Olivier Markowitch

An intensive survey of fair non-repudiation protocols

With the phenomenal growth of the Internet and open networks in general, security services, such as non-repudiation, become crucial to many applications. Non-repudiation services must ensure that when Alice sends some information to Bob over a network, neither Alice nor Bob can deny having participated in a part or the whole of this communication. Therefore a fair non-repudiation protocol has to generate non-repudiation of origin evidences intended to Bob, and non-repudiation of receipt evidences destined to Alice. In this paper, we clearly define the properties a fair non-repudiation protocol must respect, and give a survey of the most important non-repudiation protocols without and with trusted third party (TTP). For the later ones we discuss the evolution of the TTPs involvement and, between others, describe the most recent protocol using a transparent TTP. We also discuss some ad-hoc problems related to the management of non-repudiation evidences.

An Efficient Strong Designated Verifier Signature Scheme

This paper proposes a designated verifier signature scheme based on the Schnorr signature and the Zheng signcryption schemes. One of the advantages of the new scheme compared with all previously proposed schemes is that it achieves the “strong designated verifier” property without encrypting any part of the signatures. This is because the designated verifier’s secret key is involved in the verification phase. Another advantage of the proposed scheme is the low communication and computational cost. Generating a signature requires only one modular exponentiation, while this amount is two for the verification. Also, a signature in our scheme is more than four times shorter than those of known designated verifier schemes.

Optimistic Fair Exchange with Transparent Signature Recovery

We propose a new protocol allowing the exchange of an item against a signature while assuring fairness. The proposed protocol, based on the Girault-Poupard-Stern signature scheme (a variation of the Schnorr scheme), assumes the existence of a trusted third party that, except in the setup phase, is involved in the protocol only when one of the parties does not follow the designated protocol or some technical problem occurs during the execution of the protocol. The interesting feature of the protocol is the low communication and computational charges required by the parties. Moreover, in case of problems during the main protocol, the trusted third party can derive the same digital signature as the one transmitted in a faultless case, rather than an affidavit or an official certificate.

An Optimistic Non-repudiation Protocol with Transparent Trusted Third Party

In this paper we consider a new and efficient optimistic nonrepudiation protocol. In a non-repudiation protocol, during which Alice wants to transmit a message to Bob, Alice has to send a nonrepudiation of origin evidence to Bob (attesting that Alice is at the origin of the transmitted message), and Bob has to send a non-repudiation of receipt evidence to Alice (attesting Bobs receipt of the message). Classical solutions propose to use a trusted third party to help realizing the exchange without giving any significant advantage to one of the two parties. In an optimistic protocol, the trusted third party intervenes only in case of problems during the communication between Alice and Bob. Classically, in a situation where an error occurs, evidences that have been digitally signed by the TTP are issued. Although these evidences are distinct from those produced by Alice and Bob in a faultless case, they have the same value in case of a dispute. In this paper we propose a protocol where the TTP produces the same evidences that Alice and Bob should have produced in a faultless protocol execution (this prevents, after a succesful protocol execution, to determine whether the TTP was involved or not).

A Multi-party Optimistic Non-repudiation Protocol

In this paper we consider the optimistic approach of the non-repudiation protocols. We study a non-repudiation protocol with off-line trusted third party and we keep on with the definition of the multi-party non-repudiation, compare it to multi-party fair exchange and show some fundamental differences between these two problems. Finally, we generalize our protocol and propose a multi-party nonrepudiation protocol with off-line trusted third party.

A Multi-Party Non-Repudiation Protocol

We present a multi-party non-repudiation protocol, based on a group encryption scheme. We define multi-party non-repudiation, compare it to multi-party fair exchange and show some fundamental differences between these two problems. This is the first effort to generalize non-repudiation to the multi-party case. Our definitions and the resulting protocol are more general than the ones given in the only comparable work, a multi-party certified mail protocol. Finally we also give an example of a group encryption scheme that can be used with our protocol.

On fairness in exchange protocols

The aim of this paper is to give an overview of the most classical definitions of fairness in exchange protocols. We show the evolution of the definition, while putting forward that certain definitions are rather vague or too specialized. We propose a structured and generalized definition of fairness and of the security of exchange protocols.

Selective Receipt in Certified E-mail

Traditional pen and paper transactions are becoming more and more replaced by equivalent electronic services. Therefore electronic e-mail should also provide enhanced services as those provided by traditional mail. In this paper we present new optimistic protocols for certified e-mail. The major contribution of our paper is the definition of a new property, specific to certified no author-based selective receipt. This property requires that once the identity of the author of the mail is known, the receipt can not be refused any more. We present two certified email protocols respecting this property.

Power analysis attack: an approach based on machine learning

In cryptography, a side-channel attack is any attack based on the analysis of measurements related to the physical implementation of a cryptosystem. Nowadays, the possibility of collecting a large amount of observations paves the way to the adoption of machine learning techniques, i.e., techniques able to extract information and patterns from large datasets. The use of statistical techniques for side-channel attacks is not new. Techniques like the template attack have shown their effectiveness in recent years. However, these techniques rely on parametric assumptions and are often limited to small dimensionality settings, which limit their range of application. This paper explores the use of machine learning techniques to relax such assumptions and to deal with high dimensional feature vectors.

A new key-insulated signature scheme

In this paper we propose a new strong and perfectly key-insulated signature scheme, more efficient than previous proposals and whose key length is constant and independent of the number of insulated time periods. Moreover, unlike previous schemes, it becomes forward-secure when all the existing secrets at a given time period are compromised. We also present a variant forward-secure scheme in which an adversary needs to compromise a user at a second time period before being able to compute future secret keys.