicating whether the signature is exportable. Signatures which are boolean flag indicating whether the signature is exportable.
not exportable are ignored during export and import operations. If Signatures which are not exportable are ignored during export
this packet is not present the signature is assumed to be exportable. and import operations. If this packet is not present the
signature is assumed to be exportable.
Revocable (1 octet of revocability, 0 for not, 1 for revocable) Revocable (1 octet of revocability, 0 for not, 1 for revocable)
(Hashed)
Signature's revocability status. Packet body contains a boolean flag
indicating whether the signature is revocable. Signatures which are
not revocable get any later revocation signatures ignored. They
represent a commitment by the signer that he cannot revoke his
signature for the life of his key. If this packet is not present the
signature is assumed to be revocable.
Trust signature (1 octet of "level" (depth), 1 octet of trust amount) Signature's revocability status. Packet body contains a
(Hashed) boolean flag indicating whether the signature is revocable.
Signer asserts that the key is not only valid, but also trustworthy, at Signatures which are not revocable have any later revocation
the specified level. Level 0 has the same meaning as an ordinary signatures ignored. They represent a commitment by the signer
validity signature. Level 1 means that the signed key is asserted to that he cannot revoke his signature for the life of his key.
be a valid trusted introducer, with the 2nd octet of the body If this packet is not present, the signature is revocable.
specifying the degree of trust. Level 2 means that the signed key is
asserted to be trusted to issue level 1 trust signatures, i.e. that it
is a "meta introducer". Generally, a level n trust signature asserts
that a key is trusted to issue level n-1 trust signatures. The trust
amount is in a range from 0-255, interpreted such that values less than
120 indicate partial trust and values of 120 or greater indicate
complete trust. Implementations SHOULD emit values of 60 for partial
trust and 120 for complete trust.
Regular expression (null-terminated regular expression) (Hashed) Trust signature (1 octet "level" (depth), 1 octet of trust amount)
Used in conjunction with trust signature packets (of level > 0) to Signer asserts that the key is not only valid, but also
limit the scope of trust which is extended. Only signatures by the trustworthy, at the specified level. Level 0 has the same
target key on user IDs which match the regular expression in the body meaning as an ordinary validity signature. Level 1 means that
of this packet have trust extended by the trust packet. the signed key is asserted to be a valid trusted introducer,
with the 2nd octet of the body specifying the degree of trust.
Level 2 means that the signed key is asserted to be trusted to
issue level 1 trust signatures, i.e. that it is a "meta
introducer". Generally, a level n trust signature asserts that
a key is trusted to issue level n-1 trust signatures. The
trust amount is in a range from 0-255, interpreted such that
values less than 120 indicate partial trust and values of 120
or greater indicate complete trust. Implementations SHOULD
emit values of 60 for partial trust and 120 for complete trust.
Additional recipient request (1 octet of class, 1 octet of algid, Regular expression (null-terminated regular expression)
20 octets of fingerprint) (Hashed)
Key holder requests encryption to additional recipient when data is Used in conjunction with trust signature packets (of level > 0)
encrypted to this username. If the class octet contains 0x80, then the to limit the scope of trust which is extended. Only signatures
key holder strongly requests that the additional recipient be added to by the target key on user IDs which match the regular
an encryption. Implementing software may treat this subpacket in any expression in the body of this packet have trust extended by
way it sees fit. This is found only on a self-signature. the trust packet. The regular expression uses the same syntax
as the Henry Spencer's "almost public domain" regular
expression package. A description of the syntax in in a
section below.
Revocation key (1 octet of class, 1 octet of algid, 20 octets of Revocation key (1 octet of class, 1 octet of algid, 20 octets of
fingerprint) (Hashed) fingerprint)
Authorizes the specified key to issue revocation self-signatures on Authorizes the specified key to issue revocation
this key. Class octet must have bit 0x80 set, other bits are for self-signatures for this key. Class octet must have bit 0x80
set, other bits are for future expansion to other kinds of
signature authorizations. This is found on a self-signature.
future expansion to other kinds of signature authorizations. This is Authorizes the specified key to issue revocation signatures for
found on a self-signature. this key. Class octet must have bit 0x80 set. If the bit 0x40
is set, then this means that the revocation information is
sensitive. Other bits are for future expansion to other kinds
of authorizations. This is found on a self-signature.
If the "sensitive" flag is set, the keyholder feels this
subpacket contains private trust information that describes a
real-world sensitive relationship. If this flag is set,
implementations SHOULD NOT export this signature to other users
except in cases where the data needs to be available: when the
signature is being sent to the designated revoker, or when it
is accompanied by a revocation signature from that revoker.
Note that it may be appropriate to isolate this subpacket
within a separate signature so that it is not combined with
other subpackets which need to be exported.
Notation Data (4 octets of flags, 2 octets of name length, Notation Data (4 octets of flags, 2 octets of name length,
2 octets of value length, M octets of name data, 2 octets of value length, M octets of name data,
N octets of value data) (Hashed) N octets of value data)
This subpacket describes a "notation" on the signature that the issuer This subpacket describes a "notation" on the signature that the
wishes to make. The notation has a name and a value, each of which are issuer wishes to make. The notation has a name and a value,
strings of octets. There may be more than one notation in a signature. each of which are strings of octets. There may be more than
Notations can be used for any extension the issuer of the signature one notation in a signature. Notations can be used for any
cares to make. The "flags" field holds four octets of flags. All extension the issuer of the signature cares to make. The
undefined flags MUST be zero. Defined flags are: "flags" field holds four octets of flags.
All undefined flags MUST be zero. Defined flags are:
First octet: 0x80 = human-readable. This note is text, a note First octet: 0x80 = human-readable. This note is text, a note
from one person to another, and has no from one person to another, and has no
meaning to software. meaning to software.
Other octets: none. Other octets: none.
Key server preferences (N octets of flags) (Hashed) Key server preferences (N octets of flags)
This is a list of flags that indicate preferences that the key holder This is a list of flags that indicate preferences that the key
has about how the key is handled on a key server. All undefined flags holder has about how the key is handled on a key server. All
MUST be zero. undefined flags MUST be zero.
First octet: 0x80 = No-modify -- the key holder requests that First octet: 0x80 = No-modify -- the key holder requests that
this key only be modified or updated by the this key only be modified or updated by the
key holder or an authorized administrator of key holder or an authorized administrator of
the key server. the key server.
This is found only on a self-signature. This is found only on a self-signature.
Preferred key server (String) (Hashed) Preferred key server (String)
This is a URL of a key server that the key holder prefers be used for This is a URL of a key server that the key holder prefers be
updates. Note that keys with multiple user names can have a preferred used for updates. Note that keys with multiple user names can
key server for each user name. This is found only on a self-signature. have a preferred key server for each user name. Note also that
since this is a URL, the key server can actually be a copy of
the key retrieved by ftp, http, finger, etc.
Implementations SHOULD implement a "preference" and MAY implement a Primary user id (1 octet, boolean)
"request."
{{Editor's note: None of the preferences have a way to specify a This is a flag in a user id's self signature that states
negative preference (for example, I like Triple-DES, don't use whether this user id is the main user id for this key. It is
algorithm X). Tacitly, the absence of an algorithm from a set is a reasonable for an implementation to resolve ambiguities in
negative preference, but should there be an explicit way to give a preferences, etc. by referring to the primary user id. If this
negative preference? -jdcc}} flag is absent, its value is zero. If more than one user id in
a key is marked as primary, the implementation may resolve the
ambiguity in any way it sees fit.
{{Editor's note: A missing feature is to invalidate (or revoke) a user Policy URL (String)
id, rather than the entire key. Lots of people want this, and many
people have keys cluttered with old work email addresses. There is
another related issue, that that is with key rollover -- suppose I'm
retiring an old key, but I don't want to have to lose all my
certification signatures. It would be nice if there were a way for a
key to transfer itself to a new one. Lastly, if either (or both) of
these is desirable, do we handle them with a new signature type, or
with notations, which are an extension mechanism. I think that it This subpacket contains a URL of a document that describes the
makes sense to make a revocation type (because it's analogous to the policy under which the signature was issued.
other forms of revocation), but rollover might be best implemented as
an extension. --jdcc}}
{{Editor's note: PGP 3 designed, but never implemented a number of Key Flags (Octet string)
other subpacket types. They were: A signature version number; A set
of key usage flags (signing key, encryption key for communication, and
encryption key for storage); User ID of the signer; Policy URL; net
location of the key.
