distribute the role of such a validation authority among multiple servers, protecting users' anonymity even if one or a limited number of validation servers is compromised. Social trust graphs 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. Examples of such prevention techniques include SybilGuard, SybilLimit, the Advogato Trust Metric, SybilRank, and the sparsity based metric to identify Sybil clusters in a distributed P2P based reputation system. These techniques cannot prevent Sybil attacks entirely, and may be vulnerable to widespread small-scale Sybil attacks. In addition, it is not clear whether real-world online social networks will satisfy the trust or connectivity assumptions that these algorithms assume. Economic costs Alternatively, imposing economic costs as artificial barriers to entry may be used to make Sybil attacks more expensive. Proof of work, for example, requires a user to prove that they expended a certain amount of computational effort to solve a cryptographic puzzle. In Bitcoin and related permissionless cryptocurrencies, miners compete to append blocks to a blockchain and earn rewards roughly in proportion to the amount of computational effort they invest in a given time period. Investments in other resources such as storage or stake in existing cryptocurrency may similarly be used to impose economic costs. Personhood validation As an alternative to identity verification that attempts to maintain a strict "one-per-person" allocation rule, a validation 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 as in a pseudonym party – to enforce a one-to-one correspondence between online identities and real-world users. Such proof of personhood approaches have been proposed as a basis for permissionless blockchains and cryptocurrencies in which each human participant would wield exactly one vote in consensus. A variety of approaches to proof of personhood have been proposed, some with deployed implementations, although many usability and security issues remain. Application-specific defenses A number of distributed protocols have been designed with Sybil attack protection in mind. SumUp and DSybil are Sybil-resistant algorithms for online content recommendation and voting. Whānau is a Sybil-resistant distributed hash table algorithm. I2P's implementation of Kademlia also has provisions to mitigate Sybil attacks. See also Astroturfing Ballot stuffing Social bot Sockpuppetry References External links A Survey of Solutions