LibrePGP Message Format
draft-koch-librepgp-00
| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Authors | Werner Koch , Ronald Henry Tse | ||
| Last updated | 2023-11-29 | ||
| Replaces | draft-koch-openpgp-2015-rfc4880bis | ||
| RFC stream | (None) | ||
| Intended RFC status | (None) | ||
| Formats | |||
| Stream | Stream state | (No stream defined) | |
| Consensus boilerplate | Unknown | ||
| RFC Editor Note | (None) | ||
| IESG | IESG state | I-D Exists | |
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-koch-librepgp-00
Network Working Group W. Koch
Internet-Draft g10 Code GmbH
Obsoletes: 4880, 5581, 6637 (if approved) R. H. Tse
Intended status: Standards Track Ribose
Expires: 1 June 2024 29 November 2023
LibrePGP Message Format
draft-koch-librepgp-00
Abstract
This document specifies the message formats used in LibrePGP.
LibrePGP is an extension of the OpenPGP format which provides
encryption with public-key or symmetric cryptographic algorithms,
digital signatures, compression and key management.
This document is maintained in order to publish all necessary
information needed to develop interoperable applications based on the
LibrePGP 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
security flaws.
This document is based on: RFC 4880 (OpenPGP), RFC 5581 (Camellia in
OpenPGP), and RFC 6637 (Elliptic Curves in OpenPGP).
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 1 June 2024.
Koch & Tse Expires 1 June 2024 [Page 1]
Internet-Draft LibrePGP Message Format November 2023
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1. Terms . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2. General functions . . . . . . . . . . . . . . . . . . . . . . 6
2.1. Confidentiality via Encryption . . . . . . . . . . . . . 7
2.2. Authentication via Digital Signature . . . . . . . . . . 7
2.3. Compression . . . . . . . . . . . . . . . . . . . . . . . 8
2.4. Conversion to Radix-64 . . . . . . . . . . . . . . . . . 8
2.5. Signature-Only Applications . . . . . . . . . . . . . . . 9
3. Data Element Formats . . . . . . . . . . . . . . . . . . . . 9
3.1. Scalar Numbers . . . . . . . . . . . . . . . . . . . . . 9
3.2. Multiprecision Integers . . . . . . . . . . . . . . . . . 9
3.3. Key IDs . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.4. Text . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.5. Time Fields . . . . . . . . . . . . . . . . . . . . . . . 10
3.6. Keyrings . . . . . . . . . . . . . . . . . . . . . . . . 10
3.7. String-to-Key (S2K) Specifiers . . . . . . . . . . . . . 10
3.7.1. String-to-Key (S2K) Specifier Types . . . . . . . . . 10
3.7.2. String-to-Key Usage . . . . . . . . . . . . . . . . . 13
4. Packet Syntax . . . . . . . . . . . . . . . . . . . . . . . . 14
4.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 14
4.2. Packet Headers . . . . . . . . . . . . . . . . . . . . . 14
4.2.1. Old Format Packet Lengths . . . . . . . . . . . . . . 15
4.2.2. New Format Packet Lengths . . . . . . . . . . . . . . 15
4.2.3. Packet Length Examples . . . . . . . . . . . . . . . 17
4.3. Packet Tags . . . . . . . . . . . . . . . . . . . . . . . 17
5. Packet Types . . . . . . . . . . . . . . . . . . . . . . . . 19
5.1. Public-Key Encrypted Session Key Packets (Tag 1) . . . . 19
5.2. Signature Packet (Tag 2) . . . . . . . . . . . . . . . . 21
5.2.1. Signature Types . . . . . . . . . . . . . . . . . . . 21
5.2.2. Version 3 Signature Packet Format . . . . . . . . . . 23
5.2.3. Version 4 and 5 Signature Packet Formats . . . . . . 26
5.2.4. Computing Signatures . . . . . . . . . . . . . . . . 48
Koch & Tse Expires 1 June 2024 [Page 2]
Internet-Draft LibrePGP Message Format November 2023
5.3. Symmetric-Key Encrypted Session Key Packets (Tag 3) . . . 51
5.4. One-Pass Signature Packets (Tag 4) . . . . . . . . . . . 53
5.5. Key Material Packet . . . . . . . . . . . . . . . . . . . 54
5.5.1. Key Packet Variants . . . . . . . . . . . . . . . . . 54
5.5.2. Public-Key Packet Formats . . . . . . . . . . . . . . 54
5.5.3. Secret-Key Packet Formats . . . . . . . . . . . . . . 56
5.6. Algorithm-specific Parts of Keys . . . . . . . . . . . . 58
5.6.1. Algorithm-Specific Part for RSA Keys . . . . . . . . 58
5.6.2. Algorithm-Specific Part for DSA Keys . . . . . . . . 59
5.6.3. Algorithm-Specific Part for Elgamal Keys . . . . . . 59
5.6.4. Algorithm-Specific Part for ECDSA Keys . . . . . . . 59
5.6.5. Algorithm-Specific Part for EdDSA Keys . . . . . . . 60
5.6.6. Algorithm-Specific Part for ECDH Keys . . . . . . . . 60
5.6.7. Algorithm-Specific Part for ML-KEM Keys . . . . . . . 61
5.7. Compressed Data Packet (Tag 8) . . . . . . . . . . . . . 61
5.8. Symmetrically Encrypted Data Packet (Tag 9) . . . . . . . 62
5.9. Marker Packet (Obsolete Literal Packet) (Tag 10) . . . . 63
5.10. Literal Data Packet (Tag 11) . . . . . . . . . . . . . . 63
5.11. Trust Packet (Tag 12) . . . . . . . . . . . . . . . . . . 65
5.12. User ID Packet (Tag 13) . . . . . . . . . . . . . . . . . 65
5.13. User Attribute Packet (Tag 17) . . . . . . . . . . . . . 65
5.13.1. The Image Attribute Subpacket . . . . . . . . . . . 66
5.13.2. User ID Attribute Subpacket . . . . . . . . . . . . 67
5.14. Sym. Encrypted Integrity Protected Data Packet (Tag
18) . . . . . . . . . . . . . . . . . . . . . . . . . . 67
5.15. Modification Detection Code Packet (Tag 19) . . . . . . . 70
5.16. OCB Encrypted Data Packet (Tag 20) . . . . . . . . . . . 71
5.16.1. EAX Mode . . . . . . . . . . . . . . . . . . . . . . 72
5.16.2. OCB Mode . . . . . . . . . . . . . . . . . . . . . . 73
6. Radix-64 Conversions . . . . . . . . . . . . . . . . . . . . 73
6.1. An Implementation of the CRC-24 in "C" . . . . . . . . . 74
6.2. Forming ASCII Armor . . . . . . . . . . . . . . . . . . . 74
6.3. Encoding Binary in Radix-64 . . . . . . . . . . . . . . . 77
6.4. Decoding Radix-64 . . . . . . . . . . . . . . . . . . . . 78
6.5. Examples of Radix-64 . . . . . . . . . . . . . . . . . . 79
6.6. Example of an ASCII Armored Message . . . . . . . . . . . 79
7. Cleartext Signature Framework . . . . . . . . . . . . . . . . 80
7.1. Dash-Escaped Text . . . . . . . . . . . . . . . . . . . . 80
8. Regular Expressions . . . . . . . . . . . . . . . . . . . . . 81
9. Constants . . . . . . . . . . . . . . . . . . . . . . . . . . 82
9.1. Public-Key Algorithms . . . . . . . . . . . . . . . . . . 82
9.2. ECC Curve OID . . . . . . . . . . . . . . . . . . . . . . 83
9.3. Symmetric-Key Algorithms . . . . . . . . . . . . . . . . 85
9.4. Compression Algorithms . . . . . . . . . . . . . . . . . 86
9.5. Hash Algorithms . . . . . . . . . . . . . . . . . . . . . 86
9.6. Encryption Modes . . . . . . . . . . . . . . . . . . . . 87
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 87
10.1. New String-to-Key Specifier Types . . . . . . . . . . . 88
Koch & Tse Expires 1 June 2024 [Page 3]
Internet-Draft LibrePGP Message Format November 2023
10.2. New Packets . . . . . . . . . . . . . . . . . . . . . . 88
10.2.1. User Attribute Types . . . . . . . . . . . . . . . . 88
10.2.2. Image Format Subpacket Types . . . . . . . . . . . . 89
10.2.3. New Signature Subpackets . . . . . . . . . . . . . . 89
10.2.4. New Packet Versions . . . . . . . . . . . . . . . . 92
10.3. New Algorithms . . . . . . . . . . . . . . . . . . . . . 92
10.3.1. Public-Key Algorithms . . . . . . . . . . . . . . . 92
10.3.2. Symmetric-Key Algorithms . . . . . . . . . . . . . . 93
10.3.3. Hash Algorithms . . . . . . . . . . . . . . . . . . 93
10.3.4. Compression Algorithms . . . . . . . . . . . . . . . 94
11. Packet Composition . . . . . . . . . . . . . . . . . . . . . 94
11.1. Transferable Public Keys . . . . . . . . . . . . . . . . 94
11.2. Transferable Secret Keys . . . . . . . . . . . . . . . . 96
11.3. LibrePGP Messages . . . . . . . . . . . . . . . . . . . 96
11.4. Detached Signatures . . . . . . . . . . . . . . . . . . 97
12. Enhanced Key Formats . . . . . . . . . . . . . . . . . . . . 97
12.1. Key Structures . . . . . . . . . . . . . . . . . . . . . 97
12.2. Key IDs and Fingerprints . . . . . . . . . . . . . . . . 99
13. Elliptic Curve Cryptography . . . . . . . . . . . . . . . . . 100
13.1. Supported ECC Curves . . . . . . . . . . . . . . . . . . 101
13.2. ECDSA and ECDH Conversion Primitives . . . . . . . . . . 101
13.3. EdDSA Point Format . . . . . . . . . . . . . . . . . . . 102
13.4. Key Derivation Function . . . . . . . . . . . . . . . . 102
13.5. ECDH Algorithm . . . . . . . . . . . . . . . . . . . . . 103
13.5.1. ECDH Parameters . . . . . . . . . . . . . . . . . . 105
14. Post-Quantum Cryptography . . . . . . . . . . . . . . . . . . 106
14.1. Kyber Algorithm . . . . . . . . . . . . . . . . . . . . 106
15. Notes on Algorithms . . . . . . . . . . . . . . . . . . . . . 106
15.1. PKCS#1 Encoding in LibrePGP . . . . . . . . . . . . . . 107
15.1.1. EME-PKCS1-v1_5-ENCODE . . . . . . . . . . . . . . . 107
15.1.2. EME-PKCS1-v1_5-DECODE . . . . . . . . . . . . . . . 107
15.1.3. EMSA-PKCS1-v1_5 . . . . . . . . . . . . . . . . . . 108
15.2. Symmetric Algorithm Preferences . . . . . . . . . . . . 109
15.3. Other Algorithm Preferences . . . . . . . . . . . . . . 110
15.3.1. Compression Preferences . . . . . . . . . . . . . . 110
15.3.2. Hash Algorithm Preferences . . . . . . . . . . . . . 111
15.4. Plaintext . . . . . . . . . . . . . . . . . . . . . . . 111
15.5. RSA . . . . . . . . . . . . . . . . . . . . . . . . . . 111
15.6. DSA . . . . . . . . . . . . . . . . . . . . . . . . . . 112
15.7. Elgamal . . . . . . . . . . . . . . . . . . . . . . . . 112
15.8. EdDSA . . . . . . . . . . . . . . . . . . . . . . . . . 112
15.9. Reserved Algorithm Numbers . . . . . . . . . . . . . . . 113
15.10. LibrePGP CFB Mode . . . . . . . . . . . . . . . . . . . 113
15.11. Private or Experimental Parameters . . . . . . . . . . . 114
15.12. Meta-Considerations for Expansion . . . . . . . . . . . 115
16. Security Considerations . . . . . . . . . . . . . . . . . . . 115
17. Compatibility Profiles . . . . . . . . . . . . . . . . . . . 120
17.1. LibrePGP ECC Profile . . . . . . . . . . . . . . . . . . 120
Koch & Tse Expires 1 June 2024 [Page 4]
Internet-Draft LibrePGP Message Format November 2023
18. Implementation Nits . . . . . . . . . . . . . . . . . . . . . 121
19. References . . . . . . . . . . . . . . . . . . . . . . . . . 122
19.1. Normative References . . . . . . . . . . . . . . . . . . 122
19.2. Informative References . . . . . . . . . . . . . . . . . 126
Appendix A. Test vectors . . . . . . . . . . . . . . . . . . . . 127
A.1. Sample EdDSA key . . . . . . . . . . . . . . . . . . . . 127
A.2. Sample EdDSA signature . . . . . . . . . . . . . . . . . 127
A.3. Sample OCB encryption and decryption . . . . . . . . . . 128
A.3.1. Sample Parameters . . . . . . . . . . . . . . . . . . 128
A.3.2. Sample symmetric-key encrypted session key packet
(v5) . . . . . . . . . . . . . . . . . . . . . . . . 128
A.3.3. Starting OCB decryption of CEK . . . . . . . . . . . 129
A.3.4. Sample OCB Encrypted Data packet . . . . . . . . . . 129
A.3.5. Decryption of data . . . . . . . . . . . . . . . . . 130
A.3.6. Complete OCB encrypted packet sequence . . . . . . . 130
Appendix B. ECC Point compression flag bytes . . . . . . . . . . 131
Appendix C. Changes since RFC-4880 . . . . . . . . . . . . . . . 131
Appendix D. Acknowledgments . . . . . . . . . . . . . . . . . . 133
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 133
1. Introduction
This document provides information on the message-exchange packet
formats used by LibrePGP to provide encryption, decryption, signing,
and key management functions. It is a revision of RFC 4880, "OpenPGP
Message Format", which is a revision of RFC 2440, which itself
replaces RFC 1991, "PGP Message Exchange Formats" [RFC1991] [RFC2440]
[RFC4880].
LibrePGP is fully compatible to the OpenPGP specification it is based
on: RFC 4880 (OpenPGP), RFC 5581 (Camellia cipher), and RFC 6637 (ECC
for OpenPGP).
1.1. Terms
* LibrePGP - This is a term for security software that is based on
OpenPGP with updates for newer algorithms and an advertency to
long-term stability and critical deployments. It is formalized in
this document.
* OpenPGP - This is a term for security software that uses PGP 5 as
a basis.
* PGP - Pretty Good Privacy. PGP is a family of software systems
developed by Philip R. Zimmermann from which OpenPGP is based.
Koch & Tse Expires 1 June 2024 [Page 5]
Internet-Draft LibrePGP Message Format November 2023
* PGP 2 - This version of PGP has many variants; where necessary a
more detailed version number is used here. PGP 2 uses only RSA,
MD5, and IDEA for its cryptographic transforms. An informational
RFC, RFC 1991, was written describing this version of PGP.
* PGP 5 - This version of PGP is formerly known as "PGP 3" in the
community. It has new formats and corrects a number of problems
in the PGP 2 design. It is referred to here as PGP 5 because that
software was the first release of the "PGP 3" code base.
* GnuPG - GNU Privacy Guard, also called GPG, is the leading Open
Source implementation of OpenPGP and LibrePGP and has been
developed along with the OpenPGP standard since 1997.
* RNP - Ribose Network PGP is a newer OpenPGP and LibrePGP
implemention and prominently used by the mail client Thunderbird.
"PGP" is a trademark of CA, INC. The use of this, or any other,
marks is solely for identification purposes. The terms "OpenPGP" and
"LibrePGP" refer 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", "EXPERT REVIEW", "SPECIFICATION
REQUIRED", "RFC REQUIRED", and "IETF REVIEW" that appear in this
document when used to describe namespace allocation are to be
interpreted as described in [RFC8126].
2. General functions
LibrePGP provides data integrity services for messages and data files
by using these core technologies:
* digital signatures
* encryption
* compression
* Radix-64 conversion
In addition, LibrePGP provides key management and certificate
services, but many of these are beyond the scope of this document.
Koch & Tse Expires 1 June 2024 [Page 6]
Internet-Draft LibrePGP Message Format November 2023
2.1. Confidentiality via Encryption
LibrePGP 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 LibrePGP 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 LibrePGP 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 LibrePGP decrypts the session key using the
recipient's private key.
6. The receiving LibrePGP 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:
Koch & Tse Expires 1 June 2024 [Page 7]
Internet-Draft LibrePGP Message Format November 2023
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
LibrePGP 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 and LibrePGP 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
LibrePGP'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 LibrePGP's native raw binary octets
through channels that are not safe to raw binary data, a printable
encoding of these binary octets is needed. LibrePGP 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.
Koch & Tse Expires 1 June 2024 [Page 8]
Internet-Draft LibrePGP Message Format November 2023
2.5. Signature-Only Applications
LibrePGP 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. Data Element Formats
This section describes the data elements used by LibrePGP.
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].
Koch & Tse Expires 1 June 2024 [Page 9]
Internet-Draft LibrePGP Message Format November 2023
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
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.
3.7.1. String-to-Key (S2K) Specifier Types
There are three types of S2K specifiers currently supported, and some
reserved values:
Koch & Tse Expires 1 June 2024 [Page 10]
Internet-Draft LibrePGP Message Format November 2023
+============+==========================+
| ID | S2K Type |
+============+==========================+
| 0 | Simple S2K |
+------------+--------------------------+
| 1 | Salted S2K |
+------------+--------------------------+
| 2 | Reserved value |
+------------+--------------------------+
| 3 | Iterated and Salted S2K |
+------------+--------------------------+
| 100 to 110 | Private/Experimental S2K |
+------------+--------------------------+
Table 1
These are described in the following Sections.
3.7.1.1. Simple S2K
This directly hashes the string to produce the key data. See below
for how this hashing is done.
Octet 0: 0x00
Octet 1: hash algorithm
Simple S2K hashes the passphrase to produce the session key. The
manner in which this is done depends on the size of the session key
(which will depend on the cipher used) and the size of the hash
algorithm's output. If the hash size is greater than the session key
size, the high-order (leftmost) octets of the hash are used as the
key.
If the hash size is less than the key size, multiple instances of the
hash context are created --- enough to produce the required key data.
These instances are preloaded with 0, 1, 2, ... octets of zeros (that
is to say, the first instance has no preloading, the second gets
preloaded with 1 octet of zero, the third is preloaded with two
octets of zeros, and so forth).
As the data is hashed, it is given independently to each hash
context. Since the contexts have been initialized differently, they
will each produce different hash output. Once the passphrase is
hashed, the output data from the multiple hashes is concatenated,
first hash leftmost, to produce the key data, with any excess octets
on the right discarded.
Koch & Tse Expires 1 June 2024 [Page 11]
Internet-Draft LibrePGP Message Format November 2023
3.7.1.2. Salted S2K
This includes a "salt" value in the S2K specifier --- some arbitrary
data --- that gets hashed along with the passphrase string, to help
prevent dictionary attacks.
Octet 0: 0x01
Octet 1: hash algorithm
Octets 2-9: 8-octet salt value
Salted S2K is exactly like Simple S2K, except that the input to the
hash function(s) consists of the 8 octets of salt from the S2K
specifier, followed by the passphrase.
3.7.1.3. Iterated and Salted S2K
This includes both a salt and an octet count. The salt is combined
with the passphrase and the resulting value is hashed repeatedly.
This further increases the amount of work an attacker must do to try
dictionary attacks.
Octet 0: 0x03
Octet 1: hash algorithm
Octets 2-9: 8-octet salt value
Octet 10: count, a one-octet, coded value
The count is coded into a one-octet number using the following
formula:
#define EXPBIAS 6
count = ((Int32)16 + (c & 15)) << ((c >> 4) + EXPBIAS);
The above formula is in C, where "Int32" is a type for a 32-bit
integer, and the variable "c" is the coded count, Octet 10.
Iterated-Salted S2K hashes the passphrase and salt data multiple
times. The total number of octets to be hashed is specified in the
encoded count in the S2K specifier. Note that the resulting count
value is an octet count of how many octets will be hashed, not an
iteration count.
Koch & Tse Expires 1 June 2024 [Page 12]
Internet-Draft LibrePGP Message Format November 2023
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
dictionary attacks.
3.7.2.1. Secret-Key Encryption
An S2K specifier can be stored in the secret keyring to specify how
to convert the passphrase to a key that unlocks the secret data.
Older versions of PGP just stored a cipher algorithm octet preceding
the secret data or a zero to indicate that the secret data was
unencrypted. The MD5 hash function was always used to convert the
passphrase to a key for the specified cipher algorithm.
For compatibility, when an S2K specifier is used, the special value
253, 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
possibilities:
0: secret data is unencrypted (no passphrase)
255, 254, or 253: 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.
Koch & Tse Expires 1 June 2024 [Page 13]
Internet-Draft LibrePGP Message Format November 2023
3.7.2.2. Symmetric-Key Message Encryption
LibrePGP 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 always used IDEA with Simple string-to-key conversion when
encrypting a message with a symmetric algorithm. This is deprecated,
but MAY be used for backward-compatibility.
4. Packet Syntax
This section describes the packets used by LibrePGP.
