OpenPGP Message Format

1.1. Terms

• OpenPGP - This is a term for security software that uses PGP 5.x as a basis, formalized in [RFC2440] and this document.¶
• PGP - Pretty Good Privacy. PGP is a family of software systems developed by Philip R. Zimmermann from which OpenPGP is based.¶
• PGP 2.6.x - This version of PGP has many variants, hence the term PGP 2.6.x. It used only RSA, MD5, and IDEA for its cryptographic transforms. An informational RFC, [RFC1991], was written describing this version of PGP.¶
• PGP 5.x - This version of PGP is formerly known as "PGP 3" in the community and also in the predecessor of this document, [RFC1991]. It has new formats and corrects a number of problems in the PGP 2.6.x design. It is referred to here as PGP 5.x because that software was the first release of the "PGP 3" code base.¶
• GnuPG - GNU Privacy Guard, also called GPG. GnuPG is an OpenPGP implementation that avoids all encumbered algorithms. Consequently, early versions of GnuPG did not include RSA public keys. GnuPG may or may not have (depending on version) support for IDEA or other encumbered algorithms.¶

"PGP", "Pretty Good", and "Pretty Good Privacy" are trademarks of PGP Corporation and are used with permission. The term "OpenPGP" refers to the protocol described in this and related documents.¶

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].¶

The key words "PRIVATE USE", "HIERARCHICAL ALLOCATION", "FIRST COME FIRST SERVED", "EXPERT REVIEW", "SPECIFICATION REQUIRED", "IESG APPROVAL", "IETF CONSENSUS", and "STANDARDS ACTION" that appear in this document when used to describe namespace allocation are to be interpreted as described in [RFC2434].¶

2.1. Confidentiality via Encryption

OpenPGP combines symmetric-key encryption and public-key encryption to provide confidentiality. When made confidential, first the object is encrypted using a symmetric encryption algorithm. Each symmetric key is used only once, for a single object. A new "session key" is generated as a random number for each object (sometimes referred to as a session). 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 OpenPGP generates a random number to be used as a session key for this message only.¶
3. The session key is encrypted using each recipient's public key. These "encrypted session keys" start the message.¶
4. The sending OpenPGP encrypts the message using the session key, which forms the remainder of the message. Note that the message is also usually compressed.¶
5. The receiving OpenPGP decrypts the session key using the recipient's private key.¶
6. The receiving OpenPGP decrypts the message using the session key. If the message was compressed, it will be decompressed.¶

With symmetric-key encryption, an object may be encrypted with a symmetric key derived from a passphrase (or other shared secret), or a two-stage mechanism similar to the public-key method described above in which a session key is itself encrypted with a symmetric algorithm keyed from a shared secret.¶

Both digital signature and confidentiality services may be applied to the same message. First, a signature is generated for the message and attached to the message. Then the message plus signature is encrypted using a symmetric session key. Finally, the session key is encrypted using public-key encryption and prefixed to the encrypted block.¶

2.2. Authentication via Digital Signature

The digital signature uses a hash code or message digest algorithm, and a public-key signature algorithm. The sequence is as follows:¶

1. The sender creates a message.¶
2. The sending software generates a hash code of the message.¶
3. The sending software generates a signature from the hash code using the sender's private key.¶
4. The binary signature is attached to the message.¶
5. The receiving software keeps a copy of the message signature.¶
6. The receiving software generates a new hash code for the received message and verifies it using the message's signature. If the verification is successful, the message is accepted as authentic.¶

2.3. Compression

OpenPGP implementations SHOULD compress the message after applying the signature but before encryption.¶

If an implementation does not implement compression, its authors should be aware that most OpenPGP messages in the world are compressed. Thus, it may even be wise for a space-constrained implementation to implement decompression, but not compression.¶

Furthermore, compression has the added side effect that some types of attacks can be thwarted by the fact that slightly altered, compressed data rarely uncompresses without severe errors. This is hardly rigorous, but it is operationally useful. These attacks can be rigorously prevented by implementing and using Modification Detection Codes as described in sections following.¶

2.4. Conversion to Radix-64

OpenPGP's underlying native representation for encrypted messages, signature certificates, and keys is a stream of arbitrary octets. Some systems only permit the use of blocks consisting of seven-bit, printable text. For transporting OpenPGP's native raw binary octets through channels that are not safe to raw binary data, a printable encoding of these binary octets is needed. OpenPGP provides the service of converting the raw 8-bit binary octet stream to a stream of printable ASCII characters, called Radix-64 encoding or ASCII Armor.¶

Implementations SHOULD provide Radix-64 conversions.¶

2.5. Signature-Only Applications

OpenPGP is designed for applications that use both encryption and signatures, but there are a number of problems that are solved by a signature-only implementation. Although this specification requires both encryption and signatures, it is reasonable for there to be subset implementations that are non-conformant only in that they omit encryption.¶

3.1. Scalar Numbers

Scalar numbers are unsigned and are always stored in big-endian format. Using n[k] to refer to the kth octet being interpreted, the value of a two-octet scalar is ((n[0] << 8) + n[1]). The value of a four-octet scalar is ((n[0] << 24) + (n[1] << 16) + (n[2] << 8) + n[3]).¶

3.2. Multiprecision Integers

Multiprecision integers (also called MPIs) are unsigned integers used to hold large integers such as the ones used in cryptographic calculations.¶

An MPI consists of two pieces: a two-octet scalar that is the length of the MPI in bits followed by a string of octets that contain the actual integer.¶

These octets form a big-endian number; a big-endian number can be made into an MPI by prefixing it with the appropriate length.¶

Examples:¶

(all numbers are in hexadecimal)¶

The string of octets [00 01 01] forms an MPI with the value 1. The string [00 09 01 FF] forms an MPI with the value of 511.¶

Additional rules:¶

The size of an MPI is ((MPI.length + 7) / 8) + 2 octets.¶

The length field of an MPI describes the length starting from its most significant non-zero bit. Thus, the MPI [00 02 01] is not formed correctly. It should be [00 01 01].¶

