16 March 1998
Source:
ftp://ietf.org/internet-drafts/draft-ietf-openpgp-formats-01.txt
Thanks to Jim Gillogly
Network Working Group Jon Callas
Category: INTERNET-DRAFT Network Associates
draft-ietf-openpgp-formats-01.txt Lutz Donnerhacke
Expires Aug 1998 IN-Root-CA Individual Network e.V.
March 1997 Hal Finney
Network Associates
Rodney Thayer
Sable Technology
OP Formats - OpenPGP Message Format
draft-ietf-openpgp-formats-01.txt
Copyright 1998 by The Internet Society. All Rights Reserved.
Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas, and
its working groups. Note that other groups may also distribute working
documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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material or to cite them other than as "work in progress."
To view the entire list of current Internet-Drafts, please check the
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ftp.nis.garr.it (Southern Europe), munnari.oz.au (Pacific Rim),
ftp.ietf.org (US East Coast), or ftp.isi.edu (US West Coast).
Abstract
This document is maintained in order to publish all necessary
information needed to develop interoperable applications based on the
OP 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 storing and implementation questions albeit it is
necessary to avoid security flaws.
Open-PGP software uses a combination of strong public-key and
conventional 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 OP.
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Table of Contents
1. Introduction
1.1 Terms
2. General functions
2.1 Confidentiality via Encryption
2.2 Authentication via Digital signature
2.3 Compression
2.4 Conversion to Radix-64
3. Data Element Formats
3.1 Scalar numbers
3.2 Multi-Precision Integers
3.3 Key IDs
3.4 Text
3.5 Time fields
3.6 String-to-key (S2K) specifiers
3.6.1 String-to-key (S2k) specifier types
3.6.1.1 Simple S2K
3.6.1.2 Salted S2K
3.6.1.3 Iterated and Salted S2K
3.6.2 String-to-key usage
3.6.2.1 Secret key encryption
3.6.2.2 Conventional message encryption
3.6.3 String-to-key algorithms
3.6.3.1 Simple S2K algorithm
3.6.3.2 Salted S2K algorithm
3.6.3.3 Iterated-Salted S2K algorithm
4. Packet Syntax
4.1 Overview
4.2 Packet Headers
4.3 Packet Tags
5. Packet Types
5.1 Public-Key Encrypted Session Key Packets (Tag 1)
5.2 Signature Packet (Tag 2)
5.2.1 Version 3 Signature Packet Format
5.2.2 Version 4 Signature Packet Format
5.2.2.1 Signature Subpacket Specification
5.2.2.2 Signature Subpacket Types
5.2.3 Signature Types
5.2.4 Computing Signatures
5.3 Symmetric-Key Encrypted Session-Key Packets (Tag 3)
5.4 One-Pass Signature Packets (Tag 4)
5.5 Key Material Packet
5.5.1 Key Packet Variants
5.5.1.1 Public Key Packet (Tag 6)
5.5.1.2 Public Subkey Packet (Tag 14)
5.5.1.3 Secret Key Packet (Tag 5)
5.5.1.4 Secret Subkey Packet (Tag 7)
5.5.2 Public Key Packet Formats
5.5.3 Secret Key Packet Formats
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5.6 Compressed Data Packet (Tag 8)
5.7 Symmetrically Encrypted Data Packet (Tag 9)
5.8 Marker Packet (Obsolete Literal Packet) (Tag 10)
5.9 Literal Data Packet (Tag 11)
5.10 Trust Packet (Tag 12)
5.11 User ID Packet (Tag 13)
6. Radix-64 Conversions
6.1 An Implementation of the CRC-24 in "C"
6.2 Forming ASCII Armor
6.3 Encoding Binary in Radix-64
6.4 Decoding Radix-64
6.5 Examples of Radix-64
6.6 Example of an ASCII Armored Message
7. Cleartext signature framework
8. Regular expressions
9. Constants
9.1 Public Key Algorithms
9.2 Symmetric Key Algorithms
9.3 Compression Algorithms
9.4 Hash Algorithms
10. Packet Composition
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
This document provides information on the message-exchange packet
formats used by OP to provide encryption, decryption, signing, key
management and functions. It builds on the foundation provided RFC
1991 "PGP Message Exchange Formats."
