The LOCKSS project has developed and deployed in a worldwide test a peer-to-peer system for preserving access to journals and other archival information published on the Web. It consists of a large number of independent, low-cost, persistent web caches that cooperate to detect and repair damage to their content by voting in "opinion polls." Based on this experience, we present a design for and simulations of a novel protocol for voting in systems of this kind. It incorporates rate limitation and intrusion detection to ensure that even some very powerful adversaries attacking over many years have only a small probability of causing irrecoverable damage before being detected.

The LOCKSS project has developed and deployed in a worldwide test a peer-to-peer system for preserving access to journals and other archival information published on the Web. It consists of a large number of independent, low-cost, persistent web caches that cooperate to detect and repair damage to their content by voting in "opinion polls." Based on this experience, we present a design for and simulations of a novel protocol for voting in systems of this kind. It incorporates rate limitation and intrusion detection to ensure that even some very powerful adversaries attacking over many years have only a small probability of causing irrecoverable damage before being detected.

Peer-to-peer systems in which the peers are truly autonomous have valuable properties, including resistance to certain forms of organizational failure and legal attack. Unfortunately, they can be vulnerable to malign peers. In the context of the LOCKSS system, a peer-to-peer digital preservation system for e-journals, we describe a set of techniques that enable a large population of autonomous peers to resist attack by a substantial minority of malign peers endowed with unlimited computational resources. LOCKSS peers are able to detect attacks and alert the community of peer operators before damage becomes irreversible. These techniques include rate limitation and making peers "pay" certain costs by demanding proofs of effort from them.

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