Unused bits of an MPI MUST be zero.¶

Also note that when an MPI is encrypted, the length refers to the plaintext MPI. It may be ill-formed in its ciphertext.¶

3.3. Key IDs

A Key ID is an eight-octet scalar that identifies a key. Implementations SHOULD NOT assume that Key IDs are unique. Section 12 describes how Key IDs are formed.¶

3.4. Text

Unless otherwise specified, the character set for text is the UTF-8 [RFC3629] 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. Keyrings

A keyring is a collection of one or more keys in a file or database. Traditionally, a keyring is simply a sequential list of keys, but may be any suitable database. It is beyond the scope of this standard to discuss the details of keyrings or other databases.¶

3.7. String-to-Key (S2K) Specifiers

String-to-key (S2K) specifiers are used to convert passphrase strings into symmetric-key encryption/decryption keys. They are used in two places, currently: to encrypt the secret part of private keys in the private keyring, and to convert passphrases to encryption keys for symmetrically encrypted messages.¶

  Octet 0:        0x00
  Octet 1:        hash algorithm

  Octet 0:        0x01
  Octet 1:        hash algorithm
  Octets 2-9:     8-octet salt value

  Octet  0:        0x03
  Octet  1:        hash algorithm
  Octets 2-9:      8-octet salt value
  Octet  10:       count, a one-octet, coded value

  #define EXPBIAS 6
      count = ((Int32)16 + (c & 15)) << ((c >> 4) + EXPBIAS);

  0:           secret data is unencrypted (no passphrase)
  255 or 254:  followed by algorithm octet and S2K specifier
  Cipher alg:  use Simple S2K algorithm using MD5 hash

4.1. Overview

An OpenPGP message is constructed from a number of records that are traditionally called packets. A packet is a chunk of data that has a tag specifying its meaning. An OpenPGP message, keyring, certificate, and so forth consists of a number of packets. Some of those packets may contain other OpenPGP packets (for example, a compressed data packet, when uncompressed, contains OpenPGP packets).¶

Each packet consists of a packet header, followed by the packet body. The packet header is of variable length.¶

4.2. Packet Headers

The first octet of the packet header is called the "Packet Tag". It determines the format of the header and denotes the packet contents. The remainder of the packet header is the length of the packet.¶

Note that the most significant bit is the leftmost bit, called bit 7. A mask for this bit is 0x80 in hexadecimal.¶

       ┌───────────────┐
  PTag │7 6 5 4 3 2 1 0│
       └───────────────┘
  Bit 7 -- Always one
  Bit 6 -- New packet format if set

PGP 2.6.x only uses old format packets. Thus, software that interoperates with those versions of PGP must only use old format packets. If interoperability is not an issue, the new packet format is RECOMMENDED. Note that old format packets have four bits of packet tags, and new format packets have six; some features cannot be used and still be backward-compatible.¶

Also note that packets with a tag greater than or equal to 16 MUST use new format packets. The old format packets can only express tags less than or equal to 15.¶

Old format packets contain:¶

  Bits 5-2 -- packet tag
  Bits 1-0 -- length-type

New format packets contain:¶

  Bits 5-0 -- packet tag

  bodyLen = 1st_octet;

  bodyLen = ((1st_octet - 192) << 8) + (2nd_octet) + 192

  bodyLen = (2nd_octet << 24) | (3rd_octet << 16) |
            (4th_octet << 8)  | 5th_octet

  partialBodyLen = 1 << (1st_octet & 0x1F);

4.3. Packet Tags

The packet tag denotes what type of packet the body holds. Note that old format headers can only have tags less than 16, whereas new format headers can have tags as great as 63. The defined tags (in decimal) are as follows:¶

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 Public-Key Encrypted Session Key packets and/or Symmetric-Key Encrypted Session Key packets 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 eight-octet number that gives the Key ID of the public key to which the session key is encrypted. If the session key is encrypted to a subkey, then the Key ID of this subkey is used here instead of the Key ID of the primary key.¶
• 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 encoded as described in PKCS#1 block encoding EME-PKCS1-v1_5 in Section 7.2.1 of [RFC3447] to form the "m" value used in the formulas above. See Section 13.1 in this document for notes on OpenPGP's use of PKCS#1.¶

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 a new PKCS#1 encoding 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 SHOULD accept V3 signatures. Implementations SHOULD generate V4 signatures.¶

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.¶

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]

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 Public-Key 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 passphrases. 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 ensure 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.¶

5.6. 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 a literal data packet.¶

The body of this packet consists of:¶

• One octet that gives the algorithm used to compress the packet.¶
• Compressed data, which makes up the remainder of the packet.¶

A Compressed Data Packet's body contains an block that compresses some set of packets. See Section 11 for details on how messages are formed.¶

ZIP-compressed packets are compressed with raw [RFC1951] 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 [RFC1950] ZLIB-style blocks.¶

BZip2-compressed packets are compressed using the BZip2 [BZ2] algorithm.¶

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 contains other packets (usually a literal data packet or compressed data packet, but in theory other Symmetrically Encrypted Data packets or sequences of packets that form whole OpenPGP messages).¶

The body of this packet consists of:¶

• Encrypted data, the output of the selected symmetric-key cipher operating in OpenPGP's variant of Cipher Feedback (CFB) mode.¶

The symmetric cipher used may be specified in a 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, though this use is deprecated.¶

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 string of length equal to the block size of the cipher plus two to the data before it is encrypted. The first block-size octets (for example, 8 octets for a 64-bit block length) are random, and the following two octets are copies of the last two octets of the IV. For example, in an 8-octet block, octet 9 is a repeat of octet 7, and octet 10 is a repeat of octet 8. In a cipher of length 16, octet 17 is a repeat of octet 15 and octet 18 is a repeat of octet 16. As a pedantic clarification, in both these examples, we consider the first octet to be numbered 1.¶

After encrypting the first block-size-plus-two octets, the CFB state is resynchronized. The last block-size octets of ciphertext are passed through the cipher and the block boundary is reset.¶

The repetition of 16 bits in the random data prefixed to the message allows the receiver to immediately check whether the session key is incorrect. See Section 14 for hints on the proper use of this "quick check".¶

