@pcd/gpc

A library for creating and verifying zero-knowledge proofs using General Purpose Circuits. For a full introduction, see the Developer Site.

POD libraries enable any app to create zero-knowledge proofs of cryptographic data. A POD could represent your ticket to an event, a secure message, a collectible badge, or an item in a role-playing game. Using PODs, developers can create ZK-enabled apps without the effort and risk of developing custom cryptography.

ZK proofs about PODs use General Purpose Circuits (GPC) which can 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.

PODs and GPCs can be used in Zupass, or in your own apps without Zupass.

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 },
        weaponType: { isRevealed: false, isMemberOf: "favoriteWeapons" },
        owner: { isRevealed: false, isOwnerID: true }
      }
    }
  }
};
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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).

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 constraint 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.

Entry Points

• The GPCProofConfig type configures a proof. Start there to learn what you can prove about PODs.
• The gpcProve and gpcVerify functions allow you to generate and validate GPC proofs.
• The gpcArtifactDownloadURL function can help you find the right place to get the necessary binary artifacts (proving key, verification key, witness generator) to perform proof calculations.

For more details on usage, check out the tutorial code.

Related Packages

• For information about making POD objects, see the @pcd/pod package.

• To see how GPC proofs are created and stored in the Zupass app, see the @pcd/gpc-pcd package.

• To find the binaries required to prove and verify, see the @pcd/proto-pod-gpc-artifacts package. Since these artifacts are large and numerous, you generally won't want to include this package directly into your app bundle.

Package Installation

This package will work either in browser or in a Node.js server. Packaging for a browser requires polyfill for some Node modules, including buffer and constants.

There is a known issue with a dependency fastfile which can be resolved by polyfilling constants as you can see in this example.

Stability and Security

POD and GPC libraries are in beta and subject to change. We encourage devs to try them out and use them for apps, but be aware that updates will come in future.

GPC proofs are considered ephemeral (for now), primarily intended for transactional use cases. Saved proofs may not be verifiable with future versions of code.

These libraries should not be considered secure enough for highly-sensitive use cases yet. The cryptography, circuits, and configuration compiler have not been audited. The proving/verification keys were generated in good faith by a single author, but are not the result of a distributed trusted setup ceremony.