OpenPGP Signatures
draft-gallagher-openpgp-signatures-03
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| Last updated | 2026-06-04 | ||
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draft-gallagher-openpgp-signatures-03
openpgp A. Gallagher, Ed.
Internet-Draft PGPKeys.EU
Updates: 9580 (if approved) 4 June 2026
Intended status: Standards Track
Expires: 6 December 2026
OpenPGP Signatures
draft-gallagher-openpgp-signatures-03
Abstract
This document specifies several updates and clarifications to the
grammar and semantics of OpenPGP signatures.
About This Document
This note is to be removed before publishing as an RFC.
The latest revision of this draft can be found at
https://andrewgdotcom.gitlab.io/openpgp-signatures. Status
information for this document may be found at
https://datatracker.ietf.org/doc/draft-gallagher-openpgp-signatures/.
Discussion of this document takes place on the OpenPGP Working Group
mailing list (mailto:openpgp@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/openpgp/. Subscribe at
https://www.ietf.org/mailman/listinfo/openpgp/.
Source for this draft and an issue tracker can be found at
https://gitlab.com/andrewgdotcom/openpgp-signatures.
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
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Internet-Drafts are draft documents valid for a maximum of six months
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on 6 December 2026.
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Copyright Notice
Copyright (c) 2026 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 . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3
3. Signature Types . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Timestamp Signature (0x40) . . . . . . . . . . . . . . . 4
3.2. Third-Party Confirmation Signature (0x50) . . . . . . . . 5
3.2.1. Terminology Subtleties . . . . . . . . . . . . . . . 6
4. Signature Packets . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Recursive Embedding Inside Signature Subpackets . . . . . 6
4.2. Subpackets with Conflicting Information . . . . . . . . . 7
4.2.1. Multiple Revocation Key (type 12) Subpackets . . . . 7
4.2.2. Multiple Preferred Key Server (type 24) Subpackets . 7
4.2.3. Multiple Embedded Signature (type 32) Subpackets . . 7
4.2.4. Multiple Issuer Fingerprint (type 33) Subpackets . . 8
4.2.5. Multiple Key Block (type 38, experimental)
Subpackets . . . . . . . . . . . . . . . . . . . . . 8
4.3. Deprecation of the "Revocable" Signature Subpacket (type
7) . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
4.3.1. Non-functionality of the "Revocable" Signature
Subpacket . . . . . . . . . . . . . . . . . . . . . . 8
4.4. Deprecation of the Signature Target Subpacket (type
31) . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
4.4.1. Non-functionality of the "Signature Target" Signature
Subpacket . . . . . . . . . . . . . . . . . . . . . . 9
5. Signature Categories . . . . . . . . . . . . . . . . . . . . 11
5.1. Key Flags . . . . . . . . . . . . . . . . . . . . . . . . 13
5.2. Authentication Signatures . . . . . . . . . . . . . . . . 14
6. Signature Subpacket Categories . . . . . . . . . . . . . . . 14
6.1. General subpackets. . . . . . . . . . . . . . . . . . . . 14
6.2. Context subpackets. . . . . . . . . . . . . . . . . . . . 15
6.2.1. Direct subpackets. . . . . . . . . . . . . . . . . . 15
6.2.2. Revocation subpackets. . . . . . . . . . . . . . . . 15
6.2.3. Key Binding subpackets. . . . . . . . . . . . . . . . 16
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6.2.4. First-party Certification subpackets. . . . . . . . . 16
6.2.5. Third-party Certification subpackets. . . . . . . . . 16
6.2.6. Literal Data subpackets. . . . . . . . . . . . . . . 16
6.2.7. Attribute Value subpackets. . . . . . . . . . . . . . 17
6.3. Subpackets summary . . . . . . . . . . . . . . . . . . . 17
6.4. Guidance for management of the Signature Subpacket
Registry . . . . . . . . . . . . . . . . . . . . . . . . 20
6.5. Unhashed Subpacket Deduplication . . . . . . . . . . . . 20
7. Security Considerations . . . . . . . . . . . . . . . . . . . 20
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
8.1. OpenPGP Signature Types Registry . . . . . . . . . . . . 20
8.2. OpenPGP Key Flags Registry . . . . . . . . . . . . . . . 21
8.3. OpenPGP Signature Subpacket Types Registry . . . . . . . 22
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 22
9.1. Normative References . . . . . . . . . . . . . . . . . . 22
9.2. Informative References . . . . . . . . . . . . . . . . . 23
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . 24
Appendix B. Document History . . . . . . . . . . . . . . . . . . 24
B.1. Changes Between draft-gallagher-openpgp-signatures-02 and
draft-gallagher-openpgp-signatures-03 . . . . . . . . . . 24
B.2. Changes Between draft-gallagher-openpgp-signatures-01 and
draft-gallagher-openpgp-signatures-02 . . . . . . . . . . 24
B.3. Changes Between draft-gallagher-openpgp-signatures-00 and
draft-gallagher-openpgp-signatures-01 . . . . . . . . . . 25
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 25
1. Introduction
OpenPGP signatures have a rich vocabulary, however this is often
under-specified. This document attempts to address this by:
* Expanding on specifications where [RFC9580] does not fully
describe the existing or expected behaviour of deployed
implementations.
