Challenge-Response Attack on a Key Transport Protocol based on a Ring Locking Technique

Key transport is one of the fundamental issues in modern cryptography. Recently Khayat proposed a multiple agent distributed version of Shamir´s 3-pass no-key protocol. In this paper, based on a ring locking technique a challenge-response attack on Khayat´s protocol is mounted to capture its vulnerability to CCA1 and CCA2. The cryptanalysis shows that the attack protocol is efficient (without expensive on-line computation of modular inverses), scalable (independent of the size of trustees), and feasible on the Internet environment. One of its novel features is that the active attacker can conceal some trace of his cyber crime by means of the commutative property of index multiplication in iterative modular exponentiations.