4.1. Overview
An LibrePGP 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 LibrePGP message, keyring,
certificate, and so forth consists of a number of packets. Some of
those packets may contain other LibrePGP packets (for example, a
compressed data packet, when uncompressed, contains LibrePGP
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
Koch & Tse Expires 1 June 2024 [Page 14]
Internet-Draft LibrePGP Message Format November 2023
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
4.2.1. Old Format Packet Lengths
The meaning of the length-type in old format packets is:
0 The packet has a one-octet length. The header is 2 octets long.
1 The packet has a two-octet length. The header is 3 octets long.
2 The packet has a four-octet length. The header is 5 octets long.
3 The packet is of indeterminate length. The header is 1 octet
long, and the implementation must determine how long the packet
is. If the packet is in a file, this means that the packet
extends until the end of the file. In general, an implementation
SHOULD NOT use indeterminate-length packets except where the end
of the data will be clear from the context, and even then it is
better to use a definite length, or a new format header. The new
format headers described below have a mechanism for precisely
encoding data of indeterminate length.
4.2.2. New Format Packet Lengths
New format packets have four possible ways of encoding length:
1. A one-octet Body Length header encodes packet lengths of up to
191 octets.
2. A two-octet Body Length header encodes packet lengths of 192 to
8383 octets.
Koch & Tse Expires 1 June 2024 [Page 15]
Internet-Draft LibrePGP Message Format November 2023
3. A five-octet Body Length header encodes packet lengths of up to
4,294,967,295 (0xFFFFFFFF) octets in length. (This actually
encodes a four-octet scalar number.)
4. When the length of the packet body is not known in advance by the
issuer, Partial Body Length headers encode a packet of
indeterminate length, effectively making it a stream.
4.2.2.1. One-Octet Lengths
A one-octet Body Length header encodes a length of 0 to 191 octets.
This type of length header is recognized because the one octet value
is less than 192. The body length is equal to:
bodyLen = 1st_octet;
4.2.2.2. Two-Octet Lengths
A two-octet Body Length header encodes a length of 192 to 8383
octets. It is recognized because its first octet is in the range 192
to 223. The body length is equal to:
bodyLen = ((1st_octet - 192) << 8) + (2nd_octet) + 192
4.2.2.3. Five-Octet Lengths
A five-octet Body Length header consists of a single octet holding
the value 255, followed by a four-octet scalar. The body length is
equal to:
bodyLen = (2nd_octet << 24) | (3rd_octet << 16) |
(4th_octet << 8) | 5th_octet
This basic set of one, two, and five-octet lengths is also used
internally to some packets.
4.2.2.4. Partial Body Lengths
A Partial Body Length header is one octet long and encodes the length
of only part of the data packet. This length is a power of 2, from 1
to 1,073,741,824 (2 to the 30th power). It is recognized by its one
octet value that is greater than or equal to 224, and less than 255.
The Partial Body Length is equal to:
partialBodyLen = 1 << (1st_octet & 0x1F);
Koch & Tse Expires 1 June 2024 [Page 16]
Internet-Draft LibrePGP Message Format November 2023
Each Partial Body Length header is followed by a portion of the
packet body data. The Partial Body Length header specifies this
portion's length. Another length header (one octet, two-octet, five-
octet, or partial) follows that portion. The last length header in
the packet MUST NOT be a Partial Body Length header. Partial Body
Length headers may only be used for the non-final parts of the
packet.
Note also that the last Body Length header can be a zero-length
header.
An implementation MAY use Partial Body Lengths for data packets, be
they literal, compressed, or encrypted. The first partial length
MUST be at least 512 octets long. Partial Body Lengths MUST NOT be
used for any other packet types.
4.2.3. Packet Length Examples
These examples show ways that new format packets might encode the
packet lengths.
A packet with length 100 may have its length encoded in one octet:
0x64. This is followed by 100 octets of data.
A packet with length 1723 may have its length encoded in two octets:
0xC5, 0xFB. This header is followed by the 1723 octets of data.
A packet with length 100000 may have its length encoded in five
octets: 0xFF, 0x00, 0x01, 0x86, 0xA0.
It might also be encoded in the following octet stream: 0xEF, first
32768 octets of data; 0xE1, next two octets of data; 0xE0, next one
octet of data; 0xF0, next 65536 octets of data; 0xC5, 0xDD, last 1693
octets of data. This is just one possible encoding, and many
variations are possible on the size of the Partial Body Length
headers, as long as a regular Body Length header encodes the last
portion of the data.
Please note that in all of these explanations, the total length of
the packet is the length of the header(s) plus the length of the
body.
4.3. Packet Tags
The packet tag denotes what type of packet the body holds. Note that
old format headers can only have tags less than 16, whereas new
format headers can have tags as great as 63. The defined tags (in
decimal) are as follows:
Koch & Tse Expires 1 June 2024 [Page 17]
Internet-Draft LibrePGP Message Format November 2023
+==========+====================================================+
| Tag | Packet Type |
+==========+====================================================+
| 0 | Reserved - a packet tag MUST NOT have this value |
+----------+----------------------------------------------------+
| 1 | Public-Key Encrypted Session Key Packet |
+----------+----------------------------------------------------+
| 2 | Signature Packet |
+----------+----------------------------------------------------+
| 3 | Symmetric-Key Encrypted Session Key Packet |
+----------+----------------------------------------------------+
| 4 | One-Pass Signature Packet |
+----------+----------------------------------------------------+
| 5 | Secret-Key Packet |
+----------+----------------------------------------------------+
| 6 | Public-Key Packet |
+----------+----------------------------------------------------+
| 7 | Secret-Subkey Packet |
+----------+----------------------------------------------------+
| 8 | Compressed Data Packet |
+----------+----------------------------------------------------+
| 9 | Symmetrically Encrypted Data Packet |
+----------+----------------------------------------------------+
| 10 | Marker Packet |
+----------+----------------------------------------------------+
| 11 | Literal Data Packet |
+----------+----------------------------------------------------+
| 12 | Trust Packet |
+----------+----------------------------------------------------+
| 13 | User ID Packet |
+----------+----------------------------------------------------+
| 14 | Public-Subkey Packet |
+----------+----------------------------------------------------+
| 17 | User Attribute Packet |
+----------+----------------------------------------------------+
| 18 | Sym. Encrypted and Integrity Protected Data Packet |
+----------+----------------------------------------------------+
| 19 | Modification Detection Code Packet |
+----------+----------------------------------------------------+
| 20 | OCB Encrypted Data Packet |
+----------+----------------------------------------------------+
| 21 | Reserved |
+----------+----------------------------------------------------+
| 26 | Reserved (CMS Encrypted Session Key Packet) |
+----------+----------------------------------------------------+
| 60 to 63 | Private or Experimental Values |
+----------+----------------------------------------------------+
Koch & Tse Expires 1 June 2024 [Page 18]
Internet-Draft LibrePGP Message Format November 2023
Table 2
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 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
LibrePGP 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.
Algorithm-Specific Fields for ECDH encryption:
- MPI of an EC point representing an ephemeral public key.
Koch & Tse Expires 1 June 2024 [Page 19]
Internet-Draft LibrePGP Message Format November 2023
- a one-octet size, followed by a symmetric key encoded using the
method described in Section 13.5.
Algorithm-Specific Fields for ML-KEM (Kyber) encryption: {Note:
This part is not finalized and subject to change}
- A fixed-length octet string representing an ECC ephemeral
public key in the format associated with the curve. For ML-
KEM-768+X25519 these are 32 octets, for ML-KEM-1024+X448 these
are 56 octets.
- A fixed-length octet string of the ML-KEM ciphertext, whose
length depends on the algorithm. For ML-KEM-768+X25519 these
are 1088 octets, for ML-KEM-1024+X448 these are 1568 octets.
- A one-octet algorithm identifier which must indicate one of the
AES algorithms (7, 8, or 9).
- a one-octet size, followed by a symmetric key wrapped using the
method described in Section 14.1.
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 15.1 of
this document for notes on LibrePGP'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 or 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.
Note that the confidentiality of a message is only post-quantum
secure when encrypting to multiple recipients iff only ML-KEM
algorithms are used for all recipients.
Koch & Tse Expires 1 June 2024 [Page 20]
Internet-Draft LibrePGP Message Format November 2023
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.
Three versions of Signature packets are defined. Version 3 provides
basic signature information, while versions 4, 5, and 6 provide an
expandable format with subpackets that can specify more information
about the signature. PGP 2.6.x only accepts version 3 signatures.
Implementations MUST generate version 5 signatures when using a
version 5 key. Version 6 signature MAY be generated if larger
subpacket data needs to be used. Implementations SHOULD generate V4
signatures with version 4 keys. Implementations MUST NOT create
version 3 signatures; they MAY accept version 3 signatures.
5.2.1. Signature Types
There are a number of possible meanings for a signature, which are
indicated in a signature type octet in any given signature. Please
note that the vagueness of these meanings is not a flaw, but a
feature of the system. Because LibrePGP places final authority for
validity upon the receiver of a signature, it may be that one
signer's casual act might be more rigorous than some other
authority's positive act. See Section 5.2.4, "Computing Signatures",
for detailed information on how to compute and verify signatures of
each type.
These meanings are as follows:
0x00 Signature of a binary document. This means the signer owns it,
created it, or certifies that it has not been modified.
0x01 Signature of a canonical text document. 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>.
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.
Koch & Tse Expires 1 June 2024 [Page 21]
Internet-Draft LibrePGP Message Format November 2023
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.
Most LibrePGP implementations make their "key signatures" as 0x10
certifications. Some implementations can issue 0x11-0x13
certifications, but few differentiate between the types.
0x16 Attested Key Signature. This signature is issued by the
primary key over itself and its User ID (or User Attribute). It
MUST contain an "Attested Certifications" subpacket and a
"Signature Creation Time" subpacket. This type of key signature
does not replace or override any standard certification
(0x10-0x13).
Only the most recent Attestation Key Signature is valid for any
given <key,userid> pair. If more than one Certification
Attestation Key Signature is present with the same Signature
Creation Time, the set of attestations should be treated as the
union of all "Attested Certifications" subpackets from all such
signatures with the same timestamp.
0x18 Subkey Binding Signature. This signature is a statement by the
top-level signing key that indicates that it owns the subkey.
This signature is calculated directly on the primary key and
subkey, and not on any User ID or other packets. A signature that
binds a signing subkey MUST have an Embedded Signature subpacket
in this binding signature that contains a 0x19 signature made by
the signing subkey on the primary key and subkey.
0x19 Primary Key Binding Signature. This signature is a statement
by a signing subkey, indicating that it is owned by the primary
key and subkey. This signature is calculated the same way as a
0x18 signature: directly on the primary key and subkey, and 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
Koch & Tse Expires 1 June 2024 [Page 22]
Internet-Draft LibrePGP Message Format November 2023
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) or direct-key signature (0x1F). It should be issued
by the same key that issued the revoked signature or an authorized
revocation key. The signature is computed over the same data as
the certificate that it revokes, and should have a later creation
date than that certificate.
0x40 Timestamp signature. This signature is only meaningful for the
timestamp contained in it.
0x50 Third-Party Confirmation signature. This signature is a
signature over some other LibrePGP Signature packet(s). It is
analogous to a notary seal on the signed data. A third-party
signature SHOULD include Signature Target subpacket(s) to give
easy identification. Note that we really do mean SHOULD. There
are plausible uses for this (such as a blind party that only sees
the signature, not the key or source document) that cannot include
a target subpacket.
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.
Koch & Tse Expires 1 June 2024 [Page 23]
Internet-Draft LibrePGP Message Format November 2023
* 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 multiprecision integers comprising the signature.
This portion is algorithm specific, as described below.
The concatenation of the data to be signed, the signature type,
and creation time from the Signature packet (5 additional octets)
is hashed. 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 way to reject some
invalid signatures without performing a signature verification.
Algorithm-Specific Fields for RSA signatures:
- Multiprecision integer (MPI) of RSA signature value m**d mod n.
Algorithm-Specific Fields for DSA and ECDSA signatures:
- MPI of DSA or ECDSA value r.
- MPI of DSA or ECDSA 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 signatures than for RSA signatures.
With RSA signatures, the hash value is encoded using PKCS#1 encoding
type EMSA-PKCS1-v1_5 as described in Section 9.2 of RFC 3447. 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 as follows:
Koch & Tse Expires 1 June 2024 [Page 24]
Internet-Draft LibrePGP Message Format November 2023
- MD5: 0x2A, 0x86, 0x48, 0x86, 0xF7, 0x0D, 0x02, 0x05
- RIPEMD-160: 0x2B, 0x24, 0x03, 0x02, 0x01
- SHA-1: 0x2B, 0x0E, 0x03, 0x02, 0x1A
- SHA2-224: 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x04
- SHA2-256: 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x01
- SHA2-384: 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x02
- SHA2-512: 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x03
The ASN.1 Object Identifiers (OIDs) are as follows:
- MD5: 1.2.840.113549.2.5
- RIPEMD-160: 1.3.36.3.2.1
- SHA-1: 1.3.14.3.2.26
- SHA2-224: 2.16.840.1.101.3.4.2.4
- SHA2-256: 2.16.840.1.101.3.4.2.1
- SHA2-384: 2.16.840.1.101.3.4.2.2
- SHA2-512: 2.16.840.1.101.3.4.2.3
The full hash prefixes for these are as follows:
Koch & Tse Expires 1 June 2024 [Page 25]
Internet-Draft LibrePGP Message Format November 2023
- 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
- SHA2-224: 0x30, 0x2D, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86,
0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x04, 0x05,
0x00, 0x04, 0x1C
- SHA2-256: 0x30, 0x31, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86,
0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x01, 0x05,
0x00, 0x04, 0x20
- SHA2-384: 0x30, 0x41, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86,
0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x02, 0x05,
0x00, 0x04, 0x30
- SHA2-512: 0x30, 0x51, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86,
0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x03, 0x05,
0x00, 0x04, 0x40
DSA signatures MUST use hashes that are equal in size to the number
of bits of q, the group generated by the DSA key's generator value.
If the output size of the chosen hash is larger than the number of
bits of q, the hash result is truncated to fit by taking the number
of leftmost bits equal to the number of bits of q. This (possibly
truncated) hash function result is treated as a number and used
directly in the DSA signature algorithm.
5.2.3. Version 4 and 5 Signature Packet Formats
The body of a V4, V5, and V6 Signature packet contains:
* One-octet version number. This is 4 for V4 signatures, 5 for V5
signatures, and 6 vor V6 signature.
* One-octet signature type.
* One-octet public-key algorithm.
* One-octet hash algorithm.
Koch & Tse Expires 1 June 2024 [Page 26]
Internet-Draft LibrePGP Message Format November 2023
* Two-octet scalar octet count for following hashed subpacket data.
For V6 signatures this is a four-octet field. 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 set (zero or more subpackets).
* Two-octet scalar octet count for the following unhashed subpacket
data. For V6 signatures this is a four-octet field. 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 set (zero or more subpackets).
* Two-octet field holding the left 16 bits of the signed hash value.
* Only for V6 signatures, a variable-length field containing:
- A one-octet salt size. If salted signatures are used the value
SHOULD match the digest length of the hash algorithm. The
common use case is not to use salted signatures.
- The salt; a random value of the specified size.
* One or more multiprecision integers comprising the signature.
This portion is algorithm specific:
Algorithm-Specific Fields for RSA signatures:
- Multiprecision integer (MPI) of RSA signature value m**d mod n.
Algorithm-Specific Fields for DSA or ECDSA signatures:
- MPI of DSA or ECDSA value r.
- MPI of DSA or ECDSA value s.
Algorithm-Specific Fields for EdDSA signatures:
- MPI of an EC point r.
- EdDSA value s, in MPI, in the little endian representation.
The format of R and S for use with EdDSA is described in [RFC8032].
A version 3 signature MUST NOT be created and MUST NOT be used with
EdDSA.
Koch & Tse Expires 1 June 2024 [Page 27]
Internet-Draft LibrePGP Message Format November 2023
The concatenation of the data being signed and the signature data
from the version number through the hashed subpacket data (inclusive)
is hashed. The resulting hash value is what is signed. The high 16
bits (first two octets) of the hash are included in the Signature
packet to provide a way to reject some invalid signatures without
performing a signature verification.
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 difference between a V4 and V5 signature is that the latter
includes additional meta data.
The algorithms for converting the hash function result to a signature
are described in a section below.
5.2.3.1. Signature Subpacket Specification
A subpacket data set consists of zero or more Signature subpackets.
In Signature packets, the subpacket data set is preceded by a two-
octet scalar count of the length in octets of all the subpackets. A
pointer incremented by this number will skip over the subpacket data
set.
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:
Koch & Tse Expires 1 June 2024 [Page 28]
Internet-Draft LibrePGP Message Format November 2023
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:
+============+========================================+
| Type | Description |
+============+========================================+
| 0 | Reserved |
+------------+----------------------------------------+
| 1 | Reserved |
+------------+----------------------------------------+
| 2 | Signature Creation Time |
+------------+----------------------------------------+
| 3 | Signature Expiration Time |
+------------+----------------------------------------+
| 4 | Exportable Certification |
+------------+----------------------------------------+
| 5 | Trust Signature |
+------------+----------------------------------------+
| 6 | Regular Expression |
+------------+----------------------------------------+
| 7 | Revocable |
+------------+----------------------------------------+
| 8 | Reserved |
+------------+----------------------------------------+
| 9 | Key Expiration Time |
+------------+----------------------------------------+
| 10 | Placeholder for backward compatibility |
+------------+----------------------------------------+
| 11 | Preferred Symmetric Algorithms |
+------------+----------------------------------------+
| 12 | Revocation Key |
+------------+----------------------------------------+
| 13 to 15 | Reserved |
+------------+----------------------------------------+
| 16 | Issuer |
+------------+----------------------------------------+
| 17 to 19 | Reserved |
Koch & Tse Expires 1 June 2024 [Page 29]
Internet-Draft LibrePGP Message Format November 2023
+------------+----------------------------------------+
| 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 URI |
+------------+----------------------------------------+
| 27 | Key Flags |
+------------+----------------------------------------+
| 28 | Signer's User ID |
+------------+----------------------------------------+
| 29 | Reason for Revocation |
+------------+----------------------------------------+
| 30 | Features |
+------------+----------------------------------------+
| 31 | Signature Target |
+------------+----------------------------------------+
| 32 | Embedded Signature |
+------------+----------------------------------------+
| 33 | Issuer Fingerprint |
+------------+----------------------------------------+
| 34 | Preferred Encryption Modes |
+------------+----------------------------------------+
| 35 | Intended Recipient Fingerprint |
+------------+----------------------------------------+
| 37 | Attested Certifications |
+------------+----------------------------------------+
| 38 | Key Block |
+------------+----------------------------------------+
| 39 | Reserved |
+------------+----------------------------------------+
| 40 | Literal Data Meta Hash |
+------------+----------------------------------------+
| 41 | Trust Alias |
+------------+----------------------------------------+
| 100 to 110 | Private or experimental |
+------------+----------------------------------------+
Table 3
Koch & Tse Expires 1 June 2024 [Page 30]
Internet-Draft LibrePGP Message Format November 2023
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 the four preferred algorithm
subpackets (11, 21, 22, and 34), as well as the "Reason for
Revocation" subpacket. Note, however, that if an implementation
chooses not to implement some of the preferences, it is required to
behave in a polite manner to respect the wishes of those users who do
implement these 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
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 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. Notes on Self-Signatures
A self-signature is a binding signature made by the key to which the
signature refers. 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 user name, 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.
Koch & Tse Expires 1 June 2024 [Page 31]
Internet-Draft LibrePGP Message Format November 2023
Implementing software should interpret a self-signature's preference
subpackets as narrowly as possible. For example, suppose a key has
two user names, Alice and Bob. Suppose that Alice prefers the
symmetric algorithm AES-256, and Bob prefers Camellia-256 or AES-128.
If the software locates this key via Alice's name, then the preferred
algorithm is AES-256; if software locates the key via Bob's name,
then the preferred algorithm is Camellia-256. If the key is located
by Key ID, the algorithm of the primary User ID of the key provides
the preferred symmetric algorithm.
Revoking a self-signature or allowing it to expire has a semantic
meaning that varies with the signature type. Revoking the self-
signature on a User ID effectively retires that user name. The self-
signature is a statement, "My name X is tied to my signing key K" and
is corroborated by other users' certifications. If another user
revokes their certification, they are effectively saying that they no
longer believe that name and that key are tied together. Similarly,
if the users themselves revoke their self-signature, then the users
no longer go by that name, no longer have that email address, etc.
Revoking a binding signature effectively retires that subkey.
Revoking a direct-key signature cancels that signature. Please see
the "Reason for Revocation" subpacket (Section 5.2.3.25) for more
relevant detail.
Since a self-signature contains important information about the key's
use, an implementation SHOULD allow the user to rewrite the self-
signature, and important information in it, such as preferences and
key expiration.
It is good practice to verify that a self-signature imported into an
implementation doesn't advertise features that the implementation
doesn't support, rewriting the signature as appropriate.
An implementation that encounters multiple self-signatures on the
same object may resolve the ambiguity in any way it sees fit, but it
is RECOMMENDED that priority be given to the most recent self-
signature.
5.2.3.4. Signature Creation Time
(4-octet time field)
The time the signature was made.
MUST be present in the hashed area.
