Well, I can't be sure that I'm not misunderstanding something either. For the most part, I agree with Dimitrios that challenges with proof of origin are part of the solution to Mafia Fraud attacks. My main point is that I don't think simply signing the challenge is enough. Let me try to restate things symbolically. Nominally, in the naive case, Dave would present Alice with a challenge, X, and Alice would transform & return the challenge: X'. This, as we know, is vulnerable to the Mafia Fraud. What I believe Dimitrios is proposing is for Dave to present both the challenge and a signature on the challenge: {X, S_dave(X)}. Then, Alice would verify that the signature corresponds to the person she thinks she's talking to, and if so she can return the transformed challenge X'. I'm essentially contending that Dave needs to verify that Alice did indeed see the challenge & signature he presented. Consider Mafia Fraud against the above scenario. Dave presents {X, S_dave(X)} to Carol, who forwards it to Bob. Now, Bob can re-sign the challenge himself, and present {X, S_bob(X)} to Alice. Alice will happily verify that the challenge comes from Bob, and return X' to Bob, who then passes it to Carol & then on to Dave. The fraud is successful, because Dave can't tell that Alice saw Bob's signature on the challenge and not his own. So the X' that Alice computes must be a function f(X, S_dave(X)) on both the challenge and the signature. (If, in the naive case, X'=S_alice(X), then to truly prevent the fraud we need X'=S_alice(X,S_dave(X)).) Now the fraud fails because Alice would compute X'=f(X, S_bob(X)), and so Dave (not Alice) would detect the fraud. So it's not enough for Dave to simply sign the challenge & for Alice to verify that signature. Alice must prove to Dave that she saw his signature and not somebody else's. BTW, without giving it any thought, I believe this scheme is safe against replay attacks (because Dave generates a new challenge every time). Does anybody have any thoughts about that? Marc --------------------------------------------------------------------- The Cryptography Mailing List Unsubscribe by sending "unsubscribe cryptography" to [EMAIL PROTECTED]

- Zero Knowledge Identity Proofs Michael Conlen
- Re: Zero Knowledge Identity Proofs Helger Lipmaa
- Re: Zero Knowledge Identity Proofs Dimitrios . Petropoulos
- Re: Zero Knowledge Identity Proofs Marc Branchaud

- Re: Zero Knowledge Identity Proofs Dimitrios . Petropoulos
- Re: Zero Knowledge Identity Proofs Marc Branchaud

- Re: Zero Knowledge Identity Proofs Dimitrios . Petropoulos
- Re: Zero Knowledge Identity Proofs Dimitrios . Petropoulos