1.1 Terms
OP - 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 OP is based.
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.
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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
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
release of the "PGP 3" code base.
"PGP", "Pretty Good", and "Pretty Good Privacy" are trademarks of
Network Associates, Inc.
2. General functions
OP provides data integrity services for messages and data files by
using these core technologies:
-digital signature
-encryption
-compression
-radix-64 conversion
In addition, OP provides key management and certificate services.
2.1 Confidentiality via Encryption
OP offers two encryption options to provide confidentiality:
conventional (symmetric-key) encryption and public key encryption.
With public-key encryption, the message is actually encrypted using a
conventional encryption algorithm. In this mode, each conventional key
is used only once. That is, a new key is generated as a random number
for each message. Since it is used 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 OP 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 OP 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 OP decrypts the session key using the recipient's
private key.
6. The receiving OP decrypts the message using the session key.
If the message was compressed, it will be decompressed.
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 conventional session key. Finally, the session key is
encrypted using public-key encryption and prepended to the encrypted
block.
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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.
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
OP implementations MAY compress the message after applying the
signature but before encryption.
2.4 Conversion to Radix-64
OP'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 OP's native raw binary octets through channels that
are not safe to raw binary data, a printable encoding of these binary
octets is needed. OP 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,
RFC2015. An application that implements OP for messaging SHOULD also
implement OpenPGP-MIME.
3. Data Element Formats
This section describes the data elements used by OP.
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]).
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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.
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 number 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 [RFC2044] encoding of
Unicode [ISO10646].
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 conventional 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
conventionally encrypted messages.
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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
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
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: 0x04
Octet 1: hash algorithm
Octets 2-9: 8-octet salt value
Octets 10-13: count, a four-octet, unsigned value
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.
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.
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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 is provided for backward compatibility; it should be understood,
but not generated.
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 Conventional message encryption
PGP 2.X always used IDEA with Simple string-to-key conversion when
conventionally encrypting a message. PGP 5 can create a Conventional
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,
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
to be decrypted either with a passphrase or a public key.
3.6.3 String-to-key algorithms
3.6.3.1 Simple S2K 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 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
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leftmost, to produce the key data, with any excess octets on the right
discarded.
3.6.3.2 Salted S2K algorithm
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.
3.6.3.3 Iterated-Salted S2K algorithm
Iterated-Salted S2K hashes the passphrase and salt data multiple times.
The total number of octets to be hashed is specified in the four-octet
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. After the hashing is done
the data is unloaded from the hash context(s) as with the other S2K
algorithms.
4. Packet Syntax
This section describes the packets used by OP.
4.1 Overview
An OP 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 OP message, keyring, certificate, and
so forth consists of a number of packets. Some of those packets may
contain other OP packets (for example, a compressed data packet, when
uncompressed, contains OP 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.
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+---------------+
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
backwards-compatible.
Old format packets contain:
Bits 5-2 -- content tag
Bits 1-0 - length-type
New format packets contain:
Bits 5-0 -- content tag
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 byte long,
and the application 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 application should not use indeterminate
length packets except where the end of the data will be clear from the
context.
New format packets have three possible ways of encoding length. A
one-octet Body Length header encodes packet lengths of up to 191
octets, and a two-octet Body Length header encodes packet lengths of
192 to 8383 octets. For cases where longer packet body lengths are
needed, or where the length of the packet body is not known in advance
by the issuer, Partial Body Length headers can be used. These are
one-octet length headers that encode the length of only part of the
data packet.
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) follows that
portion. The last length header in the packet must always be a regular
Body Length header. Partial Body Length headers may only be used for
the non-final parts of the packet.
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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 = length_octet;
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) * 256 + (2nd_octet) + 192
A Partial Body Length header is one octet long and encodes a length
which is a power of 2, from 1 to 2147483648 (2 to the 31st power). It
is recognized because its one octet value is greater than or equal to
224. The partial body length is equal to:
partialBodyLen = 1 << (length_octet & 0x1f);
Examples:
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 might be encoded in the following octet
stream: 0xE1, first two octets of data, 0xE0, next one octet of data,
0xEF, next 32768 octets 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.
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 packets can only have tags less than 16, whereas new format
packets can have tags as great as 63. The defined tags (in decimal)
are:
0 -- Reserved. A packet must not have a tag with this value.