Koch & Tse Expires 1 June 2024 [Page 32]
Internet-Draft LibrePGP Message Format November 2023
5.2.3.5. Issuer
(8-octet Key ID)
The LibrePGP Key ID of the key issuing the signature. If the version
of that key is greater than 4, this subpacket MUST NOT be included in
the signature.
5.2.3.6. 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.
5.2.3.7. Preferred Symmetric Algorithms
(array 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 are in Section 9. This is only found on a self-
signature.
5.2.3.8. Preferred Encryption Modes
(array of one-octet values)
This is a deprecated subpacket with encryption mode numbers to
indicate which modes the key holder prefers to use with OCB Encrypted
Data Pakets. Implementations SHOULD ignore this subpacket and assume
OCB. The subpacket body is an ordered list of octets with the most
preferred listed first. It is assumed that only modes listed are
supported by the recipient's software. Mode numbers are in
Section 9.6. This is only found on a self-signature. Note that
support for the OCB Encrypted Data packet in the general is indicated
by a Feature Flag.
5.2.3.9. Preferred Hash Algorithms
(array of one-octet values)
Koch & Tse Expires 1 June 2024 [Page 33]
Internet-Draft LibrePGP Message Format November 2023
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 9.5. This is only found on a self-signature.
5.2.3.10. 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 9.4. 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.
5.2.3.11. 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.
5.2.3.12. 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.
Koch & Tse Expires 1 June 2024 [Page 34]
Internet-Draft LibrePGP Message Format November 2023
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.
5.2.3.13. Revocable
(1 octet of revocability, 0 for not, 1 for revocable)
Signature's revocability status. The 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.
5.2.3.14. 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.
5.2.3.15. Regular Expression
(null-terminated regular expression)
Koch & Tse Expires 1 June 2024 [Page 35]
Internet-Draft LibrePGP Message Format November 2023
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 [REGEX] package. A
description of the syntax is found in Section 8 below.
5.2.3.16. Trust Alias
(String)
This subpacket allows a keyholder to state an alias for a mail
address in the User ID. The value is expected to be the UTF-8
encoded addr-spec part of a mail address.
If used in conjunction with a Trust Signature subpacket of level > 0
and a Regular Expression subpacket the User ID is not considered and
the trust is only extended if at least one Trust Alias matches the
regular expression.
If this subpacket is found on a self-signature, a signature done with
a Trust Signature subpacket of level > 0 and a Regular Expression
subpacket is computed over the User ID, an octet 0x00, and the value
of the Trust Alias subpacket. If more than one Trust Alias subpacket
exists all those subpackets are first sorted using binary ordering
and then concatenated using an octet 0x00 as separator.
This subpacket is useful to handle the common case that mails with
and without subdomains are used. For example alice@example.org is
her canonical address but she receives mail also under the address
alice@lab.example.org. All other properties are identical for the
second address and thus a second User ID packet is not useful here.
5.2.3.17. Revocation Key
(1 octet of class, 1 octet of public-key algorithm ID, 20 or 32
octets of fingerprint)
V4 keys use the full 20 octet fingerprint; V4 keys with the Class
octet bit 0x20 set use the extended 32 octet v4 fingerprint; V5 keys
use the full 32 octet fingerprint.
Koch & Tse Expires 1 June 2024 [Page 36]
Internet-Draft LibrePGP Message Format November 2023
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. Bit
0x20 is used as described above. Other bits are for future expansion
to other kinds of authorizations. This is only found on a direct-key
self-signature (type 0x1f). The use on other types of self-
signatures is unspecified.
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.
5.2.3.18. 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 as follows:
First octet: 0x80 = human-readable. This note value is text.
Other octets: none.
Notation names are arbitrary strings encoded in UTF-8. They reside
in two namespaces: The IETF namespace and the user namespace.
The IETF namespace is registered with IANA. These names MUST NOT
contain the "@" character (0x40). This is a tag for the user
namespace.
Names in the user namespace consist of a UTF-8 string tag followed by
"@" followed by a DNS domain name. Note that the tag MUST NOT
contain an "@" character. For example, the "sample" tag used by
Example Corporation could be "sample@example.com".
Koch & Tse Expires 1 June 2024 [Page 37]
Internet-Draft LibrePGP Message Format November 2023
Names in a user space are owned and controlled by the owners of that
domain. Obviously, it's bad form to create a new name in a DNS space
that you don't own.
Since the user namespace is in the form of an email address,
implementers MAY wish to arrange for that address to reach a person
who can be consulted about the use of the named tag. Note that due
to UTF-8 encoding, not all valid user space name tags are valid email
addresses.
If there is a critical notation, the criticality applies to that
specific notation and not to notations in general.
The following subsections define a set of standard notations.
5.2.3.18.1. The 'charset' Notation
The "charset" notation is a description of the character set used to
encode the signed plaintext. The default value is "UTF-8". If used,
the value MUST be encoded as human readable and MUST be present in
the hashed subpacket section of the signature. This notation is
useful for cleartext signatures in cases where it is not possible to
encode the text in UTF-8. By having the used character set a part of
the signed data, attacks exploiting different representation of code
points will be mitigated.
5.2.3.18.2. The 'manu' Notation
The "manu" notation is a string that declares the device
manufacturer's name. The certifier key is asserting this string
(which may or may not be related to the User ID of the certifier's
key).
5.2.3.18.3. The 'make' Notation
This notation defines the product make. It is a free form string.
5.2.3.18.4. The 'model' Notation
This notation defines the product model name/number. It is a free
form string.
5.2.3.18.5. The 'prodid' Notation
This notation contains the product identifier. It is a free form
string.
Koch & Tse Expires 1 June 2024 [Page 38]
Internet-Draft LibrePGP Message Format November 2023
5.2.3.18.6. The 'pvers' Notation
This notation defines the product version number (which could be a
release number, year, or some other identifier to differentiate
different versions of the same make/model). It is a free form
string.
5.2.3.18.7. The 'lot' Notation
This notation defines the product lot number (which is an indicator
of the batch of product). It is a free form string.
5.2.3.18.8. The 'qty' Notation
This notation defines the quantity of items in this package. It is a
decimal integer representation with no punctuation, e.g. "10",
"1000", "10000", etc.
5.2.3.18.9. The 'loc' and 'dest' Notations
The "loc" and 'dest' notations declare a GeoLocation as defined by
RFC 5870 [RFC5870] but without the leading "geo:" header. For
example, if you had a GeoLocation URI of "geo:13.4125,103.8667" you
would encode that in these notations as "13.4125,103.8667".
The 'loc' notation is meant to encode the geo location where the
signature was made. The 'dest' notation is meant to encode the geo
location where the device is "destined" (i.e., a "destination" for
the device).
5.2.3.18.10. The 'hash' Notation
A 'hash' notation is a means to include external data in the contents
of a signature without including the data itself. This is done by
hashing the external data separately and then including the data's
name and hash in the signature via this notation. This is useful,
for example, to have an external "manifest," "image," or other data
that might not be vital to the signature itself but still needs to be
protected and authenticated without requiring a second signature.
The 'hash' notation has the following structure:
* A single byte specifying the length of the name of the hashed
data.
* A UTF-8 string of the name of the hashed data.
* A single byte specifying the hash algorithm (see section 9.4).
Koch & Tse Expires 1 June 2024 [Page 39]
Internet-Draft LibrePGP Message Format November 2023
* The binary hash output of the hashed data using the specified
algorithm. (The length of this data is implicit based on the
algorithm specified).
Due to its nature a 'hash' notation is not human readable and MUST
NOT be marked as such when used.
5.2.3.19. Key Server Preferences
(N octets of flags)
This is a list of one-bit 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.
If No-modify is set on the most recent self-sig over a User ID,
then a keyserver should only redistribute those third-party
certifications over that User ID that have been attested to in the
most recent Attestation Key Signature packet (see "Attested
Certifications" below).
This is found only on a self-signature.
5.2.3.20. Preferred Key Server
(String)
This is a URI 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 URI, the
key server can actually be a copy of the key retrieved by ftp, http,
finger, etc.
5.2.3.21. Primary User ID
(1 octet, Boolean)
Koch & Tse Expires 1 June 2024 [Page 40]
Internet-Draft LibrePGP Message Format November 2023
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, but
it is RECOMMENDED that priority be given to the User ID with the most
recent self-signature.
When appearing on a self-signature on a User ID packet, this
subpacket applies only to User ID packets. When appearing on a self-
signature on a User Attribute packet, this subpacket applies only to
User Attribute packets. That is to say, there are two different and
independent "primaries" --- one for User IDs, and one for User
Attributes.
5.2.3.22. Policy URI
(String)
This subpacket contains a URI of a document that describes the policy
under which the signature was issued.
5.2.3.23. Key Flags
(N octets of flags)
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 as follows:
First octet:
Koch & Tse Expires 1 June 2024 [Page 41]
Internet-Draft LibrePGP Message Format November 2023
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.
0x20 - This key may be used for authentication.
0x80 - The private component of this key may be in the possession
of more than one person.
Second octet:
0x04 - This key may be used to encrypt communications or storage.
0x08 - This key may be used for timestamping.
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" (1st,0x10) and "group key" (1st,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.
The "restricted encryption key" (2nd,0x04) does not take part in any
automatic selection of encryption keys. It is only found on a subkey
signature (type 0x18), one that refers to the key the flag applies
to.
Koch & Tse Expires 1 June 2024 [Page 42]
Internet-Draft LibrePGP Message Format November 2023
5.2.3.24. Signer's User ID
(String)
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.
This subpacket is not appropriate to use to refer to a User Attribute
packet.
5.2.3.25. 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:
+=========+==================================+
| Code | Reason |
+=========+==================================+
| 0 | No reason specified (key |
| | revocations or cert revocations) |
+---------+----------------------------------+
| 1 | Key is superseded (key |
| | revocations) |
+---------+----------------------------------+
| 2 | Key material has been |
| | compromised (key revocations) |
+---------+----------------------------------+
| 3 | Key is retired and no longer |
| | used (key revocations) |
+---------+----------------------------------+
| 32 | User ID information is no longer |
| | valid (cert revocations) |
+---------+----------------------------------+
| 100-110 | Private Use |
+---------+----------------------------------+
Table 4
Koch & Tse Expires 1 June 2024 [Page 43]
Internet-Draft LibrePGP Message Format November 2023
Following the revocation code is a string of octets that 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.
An implementation SHOULD implement this subpacket, include it in all
revocation signatures, and interpret revocations appropriately.
There are important semantic differences between the reasons, and
there are thus important reasons for revoking signatures.
If a key has been revoked because of a compromise, all signatures
created by that key are suspect. However, if it was merely
superseded or retired, old signatures are still valid. If the
revoked signature is the self-signature for certifying a User ID, a
revocation denotes that that user name is no longer in use. Such a
revocation SHOULD include a 0x20 code.
Note that any signature may be revoked, including a certification on
some other person's key. There are many good reasons for revoking a
certification signature, such as the case where the keyholder leaves
the employ of a business with an email address. A revoked
certification is no longer a part of validity calculations.
5.2.3.26. Features
(N octets of flags)
The Features subpacket denotes which advanced LibrePGP features a
user's implementation supports. This is so that as features are
added to LibrePGP that cannot be backwards-compatible, a user can
state that they can use that feature. The flags are single bits that
indicate that a given feature is supported.
This subpacket is similar to a preferences subpacket, and only
appears in a self-signature.
An implementation SHOULD NOT use a feature listed when sending to a
user who does not state that they can use it.
Defined features are as follows:
First octet:
Koch & Tse Expires 1 June 2024 [Page 44]
Internet-Draft LibrePGP Message Format November 2023
0x01 - Modification Detection (packets 18 and 19)
0x02 - OCB Encrypted Data Packet (packet 20) and version 5
Symmetric-Key Encrypted Session Key Packets (packet 3)
0x04 - Version 5 Public-Key Packet format and corresponding new
fingerprint format
If an implementation implements any of the defined features, it
SHOULD implement the Features subpacket, too.
An implementation may freely infer features from other suitable
implementation-dependent mechanisms.
5.2.3.27. Signature Target
(1 octet public-key algorithm, 1 octet hash algorithm, N octets hash)
This subpacket identifies a specific target signature to which a
signature refers. For revocation signatures, this subpacket provides
explicit designation of which signature is being revoked. For a
third-party or timestamp signature, this designates what signature is
signed. All arguments are an identifier of that target signature.
The N octets of hash data MUST be the size of the hash of the
signature. For example, a target signature with a SHA-1 hash MUST
have 20 octets of hash data.
5.2.3.28. Embedded Signature
(1 signature packet body)
This subpacket contains a complete Signature packet body as specified
in Section 5.2 above. It is useful when one signature needs to refer
to, or be incorporated in, another signature.
5.2.3.29. Issuer Fingerprint
(1 octet key version number, N octets of fingerprint)
The LibrePGP Key fingerprint of the key issuing the signature. This
subpacket SHOULD be included in all signatures. If the version of
the issuing key is 4 and an Issuer subpacket is also included in the
signature, the key ID of the Issuer subpacket MUST match the low 64
bits of the fingerprint.
Note that the length N of the fingerprint for a version 4 key is 20
octets; for a version 5 key N is 32.
Koch & Tse Expires 1 June 2024 [Page 45]
Internet-Draft LibrePGP Message Format November 2023
5.2.3.30. Intended Recipient Fingerprint
(1 octet key version number, N octets of fingerprint)
The LibrePGP Key fingerprint of the intended recipient primary key.
If one or more subpackets of this type are included in a signature,
it SHOULD be considered valid only in an encrypted context, where the
key it was encrypted to is one of the indicated primary keys, or one
of their subkeys. This can be used to prevent forwarding a signature
outside of its intended, encrypted context.
Note that the length N of the fingerprint for a version 4 key is 20
octets; for a version 5 key N is 32.
5.2.3.31. Attested Certifications
(N octets of certification digests)
This subpacket MUST only appear as a hashed subpacket of an
Attestation Key Signature. It has no meaning in any other signature
type. It is used by the primary key to attest to a set of third-
party certifications over the associated User ID or User Attribute.
This enables the holder of an LibrePGP primary key to mark specific
third-party certifications as re-distributable with the rest of the
Transferable Public Key (see the "No-modify" flag in "Key Server
Preferences", above). Implementations MUST include exactly one
Attested Certification subpacket in any generated Attestation Key
Signature.
The contents of the subpacket consists of a series of digests using
the same hash algorithm used by the signature itself. Each digest is
made over one third-party signature (any Certification, i.e.,
signature type 0x10-0x13) that covers the same Primary Key and User
ID (or User Attribute). For example, an Attestation Key Signature
made by key X over User ID U using hash algorithm SHA256 might
contain an Attested Certifications subpacket of 192 octets (6*32
octets) covering six third-party certification Signatures over <X,U>.
They SHOULD be ordered by binary hash value from low to high (e.g., a
hash with hexadecimal value 037a... precedes a hash with value
0392..., etc). The length of this subpacket MUST be an integer
multiple of the length of the hash algorithm used for the enclosing
Attestation Key Signature.
The listed digests MUST be calculated over the third-party
certification's Signature packet as described in the "Computing
Signatures" section, but without a trailer: the hash data starts with
the octet 0x88, followed by the four-octet length of the Signature,
and then the body of the Signature packet. (Note that this is an
Koch & Tse Expires 1 June 2024 [Page 46]
Internet-Draft LibrePGP Message Format November 2023
old-style packet header for a Signature packet with the length-of-
length field set to zero.) The unhashed subpacket data of the
Signature packet being hashed is not included in the hash, and the
unhashed subpacket data length value is set to zero.
If an implementation encounters more than one such subpacket in an
Attestation Key Signature, it MUST treat it as a single Attested
Certifications subpacket containing the union of all hashes.
The Attested Certifications subpacket in the most recent Attestation
Key Signature over a given User ID supersedes all Attested
Certifications subpackets from any previous Attestation Key
Signature. However, note that if more than one Attestation Key
Signatures has the same (most recent) Signature Creation Time
subpacket, implementations MUST consider the union of the
attestations of all Attestation Key Signatures (this allows the
keyholder to attest to more third-party certifications than could fit
in a single Attestation Key Signature).
If a keyholder Alice has already attested to third-party
certifications from Bob and Carol and she wants to add an attestation
to a certification from David, she should issue a new Attestation Key
Signature (with a more recent Signature Creation timestamp) that
contains an Attested Certifications subpacket covering all three
third-party certifications.
If she later decides that she does not want Carol's certification to
be redistributed with her certificate, she can issue a new
Attestation Key Signature (again, with a more recent Signature
Creation timestamp) that contains an Attested Certifications
subpacket covering only the certifications from Bob and David.
Note that Certification Revocation Signatures are not relevant for
Attestation Key Signatures. To rescind all attestations, the primary
key holder needs only to publish a more recent Attestation Key
Signature with an empty Attested Certifications subpacket.
5.2.3.32. Key Block
(1 octet with value 0, N octets of key data)
This subpacket MAY be used to convey key data along with a signature
of class 0x00, 0x01, or 0x02. It MUST contain the key used to create
the signature; either as the primary key or as a subkey. The key
SHOULD contain a primary or subkey capable of encryption and the
entire key must be a valid LibrePGP key including at least one User
ID packet and the corresponding self-signatures.
Koch & Tse Expires 1 June 2024 [Page 47]
Internet-Draft LibrePGP Message Format November 2023
Implementations MUST ignore this subpacket if the first octet does
not have a value of zero or if the key data does not represent a
valid transferable public key.
5.2.3.33. Literal Data Meta Hash
(1 octet with value 0, 32 octets hash value)
This subpacket MAY be used to protect the meta data from the Literal
Data Packet with V4 signatures. The hash is computed using SHA2-256
from this data:
* the one-octet content format,
* the file name as a string (one octet length, followed by the file
name),
* a four-octet number that indicates a date.
These three data items need to mirror the corresponding values of the
Literal Data packet. Implementations encountering this subpacket
must re-create the hash from the received Literal Data packet and
compare them. If the hash values do not match or if this packet is
used in a V5 signature the signature MUST be deemed as invalid.
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.
When a V6 signature with a salt is made, the salt is hashed first.
For binary document signatures (type 0x00), the document data is
hashed directly. For text document signatures (type 0x01), the
document is canonicalized by converting line endings to <CR><LF>, and
the resulting data is hashed.
When a V4 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; when a V5 or V6 signature is made over a key, the
hash data starts with the octet 0x9a for V5 and 0x9b for V6, followed
by a four-octet length of the key, and then body of the key packet.
A subkey binding signature (type 0x18) or primary key binding
signature (type 0x19) then hashes the subkey using the same format as
the main key (also using 0x99 or 0x9a as the first octet). Primary
key revocation signatures (type 0x20) hash only the key being
revoked. Subkey revocation signature (type 0x28) hash first the
primary key and then the subkey being revoked.
Koch & Tse Expires 1 June 2024 [Page 48]
Internet-Draft LibrePGP Message Format November 2023
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 User ID or attribute
packet packet, without any header. A V4 or V5 certification hashes
the constant 0xB4 for User ID certifications or the constant 0xD1 for
User Attribute certifications, followed by a four-octet number giving
the length of the User ID or User Attribute data, and then the User
ID or User Attribute data.
An Attestation Key Signature (0x16) hashes the same data body that a
standard certification signature does: primary key, followed by User
ID or User Attribute.
When a signature is made over a Signature packet (type 0x50, "Third-
Party Confirmation signature"), the hash data starts with the octet
0x88, followed by the four-octet length of the signature, and then
the body of the Signature packet. (Note that this is an old-style
packet header for a Signature packet with the length-of-length field
set to zero.) The unhashed subpacket data of the Signature packet
being hashed is not included in the hash, and the unhashed subpacket
data length value is set to zero.
Once the data body is hashed, then a trailer is hashed. This trailer
depends on the version of the signature.
* 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 and a final extra trailer. Thus, the hashed fields are:
- the signature version (0x04),
- the signature type,
- the public-key algorithm,
- the hash algorithm,
- the hashed subpacket length,
- the hashed subpacket body,
- the two octets 0x04 and 0xFF,
Koch & Tse Expires 1 June 2024 [Page 49]
Internet-Draft LibrePGP Message Format November 2023
- a four-octet big-endian number that is the length of the hashed
data from the Signature packet stopping right before the 0x04,
0XFF octets.
The four-octet big-endian number is considered to be an
unsigned integer modulo 2^32.
* A V5 signature hashes the packet body starting from its first
field, the version number, through the end of the hashed subpacket
data and a final extra trailer. Thus, the hashed fields are:
- the signature version (0x05),
- the signature type,
- the public-key algorithm,
- the hash algorithm,
- the hashed subpacket length,
- the hashed subpacket body,
- Only for document signatures (type 0x00 or 0x01) the following
three data items are hashed here:
- the one-octet content format,
- the file name as a string (one octet length, followed by
the file name),
- a four-octet number that indicates a date,
- the two octets 0x05 and 0xFF,
- a eight-octet big-endian number that is the length
of the hashed data from the Signature packet
stopping right before the 0x05, 0XFF octets.
The three data items hashed for document signatures need to mirror
the values of the Literal Data packet. Note that for a detached
signatures this means to hash 6 0x00 octets and for a cleartext
signature this means to hash a 't' followed by 5 0x00 octets.
