International Association for Cryptologic Research

International Association
for Cryptologic Research

CryptoDB

Christian Cachin

Publications

Year
Venue
Title
2006
EPRINT
Parsimonious Asynchronous Byzantine-Fault-Tolerant Atomic Broadcast
HariGovind V. Ramasamy Christian Cachin
Atomic broadcast is a communication primitive that allows a group of n parties to deliver a common sequence of payload messages despite the failure of some parties. We address the problem of asynchronous atomic broadcast when up to t < n/3 parties may exhibit Byzantine behavior. We provide the first protocol with an amortized expected message complexity of O(n) per delivered payload. The most efficient previous solutions are the BFT protocol by Castro and Liskov and the KS protocol by Kursawe and Shoup, both of which have message complexity O(n^2). Like the BFT and KS protocols, our protocol is optimistic and uses inexpensive mechanisms during periods when no faults occur; when network instability or faults are detected, it switches to a more expensive recovery mode. The key idea of our solution is to replace reliable broadcast in the KS protocol by consistent broadcast, which reduces the message complexity from O(n^2) to O(n) in the optimistic mode. But since consistent broadcast provides weaker guarantees than reliable broadcast, our recovery mode incorporates novel techniques to ensure that safety and liveness are always satisfied.
2005
TCC
2005
JOFC
2005
EPRINT
Secure Key-Updating for Lazy Revocation
Michael Backes Christian Cachin Alina Oprea
We consider the problem of efficient key management and user revocation in cryptographic file systems that allow shared access to files. A performance-efficient solution to user revocation in such systems is lazy revocation, a method that delays the re-encryption of a file until the next write to that file. We formalize the notion of key-updating schemes for lazy revocation, an abstraction to manage cryptographic keys in file systems with lazy revocation, and give a security definition for such schemes. We give two composition methods that combine two secure key-updating schemes into a new secure scheme that permits a larger number of user revocations. We prove the security of two slightly modified existing constructions and propose a novel binary tree construction that is also provable secure in our model. Finally, we give a systematic analysis of the computational and communication complexity of the three constructions and show that the novel construction improves the previously known constructions.
2003
EPRINT
Public-Key Steganography with Active Attacks
Michael Backes Christian Cachin
A complexity-theoretic model for public-key steganography with active attacks is introduced. The notion of steganographic security against adaptive chosen-covertext attacks (SS-CCA) and a relaxation called steganographic security against publicly-detectable replayable adaptive chosen-covertext attacks (SS-PDR-CCA) are formalized. These notions are closely related to CCA-security and PDR-CCA-security for public-key cryptosystems. In particular, it is shown that any SS-(PDR-)CCA stegosystem is a (PDR-)CCA-secure public-key cryptosystem and that an SS-PDR-CCA stegosystem can be realized from any PDR-CCA-secure public-key cryptosystem with pseudorandom ciphertexts.
2002
EPRINT
Asynchronous Verifiable Secret Sharing and Proactive Cryptosystems
Verifiable secret sharing is an important primitive in distributed cryptography. With the growing interest in the deployment of threshold cryptosystems in practice, the traditional assumption of a synchronous network has to be reconsidered and generalized to an asynchronous model. This paper proposes the first \emph{practical} verifiable secret sharing protocol for asynchronous networks. The protocol creates a discrete logarithm-based sharing and uses only a quadratic number of messages in the number of participating servers. It yields the first asynchronous Byzantine agreement protocol in the standard model whose efficiency makes it suitable for use in practice. Proactive cryptosystems are another important application of verifiable secret sharing. The second part of this paper introduces proactive cryptosystems in asynchronous networks and presents an efficient protocol for refreshing the shares of a secret key for discrete logarithm-based sharings.
2001
CRYPTO
2001
EPRINT
Secure and Efficient Asynchronous Broadcast Protocols
Reliable broadcast protocols are a fundamental building block for implementing replication in fault-tolerant distributed systems. This paper addresses secure service replication in an asynchronous environment with a static set of servers, where a malicious adversary may corrupt up to a threshold of servers and controls the network. We develop a formal model using concepts from modern cryptography, present modular definitions for several broadcast problems, including reliable, atomic, and secure causal broadcast, and present protocols implementing them. Reliable broadcast is a basic primitive, also known as the Byzantine generals problem, providing agreement on a delivered message. Atomic broadcast imposes additionally a total order on all delivered messages. We present a randomized atomic broadcast protocol based on a new, efficient multi-valued asynchronous Byzantine agreement primitive with an external validity condition. Apparently, no such efficient asynchronous atomic broadcast protocol maintaining liveness and safety in the Byzantine model has appeared previously in the literature. Secure causal broadcast extends atomic broadcast by encryption to guarantee a causal order among the delivered messages. Threshold-cryptographic protocols for signatures, encryption, and coin-tossing also play an important role.
2000
CRYPTO
2000
EPRINT
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.
2000
EPRINT
Random Oracles in Constantinople: Practical Asynchronous Byzantine Agreement using Cryptography
Christian Cachin Klaus Kursawe Victor Shoup
Byzantine agreement requires a set of parties in a distributed system to agree on a value even if some parties are corrupted. A new protocol for Byzantine agreement in a completely asynchronous network is presented that makes use of cryptography, specifically of threshold signatures and coin-tossing protocols. These cryptographic protocols have practical and provably secure implementations in the ``random oracle'' model. In particular, a coin-tossing protocol based on the Diffie-Hellman problem is presented and analyzed. The resulting asynchronous Byzantine agreement protocol is both practical and theoretically nearly optimal because it tolerates the maximum number of corrupted parties, runs in constant expected time, has message and communication complexity close to the optimum, and uses a trusted dealer only in a setup phase, after which it can process a virtually unlimited number of transactions. The protocol is formulated as a transaction processing service in a cryptographic security model, which differs from the standard information-theoretic formalization and may be of independent interest.
1999
EUROCRYPT
1998
EUROCRYPT
1997
CRYPTO
1997
EUROCRYPT
1997
JOFC
1994
EUROCRYPT

Program Committees

Eurocrypt 2011
Crypto 2010
PKC 2009
Eurocrypt 2004 (Program chair)
Eurocrypt 2003
Asiacrypt 2003
Eurocrypt 2002
Crypto 2000
Eurocrypt 2000