Semaphore Protocol

Part 1: Setting up the Docker Environment

This example uses TypeScript and the Snarkyjs zk library developed by the O(1) Labs team. You can pull a pre-built dockerfile will all required environment dependencies:

docker pull tthebc01/zksnark-playground

and run it like this:

docker run -it --rm --entrypoint bash -v /path/to/circom-playground:/root/playground tthebc01/zksnark-playground

This will start a bash session in the playground environment. Next cd to the directory containing this example:

cd /root/playground/examples/snickerdoodle-semaphore

Once you are in the example folder called snickerdoodle-semaphore, you need to install dependencies and build the project:

npm install
npm run build

Now you are ready to experiment.

Part 2: The Circuit

The Semaphore circuit itself is contained in Semaphore.ts. This file exports three things:

1. A Merkle Tree Witness object called MyMerkleWitness that is configured for an arity of 16 (i.e. 2^16 possible leaves). This will be used to compute the secret membership path belonging to a prover.

2. A special identity structure that takes an identityNullifier and an identityTrapdoor, both of which are random Field elements that must be kept secret. Taking the Poseidon hash of these two quantities and then hashing that value again results in a public identity commitment.

3. The Semaphore circuit implementation which allows a prover to prove their membership to a group while preventing replays and message tampering.

Part 3: How to Run the Circuit

The usage of the Semaphore circuit is demonstrated in run_semaphore.ts. First a kaypair object must be created for the circuit by calling .generateKeypair() on the circuit object. This keypair is used in its entirety to compute a proof.

The verification key can be extracted from the keypair object by calling .verificationKey() on the keypair. The verification key is one of the inputs needed for proof verification.

If you've already run npm run build, you can run the example script like this:

node run build/src/run_semaphore.js