[openpgp] Re: Algorithms vs packets in forwarding and PSK

Hi Falko,

Thanks for the thorough response. The framework you propose seems sensible to me.

I'll pick on some details re. the persistent symmetric keys draft:

On Thursday, September 4th, 2025 at 14:24, Falko Strenzke <[email protected]> wrote:

> - define new public key packets, since they can't be used for encryption or signature verification

The current draft doesn't allow public key packets to be used with persistent symmetric key algorithms at all, since there isn't any use for them. Do you think that should change?

> - define new "MAC" packets: a MAC (or the authentication feature of AEAD) is not equivalent to a public key signature. It doesn't provide non-repudiation. Further, any user from a group that shares the same MAC key can forge "signatures" for any member of the group. Accordingly, as I argued in my review [3] of [2], receiving clients might have to treat MACs different than public-key signatures when displaying messages. This seems to suggest that new packet types are needed.

I want to note that this argument is somewhat specific to the scenario when you're sharing persistent symmetric key material between multiple users. I'll concede that that may be useful, though it wasn't originally the use case I had in mind, but I'm not sure if we should then base the decision to define a new packet on the semantics in that use case alone.

Beyond that, the semantics are the same if you share an asymmetric private key in a group of users, no? You lose non-repudiation in that case, too. So I don't think this argument is actually specific to symmetric keys.

(The only difference is that since there's no public key, you might get tempted to share the private key, but then it's the act of doing so that changes the security properties, not the fact that it's a symmetric key.)

Best,
Daniel

P.S. Aron is on vacation for the coming 10 days, so it may take him some time to respond to the forwarding part.

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