Group signatures are a useful cryptographic construct for privacy-preserving non-repudiable authentication, and there have been many group signature schemes. In this paper, we introduce a variant of group signatures that offers two new security properties called leak-freedom and immediate-revocation. Intuitively, the former ensures that an insider (i.e., an authorized but malicious signer) be unable to convince an outsider (e.g., signature receiver) that she indeed signed a certain message;

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... eas the latter ensures that the authorization for a user to issue group signatures can be immediately revoked * An earlier version appeared in the Proceedings of IEEE ICDCS'04 [32] .