libsemaphore

libsemaphore is a helper library for Semaphore written in Typescript. Any dApp written in Javascript or Typescript should use it as it provides useful abstractions over common tasks and objects, such as identities and proof generation.

Note that only v1.0.14 and above works with the Semaphore code in this repository. v0.0.x is compatible with the pre-audited Semaphore code.

Available types, interfaces, and functions

Types

SnarkBigInt

A big integer type compatible with the snarkjs library. Note that it is not advisable to mix variables of this type with bigNumbers or BigInts. Encapsulates snarkjs.bigInt.

EddsaPrivateKey

An EdDSA private key which should be 32 bytes long. Encapsulates a Buffer.

EddsaPublicKey

An EdDSA public key. Encapsulates an array of SnarkBigInts.

SnarkProvingKey

A proving key, which when used with a secret witness, generates a zk-SNARK proof about said witness. Encapsulates a Buffer.

SnarkVerifyingKey

A verifying key which when used with public inputs to a zk-SNARK and a SnarkProof, can prove the proof's validity. Encapsulates a Buffer.

SnarkWitness

The secret inputs to a zk-SNARK. Encapsulates an array of SnarkBigInts.

SnarkPublicSignals

The public inputs to a zk-SNARK. Encapsulates an array of SnarkBigInts.

Interfaces

EddsaKeyPair

Encapsulates an EddsaPublicKey and an EddsaPrivateKey.

interface EddsaKeyPair {
    pubKey: EddsaPublicKey,
    privKey: EddsaPrivateKey,
}

Identity

Encapsulates all information required to generate an identity commitment, and is crucial to creating SnarkProofs to broadcast signals.

interface Identity {
    keypair: EddsaKeyPair,
    identityNullifier: SnarkBigInt,
    identityTrapdoor: SnarkBigInt,
}

SnarkProof

Note that broadcastSignal() accepts a uint256[8] array for its _proof parameter. See genBroadcastSignalParams().

interface SnarkProof {
    pi_a: SnarkBigInt[]
    pi_b: SnarkBigInt[][]
    pi_c: SnarkBigInt[]
}

Functions

genPubKey(privKey: EddsaPrivateKey): EddsaPublicKey

Generates a public EdDSA key from a supplied private key. To generate a private key, use crypto.randomBytes(32) where crypto is the built-in Node or browser module.

genIdentity(): Identity

This is a convenience function to generate a fresh and random Identity. That is, the 32-byte private key for the EddsaKeyPair is randomly generated, as are the distinct 31-byte identity nullifier and the 31-byte identity trapdoor values.

serialiseIdentity(identity: Identity): string

Converts an Identity into a JSON string which looks like this:

["e82cc2b8654705e427df423c6300307a873a2e637028fab3163cf95b18bb172e","a02e517dfb3a4184adaa951d02bfe0fe092d1ee34438721d798db75b8db083","15c6540bf7bddb0616984fccda7e954a0fb5ea4679ac686509dc4bd7ba9c3b"]

You can also spell this function as serializeIdentity.

To convert this string back into an Identity, use unSerialiseIdentity().

unSerialiseIdentity(string: serialisedId): Identity

Converts the string output of serialiseIdentity() to an Identity.

You can also spell this function as unSerializeIdentity.

genIdentityCommitment(identity: Identity): SnarkBigInt

Generates an identity commitment, which is the hash of the public key, the identity nullifier, and the identity trapdoor.

async genProof(witness: SnarkWitness, provingKey: SnarkProvingKey): SnarkProof

Generates a SnarkProof, which can be sent to the Semaphore contract's broadcastSignal() function. It can also be verified off-chain using verifyProof() below.

genPublicSignals(witness: SnarkWitness, circuit: SnarkCircuit): SnarkPublicSignals

Extracts the public signals to be supplied to the contract or verifyProof().

verifyProof(verifyingKey: SnarkVerifyingKey, proof: SnarkProof, publicSignals: SnarkPublicSignals): boolean

Returns true if the given proof is valid, given the correct verifying key and public signals.

Returns false otherwise.

signMsg(privKey: EddsaPrivateKey, msg: SnarkBigInt): EdDSAMiMcSpongeSignature)

Encapsualtes circomlib.eddsa.signMiMCSponge to sign a message msg using private key privKey.

verifySignature(msg: SnarkBigInt, signature: EdDSAMiMcSpongeSignature, pubKey: EddsaPublicKey): boolean

Returns true if the cryptographic signature of the signed msg is from the private key associated with pubKey.

Returns false otherwise.

setupTree(levels: number, prefix: string): MerkleTree

Returns a Merkle tree created using semaphore-merkle-tree with the same number of levels which the Semaphore zk-SNARK circuit expects. This tree is also configured to use MimcSpongeHasher, which is also what the circuit expects.

levels sets the number of levels of the tree. A tree with 20 levels, for instance, supports up to 1048576 deposits.

genCircuit(circuitDefinition: any)

Returns a new snarkjs.Circuit(circuitDefinition). The circuitDefinition object should be the JSON.parsed result of the circom command which converts a .circom file to a .json file.

async genWitness(...)

This function has the following signature:

const genWitness = async (
    signal: string,
    circuit: SnarkCircuit,
    identity: Identity,
    idCommitments: SnarkBigInt[] | BigInt[] | ethers.utils.BigNumber[],
    treeDepth: number,
    externalNullifier: SnarkBigInt,
)

• signal is the string you wish to broadcast.
• circuit is the output of genCircuit().
• identity is the Identity whose identity commitment you want to prove is in the set of registered identities.
• idCommitments is an array of registered identity commmitments; i.e. the leaves of the tree.
• treeDepth is the number of levels which the Merkle tree used has
• externalNullifier is the current external nullifier

It returns an object as such:

• witness: The witness to pass to genProof().
• signal: The computed signal for Semaphore. This is the hash of the recipient's address, relayer's address, and fee.
• signalHash: The hash of the computed signal.
• msg: The hash of the external nullifier and the signal hash
• signature: The signature on the above msg.
• tree: The Merkle tree object after it has been updated with the identity commitment
• identityPath: The Merkle path to the identity commmitment
• identityPathIndex: The leaf index of the identity commitment
• identityPathElements: The elements along the above Merkle path

Only witness is essential to generate the proof; the other data is only useful for debugging and additional off-chain checks, such as verifying the signature and the Merkle tree root.

formatForVerifierContract = (proof: SnarkProof, publicSignals: SnarkPublicSignals

Converts the data in proof and publicSignals to strings and rearranges elements of proof.pi_b so that snarkjs's verifier.sol will accept it. To be specific, it returns an object as such:

{
    a: [ proof.pi_a[0].toString(), proof.pi_a[1].toString() ],
    b: [ 
         [ proof.pi_b[0][1].toString(), proof.pi_b[0][0].toString() ],
         [ proof.pi_b[1][1].toString(), proof.pi_b[1][0].toString() ],
    ],
    c: [ proof.pi_c[0].toString(), proof.pi_c[1].toString() ],
    input: publicSignals.map((x) => x.toString()),
}

stringifyBigInts = (obj: any) => object

Encapsulates snarkjs.stringifyBigInts(). Makes it easy to convert SnarkProofs to JSON.

unstringifyBigInts = (obj: any) => object

Encapsulates snarkjs.unstringifyBigInts(). Makes it easy to convert JSON to SnarkProofs.

genExternalNullifier = (plaintext: string) => string

Each external nullifier must be at most 29 bytes large. This function keccak-256-hashes a given plaintext, takes the last 29 bytes, and pads it (from the start) with 0s, and returns the resulting hex string.