An Improved Updatable Signature Scheme with Weakened Token

Updatable signatures (US) play a crucial role in cloud storage scenarios by providing the functionality to update signatures and resisting key compromise attacks. In PKC 2021, Cini et al. proposed the first updatable signature scheme, where an update token was granted more power than necessary. To restrict access to the token, various techniques such as leakage profiles were employed. Without such restrictions, an adversary could easily forge a new valid signature without compromising a new signing key. In this paper, we improve Cini et al.’s signature and its security via weakening the functionality of update token. Specifically, we propose a change in the way the update token is generated, involving both the signer and the server. Initially, a Diffie-Hellman key exchange protocol is executed between the signer and the server to establish a secret value. This secret value is then embedded by the signer into the update token generation process, ensuring that only those possessing the secret value can update a signature using the update token. Furthermore, we employ the technique of indistinguishability obfuscation to minimize the information leaked by the weakened token. This enhancement allows the improved updatable signature scheme to achieve no-directional key update and uni-directional signature update. Finally, we present an enhanced security model without leakage profiles restricting an adversary, where the adversary is allowed to corrupt update tokens and signing keys freely except a signing key at the challenge epoch, and prove the improved scheme to be secure.