Certification signatures

Signatures make up the magic of OpenPGP. They act as the syntax that allows forming and interpreting complex statements about data and identities. Without signatures there would only be loose keys, impossible to associate with their owner. Signatures are the glue that allows for keys, subkeys and identities to be assembled into hierarchical certificates and for messages to gain authenticity.

:class: warning

  - Purpose of a signature
  - Meaning of different signature types, nuances of subpackets
  - Can we have a "catalogue" of statements a user might want to make, mapping these to archetypical signatures?
  - Revocation; Hard vs. Soft

Terminology


%%{ init: { 'flowchart': { 'curve': '' } } }%%
flowchart LR
    signature(OpenPGP Signature)
    3rdcert(Third-Party Certification)
    data(Data Signature)
    selfcert(Self Certification)
    certification(Certification)

    signature --> data & selfcert & 3rdcert & confsig & standalone & timestamp
    data --> binary & text
    selfcert --> skbind & skrev
    selfcert & 3rdcert --> certification
    certification --> uidcert & uidrev & dksig & krev
    subgraph Siganture Types and Targets
        standalone[0x02: Standalone]
        subgraph Signature Packet
            confsig[0x50: Third-Party Confirmation]
            timestamp[0x40: Timestamp]
        end
        subgraph Data
            binary[0x00: Binary Data]
            text[0x01: Canonical Text]
        end
        subgraph Primary Key + User ID / Attr. Packet
            uidcert[
                0x10: Generic Certification
                0x11: Persona Certification
                0x12: Casual Certification
                0x13: Positive Certification
            ]
            uidrev[0x30: Certification Revocation]
        end
        subgraph Primary Key
            dksig[0x1F: Direct-Key Signature]
            krev[0x20: Key Revocation]
        end
        subgraph Primary + Subkey
            skbind[
                0x18: Subkey Binding
                0x19: Primary Key Binding
            ]
            skrev[0x28: Subkey Revocation]
        end
    end

The term signature can have multiple meanings in the context of the OpenPGP specification. Cryptographic keys create raw signatures which are byte sequences calculated according to some signature scheme. OpenPGP packs these raw signatures up into OpenPGP signature packets, which carry additional information in the form of signature subpackets. For the purpose of this document, the term signature will refer to an OpenPGP signature packet (tag 2).

OpenPGP signatures can be separated into data signatures and certifications. A data signature serves the purpose to cryptographically guarantee the authenticity (and implicitly also the integrity) of a message, e.g. an email or a file, while a certification is used to attach metadata or subkeys to a certificate. Data signatures are always calculated by keys carrying the Signing key flag. Different types of signatures are distinguished by a signature type code and are calculated in different ways. Signatures can either be distributed standalone as detached signatures, or can be inlined with OpenPGP data, such as an OpenPGP message or a key or certificate.

Data signatures (type 0x00 and 0x01) are created by hashing the message content and calculating a cryptographic signature over the hash. You can read more about data signatures in the next chapter. The result is packed up into an OpenPGP signature packet, which can either be included in the OpenPGP message (TODO: See section about forming messages, cleartext signature framework), or distributed separately as a so called detached signature. Data signatures are always calculated using a Signing key.

Certifications are separated into self-certifications and third-party certifications. A certification made by a key over components of the same certificate is referred to as a self-certification. :::{note} The Ccertify Others key flag is not required in order to issue self-certifications. It is only necessary to issue valid third-party certifications. ::: A typical use-case for a self-certification is to attach a User ID, such as a name and email address to a certificate. This is done by calculating the signature over the User ID and the public primary key. The resulting User ID certification (typically type 0x13, potentially type 0x10-0x12) can then be inserted into the certificate, right after the User ID packet.

Other examples for self-signatures are binding signatures for subkeys. In order to add an OpenPGP subkey to a certificate, a subkey binding signature is calculated over the public primary key, followed by the public subkey. The resulting subkey binding signature (type 0x18) can then be inserted into the certificate right after the subkey. If the subkey itself is intended to be used as a Signing key, an extra step is required. To prevent an attacker from being able to "adopt" a victims signing subkey and then being able to claim to be the origin of signatures in fact made by victim, subkey binding signatures for signing subkeys need to include an embedded "back signature" (formally known as primary key binding signature) made by the signing key itself.

Certifications over User IDs can also be used to certify certificates of third-parties. If Alice is certain that Bob Baker <bob@example.com> controls the key 0xB0B, she can create a User ID certification signature for that identity and send it to Bob. Bob can then add this signature to his certificate. TODO: More WoT.

Another important category of signatures are revocations. A revocation is used to retract the statement formed by a prior signature. A subkey revocation signature revokes a prior subkey binding signature, while a certification revocation revokes a certification signature. Typical use-cases for revocations are marking certificates or individual subkeys as unusable, or marking User IDs as no longer used.

