[openpgp] Key Superseded signature type

[Aron Wussler <[e-mail]>]

Hi everyone,

I've got another small proposal for the crypto-refresh, also coming from the OpenPGP summit.

We've been discussing about the options for the v4 -> v5 transition (I guess v6 now?), and I'm a big fan of having multiple independently generated certificates, and allowing a smooth transition between them.

In OpenPGP we don't have a mechanism to signal a superseded or deprecated key, and I fear that if we all wait for v6 keys to be understood from the majority of software and then atomically switch to v6 revoking our old v4 keys we'll wait for a long time. The alternative is to have two cross-signed certificates, and publishing them both, hoping for people to use the newer one.

With this proposal, my objective is to have a standardized way to tackle this, by introducing a new signature type that signals key deprecation with a pointer to the new key.
This would automatically extend to future transitions (e.g. PQC), and allow for an easier key rotation (It would be allowed on hard revocation too).

https://gitlab.com/openpgp-wg/rfc4880bis/-/merge_requests/222

All kind of feedback is welcome, whether this means
(1) not introducing this feature at all
(2) introducing this feature but later
(3) introducing this feature now

Cheers,
Aron

--
Aron Wussler
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