Degree Name

Masters of Computer Science by Research


School of Information Technology and Computer Science - Faculty of Informatics


Three separate credential systems, namely Secret Handshakes (SH), Oblivious Signature-Based Envelopes (OSBE) and Hidden Credentials, have been introduced in recent years. These credential systems are very useful in anonymous communication as they have an interesting common feature which is the ability to combine encryption with access control. This feature allows participants to protect their credentials from being disclosed while running the protocols, which makes these credential systems a natural fit for privacy-preserving and anonymity-oriented applications. Since these systems have many similarities, interest has arisen in converting them from one to another. Consequently, a series of OSBE schemes based on ElGamal family signatures was proposed, along with a generic construction of SH from OSBE. According to this generic construction, any ElGamal family signature based OSBE scheme can be converted to SH within three communication moves, with the exception of the ElGamal and DSA signatures. To complement the previous result, we propose two three-move SH schemes based on ElGamal and DSA signatures, respectively. Furthermore, we consider the question of extending the two-party SH to a multi-party setting. We observe that almost all of the SH schemes can be constructed from particular key agreement schemes. Hence we implement an efficient ID-based Authenticated Group Key Agreement (AGKA) scheme, from which we can construct a multi-party SH scheme. Very recently, a new multi-party SH scheme has been proposed based on an unauthenticated group key agreement scheme ahead of our implementation. However, we note that there exists a drawback in this scheme, which may cause the leakage of a valid member's group affiliation in a failed multi-party SH protocol. Therefore, we propose a Group Secret Handshake (GSH) scheme that resists against this attack, and prove that our scheme is secure.

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