Cryptanalysis of Two New Signature Schemes.

Group signature and blind signature are very important primitives in cryptography. A group signature scheme allows a group member to sign messages anonymously on behalf of the group and a blind signature scheme can ensure anonymity of the sender of a message. Recently, S. Xia and J. You [14] proposed a group signature scheme with strong separability in which the revocation manager can work without the involvement of the membership manager and J.J-R. Chen and A.P. Chen [5] proposed a blind signature scheme based on dual complexities (which combines factorization and discrete logarithm problem). In this paper, we give a universal forgery attack on Xia-You's group signature scheme which any one (not necessarily a group member) can produce a valid group signature on an arbitrary message, and it is untraceable by the group revocation manager. For Chen-Chen's blind signature scheme, we show that it could not meet the untraceability property of a blind signature, i.e., it could not ensure anonymity of the user.