Some of these options are things the WG has talked about as being a This subpacket contains a list of binary flags that hold
Good Thing -- like flags denoting if a key is a comm key or a storage information about a key. It is a string of octets, and an
key. My design of such a feature would be different than the other implementation MUST NOT assume a fixed size. This is so it can
one, though. I think it would be a great idea to have a URL that's a grow over time. If a list is shorter than an implementation
location to find the key, so people who prefer to have a web, ftp, or expects, the unstated flags are considered to be zero. The
finger location can use those. However, some of them (like a URL) are defined flags are:
also perfect for designing in with extensions. After all, we only have
128 subpacket constants.
First octet:
0x01 - This key may be used to certify other keys.
0x02 - This key may be used to sign data.
0x04 - This key may be used to encrypt communications.
0x08 - This key may be used to encrypt storage.
0x10 - The private component of this key may have been split by
a secret-sharing mechanism.
0x80 - The private component of this key may be in the posession
of more than one person.
Usage notes:
The flags in this packet may appear in self-signatures or in
certification signatures. They mean different things depending
on who is making the statement -- for example, a certification
signature that has the "sign data" flag is stating that the
certification is for that use. On the other hand, the
"communications encryption" flag in a self-signature is stating
a preference that a given key be used for communications. Note
however, that it is a thorny issue to determine what is
"communications" and what is "storage." This decision is left
wholly up to the implementation; the authors of this document
do not claim any special wisdom on the issue, and realize that
accepted opinion may change.
The "split key" (0x10) and "group key" (0x80) flags are placed
on a self-signature only; they are meaningless on a
certification signature. They SHOULD be placed only on a
direct-key signature (type 0x1f) or a subkey signature (type
0x18), one that refers to the key the flag applies to.
Signer's User ID
This subpacket allows a keyholder to state which user id is
responsible for the signing. Many keyholders use a single key
for different purposes, such as business communications as well
as personal communications. This subpacket allows such a
keyholder to state which of their roles is making a signature.
Implementations SHOULD implement "preferences".
5.2.3 Signature Types 5.2.3 Signature Types
There are a number of possible meanings for a signature, which are There are a number of possible meanings for a signature, which are
specified in a signature type octet in any given signature. These specified in a signature type octet in any given signature. These
meanings are: meanings are:
- 0x00: Signature of a binary document. - 0x00: Signature of a binary document.
Typically, this means the signer owns it, created it, or certifies that Typically, this means the signer owns it, created it, or certifies that
it has not been modified. it has not been modified.
- 0x01: Signature of a canonical text document. - 0x01: Signature of a canonical text document.
Typically, this means the signer owns it, created it, or certifies that Typically, this means the signer owns it, created it, or certifies that
it has not been modified. The signature will be calculated over the it has not been modified. The signature will be calculated over the
textual data with its line endings converted to <CR><LF>. text data with its line endings converted to <CR><LF>.
- 0x02: Standalone signature. - 0x02: Standalone signature.
This signature is a signature of only its own subpacket contents. It This signature is a signature of only its own subpacket contents. It
is calculated identically to a signature over a zero-length binary is calculated identically to a signature over a zero-length binary
document. document. Note that it doesn't make sense to have a V3 standalone
signature.
- 0x10: The certification of a User ID and Public Key packet.
- 0x10: The generic certification of a User ID and Public Key
packet.
The issuer of this certification does not make any particular assertion The issuer of this certification does not make any particular assertion
as to how well the certifier has checked that the owner of the key is as to how well the certifier has checked that the owner of the key is
in fact the person described by the user ID. Note that all PGP "key in fact the person described by the user ID. Note that all PGP "key
signatures" are this type of certification. signatures" are this type of certification.
- 0x11: This is a persona certification of a User ID and - 0x11: This is a persona certification of a User ID and
Public Key packet. Public Key packet.
It means that the issuer of this certification has not done any
verification of the claim that the owner of this key is the user ID
specified. Note that no released version of PGP has generated this The issuer of this certification has not done any verification of the
type of certification. claim that the owner of this key is the user ID specified.
- 0x12: This is the casual certification of a User ID and - 0x12: This is the casual certification of a User ID and
Public Key packet. Public Key packet.
It means that the issuer of this certification has done some casual
verification of the claim of identity. Note that no version of PGP has The issuer of this certification has done some casual verification of
generated this type of certification, nor is there any definition of the claim of identity.
what constitutes a casual certification.
- 0x13: This is the positive certification of a User ID and - 0x13: This is the positive certification of a User ID and
Public Key packet. Public Key packet.
It means that the issuer of this certification has done substantial
verification of the claim of identity. Note that no version of PGP has
generated this type of certification, nor is there any definition of
what constitutes a positive certification. Please also note that the
vagueness of these certification systems is not a flaw, but a feature
of the system. Because PGP places final authority for validity upon
the receiver of a certification, it may be that one authority's casual
certification might be more rigorous than some other authority's
positive certification.
{{Editor's note: While there is a scale of identification signatures The issuer of this certification has done substantial verification of
in the range 0x10 to 0x13, most of them have never been implemented or the claim of identity.
used. Current implementations only use 0x10, the "generic
certification." Should the others be removed? RFC 1991 went to some Please note that the vagueness of these certification claims is not a
trouble to explain which ones were defined but not implemented, or read flaw, but a feature of the system. Because PGP places final authority
but not generated. I think we should not do that. If we define them, for validity upon the receiver of a certification, it may be that one
they should be MAY features at the very least. If we're not going to authority's casual certification might be more rigorous than some other
use them, they shouldn't be in the spec. --jdcc}} authority's positive certification. These classifications allow a
certification authority to issue fine-grained claims.
- 0x18: This is used for a signature by a signature key to bind a - 0x18: This is used for a signature by a signature key to bind a
subkey which will be used for encryption. subkey which will be used for encryption.
The signature is calculated directly on the subkey itself, not on any The signature is calculated directly on the subkey itself, not on any
User ID or other packets. User ID or other packets.
- 0x1f: Signature directly on a key
This signature is calculated directly on a key. It binds the
information in the signature subpackets to the key, and is appropriate
to be used for subpackets which provide information about the key, such
as the revocation key subpacket. It is also appropriate for statements
that non-self certifiers want to make about the key itself, rather than
the binding between a key and a name.
- 0x20: This signature is used to revoke a key. - 0x20: This signature is used to revoke a key.
The signature is calculated directly on the key being revoked. A The signature is calculated directly on the key being revoked. A
revoked key is not to be used. Only revocation signatures by the key revoked key is not to be used. Only revocation signatures by the key
being revoked, or by an authorized revocation key, should be being revoked, or by an authorized revocation key, should be
considered. considered.
- 0x28: This is used to revoke a subkey. - 0x28: This is used to revoke a subkey.
The signature is calculated directly on the subkey being revoked. A The signature is calculated directly on the subkey being revoked. A
revoked subkey is not to be used. Only revocation signatures by the revoked subkey is not to be used. Only revocation signatures by the
top-level signature key which is bound to this subkey, or by an top-level signature key which is bound to this subkey, or by an
authorized revocation key, should be considered. authorized revocation key, should be considered.
- 0x30: This signature revokes an earlier user ID certification - 0x30: This signature revokes an earlier user ID certification
signature (signature class 0x10 - 0x13). signature (signature class 0x10 through 0x13).
It should be issued by the same key which issued the revoked signature, It should be issued by the same key which issued the revoked signature,
and should have a later creation date. and should have a later creation date than the signature it revokes.
- 0x40: Timestamp signature. - 0x40: Timestamp signature.
