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Verifiable randomness

You have an option to use random values in contracts via Verifiable Random Functions.
This may be a very useful feature in gaming, dynamic NFT generation, etc.

We're using the Google Key Transparency VRF implementation.

Interaction creation​

In order to use the VRF, the Warp Sequencer must be used with the vrf option set to true:

await contract.writeInteraction(
      { function: 'generateNFT' },
      { vrf: true });

Contract code​

An interaction transaction generated with vrf option gives access to specific methods via the SmartWeave global object:

  • SmartWeave.vrf.data
  • SmartWeave.vrf.value
  • SmartWeave.vrf.randomInt(maxValue: number)

The last method (randomInt) allows to generate a pseudo-random (but deterministic) value based on the VRF data generated during interaction transaction registration. It generates a random integer value in a range [1, maxValue].

Example​

A full example of using a VRF in a simple guessing game is available here.

Verification​

In order to verify the transaction's VRF data:

function verifyVrf(vrf: VrfData, sortKey: string, arweave: Arweave): boolean {
    const keys = EC.keyFromPublic(vrf.pubkey, 'hex');

    let hash;
    try {
      // ProofHoHash throws its own 'invalid vrf' exception
      hash = ProofHoHash(
        keys.getPublic(),
        arweave.utils.stringToBuffer(sortKey),
        arweave.utils.b64UrlToBuffer(vrf.proof)
      );
    } catch (e: any) {
      return false;
    }

    return arweave.utils.bufferTob64Url(hash) == vrf.index;
}
info

The Warp Contracts SDK makes such verification automatically.