Provably Secure Key-Updating Schemes in Identity-Based Systems

In this pa.per, we present Key-Updating Schemes in identity-based (identification or signature) systems, and consider the security of the schemes. We ’ propose two kinds of key-updating schemes, i.e., one is sequential type and the other is parallel type, and show that both schemes are equivalent to each other in a polynomial time sense, i.e., there elrists a deterministic polynomial time algorithm that transforms the sequential key-updating scheme to the parallel one, and vice versa. We also show that even if any polynomially many entities conspire to find a secret-key of any other entities, both key-updating schemes are provably secure against polynomially many times key-updating if _ decrypting RSX is hard. I.B. Damgard (Ed.): Advances in Cryptology EUROCRYPT ‘90, LNCS 473, pp. 16-30, 1991. 0 Springer-Verlag Berlin Heidelberg 1991