[bitcoindev] [BIP Proposal] OP_TWEAKADD

[bitcoindev] [BIP Proposal] OP_TWEAKADD

[jeremy]

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Hi all,

I've made a draft BIP writeup of an (often discussed) simple opcode, 
OP_TWEAKADD, deployable as an OP_SUCCESSx upgrade.

https://github.com/bitcoin/bips/pull/1944

This opcode is relatively simple. The main design choices are:

1) Verify v.s. Push semantics -- Push, for succinctness on-chain
2) Argument order -- Key on top, for tweak in witness
3) Plain tweak or something else -- Plain tweak, if hashing is desirable 
the user can do it. The most flexible is to do a plain tweak. Future work 
could add TapTree opcodes to construct taproot tweaks.

Feedback and discussion are welcome.

Best,

Jeremy

[^1] OP_SHA256 in these example prevents key-cancellation.

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[bitcoindev] Re: [BIP Proposal] OP_TWEAKADD

[jeremy]

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realized that I accidentally didn't post a few notable examples. I've left 
these out of the BIP largely, but could include more examples as desired:


*Tweak Reveal Scripts:*
OP_TWEAKADD composes, for example, with OP_CSFS and OP_IKEY which have been 
proposed separately in BIP-348, BIP-349.
```
witness: <sig> <msg> <tweak>
program: OP_SHA256^1 OP_IKEY OP_TWEAKADD OP_CSFS
```
or
```
witness: <sig> <tweak>
program: OP_SHA256 OP_IKEY OP_TWEAKADD OP_CHECKSIG
```



*Proof-of-Signing-Order & Transaction "refinement":*

A signs strictly after B, with B's signature fully committed. 

```
witness: <sig A> <sig B>
program: DUP TOALT <B> CHECKSIGVERIFY FROMALT SHA256 <A> OP_TWEAKADD 
OP_CHECKSIG
```

A is bound to B's signature, so B fixes whatever details for A.

N.B. B may use any sighash combination mode, so A *refines* B's signature.

*Delegation:*

Key A signs tweaked by key B, key B signs whatever. 

Key A can use a very limited (e.g. SIGHASH_NONE) sighash mode.

```
witness: <sig A> <sig B> <B>
program: DUP TOALT CHECKSIGVERIFY FROMALT SHA256 <A> OP_TWEAKADD OP_CHECKSIG
```

*Target Tweak:*

proves that <t> is known such that `tG + k1G = k2G`

```
witness: <t>
program: <k1G> OP_TWEAKADD <k2G> OP_EQUAL
```

```
witness: <t>
program: SHA256 <k1G> OP_TWEAKADD <k2G> OP_EQUAL
```
Can be used, if k2G is a Taproot output, to "force" disclosure/presence of 
a TapTweak



*Key Reveal:*

Use Target Tweak to "program" a key reveal contract.

Proves knowledge of discrete log of T = tG.

Take T and compute T+G = (t+1)G.

```
witness: <t>
program: <G> OP_TWEAKADD <T+G>
```

[^1] OP_SHA256 in these examples prevents key-cancellation.


On Saturday, August 23, 2025 at 1:36:44 PM UTC-4 jeremy wrote:

> Hi all,
>
> I've made a draft BIP writeup of an (often discussed) simple opcode, 
> OP_TWEAKADD, deployable as an OP_SUCCESSx upgrade.
>
> https://github.com/bitcoin/bips/pull/1944
>
> This opcode is relatively simple. The main design choices are:
>
> 1) Verify v.s. Push semantics -- Push, for succinctness on-chain
> 2) Argument order -- Key on top, for tweak in witness
> 3) Plain tweak or something else -- Plain tweak, if hashing is desirable 
> the user can do it. The most flexible is to do a plain tweak. Future work 
> could add TapTree opcodes to construct taproot tweaks.
>
> Feedback and discussion are welcome.
>
> Best,
>
> Jeremy
>
> [^1] OP_SHA256 in these example prevents key-cancellation.
>

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