Align minting policy

[ch1bo]

Why

The minting policy ensures security of a Hydra Head on-chain by:

• providing a unique identity to a Head (HeadId)
• helps in identifying a "proper" Head, i.e. one that follows the rules
• able to tell a proper Head from a faked one apart
• want to limit necessary knowledge required by users/implementations off-chain

Within this feature we want to make observing arbitrary, proper Heads correct and as simple as possible. While observing and deciding whether to interact with a head in the hydra-node may still involve some off-chain checks.

The overall strategy should be that off-chain code ensures that minting policy ran for a given InitTx.

What

We want to implement a "full" minting policy to do some checks on-chain and simplify the off-chain handling. See #718 for some more details and rationale.

1. ✔ Ensure minting policy mu_head is parameterized by an out-ref, it's policy id / currency symbol then is the unique headId

   • to enforce on-chain check that out-ref is spent when minting any token under this policy

2. ✔ Ensure out-ref and the headId are in the datum of the first output of the transaction which mints tokens.

3. ✔ Off-chain, we use the out-ref and headId from the datum to check whether headId == hash(mu_head(out-ref)) holds.

   • need to check there's a matching ST too

4. ✔️ Ensure the minting policy mu_head also checks on-chain when minting:

   • There is single state token that is paid into v_head, which ensures continuity
   • The the number of minted PTs == number of participants evident from the datum

5. ✔️ Do not restrict burning on the mu_head at all. It is ensured by the v_head validator, when tokens of a specific headId may be burned.

6. ✔️ Additional checks we decide to do off-chain still (when constructing and observing the init tx) :

   • the pub key hashes of all configured participants == the token names of PTs
   • the contestation period in the datum matches the configured one
   • the Hydra keys (parties) in the datum matches the configured keys

How

• Update the spec to reflect these changes (and rationale)
• Implement the on-chain checks as specified above
  • Using mutation tests on the init and abort transactions to verify correct working of mu_head
  • Update mutation test expectations on burning tokens as we not do these checks anymore.
• Update initTx and observeInitTx to satisfy and do the off-chain checks above
  • Using state-full tx interface of intialize and observeInit functions and verify in StateSpec