Initial outline and old notes

(Rough merge of two precursor projects by Heiko, and outline notes by Paul)

book/source/06-signatures.md

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