The transition to post-quantum cryptography has been an enormous challenge and effort for cryptographers over the last decade, with impressive results such as the future NIST standards. However, the latter has so far only considered central cryptographic mechanisms (signatures or KEM) and not more advanced ones, e.g., targeting privacy-preserving applications. Of particular interest is the family of solutions called blind signatures, group signatures and anonymous credentials, for which standards already exist, and which are deployed in billions of devices. Such a family does not have, at this stage, an efficient post-quantum counterpart although very recent works improved this state of affairs by offering two different alternatives: either one gets a system with rather large elements but a security proved under standard assumptions or one gets a more efficient system at the cost of ad-hoc interactive assumptions or weaker security models. Moreover, all these works have only considered size complexity without implementing the quite complex building blocks their systems are composed of. In other words, the practicality of such systems is still very hard to assess, which is a problem if one envisions a post-quantum transition for the corresponding systems/standards.

Anonymous credentials are cryptographic mechanisms enabling users to authenticate themselves with a fine-grained control on the information they leak in the process. They have been the topic of countless papers which have improved the performance of such mechanisms or proposed new schemes able to prove ever-more complex statements about the attributes certified by those credentials. However, although these papers have studied in depth the problem of the information leaked by the credential and/or the attributes, almost all of them have surprisingly overlooked the information one may infer from the knowledge of the credential issuer. In this paper we address this problem by showing how one can efficiently hide the actual issuer of a credential within a set of potential issuers. The novelty of our work is that we do not resort to zero-knowledge proofs but instead we show how one can tweak Pointcheval-Sanders signatures to achieve this issuer-hiding property in a compact way. This results in an efficient anonymous credential system that indeed provides a complete control of the information leaked in the authentication process. Our construction is moreover modular and can then fit a wide spectrum of applications, notably for Self-Sovereign Identity (SSI) systems.

EPID systems are anonymous authentication mechanisms which are standardized by ISO/IEC and massively deployed in Intel processors. They are related to the large family of privacy-preserving signatures but differ in that they provide a very pragmatic way of revoking members. Concretely, a member ℘ can be revoked by simply placing one of its signatures in a so-called signature revocation list SRL. Once this is done, every signer will have to include in its future signatures a proof that it has not generated any element of SRL, which implicitly revokes ℘.

Divisible e‐cash systems allow users to withdraw a unique coin of value 2 n units from a bank, but then to spend it in several times to distinct merchants. In such a system, whereas users want anonymity of their transactions, the bank wants to prevent, or at least detect, double‐spending, and trace defrauders. While this primitive was introduced two decades ago, quite a few (really) anonymous constructions have been proposed. In addition, all but one were just proven secure in the random oracle model, but still with either weak security models or quite complex settings and thus costly constructions. The unique proposal, secure in the standard model, appeared recently and is unpractical. As evidence, the authors left the construction of an efficient scheme secure in this model as an open problem. In this study, the authors answer it with the first efficient divisible e‐cash system secure in the standard model. It is based on a new way of building the coins, with a unique and public global tree structure for all the coins. Actually, they propose two constructions which offer a tradeoff between efficiency and security. They both achieve constant time for withdrawing and spending amounts of 2 ℓ units, while allowing the bank to quickly detect double‐spendings by a simple comparison of the serial numbers of deposited coins to the ones of previously spent coins.