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Network Working Group J. Callas
Request for Comments: 2440 Network Associates
Category: Standards Track L. Donnerhacke
IN-Root-CA Individual Network e.V.
H. Finney
Network Associates
R. Thayer
EIS Corporation
November 1998
OpenPGP Message Format
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (1998). All Rights Reserved.
IESG Note
This document defines many tag values, yet it doesn't describe a
mechanism for adding new tags (for new features). Traditionally the
Internet Assigned Numbers Authority (IANA) handles the allocation of
new values for future expansion and RFCs usually define the procedure
to be used by the IANA. However, there are subtle (and not so
subtle) interactions that may occur in this protocol between new
features and existing features which result in a significant
reduction in over all security. Therefore, this document does not
define an extension procedure. Instead requests to define new tag
values (say for new encryption algorithms for example) should be
forwarded to the IESG Security Area Directors for consideration or
forwarding to the appropriate IETF Working Group for consideration.
Abstract
This document is maintained in order to publish all necessary
information needed to develop interoperable applications based on the
OpenPGP format. It is not a step-by-step cookbook for writing an
application. It describes only the format and methods needed to read,
check, generate, and write conforming packets crossing any network.
It does not deal with storage and implementation questions. It does,
Callas, et. al. Standards Track [Page 1]
RFC 2440 OpenPGP Message Format November 1998
however, discuss implementation issues necessary to avoid security
flaws.
Open-PGP software uses a combination of strong public-key and
symmetric cryptography to provide security services for electronic
communications and data storage. These services include
confidentiality, key management, authentication, and digital
signatures. This document specifies the message formats used in
OpenPGP.
Table of Contents
Status of this Memo 1
IESG Note 1
Abstract 1
Table of Contents 2
1. Introduction 4
1.1. Terms 5
2. General functions 5
2.1. Confidentiality via Encryption 5
2.2. Authentication via Digital signature 6
2.3. Compression 7
2.4. Conversion to Radix-64 7
2.5. Signature-Only Applications 7
3. Data Element Formats 7
3.1. Scalar numbers 8
3.2. Multi-Precision Integers 8
3.3. Key IDs 8
3.4. Text 8
3.5. Time fields 9
3.6. String-to-key (S2K) specifiers 9
3.6.1. String-to-key (S2k) specifier types 9
3.6.1.1. Simple S2K 9
3.6.1.2. Salted S2K 10
3.6.1.3. Iterated and Salted S2K 10
3.6.2. String-to-key usage 11
3.6.2.1. Secret key encryption 11
3.6.2.2. Symmetric-key message encryption 11
4. Packet Syntax 12
4.1. Overview 12
4.2. Packet Headers 12
4.2.1. Old-Format Packet Lengths 13
4.2.2. New-Format Packet Lengths 13
4.2.2.1. One-Octet Lengths 14
4.2.2.2. Two-Octet Lengths 14
4.2.2.3. Five-Octet Lengths 14
4.2.2.4. Partial Body Lengths 14
4.2.3. Packet Length Examples 14
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RFC 2440 OpenPGP Message Format November 1998
4.3. Packet Tags 15
5. Packet Types 16
5.1. Public-Key Encrypted Session Key Packets (Tag 1) 16
5.2. Signature Packet (Tag 2) 17
5.2.1. Signature Types 17
5.2.2. Version 3 Signature Packet Format 19
5.2.3. Version 4 Signature Packet Format 21
5.2.3.1. Signature Subpacket Specification 22
5.2.3.2. Signature Subpacket Types 24
5.2.3.3. Signature creation time 25
5.2.3.4. Issuer 25
5.2.3.5. Key expiration time 25
5.2.3.6. Preferred symmetric algorithms 25
5.2.3.7. Preferred hash algorithms 25
5.2.3.8. Preferred compression algorithms 26
5.2.3.9. Signature expiration time 26
5.2.3.10.Exportable Certification 26
5.2.3.11.Revocable 27
5.2.3.12.Trust signature 27
5.2.3.13.Regular expression 27
5.2.3.14.Revocation key 27
5.2.3.15.Notation Data 28
5.2.3.16.Key server preferences 28
5.2.3.17.Preferred key server 29
5.2.3.18.Primary user id 29
5.2.3.19.Policy URL 29
5.2.3.20.Key Flags 29
5.2.3.21.Signer's User ID 30
5.2.3.22.Reason for Revocation 30
5.2.4. Computing Signatures 31
5.2.4.1. Subpacket Hints 32
5.3. Symmetric-Key Encrypted Session-Key Packets (Tag 3) 32
5.4. One-Pass Signature Packets (Tag 4) 33
5.5. Key Material Packet 34
5.5.1. Key Packet Variants 34
5.5.1.1. Public Key Packet (Tag 6) 34
5.5.1.2. Public Subkey Packet (Tag 14) 34
5.5.1.3. Secret Key Packet (Tag 5) 35
5.5.1.4. Secret Subkey Packet (Tag 7) 35
5.5.2. Public Key Packet Formats 35
5.5.3. Secret Key Packet Formats 37
5.6. Compressed Data Packet (Tag 8) 38
5.7. Symmetrically Encrypted Data Packet (Tag 9) 39
5.8. Marker Packet (Obsolete Literal Packet) (Tag 10) 39
5.9. Literal Data Packet (Tag 11) 40
5.10. Trust Packet (Tag 12) 40
5.11. User ID Packet (Tag 13) 41
6. Radix-64 Conversions 41
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RFC 2440 OpenPGP Message Format November 1998
6.1. An Implementation of the CRC-24 in "C" 42
6.2. Forming ASCII Armor 42
6.3. Encoding Binary in Radix-64 44
6.4. Decoding Radix-64 46
6.5. Examples of Radix-64 46
6.6. Example of an ASCII Armored Message 47
7. Cleartext signature framework 47
7.1. Dash-Escaped Text 47
8. Regular Expressions 48
9. Constants 49
9.1. Public Key Algorithms 49
9.2. Symmetric Key Algorithms 49
9.3. Compression Algorithms 50
9.4. Hash Algorithms 50
10. Packet Composition 50
10.1. Transferable Public Keys 50
10.2. OpenPGP Messages 52
10.3. Detached Signatures 52
11. Enhanced Key Formats 52
11.1. Key Structures 52
11.2. Key IDs and Fingerprints 53
12. Notes on Algorithms 54
12.1. Symmetric Algorithm Preferences 54
12.2. Other Algorithm Preferences 55
12.2.1. Compression Preferences 56
12.2.2. Hash Algorithm Preferences 56
12.3. Plaintext 56
12.4. RSA 56
12.5. Elgamal 57
12.6. DSA 58
12.7. Reserved Algorithm Numbers 58
12.8. OpenPGP CFB mode 58
13. Security Considerations 59
14. Implementation Nits 60
15. Authors and Working Group Chair 62
16. References 63
17. Full Copyright Statement 65
1. Introduction
This document provides information on the message-exchange packet
formats used by OpenPGP to provide encryption, decryption, signing,
and key management functions. It builds on the foundation provided in
RFC 1991 "PGP Message Exchange Formats."
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RFC 2440 OpenPGP Message Format November 1998
1.1. Terms
* OpenPGP - This is a definition for security software that uses
PGP 5.x as a basis.
* PGP - Pretty Good Privacy. PGP is a family of software systems
developed by Philip R. Zimmermann from which OpenPGP is based.
* PGP 2.6.x - This version of PGP has many variants, hence the term
PGP 2.6.x. It used only RSA, MD5, and IDEA for its cryptographic
transforms. An informational RFC, RFC 1991, was written
describing this version of PGP.
