Align implementation with HeadV1 protocol specification

[ch1bo]

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Why

In the work stream to mature the Hydra Head protocol enough to aim for a mainnet release, we have been running our implementation against a formal model and created an updated specifiction (because the protocol evolved since publishing the peer-reviewed paper).

These prior works will/did highlight gaps in our implementation of the protocol behavior and/or the scripts that should ensure protocol security on the layer 1.

What

This work package is about addressing and closing these first gaps:

• Make sure the on-chain code (validators) is checking what is specified
• Make sure the off-chain code (HeadLogic + chain layer) is working as specified

How

• Go through all the gaps and fix them (detail this step)
• Update or create tests corresponding to each item and link them from the gap list closed in this work package

Gaps identified so far

Transition security

• Head ID needs to be checked in each v_head transition
  • we wouldn't know wheter some NFT is indeed an ST, so we need to keep the head currency id in the datum
• ST in output value not checked right now
• Upper tx validity could be very big on close/contest (e.g. close but have deadline in 10 years)
  • enforce a bound using T_max - T_min < some slots
• Check committed outputs are reimbursed on v_head instead of v_commit
  • individual checks would "collapse" if two identical TxOuts were committed and only one needs to be reimbursed to pass
• Validator can assume first output is v_head output (simplification)
• Snapshot number and utxo hash need not be in the redeemer on close/contest
  • the output datum needs to include them and we can check using that
  • only the signature is enough
• Do not allow contest by closing party and only allow contest once
  • need to collect who contested in the datum and check it
  • needed? leads to upper bound of T_final when we do this 👇
• Deadline handling changed between paper and V1 spec -> deadline gets "pushed out" on each contest
  • this makes the contestation period easier to determine and scales with number of parties

Protocol logic

• Sign network messages / authenticate them to be sure they are from a given Party (Hydra VK)
• Not send transactions with ReqSn, but only transaction ids (Hashes of transactions)
  • Book-keep transactions seend via ReqTx
  • ReqSn handling would then need to resolve TxId -> Tx
• Decision when to snapshot is different than in spec
  • Best case: contrived and refactor it
  • Worst case: actually different and not clear which is correct
• "Detect cheating" if we see a ReqSn or AckSn with s > s_seen + 1
• Actually check the the aggregated signature and "detect cheating" if not

[ch1bo]

Marked 🔴 as not yet discussed. Depends on outcomes of #194 #448.

[abailly-iohk]

Marked amber as the list of gaps appears somewhat clearer (and solvable...)

[ch1bo]

When running hydra heads we might have discovered another gap / at least an annoyance: #612