{{Editor's note: The timestamp signature is left over from RFC 1991, This signature is only meaningful for the timestamp contained in it.
and has never been fully designed nor implemented. Is this the sort of
thing best handled by notations? --jdcc}}
{{Editor's note: It would be nice to have a signature that applied to 5.2.4 Computing Signatures
the key alone, rather than a key plus a user name. Perhaps this is
best done with a notation. --jdcc}}
{{Editor's note: There is presently no way for a key-signer (a.k.a. All signatures are formed by producing a hash over the signature data,
certifier) to sign a main key along with a subkey. There are a number and then using the resulting hash in the signature algorithm.
of useful situations for a set of keys (main plus subkeys) to all be
signed together. How do we solve this? --jdcc}}
5.3 Conventional Encrypted Session-Key Packets (Tag 3) The signature data is simple to compute for document signatures (types
0x00 and 0x01), for which the document itself is the data. For
standalone signatures, this is a null string.
The Conventional Encrypted Session Key packet holds the When a signature is made over a key, the hash data starts with the
octet 0x99, followed by a two-octet length of the key, and then body of
the key packet. (Note that this is an old-style packet header for a key
packet with two-octet length.) A subkey signature (type 0x18) then
hashes the subkey, using the same format as the main key. Key
revocation signatures (types 0x20 and 0x28) hash only the key being
revoked.
A certification signature (type 0x10 through 0x13) then hashes the user
name being bound to the key. A V3 certification hashes the contents of
the name packet, without any header. A V4 certification hashes the
constant 0xd4 (which is an old-style CTB with the length-of-length set
to zero), a four-octet number giving the length of the username, and
then the username data.
Once the data body is hashed, then a trailer is hashed. A V3 signature
hashes five octets of the packet body, starting from the signature type
field. This data is the signature type, followed by the four-octet
signature time. A V4 signature hashes the packet body starting from
its first field, the version number, through the end of the hashed
subpacket data. Thus, the fields hashed are the signature version, the
signature type, the public key algorithm, the hash algorithm, the
hashed subpacket length, and the hashed subpacket body.
After all this has been hashed, the resulting hash field is used in the
signature algorithm, and placed at the end of the signature packet.
5.3 Symmetric-Key Encrypted Session-Key Packets (Tag 3)
The Symmetric-Key Encrypted Session Key packet holds the
conventional-cipher encryption of a session key used to encrypt a conventional-cipher encryption of a session key used to encrypt a
message. Zero or more Encrypted Session Key packets and/or message. Zero or more Encrypted Session Key packets and/or
Conventional Encrypted Session Key packets may precede a Symmetrically Conventional Encrypted Session Key packets may precede a Symmetrically
Encrypted Data Packet that holds an encrypted message. The message is Encrypted Data Packet that holds an encrypted message. The message is
encrypted with a session key, and the session key is itself encrypted encrypted with a session key, and the session key is itself encrypted
and stored in the Encrypted Session Key packet or the Conventional and stored in the Encrypted Session Key packet or the Conventional
Encrypted Session Key packet. Encrypted Session Key packet.
If the Symmetrically Encrypted Data Packet is preceded by one or more If the Symmetrically Encrypted Data Packet is preceded by one or more
Conventional Encrypted Session Key packets, each specifies a passphrase Symmetric-Key Encrypted Session Key packets, each specifies a
which may be used to decrypt the message. This allows a message to be passphrase which may be used to decrypt the message. This allows a
encrypted to a number of public keys, and also to one or more pass message to be encrypted to a number of public keys, and also to one or
phrases. This packet type is new, and is not generated by PGP 2.x or more pass phrases. This packet type is new, and is not generated by
PGP 5.0. PGP 2.x or PGP 5.0.
The body of this packet consists of: The body of this packet consists of:
- A one-octet version number. The only currently defined version is - A one-octet version number. The only currently defined version is
4. 4.
- A one-octet number describing the symmetric algorithm used. - A one-octet number describing the symmetric algorithm used.
- A string-to-key (S2K) specifier, length as defined above. - A string-to-key (S2K) specifier, length as defined above.
- Optionally, the encrypted session key itself, which is decrypted - Optionally, the encrypted session key itself, which is decrypted
with the string-to-key object. with the string-to-key object.
If the encrypted session key is not present (which can be detected on If the encrypted session key is not present (which can be detected on
the basis of packet length and S2K specifier size), then the S2K the basis of packet length and S2K specifier size), then the S2K
algorithm applied to the passphrase produces the session key for algorithm applied to the passphrase produces the session key for
decrypting the file, using the symmetric cipher algorithm from the decrypting the file, using the symmetric cipher algorithm from the
Conventional Encrypted Session Key packet. Symmetric-Key Encrypted Session Key packet.
If the encrypted session key is present, the result of applying the S2K If the encrypted session key is present, the result of applying the S2K
algorithm to the passphrase is used to decrypt just that encrypted algorithm to the passphrase is used to decrypt just that encrypted
session key field, using CFB mode with an IV of all zeros. The session key field, using CFB mode with an IV of all zeros. The
decryption result consists of a one-octet algorithm identifier that decryption result consists of a one-octet algorithm identifier that
specifies the conventional encryption algorithm used to encrypt the specifies the conventional encryption algorithm used to encrypt the
following Symmetrically Encrypted Data Packet, followed by the session following Symmetrically Encrypted Data Packet, followed by the session
key octets themselves. key octets themselves.
Note: because an all-zero IV is used for this decryption, the S2K Note: because an all-zero IV is used for this decryption, the S2K
specifier MUST use a salt value, either a a Salted S2K or an specifier MUST use a salt value, either a a Salted S2K or an
Iterated-Salted S2K. The salt value will insure that the decryption Iterated-Salted S2K. The salt value will insure that the decryption
key is not repeated even if the passphrase is reused. key is not repeated even if the passphrase is reused.
5.4 One-Pass Signature Packets (Tag 4) 5.4 One-Pass Signature Packets (Tag 4)
The One-Pass Signature packet precedes the signed data and contains The One-Pass Signature packet precedes the signed data and contains
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two-octet length) of the MPIs that form the key material (public two-octet length) of the MPIs that form the key material (public
modulus n, followed by exponent e) with MD5. modulus n, followed by exponent e) with MD5.
The eight-octet key ID of the key consists of the low 64 bits of the The eight-octet key ID of the key consists of the low 64 bits of the
public modulus of an RSA key. public modulus of an RSA key.
Since the release of V3 keys, there have been a number of improvements Since the release of V3 keys, there have been a number of improvements
desired in the key format. For example, if the key ID is a function of desired in the key format. For example, if the key ID is a function of
the public modulus, it is easy for a person to create a key that has the public modulus, it is easy for a person to create a key that has
the same key ID as some existing key. Similarly, MD5 is no longer the the same key ID as some existing key. Similarly, MD5 is no longer the
preferred hash algorithm, and not hashing the length of an MPI with its preferred hash algorithm, and not hashing the length of an MPI with its
body increases the chances of a fingerprint collision. body increases the chances of a fingerprint collision.
The version 4 format is similar to the version 3 format except for the The version 4 format is similar to the version 3 format except for the
absence of a validity period. This has been moved to the signature absence of a validity period. This has been moved to the signature
packet. In addition, fingerprints of version 4 keys are calculated packet. In addition, fingerprints of version 4 keys are calculated
differently from version 3 keys, as described elsewhere. differently from version 3 keys, as described in section "Enhanced Key
Formats."
A version 4 packet contains: A version 4 packet contains:
- A one-octet version number (4). - A one-octet version number (4).
- A four-octet number denoting the time that the key was created. - A four-octet number denoting the time that the key was created.
- A one-octet number denoting the public key algorithm of this key - A one-octet number denoting the public key algorithm of this key
- A series of multi-precision integers comprising the key - A series of multi-precision integers comprising the key
material. This algorithm-specific portion is: material. This algorithm-specific portion is:
Algorithm Specific Fields for RSA public keys: Algorithm Specific Fields for RSA public keys:
- multiprecision integer (MPI) of RSA public modulus n; - multiprecision integer (MPI) of RSA public modulus n;
skipping to change at page 31, line 4 skipping to change at page 29, line 6
The Compressed Data packet contains compressed data. Typically, this The Compressed Data packet contains compressed data. Typically, this
packet is found as the contents of an encrypted packet, or following a packet is found as the contents of an encrypted packet, or following a
Signature or One-Pass Signature packet, and contains literal data Signature or One-Pass Signature packet, and contains literal data
packets. packets.
