Abstract. We introduce a short signature scheme based on the Computational Diffie-Hellman assumption on certain elliptic and hyper-elliptic curves. The signature length is half the size of a DSA signature for a similar level of security. Our short signature scheme is designed for systems where signatures are typed in by a human or signatures are sent over a low-bandwidth channel. 1

The notion of non-malleable cryptography, an extension of semantically secure cryptography, is defined. Informally, in the context of encryption the additional requirement is that given the ciphertext it is impossible to generate a different ciphertext so that the respective plaintexts are related. The same concept makes sense in the contexts of string commitment and zero-knowledge proofs of possession of knowledge. Non-malleable schemes for each of these three problems are presented. The schemes do not assume a trusted center; a user need not know anything about the number or identity of other system users. Our cryptosystem is the first proven to be secure against a strong type of chosen ciphertext attack proposed by Rackoff and Simon, in which the attacker knows the ciphertext she wishes to break and can query the decryption oracle on any ciphertext other than the target.

Let g be a primitive root modulo a prime p. It is proved that the triples (gx,gy,gxy), x,y�1,…,p�1, are uniformly distributed modulo p in the sense of H. Weyl. This result is based on the following upper bound for double exponential sums. Let ε�0 be fixed. Then p−� x,y=� exp0 2πiagx�bgy�cgxy

Proved here is the sufficiency of certain conditions to ensure the Elliptic Curve Digital Signature Algorithm (ECDSA) existentially unforgeable by adaptive chosen-message attacks. The sufficient conditions include (i) a uniformity property and collision-resistance for the underlying hash function, (ii) pseudo-randomness in the private key space for the ephemeral private key generator, (iii) generic treatment of the underlying group, and (iv) a further condition on how the ephemeral public keys are mapped into the private key space. For completeness, a brief survey of necessary security conditions is also given. Some of the necessary conditions are weaker than the corresponding sufficient conditions used in the security proofs here, but others are identical.

Verifiably encrypted signatures are used when Alice wants to sign a message for Bob but does not want Bob to possess her signature on the message until a later date. Such signatures are used in optimistic contact signing to provide fair exchange. Partially blind signature schemes are an extension of blind signature schemes that allows a signer to sign a partially blinded message that include preagreed information such as expiry date or collateral conditions in unblinded form. These signatures are used...

We investigate relations among the discrete logarithm (DL) problem, the Diffie-Hellman (DH) problem and the bilinear Diffie-Hellman (BDH) problem when we have an efficient computable non-degenerate bilinear map e : G G ! H. Under a certain assumption on the order of G, we show that the DH problem on H implies the DH problem on G, and both of them are equivalent to the BDH problem when e is weak-invertible. Moreover, we show that given the bilinear map e an injective homomorphism f : H ! G enables us to solve the DH problem on G eciently, which implies the non-existence a self-bilinear map e : G G ! G when the DH problem on G is hard. Finally we introduce a sequence of bilinear maps and its applications.

Abstract. Cheng and Uchiyama show that if one is given an elliptic curve, depending on a prime p, that is defined over a number field and has certain properties, then one can solve the Decision Diffie-Hellman Problem (DDHP) in F ∗ p in polynomial time. We show that it is unlikely that an elliptic curve with the desired properties exists. 1.