Some Digital Signature Schemes with Collective Signers

Digital signatures are one of the most important consequences of the appearance of public key cryptography, in 1976. These schemes provide authentication, integrity and non-repudiation to digital communications. Some extensions or variations of the concept of digital signature have been introduced, and many specific realizations of these new types of signature schemes have been proposed. In this thesis, we deal with the basic definitions and required security properties of traditional signature schemes and two of its extensions: distributed signature schemes and ring signature schemes. We review the state of the art in these two topics; then we propose and analyze new specific schemes for different scenarios. Namely, we first study distributed signature schemes for general access structures, based on RSA; then we show that such schemes can be used to construct other cryptographic protocols: distributed key distribution schemes and metering schemes. With respect to ring signatures, we propose schemes for both a scenario where the keys are of the Discrete Logarithm type and a scenario where the public keys of users are inferred from their personal identities. Finally, we also propose some distributed ring signature schemes, a kind of schemes which combine the concepts of distributed signatures and ring signatures. We formally prove the security of all these proposals, assuming that some mathematical problems are hard to solve. Specifically, we base the security of our schemes in the hardness of either the RSA problem, or the Discrete Logarithm problem, or the Computational Diffie-Hellman problem.