Paper 2005/418

Generic On-Line/Off-Line Threshold Signatures

Chris Crutchfield, David Molnar, David Turner, and David Wagner

Abstract

We propose on-line/off-line threshold signature schemes, in which the bulk of signature computation can take place ``off-line" during lulls in service requests. Such precomputation can help systems using threshold signatures quickly respond to requests. For example, tests of the Pond distributed file system showed that computation of a threshold RSA signature consumes roughly 86% of the time required to service writes to small files. Because a large number of writes in file systems are for small files, threshold signatures form a performance bottleneck in Pond and similar systems. We apply the ``hash-sign-switch" paradigm of Shamir and Tauman and the distributed key generation protocol of Gennaro et al. to convert any existing secure threshold digital signature scheme into a threshold on-line/off-line signature scheme. Our construction is fully distributed and requires no trusted dealers. We show that the straightforward attempt at proving security of the resulting construction runs into a subtlety that does not arise for Shamir and Tauman's construction. We resolve the subtlety and prove our signature scheme secure against a static adversary in the partially synchronous communication model under the one-more-discrete-logarithm assumption. The on-line phase of our scheme is efficient: computing a signature takes one round of communication and a few modular multiplications in the common case.