The body of this packet consists of: The body of this packet consists of:
- One octet that gives the algorithm used to compress the packet. - One octet that gives the algorithm used to compress the packet.
- The remainder of the packet is compressed data. - The remainder of the packet is compressed data.
A Compressed Data Packet's body contains an RFC1951 DEFLATE block that A Compressed Data Packet's body contains an RFC1951 DEFLATE block that
compresses some set of packets. See section "Packet Composition" for
compresses some set of packets. See section 7 for details on how details on how messages are formed.
messages are formed.
5.7 Symmetrically Encrypted Data Packet (Tag 9) 5.7 Symmetrically Encrypted Data Packet (Tag 9)
The Symmetrically Encrypted Data packet contains data encrypted with a The Symmetrically Encrypted Data packet contains data encrypted with a
conventional (symmetric-key) algorithm. When it has been decrypted, it conventional (symmetric-key) algorithm. When it has been decrypted, it
will typically contain other packets (often literal data packets or will typically contain other packets (often literal data packets or
compressed data packets). compressed data packets).
The body of this packet consists of: The body of this packet consists of:
skipping to change at page 31, line 29 skipping to change at page 29, line 30
The conventional cipher used may be specified in an Encrypted Session The conventional cipher used may be specified in an Encrypted Session
Key or Conventional Encrypted Session Key packet which precedes the Key or Conventional Encrypted Session Key packet which precedes the
Symmetrically Encrypted Data Packet. In that case, the cipher Symmetrically Encrypted Data Packet. In that case, the cipher
algorithm octet is prepended to the session key before it is encrypted. algorithm octet is prepended to the session key before it is encrypted.
If no packets of these types precede the encrypted data, the IDEA If no packets of these types precede the encrypted data, the IDEA
algorithm is used with the session key calculated as the MD5 hash of algorithm is used with the session key calculated as the MD5 hash of
the passphrase. the passphrase.
The data is encrypted in CFB mode, with a CFB shift size equal to the The data is encrypted in CFB mode, with a CFB shift size equal to the
cipher's block size [Ref]. The Initial Vector (IV) is specified as all cipher's block size. The Initial Vector (IV) is specified as all
zeros. Instead of using an IV, OP prepends a 10 octet string to the zeros. Instead of using an IV, OP prefixes a 10 octet string to the
data before it is encrypted. The first eight octets are random, and data before it is encrypted. The first eight octets are random, and
the 9th and 10th octets are copies of the 7th and 8th octets, the 9th and 10th octets are copies of the 7th and 8th octets,
respectivelly. After encrypting the first 10 octets, the CFB state is respectivelly. After encrypting the first 10 octets, the CFB state is
resynchronized if the cipher block size is 8 octets or less. The last resynchronized if the cipher block size is 8 octets or less. The last
8 octets of ciphertext are passed through the cipher and the block 8 octets of ciphertext are passed through the cipher and the block
boundary is reset. boundary is reset.
The repetition of 16 bits in the 80 bits of random data prepended to The repetition of 16 bits in the 80 bits of random data prepended to
the message allows the receiver to immediately check whether the the message allows the receiver to immediately check whether the
session key is correct. session key is correct.
skipping to change at page 31, line 52 skipping to change at page 30, line 5
5.8 Marker Packet (Obsolete Literal Packet) (Tag 10) 5.8 Marker Packet (Obsolete Literal Packet) (Tag 10)
An experimental version of PGP used this packet as the Literal packet, An experimental version of PGP used this packet as the Literal packet,
but no released version of PGP generated Literal packets with this tag. but no released version of PGP generated Literal packets with this tag.
With PGP 5.x, this packet has been re-assigned and is reserved for use With PGP 5.x, this packet has been re-assigned and is reserved for use
as the Marker packet. as the Marker packet.
The body of this packet consists of: The body of this packet consists of:
- The three octets 0x60, 0x47, 0x60 (which spell "PGP" in UTF-8). - The three octets 0x60, 0x47, 0x60 (which spell "PGP" in UTF-8).
Such a packet should be ignored on input. It may be placed at the Such a packet MUST be ignored when received. It may be placed at the
beginning of a message that uses features not available in PGP 2.6.X in beginning of a message that uses features not available in PGP 2.6.X in
order to cause that version to report that newer software necessary to order to cause that version to report that newer software necessary to
process the message. process the message.
5.9 Literal Data Packet (Tag 11) 5.9 Literal Data Packet (Tag 11)
A Literal Data packet contains the body of a message; data that is not A Literal Data packet contains the body of a message; data that is not
to be further interpreted. to be further interpreted.
The body of this packet consists of: The body of this packet consists of:
skipping to change at page 32, line 29 skipping to change at page 30, line 38
"for your eyes only". This advises that the message data is unusually "for your eyes only". This advises that the message data is unusually
sensitive, and the receiving program should process it more carefully, sensitive, and the receiving program should process it more carefully,
perhaps avoiding storing the received data to disk, for example. perhaps avoiding storing the received data to disk, for example.
- A four-octet number that indicates the modification date of the - A four-octet number that indicates the modification date of the
file, or the creation time of the packet, or a zero that indicates the file, or the creation time of the packet, or a zero that indicates the
present time. present time.
- The remainder of the packet is literal data. - The remainder of the packet is literal data.
Text data is stored with <CR><LF> text endings. This should be Text data is stored with <CR><LF> text endings (i.e. network-normal
converted to native line endings by the receiving software. line endings). These should be converted to native line endings by the
receiving software.
5.10 Trust Packet (Tag 12) 5.10 Trust Packet (Tag 12)
The Trust packet is used only within keyrings and is not normally The Trust packet is used only within keyrings and is not normally
exported. Trust packets contain data that record the user's exported. Trust packets contain data that record the user's
specifications of which key holders are trustworthy introducers, along specifications of which key holders are trustworthy introducers, along
with other information that implementing software uses for trust with other information that implementing software uses for trust
information. information.
Trust packets SHOULD NOT be emitted to output streams that are Trust packets SHOULD NOT be emitted to output streams that are
transferred to other users, and they SHOULD be ignored on any input transferred to other users, and they SHOULD be ignored on any input
other than local keyring files. other than local keyring files.
{{Editor's note: I have brushed aside the description of the old PGP
trust packets for a number of reasons. They are context dependent;
their meaning depends on the packet preceding them in a keyring.
There is also a security problem with trust packets. For example,
malicious software can write a new public key into a user's key ring
with trust packets that make it trusted.
A number of us have discussed this problem, and think that trust
information should always be self-signed to act as an integrity check,
but other people may have other solutions.
My solution is to make trust packets implementation dependent. They
are not emitted on export and ignored on import. Because of this, they
are arguably out of scope of this document anyway. Given that the PGP
implementation of trust packets has security flaws, this seems to be
the best way to deal with them.
5.11 User ID Packet (Tag 13) 5.11 User ID Packet (Tag 13)
A User ID packet consists of data which is intended to represent the A User ID packet consists of data which is intended to represent the
name and email address of the key holder. By convention, it includes name and email address of the key holder. By convention, it includes
an RFC822 mail name, but there are no restrictions on its content. The an RFC822 mail name, but there are no restrictions on its content. The
packet length in the header specifies the length of the user name. If packet length in the header specifies the length of the user name. If
it is text, it is encoded in UTF-8. it is text, it is encoded in UTF-8.
{{Editor's note: PRZ thinks there should be more types of "user ids" 6. Radix-64 Conversions
other than the traditional name, such as photos, and so on. The above
definition, which assiduously avoids saying that the content of the
packet is a counted string, is one potential way to handle it. Another
would be to explicitly state that this packet is a string, and
introduce a free-form user identification packet.
A related issue with this document is that sometimes it says "user id" As stated in the introduction, OP's underlying native representation
and sometimes "user name." We need some work here. Present plan is to for objects is a stream of arbitrary octets, and some systems desire
use "User ID" everywhere. --jdcc}} these objects to be immune to damage caused by character set
translation, data conversions, etc.
{{Editor's note: Carl Ellison pointed out to me that if we have In principle, any printable encoding scheme that met the requirements
non-exportable (local to one's own keyring) usernames that I can assign of the unsafe channel would suffice, since it would not change the
to keys I use, then essentially we have SDSI naming in PGP. This is a underlying binary bit streams of the native OP data structures. The OP
Good Thing, in my opinion, but we have to have a way to define it. standard specifies one such printable encoding scheme to ensure
interoperability.
