We present general definitions of security for multi-party cryptographic protocols, with focus on the task of evaluating a probabilistic function of the parties' inputs. We show that, with respect to these definitions, security is preserved under a natural composition operation. The definitions follow the general paradigm of known definitions; yet some substantial modifications and simplifications are introduced. The composition operation is the natural `subroutine substitution' operation, formalized by Micali and Rogaway. We consider several standard settings for multi-party protocols, including the cases of eavesdropping, Byzantine, non-adaptive and adaptive adversaries, as well as the information-theoretic and the computational models. In particular, in the computational model we provide the first definition of security of protocols that is shown to be preserved under composition.

We present an RSA threshold signature scheme. The scheme enjoys the following properties: 1. it is unforgeable and robust in the random oracle model, assuming the RSA problem is hard

OceanStore is an Internet-scale, persistent data store designed for incremental scalability, secure sharing, and long-term durability. Pond is the OceanStore prototype; it contains many of the features of a complete system including location-independent routing, Byzantine update commitment, push-based update of cached copies through an overlay multicast network, and continuous archiving to erasure-coded form. In the wide area, Pond outperforms NFS by up to a factor of 4.6 on readintensive phases of the Andrew benchmark, but underperforms NFS by as much as a factor of 7.3 on writeintensive phases. Microbenchmarks show that write performance is limited by the speed of erasure coding and threshold signature generation, two important areas of future research. Further microbenchmarks show that Pond manages replica consistency in a bandwidthefficient manner and quantify the latency cost imposed by this bandwidth savings.

this article, is such an online CA

We propose a robust proactive threshold signature scheme, a multisignature scheme and a blind signature scheme which work in any Gap Diffie-Hellman (GDH) group (where the Computational Diffie-Hellman problem is hard but the Decisional Diffie-Hellman problem is easy). Our constructions are based on the recently proposed GDH signature scheme of Boneh et al. [8]. Due to the instrumental structure of GDH groups and of the base scheme, it turns out that most of our constructions are simpler, more efficient and have more useful properties than similar existing constructions. We support all the proposed schemes with proofs under the appropriate computational assumptions, using the corresponding notions of security.

We describe efficient techniques for a number of parties to jointly generate an RSA key. At the end of the protocol an RSA modulus N = pq is publicly known. None of the parties know the factorization of N. In addition a public encryption exponent is publicly known and each party holds a share of the private exponent that enables threshold decryption. Our protocols are efficient in computation and communication. All results are presented in the honest but curious settings (passive adversary).

Several public key cryptosystems with additional homomorphic properties have been proposed so far. They allow to perform computation with encrypted data without the knowledge of any secret information. In many applications, the ability to perform decryption, i.e. the knowledge of the secret key, gives a huge power. A classical way to reduce the trust in such a secret owner, and consequently to increase the security, is to share the secret between many entities in such a way that cooperation between them is necessary to decrypt. In this paper, we propose a distributed version of the Paillier cryptosystem presented at Eurocrypt ’99. This shared scheme can for example be used in an electronic voting scheme or in a lottery where a random number related to the winning ticket has to be jointly chosen by all participants.

The ITTC project (Intrusion Tolerance via Threshold Cryptography) provides tools and an infrastructure for building intrusion tolerant applications. Rather than prevent intrusions or detect them after the fact, the ITTC system ensures that the compromise of a few system components does not compromise sensitive security information. To do so we protect cryptographic keys by distributing them across a few servers. The keys are never reconstructed at a single location. Our designs are intended to simplify the integration of ITTC into existing applications. We give examples of embedding ITTC into the Apache web server and into a Certication Authority (CA). Performance measurements on both the modied web server and the modied CA show that the architecture works and performs well. 1 Introduction To combat intrusions into a networked system one often installs intrusion detection software to monitor system behavior. Whenever an \irregular" behavior is observed the software noties an admi...

We present a new protocol for ecient distributed computation modulo a shared secret. We further present a protocol to distributively generate a random shared prime or safe prime that is much more efficient than previously known methods. This allows to distributively compute shared RSA keys, where the modulus is the product of two safe primes, much more efficiently than was previously known.

We present adaptively-secure efficient solutions to several central problems in the area of threshold cryptography. We prove these solutions to withstand adaptive attackers that choose parties for corruption at any time during the run of the protocol. In contrast, all previously known efficient protocols for these problems were proven secure only against less realistic static adversaries that choose and fix the subset of corrupted parties before the start of the protocol run. Specifically, we provide adaptively-secure solutions for distributed key generation in discrete-log based cryptosystems, and for the problem of distributed generation of DSS signatures (threshold DSS). We also show how to transform existent static solutions for threshold RSA and proactive schemes to withstand the stronger adaptive attackers. In doing so, we introduce several techniques for the design and analysis of adaptively-secure protocols that may well find further applications.