* Adding clarification where deployed implementations differ in
their interpretation of [RFC9580] and its predecessors.
* Deprecating unused or error-prone features.
This document does not specify any new wire formats.
2. Conventions and Definitions
The term "OpenPGP Certificate" is used in this document
interchangeably with "OpenPGP Transferable Public Key", as defined in
Section 10.1 of [RFC9580].
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The term "Component key" is used in this document to mean either a
primary key or subkey.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Signature Types
Several signature types are specified in incomplete, confusing or
contradictory ways. We update their specifications as follows.
3.1. Timestamp Signature (0x40)
Section 6.2.1 of [RFC1991] defined the Timestamp signature as:
<40> - time stamping ("I saw this document")
Type <40> is intended to be a signature of a signature, as a
notary seal on a signed document.
The second statement implies that a v3 0x40 sig is made over a
signature packet. But the first statement implies a signature over a
document, just with different semantics.
By Section 5.2.1.14 of [RFC9580], this has changed to:
0x40: Timestamp signature. This signature is only meaningful for
the timestamp contained in it.
This avoids the apparent contradiction of [RFC1991], but is less
informative. And there is no explicit construction given in
Section 5.2.4 of [RFC9580].
We note also that [RFC9580] introduced a Key Flag for timestamping.
This indicates that timestamping documents is sufficiently different
from signing them that separate keys should be used. This is
consistent with the idea that "I wrote this document" and "I saw this
document" are distinct statements with different consequences. This
is crucial in the case of an automated timestamping service that
makes no claims about the accuracy of document contents.
We define type 0x40 Timestamp signatures as follows:
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A type 0x40 Timestamp signature is made over a Literal Data
Packet. It is constructed and distributed in the same way as a
type 0x00 Binary Document Signature. If the message is a text
document, it MUST already be in Canonical Text form. By default a
Timestamp signature conveys no opinion about the validity of the
document; it only claims that the document existed at the
timestamp of signature creation. This interpretation MAY be
modified by adding notation subpackets, the meaning of which are
application-dependent. It can be made over an otherwise unsigned
document, or it can be one of many signatures over the same
document. The Cleartext Signature Framework MUST NOT be used with
Timestamp signatures.
Countersigning a Signature packet only (including blind
countersigning) is done using the type 0x50 Third-Party Confirmation
signature.
3.2. Third-Party Confirmation Signature (0x50)
Section 5.2.1.15 of [RFC9580] defines a Third-Party Confirmation
signature as:
This signature is a signature over some other OpenPGP Signature
packet(s). It is analogous to a notary seal on the signed data.
A Third-Party Confirmation signature SHOULD include a Signature
Target subpacket that identifies the confirmed signature.
A concrete construction is provided, but the placement and semantics
are still not well-defined. We clarify these as follows:
By default, a Third-Party Confirmation signature makes no claim
about the validity of the other signature, just its existence, and
makes no claim whatsoever about the subject of that signature.
This interpretation MAY be modified by adding notation subpackets,
the meaning of which are application-dependent. It SHOULD be
distributed in an Embedded Signature subpacket in the unhashed
area of the signature it notarizes. A Signature Target subpacket
is therefore unnecessary and SHOULD NOT be included.
See Section 4.4 for further discussion of the Signature Target
subpacket.
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3.2.1. Terminology Subtleties
Implementers should note that "Third-Party Confirmation" signatures
(type 0x50) are distinct from "third-party Certification" signatures
(types 0x10..0x13 when issued by a primary key other than the one
signed over), and beware that older RFCs do not always use
sufficiently precise terminology to distinguish them.
4. Signature Packets
Receiving implementations currently have insufficient guidance for
when to reject non-idiomatic signature packets.
4.1. Recursive Embedding Inside Signature Subpackets
Section 5.2.3 of [RFC9580] specifies two subpackets which could
recursively include a signature inside a signature:
* Embedded Signature (type 32): contains a signature packet
* Key Block (type 38, experimental): contains an entire certificate,
which may itself include signature packets
In order to prevent excessive recursion via nested signature
subpackets:
* Signatures contained within Embedded Signature subpackets MUST NOT
contain any Embedded Signature subpackets:
- An Embedded Signature subpacket MUST contain a signature of an
Embeddable signature type.
- An Embeddable signature MUST NOT contain Embedded Signature
subpackets.
- Initially, only the Primary Key Binding and Third-Party
Confirmation signature types are specified as Embeddable.
* A signature of a type other than in the Literal Data Signature
Category (Section 5) MUST NOT contain a Key Block subpacket.
A receiving implementation MUST invalidate any signature that does
not conform to the above guidance.
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4.2. Subpackets with Conflicting Information
Section 5.2.3.9 of [RFC9580] gives a receiving implementation
significant leeway in interpreting conflicting combinations of
subpackets:
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
hashed section of the signature, but it MAY use any conflict
resolution scheme that makes more sense.
We hereby tighten this guidance:
A signature MUST NOT contain more than one subpacket of any given
type in its hashed subpackets area, unless otherwise specified. A
receiving implementation MUST invalidate a signature that contains
in its hashed area more than one subpacket of any type for which
this is not explicitly permitted.