5.12 Comment Packet (Tag 16) OP's Radix-64 encoding is composed of two parts: a base64 encoding of
the binary data, and a checksum. The base64 encoding is identical to
the MIME base64 content-transfer-encoding [RFC 2045, Section 6.8]. An
OP implementation MAY use ASCII Armor to protect the raw binary data.
A Comment packet is used for holding data that is not relevant to The checksum is a 24-bit CRC converted to four characters of radix-64
software. Comment packets should be ignored. encoding by the same MIME base64 transformation, preceded by an equals
sign (=). The CRC is computed by using the generator 0x864CFB and an
initialization of 0xB704CE. The accumulation is done on the data
before it is converted to radix-64, rather than on the converted data.
A sample implementation of this algorithm is in the next section.
{{Editor's note: should? Must? What does it mean to ignore them? For The checksum with its leading equal sign MAY appear on the first line
example, if it's desirable to show a comment to a user, then how does after the Base64 encoded data.
that interact with should/must and a suitable definition of "ignore." I
believe that they MUST be ignored, but displaying them to a user is
ignoring them. Looking inside them for cryptographic content (like OP
packets) is *not* ignoring them.}}
{{Editor's note: should we put in an X.509 encapsulation packet type?}} Rationale for CRC-24: The size of 24 bits fits evenly into printable
base64. The nonzero initialization can detect more errors than a zero
initialization.
6. Constants 6.1 An Implementation of the CRC-24 in "C"
#define CRC24_INIT 0xb704ce
#define CRC24_POLY 0x1864cfb
crc24 crc_bytes(unsigned char *bytes, size_t len)
{
crc24 crc = CRC_INIT;
int i;
while (len--) {
crc ^= *bytes++;
for (i = 0; i < 8; i++) {
crc <<= 1;
if (crc & 0x1000000)
crc ^= CRC24_POLY;
}
}
return crc;
}
6.2 Forming ASCII Armor
When OP encodes data into ASCII Armor, it puts specific headers around
the data, so OP can reconstruct the data later. OP informs the user
what kind of data is encoded in the ASCII armor through the use of the
headers.
Concatenating the following data creates ASCII Armor:
- An Armor Header Line, appropriate for the type of data
- Armor Headers
- A blank (zero-length, or containing only whitespace) line
- The ASCII-Armored data
- An Armor Checksum
- The Armor Tail, which depends on the Armor Header Line.
An Armor Header Line consists of the appropriate header line text
surrounded by five (5) dashes ('-', 0x2D) on either side of the header
line text. The header line text is chosen based upon the type of data
that is being encoded in Armor, and how it is being encoded. Header
line texts include the following strings:
BEGIN PGP MESSAGE used for signed, encrypted, or
compressed files
BEGIN PGP PUBLIC KEY BLOCK used for armoring public keys
BEGIN PGP PRIVATE KEY BLOCK used for armoring private keys
BEGIN PGP MESSAGE, PART X/Y used for multi-part messages, where
the armor is split amongst Y parts,
and this is the Xth part out of Y.
BEGIN PGP MESSAGE, PART X used for multi-part messages, where
this is the Xth part of an
unspecified number of parts.
Requires the MESSAGE-ID Armor
Header to be used.
BEGIN PGP SIGNATURE used for detached signatures,
OP/MIME signatures, and signatures
following clearsigned messages
The Armor Headers are pairs of strings that can give the user or the
receiving OP message block some information about how to decode or use
the message. The Armor Headers are a part of the armor, not a part of
the message, and hence are not protected by any signatures applied to
the message.
The format of an Armor Header is that of a key-value pair. A colon
(':' 0x38) and a single space (0x20) separate the key and value. OP
should consider improperly formatted Armor Headers to be corruption of
the ASCII Armor. Unknown keys should be reported to the user, but OP
should continue to process the message.
Currently defined Armor Header Keys are:
- "Version", which states the OP Version used to encode the
message.
- "Comment", a user-defined comment.
- "MessageID", a 32-character string of printable characters. The
string must be the same for all parts of a multi-part message that
uses the "PART X" Armor Header. MessageID strings should be unique
enough that the recipient of the mail can associate all the parts
of a message with each other. A good checksum or cryptographic
hash function is sufficent.
The MessageID should not appear unless it is in a multi-part
message. If it appears at all, it MUST be computed from the message
in a deterministic fashion, rather than contain a purely random
value. This is to allow anyone to determine that the MessageID
cannot serve as a covert means of leaking cryptographic key
information.
The Armor Tail Line is composed in the same manner as the Armor Header
Line, except the string "BEGIN" is replaced by the string "END."
6.3 Encoding Binary in Radix-64
The encoding process represents 24-bit groups of input bits as output
strings of 4 encoded characters. Proceeding from left to right, a
24-bit input group is formed by concatenating three 8-bit input groups.
These 24 bits are then treated as four concatenated 6-bit groups, each
of which is translated into a single digit in the Radix-64 alphabet.
When encoding a bit stream with the Radix-64 encoding, the bit stream
must be presumed to be ordered with the most-significant-bit first.
That is, the first bit in the stream will be the high-order bit in the
first 8-bit byte, and the eighth bit will be the low-order bit in the
first 8-bit byte, and so on.
+--first octet--+-second octet--+--third octet--+
|7 6 5 4 3 2 1 0|7 6 5 4 3 2 1 0|7 6 5 4 3 2 1 0|
+-----------+---+-------+-------+---+-----------+
|5 4 3 2 1 0|5 4 3 2 1 0|5 4 3 2 1 0|5 4 3 2 1 0|
+--1.index--+--2.index--+--3.index--+--4.index--+
Each 6-bit group is used as an index into an array of 64 printable
characters from the table below. The character referenced by the index
is placed in the output string.
Value Encoding Value Encoding Value Encoding Value Encoding
0 A 17 R 34 i 51 z
1 B 18 S 35 j 52 0
2 C 19 T 36 k 53 1
3 D 20 U 37 l 54 2
4 E 21 V 38 m 55 3
5 F 22 W 39 n 56 4
6 G 23 X 40 o 57 5
7 H 24 Y 41 p 58 6
8 I 25 Z 42 q 59 7
9 J 26 a 43 r 60 8
10 K 27 b 44 s 61 9
11 L 28 c 45 t 62 +
12 M 29 d 46 u 63 /
13 N 30 e 47 v
14 O 31 f 48 w (pad) =
15 P 32 g 49 x
16 Q 33 h 50 y
The encoded output stream must be represented in lines of no more than
76 characters each.
Special processing is performed if fewer than 24 bits are available at
the end of the data being encoded. There are three possibilities:
- The last data group has 24 bits (3 octets). No special processing is
needed.
- The last data group has 16 bits (2 octets). The first two 6-bit
groups are processed as above. The third (incomplete) data group has
two zero-value bits added to it, and is processed as above. A pad
character (=) is added to the output.
- The last data group has 8 bits (1 octet). The first 6-bit group is
processed as above. The second (incomplete) data group has four
zero-value bits added to it, and is processed as above. Two pad
characters (=) are added to the output.
6.4 Decoding Radix-64
Any characters outside of the base64 alphabet are ignored in Radix-64
data. Decoding software must ignore all line breaks or other
characters not found in the table above.
In Radix-64 data, characters other than those in the table, line
breaks, and other white space probably indicate a transmission error,
about which a warning message or even a message rejection might be
appropriate under some circumstances.
Because it is used only for padding at the end of the data, the
occurrence of any "=" characters may be taken as evidence that the end
of the data has been reached (without truncation in transit). No such
assurance is possible, however, when the number of octets transmitted
was a multiple of three and no "=" characters are present.