1 -- Public-Key Encrypted Session Key Packet
2 -- Signature Packet
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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
13 -- Name Packet
14 -- Subkey Packet
15 -- Reserved
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 key used to encrypt
a message that is itself encrypted with a public key. Zero or more
Encrypted Session Key packets and/or Conventional Encrypted Session Key
packets may precede a Symmetrically Encrypted Data Packet, which 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(s). The Symmetrically Encrypted Data Packet is preceded by
one Public-Key Encrypted Session Key packet for each OP 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 DSA value g**k mod p.
- MPI of DSA value m * y**k mod p.
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The encrypted value "m" in the above formulas is derived from the
session key as follows. First the session key is prepended with a
one-octet algorithm identifier that specifies the conventional
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 octets, including the algorithm
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
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)
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, unless there is a need to generate a signature
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
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.
- 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
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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 [PKCS1]. 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:
- MD5: 0x2a, 0x86, 0x48, 0x86, 0xf7, 0x0d, 0x02, 0x05
- SHA-1: 0x2b, 0x0e, 0x03, 0x02, 0x1a
- RIPEMD-160: 0x2b, 0x24, 0x03, 0x02, 0x01
The ASN.1 OIDs are:
- MD5: 1.2.840.113549.2.5
- SHA-1: 1.3.14.3.2.26
- RIPEMD160: 1.3.36.3.2.1
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
hash function result is treated as a 160 bit number and used directly
in the DSA signature algorithm.
5.2.2 Version 4 Signature Packet Format
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 octet count for following hashed subpacket data.
- Hashed subpacket data. (zero or more subpackets)
- Two-octet octet count for following unhashed subpacket data.
- 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.
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The data being signed is hashed, and then the signature data from 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
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 above.
5.2.2.1 Signature Subpacket Specification
The subpacket fields consist of zero or more signature subpackets.
Each set of subpackets is preceded by a two-octet count of the length
of the set of subpackets.
Each subpacket consists of a subpacket header and a body. The header
consists of:
- subpacket length (1 or 2 octets):
Length includes the type octet but not this length,
1st octet < 192, then length is octet value
1st octet >= 192, then length is 2 octets and equal to
(1st octet - 192) * 256 + (2nd octet) + 192
- subpacket type (1 octet):
If bit 7 is set, subpacket understanding is critical,
2 = signature creation time,
3 = signature expiration time,
4 = exportable,
5 = trust signature,
6 = regular expression,
7 = revocable,
9 = key expiration time,
10 = placeholder for backwards 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,
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27 = key flags, 28 = Signer's user id
- subpacket specific data:
An implementation SHOULD ignore any subpacket that it does not
recognize.
Bit 7 of the subpacket type is the "critical" bit. If set, it denotes
that the subpacket is one which is critical that the evaluator of the
signature recognize. If a subpacket is encountered which is marked
critical but is unknown to the evaluating software, the evaluator
SHOULD consider the signature to be in error.
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
Several types 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 name
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
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
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 name of the key provides the default symmetric
algorithm.
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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.
Subpacket types:
Signature creation time (4 octet time field)
The time the signature was made. Always included with new
signatures.
Issuer (8 octet key ID)
The OP key ID of the key issuing the signature.
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.
Preferred symmetric algorithms (array of one-octet values)
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.
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.
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 may not have compression software.
This is only found on a self-signature.
Signature expiration time (4 octet time field)
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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.
Exportable (1 octet of exportability, 0 for not, 1 for exportable)
Signature's exportability status. Packet body contains a
boolean flag indicating whether the signature is exportable.
Signatures which are not exportable are ignored during export
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)
Signature's revocability status. Packet body contains a
boolean flag indicating whether the signature is revocable.
Signatures which 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.
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.
Regular expression (null-terminated regular expression)
Used in conjunction with trust signature packets (of level > 0)
to limit the scope of trust which is extended. Only signatures
by the target key on user IDs which match the regular
expression in the body of this packet have trust extended by
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.
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Revocation key (1 octet of class, 1 octet of algid, 20 octets of
fingerprint)
Authorizes the specified key to issue revocation
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.
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 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,
2 octets of value length, 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.
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
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key holder or an authorized administrator of
the key server.
This is found only on a self-signature.
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 names can
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.
