ETH Series in Information Security and Cryptography
edited by Ueli Maurer
Volume 5

Reto Strobl
Distributed Cryptographic Protocols in
Asynchronous Networks with Universal Composability

1st edition/ 1. Auflage 2005, 192 pages/Seiten, 64,00. ISBN 3-89649-988-2

We consider a group of parties that communicate over asynchronous point-to-point links, where the parties and the links may be attacked by an adversary. We develop new implementations of fundamental primitives that allow to secure such systems against different attacks. The security of all our solutions is based on computational assumptions and remains valid even under arbitrary composition.
The primitives we consider are group key exchange, verifiable secret sharing, and proactive verifiable secret sharing. Group key exchange allows the parties to compute a key that remains hidden from the adversary who may observe all network traffic. Our implementation of this primitive is the first one that guarantees termination for all parties even if a minority crashes silently.
Verifiable secret sharing allows a designated party to share a secret amongst all parties, such that an adversary, who controls a fraction of the parties, can neither learn the secret, nor prevent the others from reconstructing it correctly. We present the first practical asynchronous implementation of this primitive.
Proactive verifiable secret sharing offers the same functionality as verifiable secret sharing, but tolerates an adversary who may change the set of controlled parties from time to time. We present the first asynchronous implementation of this primitive.

Keywords: provable security, composition, cryptography, distributed protocols, group key exchange, secret sharing, proactive security.

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ETH Series in Information Security and Cryptography