CTV + OP_TEMPLATEHASH + OP_INPUTAMOUNTS

CTV++.md

CTV++

Two additional opcodes to consider as an addition to OP_CHECKTEMPLATEVERIFY:

• OP_TEMPLATEHASH
• OP_INPUTAMOUNTS

Together they relax the strict limitations that OP_CHECKTEMPLATEVERIFY imposes, because it explicitly commits to the output amounts and therefore implicitly commits to the spendable input amounts in a lot of cases.

OP_TEMPLATEHASH

Builds the CTV template hash from the stack.

<inputIndex>
# FOR i = inputCount-1; i >= 0; i--; LOOP
  <sequence[i]>
# END LOOP
<inputCount>
<nLockTime>
# FOR j = outputCount-1; j >= 0; j--; LOOP
  <out[j].amount>
  <out[j].script>
# END LOOP
<outputCount>
<nVersion>
OP_TEMPLATEHASH
OP_CHECKTEMPLATEVERIFY

OP_INPUTAMOUNTS

Taproot only. Consumes a 32 bit signed integer n as parameter

• n = 0 return the SUM of all input amounts with same script
• n < 0 return the SUM of last abs(n) input amounts including current input
• n > 0 return the SUM of first n input amounts of the transaction

fails if

• n < 0 and abs(n) > inputIndex + 1
• n > inputCount

<n>
OP_INPUTAMOUNTS

Example use:

This contract below allows the combining of UTXOs locked by the same script for something like a withdrawal from a Vault contract to a staging address where the relative time lock can begin. It works with any amount UTXOs unlike basic CTV Vaults. Also allows for paying fees endogenously and registering a change address. The fee paying input would sign with SIGHASH_ALL.

### Witness stack BEGIN ###

<inputIndex>
# FOR i = inputCount-1; i >= 0; i--; LOOP
  <sequence[i]>
# END LOOP
<inputCount>
<nLockTime>
<changeAmount>        # out[1].amount
<changeScriptPub>     # out[1].script

### Witness stack END ###

<0>                   # sum of all inputs with same script
OP_INPUTAMOUNTS       # out[0].amount
<stagingScriptPub>    # out[0].script 33 bytes for P2TR
<2>                   # outputCount
<2>                   # nVersion
OP_TEMPLATEHASH
OP_CHECKTEMPLATEVERIFY

Credits:

• Jeremy Rubin who have already came up with everything many years ago
• James O'Beirne for his awesome work on OP_VAULT
• Salvatore Ingala for his work on CCV/MATT and to generalize state carrying covenants
• Many others who have explored the covenant design space and CTV in particular