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 reassigned 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. The tag u (0x75) means the same as t, but also indicates that implementation believes that the literal data contains UTF-8 text.¶

  Early versions of PGP also defined a value of l as a 'local' mode for machine-local conversions. [RFC1991] incorrectly stated this local mode flag as 1 (ASCII numeral one). Both of these local modes are deprecated.¶

• File name as a string (one-octet length, followed by a file name). This may be a zero-length string. Commonly, if the source of the encrypted data is a file, this will be the name of the encrypted file. An implementation MAY consider the file name in the Literal packet to be a more authoritative name than the actual file name.¶

  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 a date associated with the literal data. Commonly, the date might be the modification date of a file, or the time the packet was created, or a zero that indicates no specific 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. The format of Trust packets is defined by a given implementation.¶

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 UTF-8 text that is intended to represent the name and email address of the key holder. By convention, it includes an [RFC2822] mail name-addr, but there are no restrictions on its content. The packet length in the header specifies the length of the User ID.¶

5.12. User Attribute Packet (Tag 17)

The User Attribute packet is a variation of the User ID packet. It is capable of storing more types of data than the User ID packet, which is limited to text. Like the User ID packet, a User Attribute packet may be certified by the key owner ("self-signed") or any other key owner who cares to certify it. Except as noted, a User Attribute packet may be used anywhere that a User ID packet may be used.¶

While User Attribute packets are not a required part of the OpenPGP standard, implementations SHOULD provide at least enough compatibility to properly handle a certification signature on the User Attribute packet. A simple way to do this is by treating the User Attribute packet as a User ID packet with opaque contents, but an implementation may use any method desired.¶

The User Attribute packet is made up of one or more attribute 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 only currently defined subpacket type is 1, signifying an image. An implementation SHOULD ignore any subpacket of a type that it does not recognize. Subpacket types 100 through 110 are reserved for private or experimental use.¶

5.13. Sym. Encrypted Integrity Protected Data Packet (Tag 18)

The Symmetrically Encrypted Integrity Protected Data packet is a variant of the Symmetrically Encrypted Data packet. It is a new feature created for OpenPGP that addresses the problem of detecting a modification to encrypted data. It is used in combination with a Modification Detection Code packet.¶

There is a corresponding feature in the features Signature subpacket that denotes that an implementation can properly use this packet type. An implementation MUST support decrypting these packets and SHOULD prefer generating them to the older Symmetrically Encrypted Data packet when possible. Since this data packet protects against modification attacks, this standard encourages its proliferation. While blanket adoption of this data packet would create interoperability problems, rapid adoption is nevertheless important. An implementation SHOULD specifically denote support for this packet, but it MAY infer it from other mechanisms.¶

For example, an implementation might infer from the use of a cipher such as Advanced Encryption Standard (AES) or Twofish that a user supports this feature. It might place in the unhashed portion of another user's key signature a Features subpacket. It might also present a user with an opportunity to regenerate their own self-signature with a Features subpacket.¶

This packet contains data encrypted with a symmetric-key algorithm and protected against modification by the SHA-1 hash algorithm. When it has been decrypted, it will typically contain other packets (often a Literal Data packet or Compressed Data packet). The last decrypted packet in this packet's payload MUST be a Modification Detection Code packet.¶

The body of this packet consists of:¶

• A one-octet version number. The only currently defined value is 1.¶
• Encrypted data, the output of the selected symmetric-key cipher operating in Cipher Feedback mode with shift amount equal to the block size of the cipher (CFB-n where n is the block size).¶

The symmetric cipher used MUST be specified in a Public-Key or Symmetric-Key Encrypted Session Key packet that precedes the Symmetrically Encrypted Data packet. In either case, the cipher algorithm octet is prefixed to the session key before it is encrypted.¶

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 an octet string to the data before it is encrypted. The length of the octet string equals the block size of the cipher in octets, plus two. The first octets in the group, of length equal to the block size of the cipher, are random; the last two octets are each copies of their 2nd preceding octet. For example, with a cipher whose block size is 128 bits or 16 octets, the prefix data will contain 16 random octets, then two more octets, which are copies of the 15th and 16th octets, respectively. Unlike the Symmetrically Encrypted Data Packet, no special CFB resynchronization is done after encrypting this prefix data. See Section 13.9 for more details.¶

The repetition of 16 bits in the random data prefixed to the message allows the receiver to immediately check whether the session key is incorrect.¶

The plaintext of the data to be encrypted is passed through the SHA-1 hash function, and the result of the hash is appended to the plaintext in a Modification Detection Code packet. The input to the hash function includes the prefix data described above; it includes all of the plaintext, and then also includes two octets of values 0xD3, 0x14. These represent the encoding of a Modification Detection Code packet tag and length field of 20 octets.¶

The resulting hash value is stored in a Modification Detection Code (MDC) packet, which MUST use the two octet encoding just given to represent its tag and length field. The body of the MDC packet is the 20-octet output of the SHA-1 hash.¶

The Modification Detection Code packet is appended to the plaintext and encrypted along with the plaintext using the same CFB context.¶

During decryption, the plaintext data should be hashed with SHA-1, including the prefix data as well as the packet tag and length field of the Modification Detection Code packet. The body of the MDC packet, upon decryption, is compared with the result of the SHA-1 hash.¶

Any failure of the MDC indicates that the message has been modified and MUST be treated as a security problem. Failures include a difference in the hash values, but also the absence of an MDC packet, or an MDC packet in any position other than the end of the plaintext. Any failure SHOULD be reported to the user.¶

Note: future designs of new versions of this packet should consider rollback attacks since it will be possible for an attacker to change the version back to 1.¶