* PGP 5.x - This version of PGP is formerly known as "PGP 3" in the
community and also in the predecessor of this document, RFC 1991.
It has new formats and corrects a number of problems in the PGP
2.6.x design. It is referred to here as PGP 5.x because that
software was the first release of the "PGP 3" code base.
"PGP", "Pretty Good", and "Pretty Good Privacy" are trademarks of
Network Associates, Inc. and are used with permission.
This document uses the terms "MUST", "SHOULD", and "MAY" as defined
in RFC 2119, along with the negated forms of those terms.
2. General functions
OpenPGP provides data integrity services for messages and data files
by using these core technologies:
- digital signatures
- encryption
- compression
- radix-64 conversion
In addition, OpenPGP provides key management and certificate
services, but many of these are beyond the scope of this document.
2.1. Confidentiality via Encryption
OpenPGP uses two encryption methods to provide confidentiality:
symmetric-key encryption and public key encryption. With public-key
encryption, the object is encrypted using a symmetric encryption
algorithm. Each symmetric key is used only once. A new "session key"
is generated as a random number for each message. Since it is used
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RFC 2440 OpenPGP Message Format November 1998
only once, the session key is bound to the message and transmitted
with it. To protect the key, it is encrypted with the receiver's
public key. The sequence is as follows:
1. The sender creates a message.
2. The sending OpenPGP generates a random number to be used as a
session key for this message only.
3. The session key is encrypted using each recipient's public key.
These "encrypted session keys" start the message.
4. The sending OpenPGP encrypts the message using the session key,
which forms the remainder of the message. Note that the message
is also usually compressed.
5. The receiving OpenPGP decrypts the session key using the
recipient's private key.
6. The receiving OpenPGP decrypts the message using the session key.
If the message was compressed, it will be decompressed.
With symmetric-key encryption, an object may be encrypted with a
symmetric key derived from a passphrase (or other shared secret), or
a two-stage mechanism similar to the public-key method described
above in which a session key is itself encrypted with a symmetric
algorithm keyed from a shared secret.
Both digital signature and confidentiality services may be applied to
the same message. First, a signature is generated for the message and
attached to the message. Then, the message plus signature is
encrypted using a symmetric session key. Finally, the session key is
encrypted using public-key encryption and prefixed to the encrypted
block.
2.2. Authentication via Digital signature
The digital signature uses a hash code or message digest algorithm,
and a public-key signature algorithm. The sequence is as follows:
1. The sender creates a message.
2. The sending software generates a hash code of the message.
3. The sending software generates a signature from the hash code
using the sender's private key.
4. The binary signature is attached to the message.
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RFC 2440 OpenPGP Message Format November 1998
5. The receiving software keeps a copy of the message signature.
6. The receiving software generates a new hash code for the
received message and verifies it using the message's signature.
If the verification is successful, the message is accepted as
authentic.
2.3. Compression
OpenPGP implementations MAY compress the message after applying the
signature but before encryption.
2.4. Conversion to Radix-64
OpenPGP's underlying native representation for encrypted messages,
signature certificates, and keys is a stream of arbitrary octets.
Some systems only permit the use of blocks consisting of seven-bit,
printable text. For transporting OpenPGP's native raw binary octets
through channels that are not safe to raw binary data, a printable
encoding of these binary octets is needed. OpenPGP provides the
service of converting the raw 8-bit binary octet stream to a stream
of printable ASCII characters, called Radix-64 encoding or ASCII
Armor.
Implementations SHOULD provide Radix-64 conversions.
Note that many applications, particularly messaging applications,
will want more advanced features as described in the OpenPGP-MIME
document, RFC 2015. An application that implements OpenPGP for
messaging SHOULD implement OpenPGP-MIME.
2.5. Signature-Only Applications
OpenPGP is designed for applications that use both encryption and
signatures, but there are a number of problems that are solved by a
signature-only implementation. Although this specification requires
both encryption and signatures, it is reasonable for there to be
subset implementations that are non-comformant only in that they omit
encryption.
3. Data Element Formats
This section describes the data elements used by OpenPGP.
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RFC 2440 OpenPGP Message Format November 1998
3.1. Scalar numbers
Scalar numbers are unsigned, and are always stored in big-endian
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
four-octet scalar is ((n[0] << 24) + (n[1] << 16) + (n[2] << 8) +
n[3]).
3.2. Multi-Precision Integers
Multi-Precision Integers (also called MPIs) are unsigned integers
used to hold large integers such as the ones used in cryptographic
calculations.
An MPI consists of two pieces: a two-octet scalar that is the length
of the MPI in bits followed by a string of octets that contain the
actual integer.
These octets form a big-endian number; a big-endian number can be
made into an MPI by prefixing it with the appropriate length.
Examples:
(all numbers are in hexadecimal)
The string of octets [00 01 01] forms an MPI with the value 1. The
string [00 09 01 FF] forms an MPI with the value of 511.
Additional rules:
The size of an MPI is ((MPI.length + 7) / 8) + 2 octets.
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
correctly. It should be [00 01 01].
3.3. Key IDs
A Key ID is an eight-octet scalar that identifies a key.
Implementations SHOULD NOT assume that Key IDs are unique. The
section, "Enhanced Key Formats" below describes how Key IDs are
formed.
3.4. Text
The default character set for text is the UTF-8 [RFC2279] encoding of
Unicode [ISO10646].
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RFC 2440 OpenPGP Message Format November 1998
3.5. Time fields
A time field is an unsigned four-octet number containing the number
of seconds elapsed since midnight, 1 January 1970 UTC.
3.6. String-to-key (S2K) specifiers
String-to-key (S2K) specifiers are used to convert passphrase strings
into symmetric-key encryption/decryption keys. They are used in two
places, currently: to encrypt the secret part of private keys in the
private keyring, and to convert passphrases to encryption keys for
symmetrically encrypted messages.
3.6.1. String-to-key (S2k) specifier types
There are three types of S2K specifiers currently supported, as
follows:
3.6.1.1. Simple S2K
This directly hashes the string to produce the key data. See below
for how this hashing is done.
Octet 0: 0x00
Octet 1: hash algorithm
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
(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
session key size, the high-order (leftmost) octets of the hash are
used as the key.
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.
These instances are preloaded with 0, 1, 2, ... octets of zeros (that
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 of zeros, and so forth).
As the data is hashed, it is given independently to each hash
context. Since the contexts have been initialized differently, they
will each produce different hash output. Once the passphrase is
hashed, the output data from the multiple hashes is concatenated,
first hash leftmost, to produce the key data, with any excess octets
on the right discarded.
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RFC 2440 OpenPGP Message Format November 1998
3.6.1.2. Salted S2K
This includes a "salt" value in the S2K specifier -- some arbitrary
data -- that gets hashed along with the passphrase string, to help
prevent dictionary attacks.
Octet 0: 0x01
Octet 1: hash algorithm
Octets 2-9: 8-octet salt value
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
specifier, followed by the passphrase.
3.6.1.3. Iterated and Salted S2K
This includes both a salt and an octet count. The salt is combined
with the passphrase and the resulting value is hashed repeatedly.
This further increases the amount of work an attacker must do to try
dictionary attacks.
Octet 0: 0x03
Octet 1: hash algorithm
Octets 2-9: 8-octet salt value
Octet 10: count, a one-octet, coded value
The count is coded into a one-octet number using the following
formula:
#define EXPBIAS 6
count = ((Int32)16 + (c & 15)) << ((c >> 4) + EXPBIAS);
The above formula is in C, where "Int32" is a type for a 32-bit
integer, and the variable "c" is the coded count, Octet 10.
