Degree Name

Master of Computer Science by Research


School of Information Technology & Computer Science - Faculty of Informatics


A ring signature is a cryptographic primitive that enables a signer to produce a signature without revealing his or her identity. In this thesis, we propose two ring signature schemes for privacy-preserving applications over the Internet.

First we design a protocol that enables a ring signer to receive an acknowledgement from the verifer. We propose two constructions. With the basic construction, the verifer can send a message back to the original signer while keeping the latter's identity hidden. Additionally, the verifer is assured that the signer is indeed the user in a group. We then extend our basic construction to a multi-party scenario. In the second construction, the verifer can discern a certain number of signers involved in the specific signature while their identities remain anonymous. We also investigate the possible applications of our schemes, such as in E-Commerce and Pay-TV.

Then, we introduce the concept of identity-based anonymous designated ring signatures, which has not been studied before. This concept extends the existing notion of ring signatures in two ways: frstly, it allows a member of the ring to sign a message directed to a designated verifer. Secondly, we enable the concept of anonymous designated verifer. We show that it has useful applications in Peer-to-Peer networks and provide a construction based on bilinear pairings. Furthermore, we formulate a security model and prove the security of our proposed construction against a chosen message attack. We extend the scheme to construct a convertible version of the previous scheme, which enables a designated verifer to prove its participation in a particular session in case of dispute.

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