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.
Policy URL (String)
This subpacket contains a URL of a document that describes the
policy under which the signature was issued.
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.
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:
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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
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.
- 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 will be calculated over the
text data with its line endings converted to <CR><LF>.
- 0x02: Standalone signature.
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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: The 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: This is a 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: This is the 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: This is the 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: This is used for a signature by a signature key to bind a
subkey which will be used for encryption.
The signature is calculated directly on the subkey itself, not on any
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.
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- 0x20: This signature is used to revoke a key.
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.
- 0x28: This is used to revoke a subkey.
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 which is bound to this subkey, or by an
authorized revocation key, should be considered.
- 0x30: This signature revokes an earlier user ID certification
signature (signature class 0x10 through 0x13).
It should be issued by the same key which issued the revoked signature,
and 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.
5.2.4 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) 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
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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
message. Zero or more Encrypted Session Key packets and/or
Conventional 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 Conventional
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 which may be used to decrypt the message. This allows a
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 conventional 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 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.
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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:
- A one-octet version number. The current version is 3.
- A one-octet signature type. Signature types are described
in section 5.2.3.
- 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 which describes another
signature to be applied to the same message data.
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 OP key
(sometimes called an OP 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 backwards compatibility.
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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 PGP 2.6. Version 2 packets are identical in format to
Version 3 packets, but are generated by PGP 2.5 or before. PGP 5.0
introduces version 4 packets, with new fields and semantics. PGP 2.6.X
will not accept key-material packets with versions greater than 3.
OP 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.
The fingerprint of the key is formed by hashing the body (but not the
two-octet length) of the MPIs that form the key material (public
modulus n, followed by exponent e) with MD5.
The eight-octet key ID of the key consists of the low 64 bits of the
public modulus of an RSA key.
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
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
preferred hash algorithm, and not hashing the length of an MPI with its
body increases the chances of a fingerprint collision.
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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;
- 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 conventional encryption algorithm specifier.
- [Optional] If string-to-key usage octet was 255, a one-octet
conventional 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.
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- 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.
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 backwards 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 RSA keys than with other key formats.
With RSA 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 non-RSA 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 RSA
keys, the checksum is stored in the clear. With non-RSA 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.
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A Compressed Data Packet's body contains an RFC1951 DEFLATE block that
compresses some set of packets. See section "Packet Composition" for
details on how messages are formed.
5.7 Symmetrically Encrypted Data Packet (Tag 9)
The Symmetrically Encrypted Data packet contains data encrypted with a
conventional (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 conventional cipher
operating in PGP's variant of Cipher Feedback (CFB) mode.
The conventional cipher used may be specified in an Encrypted Session
Key or Conventional Encrypted Session Key packet which precedes the
Symmetrically Encrypted Data Packet. In that case, the cipher
algorithm octet is prepended 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, OP 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,
respectivelly. 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 prepended to
the message allows the receiver to immediately check whether the
session key is correct.
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.
The body of this packet consists of:
- The three octets 0x60, 0x47, 0x60 (which spell "PGP" in UTF-8).
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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 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, 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.
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5.11 User ID Packet (Tag 13)
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
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
it is text, it is encoded in UTF-8.
6. Radix-64 Conversions
As stated in the introduction, OP'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 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.
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.
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++;
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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
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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.
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+--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.
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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-----
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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.
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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.
Note that these tables are not exhaustive lists; an implementation MAY
implement an algorithm not on these lists.
9.1 Public Key Algorithms
1 - RSA (Encrypt or Sign)
2 - RSA Encrypt-Only
3 - RSA Sign-Only
16 - Elgamal, see [ELGAMAL]
17 - DSA (Digital Signature Standard)
18 - Elliptic Curve
19 - ECDSA
21 - Diffie-Hellman (X9.42)
100 to 110 - Private/Experimental algorithm.
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Implementations MUST implement DSA for signatures, and Elgamal for
encryption. Implementations SHOULD implement RSA encryption.
Implementations MAY implement any other algorithm.
9.2 Symmetric Key Algorithms
0 - Plaintext
1 - IDEA
2 - Triple-DES (DES-EDE, as per spec -
168 bit key derived from 192)
3 - CAST5 (128 bit key)
4 - Blowfish (128 bit key, 16 rounds)
5 - ROT-N (128 bit N)
6 - SAFER-SK128
7 - DES/SK
100 to 110 - Private/Experimental algorithm.