• NON-NORMATIVE EXPLANATION¶
• The MDC system, as packets 18 and 19 are called, were created to provide an integrity mechanism that is less strong than a signature, yet stronger than bare CFB encryption.¶
• It is a limitation of CFB encryption that damage to the ciphertext will corrupt the affected cipher blocks and the block following. Additionally, if data is removed from the end of a CFB-encrypted block, that removal is undetectable. (Note also that CBC mode has a similar limitation, but data removed from the front of the block is undetectable.)¶
• The obvious way to protect or authenticate an encrypted block is to digitally sign it. However, many people do not wish to habitually sign data, for a large number of reasons beyond the scope of this document. Suffice it to say that many people consider properties such as deniability to be as valuable as integrity.¶
• OpenPGP addresses this desire to have more security than raw encryption and yet preserve deniability with the MDC system. An MDC is intentionally not a MAC. Its name was not selected by accident. It is analogous to a checksum.¶
• Despite the fact that it is a relatively modest system, it has proved itself in the real world. It is an effective defense to several attacks that have surfaced since it has been created. It has met its modest goals admirably.¶
• Consequently, because it is a modest security system, it has modest requirements on the hash function(s) it employs. It does not rely on a hash function being collision-free, it relies on a hash function being one-way. If a forger, Frank, wishes to send Alice a (digitally) unsigned message that says, "I've always secretly loved you, signed Bob", it is far easier for him to construct a new message than it is to modify anything intercepted from Bob. (Note also that if Bob wishes to communicate secretly with Alice, but without authentication or identification and with a threat model that includes forgers, he has a problem that transcends mere cryptography.)¶
• Note also that unlike nearly every other OpenPGP subsystem, there are no parameters in the MDC system. It hard-defines SHA-1 as its hash function. This is not an accident. It is an intentional choice to avoid downgrade and cross-grade attacks while making a simple, fast system. (A downgrade attack would be an attack that replaced SHA-256 with SHA-1, for example. A cross-grade attack would replace SHA-1 with another 160-bit hash, such as RIPE-MD/160, for example.)¶
• However, given the present state of hash function cryptanalysis and cryptography, it may be desirable to upgrade the MDC system to a new hash function. See Section 13.11 for guidance.¶

5.14. Modification Detection Code Packet (Tag 19)

The Modification Detection Code packet contains a SHA-1 hash of plaintext data, which is used to detect message modification. It is only used with a Symmetrically Encrypted Integrity Protected Data packet. The Modification Detection Code packet MUST be the last packet in the plaintext data that is encrypted in the Symmetrically Encrypted Integrity Protected Data packet, and MUST appear in no other place.¶

A Modification Detection Code packet MUST have a length of 20 octets.¶

The body of this packet consists of:¶

• A 20-octet SHA-1 hash of the preceding plaintext data of the Symmetrically Encrypted Integrity Protected Data packet, including prefix data, the tag octet, and length octet of the Modification Detection Code packet.¶

Note that the Modification Detection Code packet MUST always use a new format encoding of the packet tag, and a one-octet encoding of the packet length. The reason for this is that the hashing rules for modification detection include a one-octet tag and one-octet length in the data hash. While this is a bit restrictive, it reduces complexity.¶

6.1. An Implementation of the CRC-24 in "C"

#define CRC24_INIT 0xB704CEL
#define CRC24_POLY 0x1864CFBL

typedef long crc24;
crc24 crc_octets(unsigned char *octets, size_t len)
{
    crc24 crc = CRC24_INIT;
    int i;
    while (len--) {
        crc ^= (*octets++) << 16;
        for (i = 0; i < 8; i++) {
            crc <<= 1;
            if (crc & 0x1000000)
                crc ^= CRC24_POLY;
        }
    }
    return crc & 0xFFFFFFL;
}

6.2. Forming ASCII Armor

When OpenPGP encodes data into ASCII Armor, it puts specific headers around the Radix-64 encoded data, so OpenPGP can reconstruct the data later. An OpenPGP implementation MAY use ASCII armor to protect raw binary data. OpenPGP informs the user what kind of data is encoded in the ASCII armor through the use of the headers.¶

Concatenating the following data creates ASCII Armor:¶

• An Armor Header Line, appropriate for the type of data¶
• Armor Headers¶
• A blank (zero-length, or containing only whitespace) line¶
• The ASCII-Armored data¶
• An Armor Checksum¶
• The Armor Tail, which depends on the Armor Header Line¶

An Armor Header Line consists of the appropriate header line text surrounded by five (5) dashes (-, 0x2D) on either side of the header line text. The header line text is chosen based upon the type of data that is being encoded in Armor, and how it is being encoded. Header line texts include the following strings:¶

BEGIN PGP MESSAGE

Used for signed, encrypted, or compressed files.¶

BEGIN PGP PUBLIC KEY BLOCK

Used for armoring public keys.¶

BEGIN PGP PRIVATE KEY BLOCK

Used for armoring private keys.¶

BEGIN PGP MESSAGE, PART X/Y

Used for multi-part messages, where the armor is split amongst Y parts, and this is the Xth part out of Y.¶

BEGIN PGP MESSAGE, PART X

Used for multi-part messages, where this is the Xth part of an unspecified number of parts. Requires the MESSAGE-ID Armor Header to be used.¶

BEGIN PGP SIGNATURE

Used for detached signatures, OpenPGP/MIME signatures, and cleartext signatures. Note that PGP 2.x uses BEGIN PGP MESSAGE for detached signatures.¶

Note that all these Armor Header Lines are to consist of a complete line. That is to say, there is always a line ending preceding the starting five dashes, and following the ending five dashes. The header lines, therefore, MUST start at the beginning of a line, and MUST NOT have text other than whitespace following them on the same line. These line endings are considered a part of the Armor Header Line for the purposes of determining the content they delimit. This is particularly important when computing a cleartext signature (see below).¶

The Armor Headers are pairs of strings that can give the user or the receiving OpenPGP implementation some information about how to decode or use the message. The Armor Headers are a part of the armor, not a part of the message, and hence are not protected by any signatures applied to the message.¶

The format of an Armor Header is that of a key-value pair. A colon (: 0x38) and a single space (0x20) separate the key and value. OpenPGP should consider improperly formatted Armor Headers to be corruption of the ASCII Armor. Unknown keys should be reported to the user, but OpenPGP should continue to process the message.¶

