Updatable Signature Scheme With Weakened Token and No-Directional Key Update

Updatable signature (US) resists key compromise attacks and is integral in numerous fields that require authentication. However, previous US schemes granted excessive power to update token, which cannot achieve no-directional key update. In this paper, we improve Cini et al.'s US scheme by weakening update token. The improvement lies in changing the mode of generating an update token, from the signer alone to the signer and the server cooperatively. Specifically, they first negotiate a secret value, which is then used by the signer to generate an update token. This mode ensures that only the entity who owns the secret value can use the update token to update a signature. Furthermore, we employ indistinguishability obfuscation to minimize information leakage through update token. These advancements make the improved updatable signature scheme achieve no-directional key update and unidirectional signature update. Finally, we present an enhanced security model, where an adversary is permitted to freely corrupt update tokens and signing keys, except the signing key at challenge epoch, and then prove the improved scheme to be unforgeable based on CDH hardness.