---------- Forwarded message ---------- Date: Tue, 3 Jul 2001 17:48:31 +0300 (EET DST) From: Helger Lipmaa <helger@tml.hut.fi> To: Panayiotis Kotzanikolaou <pkotzani@unipi.gr> Cc: cryptography@wasabisystems.com Subject: Re: Sender and receiver non-repudiation On Tue, 3 Jul 2001, Panayiotis Kotzanikolaou wrote:

The problem in this scheme is that Bob signs and sends the proof after he has received M. Bob can receive M and never send a receipt.

By using a trusted delivery service, it is easy to produce non-repudiation evidence both for the sender and the receiver. Is there any cryptographic protocol that "forces" Bob to produce non-repudiation evidence during execution?

Answering only to this concrete question: the kind of things you want are accomplished by contract signing protocols. A few recent papers: Optimistic Protocols for Fair Exchange(N. Asokan, Matthis Schunter, Michael Waidner, 1996) Optimal Efficiency of Optimistic Contract Signing(Birgit Pfitzmann, Matthias Schunter, Michael Waidner, PODC 98) Optimistic synchronous multi-party contract signing(N. Asokan, Birgit Baum-Waidner, Matthias Schunter, and Michael Waidner, dec 1998) Asynchronous protocols for optimistic fair exchange(N. Asokan, Victor Shoup, Michael Waidner, may 1998) Optimistic asynchronous multi-party contract signing(Birgit Baum-Waidner and Michael Waidner, nov 1998) Optimistic fair exchange of digital signatures(N. Askon, Victor Shoup, Michael Waidner, oct 1999) Abuse-free Optimistic Contract Signing(Juan Garay, Markus Jakobsson, Phil MacKenzie, CRYPTO '99) Abuse-free Multi-party Contract Signing( Juan Garay, Phil MacKenzie, DISC '99) Provable Secure Certified Mail(Birgit Pfitzmann, Matthias Schunter, Michael Waidner, 2000) Analysis of Abuse-Free Contract Signing( Vitaly Shmatikov, John C. Mitchell, 2000) 06/23/01 Optimistic Asynchronous Multi-Party Contract Signing with Reduced Number of Rounds(Birgit Baum-Waidner, eprint 2001/044) (you can find links to them at http://www.tml.hut.fi/~helger/crypto/link/protocols/contract.html) Answering more generally: it does not help you to get non-repudiation. Nothing does, unless Bob is able to prove that you signed this document consiously, being sober, and generally, that you MEANT to sign it. Helger --------------------------------------------------------------------- The Cryptography Mailing List Unsubscribe by sending "unsubscribe cryptography" to majordomo@wasabisystems.com -- ____________________________________________________________________ Whereof one cannot speak, thereof one must be silent. Ludwig Wittgenstein The Armadillo Group ,::////;::-. James Choate Austin, Tx /:'///// ``::>/|/ ravage@ssz.com www.ssz.com .', |||| `/( e\ 512-451-7087 -====~~mm-'`-```-mm --'- --------------------------------------------------------------------