Improved server assisted signatures

Verifiable-server assisted signatures were proposed in the past but they could not totally eliminate public key operations for the signer. In this paper, we propose a new alternative called SAOTS (server assisted one-time signatures) where just like proxy signatures generating a public key signature is possible without performing any public key operations at all. This feature results in both computational efficiency and implementation simplicity (e.g. a reduction in the code size) of the proposed protocol. In addition, SAOTS is a more promising approach since the signature is indistinguishable from a standard signature, no storage is necessary for the signer to prove the server's cheating and the protocol works in less number of rounds (two instead of three). On the other hand, the drawback of SAOTS is the increased bandwidth requirement between the sender and server.