Alex Escala

Revocable attribute-based signatures with adaptive security in the standard model

An attribute-based signature with respect to a signing policy, chosen ad-hoc by the signer, convinces the verifier that the signer holds a subset of attributes satisfying that signing policy. The verifier must obtain no other information about the identity of the signer or the attributes he holds. This primitive has many applications in real scenarios requiring both authentication and anonymity/privacy properties. We propose in this paper the first attribute-based signature scheme satisfying at the same time the following properties: (1) it admits general signing policies, (2) it is proved secure against fully adaptive adversaries, in the standard model, and (3) the number of elements in a signature depends only on the size of the signing policy. Furthermore, our scheme enjoys the additional property of revocability: an external judge can break the anonymity of a signature, when necessary. This property may be very interesting in real applications where authorities are unwilling to allow full anonymity of users.

Fine-Tuning Groth-Sahai Proofs

Groth-Sahai proofs are efficient non-interactive zero-knowledge proofs that have found widespread use in pairing-based cryptography. We propose efficiency improvements of Groth-Sahai proofs in the SXDH setting, which is the one that yields the most efficient non-interactive zero-knowledge proofs. We replace some of the commitments with ElGamal encryptions, which reduces the provers computation and for some types of equations reduces the proof size.Groth-Sahai proofs are zero-knowledge when no public elements are paired to each other. We observe that they are also zero-knowledge when base elements for the groups are paired to public constants.The provers computation can be reduced by letting her pick her own common reference string. By giving a proof she has picked a valid common reference string this does not compromise soundness.We define a type-based commit-and-prove scheme, which allows commitments to be reused in many different proofs.

An Algebraic Framework for Diffie---Hellman Assumptions

We put forward a new algebraic framework to generalize and analyze Diffie–Hellman like decisional assumptions which allows us to argue about security and applications by considering only algebraic properties. Our

Vote Validatability in Mix-Net-Based eVoting

\mathcal {D}_{\ell ,k}\text{- }\textsf {MDDH}

Identity-Based Lossy Trapdoor Functions: New Definitions, Hierarchical Extensions, and Implications

Dℓ,k-MDDH Assumption states that it is hard to decide whether a vector in

Hierarchical Identity-Based Lossy Trapdoor Functions

\mathbb {G}^\ell