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This paper describes a Secure INtrusion-Tolerant Replication Architecture (SINTRA) for coordination in asynchronous networks subject to Byzantine faults. SINTRA supplies a number of group communication primitives, such as binary and multi-valued Byzantine agreement, reliable and consistent broadcast, and an atomic broadcast channel. Atomic broadcast immediately provides secure state-machine replication. The protocols are designed for an asynchronous wide-area network, such as the Internet, where messages may be delayed indefinitely, the servers do not have access to a common clock, and up to one third of the servers may fail in potentially malicious ways. Security is achieved through the use of threshold public-key cryptography, in particular through a cryptographic common coin based on the Diffie-Hellman problem that underlies the randomized protocols in SINTRA. The implementation of SINTRA in Java is described and timing measurements are given for a test-bed of servers distributed over three continents. They show that extensive use of public-key cryptography does not impose a large overhead for secure coordination in wide-area networks.