Multiple copies of the following subpacket types are already
explicitly permitted:
* Issuer Key ID (type 16) Section 5.2.3.9 of [RFC9580]
* Notation Data (type 20) Section 5.2.3.24 of [RFC9580]
* Intended Recipient Fingerprint (type 35) Section 5.2.3.36 of
[RFC9580]
In addition, the following interpretations are natural extensions of
specified behaviour, and hereby permitted:
4.2.1. Multiple Revocation Key (type 12) Subpackets
If multiple Revocation Key subpackets are present, any of the listed
keys MAY generate key revocation signatures.
4.2.2. Multiple Preferred Key Server (type 24) Subpackets
If multiple Preferred Key Server subpackets are present, updates MAY
be obtained from any of the listed keyservers.
4.2.3. Multiple Embedded Signature (type 32) Subpackets
If multiple Embedded Signature subpackets are present, a receiving
implementation MAY attempt to process them all in turn.
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4.2.4. Multiple Issuer Fingerprint (type 33) Subpackets
Multiple Issuer Fingerprint subpackets are permitted, with the same
interpretation as multiple Issuer Key ID subpackets.
Note however that Section 5.2.3.35 of [RFC9580] states of the Issuer
Fingerprint subpacket:
If the version of the issuing key is 4 and an Issuer Key ID
subpacket (Section 5.2.3.12) is also included in the signature,
the Key ID of the Issuer Key ID subpacket MUST match the low 64
bits of the fingerprint.
Generalizing to multiple subpackets, we replace this with:
If both Issuer Key ID and Issuer Fingerprint subpackets are
included in a signature then each Issuer Key ID subpacket MUST
match the low 64 bits of only one v4 Issuer Fingerprint subpacket,
and all v4 Issuer Fingerprint subpackets MUST have a corresponding
Key ID subpacket.
4.2.5. Multiple Key Block (type 38, experimental) Subpackets
If multiple Key Block subpackets are present, a receiving
implementation MAY attempt to process them all in turn.
4.3. Deprecation of the "Revocable" Signature Subpacket (type 7)
The "Revocable" subpacket is not commonly supported, and when used as
described is effectively non-functional. It is hereby deprecated.
4.3.1. Non-functionality of the "Revocable" Signature Subpacket
Section 5.2.3.20 of [RFC9580] states:
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.
But this is not an effective constraint on the key owner's future
behaviour:
* Since there is no such thing as a document revocation signature,
"revocability" is only applicable to self-signatures and third
party certifications.
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* If a key is compromised, then the timestamp on any signature can
be trivially backdated, so the timestamps on any signatures cannot
be relied upon. Since "revocability" only concerns "later
revocation signatures", it is only meaningful for signatures made
by valid or soft-revoked keys.
* A soft revocation of a self-signature or third-party certification
is functionally equivalent to a later signature with an expiry
date, which is not covered by the "Revocable" semantics.
* A hard revocation has the same semantics regardless of its
creation date. In particular, an escrowed revocation signature
(such as the revocation signatures commonly made at key generation
time) will typically have a creation time significantly in the
past compared to when it is published. A "non-revocable"
certification created after the escrowed revocation sig cannot
prevent the escrowed revocation taking effect.
Therefore any "non-revocable" signature can still be effectively
"revoked" by one of the following unremarkable events:
* by a later signature with an explicit expiry date, which has the
same practical effect as a soft revocation,
* or by an escrowed hard revocation, which has the same practical
effect as a later hard revocation.
The "revocable" subpacket is therefore non-functional.
4.4. Deprecation of the Signature Target Subpacket (type 31)
The Signature Target subpacket (Section 5.2.3.33 of [RFC9580])
fulfils the following roles:
* In a Timestamp or Third-Party Confirmation signature, it
identifies the signature that is being countersigned
* In a Revocation signature, it identifies the signature being
revoked
It is imprecisely defined and does not uniquely identify a particular
Signature subpacket, and so when used as described is effectively
non-functional. It is hereby deprecated.
4.4.1. Non-functionality of the "Signature Target" Signature Subpacket
The Signature Target subpacket is imprecisely defined:
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(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 Confirmation 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 signature's
hash. For example, a target signature with a SHA-1 hash MUST have
20 octets of hash data.
It is unclear from this text alone whether the hash field refers to
the digest that the target signature is made over, or a digest over
the resulting signature packet. The definition of a Third-Party
Confirmation signature in Section 5.2.1 of [RFC4880] gives us a hint
however:
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.
(Beware that "third-party signature" in the above should be read as
"Third-Party Confirmation signature"; see Section 3.2.1.)
The only way that a blind party would be unable to generate a
Signature Target subpacket is if the hash is the digest that the
original signature was made over. But if so it is not a unique
identifier of a signature packet, since multiple distinct signatures
can be made over the exact same material, including subpackets. In
particular, if there was no Issuer Key ID or Issuer Fingerprint
subpacket in the target signature's hashed area, a Signature Target
subpacket could not distinguish between the original signature or an
otherwise valid one issued by a completely different signing key.