After all this has been hashed in a single hash context, the
resulting hash field is used in the signature algorithm and placed at
the end of the Signature packet.
Koch & Tse Expires 1 June 2024 [Page 50]
Internet-Draft LibrePGP Message Format November 2023
5.2.4.1. Subpacket Hints
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 a 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 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 or PGP version 5.0.
A version 4 Symmetric-Key Encrypted Session Key packet consists of:
* A one-octet version number with value 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.
Koch & Tse Expires 1 June 2024 [Page 51]
Internet-Draft LibrePGP Message Format November 2023
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 message, 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.
A version 5 Symmetric-Key Encrypted Session Key packet consists of:
* A one-octet version number with value 5.
* A one-octet cipher algorithm.
* A one-octet encryption mode number which MUST be 2 to indicate
OCB.
* A string-to-key (S2K) specifier, length as defined above.
* A starting initialization vector of size specified by the mode.
* The encrypted session key itself, which is decrypted with the
string-to-key object using the given cipher and encryption mode.
* An authentication tag for the encryption mode.
The encrypted session key is encrypted using the encryption mode
specified for the OCB Encrypted Packet. Note that no chunks are used
and that there is only one authentication tag. The Packet Tag in new
format encoding (bits 7 and 6 set, bits 5-0 carry the packet tag),
the packet version number, the cipher algorithm octet, and the mode
octet are given as additional data. For example, the additional data
used with OCB and AES-128 consists of the octets 0xC3, 0x05, 0x07,
and 0x02.
Koch & Tse Expires 1 June 2024 [Page 52]
Internet-Draft LibrePGP Message Format November 2023
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 currently specified versions are
3 and 6.
* 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.
* Only for V6 signatures, a variable-length field containing:
- A one-octet salt size. If salted signatures are used the value
SHOULD match the digest length of the hash algorithm. The
common use case is not to use salted signatures.
- The salt; a random value of the specified size. The value MUST
match the salt field of the corresponding Signature packet.
* Only for V3 signatures: an eight-octet number holding the Key ID
of the signing key.
* Only for V6 sigmatures: a one octet key version number and N
octets of the fingerprint of the signing key. Note that the
length N of the fingerprint for a version 6 key is 32.
* 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.
Koch & Tse Expires 1 June 2024 [Page 53]
Internet-Draft LibrePGP Message Format November 2023
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 LibrePGP
key (sometimes called an LibrePGP 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 version 2.6, 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 version 2.6 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 three versions of key-material packets. Version 3 packets
were first generated by PGP version 2.6. Version 4 keys first
appeared in PGP 5 and are the preferred key version for LibrePGP.
LibrePGP implementations MUST create keys with version 4 format. V3
keys are deprecated; an implementation MUST NOT generate a V3 key,
but MAY accept it.
Koch & Tse Expires 1 June 2024 [Page 54]
Internet-Draft LibrePGP Message Format November 2023
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 multiprecision integers comprising the key material:
- a multiprecision integer (MPI) of RSA public modulus n;
- an MPI of RSA public encryption exponent e.
V3 keys are deprecated. They contain three weaknesses. 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, there is an increased opportunity
for fingerprint collisions. Third, there are weaknesses in the MD5
hash algorithm that make developers prefer other algorithms. See
below for a fuller discussion of Key IDs and fingerprints.
V2 keys are identical to the deprecated V3 keys except for the
version number. An implementation MUST NOT generate them and MAY
accept or reject them as it sees fit.
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 the
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 values comprising the key material. This is
algorithm-specific and described in Section 5.6.
Koch & Tse Expires 1 June 2024 [Page 55]
Internet-Draft LibrePGP Message Format November 2023
The version 5 format is similar to the version 4 format except for
the addition of a count for the key material. This count helps
parsing secret key packets (which are an extension of the public key
packet format) in the case of an unknown algoritm. In addition,
fingerprints of version 5 keys are calculated differently from
version 4 keys, as described in the section "Enhanced Key Formats".
A version 5 packet contains:
* A one-octet version number (5).
* 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 four-octet scalar octet count for the following public key
material.
* A series of values comprising the public key material. This is
algorithm-specific and described in Section 5.6.
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, usually in encrypted form.
The packet contains:
* A Public-Key or Public-Subkey packet, as described above.
* One octet indicating string-to-key usage conventions. Zero
indicates that the secret-key data is not encrypted. 255 or 254
indicates that a string-to-key specifier is being given. Any
other value is a symmetric-key encryption algorithm identifier. A
version 5 packet MUST NOT use the value 255.
* Only for a version 5 packet, a one-octet scalar octet count of the
next 4 optional fields.
* [Optional] If string-to-key usage octet was 255, 254, or 253, a
one-octet symmetric encryption algorithm.
* [Optional] If string-to-key usage octet was 253, an octet with the
values 0x02 to indicate the OCB encryption mode.
Koch & Tse Expires 1 June 2024 [Page 56]
Internet-Draft LibrePGP Message Format November 2023
* [Optional] If string-to-key usage octet was 255, 254, or 253, 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 (string-to-key usage octet
not zero), an Initial Vector (IV) of the same length as the
cipher's block size. If string-to-key usage octet was 253 the IV
is used as the nonce for the OCB mode. If the OCB mode requires a
shorter nonce, the high-order bits of the IV are used and the
remaining bits MUST be zero.
* Only for a version 5 packet, a four-octet scalar octet count for
the following secret key material. This includes the encrypted
SHA-1 hash or OCB tag if the string-to-key usage octet is 254 or
253.
* Plain or encrypted series of values comprising the secret key
material. This is algorithm-specific and described in section
Section 5.6. Note that if the string-to-key usage octet is 254, a
20-octet SHA-1 hash of the plaintext of the algorithm-specific
portion is appended to plaintext and encrypted with it. If the
string-to-key usage octet is 253, then the OCB authentication tag
is part of that data.
* If the string-to-key usage octet is zero or 255, then a two-octet
checksum of the plaintext of the algorithm-specific portion (sum
of all octets, mod 65536).
Note that the version 5 packet format adds two count values to help
parsing packets with unknown S2K or public key algorithms.
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 MUST use a string-to-key
specifier; the simple hash is for backward compatibility and is
deprecated, though implementations MAY continue to use existing
private keys in the old format. 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.
Koch & Tse Expires 1 June 2024 [Page 57]
Internet-Draft LibrePGP Message Format November 2023
With V4 and V5 keys, a simpler method is used. All secret MPI values
are encrypted, including the MPI bitcount prefix.
If the string-to-key usage octet is 253, the encrypted MPI values are
encrypted as one combined plaintext using OCB mode. Note that no
chunks are used and that there is only one authentication tag. The
Packet Tag in new format encoding (bits 7 and 6 set, bits 5-0 carry
the packet tag), the cipher algorithm octet, an octet with value 0x02
(to indicate OCB mode), followed by the public-key packet fields,
starting with its packet version number are given as additional data.
For example, the additional data used with AES-128 in a Secret-Key
Packet of version 4 consists of the octets 0xC5, 0x07, 0x02, 0x04,
followed by the creation time field up to the last value of the
public-key material; in a Secret-Subkey Packet the first octet would
be 0xC7.
The two-octet 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. However, this checksum is
deprecated; an implementation SHOULD NOT use it, but should rather
use the SHA-1 hash denoted with a usage octet of 254. The reason for
this is that there are some attacks that involve undetectably
modifying the secret key. If the string-to-key usage octet is 253 no
checksum or SHA-1 hash is used but the authentication tag of the OCB
mode follows.
5.6. Algorithm-specific Parts of Keys
The public and secret key format specifies algorithm-specific parts
of a key. The following sections describe them in detail.
5.6.1. Algorithm-Specific Part for RSA Keys
The public key is this series of multiprecision integers:
* MPI of RSA public modulus n;
* MPI of RSA public encryption exponent e.
The secret key is this series of multiprecision integers:
* MPI of RSA secret exponent d;
* MPI of RSA secret prime value p;
Koch & Tse Expires 1 June 2024 [Page 58]
Internet-Draft LibrePGP Message Format November 2023
* MPI of RSA secret prime value q (p < q);
* MPI of u, the multiplicative inverse of p, mod q.
5.6.2. Algorithm-Specific Part for DSA Keys
The public key is this series of multiprecision integers:
* 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 mod p where x is secret).
The secret key is this single multiprecision integer:
* MPI of DSA secret exponent x.
5.6.3. Algorithm-Specific Part for Elgamal Keys
The public key is this series of multiprecision integers:
* MPI of Elgamal prime p;
* MPI of Elgamal group generator g;
* MPI of Elgamal public key value y (= g**x mod p where x is
secret).
The secret key is this single multiprecision integer:
* MPI of Elgamal secret exponent x.
5.6.4. Algorithm-Specific Part for ECDSA Keys
The public key is this series of values:
* a variable-length field containing a curve OID, formatted as
follows:
- a one-octet size of the following field; values 0 and 0xFF are
reserved for future extensions,
- the octets representing a curve OID, defined in Section 9.2;
* a MPI of an EC point representing a public key.
Koch & Tse Expires 1 June 2024 [Page 59]
Internet-Draft LibrePGP Message Format November 2023
The secret key is this single multiprecision integer:
* MPI of an integer representing the secret key, which is a scalar
of the public EC point.
5.6.5. Algorithm-Specific Part for EdDSA Keys
The public key is this series of values:
* a variable-length field containing a curve OID, formatted as
follows:
- a one-octet size of the following field; values 0 and 0xFF are
reserved for future extensions,
- the octets representing a curve OID, defined in Section 9.2;
* a MPI of an EC point representing a public key Q as described
under EdDSA Point Format below.
The secret key is this single multiprecision integer:
* MPI of an integer representing the secret key, which is a scalar
of the public EC point.
5.6.6. Algorithm-Specific Part for ECDH Keys
The public key is this series of values:
* a variable-length field containing a curve OID, formatted as
follows:
- a one-octet size of the following field; values 0 and 0xFF are
reserved for future extensions,
- the octets representing a curve OID, defined in Section 9.2;
* a MPI of an EC point representing a public key;
* a variable-length field containing KDF parameters, formatted as
follows:
- a one-octet size of the following fields; values 0 and 0xff are
reserved for future extensions;
- a one-octet value 1, reserved for future extensions;
- a one-octet hash function ID used with a KDF;
Koch & Tse Expires 1 June 2024 [Page 60]
Internet-Draft LibrePGP Message Format November 2023
- a one-octet algorithm ID for the symmetric algorithm used to
wrap the symmetric key used for the message encryption; see
Section 13.5 for details.
Observe that an ECDH public key is composed of the same sequence of
fields that define an ECDSA key, plus the KDF parameters field.
The secret key is this single multiprecision integer:
* MPI of an integer representing the secret key, which is a scalar
of the public EC point.
5.6.7. Algorithm-Specific Part for ML-KEM Keys
{Note: This part is not finalized and subject to change}
The public key is this series of values:
* A fixed-length octet string representing an ECC public key in the
format associated with the curve. For ML-KEM-768+X25519 these are
32 octets, for ML-KEM-1024+X448 these are 56 octets.
* A fixed-length octet string containing the ML-KEM public key,
whose length depends on the algorithm. For ML-KEM-768+X25519
these are 1184 octets, for ML-KEM-1024+X448 these are 1568 octets.
The secret key is this series of values:
* A fixed-length octet string of the encoded secret scalar, whose
encoding and length depend on the algorithm. For ML-KEM-
768+X25519 these are 32 octets, for ML-KEM-1024+X448 these are 56
octets.
* A fixed-length octet string containing the ML-KEM secret key,
whose length depends on the algorithm. For ML-KEM-768+X25519
these are 2400 octets, for ML-KEM-1024+X448 these are 3168 octets.
Observe that the format of the ECC keys differ from the format used
with ECDH. This has been chosen to avoid prefix octets.
5.7. 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:
Koch & Tse Expires 1 June 2024 [Page 61]
Internet-Draft LibrePGP Message Format November 2023
* 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 "Packet Composition" for details on
how messages are formed.
ZIP-compressed packets are compressed with raw RFC 1951 [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 RFC 1950 [RFC1950] ZLIB-
style blocks.
BZip2-compressed packets are compressed using the BZip2 [BZ2]
algorithm.
5.8. 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 LibrePGP messages).
This packet is obsolete. An implementation MUST NOT create this
packet. An implementation MAY process such a packet but it MUST
return a clear diagnostic that a non-integrity protected packet has
been processed. The implementation SHOULD also return an error in
this case and stop processing.
The body of this packet consists of:
* Encrypted data, the output of the selected symmetric-key cipher
operating in LibrePGP'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.
Koch & Tse Expires 1 June 2024 [Page 62]
Internet-Draft LibrePGP Message Format November 2023
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, LibrePGP 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 the "Security Considerations" section for hints on
the proper use of this "quick check".
5.9. 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, 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
version 2.6 in order to cause that version to report that newer
software is necessary to process the message.
5.10. 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.
Koch & Tse Expires 1 June 2024 [Page 63]
Internet-Draft LibrePGP Message Format November 2023
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.
If it is a m (0x6d), then it contains a MIME message body part
[RFC2045].
Early versions of PGP also defined a value of l as a 'local' mode
for machine-local conversions. RFC 1991 [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.
Note that V3 and V4 signatures do not include the formatting octet,
the file name, and the date field of the literal packet in a
signature hash and thus are not protected against tampering in a
signed document. In contrast V5 signatures include them.
Koch & Tse Expires 1 June 2024 [Page 64]
Internet-Draft LibrePGP Message Format November 2023
5.11. 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.
5.12. 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 RFC 2822 [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.
5.13. 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 LibrePGP
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 following table lists the currently known subpackets:
Koch & Tse Expires 1 June 2024 [Page 65]
Internet-Draft LibrePGP Message Format November 2023
+=========+=============================+
| Type | Attribute Subpacket |
+=========+=============================+
| 1 | Image Attribute Subpacket |
+---------+-----------------------------+
| [TBD1] | User ID Attribute Subpacket |
+---------+-----------------------------+
| 100-110 | Private/Experimental Use |
+---------+-----------------------------+
Table 5
An implementation SHOULD ignore any subpacket of a type that it does
not recognize.
5.13.1. The Image Attribute Subpacket
The Image Attribute subpacket is used to encode an image, presumably
(but not required to be) that of the key owner.
The Image Attribute subpacket begins with an image header. The first
two octets of the image header contain the length of the image
header. Note that unlike other multi-octet numerical values in this
document, due to a historical accident this value is encoded as a
little-endian number. The image header length is followed by a
single octet for the image header version. The only currently
defined version of the image header is 1, which is a 16-octet image
header. The first three octets of a version 1 image header are thus
0x10, 0x00, 0x01.
The fourth octet of a version 1 image header designates the encoding
format of the image. The only currently defined encoding format is
the value 1 to indicate JPEG. Image format types 100 through 110 are
reserved for private or experimental use. The rest of the version 1
image header is made up of 12 reserved octets, all of which MUST be
set to 0.
The rest of the image subpacket contains the image itself. As the
only currently defined image type is JPEG, the image is encoded in
the JPEG File Interchange Format (JFIF), a standard file format for
JPEG images [JFIF].
An implementation MAY try to determine the type of an image by
examination of the image data if it is unable to handle a particular
version of the image header or if a specified encoding format value
is not recognized.
Koch & Tse Expires 1 June 2024 [Page 66]
Internet-Draft LibrePGP Message Format November 2023
5.13.2. User ID Attribute Subpacket
A User ID Attribute subpacket has type [IANA --- assignment TBD1].
A User ID Attribute subpacket, just like 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 RFC 2822
[RFC2822] mail name-addr, but there are no restrictions on its
content. For devices using LibrePGP for device certificates, it may
just be the device identifier. The packet length in the header
specifies the length of the User ID.
Because User Attribute subpackets can be used anywhere a User ID
packet can be used, implementations MAY choose to trust a signed User
Attribute subpacket that includes a User ID Attribute subpacket.
5.14. 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 LibrePGP 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.
Koch & Tse Expires 1 June 2024 [Page 67]
Internet-Draft LibrePGP Message Format November 2023
The body of this packet consists of:
* A one-octet version number. The only defined value is 1. There
won't be any future versions of this packet because the MDC system
has been superseded by the OCB Encrypted Data packet.
* 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, LibrePGP 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 "LibrePGP CFB Mode" below 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.
Koch & Tse Expires 1 June 2024 [Page 68]
Internet-Draft LibrePGP Message Format November 2023
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.
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.
LibrePGP 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
Koch & Tse Expires 1 June 2024 [Page 69]
Internet-Draft LibrePGP Message Format November 2023
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 LibrePGP 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 SHA2-256 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, no update will be needed because the MDC will be
replaced by the OCB encryption described in this document.
5.15. 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.
Koch & Tse Expires 1 June 2024 [Page 70]
Internet-Draft LibrePGP Message Format November 2023
5.16. OCB Encrypted Data Packet (Tag 20)
This packet contains data encrypted with an authenticated encryption
and additional data (AEAD) construction. When it has been decrypted,
it will typically contain other packets (often a Literal Data packet
or Compressed Data packet).
The body of this packet consists of:
* A one-octet version number. The only currently defined value is
1.
* A one-octet cipher algorithm.
* A one-octet encryption mode octet with the fixed value 0x02. If
decryption using the EAX mode is supported this octet may have the
value 0x01.
* A one-octet chunk size.
* A starting initialization vector of size specified by the
encryption mode (15 octets for OCB).
* Encrypted data, the output of the selected symmetric-key cipher
operating in the given encryption mode.
* A final, summary authentication tag for the encryption mode (16
octets for OCB).
An OCB Encrypted Data packet consists of one or more chunks of data.
The plaintext of each chunk is of a size specified using the chunk
size octet using the method specified below.
The encrypted data consists of the encryption of each chunk of
plaintext, followed immediately by the relevant authentication tag.
If the last chunk of plaintext is smaller than the chunk size, the
ciphertext for that data may be shorter; it is nevertheless followed
by a full authentication tag.
For each chunk, the AEAD construction is given the Packet Tag in new
format encoding (bits 7 and 6 set, bits 5-0 carry the packet tag),
version number, cipher algorithm octet, encryption mode octet, chunk
size octet, and an eight-octet, big-endian chunk index as additional
data. The index of the first chunk is zero. For example, the
additional data of the first chunk using OCB and AES-128 with a chunk
size of 64 kiByte consists of the octets 0xD4, 0x01, 0x07, 0x02,
0x10, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, and 0x00.
Koch & Tse Expires 1 June 2024 [Page 71]
Internet-Draft LibrePGP Message Format November 2023
After the final chunk, the encryption mode is used to produce a final
authentication tag encrypting the empty string. This AEAD instance
is given the additional data specified above, plus an eight-octet,
big-endian value specifying the total number of plaintext octets
encrypted. This allows detection of a truncated ciphertext. Please
note that the big-endian number representing the chunk index in the
additional data is increased accordingly, although it's not really a
chunk.
The chunk size octet specifies the size of chunks using the following
formula (in C), where c is the chunk size octet:
chunk_size = ((uint64_t)1 << (c + 6))
To facilitate interoperability between a wide variety of
implementations, from constrained to large compute environments, a
chunk size maximum is specified: An implementation MUST accept chunk
size octets with values from 0 to 16. An implementation MUST NOT
create data with a chunk size octet value larger than 16 (4 MiB
chunks).
A new random initialization vector MUST be used for each message.
Failure to do so for each message will lead to a catastrophic failure
depending on the used encryption mode.
5.16.1. EAX Mode
The EAX algorithm can only use block ciphers with 16-octet blocks.
The starting initialization vector and authentication tag are both 16
octets long.
The starting initialization vector for this mode MUST be unique and
unpredictable.
The nonce for EAX mode is computed by treating the starting
initialization vector as a 16-octet, big-endian value and exclusive-
oring the low eight octets of it with the chunk index.
The security of EAX requires that the nonce is never reused, hence
the requirement that the starting initialization vector be unique.
EAX mode is deprecated due to the far better properties of the OCB
mode. Implementations may use EAX mode only for decryption of
existing data.
Koch & Tse Expires 1 June 2024 [Page 72]
Internet-Draft LibrePGP Message Format November 2023
5.16.2. OCB Mode
The OCB Authenticated-Encryption Algorithm used in this document is
defined in [RFC7253].
OCB usage requires specification of the following parameters:
* a blockcipher that operate on 128-bit (16-octet) blocks
* an authentication tag length of 16 octets
* a nonce of 15 octets long (which is the longest nonce allowed
specified by [RFC7253])
* an initialization vector of at least 15 octets long
In the case that the initialization vector is longer than 15 octets
(such as in Section 5.5.1.3, only the 15 leftmost octets are used in
calculations; the remaining octets MUST be considered as zero.
The nonce for OCB mode is computed by the exclusive-oring of the
initialization vector as a 15-octet, big endian value, against the
chunk index.
Security of OCB mode depends on the non-repeated nature of nonces
used for the same key on distinct plaintext [RFC7253]. Therefore the
initialization vector per message MUST be distinct, and OCB mode
SHOULD only be used in environments when there is certainty to
fulfilling this requirement.
