It s a dangerous digital world out there. Security and antivirus software is important for any network. One way security can break down is in a Sybil attack. Named after the case study of a woman with multiple personality disorder, a Sybil attack is a type of security threat when a node in a network claims multiple identities.

Most networks, like a peer-to-peer network, rely on assumptions of identity, where each computer represents one identity. A Sybil attack happens when an insecure computer is hijacked to claim multiple identities. Problems arise when a reputation system (such as a file-sharing reputation on a torrent network) is tricked into thinking that an attacking computer has a disproportionally large influence. Similarly, an attacker with many identities can use them to act maliciously, by either stealing information or disrupting communication. It is important to recognize a Sybil attack and note its danger in order to protect yourself from being a target.

First described by Microsoft researcher John Douceur, a Sybil attack relies on the fact that a network of computers cannot ensure that each unknown computing element is a distinct, physical computer. A number of authorities have attempted to establish the identity of computers on a network (or nodes) by using certification software such as VeriSign, employing IP addresses to identify nodes, requiring passwords and usernames, and so forth. However, impersonation, both in the real and digital worlds, is commonplace. Friends may share passwords, communities may share website registrations and some services provide a single IP address that is shared among users.

Sybil attacks have appeared in many scenarios, with wide implications for security, safety and trust. For example, an internet poll can be rigged using multiple IP addresses to submit a large number of votes. Some companies have also used Sybil attacks to gain better ratings on Google Page Rank. Reputation systems like eBay's have also been victims of this type of attack.

There are few sure-fire ways to protect a network from a Sybil attack, but there is a wide range of literature dedicated to discussing options for protection and verification of computing identities. One way is by using trusted certification in which a single, central authority establishes and verifies each identity via a certificate. Trusted certification is not foolproof, however, and it can use up large amounts of resources and bottleneck traffic on the network.

Another option is called resource testing. The aim of resource testing is to determine whether a collection of identities posses fewer resources than they would if they were independent. Resource testing scans computing power, storage space, network bandwidth and other parameters to determine if the collection is from a single, Sybil-attacking computer or a series of true identities.

Utilizing trusted devices is similar to using trusted certification to defend against a Sybil attack. In this case, identities are associated to specific hardware devices. Similar to a central authority creating certificates, there are few ways to prevent an attacker from attaining multiple devices.

It is important to know what threats are out there. In a typical home or office setting, a Sybil attack may not have as much direct effect as a virus or Trojan attack, but this type of attack can affect the fabric of internet commerce and communication. Understanding what a Sybil attack is and how to spot one is essential for any savvy internet user.

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