Quantifying Resistance to the Sybil Attack

Sybil attacks have been shown to be unpreventable except under the protection of a vigilant central authority. We use an economic analysis to show quantitatively that some applications and protocols are more robust against the attack than others. In our approach, for each distributed application and an attacker objective, there is a critical value that determines the costeffectiveness of the attack. A Sybil attack is worthwhile only when the critical value is exceeded by the ratio of the value of the attacker’s goal to the cost of identities. We show that for many applications, successful Sybil attacks may be expensive even when the Sybil attack cannot be prevented. Specifically, we propose the use of a recurring fee as a deterrent against the Sybil attack. As a detailed example, we look at four variations of the Sybil attack against a recurring fee based onion routing anonymous routing network and quantify its vulnerability.