Iterated-Salted S2K hashes the passphrase and salt data multiple
times. The total number of octets to be hashed is specified in the
encoded count in the S2K specifier. 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
algorithms, depending on how many octets of key data are needed.
Then the salt, followed by the passphrase data is repeatedly hashed
until 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 salt plus passphrase, the full salt plus passphrase
will be hashed even though that is greater than the octet count.
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RFC 2440 OpenPGP Message Format November 1998
After the hashing is done the data is unloaded from the hash
context(s) as with the other S2K algorithms.
3.6.2. String-to-key usage
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 convert the passphrase to a key that unlocks the secret data.
Older versions of PGP just stored a cipher algorithm octet preceding
the 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 key for the specified cipher algorithm.
For compatibility, when an S2K specifier is used, the special value
255 is stored in the position where the hash algorithm octet would
have been in the old data structure. This is then followed
immediately by a one-octet algorithm identifier, and then by the S2K
specifier as encoded above.
Therefore, preceding the secret data there will be one of these
possibilities:
0: secret data is unencrypted (no pass phrase)
255: followed by algorithm octet and S2K specifier
Cipher alg: use Simple S2K algorithm using MD5 hash
This last possibility, the cipher algorithm number with an implicit
use of MD5 and IDEA, is provided for backward compatibility; it MAY
be understood, but SHOULD NOT be generated, and is deprecated.
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
values themselves.
3.6.2.2. Symmetric-key message encryption
OpenPGP can create a Symmetric-key Encrypted Session Key (ESK) packet
at the front of a message. This is used to allow S2K specifiers to
be used for the passphrase conversion or to create messages with a
mix of symmetric-key ESKs and public-key ESKs. This allows a message
to be decrypted either with a passphrase or a public key.
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RFC 2440 OpenPGP Message Format November 1998
PGP 2.X always used IDEA with Simple string-to-key conversion when
encrypting a message with a symmetric algorithm. This is deprecated,
but MAY be used for backward-compatibility.
4. Packet Syntax
This section describes the packets used by OpenPGP.
4.1. Overview
An OpenPGP message is constructed from a number of records that are
traditionally called packets. A packet is a chunk of data that has a
tag specifying its meaning. An OpenPGP message, keyring, certificate,
and so forth consists of a number of packets. Some of those packets
may contain other OpenPGP packets (for example, a compressed data
packet, when uncompressed, contains OpenPGP packets).
Each packet consists of a packet header, followed by the packet body.
The packet header is of variable length.
4.2. Packet Headers
The first octet of the packet header is called the "Packet Tag." It
determines the format of the header and denotes the packet contents.
The remainder of the packet header is the length of the packet.
Note that the most significant bit is the left-most bit, called bit
7. A mask for this bit is 0x80 in hexadecimal.
+---------------+
PTag |7 6 5 4 3 2 1 0|
+---------------+
Bit 7 -- Always one
Bit 6 -- New packet format if set
PGP 2.6.x only uses old format packets. Thus, software that
interoperates with those versions of PGP must only use old format
packets. If interoperability is not an issue, either format may be
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 backward-compatible.
Old format packets contain:
Bits 5-2 -- content tag
Bits 1-0 - length-type
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RFC 2440 OpenPGP Message Format November 1998
New format packets contain:
Bits 5-0 -- content tag
4.2.1. Old-Format Packet Lengths
The meaning of the length-type in old-format packets is:
0 - The packet has a one-octet length. The header is 2 octets long.
1 - The packet has a two-octet length. The header is 3 octets long.
2 - The packet has a four-octet length. The header is 5 octets long.
3 - The packet is of indeterminate length. The header is 1 octet
long, and the implementation must determine how long the packet
is. If the packet is in a file, this means that the packet
extends until the end of the file. In general, an implementation
SHOULD NOT use indeterminate length packets except where the end
of the data will be clear from the context, and even then it is
better to use a definite length, or a new-format header. The
new-format headers described below have a mechanism for precisely
encoding data of indeterminate length.
4.2.2. New-Format Packet Lengths
New format packets have four possible ways of encoding length:
1. A one-octet Body Length header encodes packet lengths of up to
191 octets.
2. A two-octet Body Length header encodes packet lengths of 192 to
8383 octets.
3. A five-octet Body Length header encodes packet lengths of up to
4,294,967,295 (0xFFFFFFFF) octets in length. (This actually
encodes a four-octet scalar number.)
4. When the length of the packet body is not known in advance by the
issuer, Partial Body Length headers encode a packet of
indeterminate length, effectively making it a stream.
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RFC 2440 OpenPGP Message Format November 1998
4.2.2.1. One-Octet Lengths
A one-octet Body Length header encodes a length of from 0 to 191
octets. This type of length header is recognized because the one
octet value is less than 192. The body length is equal to:
bodyLen = 1st_octet;
4.2.2.2. Two-Octet Lengths
A two-octet Body Length header encodes a length of from 192 to 8383
octets. It is recognized because its first octet is in the range 192
to 223. The body length is equal to:
bodyLen = ((1st_octet - 192) << 8) + (2nd_octet) + 192
4.2.2.3. Five-Octet Lengths
A five-octet Body Length header consists of a single octet holding
the value 255, followed by a four-octet scalar. The body length is
equal to:
bodyLen = (2nd_octet << 24) | (3rd_octet << 16) |
(4th_octet << 8) | 5th_octet
4.2.2.4. Partial Body Lengths
A Partial Body Length header is one octet long and encodes the length
of only part of the data packet. This length is a power of 2, from 1
to 1,073,741,824 (2 to the 30th power). It is recognized by its one
octet value that is greater than or equal to 224, and less than 255.
The partial body length is equal to:
partialBodyLen = 1 << (1st_octet & 0x1f);
Each Partial Body Length header is followed by a portion of the
packet body data. The Partial Body Length header specifies this
portion's length. Another length header (of one of the three types --
one octet, two-octet, or partial) follows that portion. The last
length header in the packet MUST NOT be a partial Body Length header.
Partial Body Length headers may only be used for the non-final parts
of the packet.
4.2.3. Packet Length Examples
These examples show ways that new-format packets might encode the
packet lengths.
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RFC 2440 OpenPGP Message Format November 1998
A packet with length 100 may have its length encoded in one octet:
0x64. This is followed by 100 octets of data.
A packet with length 1723 may have its length coded in two octets:
0xC5, 0xFB. This header is followed by the 1723 octets of data.
A packet with length 100000 may have its length encoded in five
octets: 0xFF, 0x00, 0x01, 0x86, 0xA0.
It might also be encoded in the following octet stream: 0xEF, first
32768 octets of data; 0xE1, next two octets of data; 0xE0, next one
octet of data; 0xF0, next 65536 octets of data; 0xC5, 0xDD, last 1693
octets of data. This is just one possible encoding, and many
variations are possible on the size of the Partial Body Length
headers, as long as a regular Body Length header encodes the last
portion of the data. Note also that the last Body Length header can
be a zero-length header.
An implementation MAY use Partial Body Lengths for data packets, be
they literal, compressed, or encrypted. The first partial length MUST
be at least 512 octets long. Partial Body Lengths MUST NOT be used
for any other packet types.
Please note that in all of these explanations, the total length of
the packet is the length of the header(s) plus the length of the
body.
