Anonymous Services using Smart Cards and Cryptography.

More and more services provided by Internet pose a problem of privacy and anonymity. One cryptographic tool that could be used for solving this problem is the group signature \cite{acjt00,cg02,cvh91}. Each member of the group is able to anonymously produce a signature on behalf of the group and a designated authority can, in some cases, revoke this anonymity. During the last decade, many anonymous services using this concept have been proposed: electronic auctions \cite{nt00}, electronic cash systems \cite{t99,mb01,ct03b}, anonymous credentials \cite{cl01}. But for some other services where the anonymity is essential (such as electronic voting or call for tenders), group signature schemes cannot be applied as they are. For this reason, the authors of \cite{ct03a} proposed a variant that is partially linkable and not openable, called list signature scheme. In this paper, we first improve the cryptographic tool of \cite{ct03a} by proposing some optional modifications of list signature schemes such as anonymity revocation. We then propose more efficient list signature schemes, by using a smart card to produce the signature. We finally propose some concrete implementations of our proposals. As a result, we obtain more efficient solutions that are useful in many more services.