Ring signature based certificateless cryptography and nominative signature

Abstract

Ring signature, formally introduced by Rivest, Shamir, and Tauman in 2001 is a type of signature schemes similar to group signature schemes but have two distinct properties: spontaneity and anonymity. Spontaneity ensures that no participation or collaboration of other ring members is needed for the actual signer to form a ring (or group) of members and generate a signature on behalf of this ring. Anonymity means signer-ambiguous in the sense that a signature can be verified as coming from a certain ring, without revealing the identity of the actual signer. Due to ring signature’s distinct and interesting properties, it has been studied extensively, and there are many applications of ring signature proposed. These include secret leaking, which is the original motivation of ring signature, and ad hoc group authentication. A special case of ring signature is two-member ring signature which inherits all the properties of ring signature but has only two members involved. One prominent application of this special case is the concurrent signature. Another interesting application is the construction of designated verifier signature. Given these application, a natural question is whether we could further apply two-member ring signature to other applications. We answer this question positively by proposing a new application of two-member ring signature. We apply the techniques to the construction of Nominative Signature and show that we are able to build a provably secure while efficient nominative signature scheme with some properties which cannot be realized by comparable constructions. In a nominative signature scheme, there are three parties involved, namely nominator, nominee and verifier. Unlike other non-self-authenticating signature schemes such as Undeniable Signature, only NS is able to dethrone the ability of proving the validity of a signature from the signer who chooses the message (or Nominator). Due to this special property, it is considered to be more suitable for User Certification System than Universal Designated Verifier Signature. However for the past few years, only a few secure and efficient NS constructions which satisfy all NS properties have been proposed and they all require multiple rounds of interaction between the nominator and the nominee which is inefficient. In this thesis, we propose for the first time a one-move NS scheme based on two-member ring signature which satisfies all the properties of NS and is secure under NS’s strongest security models. Besides the application of the special two-member ring signature, we also work on another interesting problem of ring signature. Currently, all ring signature schemes are either built under the PKI (Public Key Infrastructure) or the Identity-Based setting. In the setting where there is no certificate while without key escrow, for example, in the certificateless cryptographic setting, we find that the main problem of constructing a ring signature is that there is no satisfactory security model available. When comparing with the research work on other aspects of certificateless cryptography, such as certificateless encryption, the work on certificateless ring signature is inadequate. In the second part of this thesis therefore, we focus ourselves on the security definition and construction of certificateless ring signature. Our results show that they are currently the strongest models and the most efficient constructions available.

Date of Award	2 Oct 2008
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Shek Duncan WONG (Supervisor)