4.3. Packet Tags
The packet tag denotes what type of packet the body holds. Note that
old format headers can only have tags less than 16, whereas new
format headers can have tags as great as 63. The defined tags (in
decimal) are:
0 -- Reserved - a packet tag must not have this value
1 -- Public-Key Encrypted Session Key Packet
2 -- Signature Packet
3 -- Symmetric-Key Encrypted Session Key Packet
4 -- One-Pass Signature Packet
5 -- Secret Key Packet
6 -- Public Key Packet
7 -- Secret Subkey Packet
8 -- Compressed Data Packet
9 -- Symmetrically Encrypted Data Packet
10 -- Marker Packet
11 -- Literal Data Packet
12 -- Trust Packet
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RFC 2440 OpenPGP Message Format November 1998
13 -- User ID Packet
14 -- Public Subkey Packet
60 to 63 -- Private or Experimental Values
5. Packet Types
5.1. Public-Key Encrypted Session Key Packets (Tag 1)
A Public-Key Encrypted Session Key packet holds the session key used
to encrypt a message. Zero or more Encrypted Session Key packets
(either Public-Key or Symmetric-Key) may precede a Symmetrically
Encrypted Data Packet, which holds an encrypted message. The message
is encrypted with the session key, and the session key is itself
encrypted and stored in the Encrypted Session Key packet(s). The
Symmetrically Encrypted Data Packet is preceded by one Public-Key
Encrypted Session Key packet for each OpenPGP key to which the
message is encrypted. The recipient of the message finds a session
key that is encrypted to their public key, decrypts the session key,
and then uses the session key to decrypt the message.
The body of this packet consists of:
- A one-octet number giving the version number of the packet type.
The currently defined value for packet version is 3. An
implementation should accept, but not generate a version of 2,
which is equivalent to V3 in all other respects.
- An eight-octet number that gives the key ID of the public key
that the session key is encrypted to.
- A one-octet number giving the public key algorithm used.
- A string of octets that is the encrypted session key. This string
takes up the remainder of the packet, and its contents are
dependent on the public key algorithm used.
Algorithm Specific Fields for RSA encryption
- multiprecision integer (MPI) of RSA encrypted value m**e mod n.
Algorithm Specific Fields for Elgamal encryption:
- MPI of Elgamal (Diffie-Hellman) value g**k mod p.
- MPI of Elgamal (Diffie-Hellman) value m * y**k mod p.
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The value "m" in the above formulas is derived from the session key
as follows. First the session key is prefixed with a one-octet
algorithm identifier that specifies the symmetric encryption
algorithm used to encrypt the following Symmetrically Encrypted Data
Packet. Then a two-octet checksum is appended which is equal to the
sum of the preceding session key octets, not including the algorithm
identifier, modulo 65536. This value is then padded as described in
PKCS-1 block type 02 [RFC2313] to form the "m" value used in the
formulas above.
Note that when an implementation forms several PKESKs with one
session key, forming a message that can be decrypted by several keys,
the implementation MUST make new PKCS-1 padding for each key.
An implementation MAY accept or use a Key ID of zero as a "wild card"
or "speculative" Key ID. In this case, the receiving 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)
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 of text, and a signature that is a certification of a user ID.
Two versions of signature packets are defined. Version 3 provides
basic signature information, while version 4 provides an expandable
format with subpackets that can specify more information about the
signature. PGP 2.6.x only accepts version 3 signatures.
Implementations MUST accept V3 signatures. Implementations SHOULD
generate V4 signatures. Implementations MAY generate a V3 signature
that can be verified by PGP 2.6.x.
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. Signature Types
There are a number of possible meanings for a signature, which are
specified in a signature type octet in any given signature. These
meanings are:
0x00: Signature of a binary document.
Typically, this means the signer owns it, created it, or
certifies that it has not been modified.
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0x01: Signature of a canonical text document.
Typically, this means the signer owns it, created it, or
certifies that it has not been modified. The signature is
calculated over the text data with its line endings converted
to <CR><LF> and trailing blanks removed.
0x02: Standalone signature.
This signature is a signature of only its own subpacket
contents. It is calculated identically to a signature over a
zero-length binary document. Note that it doesn't make sense to
have a V3 standalone signature.
0x10: Generic certification of a User ID and Public Key packet.
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 in fact the person described by the user
ID. Note that all PGP "key signatures" are this type of
certification.
0x11: Persona certification of a User ID and Public Key packet.
The issuer of this certification has not done any verification
of the claim that the owner of this key is the user ID
specified.
0x12: Casual certification of a User ID and Public Key packet.
The issuer of this certification has done some casual
verification of the claim of identity.
0x13: Positive certification of a User ID and Public Key packet.
The issuer of this certification has done substantial
verification of the claim of identity.
Please note that the vagueness of these certification claims 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. These classifications allow
a certification authority to issue fine-grained claims.
0x18: Subkey Binding Signature
This signature is a statement by the top-level signing key
indicates that it owns the subkey. This signature is calculated
directly on the subkey itself, not on any User ID or other
packets.
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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 that 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: Key revocation signature
The signature is calculated directly on the key being revoked.
A revoked key is not to be used. Only revocation signatures by
the key being revoked, or by an authorized revocation key,
should be considered valid revocation signatures.
0x28: Subkey revocation signature
The signature is calculated directly on the subkey being
revoked. A revoked subkey is not to be used. Only revocation
signatures by the top-level signature key that is bound to this
subkey, or by an authorized revocation key, should be
considered valid revocation signatures.
0x30: Certification revocation signature
This signature revokes an earlier user ID certification
signature (signature class 0x10 through 0x13). It should be
issued by the same key that issued the revoked signature or an
authorized revocation key The signature should have a later
creation date than the signature it revokes.
0x40: Timestamp signature.
This signature is only meaningful for the timestamp contained
in it.
5.2.2. Version 3 Signature Packet Format
The body of a version 3 Signature Packet contains:
- One-octet version number (3).
- One-octet length of following hashed material. MUST be 5.
- One-octet signature type.
- Four-octet creation time.
- Eight-octet key ID of signer.
- One-octet public key algorithm.
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- One-octet hash algorithm.
- Two-octet field holding left 16 bits of signed hash value.
- One or more multi-precision integers comprising the signature.
This portion is algorithm specific, as described below.
The data being signed is hashed, and then the signature type and
creation time from the signature packet are hashed (5 additional
octets). The resulting hash value is used in the signature
algorithm. The high 16 bits (first two octets) of the hash are
included in the signature packet to provide a quick test to reject
some invalid signatures.
Algorithm Specific Fields for RSA signatures:
- multiprecision integer (MPI) of RSA signature value m**d.
Algorithm Specific Fields for DSA signatures:
- MPI of DSA value r.
- MPI of DSA value s.
The signature calculation is based on a hash of the signed data, as
described above. The details of the calculation are different for
DSA signature than for RSA signatures.
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
DigestInfo, and then padded using PKCS-1 block type 01 [RFC2313].