6. Radix-64 Conversions
As stated in the introduction, LibrePGP's underlying native
representation for objects is a stream of arbitrary octets, and some
systems desire these objects to be immune to damage caused by
character set translation, data conversions, etc.
In principle, any printable encoding scheme that met the requirements
of the unsafe channel would suffice, since it would not change the
underlying binary bit streams of the native LibrePGP data structures.
The LibrePGP standard specifies one such printable encoding scheme to
ensure interoperability.
LibrePGP's Radix-64 encoding is composed of two parts: a base64
encoding of the binary data and a checksum. The base64 encoding is
identical to the MIME base64 content-transfer-encoding [RFC2045].
Koch & Tse Expires 1 June 2024 [Page 73]
Internet-Draft LibrePGP Message Format November 2023
The checksum is a 24-bit Cyclic Redundancy Check (CRC) converted to
four characters of radix-64 encoding by the same MIME base64
transformation, preceded by an equal sign (=). The CRC is computed
by using the generator 0x864CFB and an initialization of 0xB704CE.
The accumulation is done on the data before it is converted to radix-
64, rather than on the converted data. A sample implementation of
this algorithm is in the next section.
The checksum with its leading equal sign MAY appear on the first line
after the base64 encoded data.
Rationale for CRC-24: The size of 24 bits fits evenly into printable
base64. The nonzero initialization can detect more errors than a
zero initialization.
6.1. An Implementation of the CRC-24 in "C"
<CODE BEGINS>
#define CRC24_INIT 0xB704CEL
#define CRC24_POLY 0x864CFBL
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;
}
<CODE ENDS>
6.2. Forming ASCII Armor
When LibrePGP encodes data into ASCII Armor, it puts specific headers
around the Radix-64 encoded data, so LibrePGP can reconstruct the
data later. An LibrePGP implementation MAY use ASCII armor to
protect raw binary data. LibrePGP 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:
Koch & Tse Expires 1 June 2024 [Page 74]
Internet-Draft LibrePGP Message Format November 2023
* An Armor Header Line, appropriate for the type of data
* Armor Headers
* A blank 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, LibrePGP/MIME
signatures, and cleartext signatures. Note that PGP 2 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 --- space (0x20), tab (0x09)
or carriage return (0x0d) --- 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).
Koch & Tse Expires 1 June 2024 [Page 75]
Internet-Draft LibrePGP Message Format November 2023
The Armor Headers are pairs of strings that can give the user or the
receiving LibrePGP 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.
LibrePGP should consider improperly formatted Armor Headers to be
corruption of the ASCII Armor. Unknown keys should be reported to
the user, but LibrePGP 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 LibrePGP implementation and version
used to encode the message.
* "Comment", a user-defined comment. LibrePGP 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.
* "Hash", a comma-separated list of hash algorithms used in this
message. This is used only in cleartext signed messages.
* "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.
Koch & Tse Expires 1 June 2024 [Page 76]
Internet-Draft LibrePGP Message Format November 2023
* "Charset", a description of the character set that the plaintext
is in. Please note that LibrePGP 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 blank line can either be zero-length or contain only whitespace,
that is spaces (0x20), tabs (0x09) or carriage returns (0x0d).
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.
Koch & Tse Expires 1 June 2024 [Page 77]
Internet-Draft LibrePGP Message Format November 2023
Value Encoding Value Encoding Value Encoding Value Encoding
0 A 17 R 34 i 51 z
1 B 18 S 35 j 52 0
2 C 19 T 36 k 53 1
3 D 20 U 37 l 54 2
4 E 21 V 38 m 55 3
5 F 22 W 39 n 56 4
6 G 23 X 40 o 57 5
7 H 24 Y 41 p 58 6
8 I 25 Z 42 q 59 7
9 J 26 a 43 r 60 8
10 K 27 b 44 s 61 9
11 L 28 c 45 t 62 +
12 M 29 d 46 u 63 /
13 N 30 e 47 v
14 O 31 f 48 w (pad) =
15 P 32 g 49 x
16 Q 33 h 50 y
The encoded output stream must be represented in lines of no more
than 76 characters each.
Special processing is performed if fewer than 24 bits are available
at the end of the data being encoded. There are three possibilities:
1. The last data group has 24 bits (3 octets). No special
processing is needed.
2. The last data group has 16 bits (2 octets). The first two
6-bit groups are processed as above. The third (incomplete)
data group has two zero-value bits added to it, and is
processed as above. A pad character (=) is added to the
output.
3. The last data group has 8 bits (1 octet). The first 6-bit
group is processed as above. The second (incomplete) data
group has four zero-value bits added to it, and is processed
as above. Two pad characters (=) are added to the output.
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.
Koch & Tse Expires 1 June 2024 [Page 78]
Internet-Draft LibrePGP Message Format November 2023
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 01111110
6-bit: 000101 001111 101110 011100 | 000000 111101 100101 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.
Koch & Tse Expires 1 June 2024 [Page 79]
Internet-Draft LibrePGP Message Format November 2023
7. Cleartext Signature Framework
It is desirable to be able to sign a textual octet stream without
ASCII armoring the stream itself, so the signed text is still
readable without special software. In order to bind a signature to
such a cleartext, this framework is used, which follows the same
basic format and restrictions as the ASCII armoring described above
in "Forming ASCII Armor" (Section 6.2). (Note that this framework is
not intended to be reversible. RFC 3156 [RFC3156] defines another
way to sign cleartext messages for environments that support MIME.)
The cleartext signed message consists of:
* The cleartext header -----BEGIN PGP SIGNED MESSAGE----- on a
single line,
* One or more "Hash" Armor Headers,
* Exactly one blank line not included into the message digest,
* The dash-escaped cleartext that is included into the message
digest,
* The ASCII armored signature(s) including the -----BEGIN PGP
SIGNATURE----- Armor Header and Armor Tail Lines.
If the "Hash" Armor Header is given, the specified message digest
algorithm(s) are used for the signature. If there are no such
headers, MD5 is used. If MD5 is the only hash used, then an
implementation MAY omit this header for improved V2.x compatibility.
If more than one message digest is used in the signature, the "Hash"
armor header contains a comma-delimited list of used message digests.
Current message digest names are described below with the algorithm
IDs.
An implementation SHOULD add a line break after the cleartext, but
MAY omit it if the cleartext ends with a line break. This is for
visual clarity.
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
Koch & Tse Expires 1 June 2024 [Page 80]
Internet-Draft LibrePGP Message Format November 2023
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), tabs (0x09) or
carriage returns (0x0d) --- at the end of any line is removed when
the cleartext signature is generated and verified.
8. Regular Expressions
A regular expression is zero or more branches, separated by |. It
matches anything that matches one of the branches.
A branch is zero or more pieces, concatenated. It matches a match
for the first, followed by a match for the second, etc.
A piece is an atom possibly followed by *, +, or ?. An atom followed
by * matches a sequence of 0 or more matches of the atom. An atom
followed by + matches a sequence of 1 or more matches of the atom.
An atom followed by ? matches a match of the atom, or the null
string.
An atom is a regular expression in parentheses (matching a match for
the regular expression), a range (see below), . (matching any single
character), ^ (matching the null string at the beginning of the input
string), $ (matching the null string at the end of the input string),
a \ followed by a single character (matching that character), or a
single character with no other significance (matching that
character).
Koch & Tse Expires 1 June 2024 [Page 81]
Internet-Draft LibrePGP Message Format November 2023
A range is a sequence of characters enclosed in []. It normally
matches any single character from the sequence. If the sequence
begins with ^, it matches any single character not from the rest of
the sequence. If two characters in the sequence are separated by -,
this is shorthand for the full list of ASCII characters between them
(e.g., [0-9] matches any decimal digit). To include a literal ] in
the sequence, make it the first character (following a possible ^).
To include a literal -, make it the first or last character.
9. Constants
This section describes the constants used in LibrePGP.
Note that these tables are not exhaustive lists; an implementation
MAY implement an algorithm not on these lists, so long as the
algorithm numbers are chosen from the private or experimental
algorithm range.
See the section "Notes on Algorithms" below for more discussion of
the algorithms.
9.1. Public-Key Algorithms
+=======+===================================================+
| ID | Algorithm |
+=======+===================================================+
| 1 | RSA (Encrypt or Sign) [HAC] |
+-------+---------------------------------------------------+
| 2 | RSA Encrypt-Only [HAC] |
+-------+---------------------------------------------------+
| 3 | RSA Sign-Only [HAC] |
+-------+---------------------------------------------------+
| 16 | Elgamal (Encrypt-Only) [ELGAMAL] [HAC] |
+-------+---------------------------------------------------+
| 17 | DSA (Digital Signature Algorithm) [FIPS186] [HAC] |
+-------+---------------------------------------------------+
| 18 | ECDH public key algorithm |
+-------+---------------------------------------------------+
| 19 | ECDSA public key algorithm [FIPS186] |
+-------+---------------------------------------------------+
| 20 | Reserved (formerly Elgamal Encrypt or Sign) |
+-------+---------------------------------------------------+
| 21 | Reserved for Diffie-Hellman (X9.42, as defined |
| | for IETF-S/MIME) |
+-------+---------------------------------------------------+
| 22 | EdDSA [RFC8032] |
+-------+---------------------------------------------------+
| 23 | Reserved for AEDH |
Koch & Tse Expires 1 June 2024 [Page 82]
Internet-Draft LibrePGP Message Format November 2023
+-------+---------------------------------------------------+
| 24 | Reserved for AEDSA |
+-------+---------------------------------------------------+
| 100-- | Private/Experimental algorithm |
| 110 | |
+-------+---------------------------------------------------+
Table 6
Implementations MUST implement RSA (1) and ECDSA (19) for signatures,
and RSA (1) and ECDH (18) for encryption. Implementations SHOULD
implement EdDSA (22) keys.
RSA Encrypt-Only (2) and RSA Sign-Only (3) are deprecated and SHOULD
NOT be generated, but may be interpreted. See Section 15.5. See
Section 15.9 for notes on Elgamal Encrypt or Sign (20), and X9.42
(21). Implementations MAY implement any other algorithm.
Note that implementations conforming to previous versions of this
standard (RFC-4880) have DSA (17) and Elgamal (16) as its only MUST-
implement algorithm.
A compatible specification of ECDSA is given in [RFC6090] as "KT-I
Signatures" and in [SEC1]; ECDH is defined in Section 13.5 this
document.
9.2. ECC Curve OID
The parameter curve OID is an array of octets that define a named
curve. The table below specifies the exact sequence of bytes for
each named curve referenced in this document:
Koch & Tse Expires 1 June 2024 [Page 83]
Internet-Draft LibrePGP Message Format November 2023
+========================+=====+=================+=================+
| ASN.1 Object | OID | Curve OID bytes | Curve name |
| Identifier | len | in hexadecimal | |
| | | representation | |
+========================+=====+=================+=================+
| 1.2.840.10045.3.1.7 | 8 | 2A 86 48 CE 3D | NIST P-256 |
| | | 03 01 07 | |
+------------------------+-----+-----------------+-----------------+
| 1.3.132.0.34 | 5 | 2B 81 04 00 22 | NIST P-384 |
+------------------------+-----+-----------------+-----------------+
| 1.3.132.0.35 | 5 | 2B 81 04 00 23 | NIST P-521 |
+------------------------+-----+-----------------+-----------------+
| 1.3.36.3.3.2.8.1.1.7 | 9 | 2B 24 03 03 02 | brainpoolP256r1 |
| | | 08 01 01 07 | |
+------------------------+-----+-----------------+-----------------+
| 1.3.36.3.3.2.8.1.1.11 | 9 | 2B 24 03 03 02 | brainpoolP384r1 |
| | | 08 01 01 0B | |
+------------------------+-----+-----------------+-----------------+
| 1.3.36.3.3.2.8.1.1.13 | 9 | 2B 24 03 03 02 | brainpoolP512r1 |
| | | 08 01 01 0D | |
+------------------------+-----+-----------------+-----------------+
| 1.3.6.1.4.1.11591.15.1 | 9 | 2B 06 01 04 01 | Ed25519 |
| | | DA 47 0F 01 | |
+------------------------+-----+-----------------+-----------------+
| 1.3.6.1.4.1.3029.1.5.1 | 10 | 2B 06 01 04 01 | Curve25519 |
| | | 97 55 01 05 01 | |
+------------------------+-----+-----------------+-----------------+
| 1.3.101.112 | 3 | 2B 65 70 | Ed25519(1) |
+------------------------+-----+-----------------+-----------------+
| 1.3.102.110 | 3 | 2B 65 6E | Curve25519(1) |
+------------------------+-----+-----------------+-----------------+
| 1.3.101.113 | 3 | 2B 65 71 | Ed448 |
+------------------------+-----+-----------------+-----------------+
| 1.3.101.111 | 3 | 2B 65 6F | X448 |
+------------------------+-----+-----------------+-----------------+
Table 7
The sequence of octets in the third column is the result of applying
the Distinguished Encoding Rules (DER) to the ASN.1 Object Identifier
with subsequent truncation. The truncation removes the two fields of
encoded Object Identifier. The first omitted field is one octet
representing the Object Identifier tag, and the second omitted field
is the length of the Object Identifier body. For example, the
complete ASN.1 DER encoding for the NIST P-256 curve OID is "06 08 2A
86 48 CE 3D 03 01 07", from which the first entry in the table above
is constructed by omitting the first two octets. Only the truncated
sequence of octets is the valid representation of a curve OID.
Koch & Tse Expires 1 June 2024 [Page 84]
Internet-Draft LibrePGP Message Format November 2023
The alternative OIDs for Ed25519 and Curve25519 marked with (1)
SHOULD only be used with v5 keys.
9.3. Symmetric-Key Algorithms
+=======+======================================+
| ID | Algorithm |
+=======+======================================+
| 0 | Plaintext or unencrypted data |
+-------+--------------------------------------+
| 1 | IDEA [IDEA] |
+-------+--------------------------------------+
| 2 | TripleDES (DES-EDE, [SCHNEIER] [HAC] |
| | - 168 bit key derived from 192) |
+-------+--------------------------------------+
| 3 | CAST5 (128 bit key, as per |
| | [RFC2144]) |
+-------+--------------------------------------+
| 4 | Blowfish (128 bit key, 16 rounds) |
| | [BLOWFISH] |
+-------+--------------------------------------+
| 5 | Reserved |
+-------+--------------------------------------+
| 6 | Reserved |
+-------+--------------------------------------+
| 7 | AES with 128-bit key [AES] |
+-------+--------------------------------------+
| 8 | AES with 192-bit key |
+-------+--------------------------------------+
| 9 | AES with 256-bit key |
+-------+--------------------------------------+
| 10 | Twofish with 256-bit key [TWOFISH] |
+-------+--------------------------------------+
| 11 | Camellia with 128-bit key [RFC3713] |
+-------+--------------------------------------+
| 12 | Camellia with 192-bit key |
+-------+--------------------------------------+
| 13 | Camellia with 256-bit key |
+-------+--------------------------------------+
| 100-- | Private/Experimental algorithm |
| 110 | |
+-------+--------------------------------------+
Table 8
Implementations MUST implement AES-128. Implementations SHOULD
implement AES-256. Implementations that interoperate with RFC-4880
implementations need to support TripleDES and CAST5. Implementations
Koch & Tse Expires 1 June 2024 [Page 85]
Internet-Draft LibrePGP Message Format November 2023
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.
9.4. Compression Algorithms
+==========+================================+
| ID | Algorithm |
+==========+================================+
| 0 | Uncompressed |
+----------+--------------------------------+
| 1 | ZIP [RFC1951] |
+----------+--------------------------------+
| 2 | ZLIB [RFC1950] |
+----------+--------------------------------+
| 3 | BZip2 [BZ2] |
+----------+--------------------------------+
| 100--110 | Private/Experimental algorithm |
+----------+--------------------------------+
Table 9
Implementations MUST implement uncompressed data. Implementations
SHOULD implement ZLIB. For interoperability reasons implementations
SHOULD be able to decompress using ZIP. Implementations MAY
implement any other algorithm.
9.5. Hash Algorithms
+==========+================================+=============+
| ID | Algorithm | Text Name |
+==========+================================+=============+
| 1 | MD5 [HAC] | "MD5" |
+----------+--------------------------------+-------------+
| 2 | SHA-1 [FIPS180] | "SHA1" |
+----------+--------------------------------+-------------+
| 3 | RIPE-MD/160 [HAC] | "RIPEMD160" |
+----------+--------------------------------+-------------+
| 4 | Reserved | |
+----------+--------------------------------+-------------+
| 5 | Reserved | |
+----------+--------------------------------+-------------+
| 6 | Reserved | |
+----------+--------------------------------+-------------+
| 7 | Reserved | |
+----------+--------------------------------+-------------+
| 8 | SHA2-256 [FIPS180] | "SHA256" |
+----------+--------------------------------+-------------+
Koch & Tse Expires 1 June 2024 [Page 86]
Internet-Draft LibrePGP Message Format November 2023
| 9 | SHA2-384 [FIPS180] | "SHA384" |
+----------+--------------------------------+-------------+
| 10 | SHA2-512 [FIPS180] | "SHA512" |
+----------+--------------------------------+-------------+
| 11 | SHA2-224 [FIPS180] | "SHA224" |
+----------+--------------------------------+-------------+
| 12 | SHA3-256 [FIPS202] | "SHA3-256" |
+----------+--------------------------------+-------------+
| 13 | Reserved | |
+----------+--------------------------------+-------------+
| 14 | SHA3-512 [FIPS202] | "SHA3-512" |
+----------+--------------------------------+-------------+
| 100--110 | Private/Experimental algorithm | |
+----------+--------------------------------+-------------+
Table 10
Implementations MUST implement SHA2-256. Implementations MAY
implement other algorithms. Implementations SHOULD NOT create
messages which require the use of SHA-1 with the exception of
computing version 4 key fingerprints and for purposes of the MDC
packet. Implementations SHOULD NOT use MD5 or RIPE-MD/160.
9.6. Encryption Modes
+====+===============+
| ID | Mode |
+====+===============+
| 1 | EAX [EAX] |
+----+---------------+
| 2 | OCB [RFC7253] |
+----+---------------+
Table 11
Implementations MUST implement OCB if they support the packet 20 (OCB
Encrypted Data Packet). Implementations MAY implement EAX only for
decryption and only for backward compatibility with former drafts of
this specification.
10. IANA Considerations
LibrePGP is highly parameterized, and consequently there are a number
of considerations for allocating parameters for extensions. This
section describes how IANA should look at extensions to the protocol
as described in this document.
Koch & Tse Expires 1 June 2024 [Page 87]
Internet-Draft LibrePGP Message Format November 2023
{ FIXME: Also add forward references, like "The list of S2K specifier
types is maintained by IANA as described in Section 10." }
10.1. New String-to-Key Specifier Types
LibrePGP 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 SPECIFICATION REQUIRED method,
as described in [RFC8126].
10.2. New Packets
Major new features of LibrePGP 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 RFC REQUIRED method, as described in [RFC8126].
10.2.1. User Attribute Types
The User Attribute packet permits an extensible mechanism for other
types of certificate identification. This specification creates a
registry of User Attribute types. The registry includes the User
Attribute type, the name of the User Attribute, and a reference to
the defining specification. The initial values for this registry can
be found in Section 5.13. Adding a new User Attribute type MUST be
done through the SPECIFICATION REQUIRED method, as described in
[RFC8126].
This document requests that IANA register the User ID Attribute Type
found in Section 5.13.2:
+=======+===========+===============+
| Value | Attribute | Reference |
+=======+===========+===============+
| 1 | Image | This Document |
+-------+-----------+---------------+
Table 12
Koch & Tse Expires 1 June 2024 [Page 88]
Internet-Draft LibrePGP Message Format November 2023
10.2.2. Image Format Subpacket Types
Within User Attribute packets, there is an extensible mechanism for
other types of image-based User Attributes. This specification
creates a registry of Image Attribute subpacket types. The registry
includes the Image Attribute subpacket type, the name of the Image
Attribute subpacket, and a reference to the defining specification.
The initial values for this registry can be found in Section 5.13.1.
Adding a new Image Attribute subpacket type MUST be done through the
SPECIFICATION REQUIRED method, as described in [RFC8126].
10.2.3. New Signature Subpackets
LibrePGP signatures contain a mechanism for signed (or unsigned) data
to be added to them for a variety of purposes in the Signature
subpackets as discussed in Section 5.2.3.1. This specification
creates a registry of Signature subpacket types. The registry
includes the Signature subpacket type, the name of the subpacket, and
a reference to the defining specification. The initial values for
this registry can be found in Section 5.2.3.1. Adding a new
Signature subpacket MUST be done through the SPECIFICATION REQUIRED
method, as described in [RFC8126].
10.2.3.1. Signature Notation Data Subpackets
LibrePGP signatures further contain a mechanism for extensions in
signatures. These are the Notation Data subpackets, which contain a
key/value pair. Notations contain a user space that is completely
unmanaged and an IETF space.
This specification creates a registry of Signature Notation Data
types. The registry includes the Signature Notation Data type, the
name of the Signature Notation Data, its allowed values, and a
reference to the defining specification. The initial values for this
registry can be found in Section 5.2.3.18. Adding a new Signature
Notation Data subpacket MUST be done through the SPECIFICATION
REQUIRED method, as described in [RFC8126].
