Sybil attack

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

Description

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". More than one identity can correspond to a single entity (mathematically expressed, 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.

Prevention

Validation techniques can be used to prevent Sybil attacks and dismiss masquerading hostile entities. A local entity may accept 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.

Identity-based validation techniques generally provide accountability at the expense of anonymity, which can be an undesirable tradeoff especially in online forums that wish to permit censorship-free information exchange and open discussion of sensitive topics. A validation authority can attempt to preserve users' anonymity by refusing to perform reverse lookups, but this approach makes the validation authority a prime target for attack. Alternatively, the authority can use some mechanism other than knowledge of a user's real identity - such as verification of an "unidentified" person's physical presence at a particular place and time - to enforce a one-to-one correspondence between online identities and real-world users.

Sybil prevention techniques based on the connectivity characteristics of social graphs can also limit the extent of damage that can be caused by a given sybil attacker while preserving anonymity, though these techniques cannot prevent sybil attacks entirely, and may be vulnerable to widespread small-scale sybil attacks.

References

* [http://www.cs.rice.edu/Conferences/IPTPS02/101.pdf The Sybil Attack by John R. Douceur]
* [http://portal.acm.org/citation.cfm?id=1159945 SybilGuard: defending against sybil attacks via social networks by Haifeng Yu]
* [http://thrackle.eas.asu.edu/users/goran/papers/dc06.pdf On the establishment of distinct identities in overlay networks by Rida A. Bazzi and Goran Konjevod]
* [http://pdos.csail.mit.edu/papers/accountable-pseudonyms-socialnets08.pdf An Offline Foundation for Online Accountable Pseudonyms by Bryan Ford and Jacob Strauss]
* [http://pdos.csail.mit.edu/papers/sybil-dht-socialnets08.pdf A Sybil-proof one-hop DHT by Chris Lesniewski-Laas]