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Questions de Sécurité et de Vie Privée autour des Protocoles d’Identification de Personnes et d’Objets Bruno Kindarji To cite this version: Bruno Kindarji. Questions de Sécurité et de Vie Privée autour des Protocoles d’Identification de Personnes et d’Objets. domain_other. Télécom ParisTech, 2010. Français. pastel-00006233 HAL Id: pastel-00006233 https://pastel.archives-ouvertes.fr/pastel-00006233 Submitted on 8 Jul 2010 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. i ii Those who would give up Essential Liberty to purchase a little Temporary Safety, deserve neither Liberty nor Safety. - - Benjamin Franklin Foreword The context. Identification of people, devices, or patterns is a very broad subject, on which many research efforts are being deployed. By identification, we mean auto- matic identification by means of algorithms, protocols, making use of sensors and computers. In other words, the identification is a procedure in which the subject who needs to be identified does not directly provide its identity. This definition, intently vague and general, can be applied to many real-life situa- tions. For example, a night guard at a military facility can recognize the high ranking officers who are granted access to the building. However, should there be a high turnover of either the officers or the night guards, then this security measure becomes inefficient. The officer now needs to present an identifying element in order to gain access. This is the classical step to switch from a situation where the security is based on who I am to a situation where what matters is either what I know or what I have. The advantage of the first choice is that there is no need to carry a burdening equipment to enter; however, (classical) knowledge is transmittable, while a well thought design can make reproducibility of security elements hard or impossible. About Biometrics and Privacy The first research axis of this manuscript is to revert to what is commonly called the third factor (what we are) even though we saw that this is actually the first factor. The global research effort on automatic identification is justified by the need to make a safer use of a whole range of technologies that were invented these last decades. From physical threats against strategic targets, to privacy risks when using a specific virtual service, the range of real-life menaces to take into account is wide. While the protection against physical violence is mostly oriented towards the verification of everyone’s identity, it also requires everyone to disclose as much information as possible. However, more disclosure also means less privacy. iii iv Privacy is the ability of an individual to seclude information about himself from the incursions of the others [188]. Defining this property is not an easy task for scientists; however some attempts were made in this direction. Once this was done, achieving privacy, while ensuring the maximal security and the minimal annoyance for the user, is a major scientific challenge. The solution that we naturally considered for this purpose is the use of biometrics as a means of identification. Biometrics are fascinating elements of identification embedded in the human body; using them properly can lead to impressive levels of precision and accuracy. However, adding biometrics to the privacy equation raises even more challenges, as these identifying elements lead to identity theft, or more simply, disclosure of someone’s habits. To prevent these risks, few solutions existed at the beginning of this thesis. A large part of this work consists in studying the implications of using biometrics in security protocol. In particular, the storage and transmission of biometric templates must be handled with specific precautions. Existing Solutions for Template Protection We consider in this work that it is possible for the transmission of biometric templates to be made as se- cure as any transmission, as long as the adequate cryptographic infrastructure is chosen. The first part of this document studies the state-of-the-art solutions for template storage, which is the Achilles’ heel of biometric systems. In par- ticular, solutions such as Secure Sketches [27, 28] and Cancelable Biometrics [31, 32] are considered, in order to evaluate how good the associated algo- rithms behave on real-life data, and also how strong the underlying security is. We explicitly exhibit limitations that are inherent to these methods. Propositions for Secure Biometric Identification In the second part of this document, we propose new methods for using biometrics in a secure setting, and more specifically, in the context of biometric identification. We propose a new cryptographic primitive, called Error-Tolerant Searchable En- cryption [33], which is a generalization of Searchable Encryption. The appli- cation of Error-Tolerant Searchable Encryption to biometric data can lead to protocols that enable (secure-) biometric identification [34]. This aforementioned primitive was designed in the spirit of public-key cryptography. However, in order to preserve as much privacy as possible, the cryptographic requirements that were raised led to expensive computation on the database. In particular, the cryptographic operations that need to be done must be linear in the number of enrolled people; if this number is about the size of a national population, the computations are already too costly to be practical. Another line of work to solve this issue consists in applying symmetric cryptography, and different data structures for the storage [2]. The security properties that can be derived are different, and somewhat v less protective. Nevertheless, the efficiency of this scheme is much greater, which makes that scheme interesting to consider. Finally, we show how it is possible to investigate different strategies for biometrics by changing models. One example is to apply time-dependent functions to biometric templates [32]; if the function family is well chosen, then this can lead to an interesting application that we call Anonymous Iden- tification. Another example is to dedicate biometric hardware to improve the accuracy of a biometric system; with a specific Match-on-Card technology, we are able to deploy a secure identification scheme that requires limited crypto- graphic requirements, and proves to be very efficient [30]. Solutions for Wireless Communication and Device Identification The second research axis that motivated our study was the enforcement of privacy while being in a secure setting. Modern cryptography’s characteristic is to provide computing primitives that enable people to transmit information in a secure way, and this can be both for military and civilian purposes. This thesis makes explicit several situations where security and privacy are desirable, and sometimes achievable. The link with identification will be made clear at that time. Minimal-cost Identification Protocols Setting aside the noisy character of biometrics, one can ask the question of the overall complexity of identifi- cation protocols. In the case of noisy data, the overall performance can be expressed by the amount of data that did not perform well. With exact data, we can focus on the communication cost of identification protocols. We stud- ied the costs of identification protocols [48], and most especially the League Problem: how many bits must me transmitted from one partner to the other, when there is prior – but non-shared – information on the data. We then showed that it is possible to outperform the optimal solutions if we allow a small error-probability. This involves several techniques, such as deploying Identification Codes, a barely known though interesting coding primitive. Private Interrogation of Devices Finally, identification codes can be used in a very different way, in order to beckon an element from a set of low-cost wireless devices. Indeed, there are situations in real-life where the question is not “who is in front of me?”, but rather “Is Alice somewhere in the neighbourhood?”. For that purpose, we deploy some identification codes, and we also state the cryptographic conditions for the privacy of these elements[35]. Here, the security rests on the unique ability of a wireless sensor to know the identity of the element interrogated; we show that a construction using coding theory over finite fields can be simple yet efficient. vi In order to evaluate the security of that scheme, we use a common com- putational assumption, known as the Polynomial Reconstruction Problem. In order to test further the strength of this assumption in this context, we go further and look for the decoding possibility of Maximum-Distance Separable codes – such as Reed-Solomon codes. That lead us to the study of the thresh- old of these codes, closely related to their list-decoding capacity. After showing that the behaviour of q-ary codes around their threshold is the same as binary codes, we show how to explicitly estimate the threshold, asymptotically and for codes of finite yet reasonable length. Overview This work investigates several means for achieving identification. Authenti- cation is usually preferred to identification because the paradigm is easier to accomplish; however, we discovered during the course of research that there can be efficient ways to deal with the difficulties of identification. The main issue that is still to be improved is the error rate that is likely to arise as soon as probabilistic methods are used, be it for biometric applications, or for exact data that use probabilistic algorithms.

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