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59 lines
2.7 KiB
Markdown
59 lines
2.7 KiB
Markdown
# Signatures as "statments"
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```
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- Purpose of a signature
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- Meaning of different signature types, nuances of subpackets
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- Can we have a "catalogue" of statements a user might want to make, mapping these to archetypical signatures?
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- Revocation; Hard vs. Soft
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```
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## Certifications (third party signatures on keys)
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A certification is a machine-readable statement about a (public) key, made by a third party.
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In OpenPGP, certifications are implemented as
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[Signature Packets](https://datatracker.ietf.org/doc/html/rfc4880#section-5.2).
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More specifically, certifications in OpenPGP are usually modelled as "third party binding signatures".
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Typically, certifications in OpenPGP work like this: Alice checks that a key `0x1234...` belongs to Bob, who uses the
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email address `bob@example.org`. After making sure that the key `0x1234...` and the digital identity `bob@example.org`
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are meaningfully linked, she creates a certification stating that the key and the identity are linked.
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Such a certification can serve two purposes:
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1) Alice's OpenPGP software can now reason about Bob's key, and thus show that `0x1234...` is a good key to use for
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interacting with `bob@example.org`.
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2) Other parties can observe Alice's certification and derive some amount of confidence in Bob's key from it.
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For example, Carol might not easily be able to check if `0x1234...` is Bob's key, but she might consider Alice's
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certification for Bob's key sufficient evidence.
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Carol may decide to systematically rely on Alice's certifications. Then we say that Carol uses Alice as a
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"trusted introducer". That is, Carol *delegates* part of her authentication decisions to Alice.
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### Regular certifications
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Are a cryptographic statement that binds a User ID and a Key (via its fingerprint) together.
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Have a SignatureType in `GenericCertification, PersonaCertification, CasualCertification, PositiveCertification`.
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### Trust signatures (using a key as "trusted introducer")
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A "trust signature" has two additional parameters: a `depth` and a `level`.
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#### Alternative model: direct key signatures for pure delegation
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This is useful for using 0xB as a trusted introducer without asserting that 0xB is Bob
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(when a tsig is on a User ID, it is necessarily *also* a vouch about the binding).
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The logical place to store a tsig that is not also a vouch about a binding is a direct key signature
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(however, GnuPG does probably not respect such tsigs).
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The [OpenPGP Web of Trust](https://sequoia-pgp.gitlab.io/sequoia-wot/) spec allows such direct key signatures.
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SignatureType is `DirectKey`
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In Sequoia, roughly:
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```
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SignatureBuilder::new(SignatureType::GenericCertification).set_trust_signature(..).sign_direct_key(&mut your_signer, &signee_cert.primary_key())
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```
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