Security Analysis and Improvement of Partially Blind Proxy Re-signature Scheme

As a new concept proposed recently, partially blind proxy re-signature scheme (PBPRSS) holds the advantages of the partially blind signature and proxy re-signature. In PBPRSS, a semi-trust third party called the proxy can re-sign a message with a piece of common information between the delegator and the proxy by a re-signature key. In this paper, we analyze the first PBPRSS proposed in 2018 and point out that the PBPRSS does not satisfy the security property of unforgeability. There exist two types of attacks. One is forgery attack from anyone user. In this type of attack, the attacker can forge a blind signature of the delegator without known the private key of DA and then sends the forged blind signature to PB for a re-signature. The forged blind signature can pass the verification successfully from the PB. Another is forgery attack from the delegator DA. In this type of attack, DA as the attacker can forge a partially blind proxy re-signature on any message without known the re-signature key after DA execute a formal interaction between the DA and PB. Finally, we also give a simple analysis on the existing problem and an improvement idea.