The Signature Target subpacket is therefore not functional when used
in a Third-Party Confirmation signature. A more reliable mechanism
for identifying the target of a Third-Party Confirmation signature is
to include it an Embedded Signature subpacket, directly in the
unhashed area of the signature being countersigned.
The other specified use for the Signature Target subpacket is in a
revocation signature. Certification Revocations are customarily
understood to mean "I retract all my previous statements that this
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key is related to this user" (Section 6.2.1 of [RFC1991]), so a
Certification Revocation is not specific to any particular
Certification signature. No other signature types can be revoked -
primary key and subkey revocation signatures revoke the key, not the
previous binding signature(s), and so are not specific to any
particular binding signatures either.
The Signature Target subpacket is therefore not functional when used
in a revocation signature. It is normally sufficient to distribute a
revocation signature in an OpenPGP certificate or revocation
certificate.
Timestamp signatures as specified in Section 3.1 do not require a
Signature Target subpacket, since the signed message grammar
identifies the material being signed over.
We therefore deprecate the Signature Target subpacket in all
contexts.
5. Signature Categories
Signature Type code points are spaced out into identifiable ranges of
types with similar semantics. These also mostly correspond to the
various Key Flags. These ranges and their mapping to the Key Flags
are not specified in [RFC9580].
We define Signature Categories to cover each range of type values:
* Literal Data Signature Category (0x00..0x07)
- 0x00 Signature over a Binary document
- 0x01 Signature over a Canonical Text document
- 0x02 Standalone signature
* Unassigned (0x08..0x0F)
* Certification Category (0x10..0x17)
- 0x10 Generic certification
- 0x11 Persona certification
- 0x12 Casual certification
- 0x13 Positive certification
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- (0x16 Approved certifications)
* Key Binding Category (0x18..0x1F)
- 0x18 Subkey binding
- 0x19 Primary key binding
- 0x1F Direct key
* Primary Key Revocation Category (0x20..0x27)
- 0x20 Primary Key revocation
* Subkey Revocation Category (0x28..0x2F)
- 0x28 Subkey revocation
* Certification Revocation Category (0x30..0x37)
- 0x30 Certification revocation
* Unassigned (0x38..0x3F)
* Timestamping Category (0x40..0x47)
- 0x40 Timestamp
* Unassigned (0x48..0x4F)
* Countersignature Category (0x50..0x57)
- 0x50 Third-Party confirmation
* Unassigned (0x58..0x5F)
* Private and Experimental Range (0x60..0x6F)
* Unassigned (0x70..0xFE)
* RESERVED (0xFF)
The Standalone signature type is effectively a "signature over an
empty document". The Direct Key signature type contains key usage
preference subpackets, similar to the Subkey Binding signature type,
and is therefore effectively a "self-binding signature".
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We have defined a Private and Experimental signature type range.
This is 0x60..0x6F (96..111) for consistency with the existing
private and experimental range in other registries. It does not form
a Category and does not have a corresponding Key Flag.
Self-certifications over v4 Primary User IDs are used to convey the
same information as Key Binding signatures. Therefore, unless
specifically stated otherwise, any stipulations that apply to Key
Binding signatures also apply to self-certifications over v4 Primary
User IDs.
5.1. Key Flags
A Key Flags subpacket SHOULD be included in a Direct Key or Subkey
Binding signature (or for v4 keys, a self-certification over the
primary User ID). It applies only to a single key material packet;
for a Direct Key signature (or primary User ID self-cert) it applies
to the primary key only, and for a Subkey Binding signature, it
applies only to that subkey.
Previously, it was also specified for use in third-party
Certification Signatures. This is not widely supported and is hereby
deprecated.
The following Key Flags permit the creation of signatures in one or
more Signature Categories:
* 0x01.. Third-party signatures in the Certification and
Certification Revocation Categories
* 0x02.. Literal Data Signature Category
* 0x0008.. Timestamping Category
* ((TBC)) Countersignature Category
The following exceptional usages are always permitted regardless of
Key Flags:
* Primary keys are always permitted to make self-signatures in the
Certification, Key Binding, Certification Revocation, Primary Key
Revocation and Subkey Revocation Categories.
* Subkeys with signing-capable algorithms are always permitted to
make Primary Key Binding signatures.
* Any key with a signing-capable algorithm is permitted to make a
signature in the Private and Experimental range.
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Otherwise:
* A signature made by a key that does not have the corresponding Key
Flag MUST be considered invalid.
* A key with no Key Flags subpacket MUST NOT create signatures.
Section 5.2.1.10 of [RFC9580] also explicitly allows keys with the
0x01 Key Flag to create third-party 0x1F Direct Key Signatures.
These are used for trust delegation in [SQ-WOT].
5.2. Authentication Signatures
OpenPGP defines no authentication signature types, but does have an
authentication Key Flag. Traditionally, authentication is performed
by converting the key material into that of another protocol (usually
OpenSSH) and performing authentication in that protocol.
Beware that cross-protocol usage can be exploited to evade the domain
separation protections of Key Flags. For example, there is no
distinction between document signing, certification and
authentication usage in OpenSSH, and once converted an OpenPGP
authentication key may be used as a OpenSSH CA or to sign git
commits.