This document requests IANA register the following Signature Notation
Data types:
Koch & Tse Expires 1 June 2024 [Page 89]
Internet-Draft LibrePGP Message Format November 2023
+================+=========+====================+==================+
| Allowed Values | Name | Type | Reference |
+================+=========+====================+==================+
| A String | charset | Character Set | This Doc Section |
| | | | 5.2.3.18.1 |
+----------------+---------+--------------------+------------------+
| Any String | manu | Manufacturer Name | This Doc Section |
| | | | 5.2.3.18.2 |
+----------------+---------+--------------------+------------------+
| Any String | make | Product Make | This Doc Section |
| | | | 5.2.3.18.3 |
+----------------+---------+--------------------+------------------+
| Any String | model | Product Model | This Doc Section |
| | | | 5.2.3.18.4 |
+----------------+---------+--------------------+------------------+
| Any String | prodid | Product ID | This Doc Section |
| | | | 5.2.3.18.5 |
+----------------+---------+--------------------+------------------+
| Any String | pvers | Product Version | This Doc Section |
| | | | 5.2.3.18.6 |
+----------------+---------+--------------------+------------------+
| Any String | lot | Product Lot Number | This Doc Section |
| | | | 5.2.3.18.7 |
+----------------+---------+--------------------+------------------+
| Decimal | qty | Package Quantity | This Doc Section |
| Integer String | | | 5.2.3.18.8 |
+----------------+---------+--------------------+------------------+
| A geo: URI | loc | Current | This Doc Section |
| without the | | Geolocation | 5.2.3.18.9 |
| "geo:" | | Latitude/Longitude | |
+----------------+---------+--------------------+------------------+
| A geo: URI | dest | Destination | This Doc Section |
| without the | | Geolocation | 5.2.3.18.9 |
| "geo:" | | Latitude/Longitude | |
+----------------+---------+--------------------+------------------+
| Hash Notation | hash | The Hash of | This Doc Section |
| data | | external data | 5.2.3.18.10 |
+----------------+---------+--------------------+------------------+
Table 13
Koch & Tse Expires 1 June 2024 [Page 90]
Internet-Draft LibrePGP Message Format November 2023
10.2.3.2. Signature Notation Data Subpacket Notation Flags
This specification creates a new registry of Signature Notation Data
Subpacket Notation Flags. The registry includes the columns "Flag",
"Description", "Security Recommended", "Interoperability
Recommended", and "Reference". The initial values for this registry
can be found in Section 5.2.3.18. Adding a new item MUST be done
through the SPECIFICATION REQUIRED method, as described in [RFC8126].
10.2.3.3. Key Server Preference Extensions
LibrePGP signatures contain a mechanism for preferences to be
specified about key servers. This specification creates a registry
of key server preferences. The registry includes the key server
preference, the name of the preference, and a reference to the
defining specification. The initial values for this registry can be
found in Section 5.2.3.19. Adding a new key server preference MUST
be done through the SPECIFICATION REQUIRED method, as described in
[RFC8126].
10.2.3.4. Key Flags Extensions
LibrePGP signatures contain a mechanism for flags to be specified
about key usage. This specification creates a registry of key usage
flags. The registry includes the key flags value, the name of the
flag, and a reference to the defining specification. The initial
values for this registry can be found in Section 5.2.3.23. Adding a
new key usage flag MUST be done through the SPECIFICATION REQUIRED
method, as described in [RFC8126].
10.2.3.5. Reason for Revocation Extensions
LibrePGP signatures contain a mechanism for flags to be specified
about why a key was revoked. This specification creates a registry
of "Reason for Revocation" flags. The registry includes the "Reason
for Revocation" flags value, the name of the flag, and a reference to
the defining specification. The initial values for this registry can
be found in Section 5.2.3.25. Adding a new feature flag MUST be done
through the SPECIFICATION REQUIRED method, as described in [RFC8126].
10.2.3.6. Implementation Features
LibrePGP signatures contain a mechanism for flags to be specified
stating which optional features an implementation supports. This
specification creates a registry of feature-implementation flags.
The registry includes the feature-implementation flags value, the
name of the flag, and a reference to the defining specification. The
initial values for this registry can be found in Section 5.2.3.26.
Koch & Tse Expires 1 June 2024 [Page 91]
Internet-Draft LibrePGP Message Format November 2023
Adding a new feature-implementation flag MUST be done through the
SPECIFICATION REQUIRED method, as described in [RFC8126].
Also see Section 15.12 for more information about when feature flags
are needed.
10.2.4. New Packet Versions
The core LibrePGP packets all have version numbers, and can be
revised by introducing a new version of an existing packet. This
specification creates a registry of packet types. The registry
includes the packet type, the number of the version, and a reference
to the defining specification. The initial values for this registry
can be found in Section 5. Adding a new packet version MUST be done
through the RFC REQUIRED method, as described in [RFC8126].
10.3. New Algorithms
Section 9 lists the core algorithms that LibrePGP 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 LibrePGP-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.
10.3.1. Public-Key Algorithms
LibrePGP specifies a number of public-key algorithms. This
specification creates a registry of public-key algorithm identifiers.
The registry includes the algorithm name, its key sizes and
parameters, and a reference to the defining specification. The
initial values for this registry can be found in Section 9.1. Adding
a new public-key algorithm MUST be done through the SPECIFICATION
REQUIRED method, as described in [RFC8126].
This document requests IANA register the following new public-key
algorithm:
Koch & Tse Expires 1 June 2024 [Page 92]
Internet-Draft LibrePGP Message Format November 2023
+====+============================+========================+
| ID | Algorithm | Reference |
+====+============================+========================+
| 22 | EdDSA public key algorithm | This doc, Section 15.8 |
+----+----------------------------+------------------------+
| 23 | Reserved for AEDH | This doc |
+----+----------------------------+------------------------+
| 24 | Reserved for AEDSA | This doc |
+----+----------------------------+------------------------+
Table 14
[Notes to RFC-Editor: Please remove the table above on publication.
It is desirable not to reuse old or reserved algorithms because some
existing tools might print a wrong description. A higher number is
also an indication for a newer algorithm. As of now 22 is the next
free number.]
10.3.2. Symmetric-Key Algorithms
LibrePGP specifies a number of symmetric-key algorithms. This
specification creates a registry of symmetric-key algorithm
identifiers. The registry includes the algorithm name, its key sizes
and block size, and a reference to the defining specification. The
initial values for this registry can be found in Section 9.3. Adding
a new symmetric-key algorithm MUST be done through the SPECIFICATION
REQUIRED method, as described in [RFC8126].
10.3.3. Hash Algorithms
LibrePGP specifies a number of hash algorithms. This specification
creates a registry of hash algorithm identifiers. The registry
includes the algorithm name, a text representation of that name, its
block size, an OID hash prefix, and a reference to the defining
specification. The initial values for this registry can be found in
Section 9.5 for the algorithm identifiers and text names, and
Section 9.2 for the OIDs and expanded signature prefixes. Adding a
new hash algorithm MUST be done through the SPECIFICATION REQUIRED
method, as described in [RFC8126].
This document requests IANA register the following hash algorithms:
Koch & Tse Expires 1 June 2024 [Page 93]
Internet-Draft LibrePGP Message Format November 2023
+====+===========+===========+
| ID | Algorithm | Reference |
+====+===========+===========+
| 12 | SHA3-256 | This doc |
+----+-----------+-----------+
| 13 | Reserved | |
+----+-----------+-----------+
| 14 | SHA3-512 | This doc |
+----+-----------+-----------+
Table 15
[Notes to RFC-Editor: Please remove the table above on publication.
It is desirable not to reuse old or reserved algorithms because some
existing tools might print a wrong description. The ID 13 has been
reserved so that the SHA3 algorithm IDs align nicely with their SHA2
counterparts.]
10.3.4. Compression Algorithms
LibrePGP specifies a number of compression algorithms. This
specification creates a registry of compression algorithm
identifiers. The registry includes the algorithm name and a
reference to the defining specification. The initial values for this
registry can be found in Section 9.4. Adding a new compression key
algorithm MUST be done through the SPECIFICATION REQUIRED method, as
described in [RFC8126].
11. Packet Composition
LibrePGP packets are assembled into sequences in order to create
messages and to transfer keys. Not all possible packet sequences are
meaningful and correct. This section describes the rules for how
packets should be placed into sequences.
11.1. Transferable Public Keys
LibrePGP 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, one or more Signature packets
(certifications and attestation key signatures)
Koch & Tse Expires 1 June 2024 [Page 94]
Internet-Draft LibrePGP Message Format November 2023
* Zero or more User Attribute packets
* After each User Attribute packet, one or more Signature packets
(certifications and attestation key signatures)
* 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 one 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. Intermixed with these certifications may be Attestation Key
Signature packets issued by the primary key over the same User ID and
Public Key packet. The most recent of these is used to attest to
third-party certifications over the associated 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 one 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 or
V5 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.
Koch & Tse Expires 1 June 2024 [Page 95]
Internet-Draft LibrePGP Message Format November 2023
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
LibrePGP 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. LibrePGP Messages
An LibrePGP message is a packet or sequence of packets that
corresponds to the following grammatical rules (comma represents
sequential composition, and vertical bar separates alternatives):
Koch & Tse Expires 1 June 2024 [Page 96]
Internet-Draft LibrePGP Message Format November 2023
LibrePGP 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 :- OCB Encrypted Data Packet |
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,
LibrePGP Message, Corresponding Signature Packet.
Signed Message :- Signature Packet, LibrePGP 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 LibrePGP
Message.
11.4. Detached Signatures
Some LibrePGP 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. Enhanced Key Formats
12.1. Key Structures
The format of an LibrePGP 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 ...] ...]
Koch & Tse Expires 1 June 2024 [Page 97]
Internet-Draft LibrePGP Message Format November 2023
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 LibrePGP 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 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 SHOULD be a key capable of
certification. There are cases, such as device certificates, where
the primary key may not be capable of certification. A primary key
capable of making signatures SHOULD be accompanied by either a
certification signature (on a User ID or User Attribute) or a
signature directly on the key.
Implementations SHOULD accept encryption-only primary keys without a
signature. It also SHOULD allow importing any key accompanied either
by a certification signature or a signature on itself. It MAY accept
signature-capable primary keys without an accompanying signature.
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.
Koch & Tse Expires 1 June 2024 [Page 98]
Internet-Draft LibrePGP Message Format November 2023
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) two-octet scalar octet count of (b)-(e)
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).
A V5 fingerprint is the 256-bit SHA2-256 hash of the octet 0x9A,
followed by the four-octet packet length, followed by the entire
Public-Key packet starting with the version field. The Key ID is the
high-order 64 bits of the fingerprint. Here are the fields of the
hash material, with the example of a DSA key:
Koch & Tse Expires 1 June 2024 [Page 99]
Internet-Draft LibrePGP Message Format November 2023
a.1) 0x9A (1 octet)
a.2) four-octet scalar octet count of (b)-(f)
b) version number = 5 (1 octet);
c) timestamp of key creation (4 octets);
d) algorithm (1 octet): 17 = DSA (example);
e) four-octet scalar octet count for the following key material;
f) algorithm-specific fields.
Algorithm-Specific Fields for DSA keys (example):
f.1) MPI of DSA prime p;
f.2) MPI of DSA group order q (q is a prime divisor of p-1);
f.3) MPI of DSA group generator g;
f.4) MPI of DSA public-key value y (= g**x 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, V4, and V5 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 (V3 and V4 key) or
0x9A (V5 key) as the first octet (even though this is not a valid
packet ID for a public subkey).
13. Elliptic Curve Cryptography
This section descripes algorithms and parameters used with Elliptic
Curve Cryptography (ECC) keys. A thorough introduction to ECC can be
found in [KOBLITZ].
Koch & Tse Expires 1 June 2024 [Page 100]
Internet-Draft LibrePGP Message Format November 2023
13.1. Supported ECC Curves
This document references six named prime field curves, defined in
[FIPS186] as "Curve P-256", "Curve P-384", and "Curve P-521"; and
defined in [RFC5639] as "brainpoolP256r1" "brainpoolP384r1", and
"brainpoolP512r1". Further curves "Curve25519" and "Curve448",
defined in [RFC7748] are referenced for use with Ed25519/Ed448 (EdDSA
signing) and X25519/X448 (ECDH encryption).
The named curves are referenced as a sequence of bytes in this
document, called throughout, curve OID. Section 9.2 describes in
detail how this sequence of bytes is formed.
13.2. ECDSA and ECDH Conversion Primitives
This document defines the uncompressed point format for ECDSA and
ECDH and a custom compression format for certain curves. The point
is encoded in the Multiprecision Integer (MPI) format.
For an uncompressed point the content of the MPI is:
B = 04 || x || y
where x and y are coordinates of the point P = (x, y), each encoded
in the big-endian format and zero-padded to the adjusted underlying
field size. The adjusted underlying field size is the underlying
field size that is rounded up to the nearest 8-bit boundary. This
encoding is compatible with the definition given in [SEC1].
For a custom compressed point the content of the MPI is:
B = 40 || x
where x is the x coordinate of the point P encoded to the rules
defined for the specified curve. This format is used for ECDH keys
based on curves expressed in Montgomery form.
Therefore, the exact size of the MPI payload is 515 bits for "Curve
P-256", 771 for "Curve P-384", 1059 for "Curve P-521", and 263 for
Curve25519.
Even though the zero point, also called the point at infinity, may
occur as a result of arithmetic operations on points of an elliptic
curve, it SHALL NOT appear in data structures defined in this
document.
Koch & Tse Expires 1 June 2024 [Page 101]
Internet-Draft LibrePGP Message Format November 2023
If other conversion methods are defined in the future, a compliant
application MUST NOT use a new format when in doubt that any
recipient can support it. Consider, for example, that while both the
public key and the per-recipient ECDH data structure, respectively
defined in Section 5.6.6 and Section 5.1, contain an encoded point
field, the format changes to the field in Section 5.1 only affect a
given recipient of a given message.
13.3. EdDSA Point Format
The EdDSA algorithm defines a specific point compression format. To
indicate the use of this compression format and to make sure that the
key can be represented in the Multiprecision Integer (MPI) format the
octet string specifying the point is prefixed with the octet 0x40.
This encoding is an extension of the encoding given in [SEC1] which
uses 0x04 to indicate an uncompressed point.
For example, the length of a public key for the curve Ed25519 is 263
bit: 7 bit to represent the 0x40 prefix octet and 32 octets for the
native value of the public key.
13.4. Key Derivation Function
A key derivation function (KDF) is necessary to implement the EC
encryption. The Concatenation Key Derivation Function (Approved
Alternative 1) [SP800-56A] with the KDF hash function that is
SHA2-256 [FIPS180] or stronger is REQUIRED. See Section 17 for the
details regarding the choice of the hash function.
For convenience, the synopsis of the encoding method is given below
with significant simplifications attributable to the restricted
choice of hash functions in this document. However, [SP800-56A] is
the normative source of the definition.
// Implements KDF( X, oBits, Param );
// Input: point X = (x,y)
// oBits - the desired size of output
// hBits - the size of output of hash function Hash
// Param - octets representing the parameters
// Assumes that oBits <= hBits
// Convert the point X to the octet string:
// ZB' = 04 || x || y
// and extract the x portion from ZB'
ZB = x;
MB = Hash ( 00 || 00 || 00 || 01 || ZB || Param );
return oBits leftmost bits of MB.
Koch & Tse Expires 1 June 2024 [Page 102]
Internet-Draft LibrePGP Message Format November 2023
Note that ZB in the KDF description above is the compact
representation of X, defined in Section 4.2 of [RFC6090].
13.5. ECDH Algorithm
The method is a combination of an ECC Diffie-Hellman method to
establish a shared secret, a key derivation method to process the
shared secret into a derived key, and a key wrapping method that uses
the derived key to protect a session key used to encrypt a message.
The One-Pass Diffie-Hellman method C(1, 1, ECC CDH) [SP800-56A] MUST
be implemented with the following restrictions: the ECC CDH primitive
employed by this method is modified to always assume the cofactor as
1, the KDF specified in Section 13.4 is used, and the KDF parameters
specified below are used.
The KDF parameters are encoded as a concatenation of the following 5
variable-length and fixed-length fields, compatible with the
definition of the OtherInfo bitstring [SP800-56A]:
* a variable-length field containing a curve OID, formatted as
follows:
- a one-octet size of the following field
- the octets representing a curve OID, defined in Section 9.2
* a one-octet public key algorithm ID defined in Section 9.1
* a variable-length field containing KDF parameters, identical to
the corresponding field in the ECDH public key, formatted as
follows:
- a one-octet size of the following fields; values 0 and 0xff are
reserved for future extensions
- a one-octet value 01, reserved for future extensions
- a one-octet hash function ID used with the KDF
- a one-octet algorithm ID for the symmetric algorithm used to
wrap the symmetric key for message encryption; see Section 13.5
for details
* 20 octets representing the UTF-8 encoding of the string "Anonymous
Sender ", which is the octet sequence 41 6E 6F 6E 79 6D 6F 75 73
20 53 65 6E 64 65 72 20 20 20 20
Koch & Tse Expires 1 June 2024 [Page 103]
Internet-Draft LibrePGP Message Format November 2023
* 20 octets representing a recipient encryption subkey or a master
key fingerprint, identifying the key material that is needed for
the decryption. For version 5 keys the 20 leftmost octets of the
fingerprint are used.
The size of the KDF parameters sequence, defined above, is either 54
for the NIST curve P-256, 51 for the curves P-384 and P-521, or 56
for Curve25519.
The key wrapping method is described in [RFC3394]. KDF produces a
symmetric key that is used as a key-encryption key (KEK) as specified
in [RFC3394]. Refer to Section 16 for the details regarding the
choice of the KEK algorithm, which SHOULD be one of three AES
algorithms. Key wrapping and unwrapping is performed with the
default initial value of [RFC3394].
The input to the key wrapping method is the value "m" derived from
the session key, as described in Section 5.1, "Public-Key Encrypted
Session Key Packets (Tag 1)", except that the PKCS #1.5 padding step
is omitted. The result is padded using the method described in
[PKCS5] to the 8-byte granularity. For example, the following
AES-256 session key, in which 32 octets are denoted from k0 to k31,
is composed to form the following 40 octet sequence:
09 k0 k1 ... k31 c0 c1 05 05 05 05 05
The octets c0 and c1 above denote the checksum. This encoding allows
the sender to obfuscate the size of the symmetric encryption key used
to encrypt the data. For example, assuming that an AES algorithm is
used for the session key, the sender MAY use 21, 13, and 5 bytes of
padding for AES-128, AES-192, and AES-256, respectively, to provide
the same number of octets, 40 total, as an input to the key wrapping
method.
The output of the method consists of two fields. The first field is
the MPI containing the ephemeral key used to establish the shared
secret. The second field is composed of the following two fields:
* a one-octet encoding the size in octets of the result of the key
wrapping method; the value 255 is reserved for future extensions;
* up to 254 octets representing the result of the key wrapping
method, applied to the 8-byte padded session key, as described
above.
Note that for session key sizes 128, 192, and 256 bits, the size of
the result of the key wrapping method is, respectively, 32, 40, and
48 octets, unless the size obfuscation is used.
Koch & Tse Expires 1 June 2024 [Page 104]
Internet-Draft LibrePGP Message Format November 2023
For convenience, the synopsis of the encoding method is given below;
however, this section, [SP800-56A], and [RFC3394] are the normative
sources of the definition.
Obtain the authenticated recipient public key R
Generate an ephemeral key pair {v, V=vG}
Compute the shared point S = vR;
m = symm_alg_ID || session key || checksum || pkcs5_padding;
curve_OID_len = (byte)len(curve_OID);
Param = curve_OID_len || curve_OID || public_key_alg_ID || 03
|| 01 || KDF_hash_ID || KEK_alg_ID for AESKeyWrap || "Anonymous
Sender " || recipient_fingerprint;
Z_len = the key size for the KEK_alg_ID used with AESKeyWrap
Compute Z = KDF( S, Z_len, Param );
Compute C = AESKeyWrap( Z, m ) as per [RFC3394]
VB = convert point V to the octet string
Output (MPI(VB) || len(C) || C).
The decryption is the inverse of the method given. Note that the
recipient obtains the shared secret by calculating
S = rV = rvG, where (r,R) is the recipient's key pair.
Consistent with Section 5.16, "OCB Encrypted Data Packet (Tag 20)"
and Section 5.14, "Sym. Encrypted Integrity Protected Data Packet
(Tag 18)", OCB encryption or a Modification Detection Code (MDC) MUST
be used anytime the symmetric key is protected by ECDH.
13.5.1. ECDH Parameters
ECDH keys have a hash algorithm parameter for key derivation and a
symmetric algorithm for key encapsulation.
For v4 keys, the following algorithms SHOULD be used depending on the
curve. An implementation SHOULD only use an AES algorithm as a KEK
algorithm.
For v5 keys, the following algorithms MUST be used depending on the
curve. An implementation MUST NOT generate a v5 ECDH key over any
listed curve that uses different KDF or KEK parameters. An
implementation MUST NOT encrypt any message to a v5 ECDH key over a
listed curve that announces a different KDF or KEK parameter.
