Bachelor/Master Thesis
Blind signatures
A digital signature is a cryptographic primitive used to prove the authenticity and integrity of a message. Classical digital signature schemes provide strong security guarantees, but they bind the signer’s identity to the signed message. In many applications, the ability to link a signature to the signer constitutes a significant drawback. A representative example is electronic cash, where users wish to obtain digital coins signed by a bank without enabling the bank to trace how or when they are later spent. If the bank signs coins in the clear, it can record the serial numbers of those it signs and then link them to the serial numbers of coins when they are later spent. This enables the bank to reconstruct detailed histories of users’ spending, undermining the anonymity that physical cash provides.
To address this issue, David Chaum [1] introduced blind signatures, which allow a signer to sign a message without learning its content. This property enables the construction of privacy-preserving systems in which payments can be verified without compromising user privacy. Indeed, in a blind signature scheme, a user blinds a message before sending it to the signer, who produces a valid signature without learning any information about the underlying message. The user can then remove the blinding factor, obtaining a standard signature on the original message. The resulting signature can be publicly verified, yet remains unlinkable to the signing interaction. This blindness property ensures that the process of issuing a signature is unlinkable from the signature’s later use.
Blind signatures are also used in other applications, such as anonymous credentials and privacy-preserving e-voting systems.
The goal of this thesis is to study and compare different blind signature schemes with respect to their security properties, efficiency, and practical applications.
References
[1] David Chaum: Blind Signatures for Untraceable Payments. CRYPTO 1982.