 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.