Server-assisted forward-secure threshold signature.

Forward secure signature can protect the security of signatures previous to the key exposure. Server-assisted signature is a special digital signature in which the signer allies one server to produce the signatures. In this paper, server-assisted forward-secure threshold signature is proposed. The system is composed of n ₁ servers and n ₂ users. Each user and each server holds a partial secret, respectively. To produce a valid signature, users and servers need to cooperate to complete the work. The partial secrets of the users and servers are updated by a one-way function at regular intervals, while the public key is always fixed. Even if all the current partial secrets are exposed, the signatures pertaining to previous periods are still valid. [ABSTRACT FROM AUTHOR]