Implementations MUST implement Triple-DES. Implementations SHOULD
implement IDEA and CAST5.Implementations MAY implement any other
algorithm.
9.3 Compression Algorithms
0 - Uncompressed
1 - ZIP
100 to 110 - Private/Experimental algorithm.
Implementations MUST implement uncompressed data. Implementations
SHOULD implement ZIP.
9.4 Hash Algorithms
1 - MD5
2 - SHA-1
3 - RIPE-MD/160
4 - HAVAL
100 to 110 - Private/Experimental algorithm.
Implementations MUST implement SHA-1. Implementations SHOULD implement
MD5.
10. Packet Composition
OP packets are assembled into sequences in order to create messages and
to transfer keys. Not all possible packet sequences are meaningful and
correct. This describes the rules for how packets should be placed
into sequences.
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10.1 Transferable Public Keys
OP users may transfer public keys. The essential elements of a
transferable public key are:
- One Public Key packet
- Zero or more revocation signatures
- One or more User ID packets
- After each User ID packet, zero or more Signature packets
- Zero or more Subkey packets
- After each Subkey packet, one or more Signature packets
The Public Key packet occurs first. Each of the following User ID
packets provides the identity of the owner of this public key. If
there are multiple User ID packets, this corresponds to multiple means
of identifying the same unique individual user; for example, a user may
enjoy the use of more than one e-mail address, and construct a User ID
packet for each one.
Immediately following each User ID packet, there are zero or more
signature packets. Each signature packet is calculated on the
immediately preceding User ID packet and the initial Public Key packet.
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
public key belongs to the user identified by this user ID.
After the User ID packets there may be one or more Subkey packets. In
general, subkeys are provided in cases where the top-level public key
is a signature-only key. However, any V4 key may have subkeys, and the
subkeys may be encryption-only keys, signature-only keys, or
general-purpose keys.
Each Subkey packet must be followed by at least one Signature packet,
which should be of the subkey binding signature type, issued by the top
level key.
Subkey and Key packets may each be followed by a revocation Signature
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
is authorized to issue revocations via a revocation key subpacket in a
self-signature by the top level key.
Transferable public key packet sequences may be concatenated to allow
transferring multiple public keys in one operation.
10.2 OP Messages
An OP message is a packet or sequence of packets that corresponds to
the following grammatical rules (comma represents sequential
composition, and vertical bar separates alternatives):
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OP Message :- Encrypted Message | Signed Message | Compressed Message
| Literal Message.
Compressed Message :- Compressed Data Packet.
Literal Message :- Literal Data Packet.
ESK :- Pubic Key Encrypted Session Key Packet |
Conventionally Encrypted Session Key Packet.
ESK Sequence :- ESK | ESK Sequence, ESK.
Encrypted Message :- Symmetrically Encrypted Data Packet |
ESK Sequence, Symmetrically Encrypted Data Packet.
One-Pass Signed Message :- One-Pass Signature Packet, OP Message,
Signature Packet.
Signed Message :- Signature Packet, OP Message |
One-Pass Signed Message.
In addition, decrypting a Symmetrically Encrypted Data packet and
decompressing a Compressed Data packet must yield a valid OP Message.
11. Enhanced Key Formats
11.1 Key Structures
The format of V3 OP key using RSA is as follows. Entries in square
brackets are optional and ellipses indicate repetition.
RSA Public Key
[Revocation Self Signature]
User ID [Signature ...]
[User ID [Signature ...] ...]
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
many signatures.
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
primary key.
Primary-Key
[Revocation Self Signature]
[Direct Key Self Signature...]
User ID [Signature ...]
[User ID [Signature ...] ...]
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[Subkey Primary-Key-Signature]
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
either the new signature packets or the old signature packets.
In an key that has a main key and subkeys, the primary key MUST be a
key capable of signing. The subkeys may be keys of any other type, and
either version 3 or 4 of the signature packet can be used. There may
be other types of V4 keys, too. For example, there may be a single-key
RSA key in V4 format, a DSA primary key with an RSA encryption key,
etc, or RSA primary key with an Elgamal subkey.
