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Date: Wed, 25 Dec 2024 20:57:21 +0000
To: Bitcoin Development Mailing List <bitcoindev@googlegroups.com>
From: "'moonsettler' via Bitcoin Development Mailing List" <bitcoindev@googlegroups.com>
Subject: [bitcoindev] CTV++ OP_TEMPLATEHASH & OP_INPUTAMOUNTS
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Hi List,

There have been complaints voiced about being too paiful to work with CTV.
Especially when it comes to Vaults. OP_TX and OP_TXHASH/VERIFY are examples
how developers sought to overcome certain pain points. However they expand
the scope considerably and also rely on 64 bit integer arithmetic to handle
amounts. Here we aim to show and alternative approach that covers a lot of
useful ground in the in-between without state-carrying covenants or general
detailed introspection.

https://gist.github.com/moonsettler/d2369e043473c42ff7fa52878dd064a5

## 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 comining 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

BR,
moonsettler

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