A modular approach for cryptographic protocols leads to a simple design but often inefficient constructions. On the other hand, ad hoc constructions may yield efficient protocols at the cost of losing conceptual simplicity. We suggest structure-preserving commitments and signatures to overcome this dilemma and provide a way to construct modular protocols with reasonable efficiency, while retaining conceptual simplicity. We focus on schemes in bilinear groups that preserve parts of the group structure, which makes it easy to combine them with other primitives such as non-interactive zero-knowledge proofs for bilinear groups. We say that a signature scheme is structure-preserving if its verification keys, signatures, and messages are elements in a bilinear group, and the verification equation is a conjunction of pairing-product equations. If moreover the verification keys lie in the message space, we call them automorphic. We present several efficient instantiations of automorphic and structure-preserving signatures, enjoying various other additional properties, such as simulatability. Among many applications, we give three examples: adaptively secure round-optimal blind signature schemes, a group signature scheme with efficient concurrent join, and an efficient instantiation of anonymous proxy signatures. A further contribution is homomorphic trapdoor commitments to group elements which are also length reducing. In contrast, the messages of previous homomorphic trapdoor commitment schemes are exponents.

Abstract. We propose a new methodology for rational secret sharing leading to various instantiations (in both the two-party and multi-party settings) that are simple and efficient in terms of computation, share size, and round complexity. Our protocols do not require physical assumptions or simultaneous channels, and can even be run over asynchronous, pointto-point networks.We also propose new equilibrium notions (namely, computational versions of strict Nash equilibrium and stability with respect to trembles) and prove that our protocols satisfy them. These notions guarantee, roughly speaking, that at each point in the protocol there is a unique legal message a party can send. This, in turn, ensures that protocol messages cannot be used as subliminal channels, something achieved in prior work only by making strong assumptions on the communication network.

No abstract

No abstract

We introduce policy-based signatures (PBS), where a signer can only sign messages conforming to some authority-specified policy. The main requirements are unforgeability and privacy, the latter meaning that signatures not reveal the policy. PBS offers value along two fronts: (1) On the practical side, they allow a corporation to control what messages its employees can sign under the corporate key. (2) On the theoretical side, they unify existing work, capturing other forms of signatures as special cases or allowing them to be easily built. Our work focuses on definitions of PBS, proofs that this challenging primitive is realizable for arbitrary policies, efficient constructions for specific policies, and a few representative applications.

No abstract

This paper is motivated by the observation that existing security models for Direct Anonymous Attestation (DAA) have problems to the extent that insecure protocols may be deemed secure when analysed under these models. This is particularly disturbing as DAA is one of the few complex cryptographic protocols resulting from recent theoretical advances actually deployed in real life. Moreover, standardisation bodies are currently looking into designing the next generation of such protocols.Our first contribution is to identify issues in existing models for DAA and explain how these errors allow for proving security of insecure protocols. These issues are exhibited in all deployed and proposed DAA protocols (although they can often be easily fixed).Our second contribution is a new security model for a class of "pre-DAA scheme", i.e., DAA schemes where the computation on the user side takes place entirely on the trusted platform. Our model captures more accurately than any previous model the security properties demanded from DAA by the Trusted Computing Group (TCG), the group that maintains the DAA standard. Extending the model from pre-DAA to full DAA is only a matter of refining the trust models on the parties involved.Finally, we present a generic construction of a DAA protocol from new building blocks tailored for anonymous attestation. Some of them are new variations on established ideas, and may be of independent interest. We give instantiations for these building blocks that yield a DAA scheme more efficient than the one currently deployed, and as efficient as the one about to be standardised by the TCG which has no valid security proof.

Abstract. Randomizable encryption allows anyone to transform a ciphertext into a fresh ciphertext of the same message. Analogously, a randomizable signature can be transformed into a new signature on the same message. We combine randomizable encryption and signatures to a new primitive as follows: given a signature on a ciphertext, anyone, knowing neither the signing key nor the encrypted message, can randomize the ciphertext and adapt the signature to the fresh encryption, thus maintaining public verifiability. Moreover, given the decryption key and a signature on a ciphertext, one can compute ("extract") a signature on the encrypted plaintext. As adapting a signature to a randomized encryption contradicts the standard notion of unforgeability, we introduce a weaker notion stating that no adversary can, after querying signatures on ciphertexts of its choice, output a signature on an encryption of a new message. This is reasonable since, due to extractability, a signature on an encrypted message can be interpreted as an encrypted signature on the message.Using Groth-Sahai proofs and Waters signatures, we give several instantiations of our primitive and prove them secure under classical assumptions in the standard model and the CRS setting. As an application, we show how to construct an efficient non-interactive receipt-free universally verifiable e-voting scheme. In such a scheme a voter cannot prove what his vote was, which precludes vote selling. Besides, our primitive also yields an efficient round-optimal blind signature scheme based on standard assumptions, and namely for the classical Waters signature.