Lightning transactions with v3 and ephemeral anchors

Protocol Design
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14

Mermaid diagram with miniscript for @t-bast’s proposed commitment transaction format for whatever it’s worth:

classDiagram
   class FundingTx {
      [inputs...]
      out: and(pk(A), pk(B))
   }
   class CommitmentTxB {
       FundingTx
       Anchor: true()
       ABal: pk(A)
       BBal: or(pk(ARevoke), and(pk(B), older(Delay)))
       HTLCtoA: or(and(pk(A), pk(B)), and(pk(A), hash160(S)), pk(ARevoke))
       HTLCtoB: or(and(pk(A), pk(B)), and(pk(A), after(timeout)), pk(ARevoke))
   }

   class HTLCClaimB {
      HTLCtoA / HTLCtoB -- presigned by A with ACP|SINGLE
      out: or(and(pk(B), older(Delay)), pk(ARevoke))
   }
   FundingTx --|> CommitmentTxB
   CommitmentTxB --|> HTLCClaimB

If CommitmentTxB is v3, then the fact that it has an ephemeral anchor output would mean that that output has to be spent concurrently for the tx to be relayed, so Bob might broadcast Tx1 just to get CommitmentTxB funded, or might post Tx2 to also lock in some of the HTLCs that have either expired or that he knows the preimage for.

classDiagram
   class TxB1 {
       CommitmentTxB Anchor
       ConfirmedUTXO
       change: pk(B)
   }
   class TxB2 {
       CommitmentTxB Anchor
       CommitmentTxB HTLCtoA
       CommitmentTxB HTLCtoB
       ConfirmedUTXO
       change: pk(B)
       HTLCtoA.out: or(and(pk(B), older(delay)), pk(ARevoke))
       HTLCtoB.out: or(and(pk(B), older(delay)), pk(ARevoke))
   }
   CommitmentTxB --|>TxB1
   CommitmentTxB --|>TxB2

Alternatively, A might decide B’s txs don’t pay enough fees and decide to get things confirmed earlier, either by posting her own CommitmentTxA (conflicting with CommitmentTxB) in much the same way, or by creating her own spends of CommitmentTxB (conflicting with TxB1 or TxB2):

classDiagram
   class TxA0 {
       CommitmentTxB Anchor
       CommitmentTxB ABal -- all signed with ARevoke
       CommitmentTxB BBal
       CommitmentTxB HTLCtoA
       CommitmentTxB HTLCtoB
       change: pk(A)
   }
   class TxA1 {
       CommitmentTxB Anchor
       CommitmentTxB ABal
       CommitmentTxB HTLCtoA
       CommitmentTxB HTLCtoB
       change: pk(A)
   }
   CommitmentTxB --|>TxA0
   CommitmentTxB --|>TxA1

In either case, she probably doesn’t need to use any confirmed funds, as she can pay the on-chain fees directly from her channel balance immediately.

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