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Verifiable randomness

caution

Random values will be handled in a new way once the decentralized sequencer is launched. More details can be found here. For backward compatibility, the function SmartWeave.vrf.randomInt(maxValue) will still work, but it will use the new function SmartWeave.randomNumber(maxValue) under the hood.

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​

If you want the warp-contracts SDK to verify the VRF's proof:

  1. up until version 1.4.3 it is done automatically by the SDK
  2. from version 1.4.4 - you need to attach the VRF plugin first.

The verification code itself:

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;
}