Degree Name

Doctor of Philosophy


School of Computer Science and Software Engineering - Faculty of Informatics


In this thesis, we make contributions to secure and privacy preserving use of electronic credentials in three different levels.

First, we address the case in credential systems where a credential owner wants to show her credential to a verifier without taking the risk that the ability to prove ownership of her credential is transferred to the verifier. We define credential ownership proof protocols for credentials signed by standard signature schemes. We also propose proper security definitions for the protocol, aiming to protect the security of both the credential issuer and the credential owner against concurrent attacks. We give two generic constructions of credential ownership proofs based on identity-based encryption and identity based identification schemes. Furthermore, we show that signatures with credential ownership proofs are equivalent to identity-based identification schemes, in the sense that any secure construction of each implies a secure construction of the other. Moreover, we show that the GQ identification protocol yields an efficient credential ownership proof for credentials signed by the RSA signature scheme and prove the protocol concurrently-secure.

Then, we give a generic construction for universal (mutli) designated-verifier signature schemes from a large class of signature schemes, referred to as Class C. The resulting schemes are efficient and have two important properties. Firstly, they are provably DV-unforgeable, non-transferable and also non-delegatable. Secondly, the signer and the designated verifier can independently choose their cryptographic settings. We also propose a generic construction for (hierarchical) identity-based signature schemes from any signature scheme in C and prove that the construction is secure against adaptive chosen message and identity attacks. We discuss possible extensions of our constructions to identity-based ring signatures and identity-based designated-verifier signatures from any signature in C. Furthermore, we show that it is possible to combine the above constructions to obtain signatures with combined functionalities.

Finally, inspired by the recent developments in attribute-based encryption, we propose threshold attribute-based signatures (t-ABS). In a t-ABS, signers are associated with a set of attributes and verification of a signed document against a verification attribute set succeeds if the signer has a threshold number of (at least t) attributes in common with the verification attribute set. A t-ABS scheme enables a signature holder to prove possession of signatures by revealing only the relevant (to the verification attribute set) attributes of the signer, hence providing signer-attribute privacy for the signature holder. We define t-ABS schemes, formalize their security and propose two t-ABS schemes: a basic scheme secure against selective forgery and a second one secure against existential forgery, both provable in the standard model, assuming hardness of the computational Diffie-Hellman problem. We show that our basic t-ABS scheme can be augmented with two extra protocols that are used for efficiently issuing and verifying t-ABS signatures on committed values. We call the augmented scheme a threshold attribute based c-signature scheme (t-ABCS). We show how a t-ABCS scheme can be used to realize a secure threshold attribute-based anonymous credential system (t-ABACS) providing signer-attribute privacy. We propose a security model for t-ABACS and give a concrete scheme using t-ABCS scheme. Using the simulation paradigm, we prove that the credential system is secure if the t-ABCS scheme is secure.

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