This package implements various zero-knowledge circuits for the Snickerdoodle protocol. The circuits are writtin in O1js, a typescript-native zk-circuit library from O1Labs.

The primary circuit in the Snickerdoodle circuits package calculates proof of membership with response deduplication and was inspired by the Semaphore project. This circuit takes in a reasonably large anonymity set and uses a Merkle tree inclusion proof (calculated with the Poseidon hash function) to prove that a user belongs to set. The anonymity set is nominally written to a public blockchain network so that an end-user client can fetch it without the need of a proprietary third-party data provider.

Each member of the anonymity set commits to two secret identity values: a trapdoor value and an identity nullifier value. It is this commitment (the Poseidon hash of these two values concatenated together) that is written to the blockchain by/for each participant. The nullifier value is used in conjunction with the IPFS CID value corresponding to an SDQL query that the user's core instance is responding to. The hash of the concatenation of the identity nullifier with the IPFS CID results in a public response nullifier that allows for deduplication of responses without the need for a centralized coordinator.

Lastly, the circuit computes the square of the Poseidon hash of the response data so that the verifier can ensure that the payload was not modified after being sent over the wire.

This package also includes a helper circuit that validates if an identity commitment was constructed properly. A participant generates their secret identity nullifier and trapdoor value, produces their identity commitment with this data, then uses this circuit to prove that their identity commitment is well-formed. The circuit can also accept the Poseidon hash of an optional data payload in order to provide an additional assertion that the payload sent over the wire was not tampered with during delivery.

This circuit also serves as a quasi proof-of-work for network participants who sponsor unauthenticated optIn transactions for end-users. It leaves room to be extended for additional protection of transaction sponsors against bot account in the future.