6.5 Examples of Radix-64
Input data: 0x14fb9c03d97e
Hex: 1 4 f b 9 c | 0 3 d 9 7 e
8-bit: 00010100 11111011 10011100 | 00000011 11011001 11111110
6-bit: 000101 001111 101110 011100 | 000000 111101 100111 111110
Decimal: 5 15 46 28 0 61 37 63
Output: F P u c A 9 l /
Input data: 0x14fb9c03d9
Hex: 1 4 f b 9 c | 0 3 d 9
8-bit: 00010100 11111011 10011100 | 00000011 11011001
pad with 00
6-bit: 000101 001111 101110 011100 | 000000 111101 100100
Decimal: 5 15 46 28 0 61 36
pad with =
Output: F P u c A 9 k =
Input data: 0x14fb9c03
Hex: 1 4 f b 9 c | 0 3
8-bit: 00010100 11111011 10011100 | 00000011
pad with 0000
6-bit: 000101 001111 101110 011100 | 000000 110000
Decimal: 5 15 46 28 0 48
pad with = =
Output: F P u c A w = =
6.6 Example of an ASCII Armored Message
-----BEGIN PGP MESSAGE-----
Version: OP V0.0
owFbx8DAYFTCWlySkpkHZDKEFCXmFedmFhdn5ucpZKdWFiv4hgaHKPj5hygUpSbn
l6UWpabo8XIBAA==
=3m1o
-----END PGP MESSAGE-----
Note that this example is indented by two spaces.
7. Cleartext signature framework
It is desirable to sign a textual octet stream without ASCII armoring
the stream itself, so the signed text is still readable without special
software. In order to bind a signature to such a cleartext, this
framework is used. (Note that RFC 2015 defines another way to clear
sign messages for environments that support MIME.)
The cleartext signed message consists of:
- The cleartext header '-----BEGIN PGP SIGNED MESSAGE-----' on a
single line,
- Zero or more "Hash" Armor Headers,
- Exactly one empty line not included into the message digest,
- The dash-escaped cleartext that is included into the message digest,
- The ASCII armored signature(s) including the Armor Header and Armor
Tail Lines.
If the "Hash" armor header is given, the specified message digest
algorithm is used for the signature. If there are no such headers,
SHA-1 is used. If more than one message digest is used in the
signature, the "Hash" armor header contains a comma-delimited list of
used message digests.
Current message digest names are:
- "SHA1"
- "MD5"
- "RIPEMD160"
The cleartext content of the message must also be dash-escaped.
Dash escaped cleartext is the ordinary cleartext where every line
starting with a dash '-' (0x2D) is prefixed by the sequence dash '-'
(0x2D) and space ' ' (0x20). This prevents the parser from recognizing
armor headers of the cleartext itself. The message digest is computed
using the cleartext itself, not the dash escaped form.
As with binary signatures on text documents, a cleartext signature is
calculated on the text using canonical <CR><LF> line endings. The line
ending (i.e. the <CR><LF>) before the '-----BEGIN PGP SIGNATURE-----'
line that terminates the signed text is not considered part of the
signed text.
Also, any trailing whitespace (spaces, and tabs, 0x09) at the end of
any line is ignored when the cleartext signature is calculated.
8. Regular Expressions
A regular expression is zero or more branches, separated by `|'. It
matches anything that matches one of the branches.
A branch is zero or more pieces, concatenated. It matches a match for
the first, followed by a match for the second, etc.
A piece is an atom possibly followed by `*', `+', or `?'. An atom
followed by `*' matches a sequence of 0 or more matches of the atom.
An atom followed by `+' matches a sequence of 1 or more matches of the
atom. An atom fol- lowed by `?' matches a match of the atom, or the
null string.
An atom is a regular expression in parentheses (matching a match for
the regular expression), a range (see below), `.' (matching any single
character), `^' (matching the null string at the beginning of the input
string), `$' (matching the null string at the end of the input string),
a `\' followed by a single character (matching that char- acter), or a
single character with no other significance (matching that character).
A range is a sequence of characters enclosed in `[]'. It normally
matches any single character from the sequence. If the sequence begins
with `^', it matches any single character not from the rest of the
sequence. If two char- acters in the sequence are separated by `-',
this is shorthand for the full list of ASCII characters between them
(e.g. `[0-9]' matches any decimal digit). To include a literal `]' in
the sequence, make it the first character (following a possible `^').
To include a literal `-', make it the first or last character.
9. Constants
This section describes the constants used in OP. This section describes the constants used in OP.
Note that these tables are not exhaustive lists; an implementation MAY Note that these tables are not exhaustive lists; an implementation MAY
implement an algorithm not on these lists. implement an algorithm not on these lists.
6.1 Public Key Algorithms 9.1 Public Key Algorithms
1 - RSA (Encrypt or Sign) 1 - RSA (Encrypt or Sign)
2 - RSA Encrypt-Only 2 - RSA Encrypt-Only
3 - RSA Sign-Only 3 - RSA Sign-Only
16 - Elgamal 16 - Elgamal, see [ELGAMAL]
17 - DSA (Digital Signature Standard) 17 - DSA (Digital Signature Standard)
18 - Elliptic Curve
19 - ECDSA
21 - Diffie-Hellman (X9.42)
100 to 110 - Private/Experimental algorithm. 100 to 110 - Private/Experimental algorithm.
Implementations MUST implement DSA for signatures, and Elgamal for Implementations MUST implement DSA for signatures, and Elgamal for
encryption. Implementations SHOULD implement RSA encryption. encryption. Implementations SHOULD implement RSA encryption.
Implementations MAY implement any other algorithm. Implementations MAY implement any other algorithm.
{{Editor's note: reserve an algorithm for elliptic curve? Note that 9.2 Symmetric Key Algorithms
I've left Elgamal signatures completely unmentioned. I think this is
good. --jdcc}}
6.2 Symmetric Key Algorithms
0 - Plaintext 0 - Plaintext
1 - IDEA 1 - IDEA
2 - Triple-DES (DES-EDE, as per spec - 2 - Triple-DES (DES-EDE, as per spec -
168 bit key derived from 192) 168 bit key derived from 192)
3 - CAST5 (128 bit key) 3 - CAST5 (128 bit key)
4 - Blowfish (128 bit key) 4 - Blowfish (128 bit key, 16 rounds)
5 - ROT-N (128 bit N) 5 - ROT-N (128 bit N)
6 - SAFER-SK128 6 - SAFER-SK128
7 - DES/SK 7 - DES/SK
100 to 110 - Private/Experimental algorithm. 100 to 110 - Private/Experimental algorithm.
Implementations MUST implement Triple-DES. Implementations SHOULD Implementations MUST implement Triple-DES. Implementations SHOULD
implement IDEA and CAST5.Implementations MAY implement any other implement IDEA and CAST5.Implementations MAY implement any other
algorithm. algorithm.
6.3 Compression Algorithms 9.3 Compression Algorithms
0 - Uncompressed 0 - Uncompressed
1 - ZIP 1 - ZIP
100 to 110 - Private/Experimental algorithm. 100 to 110 - Private/Experimental algorithm.
Implementations MUST implement uncompressed data. Implementations Implementations MUST implement uncompressed data. Implementations
SHOULD implement ZIP. SHOULD implement ZIP.
6.4 Hash Algorithms 9.4 Hash Algorithms
1 - MD5 1 - MD5
2 - SHA-1 2 - SHA-1
3 - RIPE-MD/160 3 - RIPE-MD/160
4 - HAVAL 4 - HAVAL
100 to 110 - Private/Experimental algorithm. 100 to 110 - Private/Experimental algorithm.
Implementations MUST implement SHA-1. Implementations SHOULD implement Implementations MUST implement SHA-1. Implementations SHOULD implement
MD5. MD5.
7. Packet Composition 10. Packet Composition
OP packets may be assembled into sequences in order to create messages OP packets are assembled into sequences in order to create messages and
and transfer keys. Not all possible packet sequences are meaningful to transfer keys. Not all possible packet sequences are meaningful and
and correct. This describes the rules for how packets should be placed correct. This describes the rules for how packets should be placed
into sequences. into sequences.
