The unlinkability of randomization-enhanced Chaum´s blind signature scheme

The key issue in e-commerce security is digital signature. Chaum first proposed the concept of blind digital signature, and designed untraceable payments. To avoid threats from chosen-message attacks presented by Coron et al. (1999), Fan et al. (2000) proposed a randomization enhanced Chaum blind signature scheme, by injecting a random factor into messages. In this paper, we first formally define the unlinkability of the blind signature scheme. According to this definition, we prove that Fan´s scheme does not possess the unlinkablity property: after the message and signature have been revealed to the public by the sender, the signer can trace the corresponding blinded message and signature by constructing a linkage between the message and the blind message. Therefore, Fan´s scheme cannot provide true blind signatures.