Note that some transport methods are sensitive to line length. While there is a limit of 76 characters for the Radix-64 data (Section 6.3), there is no limit to the length of Armor Headers. Care should be taken that the Armor Headers are short enough to survive transport. One way to do this is to repeat an Armor Header key multiple times with different values for each so that no one line is overly long.¶

Currently defined Armor Header Keys are as follows:¶

• "Version", which states the OpenPGP implementation and version used to encode the message.¶
• "Comment", a user-defined comment. OpenPGP defines all text to be in UTF-8. A comment may be any UTF-8 string. However, the whole point of armoring is to provide seven-bit-clean data. Consequently, if a comment has characters that are outside the US-ASCII range of UTF, they may very well not survive transport.¶

• "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 sufficient.¶

  The MessageID SHOULD NOT appear unless it is in a multi-part message. If it appears at all, it MUST be computed from the finished (encrypted, signed, etc.) message in a deterministic fashion, rather than contain a purely random value. This is to allow the legitimate recipient to determine that the MessageID cannot serve as a covert means of leaking cryptographic key information.¶

• "Hash", a comma-separated list of hash algorithms used in this message. This is used only in cleartext signed messages.¶
• "Charset", a description of the character set that the plaintext is in. Please note that OpenPGP defines text to be in UTF-8. An implementation will get best results by translating into and out of UTF-8. However, there are many instances where this is easier said than done. Also, there are communities of users who have no need for UTF-8 because they are all happy with a character set like ISO Latin-5 or a Japanese character set. In such instances, an implementation MAY override the UTF-8 default by using this header key. An implementation MAY implement this key and any translations it cares to; an implementation MAY ignore it and assume all text is UTF-8.¶

The Armor Tail Line is composed in the same manner as the Armor Header Line, except the string "BEGIN" is replaced by the string "END".¶

6.3. Encoding Binary in Radix-64

The encoding process represents 24-bit groups of input bits as output strings of 4 encoded characters. Proceeding from left to right, a 24-bit input group is formed by concatenating three 8-bit input groups. These 24 bits are then treated as four concatenated 6-bit groups, each of which is translated into a single digit in the Radix-64 alphabet. When encoding a bit stream with the Radix-64 encoding, the bit stream must be presumed to be ordered with the most significant bit first. That is, the first bit in the stream will be the high-order bit in the first 8-bit octet, and the eighth bit will be the low-order bit in the first 8-bit octet, and so on.¶

┌──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.¶

The encoded output stream must be represented in lines of no more than 76 characters each.¶

Special processing is performed if fewer than 24 bits are available at the end of the data being encoded. There are three possibilities:¶

1. The last data group has 24 bits (3 octets). No special processing is needed.¶
2. The last data group has 16 bits (2 octets). The first two 6-bit groups are processed as above. The third (incomplete) data group has two zero-value bits added to it, and is processed as above. A pad character (=) is added to the output.¶
3. The last data group has 8 bits (1 octet). The first 6-bit group is processed as above. The second (incomplete) data group has four zero-value bits added to it, and is processed as above. Two pad characters (=) are added to the output.¶

6.4. Decoding Radix-64

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. Decoding software must ignore all white space.¶

Because it is used only for padding at the end of the data, the occurrence of any "=" characters may be taken as evidence that the end of the data has been reached (without truncation in transit). No such assurance is possible, however, when the number of octets transmitted was a multiple of three and no "=" characters are present.¶

6.5. Examples of Radix-64

Input data:  0x14FB9C03D97E
Hex:     1   4    F   B    9   C     | 0   3    D   9    7   E
8-bit:   00010100 11111011 10011100  | 00000011 11011001 11111110
6-bit:   000101 001111 101110 011100 | 000000 111101 100111 111110
Decimal: 5      15     46     28       0      61     37     62
Output:  F      P      u      c        A      9      l      +
Input data:  0x14FB9C03D9
Hex:     1   4    F   B    9   C     | 0   3    D   9
8-bit:   00010100 11111011 10011100  | 00000011 11011001
                                                pad with 00
6-bit:   000101 001111 101110 011100 | 000000 111101 100100
Decimal: 5      15     46     28       0      61     36
                                                   pad with =
Output:  F      P      u      c        A      9      k      =
Input data:  0x14FB9C03
Hex:     1   4    F   B    9   C     | 0   3
8-bit:   00010100 11111011 10011100  | 00000011
                                       pad with 0000
6-bit:   000101 001111 101110 011100 | 000000 110000
Decimal: 5      15     46     28       0      48
                                            pad with =      =
Output:  F      P      u      c        A      w      =      =

6.6. Example of an ASCII Armored Message

-----BEGIN PGP MESSAGE-----
Version: OpenPrivacy 0.99

yDgBO22WxBHv7O8X7O/jygAEzol56iUKiXmV+XmpCtmpqQUKiQrFqclFqUDBovzS
vBSFjNSiVHsuAA==
=njUN
-----END PGP MESSAGE-----

Note that this example has extra indenting; an actual armored message would have no leading whitespace.¶

7.1. Dash-Escaped Text

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. An implementation MAY dash-escape any line, SHOULD dash-escape lines commencing "From" followed by a space, and MUST dash-escape any line commencing in a dash. 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.¶

When reversing dash-escaping, an implementation MUST strip the string - if it occurs at the beginning of a line, and SHOULD warn on - and any character other than a space at the beginning of a line.¶

Also, any trailing whitespace -- spaces (0x20) and tabs (0x09) -- at the end of any line is removed when the cleartext signature is generated.¶

9.1. Public-Key Algorithms

Implementations MUST implement DSA for signatures, and Elgamal for encryption. Implementations SHOULD implement RSA keys (1). RSA Encrypt-Only (2) and RSA Sign-Only are deprecated and SHOULD NOT be generated, but may be interpreted. See Section 13.5. See Section 13.8 for notes on Elliptic Curve (18), ECDSA (19), Elgamal Encrypt or Sign (20), and X9.42 (21). Implementations MAY implement any other algorithm.¶

