Introduction

General Purpose Circuits (GPC) are a part of the POD standard for cryptographic data. GPC is system for easily and flexibly generating zero-knowledge proofs about POD data.

GPC libraries enable any app to create zero-knowledge proofs of cryptographic data. Using GPCs, developers can create ZK-enabled apps without the effort and risk of developing custom cryptography. General Purpose Circuitscan prove many different things about PODs without revealing all details. GPCs use human-readable configuration and pre-compiled circuits so no knowledge of circuit programming is required.

GPC is built and supported by 0xPARC, and used by projects like Zupass, PODBox, Meerkat, Devcon Passport, and more.

What is a GPC?

GPCs allow ZK proofs to be created from a simple proof configuration. You can configure your proofs using a human-readable JSON format (or equivalent TypeScript code), which is used to generate the specific circuit inputs needed for the proof.

const weaponProofConfig: GPCProofConfig = {
  pods: {
    weapon: {
      entries: {
        attack: { isRevealed: true },
        reach: { isRevealed: false,
                 inRange: { min: 5n, max 10n } },
        weaponType: { isRevealed: false,
                      isMemberOf: "proficientWeapons" },
        owner: { isRevealed: false, isOwnerID: "SemaphoreV4" }
      }
    }
  }
};

GPCs can prove properties of one POD or several PODs together. PODs can be proven to be owned by the prover, using their Semaphore identity. A GPC can constrain as many named entries as needed, whether revealed or not. For example, a proof might constrain two entries to be equal, constrain a third entry to be in a list of valid values, and reveal the value of a fourth entry.

Circuit family

The GPC library inclues a family of pre-compiled ZK circuits with different sizes and capabilities. It will automatically select the right circuit to satisfy the needs of each proof at run-time. No setup is required, and you don’t need any knowledge of circuit programming (circom, halo2, noir, etc).

Using circuits in this family to prove and verify requires downloading binary artifacts: proving keys, verification keys, and witness generators. The GPC library will automatically fetch the artifacts it needs from a download URL or local filesystem, based on your app’s configuration.

GPCs in Zupass

GPCs are the basis of proofs in new Zupass-enabled apps using the ZApp SDK. PODs and GPCs are integrated into Zupass via PCD packages. Newly-issued Zupass tickets use PODTicketPCD, while PODPCD can represent any other data such as collectible frogs. GPCPCD enables the creation of arbitrary proofs.