Koch & Tse Expires 1 June 2024 [Page 105]
Internet-Draft LibrePGP Message Format November 2023
+=================+================+=====================+
| Curve | Hash algorithm | Symmetric algorithm |
+=================+================+=====================+
| NIST P-256 | SHA2-256 | AES-128 |
+-----------------+----------------+---------------------+
| NIST P-384 | SHA2-384 | AES-192 |
+-----------------+----------------+---------------------+
| NIST P-521 | SHA2-512 | AES-256 |
+-----------------+----------------+---------------------+
| brainpoolP256r1 | SHA2-256 | AES-128 |
+-----------------+----------------+---------------------+
| brainpoolP384r1 | SHA2-384 | AES-192 |
+-----------------+----------------+---------------------+
| brainpoolP512r1 | SHA2-512 | AES-256 |
+-----------------+----------------+---------------------+
| Curve25519 | SHA2-256 | AES-128 |
+-----------------+----------------+---------------------+
| X448 | SHA2-512 | AES-256 |
+-----------------+----------------+---------------------+
Table 16
14. Post-Quantum Cryptography
{Note: This part is not finalized and subject to change}
This section descripes algorithms and parameters used with post-
quantum cryptography. Specifically, it defines composite public-key
encryption based on ML-KEM.
14.1. Kyber Algorithm
ML-KEM [FIPS203], which is also known as CRYSTALS-Kyber, is based on
the hardness of solving the learning-with-errors problem in module
lattices (MLWE). The scheme is believed to provide security against
cryptanalytic attacks by classical as well as quantum computers.
This specification defines ML-KEM only in composite combination with
ECC-based encryption schemes in order to provide a pre-quantum
security fallback.
15. Notes on Algorithms
Koch & Tse Expires 1 June 2024 [Page 106]
Internet-Draft LibrePGP Message Format November 2023
15.1. PKCS#1 Encoding in LibrePGP
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]. RFC 3447
should be treated as the ultimate authority on PKCS#1 for LibrePGP.
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.
15.1.1. EME-PKCS1-v1_5-ENCODE
Input:
k = the length in octets of the key modulus.
M = message to be encoded, an octet string of length mLen,
where mLen <= k - 11.
Output:
EM = encoded message, an octet string of length k.
Error: "message too long".
1. Length checking: If mLen > k - 11, output "message too long"
and stop.
2. Generate an octet string PS of length k - mLen - 3 consisting
of pseudo-randomly generated nonzero octets. The length of PS
will be at least eight octets.
3. Concatenate PS, the message M, and other padding to form an
encoded message EM of length k octets as
EM = 0x00 || 0x02 || PS || 0x00 || M.
4. Output EM.
15.1.2. EME-PKCS1-v1_5-DECODE
Koch & Tse Expires 1 June 2024 [Page 107]
Internet-Draft LibrePGP Message Format November 2023
Input:
EM = encoded message, an octet string
Output:
M = message, an octet string,
Error: "decryption error",
To decode an EME-PKCS1_v1_5 message, separate the encoded message EM
into an octet string PS consisting of nonzero octets and a message M
as follows
EM = 0x00 || 0x02 || PS || 0x00 || M.
If the first octet of EM does not have hexadecimal value 0x00, if the
second octet of EM does not have hexadecimal value 0x02, if there is
no octet with hexadecimal value 0x00 to separate PS from M, or if the
length of PS is less than 8 octets, output "decryption error" and
stop. See also the security note in Section 16 regarding differences
in reporting between a decryption error and a padding error.
15.1.3. EMSA-PKCS1-v1_5
This encoding method is deterministic and only has an encoding
operation.
Option:
Hash - a hash function in which hLen denotes the length in octets
of the hash function output.
Input:
M = message to be encoded.
emLen = intended length in octets of the encoded message, at least
tLen + 11, where tLen is the octet length of the DER encoding
T of a certain value computed during the encoding operation.
Output:
EM = encoded message, an octet string of length emLen.
Errors: "message too long";
"intended encoded message length too short".
Koch & Tse Expires 1 June 2024 [Page 108]
Internet-Draft LibrePGP Message Format November 2023
Steps:
1. Apply the hash function to the message M to produce a hash
value H:
H = Hash(M).
If the hash function outputs "message too long," output
"message too long" and stop.
2. Using the list in Section [](#version-3-signature-packet-format),
"Version 3 Signature Packet Format", produce an ASN.1 DER
value for the hash function used. Let T be the full hash
prefix from the list, and let tLen be the length in octets of
T.
3. If emLen < tLen + 11, output "intended encoded message length
too short" and stop.
4. Generate an octet string PS consisting of emLen - tLen - 3
octets with hexadecimal value 0xFF. The length of PS will be
at least 8 octets.
5. Concatenate PS, the hash prefix T, and other padding to form
the encoded message EM as
EM = 0x00 || 0x01 || PS || 0x00 || T.
6. Output EM.
15.2. 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 AES-128 only specified for
"alice@work.com" but Camellia-256, Twofish, and AES-128 specified for
"alice@home.org". Note that it is also possible for preferences to
be in a subkey's binding signature.
Since AES-128 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 an AES-128-only implementation. Note further that
implementations conforming to previous versions of this standard
(RFC-4880) have TripleDES as its only MUST-implement algorithm.
Koch & Tse Expires 1 June 2024 [Page 109]
Internet-Draft LibrePGP Message Format November 2023
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, AES-128, 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.
15.3. 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.
15.3.1. Compression Preferences
Compression has been an integral part of PGP since its first days.
LibrePGP and all previous versions of PGP have offered compression.
In this specification, the default is for messages to be compressed,
although an implementation is not required to do so. Consequently,
the compression preference gives a way for a keyholder to request
that messages not be compressed, presumably because they are using a
minimal implementation that does not include compression.
Additionally, this gives a keyholder a way to state that it can
support alternate algorithms.
Like the algorithm preferences, an implementation MUST NOT use an
algorithm that is not in the preference vector. If the preferences
are not present, then they are assumed to be [ZIP(1),
Uncompressed(0)].
Koch & Tse Expires 1 June 2024 [Page 110]
Internet-Draft LibrePGP Message Format November 2023
Additionally, an implementation MUST implement this preference to the
degree of recognizing when to send an uncompressed message. A robust
implementation would satisfy this requirement by looking at the
recipient's preference and acting accordingly. A minimal
implementation can satisfy this requirement by never generating a
compressed message, since all implementations can handle messages
that have not been compressed.
15.3.2. Hash Algorithm Preferences
Typically, the choice of a hash algorithm is something the signer
does, rather than the verifier, because a signer rarely knows who is
going to be verifying the signature. This preference, though, allows
a protocol based upon digital signatures ease in negotiation.
Thus, if Alice is authenticating herself to Bob with a signature, it
makes sense for her to use a hash algorithm that Bob's software uses.
This preference allows Bob to state in his key which algorithms Alice
may use.
Since SHA2-256 is the MUST-implement hash 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.
15.4. 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.
15.5. 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.
Koch & Tse Expires 1 June 2024 [Page 111]
Internet-Draft LibrePGP Message Format November 2023
15.6. 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, SHA2-224, SHA2-256, SHA2-384, or
SHA2-512 hash
* 2048-bit key, 224-bit q, SHA2-224, SHA2-256, SHA2-384, or SHA2-512
hash
* 2048-bit key, 256-bit q, SHA2-256, SHA2-384, or SHA2-512 hash
* 3072-bit key, 256-bit q, SHA2-256, SHA2-384, or SHA2-512 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.
15.7. Elgamal
An implementation SHOULD NOT implement Elgamal keys of size less than
1024 bits.
15.8. EdDSA
Although the EdDSA algorithm allows arbitrary data as input, its use
with LibrePGP requires that a digest of the message is used as input
(pre-hashed). See section Section 5.2.4, "Computing Signatures" for
details. Truncation of the resulting digest is never applied; the
resulting digest value is used verbatim as input to the EdDSA
algorithm.
Koch & Tse Expires 1 June 2024 [Page 112]
Internet-Draft LibrePGP Message Format November 2023
15.9. Reserved Algorithm Numbers
A number of algorithm IDs have been reserved for algorithms that
would be useful to use in an LibrePGP implementation, yet there are
issues that prevent an implementer from actually implementing the
algorithm. These are marked in Section 9.1, "Public-Key Algorithms",
as "reserved for".
The reserved public-key algorithm X9.42 (21) does not have the
necessary parameters, parameter order, or semantics defined. The
same is currently true for reserved public-key algorithms AEDH (23)
and AEDSA (24).
Previous versions of LibrePGP 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].
15.10. LibrePGP CFB Mode
LibrePGP 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).
LibrePGP 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.
Koch & Tse Expires 1 June 2024 [Page 113]
Internet-Draft LibrePGP Message Format November 2023
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. 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.
9. FR is encrypted to produce FRE.
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.
15.11. 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 [RFC8126].
However, implementations need to be careful with these and promote
them to full IANA-managed parameters when they grow beyond the
original, limited system.
Koch & Tse Expires 1 June 2024 [Page 114]
Internet-Draft LibrePGP Message Format November 2023
15.12. Meta-Considerations for Expansion
If LibrePGP 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.
16. Security Considerations
* As with any technology involving cryptography, you should check
the current literature to determine if any algorithms used here
have been found to be vulnerable to attack or have been found to
be too weak.
* This specification uses Public-Key Cryptography technologies. It
is assumed that the private key portion of a public-private key
pair is controlled and secured by the proper party or parties.
* Certain operations in this specification involve the use of random
numbers. An appropriate entropy source should be used to generate
these numbers (see [RFC4086]).
* The MD5 hash algorithm has been found to have weaknesses, with
collisions found in a number of cases. MD5 is deprecated for use
in LibrePGP. Implementations MUST NOT generate new signatures
using MD5 as a hash function. They MAY continue to consider old
signatures that used MD5 as valid.
* SHA2-224 and SHA2-384 require the same work as SHA2-256 and
SHA2-512, respectively. In general, there are few reasons to use
them outside of DSS compatibility. You need a situation where one
needs more security than smaller hashes, but does not want to have
the full 256-bit or 512-bit data length.
* Many security protocol designers think that it is a bad idea to
use a single key for both privacy (encryption) and integrity
(signatures). In fact, this was one of the motivating forces
Koch & Tse Expires 1 June 2024 [Page 115]
Internet-Draft LibrePGP Message Format November 2023
behind the V4 key format with separate signature and encryption
keys. If you as an implementer promote dual-use keys, you should
at least be aware of this controversy.
* The DSA algorithm will work with any hash, but is sensitive to the
quality of the hash algorithm. Verifiers should be aware that
even if the signer used a strong hash, an attacker could have
modified the signature to use a weak one. Only signatures using
acceptably strong hash algorithms should be accepted as valid.
* If you are building an authentication system, the recipient may
specify a preferred signing algorithm. However, the signer would
be foolish to use a weak algorithm simply because the recipient
requests it.
* In late summer 2002, Jallad, Katz, and Schneier published an
interesting attack on the LibrePGP protocol and some of its
implementations [JKS02]. In this attack, the attacker modifies a
message and sends it to a user who then returns the erroneously
decrypted message to the attacker. The attacker is thus using the
user as a random oracle, and can often decrypt the message.
Compressing data can ameliorate this attack. The incorrectly
decrypted data nearly always decompresses in ways that defeat the
attack. However, this is not a rigorous fix, and leaves open some
small vulnerabilities. For example, if an implementation does not
compress a message before encryption (perhaps because it knows it
was already compressed), then that message is vulnerable. Because
of this happenstance --- that modification attacks can be thwarted
by decompression errors --- an implementation SHOULD treat a
decompression error as a security problem, not merely a data
problem.
This attack can be defeated by the use of modification detection,
provided that the implementation does not let the user naively
return the data to the attacker. The modification detection is
prefereabble implemented by using the OCB Encrypted Data Packet
and only if the recipients don't supports this by use of the
Symmmetric Encrypted and Integrity Protected Data Packet. An
implementation MUST treat an authentication or MDC failure as a
security problem, not merely a data problem.
In either case, the implementation SHOULD NOT allow the user
access to the erroneous data, and MUST warn the user as to
potential security problems should that data be returned to the
sender.
Koch & Tse Expires 1 June 2024 [Page 116]
Internet-Draft LibrePGP Message Format November 2023
While this attack is somewhat obscure, requiring a special set of
circumstances to create it, it is nonetheless quite serious as it
permits someone to trick a user to decrypt a message.
Consequently, it is important that:
1. Implementers treat authentication errors, MDC errors,
decompression failures or no use of MDC or AEAD as security
problems.
2. Implementers implement OCB with all due speed and encourage
its spread.
3. Users migrate to implementations that support OCB encryption
with all due speed.
* PKCS#1 has been found to be vulnerable to attacks in which a
system that reports errors in padding differently from errors in
decryption becomes a random oracle that can leak the private key
in mere millions of queries. Implementations must be aware of
this attack and prevent it from happening. The simplest solution
is to report a single error code for all variants of decryption
errors so as not to leak information to an attacker.
* Some technologies mentioned here may be subject to government
control in some countries.
* In winter 2005, Serge Mister and Robert Zuccherato from Entrust
released a paper describing a way that the "quick check" in
LibrePGP CFB mode can be used with a random oracle to decrypt two
octets of every cipher block [MZ05]. They recommend as prevention
not using the quick check at all.
Many implementers have taken this advice to heart for any data
that is symmetrically encrypted and for which the session key is
public-key encrypted. In this case, the quick check is not needed
as the public-key encryption of the session key should guarantee
that it is the right session key. In other cases, the
implementation should use the quick check with care.
On the one hand, there is a danger to using it if there is a
random oracle that can leak information to an attacker. In
plainer language, there is a danger to using the quick check if
timing information about the check can be exposed to an attacker,
particularly via an automated service that allows rapidly repeated
queries.
Koch & Tse Expires 1 June 2024 [Page 117]
Internet-Draft LibrePGP Message Format November 2023
On the other hand, it is inconvenient to the user to be informed
that they typed in the wrong passphrase only after a petabyte of
data is decrypted. There are many cases in cryptographic
engineering where the implementer must use care and wisdom, and
this is one.
* Refer to [FIPS186], B.4.1, for the method to generate a uniformly
distributed ECC private key.
* This document explicitly discourages the use of algorithms other
than AES as a KEK algorithm because backward compatibility of the
ECDH format is not a concern. The KEK algorithm is only used
within the scope of a Public-Key Encrypted Session Key Packet,
which represents an ECDH key recipient of a message. Compare this
with the algorithm used for the session key of the message, which
MAY be different from a KEK algorithm.
Compliant applications SHOULD implement, advertise through key
preferences, and use the strongest algorithms specified in this
document.
Note that the symmetric algorithm preference list may make it
impossible to use the balanced strength of symmetric key
algorithms for a corresponding public key. For example, the
presence of the symmetric key algorithm IDs and their order in the
key preference list affects the algorithm choices available to the
encoding side, which in turn may make the adherence to the table
above infeasible. Therefore, compliance with this specification
is a concern throughout the life of the key, starting immediately
after the key generation when the key preferences are first added
to a key. It is generally advisable to position a symmetric
algorithm ID of strength matching the public key at the head of
the key preference list.
Encryption to multiple recipients often results in an unordered
intersection subset. For example, if the first recipient's set is
{A, B} and the second's is {B, A}, the intersection is an
unordered set of two algorithms, A and B. In this case, a
compliant application SHOULD choose the stronger encryption
algorithm.
Resource constraints, such as limited computational power, is a
likely reason why an application might prefer to use the weakest
algorithm. On the other side of the spectrum are applications
that can implement every algorithm defined in this document. Most
applications are expected to fall into either of two categories.
A compliant application in the second, or strongest, category
SHOULD prefer AES-256 to AES-192.
Koch & Tse Expires 1 June 2024 [Page 118]
Internet-Draft LibrePGP Message Format November 2023
SHA-1 MUST NOT be used with the ECDSA or the KDF in the ECDH
method.
MDC MUST be used when a symmetric encryption key is protected by
ECDH. None of the ECC methods described in this document are
allowed with deprecated V3 keys. A compliant application MUST
only use iterated and salted S2K to protect private keys, as
defined in Section 3.7.1.3, "Iterated and Salted S2K".
Side channel attacks are a concern when a compliant application's
use of the LibrePGP format can be modeled by a decryption or
signing oracle model, for example, when an application is a
network service performing decryption to unauthenticated remote
users. ECC scalar multiplication operations used in ECDSA and
ECDH are vulnerable to side channel attacks. Countermeasures can
often be taken at the higher protocol level, such as limiting the
number of allowed failures or time-blinding of the operations
associated with each network interface. Mitigations at the scalar
multiplication level seek to eliminate any measurable distinction
between the ECC point addition and doubling operations.
* Although technically possible, the EdDSA algorithm MUST NOT be
used with a digest algorithms weaker than SHA2-256.
LibrePGP was designed with security in mind, with many smart,
intelligent people spending a lot of time thinking about the
ramifications of their decisions. Removing the requirement for self-
certifying User ID (and User Attribute) packets on a key means that
someone could surreptitiously add an unwanted ID to a key and sign
it. If enough "trusted" people sign that surreptitious identity then
other people might believe it. The attack could wind up sending
encrypted mail destined for alice to some other target, bob, because
someone added "alice" to bob's key without bob's consent.
In the case of device certificates the device itself does not have
any consent. It is given an identity by the device manufacturer and
the manufacturer can insert that ID on the device certificate,
signing it with the manufacturer's key. If another people wants to
label the device by another name, they can do so. There is no harm
in multiple IDs, because the verification is all done based on who
has signed those IDs.
When a key can self-sign, it is still suggested to self-certify IDs,
even if it no longer required by this modification to LibrePGP. This
at least signals to recipients of keys that yes, the owner of this
key asserts that this identity belongs to herself. Note, however,
that mallet could still assert that he is 'alice' and could even
self-certify that. So the attack is not truly different. Moreover,
Koch & Tse Expires 1 June 2024 [Page 119]
Internet-Draft LibrePGP Message Format November 2023
in the case of device certificates, it's more the manufacturer than
the device that wants to assert an identity (even if the device could
self-certify).
There is no signaling whether a key is using this looser-requirement
key format. An attacker could therefore just remove the self-
signature off a published key. However one would hope that wide
publication would result in another copy still having that signature
and it being returned quickly. However, the lack of signaling also
means that a user with an application following RFC 4880 directly
would see a key following this specification as "broken" and may not
accept it.
On a different note, including the "geo" notation could leak
information about where a signer is located. However it is just an
assertion (albeit a signed assertion) so there is no verifiable truth
to the location information released. Similarly, all the rest of the
signature notations are pure assertions, so they should be taken with
the trustworthiness of the signer.
Combining the User ID with the User Attribute means that an ID and
image would not be separable. For a person this is probably not
good, but for a device it's unlikely the image will change so it
makes sense to combine the ID and image into a single signed packet
with a single signature.
17. Compatibility Profiles
17.1. LibrePGP ECC Profile
A compliant application MUST implement NIST curve P-256, SHOULD
implement NIST curve P-521, SHOULD implement brainpoolP256r1 and
brainpoolP512r1, SHOULD implement Ed25519, SHOULD implement
Curve25519, MAY implement NIST curve P-384, and MAY implement
brainpoolP384r1, as defined in Section 9.2.
A compliant application MUST implement SHA2-256 and SHOULD implement
SHA2-384 and SHA2-512. A compliant application MUST implement
AES-128 and SHOULD implement AES-256.
A compliant application SHOULD follow Section 16 regarding the choice
of the following algorithms for each curve:
* the KDF hash algorithm,
* the KEK algorithm,
Koch & Tse Expires 1 June 2024 [Page 120]
Internet-Draft LibrePGP Message Format November 2023
* the message digest algorithm and the hash algorithm used in the
key certifications,
* the symmetric algorithm used for message encryption.
It is recommended that the chosen symmetric algorithm for message
encryption be no less secure than the KEK algorithm.
18. Implementation Nits
This section is a collection of comments to help an implementer,
particularly with an eye to backward compatibility. Previous
implementations of PGP are not LibrePGP compliant. Often the
differences are small, but small differences are frequently more
vexing than large differences. Thus, this is a non-comprehensive
list of potential problems and gotchas for a developer who is trying
to be backward-compatible.
* When exporting a private key, PGP 2 generates the header "BEGIN
PGP SECRET KEY BLOCK" instead of "BEGIN PGP PRIVATE KEY BLOCK".
All previous versions ignore the implied data type, and look
directly at the packet data type.
* PGP versions 2.0 through 2.5 generated V2 Public-Key packets.
These are identical to the deprecated V3 keys except for the
version number. An implementation MUST NOT generate them and may
accept or reject them as it sees fit. Some older PGP versions
generated V2 PKESK packets (Tag 1) as well. An implementation may
accept or reject V2 PKESK packets as it sees fit, and MUST NOT
generate them.
* PGP version 2.6 will not accept key-material packets with versions
greater than 3.
* There are many ways possible for two keys to have the same key
material, but different fingerprints (and thus Key IDs). Perhaps
the most interesting is an RSA key that has been "upgraded" to V4
format, but since a V4 fingerprint is constructed by hashing the
key creation time along with other things, two V4 keys created at
different times, yet with the same key material will have
different fingerprints.