9.2. Symmetric-Key Algorithms

Implementations MUST implement TripleDES. Implementations SHOULD implement AES-128 and CAST5. Implementations that interoperate with PGP 2.6 or earlier need to support IDEA, as that is the only symmetric cipher those versions use. Implementations MAY implement any other algorithm.¶

9.3. Compression Algorithms

Implementations MUST implement uncompressed data. Implementations SHOULD implement ZIP. Implementations MAY implement any other algorithm.¶

9.4. Hash Algorithms

Implementations MUST implement SHA-1. Implementations MAY implement other algorithms. MD5 is deprecated.¶

10.1. New String-to-Key Specifier Types

OpenPGP S2K specifiers contain a mechanism for new algorithms to turn a string into a key. This specification creates a registry of S2K specifier types. The registry includes the S2K type, the name of the S2K, and a reference to the defining specification. The initial values for this registry can be found in Section 3.7.1. Adding a new S2K specifier MUST be done through the IETF CONSENSUS method, as described in [RFC2434].¶

10.2. New Packets

Major new features of OpenPGP are defined through new packet types. This specification creates a registry of packet types. The registry includes the packet type, the name of the packet, and a reference to the defining specification. The initial values for this registry can be found in Section 4.3. Adding a new packet type MUST be done through the IETF CONSENSUS method, as described in [RFC2434].¶

10.3. New Algorithms

Section 9 lists the core algorithms that OpenPGP uses. Adding in a new algorithm is usually simple. For example, adding in a new symmetric cipher usually would not need anything more than allocating a constant for that cipher. If that cipher had other than a 64-bit or 128-bit block size, there might need to be additional documentation describing how OpenPGP-CFB mode would be adjusted. Similarly, when DSA was expanded from a maximum of 1024-bit public keys to 3072-bit public keys, the revision of FIPS 186 contained enough information itself to allow implementation. Changes to this document were made mainly for emphasis.¶

11.1. Transferable Public Keys

OpenPGP users may transfer public keys. The essential elements of a transferable public key are as follows:¶

• 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 (certifications)¶
• Zero or more User Attribute packets¶
• After each User Attribute packet, zero or more Signature packets (certifications)¶
• Zero or more Subkey packets¶
• After each Subkey packet, one Signature packet, plus optionally a revocation¶

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 have more than one email address, and construct a User ID 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.¶

Within the same section as the User ID packets, there are zero or more User Attribute packets. Like the User ID packets, a User Attribute packet is followed by zero or more Signature packets calculated on the immediately preceding User Attribute packet and the initial Public-Key packet.¶

User Attribute packets and User ID packets may be freely intermixed in this section, so long as the signatures that follow them are maintained on the proper User Attribute or User ID packet.¶

After the User ID packet or Attribute packet, there may be zero 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. V3 keys MUST NOT have subkeys.¶

Each Subkey packet MUST be followed by one Signature packet, which should be a subkey binding signature issued by the top-level key. For subkeys that can issue signatures, the subkey binding signature MUST contain an Embedded Signature subpacket with a primary key binding signature (0x19) issued by the subkey on 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 that 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.¶

11.2. Transferable Secret Keys

OpenPGP users may transfer secret keys. The format of a transferable secret key is the same as a transferable public key except that secret-key and secret-subkey packets are used instead of the public key and public-subkey packets. Implementations SHOULD include self-signatures on any user IDs and subkeys, as this allows for a complete public key to be automatically extracted from the transferable secret key. Implementations MAY choose to omit the self-signatures, especially if a transferable public key accompanies the transferable secret key.¶

11.3. OpenPGP Messages

An OpenPGP message is a packet or sequence of packets that corresponds to the following grammatical rules (comma represents sequential composition, and vertical bar separates alternatives):¶

OpenPGP Message :-

Encrypted Message | Signed Message | Compressed Message | Literal Message.¶

Compressed Message :-

Compressed Data Packet.¶

Literal Message :-

Literal Data Packet.¶

ESK :-

Public-Key Encrypted Session Key Packet | Symmetric-Key Encrypted Session Key Packet.¶

ESK Sequence :-

ESK | ESK Sequence, ESK.¶

Encrypted Data :-

Symmetrically Encrypted Data Packet | Symmetrically Encrypted Integrity Protected Data Packet¶

Encrypted Message :-

Encrypted Data | ESK Sequence, Encrypted Data.¶

One-Pass Signed Message :-

One-Pass Signature Packet, OpenPGP Message, Corresponding Signature Packet.¶

Signed Message :-

Signature Packet, OpenPGP Message | One-Pass Signed Message.¶

In addition, decrypting a Symmetrically Encrypted Data packet or a Symmetrically Encrypted Integrity Protected Data packet as well as decompressing a Compressed Data packet must yield a valid OpenPGP Message.¶

11.4. Detached Signatures

Some OpenPGP applications use so-called "detached signatures". For example, a program bundle may contain a file, and with it a second file that is a detached signature of the first file. These detached signatures are simply a Signature packet stored separately from the data for which they are a signature.¶

12.1. Key Structures

The format of an OpenPGP V3 key 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. V3 keys are deprecated. Implementations MUST NOT generate new V3 keys, but MAY continue to use existing ones.¶

The format of an OpenPGP V4 key that uses multiple public keys is similar except that the other keys are added to the end as "subkeys" of the primary key.¶

Primary-Key
   [Revocation Self Signature]
   [Direct Key Signature...]
    User ID [Signature ...]
   [User ID [Signature ...] ...]
   [User Attribute [Signature ...] ...]
   [[Subkey [Binding-Signature-Revocation]
           Primary-Key-Binding-Signature] ...]

