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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.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].


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


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(
} catch (e: any) {
return false;

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

The Warp Contracts SDK makes such verification automatically.