((TODO: Guidance for the use of authentication keys should be
provided. #12))
6. Signature Subpacket Categories
Signature subpacket types may also be categorized, depending on where
they are used:
6.1. General subpackets.
These may be attached to any signature type, and define properties of
the signature itself. Some of these subpackets are self-verifying
(SV), i.e. they contain hints to locate the issuing key that can be
confirmed after the fact. It MAY be reasonable to place self-
verifying general subpackets in the unhashed area. All other general
subpackets MUST be placed in the hashed area.
Subpacket types: Signature Creation Time, Signature Expiration Time,
Issuer Key ID (SV), Notation Data, Signer's User ID, Issuer
Fingerprint (SV).
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(Notation subpackets are categorized here as general subpackets,
however the notations within them may have arbitrary semantics at the
application layer)
TODO: should Signature Expiration Time subpackets be more restricted,
e.g. to certifications? (issue #8)
6.2. Context subpackets.
These have semantics that are meaningful only when used in signatures
of a particular type or category:
6.2.1. Direct subpackets.
These are normally only meaningful in a direct self-sig (or for v4
keys, a self-cert over the primary User ID) and define usage
preferences for the certificate as a whole. They MAY be used in
self-certs over other User IDs, in which case they define usage
preferences for just that User ID (but this is not always meaningful
or universally supported). They SHOULD NOT be used elsewhere. They
MUST be placed in the hashed area.
A Direct subpacket MUST be ignored if it is in a self-cert made over
a User ID by a v6 or later primary key.
Subpacket types: Preferred Symmetric Ciphers, Revocation Key
(deprecated), Preferred Hash Algorithms, Preferred Compression
Algorithms, Key Server Preferences, Preferred Key Server, Features,
(Preferred AEAD Algorithms), Preferred AEAD Ciphersuites, Replacement
Key.
The Replacement Key subpacket MAY also be used as a key revocation
subpacket.
6.2.2. Revocation subpackets.
These are only meaningful in signatures of the Key Revocation, Subkey
Revocation or Certificate Revocation categories. They SHOULD NOT be
used elsewhere. They MUST be placed in the hashed area.
Subpacket types: Reason for Revocation, Replacement Key (Primary Key
Revocations only).
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6.2.3. Key Binding subpackets.
These are only meaningful in a signature of the Key Binding category
(or for v4 keys, a self-cert over the primary User ID) and define
properties of that particular component key. They SHOULD NOT be used
elsewhere. They MUST be placed in the hashed area.
A Key Binding subpacket MUST be ignored if it is in a self-cert over
a User ID that is not currently the primary User ID, or in a self-
cert made over a User ID by a v6 or later primary key.
Subpacket types: Key Expiration Time, Key Flags.
6.2.4. First-party Certification subpackets.
These are only meaningful in a self-certification over a User ID, and
define properties of that User ID. They SHOULD NOT be used
elsewhere. They MUST be placed in the hashed area.
Subpacket types: Primary User ID
6.2.5. Third-party Certification subpackets.
These are only meaningful in third-party certification signatures and
define properties of the Web of Trust. They SHOULD NOT be used
elsewhere. They MUST be placed in the hashed area.
Subpacket types: Exportable Certification, Trust Signature, Regular
Expression, Revocable, Policy URI, (Trust Alias).
6.2.6. Literal Data subpackets.
These are only meaningful in signatures of the Literal Data category,
and define properties of the document or message. They SHOULD NOT be
used elsewhere. Some of these subpackets are self-verifying (SV) and
MAY be placed in the unhashed area. All other Literal Data
subpackets MUST be placed in the hashed area. (Beware that the
usefulness of all of these subpackets has been questioned)
Subpacket types: Intended Recipient Fingerprint, (Key Block (SV)),
(Literal Data Metadata).
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6.2.7. Attribute Value subpackets.
These are only meaningful in signature types whose specification
explicitly requires them. They SHOULD NOT be used elsewhere. It MAY
be reasonable to place Embedded Signature subpackets in the unhashed
area. All other Attribute Value subpackets MUST be placed in the
hashed area. They have no intrinsic semantics; all semantics are
defined by the enclosing signature.
Subpacket types: Signature Target, Embedded Signature, (Delegated
Revoker), (Approved Certifications).
