Signature numérique | EPFL Graph Search

La signature numérique est un mécanisme permettant d'authentifier l'auteur d'un document électronique et d'en garantir la non-répudiation, par analogie avec la signature manuscrite d'un document papier. Elle se différencie de la signature écrite par le fait qu'elle n'est pas visuelle, mais correspond à une suite de caractères. Elle ne doit pas être confondue avec la signature électronique manuscrite. Fonctions de la signature Un mécanisme de signature numérique doit présenter les propriétés suivantes :

Il doit permettre au lecteur d'un document d'identifier la personne ou l'organisme qui a apposé sa signature (propriété d'identification).
Il doit garantir que le document n'a pas été altéré entre l'instant où l'auteur l'a signé et le moment où le lecteur le consulte (propriété d'intégrité).

Pour cela, les conditions suivantes doivent être réunies :

authentique : l'identité du signataire doit pouvoir être retrouvée de manière certaine ;
infalsifiable : la signatur

Short undeniable signatures

Jean Monnerat

Digital signatures are one of the main achievements of public-key cryptography and constitute a fundamental tool to ensure data authentication. Although their universal verifiability has the advantage to facilitate their verification by the recipient, this property may have undesirable consequences when dealing with sensitive and private information. Motivated by such considerations, undeniable signatures, whose verification requires the cooperation of the signer in an interactive way, were invented. This thesis is mainly devoted to the design and analysis of short undeniable signatures. Exploiting their online property, we can achieve signatures with a fully scalable size depending on the security requirements. To this end, we develop a general framework based on the interpolation of group elements by a group homomorphism, leading to the design of a generic undeniable signature scheme. On the one hand, this paradigm allows to consider some previous undeniable signature schemes in a unified setting. On the other hand, by selecting group homomorphisms with a small group range, we obtain very short signatures. After providing theoretical results related to the interpolation of group homomorphisms, we develop some interactive proofs in which the prover convinces a verifier of the interpolation (resp. non-interpolation) of some given points by a group homomorphism which he keeps secret. Based on these protocols, we devise our new undeniable signature scheme and prove its security in a formal way. We theoretically analyze the special class of group characters on Z*n. After studying algorithmic aspects of the homomorphism evaluation, we compare the efficiency of different homomorphisms and show that the Legendre symbol leads to the fastest signature generation. We investigate potential applications based on the specific properties of our signature scheme. Finally, in a topic closely related to undeniable signatures, we revisit the designated confirmer signature of Chaum and formally prove the security of a generalized version.

EPFL2006

State of the art in electronic payment systems

Philippe Janson

The exchange of goods conducted face-to-face between two parties dates back to before the beginning of recorded history. Traditional means of payment have always had security problems, but now electronic payments retain the same drawbacks and add some risks. Unlike paper, digital `documents' can be copied perfectly and arbitrarily often; digital signatures can be produced by anybody who knows the secret cryptographic key; a buyer's name can be associated with every payment, eliminating the anonymity of cash. Without new security measures, wide-spread electronic commerce is not viable. On the other hand, properly designed electronic payment systems can actually provide better security than traditional means of payments, in addition to flexibility. This article provides an overview of electronic payment systems, focusing on issues related to security.

1997

About Machine-Readable Travel Documents

Jean Monnerat, Serge Vaudenay, Martin Vuagnoux

Passports are now equipped with RFID chips that contain private information, biometric data, and a digital signature by issuing authorities. We review most of applicable security and privacy issues. We argue that the main privacy issue is not unauthorized access through radio channel or data skimming as claimed before, but rather the leakage of a digital signature by government authorities for private data. To fix this, we rather need the e-passport to prove the knowledge of a valid signature in a non-transferable way. Besides, several identification protocols such as GPS identification in RFID could lead to challenge semantics attacks that are privacy threats. To fix this, we also need some kind of non-transferability. In 2003, Steinfeld et al. proposed the universal designated-verifier signature (UDVS) primitive. Its drawback is in demanding verifiers to have public keys authenticated by the passport. One compromise was proposed by Baek et al. with the UDVSP primitive. We show that UDVSP does not provide non-transferability and fix it by using zero-knowledge proof of knowledge. We propose a simple method to protect Sigma-protocols against offline Mafia fraud and challenge semantics. We apply this by proposing a simple protocol based on Guillou-Quisquater identification that only requires two RSA computations and would substantially enhance the privacy of the e-passport bearer.

Springer2007

Short undeniable signatures

Jean Monnerat

EPFL2006