It is also possible to have a signature-only subkey. This permits a
primary key that collects certifications (key signatures) but is used
only used for certifying subkeys that are used for encryption and
signatures.
11.2 V4 Key IDs and Fingerprints
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
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
of the hash material, with the example of a DSA key:
a.1) 0x99 (1 byte)
a.2) high order length byte of (b)-(f) (1 byte)
a.3) low order length byte of (b)-(f) (1 byte)
b) version number = 4 (1 byte);
c) time stamp of key creation (4 bytes);
e) algorithm (1 byte):
17 = DSA;
f) Algorithm specific fields.
Algorithm Specific Fields for DSA keys (example):
f.1) MPI of DSA prime p;
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.4) MPI of DSA public key value y (= g**x where x is secret).
12. Security Considerations
As with any technology involving cryptography, you should check the
current literature to determine if any algorithms used here have been
found to be vulnerable to attack.
This specification uses Public Key Cryptography technologies.
Possession of the private key portion of a public-private key pair is
assumed to be controlled by the proper party or parties.
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Certain operations in this specification involve the use of random
numbers. An appropriate entropy source should be used to generate
these numbers. See RFC 1750.
The MD5 hash algorithm has been found to have weaknesses
(pseudo-collisions in the compress function) that make some people
deprecate its use. They consider the SHA-1 algorithm better.
If you are building an authentication system, the recipient may specify
a preferred signing algorithm. However, the signer would be foolish to
use a weak algorithm simply because the recipient requests it.
Some of the encryption algorithms mentioned in this document have been
analyzed less than others. For example, although CAST5 is presently
considered strong, it has been analyzed less than Triple-DES. Other
algorithms may have other controversies surrounding them.
Some technologies mentioned here may be subject to government control
in some countries.
13. Authors and Working Group Chair
The working group can be contacted via the current chair:
John W. Noerenberg, II
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):
Jon Callas
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@iks-jena.de
Tel: +49-3641-675642
Hal Finney
Network Associates, Inc.
4200 Bohannon Drive
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Menlo Park, CA 94025, USA
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
authors who include: Derek Atkins, Charles Breed, Dave Del Torto, Marc
Dyksterhouse, Gail Haspert, Gene Hoffman, Paul Hoffman, Raph Levine,
Colin Plumb, Will Price, William Stallings, Mark Weaver, and Philip R.
Zimmermann.
14. References
[DONNERHACKE] Donnerhacke, L., et. al, "PGP263in - an improved
international version of 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 --
Information technology -- Universal Multiple-Octet Coded Character Set
(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 Q, adopted but not yet published.
[PKCS1] RSA Laboratories, "PKCS #1: RSA Encryption Standard," version
1.5, November 1993
[RFC822] D. Crocker, "Standard for the format of ARPA Internet text
messages", RFC 822, August 1982
[RFC1423] D. Balenson, "Privacy Enhancement for Internet Electronic
Mail: Part III: Algorithms, Modes, and Identifiers", RFC 1423,
October 1993
[RFC1641] Goldsmith, D., and M. Davis, "Using Unicode with MIME", RFC
1641, Taligent inc., July 1994.
[RFC1750] Eastlake, Crocker, & Schiller., Randomness Recommendations
for Security. December 1994.
Callas, et. al. Expires Aug 1998 [Page 43]
Internet Draft OpenPGP Message Format Mar 1998
[RFC1951] Deutsch, P., DEFLATE Compressed Data Format Specification
version 1.3. May 1996.
[RFC1983] G. Malkin., Internet Users' Glossary. August 1996.
[RFC1991] Atkins, D., Stallings, W., and P. Zimmermann, "PGP Message
Exchange Formats", RFC 1991, August 1996.
[RFC2015] Elkins, M., "MIME Security with Pretty Good Privacy (PGP)",
RFC 2015, October 1996.
[RFC2044] F. Yergeau., UTF-8, a transformation format of Unicode and
ISO 10646. October 1996.
[RFC2045] Borenstein, N., and Freed, N., "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message Bodies.",
November 1996
[RFC2119] Bradner, S., Key words for use in RFCs to Indicate
Requirement Level. March 1997.
15. Full Copyright Statement
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The limited permissions granted above are perpetual and will not be
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Callas, et. al. Expires Aug 1998 [Page 44]