Christian Cachin

TCS Journal 2025 Journal Article

Synergistic knowledge

• Christian Cachin
• David Lehnherr
• Thomas Studer

Simplicial complexes are a successful model for distributed computing. They have recently been observed to provide an interesting model for epistemic multi-agent logic where the agents' local states are the main building blocks (instead of the global states). A natural generalization is to study epistemic logic on semi-simplicial sets. However, finding the appropriate modal logic for semi-simplicial models has been an open question. We answer this by introducing the logic of synergistic knowledge and establishing its soundness and completeness.

I&C Journal 2018 Journal Article

Verifying the consistency of remote untrusted services with conflict-free operations

• Christian Cachin
• Olga Ohrimenko

A group of mutually trusting clients outsources a computation to a remote server, which they do not fully trust and that may be subject to attacks. The clients do not communicate with each other and would like to verify the correctness of the remote computation and the consistency of the server's responses. This paper presents the Conflict-free Operation verification Protocol (COP) that ensures linearizability when the server is correct and preserves fork-linearizability otherwise. Clients that observe each other's operations are consistent and their operations are linearizable. If the server forks two clients by hiding an operation, however, they never again see operations of each other. COP is wait-free in the sense that when executed with a correct server, non-conflicting operations can run without waiting for other clients. The paper gives a precise model for the guarantees of COP and includes a formal analysis that these are achieved.

TCS Journal 2009 Journal Article

Preface

• Lars Arge
• Christian Cachin
• Andrzej Tarlecki

I&C Journal 2004 Journal Article

An information-theoretic model for steganography

• Christian Cachin

An information-theoretic model for steganography with a passive adversary is proposed. The adversary’s task of distinguishing between an innocent cover message C and a modified message S containing hidden information is interpreted as a hypothesis testing problem. The security of a steganographic system is quantified in terms of the relative entropy (or discrimination) between the distributions of C and S, which yields bounds on the detection capability of any adversary. It is shown that secure steganographic schemes exist in this model provided the covertext distribution satisfies certain conditions. A universal stegosystem is presented in this model that needs no knowledge of the covertext distribution, except that it is generated from independently repeated experiments.

FOCS Conference 1998 Conference Paper

Oblivious Transfer with a Memory-Bounded Receiver

• Christian Cachin
• Claude Crépeau
• Julien Marcil

We propose a protocol for oblivious transfer that is unconditionally secure under the sole assumption that the memory size of the receiver is bounded. The model assumes that a random bit string slightly larger than the receiver's memory is broadcast (either by the sender or by a third party). In our construction, both parties need memory of size in /spl theta/(n/sup 2-2/spl alpha//) for some /spl alpha/ /spl beta/>0, whereas a malicious receiver can have up to /spl gamma/N bits of memory for any /spl gamma/<1. In the course of our analysis, we provide a direct study of an interactive hashing protocol closely related to that of M. Naor et al. (1998).