Specification of Dependable Trusted Third Parties

This document describes an architecture for secure service replication in an asynchronous network like the Internet, where a malicious adversary may corrupt some servers and control the network. The underlying protocols for Byzantine agreement and for atomic broadcast rely on recent developments in threshold cryptography. These assumptions are discussed in detail and compared to related work from the last decade. A formal model using concepts from modern cryptography is developed, modular definitions for several broadcast problems are presented, including reliable, atomic, and secure causal broadcast, and 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. A randomized asynchronous atomic broadcast protocol is presented that maintains liveness and safety at the same time. It is based on a new efficient protocol for multi-valued asynchronous Byzantine agreement with an external validity condition. Secure causal broadcast extends atomic broadcast by encryption to guarantee a causal order among the delivered messages. Furthermore, it is discussed how several distributed trusted applications can be realized using such an architecture: a digital notary service, a trusted third party for fair exchange, a certification authority, and an authentication service.