This 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 included in the structure. The hexadecimal representations
for the currently defined hash algorithms are:
- MD2: 0x2A, 0x86, 0x48, 0x86, 0xF7, 0x0D, 0x02, 0x02
- MD5: 0x2A, 0x86, 0x48, 0x86, 0xF7, 0x0D, 0x02, 0x05
- RIPEMD-160: 0x2B, 0x24, 0x03, 0x02, 0x01
- SHA-1: 0x2B, 0x0E, 0x03, 0x02, 0x1A
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The ASN.1 OIDs are:
- MD2: 1.2.840.113549.2.2
- MD5: 1.2.840.113549.2.5
- RIPEMD-160: 1.3.36.3.2.1
- SHA-1: 1.3.14.3.2.26
The full hash prefixes for these are:
MD2: 0x30, 0x20, 0x30, 0x0C, 0x06, 0x08, 0x2A, 0x86,
0x48, 0x86, 0xF7, 0x0D, 0x02, 0x02, 0x05, 0x00,
0x04, 0x10
MD5: 0x30, 0x20, 0x30, 0x0C, 0x06, 0x08, 0x2A, 0x86,
0x48, 0x86, 0xF7, 0x0D, 0x02, 0x05, 0x05, 0x00,
0x04, 0x10
RIPEMD-160: 0x30, 0x21, 0x30, 0x09, 0x06, 0x05, 0x2B, 0x24,
0x03, 0x02, 0x01, 0x05, 0x00, 0x04, 0x14
SHA-1: 0x30, 0x21, 0x30, 0x09, 0x06, 0x05, 0x2b, 0x0E,
0x03, 0x02, 0x1A, 0x05, 0x00, 0x04, 0x14
DSA signatures MUST 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 hash function result is treated as a 160 bit number and used
directly in the DSA signature algorithm.
5.2.3. Version 4 Signature Packet Format
The body of a version 4 Signature Packet contains:
- One-octet version number (4).
- One-octet signature type.
- One-octet public key algorithm.
- One-octet hash algorithm.
- Two-octet scalar octet count for following hashed subpacket
data. Note that this is the length in octets of all of the hashed
subpackets; a pointer incremented by this number will skip over
the hashed subpackets.
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- Hashed subpacket data. (zero or more subpackets)
- Two-octet scalar octet count for following unhashed subpacket
data. Note that this is the length in octets of all of the
unhashed subpackets; a pointer incremented by this number will
skip over the unhashed subpackets.
- Unhashed subpacket data. (zero or more subpackets)
- Two-octet field holding left 16 bits of signed hash value.
- One or more multi-precision integers comprising the signature.
This portion is algorithm specific, as described above.
The data being signed is hashed, and then the signature data from the
version number through the hashed subpacket data (inclusive) is
hashed. The 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 some invalid signatures.
There are two fields consisting of signature subpackets. The first
field is hashed with the rest of the signature data, while the second
is unhashed. The second set of subpackets is not cryptographically
protected by the signature and should include only advisory
information.
The algorithms for converting the hash function result to a signature
are described in a section below.
5.2.3.1. Signature Subpacket Specification
The subpacket fields consist of zero or more signature subpackets.
Each set of subpackets is preceded by a two-octet scalar count of the
length of the set of subpackets.
Each subpacket consists of a subpacket header and a body. The header
consists of:
- the subpacket length (1, 2, or 5 octets)
- the subpacket type (1 octet)
and is followed by the subpacket specific data.
The length includes the type octet but not this length. Its format is
similar to the "new" format packet header lengths, but cannot have
partial body lengths. That is:
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if the 1st octet < 192, then
lengthOfLength = 1
subpacketLen = 1st_octet
if the 1st octet >= 192 and < 255, then
lengthOfLength = 2
subpacketLen = ((1st_octet - 192) << 8) + (2nd_octet) + 192
if the 1st octet = 255, then
lengthOfLength = 5
subpacket length = [four-octet scalar starting at 2nd_octet]
The value of the subpacket type octet may be:
2 = signature creation time
3 = signature expiration time
4 = exportable certification
5 = trust signature
6 = regular expression
7 = revocable
9 = key expiration time
10 = placeholder for backward compatibility
11 = preferred symmetric algorithms
12 = revocation key
16 = issuer key ID
20 = notation data
21 = preferred hash algorithms
22 = preferred compression algorithms
23 = key server preferences
24 = preferred key server
25 = primary user id
26 = policy URL
27 = key flags
28 = signer's user id
29 = reason for revocation
100 to 110 = internal or user-defined
An implementation SHOULD ignore any subpacket of a type that it does
not recognize.
Bit 7 of the subpacket type is the "critical" bit. If set, it
denotes that the subpacket is one that is critical for the evaluator
of the signature to recognize. If a subpacket is encountered that is
marked critical but is unknown to the evaluating software, the
evaluator SHOULD consider the signature to be in error.
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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.
Implementations SHOULD implement "preferences".
5.2.3.2. Signature Subpacket Types
A number of subpackets are currently defined. Some subpackets 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 id
certification made by the key itself. Note that a key may have more
than one user id, and thus may have more than one self-signature, and
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, each user ID 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
subpackets as narrowly as possible. For example, suppose a key has
two usernames, Alice and Bob. Suppose that Alice prefers the
symmetric algorithm CAST5, and Bob prefers IDEA or Triple-DES. If the
software 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 algorithm is IDEA. If the key is located by key id,
then algorithm of the default user id of the key provides the default
symmetric algorithm.
A subpacket may be found either in the hashed or unhashed subpacket
sections of a signature. If a subpacket is not hashed, then the
information in it cannot be considered definitive because it is not
part of the signature proper.
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5.2.3.3. Signature creation time
(4 octet time field)
The time the signature was made.
MUST be present in the hashed area.
5.2.3.4. Issuer
(8 octet key ID)
The OpenPGP key ID of the key issuing the signature.
5.2.3.5. Key expiration time
(4 octet time field)
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.
5.2.3.6. Preferred symmetric algorithms
(sequence of one-octet values)
Symmetric algorithm numbers that indicate which algorithms the key
holder prefers to use. The subpacket body is an ordered list of
octets with the most preferred listed first. It is assumed that only
algorithms listed are supported by the recipient's software.
Algorithm numbers in section 9. This is only found on a self-
signature.
5.2.3.7. Preferred hash algorithms
(array of one-octet values)
Message digest algorithm numbers that indicate which algorithms the
key 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.
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5.2.3.8. Preferred compression algorithms
(array of one-octet values)
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 might have
no compression software in that implementation. This is only found on
a self-signature.
5.2.3.9. Signature expiration time
(4 octet time field)
The validity period of the signature. This is the number of seconds
after the signature creation time that the signature expires. If this
is not present or has a value of zero, it never expires.
5.2.3.10. Exportable Certification
(1 octet of exportability, 0 for not, 1 for exportable)
This subpacket denotes whether a certification signature is
"exportable", to be used by other users than the signature's issuer.
The packet body contains a boolean flag indicating whether the
signature is exportable. If this packet is not present, the
certification is exportable; it is equivalent to a flag containing a
1.
Non-exportable, or "local", certifications are signatures made by a
user to mark a key as valid within that user's implementation only.
Thus, when an implementation prepares a user's copy of a key for
transport to another user (this is the process of "exporting" the
key), any local certification signatures are deleted from the key.
The receiver of a transported key "imports" it, and likewise trims
any local certifications. In normal operation, there won't be any,
assuming the import is performed on an exported key. However, there
are instances where this can reasonably happen. For example, if an
implementation allows keys to be imported from a key database in
addition to an exported key, then this situation can arise.
Some implementations do not represent the interest of a single user
(for example, a key server). Such implementations always trim local
certifications from any key they handle.
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5.2.3.11. Revocable
(1 octet of revocability, 0 for not, 1 for revocable)
Signature's revocability status. Packet body contains a boolean flag
indicating whether the signature is revocable. Signatures that are
not revocable have 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 revocable.
5.2.3.12. Trust signature
(1 octet "level" (depth), 1 octet of trust amount)
Signer asserts that the key is not only valid, but also trustworthy,
at the specified level. Level 0 has the same meaning as an ordinary
validity signature. Level 1 means that 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.
5.2.3.13. Regular expression
(null-terminated regular expression)
Used in conjunction with trust signature packets (of level > 0) to
limit the scope of trust that is extended. Only signatures by the
target key on user IDs that match the regular expression in the body
of this packet have trust extended by the trust signature subpacket.
