The SecureRing group communication protocols provide reliable ordered message delivery and group membership services despite Byzantine faults such as might be caused by modifications to the programs of a group member following illicit access to, or capture of, a group member. The protocols multicast messages to groups of processors within an asynchronous distributed system and deliver messages in a consistent total order to all members of the group. They ensure that correct members agree on changes to the membership, that correct processors are eventually included in the membership, and that processors that exhibit detectable Byzantine faults are eventually excluded from the membership. To provide these message delivery and group membership services, the protocols make use of an unreliable Byzantine fault detector. 1.

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The Eternal system supports distributed object applications that must operate continuously, without interruption of service, despite faults and despite upgrades to the hardware and the software. Based on the CORBA distributed object computing standard, the Eternal system replicates objects, invisibly and consistently, so that if one replica of an object fails, or is being upgraded, another replica is still available to provide continuous service. Through the use of interceptors, Eternal renders the object replication transparent to the applicationand also to the CORBA ORB. Consequently, Eternal is able to provide fault tolerance, and live hardware and software upgrades, for existing unmodified CORBA application programs, using unmodified commercial-off-the-shelf ORBs.

Intrusion Tolerant systems provide high-integrity and high-availability services to their clients in the face of successful attacks from an adversary. The