Najwa Aaraj

The increasing concern for identity confidentiality in the Smart City scenario has fostered research on privacy-preserving authentication based on pseudonymization. Pseudonym systems enable citizens to generate pseudo-identities and establish unlinkable anonymous accounts in cloud service providers. The citizen's identity is concealed, and his/her different anonymous accounts cannot be linked to each other. Unfortunately, current pseudonym systems require a trusted certification authority (CA) to issue the cryptographic components (e.g. credentials, secret keys, or pseudonyms) to citizens. This CA, generally a Smart City governmental entity, has the capability to grant or revoke privacy rights at will, hence posing a serious threat in case of corruption. Additionally, if the pseudonym system enables de-anonymization of misusers, a corrupted CA can jeopardize the citizens' privacy. This paper presents a novel approach to construct a pseudonym system without a trusted issuer. The CA is emulated by a set of Smart City service providers by means of secure multi-party computation (MPC), which circumvents the requirement of assuming an honest CA. The paper provides a full description of the system, which integrates an MPC protocol and a pseudonym-based signature scheme. The system has been implemented and tested.

Polynomial systems over the binary field have important applications, especially in symmetric and asymmetric cryptanalysis, multivariate-based postquantum cryptography, coding theory, and computer algebra. In this paper, we study the quantum annealing model for solving Boolean systems of multivariate equations of degree 2, usually referred to as the multivariate quadratic problem. We present different methodologies to embed the problem into a Hamiltonian that can be solved by available quantum annealing platforms. In particular, we provide three embedding options, and we highlight their differences in terms of quantum resources. Moreover, we design a machine-agnostic algorithm that adopts an iterative approach to better solve the problem Hamiltonian by repeatedly reducing the search space. Finally, we use D-Wave devices to successfully implement our methodologies on several instances of the multivariate quadratic problem.