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ch6: write "Signature versions"

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Heiko Schaefer 2023-12-09 00:32:39 +01:00
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book/source/06-signatures.md
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@ -187,10 +187,15 @@ The recipient's hash algorithm preference is defined in metadata of their certif
In this workflow, the signed hash digest is created with a hash algorithm that follows the recipient's preferences, and its intersection with the sender's capabilities and preferences.
### Explore viability of having multiple signatures, e.g. v4+v6?
### Signature versions
```{admonition} TODO
:class: warning
As described in the [RFC](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-12.html#name-signature-packet-type-id-2), the version of a generated signature packet must conform to the version of the key that issues the signature.
C-R 5.2. says: An implementation MUST generate a version 6 signature when signing with a version 6 key. An implementation MUST generate a version 4 signature when signing with a version 4 key.
```
That is:
- OpenPGP version 6 keys must generate version 6 signature packets
- OpenPGP version 4 keys must generate version 4 signature packets
Note that some historical version 3 signature packets may still be relevant for applications that handle old OpenPGP data[^sig-v3]. These version 3 signature packets will have been generated by version 4 keys.
[sig-v3]Version 4 signature packets were introduced in [RFC 2440](https://datatracker.ietf.org/doc/html/rfc2440#section-5.2) in 1998, which specifies that applications SHOULD generate v4 signature, however generation of v3 signature packets has remained allowed through [RFC 4880](https://www.rfc-editor.org/rfc/rfc4880.html#section-5.2).