The regular expression uses the same syntax as the Henry Spencer's
"almost public domain" regular expression package. A description of
the syntax is found in a section below.
5.2.3.14. Revocation key
(1 octet of class, 1 octet of algid, 20 octets of fingerprint)
Authorizes the specified key to issue revocation signatures for 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
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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 that need to be exported.
5.2.3.15. Notation Data
(4 octets of flags, 2 octets of name length (M),
2 octets of value length (N),
M octets of name data,
N octets of value data)
This subpacket describes a "notation" on the signature that the
issuer wishes to make. The notation has a name and a value, each of
which are strings of octets. There may be more than one notation in a
signature. Notations can be used for any extension the issuer of the
signature cares to make. The "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
from one person to another, and has no
meaning to software.
Other octets: none.
5.2.3.16. Key server preferences
(N octets of flags)
This is a list of flags that indicate preferences that the key holder
has about how the key is handled on a key server. All undefined flags
MUST be zero.
First octet: 0x80 = No-modify
the key holder requests that this key only be modified or updated
by the key holder or an administrator of the key server.
This is found only on a self-signature.
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5.2.3.17. Preferred key server
(String)
This is a URL of a key server that the key holder prefers be used for
updates. Note that keys with multiple user ids can have a preferred
key server for each user id. 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.
5.2.3.18. Primary user id
(1 octet, boolean)
This is a flag in a user id's self signature that states whether this
user id is the main user id for this key. It is reasonable for an
implementation to resolve ambiguities in preferences, etc. by
referring to the primary user id. If this 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.
5.2.3.19. Policy URL
(String)
This subpacket contains a URL of a document that describes the policy
that the signature was issued under.
5.2.3.20. Key Flags
(Octet string)
This subpacket contains a list of binary flags that hold information
about a key. It is a string of octets, and an implementation MUST NOT
assume a fixed size. This is so it can grow over time. If a list is
shorter than an implementation expects, the unstated flags are
considered to be zero. The defined flags are:
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.
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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 possession
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.
5.2.3.21. 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.
5.2.3.22. Reason for Revocation
(1 octet of revocation code, N octets of reason string)
This subpacket is used only in key revocation and certification
revocation signatures. It describes the reason why the key or
certificate was revoked.
The first octet contains a machine-readable code that denotes the
reason for the revocation:
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0x00 - No reason specified (key revocations or cert revocations)
0x01 - Key is superceded (key revocations)
0x02 - Key material has been compromised (key revocations)
0x03 - Key is no longer used (key revocations)
0x20 - User id information is no longer valid (cert revocations)
Following the revocation code is a string of octets which gives
information about the reason for revocation in human-readable form
(UTF-8). The string may be null, that is, of zero length. The length
of the subpacket is the length of the reason string plus one.
5.2.4. Computing Signatures
All signatures are formed by producing a hash over the signature
data, and then using the resulting hash in the signature algorithm.
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.
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) hashes the user id
being bound to the key into the hash context after the above data. A
V3 certification hashes the contents of the name packet, without any
header. A V4 certification hashes the constant 0xb4 (which is an
old-style packet header 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.
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V4 signatures also hash in a final trailer of six octets: the version
of the signature packet, i.e. 0x04; 0xFF; a four-octet, big-endian
number that is the length of the hashed data from the signature
packet (note that this number does not include these final six
octets.
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.
5.2.4.1. Subpacket Hints
An implementation SHOULD put the two mandatory subpackets, creation
time and issuer, as the first subpackets in the subpacket list,
simply to make it easier for the implementer to find them.
It is certainly possible for a signature to contain conflicting
information in subpackets. For example, a signature may contain
multiple copies of a preference or multiple expiration times. In most
cases, an implementation SHOULD use the last subpacket in the
signature, but MAY use any conflict resolution scheme that makes more
sense. Please note that we are intentionally leaving conflict
resolution to the implementer; most conflicts are simply syntax
errors, and the wishy-washy language here allows a receiver to be
generous in what they accept, while putting pressure on a creator to
be stingy in what they generate.
Some apparent conflicts may actually make sense -- for example,
suppose a keyholder has an V3 key and a V4 key that share the same
RSA key material. Either of these keys can verify a signature created
by the other, and it may be reasonable for a signature to contain an
issuer subpacket for each key, as a way of explicitly tying those
keys to the signature.
5.3. Symmetric-Key Encrypted Session-Key Packets (Tag 3)
The Symmetric-Key Encrypted Session Key packet holds the symmetric-
key encryption of a session key used to encrypt a message. Zero or
more Encrypted Session Key packets and/or Symmetric-Key Encrypted
Session Key packets may precede a Symmetrically Encrypted Data Packet
that holds an encrypted message. The message is encrypted with a
session key, and the session key is itself encrypted and stored in
the Encrypted Session Key packet or the Symmetric-Key Encrypted
Session Key packet.
If the Symmetrically Encrypted Data Packet is preceded by one or more
Symmetric-Key Encrypted Session Key packets, each specifies a
passphrase that may be used to decrypt the message. This allows a
Callas, et. al. Standards Track [Page 32]
RFC 2440 OpenPGP Message Format November 1998
message to be encrypted to a number of public keys, and also to one
or more pass phrases. This packet type is new, and is not generated
by PGP 2.x or PGP 5.0.
The body of this packet consists of:
- A one-octet version number. The only currently defined version
is 4.
- A one-octet number describing the symmetric algorithm used.
- A string-to-key (S2K) specifier, length as defined above.
- Optionally, the encrypted session key itself, which is decrypted
with the string-to-key object.
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
algorithm applied to the passphrase produces the session key for
decrypting the file, using the symmetric cipher algorithm from the
Symmetric-Key Encrypted Session Key packet.
If the encrypted session key is present, the result of applying the
S2K algorithm to the passphrase is used to decrypt just that
encrypted session key field, using CFB mode with an IV of all zeros.
The decryption result consists of a one-octet algorithm identifier
that specifies the symmetric-key encryption algorithm used to encrypt
the following Symmetrically Encrypted Data Packet, followed by the
session key octets themselves.
Note: because an all-zero IV is used for this decryption, the S2K
specifier MUST use a salt value, either a Salted S2K or an Iterated-
Salted S2K. The salt value will insure that the decryption key is
not repeated even if the passphrase is reused.
5.4. One-Pass Signature Packets (Tag 4)
The One-Pass Signature packet precedes the signed data and contains
enough information to allow the receiver to begin calculating any
hashes needed to verify the signature. It allows the Signature
Packet to be placed at the end of the message, so that the signer can
compute the entire signed message in one pass.
A One-Pass Signature does not interoperate with PGP 2.6.x or earlier.
The body of this packet consists of:
Callas, et. al. Standards Track [Page 33]
RFC 2440 OpenPGP Message Format November 1998
- A one-octet version number. The current version is 3.
- A one-octet signature type. Signature types are described in
section 5.2.1.
- A one-octet number describing the hash algorithm used.
- A one-octet number describing the public key algorithm used.
- An eight-octet number holding the key ID of the signing key.
- A one-octet number holding a flag showing whether the signature
is nested. A zero value indicates that the next packet is
another One-Pass Signature packet that describes another
signature to be applied to the same message data.
Note that if a message contains more than one one-pass signature,
then the signature packets bracket the message; that is, the first
signature packet after the message corresponds to the last one-pass
packet and the final signature packet corresponds to the first one-
pass packet.