Signature Subpackets

A cryptographic signature alone is often not expressive enough to serve certain use-cases. For this reason, the OpenPGP protocol introduced signature subpackets with rfc4880. These are well-defined data structures that can be placed as subelements into signature packets, which give additional context and meaning to a signature. Typical examples are the issuer fingerprint subpacket, which contains the fingerprint of the issuer key, or the key flags subpacket which states, what purpose a component key is intended for.

Signature subpackets can reside in two different areas of a signature packet. Subpackets in the hashed area are incorporated in the digest calculation that is done during signature calculation and are therefore covered by the cryptographic signature. In other words; hashed subpackets are authenticated. If a subpacket is placed in the unhashed area instead, it is not included in the signature calculation procedure and is therefore not protected against tampering. The unhashed area can be used to retroactively add, change or remove subpackets from a signature without invalidating it.

Due to the fact that the unhashed area doesn't provide any cryptographic guarantees, it is only intended for advisory packets, or packets that self-authenticate (e.g. the issuer fingerprint subpacket, whose "correctness" can be proven by successfully verifying the signature using the referenced issuer key). In most cases, signature subpackets are simply added into the hashed area.

Criticality

Each signature subpacket has a flag that indicates whether or not the subpacket is critical. Since different OpenPGP implementations might support subsets of the standard, it would be fatal, if for example an implementation did not understand the concept of signature expiration. Such an implementation would potentially accept an already expired signature. By marking the expiration date subpacket as critical, the user can indicate, that implementations that do not understand this type of subpacket are supposed to reject the signature as invalid.

Sections 5.2.3.11 - 5.2.3.36 give guidance on which subpackets are usually marked as critical.

Certification Recipes

As mentioned above, different signatures are used for different purposes. In this section, we will try to give guidance on how to create certain signatures by example.

Change Algorithm Preferences

In order to change what symmetric, compression, hash or AEAD algorithms are preferred by the key owner, they can issue a direct-key signature (type 0x1F) on the primary key. This signature should have the following structure:

Subpacket	Area	Critical	Mandatory	Notes
Signature Creation Time	Hashed	True	True	Current time
Issuer Fingerprint	Hashed	True or false	Strongly recommended	The primary key is the issuer
Key Flags	Hashed	True	False	Carry over key flags from previous self-signature
Features	Hashed	True	False	Carry over features from previous self-signature
Key Expiration Time	Hashed	True	False	Carry over expiration time from previous self-signature, if present
Hash Alg. Pref.	Hashed	False	False	New preferences
Comp. Alg. Pref.	Hashed	False	False	New preferences
Symm. Alg. Pref.	Hashed	False	False	New preferences
AEAD Alg. Pref.	Hashed	False	False	New preferences

Add User ID

To add (or re-bind) a User ID to a certificate, a signature of type PositiveCertification (0x13) is calculated over the primary key and User ID. The signature should have the following structure:

Subpacket	Area	Critical	Mandatory	Notes
Signature Creation Time	Hashed	True	True	Current time
Issuer Fingerprint	Hashed	True or false	Strongly Recommended	The primary key is the issuer
Primary User ID	Hashed	True	False	Optional
Signature Expiration Time	Hashed	True	False	Optional

Self-certifications over User IDs can optionally carry the same subpackets as listed in the previous table (key flags, features, algorithm preferences). This way, separate capabilities can be assigned to different identities.

Common Subpackets

There are some subpackets that are expected to be included in any type of signature.

Signature Creation Time: Every OpenPGP signature MUST contain a Signature Creation Time subpacket (2) containing the timestamp at which the signature was made. This packet MUST be present in the hashed area of the signature and SHOULD be marked as critical.
Issuer Fingerprint: In order to be able to verify a signature, the verifier needs to know, which (sub-)key was used to issue the signature in the first place. Therefore, every OpenPGP v6 signature SHOULD contain an Issuer Fingerprint subpacket (33) containing the 32 byte fingerprint of the particular component key that was used to create the signature. :::{note} The issuer key might be a subkey. ::: Since the issuer fingerprint subpacket is self-authenticating, it can either be included as a hashed or unhashed subpacket, but the authors of this book recommend to place it in the hashed area of the signature.

Potential conflicts and duplication

Since the hashed and unhashed areas of a signature are just lists of subpackets, in principle they allow duplicates of the same subpacket, which might lead to conflicts. Therefore, packets in the hashed area take precendence over the unhashed area. However, there may still be conflicts between packets in the same area, e.g. two conflicting expiration dates, etc. The specification recommends that implementations favor the last occurence of a conflicting packet in the hashed area.

In some cases, duplicate packets with conflicting content even make sense, e.g. if a signature was made by a version 4 issuer key whose key material was migrated from an older OpenPGP version such as v3. In this case, either the v3 or v4 key could be used to validate the v4 signature, but since the key ID calculation scheme was changed between v3 and v4, these identifiers would differ. Therefore, the signature could contain two isuer key ID subpackets with conflicting, but correct values.

:class: warning

- Key Flags
- Preferences
- Embedded Signature (back sig)
- Trust Signatures (amount, depth)
- Direct key signatures

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