Sybil Attack

A Sybil attack subverts a decentralised network by creating a large number of fake identities, allowing a single actor to gain disproportionate influence over the system. Named after the 1973 book about a woman with multiple personality disorder, the attack exploits any system where influence scales with the number of identities rather than with some scarce resource that cannot be cheaply duplicated.

In a peer-to-peer network where each node has one vote, an adversary who creates a thousand fake nodes controls a thousand votes; in a reputation system where each account accumulates trust independently, a network of seemingly unrelated accounts can serve the same purpose. Sybil resistance — the property of being difficult to subvert through identity multiplication — is therefore one of the core design challenges of decentralised systems, and the answers usually take the same form: tie influence to something scarce. Computation, stake, social attestation, biometrics, lived time on the network.

The Sybil problem crystallises a tension that runs through the parallel-society project: openness and integrity pull in opposite directions. A network anyone can join without verification is maximally inclusive but trivially capturable by a well-resourced adversary who simply floods it. A network demanding strong identity verification is Sybil-resistant but risks recreating the surveillance and gatekeeping it was built to escape. Bitcoin's proof-of-work ties influence to computational expenditure; proof-of-stake to economic stake; web-of-trust attestation and zero-knowledge proof-of-personhood are further points on the spectrum — each managing the same trade-off differently.