Deterministic randomness for each SPARK round

[bajtos]

ETA: 2024-07-31

Many steps of the decentralised SPARK Protocol rely on a source of deterministic randomness. This can be DRAND or Ethereum opcode PREVRANDAO.

In my limited understanding:

• In ETH, the randomness is derived from the signature or the hash of the previous block (tipset)
• In FVM, the randomness is derived from the DRAND randomness for the current epoch
  More info:
• FVM discussion: EVM Runtime: Make DIFFICULTY random filecoin-project/ref-fvm#1241
• EIP introducing PREVRANDAO opcode: https://eips.ethereum.org/EIPS/eip-4399

We need to integrate that source of randomness into SPARK, so that for each SPARK round, the following components know the randomness value:

• spark-api
• spark-evaluate
• spark (Station module)

If feasible, we think it's better & safer if the IE smart contract can store & emit the randomness value for the current round, instead of trying to find it after the round by mapping the FVM or ETH epoch to the DRAND round using the genesis timestamps.