6.3. Subpackets summary
+=====+===============+==========+========+========+=======+========================================+
|Type |Name |Category |Critical|Unhashed|Context|Notes |
+=====+===============+==========+========+========+=======+========================================+
|0 |Reserved |- | | | |never used |
+-----+---------------+----------+--------+--------+-------+----------------------------------------+
|1 |Reserved |- | | | |never used |
+-----+---------------+----------+--------+--------+-------+----------------------------------------+
|2 |Signature |General |SHOULD | | |MUST always be present in hashed area |
| |Creation Time | | | | | |
+-----+---------------+----------+--------+--------+-------+----------------------------------------+
|3 |Signature |General |SHOULD | | | |
| |Expiration Time| | | | | |
+-----+---------------+----------+--------+--------+-------+----------------------------------------+
|4 |Exportable |Third- |MUST IFF| | |boolean, default true |
| |Certification |Party |false | | | |
+-----+---------------+----------+--------+--------+-------+----------------------------------------+
|5 |Trust Signature|Third- | | | | |
| | |Party | | | | |
+-----+---------------+----------+--------+--------+-------+----------------------------------------+
|6 |Regular |Third- |SHOULD | | | |
| |Expression |Party | | | | |
+-----+---------------+----------+--------+--------+-------+----------------------------------------+
|7 |Revocable |Third- | | | |boolean, default false (Section 4.3) |
| |(deprecated) |Party | | | | |
+-----+---------------+----------+--------+--------+-------+----------------------------------------+
|8 |Reserved |- | | | |never used |
+-----+---------------+----------+--------+--------+-------+----------------------------------------+
|9 |Key Expiration |Key |SHOULD | | | |
| |Time |Binding | | | | |
+-----+---------------+----------+--------+--------+-------+----------------------------------------+
|10 |Placeholder for|- | | | |PGP.com proprietary feature |
| |backwards | | | | | |
| |compatibility | | | | | |
+-----+---------------+----------+--------+--------+-------+----------------------------------------+
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|11 |Preferred |Direct | | | | |
| |Symmetric | | | | | |
| |Ciphers | | | | | |
+-----+---------------+----------+--------+--------+-------+----------------------------------------+
|12 |Revocation Key |Direct | | | | |
| |(deprecated) | | | | | |
+-----+---------------+----------+--------+--------+-------+----------------------------------------+
|13-15|Reserved |- | | | |never used |
+-----+---------------+----------+--------+--------+-------+----------------------------------------+
|16 |Issuer Key ID |General | |MAY | |issuer fingerprint is preferred |
+-----+---------------+----------+--------+--------+-------+----------------------------------------+
|17-19|Reserved |- | | | |never used |
+-----+---------------+----------+--------+--------+-------+----------------------------------------+
|20 |Notation Data |General | | | |notations may be further classified |
+-----+---------------+----------+--------+--------+-------+----------------------------------------+
|21 |Preferred Hash |Direct | | | | |
| |Algorithms | | | | | |
+-----+---------------+----------+--------+--------+-------+----------------------------------------+
|22 |Preferred |Direct | | | | |
| |Compression | | | | | |
| |Algorithms | | | | | |
+-----+---------------+----------+--------+--------+-------+----------------------------------------+
|23 |Key Server |Direct | | | | |
| |Preferences | | | | | |
+-----+---------------+----------+--------+--------+-------+----------------------------------------+
|24 |Preferred Key |Direct | | | | |
| |Server | | | | | |
+-----+---------------+----------+--------+--------+-------+----------------------------------------+
|25 |Primary User ID|First | | | |boolean, default false |
| | |Party | | | | |
+-----+---------------+----------+--------+--------+-------+----------------------------------------+
|26 |Policy URI |Third- | | | |(effectively a human-readable notation) |
| | |Party | | | | |
+-----+---------------+----------+--------+--------+-------+----------------------------------------+
|27 |Key Flags |Key |SHOULD | | | |
| | |Binding | | | | |
+-----+---------------+----------+--------+--------+-------+----------------------------------------+
|28 |Signer's User |General | | | | |
| |ID | | | | | |
+-----+---------------+----------+--------+--------+-------+----------------------------------------+
|29 |Reason for |Revocation| | | |free text field is effectively a human- |
| |Revocation | | | | |readable notation |
+-----+---------------+----------+--------+--------+-------+----------------------------------------+
|30 |Features |Direct | | | | |
+-----+---------------+----------+--------+--------+-------+----------------------------------------+
|31 |Signature |Attr Value| | |0x50 |Section 4.4 |
| |Target | | | |3-p | |
| |(deprecated) | | | |conf | |
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+-----+---------------+----------+--------+--------+-------+----------------------------------------+
|32 |Embedded |Attr Value| |MAY |0x18 | |
| |Signature | | | |sbind | |
+-----+---------------+----------+--------+--------+-------+----------------------------------------+
|33 |Issuer |General | |MAY | | |
| |Fingerprint | | | | | |
+-----+---------------+----------+--------+--------+-------+----------------------------------------+
|34 |Reserved |- | | | | |
+-----+---------------+----------+--------+--------+-------+----------------------------------------+
|35 |Intended |Literal |SHOULD | | | |
| |Recipient |Data | | | | |
| |Fingerprint | | | | | |
+-----+---------------+----------+--------+--------+-------+----------------------------------------+
|36 |(Delegated |Attr Value|MUST | |TBD |[I-D.dkg-openpgp-revocation] |
| |Revoker) | | | | | |
+-----+---------------+----------+--------+--------+-------+----------------------------------------+
|37 |Reserved |Attr Value| | |0x16 |[I-D.dkg-openpgp-1pa3pc] |
| |(Approved | | | |1pa3pc | |
| |Certifications)| | | | | |
+-----+---------------+----------+--------+--------+-------+----------------------------------------+
|38 |Reserved (Key |Literal | |MAY | | |
| |Block) |Data | | | | |
+-----+---------------+----------+--------+--------+-------+----------------------------------------+
|39 |Preferred AEAD |Direct | | | | |
| |Ciphersuites | | | | | |
+-----+---------------+----------+--------+--------+-------+----------------------------------------+
|40 |(Literal Data |Literal | | | |[I-D.gallagher-openpgp-literal-metadata]|
| |Metadata) |Data | | | | |
+-----+---------------+----------+--------+--------+-------+----------------------------------------+
|41 |(Trust Alias) |Third- | | | | |
| | |Party | | | | |
+-----+---------------+----------+--------+--------+-------+----------------------------------------+
|TBD |Replacement Key|Direct, |SHOULD | | |[I-D.ietf-openpgp-replacementkey] |
| | |Revocation|NOT | | | |
+-----+---------------+----------+--------+--------+-------+----------------------------------------+
Table 1: OpenPGP Signature Subpacket Types
Three subpacket types are Boolean, with different default values for
when they are absent (two true, one false). It is RECOMMENDED that
these subpackets not be used to convey their default values, only the
non-default value. The default value SHOULD instead be conveyed by
the absence of the subpacket.