7.1 Transferable Public Keys 10.1 Transferable Public Keys
OP users may transfer public keys. The essential elements of a OP users may transfer public keys. The essential elements of a
transferable public key are: transferable public key are:
- One Public Key packet - One Public Key packet
- Zero or more revocation signatures - Zero or more revocation signatures
- One or more User ID packets - One or more User ID packets
- After each User ID packet, zero or more Signature packets - After each User ID packet, zero or more Signature packets
- Zero or more Subkey packets - Zero or more Subkey packets
- After each Subkey packet, one or more Signature packets - After each Subkey packet, one or more Signature packets
skipping to change at page 35, line 38 skipping to change at page 39, line 31
enjoy the use of more than one e-mail address, and construct a User ID enjoy the use of more than one e-mail address, and construct a User ID
packet for each one. packet for each one.
Immediately following each User ID packet, there are zero or more Immediately following each User ID packet, there are zero or more
signature packets. Each signature packet is calculated on the signature packets. Each signature packet is calculated on the
immediately preceding User ID packet and the initial Public Key packet. immediately preceding User ID packet and the initial Public Key packet.
The signature serves to certify the corresponding public key and user The signature serves to certify the corresponding public key and user
ID. In effect, the signer is testifying to his or her belief that this ID. In effect, the signer is testifying to his or her belief that this
public key belongs to the user identified by this user ID. public key belongs to the user identified by this user ID.
After the User ID packets there may be one or more Subkey packets. After the User ID packets there may be one or more Subkey packets. In
Subkeys are used in cases where the top-level public key is a general, subkeys are provided in cases where the top-level public key
signature-only key. The subkeys are then encryption-only keys that are is a signature-only key. However, any V4 key may have subkeys, and the
bound to the signature key. Each Subkey packet must be followed by at subkeys may be encryption-only keys, signature-only keys, or
least one Signature packet, which should be of the subkey binding general-purpose keys.
signature type, and issued by the top level key.
{{Editor's note: I think it is a good idea to have signature-only Each Subkey packet must be followed by at least one Signature packet,
subkeys, too (or even encrypt-and-sign subkeys), but no implementation which should be of the subkey binding signature type, issued by the top
does this. Should we generalize here? --jdcc}} level key.
Subkey and Key packets may each be followed by a revocation Signature Subkey and Key packets may each be followed by a revocation Signature
packet to indicate that the key is revoked. Revocation signatures are packet to indicate that the key is revoked. Revocation signatures are
only accepted if they are issued by the key itself, or by a key which only accepted if they are issued by the key itself, or by a key which
is authorized to issue revocations via a revocation key subpacket in a is authorized to issue revocations via a revocation key subpacket in a
self-signature by the top level key. self-signature by the top level key.
Transferable public key packet sequences may be concatenated to allow Transferable public key packet sequences may be concatenated to allow
transferring multiple public keys in one operation. transferring multiple public keys in one operation.
7.2 OP Messages 10.2 OP Messages
An OP message is a packet or sequence of packets that corresponds to An OP message is a packet or sequence of packets that corresponds to
the following grammatical rules (comma represents sequential the following grammatical rules (comma represents sequential
composition, and vertical bar separates alternatives): composition, and vertical bar separates alternatives):
OP Message :- Encrypted Message | Signed Message | Compressed Message OP Message :- Encrypted Message | Signed Message | Compressed Message
| Literal Message. | Literal Message.
Compressed Message :- Compressed Data Packet. Compressed Message :- Compressed Data Packet.
Literal Message :- Literal Data Packet. Literal Message :- Literal Data Packet.
ESK :- Encrypted Session Key Packet | ESK :- Pubic Key Encrypted Session Key Packet |
Conventionally Encrypted Session Key Packet. Conventionally Encrypted Session Key Packet.
ESK Sequence :- ESK | ESK Sequence, ESK. ESK Sequence :- ESK | ESK Sequence, ESK.
Encrypted Message :- Symmetrically Encrypted Data Packet | Encrypted Message :- Symmetrically Encrypted Data Packet |
ESK Sequence, Symmetrically Encrypted Data Packet. ESK Sequence, Symmetrically Encrypted Data Packet.
One-Pass Signed Message :- One-Pass Signature Packet, OP Message, One-Pass Signed Message :- One-Pass Signature Packet, OP Message,
Signature Packet. Signature Packet.
Signed Message :- Signature Packet, OP Message | Signed Message :- Signature Packet, OP Message |
One-Pass Signed Message. One-Pass Signed Message.
In addition, the decrypting a Symmetrically Encrypted Data packet and In addition, decrypting a Symmetrically Encrypted Data packet and
decompressing a Compressed Data packet must yield a valid OP Message. decompressing a Compressed Data packet must yield a valid OP Message.
8. Enhanced Key Formats 11. Enhanced Key Formats
8.1 Key Structures 11.1 Key Structures
The format of V3 OP key using RSA is as follows. Entries in square The format of V3 OP key using RSA is as follows. Entries in square
brackets are optional and ellipses indicate repetition. brackets are optional and ellipses indicate repetition.
RSA Public Key RSA Public Key
[Revocation Self Signature] [Revocation Self Signature]
User ID [Signature ...] User ID [Signature ...]
[User ID [Signature ...] ...] [User ID [Signature ...] ...]
Each signature certifies the RSA public key and the preceding user ID. Each signature certifies the RSA public key and the preceding user ID.
The RSA public key can have many user IDs and each user ID can have The RSA public key can have many user IDs and each user ID can have
many signatures. many signatures.
The format of an OP V4 key that uses two public keys is very similar The format of an OP V4 key that uses two public keys is very similar
except that the second key is added to the end as a 'subkey' of the except that the second key is added to the end as a 'subkey' of the
primary key. primary key.
Primary-Key Primary-Key
[Revocation Self Signature] [Revocation Self Signature]
[Direct Key Self Signature...]
User ID [Signature ...] User ID [Signature ...]
[User ID [Signature ...] ...] [User ID [Signature ...] ...]
[Subkey Primary-Key-Signature] [Subkey Primary-Key-Signature]
The subkey always has a single signature after it that is issued using The subkey always has a single signature after it that is issued using
the primary key to tie the two keys together. The new format can use the primary key to tie the two keys together. The new format can use
either the new signature packets or the old signature packets. either the new signature packets or the old signature packets.
In an Elgamal/DSA key, the DSA public key is the primary key, the In an key that has a main key and subkeys, the primary key MUST be a
Elgamal public key is the subkey, and either version 3 or 4 of the key capable of signing. The subkeys may be keys of any other type, and
signature packet can be used. There may be other types of V4 keys, either version 3 or 4 of the signature packet can be used. There may
too. For example, there may be a single-key RSA key in V4 format, a DSA be other types of V4 keys, too. For example, there may be a single-key
primary key with an RSA encryption key, etc, or RSA primary key with an RSA key in V4 format, a DSA primary key with an RSA encryption key,
Elgamal subkey. etc, or RSA primary key with an Elgamal subkey.
It is also possible to have a signature-only subkey. This permits a It is also possible to have a signature-only subkey. This permits a
primary key that collects certifications (key signatures) but is used primary key that collects certifications (key signatures) but is used
only used for certifying subkeys that are used for encryption and only used for certifying subkeys that are used for encryption and
signatures. signatures.
8.2 V4 Key IDs and Fingerprints 11.2 V4 Key IDs and Fingerprints
A V4 fingerprint is the 160-bit SHA-1 hash of the one-octet Packet Tag, A V4 fingerprint is the 160-bit SHA-1 hash of the one-octet Packet Tag,
followed by the two-octet packet length, followed by the entire Public followed by the two-octet packet length, followed by the entire Public
Key packet starting with the version field. The key ID is either the Key packet starting with the version field. The key ID is either the
low order 32 bits or 64 bits of the fingerprint. Here are the fields low order 32 bits or 64 bits of the fingerprint. Here are the fields
of the hash material, with the example of a DSA key: of the hash material, with the example of a DSA key:
a.1) 0x99 (1 byte) a.1) 0x99 (1 byte)
a.2) high order length byte of (b)-(f) (1 byte) a.2) high order length byte of (b)-(f) (1 byte)
a.3) low order length byte of (b)-(f) (1 byte) a.3) low order length byte of (b)-(f) (1 byte)
skipping to change at page 37, line 48 skipping to change at page 41, line 46
e) algorithm (1 byte): e) algorithm (1 byte):
17 = DSA; 17 = DSA;
f) Algorithm specific fields. f) Algorithm specific fields.