5.5. Key Material Packet
A key material packet contains all the information about a public or
private key. There are four variants of this packet type, and two
major versions. Consequently, this section is complex.
5.5.1. Key Packet Variants
5.5.1.1. Public Key Packet (Tag 6)
A Public Key packet starts a series of packets that forms an OpenPGP
key (sometimes called an OpenPGP certificate).
5.5.1.2. Public Subkey Packet (Tag 14)
A Public Subkey packet (tag 14) has exactly the same format as a
Public Key packet, but denotes a subkey. One or more subkeys may be
associated with a top-level key. By convention, the top-level key
provides signature services, and the subkeys provide encryption
services.
Note: in PGP 2.6.x, tag 14 was intended to indicate a comment packet.
This tag was selected for reuse because no previous version of PGP
ever emitted comment packets but they did properly ignore them.
Public Subkey packets are ignored by PGP 2.6.x and do not cause it to
fail, providing a limited degree of backward compatibility.
Callas, et. al. Standards Track [Page 34]
RFC 2440 OpenPGP Message Format November 1998
5.5.1.3. Secret Key Packet (Tag 5)
A Secret Key packet contains all the information that is found in a
Public Key packet, including the public key material, but also
includes the secret key material after all the public key fields.
5.5.1.4. Secret Subkey Packet (Tag 7)
A Secret Subkey packet (tag 7) is the subkey analog of the Secret Key
packet, and has exactly the same format.
5.5.2. Public Key Packet Formats
There are two versions of key-material packets. Version 3 packets
were first generated by PGP 2.6. Version 2 packets are identical in
format to Version 3 packets, but are generated by PGP 2.5 or before.
V2 packets are deprecated and they MUST NOT be generated. PGP 5.0
introduced version 4 packets, with new fields and semantics. PGP
2.6.x will not accept key-material packets with versions greater than
3.
OpenPGP implementations SHOULD create keys with version 4 format. An
implementation MAY generate a V3 key to ensure interoperability with
old software; note, however, that V4 keys correct some security
deficiencies in V3 keys. These deficiencies are described below. An
implementation MUST NOT create a V3 key with a public key algorithm
other than RSA.
A version 3 public key or public subkey packet contains:
- A one-octet version number (3).
- A four-octet number denoting the time that the key was created.
- A two-octet number denoting the time in days that this key is
valid. If this number is zero, then it does not expire.
- A one-octet number denoting the public key algorithm of this key
- A series of multi-precision integers comprising the key
material:
- a multiprecision integer (MPI) of RSA public modulus n;
- an MPI of RSA public encryption exponent e.
Callas, et. al. Standards Track [Page 35]
RFC 2440 OpenPGP Message Format November 1998
V3 keys SHOULD only be used for backward compatibility because of
three weaknesses in them. First, it is relatively easy to construct a
V3 key that has the same key ID as any other key because the key ID
is simply the low 64 bits of the public modulus. Secondly, because
the fingerprint of a V3 key hashes the key material, but not its
length, which increases the opportunity for fingerprint collisions.
Third, there are minor weaknesses in the MD5 hash algorithm that make
developers prefer other algorithms. See below for a fuller discussion
of key IDs and fingerprints.
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 packet. In addition, fingerprints of version 4 keys are
calculated differently from version 3 keys, as described in section
"Enhanced Key Formats."
A version 4 packet contains:
- A one-octet version number (4).
- 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 series of multi-precision integers comprising the key
material. This algorithm-specific portion is:
Algorithm Specific Fields for RSA public keys:
- multiprecision integer (MPI) of RSA public modulus n;
- MPI of RSA public encryption exponent e.
Algorithm Specific Fields for DSA public keys:
- MPI of DSA prime p;
- MPI of DSA group order q (q is a prime divisor of p-1);
- MPI of DSA group generator g;
- MPI of DSA public key value y (= g**x where x is secret).
Algorithm Specific Fields for Elgamal public keys:
- MPI of Elgamal prime p;
- MPI of Elgamal group generator g;
Callas, et. al. Standards Track [Page 36]
RFC 2440 OpenPGP Message Format November 1998
- MPI of Elgamal public key value y (= g**x where x is
secret).
5.5.3. Secret Key Packet Formats
The Secret Key and Secret Subkey packets contain all the data of the
Public Key and Public Subkey packets, with additional algorithm-
specific secret key data appended, in encrypted form.
The packet contains:
- A Public Key or Public Subkey packet, as described above
- One octet indicating string-to-key usage conventions. 0
indicates that the secret key data is not encrypted. 255
indicates that a string-to-key specifier is being given. Any
other value is a symmetric-key encryption algorithm specifier.
- [Optional] If string-to-key usage octet was 255, a one-octet
symmetric encryption algorithm.
- [Optional] If string-to-key usage octet was 255, a string-to-key
specifier. The length of the string-to-key specifier is implied
by its type, as described above.
- [Optional] If secret data is encrypted, eight-octet Initial
Vector (IV).
- Encrypted multi-precision integers comprising the secret key
data. These algorithm-specific fields are as described below.
- Two-octet checksum of the plaintext of the algorithm-specific
portion (sum of all octets, mod 65536).
Algorithm Specific Fields for RSA secret keys:
- multiprecision integer (MPI) of RSA secret exponent d.
- MPI of RSA secret prime value p.
- MPI of RSA secret prime value q (p < q).
- MPI of u, the multiplicative inverse of p, mod q.
Algorithm Specific Fields for DSA secret keys:
- MPI of DSA secret exponent x.
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RFC 2440 OpenPGP Message Format November 1998
Algorithm Specific Fields for Elgamal secret keys:
- MPI of Elgamal secret exponent x.
Secret MPI values can be encrypted using a passphrase. If a string-
to-key specifier is given, that describes the algorithm for
converting the passphrase to a key, else a simple MD5 hash of the
passphrase is used. Implementations SHOULD use a string-to-key
specifier; the simple hash is for backward compatibility. The cipher
for encrypting the MPIs is specified in the secret key packet.
Encryption/decryption of the secret data is done in CFB mode using
the key created from the passphrase and the Initial Vector from the
packet. A different mode is used with V3 keys (which are only RSA)
than with other key formats. With V3 keys, the MPI bit count prefix
(i.e., the first two octets) is not encrypted. Only the MPI non-
prefix data is encrypted. Furthermore, the CFB state is
resynchronized at the beginning of each new MPI value, so that the
CFB block boundary is aligned with the start of the MPI data.
With V4 keys, a simpler method is used. All secret MPI values are
encrypted in CFB mode, including the MPI bitcount prefix.
The 16-bit checksum that follows the algorithm-specific portion is
the algebraic sum, mod 65536, of the plaintext of all the algorithm-
specific octets (including MPI prefix and data). With V3 keys, the
checksum is stored in the clear. With V4 keys, the checksum is
encrypted like the algorithm-specific data. This value is used to
check that the passphrase was correct.
5.6. Compressed Data Packet (Tag 8)
The Compressed Data packet contains compressed data. Typically, this
packet is found as the contents of an encrypted packet, or following
a Signature or One-Pass Signature packet, and contains literal data
packets.
The body of this packet consists of:
- One octet that gives the algorithm used to compress the packet.
- The remainder of the packet is compressed data.
A Compressed Data Packet's body contains an block that compresses
some set of packets. See section "Packet Composition" for details on
how messages are formed.
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RFC 2440 OpenPGP Message Format November 1998
ZIP-compressed packets are compressed with raw RFC 1951 DEFLATE
blocks. Note that PGP V2.6 uses 13 bits of compression. If an
implementation uses more bits of compression, PGP V2.6 cannot
decompress it.
