Biometric systems are widely used for authentication and identification. The False Match Rate (FMR) quantifies the probability of matching a biometric template to a non-corresponding template and serves as an indicator of the system robustness against security threats. We analyze biometric systems through two main contributions. First, we study untargeted attacks, where an adversary aims to impersonate any user in the database. We compute the number of trials needed for a successful impersonation and derive the critical population size ( i.e., the maximum database size) and critical (FMR) required to maintain security against untargeted attacks as the database grows. Second, we address the biometric birthday problem, which quantifies the probability that there exists two distinct users that collide ( i.e., can impersonate each other). We compute approximate and exact probabilities of collision and derive the associated critical population size and critical (FMR) to bound the risk of biometric collisions, particularly in large-scale databases. These thresholds provide actionable insights for designing biometric systems that mitigate the risks of impersonation and biometric collisions, particularly in large-scale databases. Nevertheless, our findings show that current systems fail to meet the required security level against untargeted attacks, even in small databases, and face significant challenges with the biometric birthday problem as databases grow.

Cancelable biometric schemes aim at generating secure biometric templates by combining user specific tokens, such as password, stored secret or salt, along with biometric data. This type of transformation is constructed as a composition of a biometric transformation with a feature extraction algorithm. The security requirements of cancelable biometric schemes concern the irreversibility, unlinkability and revocability of templates, without losing in accuracy of comparison. While several schemes were recently attacked regarding these requirements, full reversibility of such a composition in order to produce colliding biometric characteristics, and specifically presentation attacks, were never demonstrated to the best of our knowledge. In this paper, we formalize these attacks for a traditional cancelable scheme with the help of integer linear programming (ILP) and quadratically constrained quadratic programming (QCQP). Solving these optimization problems allows an adversary to slightly alter its fingerprint image in order to impersonate any individual. Moreover, in an even more severe scenario, it is possible to simultaneously impersonate several individuals.

Biometric recognition encompasses two operating modes. The first one is biometric identification which consists in determining the identity of an individual based on her biometrics and requires browsing the entire database (i.e., a 1:N search). The other one is biometric authentication which corresponds to verifying claimed biometrics of an individual (i.e., a 1:1 search) to authenticate her, or grant her access to some services. The matching process is based on the similarities between a fresh and an enrolled biometric template. Considering the case of binary templates, we investigate how a highly populated database yields near-collisions, impacting the security of both the operating modes. Insight into the security of binary templates is given by establishing a lower bound on the size of templates and an upper bound on the size of a template database depending on security parameters. We provide efficient algorithms for partitioning a leaked template database in order to improve the generation of a master-template-set that can impersonates any enrolled user and possibly some future users. Practical impacts of proposed algorithms are finally emphasized with experimental studies.

Cancelable biometric schemes generate secure biometric templates by combining user specific tokens and biometric data. The main objective is to create irreversible, unlinkable, and revocable templates, with high accuracy of comparison. In this paper, we cryptanalyze two recent cancelable biometric schemes based on a particular locality sensitive hashing function, index-of-max (IoM): Gaussian Random Projection-IoM (GRP-IoM) and Uniformly Random Permutation-IoM (URP-IoM). As originally proposed, these schemes were claimed to be resistant against reversibility, authentication, and linkability attacks under the stolen token scenario. We propose several attacks against GRP-IoM and URP-IoM, and argue that both schemes are severely vulnerable against authentication and linkability attacks. We also propose better, but not yet practical, reversibility attacks against GRP-IoM. The correctness and practical impact of our attacks are verified over the same dataset provided by the authors of these two schemes.

Template protection is a crucial issue in biometrics. Many algorithms have been proposed in the literature among secure computing approaches, crypto-biometric algorithm and feature transformation schemes. The BioHashing algorithm belongs to this last category and has very interesting properties. Among them, we can cite its genericity since it could be applied on any biometric modality, the possible cancelability of the generated BioCode and its efficiency when the secret is not stolen by an impostor. Its main drawback is its weakness face to a combined attack (false acceptance with the stolen secret scenario). In this paper, we propose a transformation-based biometric template protection scheme as an improvement of the BioHashing algorithm where the projection matrix is generated by combining the secret and the biometric data. Experimental results on three biometric modalities, namely digital fingerprint, finger knuckle print and hands vein images, show the benefits of the proposed method face to attacks while keeping a good efficiency.

This paper focuses on parallel hash functions based on tree modes of operation for an inner Variable-Input-Length function. This inner function can be either a single-block-length (SBL) and prefix-free MD hash function, or a sponge-based hash function. We discuss the various forms of optimality that can be obtained when designing parallel hash functions based on trees where all leaves have the same depth. The first result is a scheme which optimizes the tree topology in order to decrease the running time. Then, without affecting the optimal running time we show that we can slightly change the corresponding tree topology so as to minimize the number of required processors as well. Consequently, the resulting scheme decreases in the first place the running time and in the second place the number of required processors.

This paper focus on incremental cryptographic schemes that solve the privacy problem introduced by Bellare, Goldreich and Goldwasser. To our knowledge, none of the schemes designed so far provide simultaneously strong privacy guarantees and byte-wise incremental operations. We propose a new method that extends a block-wise incremental cryptographic scheme into a fully byte-wise incremental one while keeping good performances. This one insures the property of perfect privacy with the same average overhead for both the size of the cryptographic form and the number of operations to perform when applying the conjugate algorithm.

We emphasize that future secure communicating systems, secured mass storages and access policies will require efficient and scalable security algorithms and protocols. More-over, parallelism will be used at quiet low level implementation of software or hardware basic mechanisms for offering efficient support to cryptographic algorithms. In this paper we concentrate on a family of generic schemes for efficient implementation of tree based hash functions. The main reason for designing a parallel algorithm based on a hash tree scheme is to obtain optimal performances when dealing with critical applications which can require tuned implementations for security aspects on multi-core target processors. Indeed, parallelism for cryptographic primitives has become a mandatory feature as imposed also by recent NIST requirements.