Since the dawn of human civilisation, information has been one of our most treasured assets. In modern societies, people store and exchanged information in digital form: we ‘talk’ in chats and save important data in the cloud. However, how can we be really sure that the person we are talking to is who we think they are, or that the messages we send will not be modified before they reach the destination?
Cryptography is a branch of computer science that aims at developing techniques for communicating and storing information securely. One of the goals of cryptography is authentication of data and users. To give an example, user authentication is what makes your car unlock only when pressing the unlock button on your car key, and not someone else’s. Data authentication relies on advanced mathematical mechanisms that extract fingerprint-like information from digital data and can tell whether the messages we receive have been manipulated or are authentic.
Cryptographic authentication is used everywhere: in our smart phones and credit cards, for online banking and e-commerce shopping, and at automatised border controls. This thesis has the objective of developing tools for data and user authentication. Concretely, it contains state-of-the art solutions that are secure even when an attacker tries to tamper with the authentication process. The application scenarios considered in this work are collaborative ones, that is, processes that involve multiple entities. For example, this thesis contains constructions that can be used by a team of researchers to perform statistics on data produced by different team members in such a way that anyone can check the correctness of the statistical results, and no one can add fake measurements or replace someone else’s.