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

[Vincent Yu <[e-mail]>]

On 03/15/2014 01:03 AM, Daniel Kahn Gillmor wrote:
> 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

Hm. Yes, scenarios like that sound like they can confuse the typical 
user and possibly lead to incorrect conclusions. It seems like it would 
be prudent for implementations to issue conspicuous errors when any 
aspect of a ring signature fails to verify, and to warn the user against 
drawing any conclusion other than the fact that the ring signature did 
not verify correctly.

But at the end of the day, the security of the scheme and the behavior 
of the implementation don't matter if users misuse them... A possibly 
more important thing to do is to provide easy-to-read references that 
users can look up. If ring signatures ever get implemented in GnuPG (or 
elsewhere), we should take care to write up clear and concise 
explanations for end users. (This is a difficult task.)

Vincent