[openpgp] SEIPDv1 algorithm obfuscation when using persistent symmetric keys

Hi all,

In the meeting today I said that for v3 PKESKs using a persistent
symmetric key, we should add a cleartext symmetric algorithm ID
indicating the algorithm used by the following SEIPDv1 packet,
like we did in X25519 and X448.

In those algorithms, we also mandated for this algorithm to be AES,
to prevent algorithm confusion attacks. However, in the case of
persistent symmetric keys, there is actually a use case for allowing
other algorithms, namely if you're re-encrypting session keys of old
messages that use a different algorithm. So, we should make sure that
the algorithm ID is covered by the integrity tag, either by adding it
to the HKDF info or additional data, or by just encrypting it together
with the session key, as we do in ECDH and RSA.

The latter may be preferable in this case, because we may not want to
reveal if a message is encrypted using IDEA or TripleDES or so, in case
one of those algorithms is ever broken. There's also no significant
downside to doing so as AEAD algorithms accept any plaintext length,
unlike AES-KW.

In addition to that, ECDH allows you to hide the session key length
using padding, to prevent distinguishing between AES-128 and AES-256,
for example. Is that something we should care about here? If so,
we could allow padding the session key with zeros, or so.

(I don't personally think that that's super important, but I'd hate for
someone to claim that symmetrically re-encrypting a session key is ever
a downgrade, for example.)

Best,
Daniel

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