Re: [openpgp] Proposal for a separable ring signature scheme compatible with RSA, DSA, and ECDSA keys

[Daniel Kahn Gillmor <[e-mail]>]

On 03/14/2014 07:42 PM, Vincent Yu wrote:
> On 03/14/2014 10:38 AM, Daniel Kahn Gillmor wrote:
>> Guidance would also be useful for implementations processing (or
>> generating) ring signatures that were made by one of a set of keys where
>> some of those keys appear to be expired or revoked.  (i haven't thought
>> this use case through in sufficient detail, but i could see
>> implementations getting tripped up here or behaving in wildly divergent
>> ways if there is no clear guidance)
> 
> I think a good general recommendation here would be to look at each
> public key individually and output the same warnings and errors that
> would be output if this were a standard signature. Are there significant
> issues that you see with this?

i'm just imagining a troubling use case in terms of UI (maybe it isn't
an issue):

 Alice and Bob have keys; Alice decides she wants to frame Bob.  Alice
makes a ring signature with her key and with Bob's key at time T over a
document that is particularly terrible.  She then sets her computer's
clock back to time T-1 and expires or revokes her own key.

Carol comes along and checks the signature on the terrible document.
her OpenPGP implementation says "this signature was made by either Alice
or Bob, but Alice's key was expired/revoked"

If Carol is naive, the implication she might take away from such a UI is
that Alice couldn't have made the signature, therefore it must have been
Bob that said the terrible thing.

I don't know how to clarify the UI to avoid giving that impression.

	--dkg