Network Working Group J. Callas
Request for Comments: 4880 PGP Corporation
Obsoletes: 1991, 2440 L. Donnerhacke
Category: Standards Track IKS GmbH
D. ShawR. ThayerNovember 2007 OpenPGP Message Format
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
This document is maintained in order to publish all necessary
information needed to develop interoperable applications based on the
OpenPGP format. It is not a step-by-step cookbook for writing an
application. It describes only the format and methods needed to
read, check, generate, and write conforming packets crossing any
network. It does not deal with storage and implementation questions.
It does, however, discuss implementation issues necessary to avoid
OpenPGP software uses a combination of strong public-key and
symmetric cryptography to provide security services for electronic
communications and data storage. These services include
confidentiality, key management, authentication, and digital
signatures. This document specifies the message formats used in
13.9. OpenPGP CFB Mode .........................................7813.10. Private or Experimental Parameters ......................7913.11. Extension of the MDC System .............................8013.12. Meta-Considerations for Expansion .......................8014. Security Considerations .......................................8115. Implementation Nits ...........................................8416. References ....................................................8616.1. Normative References .....................................8616.2. Informative References ...................................881. Introduction
This document provides information on the message-exchange packet
formats used by OpenPGP to provide encryption, decryption, signing,
and key management functions. It is a revision of RFC 2440, "OpenPGP
Message Format", which itself replaces RFC 1991, "PGP Message
Exchange Formats" [RFC1991] [RFC2440].
* OpenPGP - This is a term for security software that uses PGP 5.x
as a basis, formalized in RFC 2440 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, RFC 1991, was written
describing this version of PGP.
* PGP 5.x - This version of PGP is formerly known as "PGP 3" in the
community and also in the predecessor of this document, RFC 1991.
It has new formats and corrects a number of problems in the PGP
2.6.x design. It is referred to here as PGP 5.x because that
software was the first release of the "PGP 3" code base.
* 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. General functions
OpenPGP provides data integrity services for messages and data files
by using these core technologies:
- digital signatures
- Radix-64 conversion
In addition, OpenPGP provides key management and certificate
services, but many of these are beyond the scope of this document.
2.1. Confidentiality via Encryption
OpenPGP 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
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
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.
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
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
3. Data Element Formats
This section describes the data elements used by OpenPGP.
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 << 8) + n). The value of a
four-octet scalar is ((n << 24) + (n << 16) + (n << 8) +
3.2. Multiprecision Integers
Multiprecision integers (also called MPIs) are unsigned integers used
to hold large integers such as the ones used in cryptographic
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
These octets form a big-endian number; a big-endian number can be
made into an MPI by prefixing it with the appropriate length.
(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.
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. The
section "Enhanced Key Formats" below describes how Key IDs are
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.
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.
3.7.1. String-to-Key (S2K) Specifier Types
There are three types of S2K specifiers currently supported, and
some reserved values:
ID S2K Type
0 Simple S2K
1 Salted S2K
2 Reserved value
3 Iterated and Salted S2K
100 to 110 Private/Experimental S2K
These are described in Sections 18.104.22.168 - 22.214.171.124.
126.96.36.199. Simple S2K
This directly hashes the string to produce the key data. See below
for how this hashing is done.
Octet 0: 0x00
Octet 1: hash algorithm
Simple S2K hashes the passphrase to produce the session key. The
manner in which this is done depends on the size of the session key
(which will depend on the cipher used) and the size of the hash
algorithm's output. If the hash size is greater than the session key
size, the high-order (leftmost) octets of the hash are used as the
If the hash size is less than the key size, multiple instances of the
hash context are created -- enough to produce the required key data.
These instances are preloaded with 0, 1, 2, ... octets of zeros (that
is to say, the first instance has no preloading, the second gets
preloaded with 1 octet of zero, the third is preloaded with two
octets of zeros, and so forth).
As the data is hashed, it is given independently to each hash
context. Since the contexts have been initialized differently, they
will each produce different hash output. Once the passphrase is
hashed, the output data from the multiple hashes is concatenated,
first hash leftmost, to produce the key data, with any excess octets
on the right discarded.
188.8.131.52. Salted S2K
This includes a "salt" value in the S2K specifier -- some arbitrary
data -- that gets hashed along with the passphrase string, to help
prevent dictionary attacks.
Octet 0: 0x01
Octet 1: hash algorithm
Octets 2-9: 8-octet salt value
Salted S2K is exactly like Simple S2K, except that the input to the
hash function(s) consists of the 8 octets of salt from the S2K
specifier, followed by the passphrase.
184.108.40.206. Iterated and Salted S2K
This includes both a salt and an octet count. The salt is combined
with the passphrase and the resulting value is hashed repeatedly.
This further increases the amount of work an attacker must do to try
Octet 0: 0x03
Octet 1: hash algorithm
Octets 2-9: 8-octet salt value
Octet 10: count, a one-octet, coded value
The count is coded into a one-octet number using the following
#define EXPBIAS 6
count = ((Int32)16 + (c & 15)) << ((c >> 4) + EXPBIAS);
The above formula is in C, where "Int32" is a type for a 32-bit
integer, and the variable "c" is the coded count, Octet 10.
Iterated-Salted S2K hashes the passphrase and salt data multiple
times. The total number of octets to be hashed is specified in the
encoded count in the S2K specifier. Note that the resulting count
value is an octet count of how many octets will be hashed, not an
Initially, one or more hash contexts are set up as with the other S2K
algorithms, depending on how many octets of key data are needed.
Then the salt, followed by the passphrase data, is repeatedly hashed
until the number of octets specified by the octet count has been
hashed. The one exception is that if the octet count is less than
the size of the salt plus passphrase, the full salt plus passphrase
will be hashed even though that is greater than the octet count.
After the hashing is done, the data is unloaded from the hash
context(s) as with the other S2K algorithms.
3.7.2. String-to-Key Usage
Implementations SHOULD use salted or iterated-and-salted S2K
specifiers, as simple S2K specifiers are more vulnerable to
220.127.116.11. Secret-Key Encryption
An S2K specifier can be stored in the secret keyring to specify how
to convert the passphrase to a key that unlocks the secret data.
Older versions of PGP just stored a cipher algorithm octet preceding
the secret data or a zero to indicate that the secret data was
unencrypted. The MD5 hash function was always used to convert the
passphrase to a key for the specified cipher algorithm.
For compatibility, when an S2K specifier is used, the special value
254 or 255 is stored in the position where the hash algorithm octet
would have been in the old data structure. This is then followed
immediately by a one-octet algorithm identifier, and then by the S2K
specifier as encoded above.
Therefore, preceding the secret data there will be one of these
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
This last possibility, the cipher algorithm number with an implicit
use of MD5 and IDEA, is provided for backward compatibility; it MAY
be understood, but SHOULD NOT be generated, and is deprecated.
These are followed by an Initial Vector of the same length as the
block size of the cipher for the decryption of the secret values, if
they are encrypted, and then the secret-key values themselves.
18.104.22.168. Symmetric-Key Message Encryption
OpenPGP can create a Symmetric-key Encrypted Session Key (ESK) packet
at the front of a message. This is used to allow S2K specifiers to
be used for the passphrase conversion or to create messages with a
mix of symmetric-key ESKs and public-key ESKs. This allows a message
to be decrypted either with a passphrase or a public-key pair.
PGP 2.X always used IDEA with Simple string-to-key conversion when
encrypting a message with a symmetric algorithm. This is deprecated,
but MAY be used for backward-compatibility.