Sécurité sémantique

Résumé

La sécurité sémantique est une notion de sécurité importante dans le cadre des preuves de sécurité des protocoles cryptographiques. Cette notion a été introduite en 1984 par Shafi Goldwasser et Silvio Micali. Elle est définie indépendamment du type de cryptographie du système (c’est-à-dire symétrique ou asymétrique), mais est principalement utilisée dans les preuves des schémas à clef publique. La sécurité sémantique traduit formellement le fait qu’il doit être difficile de retrouver de l’information sur le message originel en ayant accès au chiffrement de ce message et aux informations publiques du protocole. Définition Pour atteindre la sécurité sémantique, n’importe quel adversaire ne peut dériver d'information sur un message chiffré sans connaître la clef privée. Cela se traduit par la notion d’indistinguabilité, c’est-à-dire par l’incapacité, étant donné deux messages et un chiffré d’un des deux messages, à différencier lequel des deux messages a été chiffré. En eff

Source officielle

https://fr.wikipedia.org/wiki/S%C3%A9curit%C3%A9_s%C3%A9mantique

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Le cryptosystème d'ElGamal, ou chiffrement El Gamal (ou encore système d'El Gamal) est un protocole de cryptographie asymétrique inventé par Taher Elgamal en 1984 et construit à partir du problème du

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A chosen-ciphertext attack (CCA) is an attack model for cryptanalysis where the cryptanalyst can gather information by obtaining the decryptions of chosen ciphertexts. From these pieces of informatio

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TCHo: a Hardware-Oriented Trapdoor Cipher

Jean-Philippe Aumasson, Willi Meier, Serge Vaudenay

This paper improves the Finiasz-Vaudenay construction of TCHo, a hardware-oriented public-key cryptosystem, whose security relies in the hardness of finding a low-weight multiple of a given polynomial, and on the decoding of certain noisy cyclic linear codes. Our improvement makes it possible to decrypt in polynomial time (instead of exponential time), to directly prove semantic security (instead of one-wayness), and to achieve pretty good asymptotic performances. We further build IND-CCA secure schemes using the KEM/DEM and Fujisaki-Okamoto hybrid encryption frameworks in the random oracle model. This can encrypt an arbitrary message with an overhead of about 5 Kb in less than 15 ms, on an ASIC of about 10000 gates at 4 MHz.

Springer2007

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In our daily lives, people or devices frequently need to learn their location for many reasons as some services depend on the absolute location or the proximity. The outcomes of positioning systems can have critical effects e.g., on military, emergency. Thus, the security of these systems is quite important. In this thesis, we concentrate on many security aspects of position in cryptography. The first part of this thesis focuses on the theory of distance bounding. A distance bounding protocol is a two-party authentication protocol between a prover and a verifier which considers the distance of the prover as a part of his/her credential. It aims to defeat threats by malicious provers who try to convince that they are closer to the verifier or adversaries which seek to impersonate a far-away prover. In this direction, we first study the optimal security bounds that a distance bounding protocol can achieve. We consider the optimal security bounds when we add some random delays in the distance computation and let the prover involve distance computation. Then, we focus on solving the efficiency problem of public-key distance bounding because the public-key cryptography requires much more computations than the symmetric-key cryptography. We construct two generic protocols (one without privacy, one with) which require fewer computations on the prover side compared to the existing protocols while keeping the highest security level. Then, we describe a new security model involving a tamper-resistant hardware. This model is called the secure hardware model (SHM). We define an all-in-one security model which covers all the threats of distance bounding and an appropriate privacy notion for SHM. The second part of this thesis is to fill the gap between the distance bounding and its real-world applications. We first consider contactless access control. We define an integrated security and privacy model for access control using distance bounding (DB) to defeat relay attacks. We show how a secure DB protocol can be converted to a secure contactless access control protocol. Regarding privacy (i.e., keeping anonymity in a strong sense to an active adversary), we show that the conversion does not always preserve privacy, but it is possible to study it on a case by case basis. Then, we consider contactless payment systems. We design an adversarial model and define formally the contactless payment security against malicious cards and malicious terminals. Accordingly, we design a contactless payment protocol and show its security in our security model. The last part of this thesis focuses on positioning. We consider two problems related to positioning systems: localization and proof of location. In localization, a user aims to find its position by using a wireless network. In proof of location, a user wants to prove his/her position e.g., to have access to a system or authorize itself. We first formally define the problem of localization and construct a formal security model. We describe algorithms and protocols for localization which are secure in our model. Proof of location has been considered formally by Chandran et al. in CRYPTO 2009 and it was proved that achieving security is not possible in the vanilla model. By integrating the localization and the secure hardware model, we obtain a model where we can achieve proof of location.

EPFL2018

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HELEN: a Public-key Cryptosystem Based on the LPN Problem (Extended Abstract)

Alexandre Raphaël Duc, Serge Vaudenay

We propose HELEN, a new code-based public-key cryptosystem whose security is based on the hardness of the Learning from Parity with Noise problem~(LPN) and the decisional minimum distance problem. We show that the resulting cryptosystem achieves indistinguishability under chosen plaintext attacks (IND-CPA security). Using the Fujisaki-Okamoto generic construction, HELEN achieves IND-CCA security in the random oracle model. We further propose concrete parameters.

2012

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EPFL2018