A subkey always has a single signature after it that is issued using the primary key to tie the two keys together. This binding signature may be in either V3 or V4 format, but SHOULD be V4. Subkeys that can issue signatures MUST have a V4 binding signature due to the REQUIRED embedded primary key binding signature.¶

In the above diagram, if the binding signature of a subkey has been revoked, the revoked key may be removed, leaving only one key.¶

In a V4 key, the primary key MUST be a key capable of certification. The subkeys may be keys of any other type. There may be other constructions 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, or RSA primary key with an Elgamal subkey, etc.¶

It is also possible to have a signature-only subkey. This permits a primary key that collects certifications (key signatures), but is used only for certifying subkeys that are used for encryption and signatures.¶

12.2. Key IDs and Fingerprints

For a V3 key, the eight-octet Key ID consists of the low 64 bits of the public modulus of the RSA key.¶

The fingerprint of a V3 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. Note that both V3 keys and MD5 are deprecated.¶

A V4 fingerprint is the 160-bit SHA-1 hash of the octet 0x99, followed by the two-octet packet length, followed by the entire Public-Key packet starting with the version field. The Key ID is the low-order 64 bits of the fingerprint. Here are the fields of the hash material, with the example of a DSA key:¶

a.1) 0x99 (1 octet)¶

a.2) high-order length octet of (b)-(e) (1 octet)¶

a.3) low-order length octet of (b)-(e) (1 octet)¶

b) version number = 4 (1 octet);¶

c) timestamp of key creation (4 octets);¶

d) algorithm (1 octet): 17 = DSA (example);¶

e) Algorithm-specific fields.¶

Algorithm-Specific Fields for DSA keys (example):¶

e.1) MPI of DSA prime p;¶

e.2) MPI of DSA group order q (q is a prime divisor of p-1);¶

e.3) MPI of DSA group generator g;¶

e.4) MPI of DSA public-key value y (= g**x mod p where x is secret).¶

Note that it is possible for there to be collisions of Key IDs -- two different keys with the same Key ID. Note that there is a much smaller, but still non-zero, probability that two different keys have the same fingerprint.¶

Also note that if V3 and V4 format keys share the same RSA key material, they will have different Key IDs as well as different fingerprints.¶

Finally, the Key ID and fingerprint of a subkey are calculated in the same way as for a primary key, including the 0x99 as the first octet (even though this is not a valid packet ID for a public subkey).¶

13.1. PKCS#1 Encoding in OpenPGP

This standard makes use of the PKCS#1 functions EME-PKCS1-v1_5 and EMSA-PKCS1-v1_5. However, the calling conventions of these functions has changed in the past. To avoid potential confusion and interoperability problems, we are including local copies in this document, adapted from those in PKCS#1 v2.1 [RFC3447]. [RFC3447] should be treated as the ultimate authority on PKCS#1 for OpenPGP. Nonetheless, we believe that there is value in having a self-contained document that avoids problems in the future with needed changes in the conventions.¶

  EM = 0x00 || 0x02 || PS || 0x00 || M.

13.2. Symmetric Algorithm Preferences

The symmetric algorithm preference is an ordered list of algorithms that the keyholder accepts. Since it is found on a self-signature, it is possible that a keyholder may have multiple, different preferences. For example, Alice may have TripleDES only specified for "alice@work.com" but CAST5, Blowfish, and TripleDES specified for "alice@home.org". Note that it is also possible for preferences to be in a subkey's binding signature.¶

Since TripleDES is the MUST-implement algorithm, if it is not explicitly in the list, it is tacitly at the end. However, it is good form to place it there explicitly. Note also that if an implementation does not implement the preference, then it is implicitly a TripleDES-only implementation.¶

An implementation MUST NOT use a symmetric algorithm that is not in the recipient's preference list. When encrypting to more than one recipient, the implementation finds a suitable algorithm by taking the intersection of the preferences of the recipients. Note that the MUST-implement algorithm, TripleDES, ensures that the intersection is not null. The implementation may use any mechanism to pick an algorithm in the intersection.¶

If an implementation can decrypt a message that a keyholder doesn't have in their preferences, the implementation SHOULD decrypt the message anyway, but MUST warn the keyholder that the protocol has been violated. For example, suppose that Alice, above, has software that implements all algorithms in this specification. Nonetheless, she prefers subsets for work or home. If she is sent a message encrypted with IDEA, which is not in her preferences, the software warns her that someone sent her an IDEA-encrypted message, but it would ideally decrypt it anyway.¶

13.3. Other Algorithm Preferences

Other algorithm preferences work similarly to the symmetric algorithm preference, in that they specify which algorithms the keyholder accepts. There are two interesting cases that other comments need to be made about, though, the compression preferences and the hash preferences.¶

13.4. Plaintext

Algorithm 0, "plaintext", may only be used to denote secret keys that are stored in the clear. Implementations MUST NOT use plaintext in Symmetrically Encrypted Data packets; they must use Literal Data packets to encode unencrypted or literal data.¶

13.5. RSA

There are algorithm types for RSA Sign-Only, and RSA Encrypt-Only keys. These types are deprecated. The "key flags" subpacket in a signature is a much better way to express the same idea, and generalizes it to all algorithms. An implementation SHOULD NOT create such a key, but MAY interpret it.¶

An implementation SHOULD NOT implement RSA keys of size less than 1024 bits.¶

13.6. DSA

An implementation SHOULD NOT implement DSA keys of size less than 1024 bits. It MUST NOT implement a DSA key with a q size of less than 160 bits. DSA keys MUST also be a multiple of 64 bits, and the q size MUST be a multiple of 8 bits. The Digital Signature Standard (DSS) [FIPS186] specifies that DSA be used in one of the following ways:¶

• 1024-bit key, 160-bit q, SHA-1, SHA-224, SHA-256, SHA-384, or SHA-512 hash¶
• 2048-bit key, 224-bit q, SHA-224, SHA-256, SHA-384, or SHA-512 hash¶
• 2048-bit key, 256-bit q, SHA-256, SHA-384, or SHA-512 hash¶
• 3072-bit key, 256-bit q, SHA-256, SHA-384, or SHA-512 hash¶

The above key and q size pairs were chosen to best balance the strength of the key with the strength of the hash. Implementations SHOULD use one of the above key and q size pairs when generating DSA keys. If DSS compliance is desired, one of the specified SHA hashes must be used as well. [FIPS186] is the ultimate authority on DSS, and should be consulted for all questions of DSS compliance.¶

Note that earlier versions of this standard only allowed a 160-bit q with no truncation allowed, so earlier implementations may not be able to handle signatures with a different q size or a truncated hash.¶

