  Pseudonyms are widely used in Cooperative Intelligent Transport Systems
(C-ITS) to protect the location privacy of vehicles. However, the unlinkability
nature of pseudonyms also enables Sybil attacks, where a malicious vehicle can
pretend to be multiple vehicles at the same time. In this paper, we propose a
novel protocol called zero-knowledge Proof of Distinct Identity (zk-PoDI,)
which allows a vehicle to prove that it is not the owner of another pseudonym
in the local area, without revealing its actual identity. Zk-PoDI is based on
the Diophantine equation and zk-SNARK, and does not rely on any specific
pseudonym design or infrastructure assistance. We show that zk-PoDI satisfies
all the requirements for a practical Sybil-resistance pseudonym system, and it
has low latency, adjustable difficulty, moderate computation overhead, and
negligible communication cost. We also discuss the future work of implementing
and evaluating zk-PoDI in a realistic city-scale simulation environment.
