Abstract
Cryptographic computations are often carried out on insecure devices for which the threat of key exposure raises a serious concern. In an effort to address the key exposure problem, the notion of forward security was first presented by Gunther in 1990. In a forward-secure scheme, secret keys are updated at regular periods of time; exposure of the secret key corresponding to a given time period does not enable an adversary to 'break' the scheme for any prior time period. In this paper, we first introduce forward security into certificate-based cryptography and define the security model of forward-secure certificate-based signatures (CBSs). Then we propose a forward-secure CBS scheme, which is shown to be secure against adaptive chosen message attacks under the computational Diffie-Hellman assumption in the random oracle model. Our result can be viewed as the first step toward solving the key exposure problem in CBSs and thus improving the security of the whole system.