VeeDo is a STARK-based Verifiable Delay Function (VDF) service. VeeDo works in the Vending Machine model - a user can request a randomness for a specific input, sending payment in advance. An off-chain service picks up the request, computes the randomness and generates a proof attesting to the validity of the computation. The proof is then sent to the STARK Verifier for a verification process (see the Verifier contract). The Vending contract registers the new randomness if the Verifier accepts the corresponding proof and sends the payment to the off-chain service. In case a request was not served, the user can reclaim the payment after sufficient time has passed.

The Vending contract serves as the interface and has the following API:

1. addPayment(uint256 seed, uint256 n_iter, uint256 tag, uint256 paymentAmount)
2. reclaimPayment(uint256 seed, uint256 n_iter, uint256 tag)
3. prizes(uint256 seed, uint256 n_iter) view
4. registeredRandomness(uint256 seed, uint256 n_iter) view
5. payments(address sender, uint256 seed, uint256 n_iter, uint256 tag) view

Full implementation can be found in the code.

Denote the delay function by f (see the reference code for a simple implementation example). The randomness on (seed, n_iter) is computed as follows:

1. Compute b:=keccak256(uint256 seed, string ‘veedo’).
2. Compute the delay function input input=<x_in,y_in> by taking the lowest 250 bits of b and splitting them to a pair of 125-bit elements each:
   • x_in: = b & ((1<<125)-1)
   • y_in: = b >> 125 & ((1<<125)-1)
3. Compute f on input=<x_in,y_in> for n_iter iterations.
4. Denote f(input,n_iter):=<x_out, y_out>. Randomness:= keccak256(x_out, y_out, ‘veedo’).

The proof is for the following statement: "<x_out, y_out> is the result of calculating f on input=<x_in,y_in> for n_iter iterations".