13.7. Elgamal

An implementation SHOULD NOT implement Elgamal keys of size less than 1024 bits.¶

13.8. Reserved Algorithm Numbers

A number of algorithm IDs have been reserved for algorithms that would be useful to use in an OpenPGP implementation, yet there are issues that prevent an implementer from actually implementing the algorithm. These are marked in Section 9.1 as "reserved for".¶

The reserved public-key algorithms, Elliptic Curve (18), ECDSA (19), and X9.42 (21), do not have the necessary parameters, parameter order, or semantics defined.¶

Previous versions of OpenPGP permitted Elgamal [ELGAMAL] signatures with a public-key identifier of 20. These are no longer permitted. An implementation MUST NOT generate such keys. An implementation MUST NOT generate Elgamal signatures. See [BLEICHENBACHER].¶

13.9. OpenPGP CFB Mode

OpenPGP does symmetric encryption using a variant of Cipher Feedback mode (CFB mode). This section describes the procedure it uses in detail. This mode is what is used for Symmetrically Encrypted Data Packets; the mechanism used for encrypting secret-key material is similar, and is described in the sections above.¶

In the description below, the value BS is the block size in octets of the cipher. Most ciphers have a block size of 8 octets. The AES and Twofish have a block size of 16 octets. Also note that the description below assumes that the IV and CFB arrays start with an index of 1 (unlike the C language, which assumes arrays start with a zero index).¶

OpenPGP CFB mode uses an initialization vector (IV) of all zeros, and prefixes the plaintext with BS+2 octets of random data, such that octets BS+1 and BS+2 match octets BS-1 and BS. It does a CFB resynchronization after encrypting those BS+2 octets.¶

Thus, for an algorithm that has a block size of 8 octets (64 bits), the IV is 10 octets long and octets 7 and 8 of the IV are the same as octets 9 and 10. For an algorithm with a block size of 16 octets (128 bits), the IV is 18 octets long, and octets 17 and 18 replicate octets 15 and 16. Those extra two octets are an easy check for a correct key.¶

Step by step, here is the procedure:¶

1. The feedback register (FR) is set to the IV, which is all zeros.¶
2. FR is encrypted to produce FRE (FR Encrypted). This is the encryption of an all-zero value.¶
3. FRE is xored with the first BS octets of random data prefixed to the plaintext to produce C[1] through C[BS], the first BS octets of ciphertext.¶
4. FR is loaded with C[1] through C[BS].¶
5. FR is encrypted to produce FRE, the encryption of the first BS octets of ciphertext.¶
6. The left two octets of FRE get xored with the next two octets of data that were prefixed to the plaintext. This produces C[BS+1] and C[BS+2], the next two octets of ciphertext.¶
7. (The resynchronization step) FR is loaded with C[3] through C[BS+2].¶
8. FR is encrypted to produce FRE.¶
9. FRE is xored with the first BS octets of the given plaintext, now that we have finished encrypting the BS+2 octets of prefixed data. This produces C[BS+3] through C[BS+(BS+2)], the next BS octets of ciphertext.¶
10. FR is loaded with C[BS+3] to C[BS + (BS+2)] (which is C11-C18 for an 8-octet block).¶
11. FR is encrypted to produce FRE.¶
12. FRE is xored with the next BS octets of plaintext, to produce the next BS octets of ciphertext. These are loaded into FR, and the process is repeated until the plaintext is used up.¶

13.10. Private or Experimental Parameters

S2K specifiers, Signature subpacket types, user attribute types, image format types, and algorithms described in Section 9 all reserve the range 100 to 110 for private and experimental use. Packet types reserve the range 60 to 63 for private and experimental use. These are intentionally managed with the PRIVATE USE method, as described in [RFC2434].¶

However, implementations need to be careful with these and promote them to full IANA-managed parameters when they grow beyond the original, limited system.¶

13.11. Extension of the MDC System

As described in the non-normative explanation in Section 5.13, the MDC system is uniquely unparameterized in OpenPGP. This was an intentional decision to avoid cross-grade attacks. If the MDC system is extended to a stronger hash function, care must be taken to avoid downgrade and cross-grade attacks.¶

One simple way to do this is to create new packets for a new MDC. For example, instead of the MDC system using packets 18 and 19, a new MDC could use 20 and 21. This has obvious drawbacks (it uses two packet numbers for each new hash function in a space that is limited to a maximum of 60).¶

Another simple way to extend the MDC system is to create new versions of packet 18, and reflect this in packet 19. For example, suppose that V2 of packet 18 implicitly used SHA-256. This would require packet 19 to have a length of 32 octets. The change in the version in packet 18 and the size of packet 19 prevent a downgrade attack.¶

There are two drawbacks to this latter approach. The first is that using the version number of a packet to carry algorithm information is not tidy from a protocol-design standpoint. It is possible that there might be several versions of the MDC system in common use, but this untidiness would reflect untidiness in cryptographic consensus about hash function security. The second is that different versions of packet 19 would have to have unique sizes. If there were two versions each with 256-bit hashes, they could not both have 32-octet packet 19s without admitting the chance of a cross-grade attack.¶

Yet another, complex approach to extend the MDC system would be a hybrid of the two above -- create a new pair of MDC packets that are fully parameterized, and yet protected from downgrade and cross-grade.¶

Any change to the MDC system MUST be done through the IETF CONSENSUS method, as described in [RFC2434].¶

13.12. Meta-Considerations for Expansion

If OpenPGP is extended in a way that is not backwards-compatible, meaning that old implementations will not gracefully handle their absence of a new feature, the extension proposal can be declared in the key holder's self-signature as part of the Features signature subpacket.¶

We cannot state definitively what extensions will not be upwards-compatible, but typically new algorithms are upwards-compatible, whereas new packets are not.¶

If an extension proposal does not update the Features system, it SHOULD include an explanation of why this is unnecessary. If the proposal contains neither an extension to the Features system nor an explanation of why such an extension is unnecessary, the proposal SHOULD be rejected.¶