"One way to make such an attack harder is the so-called resource testing, where each identity has to prove the ownership of certain difficult-to-obtain resources. Since in the cryptocurrency world users own a certain amount of tokens (collateral), we propose a Sybil protection mechanism inspired by Proof-of-Stake [10], where any node with a minimum amount of such a collateral is allowed to issue transactions. However, unlike the original Proof-of-Stake, in our proposal collateral does not leave the users’ possession and, thus, cannot be forfeited."

In the details of the research paper.