Unless otherwise indicated, subpackets SHOULD NOT be marked critical.
In particular, a critical subpacket that invalidates a self-signature
will leave the previous self-signature (or no self-signature!) as the
most recent valid self-signature from the PoV of some receiving
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implementations. A generating implementation MUST be sure that all
receiving implementations will behave as intended if a signature
containing a critical subpacket is invalidated. Otherwise, with the
possible exception of Literal Data signatures, it is NOT RECOMMENDED
to set the critical bit.
It is RECOMMENDED that a signature's creator places all subpackets in
the hashed area, even self-verifying subpackets for which this is not
strictly necessary. The unhashed area MAY be used for informational
subpackets attached by third parties (which can be safely stripped).
6.4. Guidance for management of the Signature Subpacket Registry
* Future boolean subpackets SHOULD NOT contain an explicit value; a
value of TRUE SHOULD be indicated by the presence of the
subpacket, and FALSE otherwise.
* Specification of new subpackets SHOULD address classification,
criticality and self-verification as outlined above.
* Subpackets SHOULD be implemented in the private/experimental area
first, then reassigned to a permanent code point.
6.5. Unhashed Subpacket Deduplication
Unhashed subpacket areas are malleable and so may have subpackets
added or removed in transit, either innocently or maliciously. A
receiving implementation SHOULD clean the unhashed area of subpackets
that are not meaningful or trustworthy outside the hashed area. If
two signature packets are bitwise identical apart from differences in
their unhashed subpacket areas, an implementation MAY merge them into
a single signature. If two unhashed subpackets in the merged
signature are bitwise identical, they MUST be deduplicated.
Otherwise, the unhashed subpacket area of the merged signature SHOULD
contain the useful subpackets from both original signatures, even if
this means multiple subpackets of the same type.
7. Security Considerations
(( TO BE COMPLETED ))
8. IANA Considerations
8.1. OpenPGP Signature Types Registry
IANA is requested to add a column to the OpenPGP Signature Types
registry, called "Embeddable". This column should be empty by
default.
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IANA is requested to register the following new entry in the
registry:
+===========+=============================+============+===========+
| ID | Name | Embeddable | Reference |
+===========+=============================+============+===========+
| 0x60-0x6F | Private or Experimental Use | | Section 5 |
+-----------+-----------------------------+------------+-----------+
Table 2: OpenPGP Signature Types (new)
IANA is requested to update the following existing entries in the
registry:
+======+========================+============+======================+
| ID | Name | Embeddable | Reference |
+======+========================+============+======================+
| 0x19 | Primary Key | Yes | [RFC9580], |
| | Binding Signature | | Section 4.1, ((TBC)) |
+------+------------------------+------------+----------------------+
| 0x40 | Timestamp | | [RFC9580], |
| | Signature | | Section 3.1 |
+------+------------------------+------------+----------------------+
| 0x50 | Third-Party | Yes | [RFC9580], |
| | Confirmation | | Section 3.2, |
| | Signature | | Section 4.1 |
+------+------------------------+------------+----------------------+
Table 3: OpenPGP Signature Types (updated)
((TODO: avoid clash between the updates to 0x19 here and in draft-
certificates))
8.2. OpenPGP Key Flags Registry
IANA is requested to register the following new entry in the OpenPGP
Key Flags registry:
+=========+============================================+===========+
| Flag | Definition | Reference |
+=========+============================================+===========+
| ((TBC)) | This key may be used to make signatures in | Section 5 |
| | the Countersignature Category (0x50..0x57) | |
+---------+--------------------------------------------+-----------+
Table 4: OpenPGP Key Flags (new)
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IANA is requested to update the following existing entries in the
registry:
+===========+=========================================+===========+
| Flag | Definition | Reference |
+===========+=========================================+===========+
| 0x01.. | This key may be used to make signatures | Section 5 |
| | over other keys, in the Certification | |
| | and Certification Revocation Categories | |
| | (0x10..0x17 and 0x30..0x37) | |
+-----------+-----------------------------------------+-----------+
| 0x02... | This key may be used to make signatures | Section 5 |
| | in the Literal Data Signature Category | |
| | (0x00..0x07) | |
+-----------+-----------------------------------------+-----------+
| 0x0008... | This key may be used to make signatures | Section 5 |
| | in the Timestamping Category | |
| | (0x40..0x47) | |
+-----------+-----------------------------------------+-----------+
Table 5: OpenPGP Key Flags (update)
8.3. OpenPGP Signature Subpacket Types Registry
IANA is requested to add columns for "Category", "Critical", and
"Self-Verifying" to the OpenPGP Signature Subpacket Types registry,
and populate them with initial values as listed in Table 1.