Algorithm Specific Fields for DSA keys (example): Algorithm Specific Fields for DSA keys (example):
f.1) MPI of DSA prime p; f.1) MPI of DSA prime p;
f.2) MPI of DSA group order q (q is a prime divisor of p-1); f.2) MPI of DSA group order q (q is a prime divisor of p-1);
f.3) MPI of DSA group generator g; f.3) MPI of DSA group generator g;
f.4) MPI of DSA public key value y (= g**x where x is secret). f.4) MPI of DSA public key value y (= g**x where x is secret).
9. Security Considerations 12. Security Considerations
As with any technology involving cryptography, you should check the As with any technology involving cryptography, you should check the
current literature to determine if any algorithms used here have been current literature to determine if any algorithms used here have been
found to be vulnerable to attack. found to be vulnerable to attack.
This specification uses Public Key Cryptography technologies. This specification uses Public Key Cryptography technologies.
Possession of the private key portion of a public-private key pair is Possession of the private key portion of a public-private key pair is
assumed to be controlled by the proper party or parties. assumed to be controlled by the proper party or parties.
Certain operations in this specification involve the use of random Certain operations in this specification involve the use of random
numbers. An appropriate entropy source should be used to generate numbers. An appropriate entropy source should be used to generate
these numbers. See RFC 1750. these numbers. See RFC 1750.
The MD5 hash algorithm has been found to have weaknesses The MD5 hash algorithm has been found to have weaknesses
(pseudo-collisions in the compress function) that make some people (pseudo-collisions in the compress function) that make some people
deprecate its use. They consider the SHA-1 algorithm better. deprecate its use. They consider the SHA-1 algorithm better.
skipping to change at page 38, line 28 skipping to change at page 42, line 25
use a weak algorithm simply because the recipient requests it. use a weak algorithm simply because the recipient requests it.
Some of the encryption algorithms mentioned in this document have been Some of the encryption algorithms mentioned in this document have been
analyzed less than others. For example, although CAST5 is presently analyzed less than others. For example, although CAST5 is presently
considered strong, it has been analyzed less than Triple-DES. Other considered strong, it has been analyzed less than Triple-DES. Other
algorithms may have other controversies surrounding them. algorithms may have other controversies surrounding them.
Some technologies mentioned here may be subject to government control Some technologies mentioned here may be subject to government control
in some countries. in some countries.
10. Authors and Working Group Chair 13. Authors and Working Group Chair
The working group can be contacted via the current chair: The working group can be contacted via the current chair:
John W. Noerenberg, II Qualcomm, Inc 6455 Lusk Blvd San Diego, CA John W. Noerenberg, II
92131 USA Email: jwn2@qualcomm.com Tel: +1 619 658 3510 Qualcomm, Inc
6455 Lusk Blvd
San Diego, CA 92131 USA
Email: jwn2@qualcomm.com
Tel: +1 619-658-3510
The principal authors of this draft are (in alphabetical order): The principal authors of this draft are (in alphabetical order):
Jon Callas Pretty Good Privacy, Inc. 555 Twin Dolphin Drive, #570 Jon Callas
Redwood Shores, CA 94065, USA Email: jon@pgp.com Tel: +1-650-596-1960 Network Associates, Inc.
4200 Bohannon Drive
Menlo Park, CA 94025, USA
Email: jon@pgp.com
Tel: +1-650-473-2860
Lutz Donnerhacke IKS GmbH Wildenbruchstr. 15 07745 Jena, Germany EMail: Lutz Donnerhacke
lutz@iks-jena.de Tel: +49-3641-675642 IKS GmbH
Wildenbruchstr. 15
07745 Jena, Germany
EMail: lutz@iks-jena.de
Tel: +49-3641-675642
Hal Finney Pretty Good Privacy, Inc. 555 Twin Dolphin Drive, #570 Hal Finney
Redwood Shores, CA 94065, USA Email: hal@pgp.com Tel: +1-650-572-0430 Network Associates, Inc.
4200 Bohannon Drive
Rodney Thayer Sable Technology Corporation 246 Walnut Street Newton, MA Menlo Park, CA 94025, USA
02160 USA Email: rodney@sabletech.com Tel: +1-617-332-7292 Email: hal@pgp.com
Rodney Thayer
Sable Technology Corporation
246 Walnut Street
Newton, MA 02160 USA
Email: rodney@sabletech.com
Tel: +1-617-332-7292
This draft also draws on much previous work from a number of other This draft also draws on much previous work from a number of other
authors who include: Derek Atkins, Charles Breed, Dave Del Torto, Marc authors who include: Derek Atkins, Charles Breed, Dave Del Torto, Marc
Dyksterhouse, Gail Haspert, Gene Hoffman, Paul Hoffman, Raph Levine, Dyksterhouse, Gail Haspert, Gene Hoffman, Paul Hoffman, Raph Levine,
Colin Plumb, Will Price, William Stallings, Mark Weaver, and Philip R. Colin Plumb, Will Price, William Stallings, Mark Weaver, and Philip R.
Zimmermann. Zimmermann.
11. References 14. References
[CAMPBELL} Campbell, Joe, "C Programmer's Guide to Serial
Communications"
[DONNERHACKE] Donnerhacke, L., et. al, "PGP263in - an improved [DONNERHACKE] Donnerhacke, L., et. al, "PGP263in - an improved
international version of PGP", international version of PGP",
ftp://ftp.iks-jena.de/mitarb/lutz/crypt/software/pgp/ ftp://ftp.iks-jena.de/mitarb/lutz/crypt/software/pgp/
[ELGAMAL] T. ElGamal, "A Public-Key Cryptosystem and a Signature
Scheme Based on Discrete Logarithms," IEEE Transactions on Information
Theory, v. IT-31, n. 4, 1985, pp. 469-472.
[ISO-10646] ISO/IEC 10646-1:1993. International Standard -- [ISO-10646] ISO/IEC 10646-1:1993. International Standard --
Information technology -- Universal Multiple-Octet Coded Character Set Information technology -- Universal Multiple-Octet Coded Character Set
(UCS) -- Part 1: Architecture and Basic Multilingual Plane. UTF-8 is (UCS) -- Part 1: Architecture and Basic Multilingual Plane. UTF-8 is
described in Annex R, adopted but not yet published. UTF-16 is described in Annex R, adopted but not yet published. UTF-16 is
described in Annex Q, adopted but not yet published. described in Annex Q, adopted but not yet published.
[PKCS1] RSA Laboratories, "PKCS #1: RSA Encryption Standard," version [PKCS1] RSA Laboratories, "PKCS #1: RSA Encryption Standard," version
1.5, November 1993 1.5, November 1993
[RFC822] D. Crocker, "Standard for the format of ARPA Internet text [RFC822] D. Crocker, "Standard for the format of ARPA Internet text
skipping to change at page 39, line 55 skipping to change at page 44, line 26
[RFC2044] F. Yergeau., UTF-8, a transformation format of Unicode and [RFC2044] F. Yergeau., UTF-8, a transformation format of Unicode and
ISO 10646. October 1996. ISO 10646. October 1996.
[RFC2045] Borenstein, N., and Freed, N., "Multipurpose Internet Mail [RFC2045] Borenstein, N., and Freed, N., "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message Bodies.", Extensions (MIME) Part One: Format of Internet Message Bodies.",
November 1996 November 1996
[RFC2119] Bradner, S., Key words for use in RFCs to Indicate [RFC2119] Bradner, S., Key words for use in RFCs to Indicate
Requirement Level. March 1997. Requirement Level. March 1997.
12. Full Copyright Statement 15. Full Copyright Statement
Copyright 1997 by The Internet Society. All Rights Reserved. Copyright 1998 by The Internet Society. All Rights Reserved.
This document and translations of it may be copied and furnished to This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it or others, and derivative works that comment on or otherwise explain it or
assist in its implementation may be prepared, copied, published and assist in its implementation may be prepared, copied, published and
distributed, in whole or in part, without restriction of any kind, distributed, in whole or in part, without restriction of any kind,
provided that the above copyright notice and this paragraph are provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing the document itself may not be modified in any way, such as by removing the
copyright notice or references to the Internet Society or other copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of developing Internet organizations, except as needed for the purpose of developing
 End of changes. 

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