Smart card-based agents for fair non-repudiation

Non-repudiation protocols are of great importance for electronic interactions. Different protocols have been proposed in this field, progressively relaxing the requirements on the computing power of both parties, and on the load and involvement of a trusted third party (TTP). Besides, there is a need to provide mobile users with a protocol adequate for ad hoc scenarios, where only unreliable channels can be guaranteed, and where the CA may not be reachable. In this paper we propose a new version of the fair non-repudiation protocol proposed by Gürgens et al. in [Sigrid Gürgens, Carsten Rudolph, Holger Vogt, On the security of fair non-repudiation protocols, in: Colin Boyd, Wenbo Mao (Eds.), ISC, Lecture Notes in Computer Science, vol. 2851, Springer, 2003, pp. 193–207]. We show unfair scenarios that can appear if one party controls the communication among the TTP and the other party. We introduce notary agents, which play the TTP role, and run our fair non-repudiation protocol. Notary agents execute inside the smart card, and sign the protocol messages with the user’s private key on his behalf. Our approach enables the protocol to run on unreliable channels, disconnected from the CA.