Forward-secure signatures with fast key update

Abstract

In regular digital signatures, once the secret key is compromised, all signatures, even those that were issued by the honest signer before the compromise, will not be trustworthy any more. Forward-secure signatures have been proposed to address this major shortcoming. We present a new forward-secure signature scheme, called KREUS, with several advantages. It has the most efficient Key Update of all known schemes, requiring just a single modular squaring. Our scheme thus enables more frequent Key Update and hence allows shorter time periods, enhancing security: fewer signatures might become invalid as a result of key compromise. In addition, the on-line component of Signing is also very efficient, consisting of a single multiplication. We precisely analyze the total signer costs and show that they are lower when the number of signatures per time period is small; the advantage of our scheme increases considerably as the number of time periods grows. Our scheme's security relies on the Strong-RSA assumption and the random-oracle-based Fiat-Shamir transform. © Springer-Verlag Berlin Heidelberg 2003.