The concrete tripartite concurrent signature scheme designed by Susilo et al is found being unambiguity In other words, any fourth party can identify who is the real signer of the publicly available valid signatures before the keystone is released In order to overcome this flaw, we proposed an improved concrete tripartite concurrent signature (iCTCS) scheme based on the ring signature Every user can choose his keystone to guarantee the fairness in the proposed scheme However, the performance analysis results show that the iCTCS scheme is existentially unforgeable under a chosen message attack in the random oracle, and satisfies correctness, ambiguity and fairness

Improved tripartite concurrent signature

At first, an convertible ID-based two-party ring signature scheme is designed, in such a scheme, after releasing of a converting information by the real signer, a ring signature can be transformed into a standard signature which can be verified publicly. And then, based on this ring signature, a convertible perfect concurrent signature protocol is proposed. Different from a general concurrent signature, there is an extra procedure called “ACONVERT” in this protocol, which can transform the exchanged ring signature into a common form, so it realizes the fair exchange of signature. At last, the security of the protocol is analyzed.

Convertible Perfect Concurrent Signature Protocol

The concept of concurrent signatures was introduced by Chen et al. in Eurocrypt 2004 which allows two entities to produce two signatures in such a way that, the signer of each signature is ambiguous from any third party?s point of view until the release of a secret, known as the keystone. Once the keystone is released, both signatures become binding to their respective signers concurrently. In the previous concurrent signature schemes, the roles of participants are asymmetrical, one party, which is called initial signer, needs to create the keystone fix and send the first ambiguous signature, the other party, which is called matching signer, responds to this initial signer by creating another ambiguous signature with the same keystone fix. This work mode may cause some unfairness. In this paper, we construct a perfect concurrent signature protocol for symmetric participants and prove its security. In our concept, the roles of participants are symmetrical. The keystone can not be decided by any participant and the two ambiguous signatures can be published in any order.

Perfect Concurrent Signature Protocol for Symmetric Participant

Key issuing protocols deal with overcoming the two inherent problems: key escrow and secure channel requirement of the identity based cryptosystems. An efficient and secure key issuing protocol enables the identity-based cryptosystems to be applicable in the real world. We analyze an threshold key issuing protocol due to Gangishetti et al. and show that the protocol suffers from the attacks of the malicious KGC and the users can deny that they have never received the key. Furthermore, we present an improved protocol, which is undeniable and secure against KGC's attacks.

Zhang Bo

Papers

Perfect Concurrent Signature Protocol for Symmetric Participant

Cryptanalysis and Improvement on an Threshold Key Issuing Protocol