IANA is requested to mark the "Revocable" subpacket entry as
"deprecated", referencing this document, Section 4.3.
9. References
9.1. Normative References
[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>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
[RFC9580] Wouters, P., Ed., Huigens, D., Winter, J., and Y. Niibe,
"OpenPGP", RFC 9580, DOI 10.17487/RFC9580, July 2024,
<https://www.rfc-editor.org/rfc/rfc9580>.
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9.2. Informative References
[I-D.dkg-openpgp-1pa3pc]
Gillmor, D. K., "First-Party Approved Third-Party
Certifications in OpenPGP", Work in Progress, Internet-
Draft, draft-dkg-openpgp-1pa3pc-02, 6 September 2024,
<https://datatracker.ietf.org/doc/html/draft-dkg-openpgp-
1pa3pc-02>.
[I-D.dkg-openpgp-revocation]
Gillmor, D. K. and A. Gallagher, "Revocation in OpenPGP",
Work in Progress, Internet-Draft, draft-dkg-openpgp-
revocation-02, 28 March 2025,
<https://datatracker.ietf.org/doc/html/draft-dkg-openpgp-
revocation-02>.
[I-D.gallagher-openpgp-literal-metadata]
Gallagher, A., "OpenPGP Literal Data Metadata Integrity",
Work in Progress, Internet-Draft, draft-gallagher-openpgp-
literal-metadata-00, 1 January 2024,
<https://datatracker.ietf.org/doc/html/draft-gallagher-
openpgp-literal-metadata-00>.
[I-D.ietf-openpgp-replacementkey]
Shaw, D. and A. Gallagher, "OpenPGP Key Replacement", Work
in Progress, Internet-Draft, draft-ietf-openpgp-
replacementkey-08, 29 May 2026,
<https://datatracker.ietf.org/doc/html/draft-ietf-openpgp-
replacementkey-08>.
[OPENPGPDEVBOOK]
"OpenPGP for Application Developers", 6 May 2024,
<https://openpgp.dev/book/>.
[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>.
[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>.
[SQ-WOT] Walfield, N., "OpenPGP Web of Trust", 3 February 2022,
<https://sequoia-pgp.gitlab.io/sequoia-wot/>.
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Appendix A. Acknowledgments
This document would not have been possible without the extensive work
of the authors of [OPENPGPDEVBOOK].
The author would also like to thank Daniel Huigens, Daniel Kahn
Gillmor, Heiko Schäfer, Neal Walfield, Justus Winter and Paul Schaub
for additional discussions and suggestions.
Appendix B. Document History
Note to RFC Editor: this section should be removed before
publication.
B.1. Changes Between draft-gallagher-openpgp-signatures-02 and draft-
gallagher-openpgp-signatures-03
* Renamed document.
* Split out certificate grammar and revocation sections into draft-
gallagher-openpgp-certificates.
* Split out message grammar section into draft-gallagher-openpgp-
messages.
* Moved sections deprecating revocable and signature target
subpackets, with minor updates.
* Minor updates to Timestamp and Third-Party Confirmation signature
guidance.
* Relaxed treatment of multiple Embedded Signature and Key Block
subpackets.
B.2. Changes Between draft-gallagher-openpgp-signatures-01 and draft-
gallagher-openpgp-signatures-02
* Merged in half of draft-dkg-openpgp-revocation and added DKG as
co-author.
* Adapted key temporal validity rules for certification signatures.
* Specified use of Persona Certifications for non-trust statements.
* Added section for Primary Key Binding signatures.
* Added sections for conflicting subpackets and conflicting
requirements.
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* Specified line ending normalization of bare LF only.
* Deprecated revocation of Direct Key signatures.
* Deprecated Signature Target subpackets.
* Added section for issues with temporary identities.
* Refactored and constrained message grammar.
* Fixed some crufty terminology.
B.3. Changes Between draft-gallagher-openpgp-signatures-00 and draft-
gallagher-openpgp-signatures-01
* Expanded temporal validity.
* Renamed "Document" and "Data Type" Signature Categories to
"Literal Data" and "Attribute Value" respectively.
* Expanded experimental range to cover 0x60..0x6F (96..111).
* Add explicit category ranges to the Key Flags registry.
* Add explicit note about when to ignore Direct and Key Binding
subpackets.
* Distinguish between signature subject and signature type-specific
data.
* Deprecated the nesting octet.
* Minor errata.
Author's Address
Andrew Gallagher (editor)
PGPKeys.EU
Email: andrewg@andrewg.com
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