* If an implementation is using zlib to interoperate with PGP 2,
then the "windowBits" parameter should be set to -13.
Koch & Tse Expires 1 June 2024 [Page 121]
Internet-Draft LibrePGP Message Format November 2023
* The 0x19 back signatures were not required for signing subkeys
until relatively recently. Consequently, there may be keys in the
wild that do not have these back signatures. Implementing
software may handle these keys as it sees fit.
* LibrePGP does not put limits on the size of public keys. However,
larger keys are not necessarily better keys. Larger keys take
more computation time to use, and this can quickly become
impractical. Different LibrePGP implementations may also use
different upper bounds for public key sizes, and so care should be
taken when choosing sizes to maintain interoperability. As of
2007 most implementations have an upper bound of 4096 bits for
RSA, DSA, and Elgamal
* ASCII armor is an optional feature of LibrePGP. The LibrePGP
working group strives for a minimal set of mandatory-to-implement
features, and since there could be useful implementations that
only use binary object formats, this is not a "MUST" feature for
an implementation. For example, an implementation that is using
LibrePGP as a mechanism for file signatures may find ASCII armor
unnecessary. LibrePGP permits an implementation to declare what
features it does and does not support, but ASCII armor is not one
of these. Since most implementations allow binary and armored
objects to be used indiscriminately, an implementation that does
not implement ASCII armor may find itself with compatibility
issues with general-purpose implementations. Moreover,
implementations of OpenPGP-MIME [RFC3156] already have a
requirement for ASCII armor so those implementations will
necessarily have support.
19. References
19.1. Normative References
[AES] NIST, "FIPS PUB 197, Advanced Encryption Standard (AES)",
November 2001,
<http://csrc.nist.gov/publications/fips/fips197/fips-
197.{ps,pdf}>.
[BLOWFISH] Schneier, B., "Description of a New Variable-Length Key,
64-Bit Block Cipher (Blowfish)", Fast Software Encryption,
Cambridge Security Workshop Proceedings Springer-Verlag,
1994, pp191-204, December 1993,
<http://www.counterpane.com/bfsverlag.html>.
[BZ2] Seward, J., "The Bzip2 and libbzip2 home page", 2010,
<http://www.bzip.org/>.
Koch & Tse Expires 1 June 2024 [Page 122]
Internet-Draft LibrePGP Message Format November 2023
[EAX] Bellare, M., Rogaway, P., and D. Wagner, "A Conventional
Authenticated-Encryption Mode", April 2003.
[ELGAMAL] Elgamal, T., "A Public-Key Cryptosystem and a Signature
Scheme Based on Discrete Logarithms", IEEE Transactions on
Information Theory v. IT-31, n. 4, 1985, pp. 469-472,
1985.
[FIPS180] National Institute of Standards and Technology, U.S.
Department of Commerce, "Secure Hash Standard (SHS), FIPS
180-4", August 2015,
<http://dx.doi.org/10.6028/NIST.FIPS.180-4>.
[FIPS186] National Institute of Standards and Technology, U.S.
Department of Commerce, "Digital Signature Standard (DSS),
FIPS 186-4", July 2013,
<http://dx.doi.org/10.6028/NIST.FIPS.186-4>.
[FIPS202] National Institute of Standards and Technology, U.S.
Department of Commerce, "SHA-3 Standard: Permutation-Based
Hash and Extendable-Output Functions, FIPS 202", August
2015, <http://dx.doi.org/10.6028/NIST.FIPS.202>.
[FIPS203] National Institute of Standards and Technology, U.S.
Department of Commerce, "Module-Lattice-Based Key-
Encapsulation Mechanism Standard", August 2023,
<https://doi.org/10.6028/NIST.FIPS.203.ipd>.
[HAC] Menezes, A. J., v Oorschot, P., and S. Vanstone, "Handbook
of Applied Cryptography", 1996.
[IDEA] Lai, X., "On the design and security of block ciphers",
ETH Series in Information Processing, J.L. Massey
(editor) Vol. 1, Hartung-Gorre Verlag Konstanz, Technische
Hochschule (Zurich), 1992.
[ISO10646] International Organization for Standardization,
"Information Technology - Universal Multiple-octet coded
Character Set (UCS) - Part 1: Architecture and Basic
Multilingual Plane", ISO Standard 10646-1, May 1993.
[JFIF] CA, E. H. M., "JPEG File Interchange Format (Version
1.02).", September 1996.
[PKCS5] RSA Laboratories, "PKCS #5 v2.0: Password-Based
Cryptography Standard", 25 March 1999.
Koch & Tse Expires 1 June 2024 [Page 123]
Internet-Draft LibrePGP Message Format November 2023
[RFC1950] Deutsch, P. and J. Gailly, "ZLIB Compressed Data Format
Specification version 3.3", RFC 1950,
DOI 10.17487/RFC1950, May 1996,
<https://www.rfc-editor.org/rfc/rfc1950>.
[RFC1951] Deutsch, P., "DEFLATE Compressed Data Format Specification
version 1.3", RFC 1951, DOI 10.17487/RFC1951, May 1996,
<https://www.rfc-editor.org/rfc/rfc1951>.
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996,
<https://www.rfc-editor.org/rfc/rfc2045>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>.
[RFC2144] Adams, C., "The CAST-128 Encryption Algorithm", RFC 2144,
DOI 10.17487/RFC2144, May 1997,
<https://www.rfc-editor.org/rfc/rfc2144>.
[RFC2822] Resnick, P., Ed., "Internet Message Format", RFC 2822,
DOI 10.17487/RFC2822, April 2001,
<https://www.rfc-editor.org/rfc/rfc2822>.
[RFC3156] Elkins, M., Del Torto, D., Levien, R., and T. Roessler,
"MIME Security with OpenPGP", RFC 3156,
DOI 10.17487/RFC3156, August 2001,
<https://www.rfc-editor.org/rfc/rfc3156>.
[RFC3394] Schaad, J. and R. Housley, "Advanced Encryption Standard
(AES) Key Wrap Algorithm", RFC 3394, DOI 10.17487/RFC3394,
September 2002, <https://www.rfc-editor.org/rfc/rfc3394>.
[RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography
Standards (PKCS) #1: RSA Cryptography Specifications
Version 2.1", RFC 3447, DOI 10.17487/RFC3447, February
2003, <https://www.rfc-editor.org/rfc/rfc3447>.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
2003, <https://www.rfc-editor.org/rfc/rfc3629>.
Koch & Tse Expires 1 June 2024 [Page 124]
Internet-Draft LibrePGP Message Format November 2023
[RFC3713] Matsui, M., Nakajima, J., and S. Moriai, "A Description of
the Camellia Encryption Algorithm", RFC 3713,
DOI 10.17487/RFC3713, April 2004,
<https://www.rfc-editor.org/rfc/rfc3713>.
[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
DOI 10.17487/RFC4086, June 2005,
<https://www.rfc-editor.org/rfc/rfc4086>.
[RFC5639] Lochter, M. and J. Merkle, "Elliptic Curve Cryptography
(ECC) Brainpool Standard Curves and Curve Generation",
RFC 5639, DOI 10.17487/RFC5639, March 2010,
<https://www.rfc-editor.org/rfc/rfc5639>.
[RFC5870] Mayrhofer, A. and C. Spanring, "A Uniform Resource
Identifier for Geographic Locations ('geo' URI)",
RFC 5870, DOI 10.17487/RFC5870, June 2010,
<https://www.rfc-editor.org/rfc/rfc5870>.
[RFC7253] Krovetz, T. and P. Rogaway, "The OCB Authenticated-
Encryption Algorithm", RFC 7253, DOI 10.17487/RFC7253, May
2014, <https://www.rfc-editor.org/rfc/rfc7253>.
[RFC7748] Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
for Security", RFC 7748, DOI 10.17487/RFC7748, January
2016, <https://www.rfc-editor.org/rfc/rfc7748>.
[RFC8032] Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
Signature Algorithm (EdDSA)", RFC 8032,
DOI 10.17487/RFC8032, January 2017,
<https://www.rfc-editor.org/rfc/rfc8032>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/rfc/rfc8126>.
[SCHNEIER] Schneier, B., "Applied Cryptography Second Edition:
protocols, algorithms, and source code in C", 1996.
[SP800-56A]
Barker, E., Johnson, D., and M. Smid, "Recommendation for
Pair-Wise Key Establishment Schemes Using Discrete
Logarithm Cryptography", NIST Special Publication 800-56A
Revision 1, March 2007.
Koch & Tse Expires 1 June 2024 [Page 125]
Internet-Draft LibrePGP Message Format November 2023
[TWOFISH] Schneier, B., Kelsey, J., Whiting, D., Wagner, D., Hall,
C., and N. Ferguson, "The Twofish Encryption Algorithm",
1999.
19.2. Informative References
[BLEICHENBACHER]
Bleichenbacher, D., "Generating ElGamal Signatures Without
Knowing the Secret Key", Lecture Notes in Computer
Science Volume 1070, pp. 10-18, 1996.
[JKS02] Jallad, K., Katz, J., and B. Schneier, "Implementation of
Chosen-Ciphertext Attacks against PGP and GnuPG", 2002,
<http://www.counterpane.com/pgp-attack.html>.
[KOBLITZ] Koblitz, N., "A course in number theory and cryptography,
Chapter VI. Elliptic Curves", ISBN 0-387-96576-9, 1997.
[MZ05] Mister, S. and R. Zuccherato, "An Attack on CFB Mode
Encryption As Used By OpenPGP", IACR ePrint Archive Report
2005/033, 8 February 2005,
<http://eprint.iacr.org/2005/033>.
[REGEX] Friedl, J., "Mastering Regular Expressions",
ISBN 0-596-00289-0, August 2002.
[RFC1991] Atkins, D., Stallings, W., and P. Zimmermann, "PGP Message
Exchange Formats", RFC 1991, DOI 10.17487/RFC1991, August
1996, <https://www.rfc-editor.org/rfc/rfc1991>.
[RFC2440] Callas, J., Donnerhacke, L., Finney, H., and R. Thayer,
"OpenPGP Message Format", RFC 2440, DOI 10.17487/RFC2440,
November 1998, <https://www.rfc-editor.org/rfc/rfc2440>.
[RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
Thayer, "OpenPGP Message Format", RFC 4880,
DOI 10.17487/RFC4880, November 2007,
<https://www.rfc-editor.org/rfc/rfc4880>.
[RFC6090] McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic
Curve Cryptography Algorithms", RFC 6090,
DOI 10.17487/RFC6090, February 2011,
<https://www.rfc-editor.org/rfc/rfc6090>.
[SEC1] Standards for Efficient Cryptography Group, "SEC 1:
Elliptic Curve Cryptography", September 2000.
Koch & Tse Expires 1 June 2024 [Page 126]
Internet-Draft LibrePGP Message Format November 2023
Appendix A. Test vectors
To help implementing this specification a non-normative example for
the EdDSA algorithm is given.
A.1. Sample EdDSA key
The secret key used for this example is:
D: 1a8b1ff05ded48e18bf50166c664ab023ea70003d78d9e41f5758a91d850f8d2
Note that this is the raw secret key used as input to the EdDSA
signing operation. The key was created on 2014-08-19 14:28:27 and
thus the fingerprint of the LibrePGP key is:
C959 BDBA FA32 A2F8 9A15 3B67 8CFD E121 9796 5A9A
The algorithm specific input parameters without the MPI length
headers are:
oid: 2b06010401da470f01
q: 403f098994bdd916ed4053197934e4a87c80733a1280d62f8010992e43ee3b2406
The entire public key packet is thus:
98 33 04 53 f3 5f 0b 16 09 2b 06 01 04 01 da 47
0f 01 01 07 40 3f 09 89 94 bd d9 16 ed 40 53 19
79 34 e4 a8 7c 80 73 3a 12 80 d6 2f 80 10 99 2e
43 ee 3b 24 06
A.2. Sample EdDSA signature
The signature is created using the sample key over the input data
"LibrePGP" on 2015-09-16 12:24:53 and thus the input to the hash
function is:
m: 4f70656e504750040016080006050255f95f9504ff0000000c
Using the SHA2-256 hash algorithm yields the digest:
d: f6220a3f757814f4c2176ffbb68b00249cd4ccdc059c4b34ad871f30b1740280
Which is fed into the EdDSA signature function and yields this
signature:
r: 56f90cca98e2102637bd983fdb16c131dfd27ed82bf4dde5606e0d756aed3366
Koch & Tse Expires 1 June 2024 [Page 127]
Internet-Draft LibrePGP Message Format November 2023
s: d09c4fa11527f038e0f57f2201d82f2ea2c9033265fa6ceb489e854bae61b404
The entire signature packet is thus:
88 5e 04 00 16 08 00 06 05 02 55 f9 5f 95 00 0a
09 10 8c fd e1 21 97 96 5a 9a f6 22 01 00 56 f9
0c ca 98 e2 10 26 37 bd 98 3f db 16 c1 31 df d2
7e d8 2b f4 dd e5 60 6e 0d 75 6a ed 33 66 01 00
d0 9c 4f a1 15 27 f0 38 e0 f5 7f 22 01 d8 2f 2e
a2 c9 03 32 65 fa 6c eb 48 9e 85 4b ae 61 b4 04
A.3. Sample OCB encryption and decryption
Encryption is performed with the string 'Hello, world!',LF and
password 'password', using AES-128 with OCB encryption.
A.3.1. Sample Parameters
S2K:
type 3
Iterations:
524288 (144), SHA2-256
Salt:
9f0b7da3e5ea6477
A.3.2. Sample symmetric-key encrypted session key packet (v5)
Packet header:
c3 3d
Version, algorithms, S2K fields:
05 07 02 03 08 9f 0b 7d a3 e5 ea 64 77 90
OCB IV:
99 e3 26 e5 40 0a 90 93 6c ef b4 e8 eb a0 8c
OCB encrypted CEK:
67 73 71 6d 1f 27 14 54 0a 38 fc ac 52 99 49 da
Koch & Tse Expires 1 June 2024 [Page 128]
Internet-Draft LibrePGP Message Format November 2023
Authentication tag:
c5 29 d3 de 31 e1 5b 4a eb 72 9e 33 00 33 db ed
A.3.3. Starting OCB decryption of CEK
The derived key is:
eb 9d a7 8a 9d 5d f8 0e c7 02 05 96 39 9b 65 08
Authenticated Data:
c3 05 07 02
Nonce:
99 e3 26 e5 40 0a 90 93 6c ef b4 e8 eb a0 8c
Decrypted CEK:
d1 f0 1b a3 0e 13 0a a7 d2 58 2c 16 e0 50 ae 44
A.3.4. Sample OCB Encrypted Data packet
Packet header:
d4 49
Version, AES-128, OCB, Chunk bits (14):
01 07 02 0e
IV:
5e d2 bc 1e 47 0a be 8f 1d 64 4c 7a 6c 8a 56
OCB Encrypted data chunk #0:
7b 0f 77 01 19 66 11 a1 54 ba 9c 25 74 cd 05 62
84 a8 ef 68 03 5c
Chunk #0 authentication tag:
62 3d 93 cc 70 8a 43 21 1b b6 ea f2 b2 7f 7c 18
Final (zero-size chunk #1) authentication tag:
d5 71 bc d8 3b 20 ad d3 a0 8b 73 af 15 b9 a0 98
Koch & Tse Expires 1 June 2024 [Page 129]
Internet-Draft LibrePGP Message Format November 2023
A.3.5. Decryption of data
Starting OCB decryption of data, using the CEK.
Chunk #0:
Authenticated data:
d4 01 07 02 0e 00 00 00 00 00 00 00 00
Nonce:
5e d2 bc 1e 47 0a be 8f 1d 64 4c 7a 6c 8a 56
Decrypted chunk #0.
Literal data packet with the string contents 'Hello, world!\n'.
cb 14 62 00 00 00 00 00 48 65 6c 6c 6f 2c 20 77
6f 72 6c 64 21 0a
Authenticating final tag:
Authenticated data:
d4 01 07 02 0e 00 00 00 00 00 00 00 01 00 00 00
00 00 00 00 16
Nonce:
5e d2 bc 1e 47 0a be 8f 1d 64 4c 7a 6c 8a 57
A.3.6. Complete OCB encrypted packet sequence
Symmetric-key encrypted session key packet (v5):
c3 3d 05 07 02 03 08 9f 0b 7d a3 e5 ea 64 77 90
99 e3 26 e5 40 0a 90 93 6c ef b4 e8 eb a0 8c 67
73 71 6d 1f 27 14 54 0a 38 fc ac 52 99 49 da c5
29 d3 de 31 e1 5b 4a eb 72 9e 33 00 33 db ed
OCB Encrypted Data packet:
d4 49 01 07 02 0e 5e d2 bc 1e 47 0a be 8f 1d 64
4c 7a 6c 8a 56 7b 0f 77 01 19 66 11 a1 54 ba 9c
25 74 cd 05 62 84 a8 ef 68 03 5c 62 3d 93 cc 70
8a 43 21 1b b6 ea f2 b2 7f 7c 18 d5 71 bc d8 3b
20 ad d3 a0 8b 73 af 15 b9 a0 98
Koch & Tse Expires 1 June 2024 [Page 130]
Internet-Draft LibrePGP Message Format November 2023
Appendix B. ECC Point compression flag bytes
This specification introduces the new flag byte 0x40 to indicate the
point compression format. The value has been chosen so that the high
bit is not cleared and thus to avoid accidental sign extension. Two
other values might also be interesting for other ECC specifications:
Flag Description
---- -----------
0x04 Standard flag for uncompressed format
0x40 Native point format of the curve follows
0x41 Only X coordinate follows.
0x42 Only Y coordinate follows.
Appendix C. Changes since RFC-4880
* Applied errata 2270, 2271, 2242, 3298.
* Added Camellia cipher from RFC 5581.
* Incorporated RFC 6637 (ECC for OpenPGP)
* Added draft-atkins-openpgp-device-certificates
* Added draft-koch-eddsa-for-openpgp-04
* Added Issuer Fingerprint signature subpacket.
* Added a v5 key and fingerprint format.
* Added OIDs for brainpool curves and Curve25519.
* Marked SHA2-256 as MUST implement.
* Marked Curve25519 and Ed25519 as SHOULD implement.
* Marked SHA-1 as SHOULD NOT be used to create messages.
* Marked MD5 as SHOULD NOT implement.
* Changed v5 key fingerprint format to full 32 octets.
* Added Literal Data Packet format octet m.
* Added Feature Flag for v5 key support.
* Added OCB Encrypted Data Packet.
Koch & Tse Expires 1 June 2024 [Page 131]
Internet-Draft LibrePGP Message Format November 2023
* Removed notes on extending the MDC packet.
* Added v5 Symmetric-Key Encrypted Session Key packet.
* Added OCB encryption of secret keys.
* Added test vectors for OCB.
* Added the Restricted Encryption key flag.
* Deprecated the Symmetrically Encrypted Data Packet.
* Suggest limitation of the OCB chunksize to 128 MiB.
* Specified the V5 signature format.
* Deprectated the creation of V3 signatures.
* Adapted terms from RFC 8126.
* Removed editorial marks and updated cross-references.
* Added the timestamping usage key flag.
* Added Intended Recipient signature subpacket.
* Added Attested Certifications signature subpacket and signature
class.
* Added Key Block signature subpacket.
* Added Literal Data Meta Hash subpacket.
Changes since draft-koch-openpgp-2015-rfc4880bis-01:
* Changed Secret-Key Packet Format for OCB mode to include the
entire public key has additional data.
* Added Trust Alias subpacket.
* Added alternative OIDs for Ed25519 and Curve25519.
Changes since draft-koch-openpgp-2015-rfc4880bis-02:
* Add ML-KEM parts from draft-wussler-openpgp-pqc-03.txt
* Changed name of the specification to LibrePGP.
Koch & Tse Expires 1 June 2024 [Page 132]
Internet-Draft LibrePGP Message Format November 2023
Appendix D. Acknowledgments
There have been a number of authors involved with the development of
the OpenPGP specification as described by RFC-4880, RFC-5581, and
RFC-6637:
Jon Callas
EMail: jon@callas.org
Lutz Donnerhacke
EMail: lutz@iks-jena.de
Hal Finney
David Shaw
EMail: dshaw@jabberwocky.com
Rodney Thayer
EMail: rodney@canola-jones.com
Andrey Jivsov
EMail: Andrey_Jivsov@symantec.com
The work to update RFC-4880 was mainly conducted by the authors of
this document and the following authors:
brian m. carlson
Email: sandals@crustytoothpaste.net
Derek Atkins
Email: derek@ihtfp.com
Daniel Kahn Gillmor
Email: dkg@fifthhorseman.net
The PQC algorithm extension was conducted by the following authors:
Stavros Kousidis
Email: stavros.kousidis@bsi.bund.de
Falko Strenzke
Email: falko.strenzke@mtg.de
Aron Wussler
Email: aron@wussler.it
Authors' Addresses
Koch & Tse Expires 1 June 2024 [Page 133]
Internet-Draft LibrePGP Message Format November 2023
Werner Koch
g10 Code GmbH
Germany
Email: wk@gnupg.org
Ronald Henry Tse
Ribose
Hong Kong
Email: ronald.tse@ribose.com
Koch & Tse Expires 1 June 2024 [Page 134]