[openpgp] Re: implicit PSK key capabilities

[Daniel Huigens <[e-mail]>]

Hi all,

Thanks dkg for the concrete MR, and thanks all for the discussion!

I agree it should be fine to permit PSKs to be used for all usages.
The PSK draft itself has domain separation between encryption and signing,
and the OpenPGP spec has domain separation between different types of signatures.
If we add a new type of signature that should already be covered,
and if we add an entirely new type of key usage that could be covered
by PSK's domain separation (but may need to be considered separately).

So, I've created an alternate MR to address both this question and the
one about revocation & expiry, as follows: [!8]

> ## Key Metadata
> 
> Because a Persistent Symmetric Key cannot include a direct-key
> signature, there is no place to include a Key Flags subpacket
> (as defined in {{Section 5.2.3.29 of RFC9580}}) to indicate the
> permissible key usages.
> Instead, Persistent Symmetric Keys are implicitly allowed to be used
> for all usages (certification, signing, encrypting communications and
> storage, and authentication).
> If a keyholder or application developer wishes to restrict the key
> usages, they may add metadata external to the key indicating the
> intended usages, instead (and process it before passing the key to
> the OpenPGP implementation).
> 
> Additionally, Persistent Symmetric Keys cannot expire, nor be revoked.
> If a keyholder or application developer no longer wishes to use a
> given key, they may either add metadata external to the key indicating
> that it should no longer be used, or destroy the key material (after
> re-encrypting and re-authenticating any data that is still relevant
> with a new key) if that is practical.

Let me know (here or in the MR) if that seems reasonable.

Best,
Daniel

[!8]: https://gitlab.com/twisstle/openpgp-persistent-symmetric-keys/-/merge_requests/8