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The Sybil attack in computer security is an attack wherein a reputation system is subverted by forging identities in peer-to-peer networks.

A Sybil attack is one in which an attacker subverts the reputation system of a peer-to-peer network by creating a large number of pseudonymous entities, using them to gain a disproportionately large influence. A reputation system's vulnerability to a Sybil attack depends on how cheaply identities can be generated. The degree to which the reputation system accepts inputs from entities that do not have a chain of trust linking them to a trusted entity, and whether the reputation system treats all entities identically. It is named after the subject of the book Sybil, a case study of a woman with multiple personality disorder.

An entity on a peer-to-peer network is a piece of software which has access to local resources. An entity advertises itself on the peer-to-peer network by presenting itself with an identity. There can be more than one identity can correspond to a single entity. In other words the mapping of identities to entities is many to one. Entities in peer-to-peer networks use multiple identities for purposes of redundancy, resource sharing, reliability and integrity. In peer-to-peer networks the identity is used as an abstraction so that a remote entity is aware of identities without necessarily knowing the correspondence of the identities with their local entities. By default, the distinct identities are assumed to correspond to a distinct local entity. In reality they may correspond to a same local entity.

A faulty node or an adversary may present itself with multiple identities in a peer-to-peer network to appear and function as distinct nodes. By becoming part of the peer-to-peer network, the adversary may then overhear communications or act maliciously. By masquerading and presenting multiple identities, the adversary can control the network substantially.

Validation techniques can be used to prevent sybil attacks and dismiss masquerading hostile entities. A local entity may accept the a remote identity based on a central authority which ensures a one-to-one correspondence between an identity and an entity and may even provide a reverse lookup . An identity may be validated either directly or indirectly. In direct validation the local entity queries the central authority to validate the remote identities. In indirect validation the local entity relies on already accepted identities which in turn vouch for the validity of the remote identity in question.

• The Sybil Attack by John R. Douceur

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