ZLIB-compressed packets are compressed with RFC 1950 ZLIB-style
blocks.
5.7. Symmetrically Encrypted Data Packet (Tag 9)
The Symmetrically Encrypted Data packet contains data encrypted with
a symmetric-key algorithm. When it has been decrypted, it will
typically contain other packets (often literal data packets or
compressed data packets).
The body of this packet consists of:
- Encrypted data, the output of the selected symmetric-key cipher
operating in PGP's variant of Cipher Feedback (CFB) mode.
The symmetric cipher used may be specified in an Public-Key or
Symmetric-Key Encrypted Session Key packet that precedes the
Symmetrically Encrypted Data Packet. In that case, the cipher
algorithm octet is prefixed to the session key before it is
encrypted. 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 the passphrase.
The data is encrypted in CFB mode, with a CFB shift size equal to the
cipher's block size. The Initial Vector (IV) is specified as all
zeros. Instead of using an IV, OpenPGP prefixes a 10-octet string to
the 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,
respectively. After encrypting the first 10 octets, the CFB state is
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 boundary is reset.
The repetition of 16 bits in the 80 bits of random data prefixed to
the message allows the receiver to immediately check whether the
session key is incorrect.
5.8. Marker Packet (Obsolete Literal Packet) (Tag 10)
An experimental version of PGP used this packet as the Literal
packet, 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 as the Marker packet.
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RFC 2440 OpenPGP Message Format November 1998
The body of this packet consists of:
- The three octets 0x50, 0x47, 0x50 (which spell "PGP" in UTF-8).
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 order to cause that version to report that newer software is
necessary to process the message.
5.9. Literal Data Packet (Tag 11)
A Literal Data packet contains the body of a message; data that is
not to be further interpreted.
The body of this packet consists of:
- A one-octet field that describes how the data is formatted.
If it is a 'b' (0x62), then the literal packet contains binary data.
If it is a 't' (0x74), then it contains text data, and thus may need
line ends converted to local form, or other text-mode changes. RFC
1991 also defined a value of 'l' as a 'local' mode for machine-local
conversions. This use is now deprecated.
- File name as a string (one-octet length, followed by file name),
if the encrypted data should be saved as a file.
If the special name "_CONSOLE" is used, the message is considered to
be "for your eyes only". This advises that the message data is
unusually sensitive, and the receiving program should process it more
carefully, perhaps avoiding storing the received data to disk, for
example.
- 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 present time.
- The remainder of the packet is literal data.
Text data is stored with <CR><LF> text endings (i.e. network-normal
line endings). These should be converted to native line endings by
the receiving software.
5.10. Trust Packet (Tag 12)
The Trust packet is used only within keyrings and is not normally
exported. Trust packets contain data that record the user's
specifications of which key holders are trustworthy introducers,
Callas, et. al. Standards Track [Page 40]
RFC 2440 OpenPGP Message Format November 1998
along with other information that implementing software uses for
trust information.
Trust packets SHOULD NOT be emitted to output streams that are
transferred to other users, and they SHOULD be ignored on any input
other than local keyring files.
5.11. User ID Packet (Tag 13)
A User ID packet consists of data that is intended to represent the
name and email address of the key holder. By convention, it includes
an RFC 822 mail name, but there are no restrictions on its content.
The packet length in the header specifies the length of the user id.
If it is text, it is encoded in UTF-8.
6. Radix-64 Conversions
As stated in the introduction, OpenPGP's underlying native
representation for objects is a stream of arbitrary octets, and some
systems desire these objects to be immune to damage caused by
character set translation, data conversions, etc.
In principle, any printable encoding scheme that met the requirements
of the unsafe channel would suffice, since it would not change the
underlying binary bit streams of the native OpenPGP data structures.
The OpenPGP standard specifies one such printable encoding scheme to
ensure interoperability.
OpenPGP'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 [RFC2231,
Section 6.8]. An OpenPGP 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. A sample implementation of this algorithm is in the next
section.
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.
Callas, et. al. Standards Track [Page 41]
RFC 2440 OpenPGP Message Format November 1998
6.1. An Implementation of the CRC-24 in "C"
#define CRC24_INIT 0xb704ceL
#define CRC24_POLY 0x1864cfbL
typedef long crc24;
crc24 crc_octets(unsigned char *octets, size_t len)
{
crc24 crc = CRC24_INIT;
int i;
while (len--) {
crc ^= (*octets++) << 16;
for (i = 0; i < 8; i++) {
crc <<= 1;
if (crc & 0x1000000)
crc ^= CRC24_POLY;
}
}
return crc & 0xffffffL;
}
6.2. Forming ASCII Armor
When OpenPGP encodes data into ASCII Armor, it puts specific headers
around the data, so OpenPGP can reconstruct the data later. OpenPGP
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:
Callas, et. al. Standards Track [Page 42]
RFC 2440 OpenPGP Message Format November 1998
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, OpenPGP/MIME signatures, and
natures following clearsigned messages. Note that PGP 2.x s BEGIN
PGP MESSAGE for detached signatures.
The Armor Headers are pairs of strings that can give the user or the
receiving OpenPGP implementation 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.
OpenPGP should consider improperly formatted Armor Headers to be
corruption of the ASCII Armor. Unknown keys should be reported to
the user, but OpenPGP should continue to process the message.
Currently defined Armor Header Keys are:
- "Version", that states the OpenPGP 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
Callas, et. al. Standards Track [Page 43]
RFC 2440 OpenPGP Message Format November 1998
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 sufficient.
- "Hash", a comma-separated list of hash algorithms used in this
message. This is used only in clear-signed messages.
- "Charset", a description of the character set that the plaintext
is in. Please note that OpenPGP defines text to be in UTF-8 by
default. An implementation will get best results by translating
into and out of UTF-8. However, there are many instances where
this is easier said than done. Also, there are communities of
users who have no need for UTF-8 because they are all happy with
a character set like ISO Latin-5 or a Japanese character set. In
such instances, an implementation MAY override the UTF-8 default
by using this header key. An implementation MAY implement this
key and any translations it cares to; an implementation MAY
ignore it and assume all text is UTF-8.
The MessageID SHOULD NOT appear unless it is in a multi-part
message. If it appears at all, it MUST be computed from the
finished (encrypted, signed, etc.) message in a deterministic
fashion, rather than contain a purely random value. This is to
allow the legitimate recipient 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 octet, and the eighth bit
will be the low-order bit in the first 8-bit octet, and so on.
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RFC 2440 OpenPGP Message Format November 1998
+--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:
1. The last data group has 24 bits (3 octets). No special
processing is needed.
2. 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.
3. 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.
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RFC 2440 OpenPGP Message Format November 1998
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 62
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 = =
Callas, et. al. Standards Track [Page 46]
RFC 2440 OpenPGP Message Format November 1998
6.6. Example of an ASCII Armored Message
-----BEGIN PGP MESSAGE-----
Version: OpenPrivacy 0.99
yDgBO22WxBHv7O8X7O/jygAEzol56iUKiXmV+XmpCtmpqQUKiQrFqclFqUDBovzS
vBSFjNSiVHsuAA==
=njUN
-----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,
- One 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 '-----BEGIN PGP
SIGNATURE-----' 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,
MD5 is used, an implementation MAY omit them for V2.x compatibility.
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 described below with the algorithm
IDs.
7.1. Dash-Escaped Text
The cleartext content of the message must also be dash-escaped.
Callas, et. al. Standards Track [Page 47]
RFC 2440 OpenPGP Message Format November 1998
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 followed 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), '