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

2014-03-15 00:26:57
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

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