RFC 2440
OpenPGP Message Format
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.
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.
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.
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.
- 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
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.
- 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:
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.
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.
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.
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.
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
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.
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.
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:
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.
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
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:
- 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.
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.
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;
- 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.
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.
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.
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,
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.
(page 41 continued on part 3)
