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Abstract. An aggregate signature is a single short string that convinces any verifier that, for all 1 ≤ i ≤ n, signer Si signed message Mi, where the n signers and n messages may all be distinct. The main motivation of aggregate signatures is compactness. However, while the aggregate signature itself may be compact, aggregate signature verification might require potentially lengthy additional information -namely, the (at most) n distinct signer public keys and the (at most) n distinct messages being signed. If the verifier must obtain and/or store this additional information, the primary benefit of aggregate signatures is largely negated.This paper initiates a line of research whose ultimate objective is to find a signature scheme in which the total information needed to verify is minimized. In particular, the verification information should preferably be as close as possible to the theoretical minimum: the complexity of describing which signer(s) signed what message(s). We move toward this objective by developing identity-based aggregate signature schemes. In our schemes, the verifier does not need to obtain and/or store various signer public keys to verify; instead, the verifier only needs a description of who signed what, along with two constant-length "tags": the short aggregate signature and the single public key of a Private Key Generator. Our scheme is secure in the random oracle model under the computational Diffie-Hellman assumption over pairing-friendly groups against an adversary that chooses its messages and its target identities adaptively.

Abstract. Most prior designated confirmer signature schemes either prove security in the random oracle model (ROM) or use general zeroknowledge proofs for NP statements (making them impractical). By slightly modifying the definition of designated confirmer signatures, Goldwasser and Waisbard presented an approach in which the Confirm and ConfirmedSign protocols could be implemented without appealing to general zero-knowledge proofs for NP statements (their Disavow protocol still requires them). The Goldwasser-Waisbard approach could be instantiated using Cramer-Shoup, GMR, or Gennaro-Halevi-Rabin signatures. In this paper, we provide an alternate generic transformation to convert any signature scheme into a designated confirmer signature scheme, without adding random oracles. Our key technique involves the use of a signature on a commitment and a separate encryption of the random string used for commitment. By adding this "layer of indirection," the underlying protocols in our schemes admit efficient instantiations (i.e., we can avoid appealing to general zero-knowledge proofs for NP statements) and furthermore the performance of these protocols is not tied to the choice of underlying signature scheme. We illustrate this using the CamenischShoup variation on Paillier's cryptosystem and Pedersen commitments. The confirm protocol in our resulting scheme requires 10 modular exponentiations (compared to 320 for Goldwasser-Waisbard) and our disavow protocol requires 41 modular exponentiations (compared to using a general zero-knowledge proof for Goldwasser-Waisbard). Previous schemes use the encryption of a signature paradigm, and thus run into problems when trying to implement the confirm and disavow protocols efficiently.

Abstract. In this paper we construct a practical group blind signature scheme. Our scheme combines the already existing notions of blind signatures and group signatures. It is an extension of Camenisch and Stadler's Group Signature Scheme [5] that adds the blindness property. We show how to use our group blind signatures to construct an electronic cash system in which multiple banks can securely distribute anonymous and untraceable e-cash. Moreover, the identity of the e-cash issuing bank is concealed, which is conceptually novel. The space, time, and communication complexities of the relevant parameters and operations are independent of the group size.

Abstract. This paper considers the problem of password-authenticated key exchange (PAKE) in a client-server setting, where the server authenticates using a stored password file, and it is desirable to maintain some degree of security even if the server is compromised. A PAKE scheme is said to be resilient to server compromise if an adversary who compromises the server must at least perform an offline dictionary attack to gain any advantage in impersonating a client. (Of course, offline dictionary attacks should be infeasible in the absence of server compromise.) One can see that this is the best security possible, since by definition the password file has enough information to allow one to play the role of the server, and thus to verify passwords in an offline dictionary attack. While some previous PAKE schemes have been proven resilient to server compromise, there was no known general technique to take an arbitrary PAKE scheme and make it provably resilient to server compromise. This paper presents a practical technique for doing so which requires essentially one extra round of communication and one signature computation/verification. We prove security in the universal composability framework by (1) defining a new functionality for PAKE with resilience to server compromise, (2) specifying a protocol combining this technique with a (basic) PAKE functionality, and (3) proving (in the random oracle model) that this protocol securely realizes the new functionality.