Re: [bitcoindev] [BIP Proposal] OP_TWEAKADD

[jeremy <jeremy.l.rubin@gmail.com>]

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Laolu,

Let me know if I miss responding to any points:

I think it'd be fine to down-cost the TWEAKADD to something lower, but I'm 
not sure on the exact amount I'd recommend, since there is a link between 
amount of data and number of ops you can do, so perhaps worth thinking 
(e.g., if a really nice use case we'd want to do 100 of in a row for some 
reason, and it uses 10 script bytes, 10 budget might be good, and it would 
be annoying if we made it 20... OTOH there is a real DoS concern with 
chaining many of these if too cheap).

Generally IIUC, the reason key generation code has to check this, is 
because you could be getting entropy from a weird source. It should be 
difficult to ever hit this with a SHA256 hashed value. E.g., something like 
the current network hashrate dedicated to finding an invalid tweak for 10 
billion years, and you would expect to find a single invalid tweak.

In our case, we'd rather not have a common paradigm have malleability, if 
using an unhashed tweak for some reason.

Cheers,

Jeremy

On Thursday, September 4, 2025 at 8:43:12 PM UTC-4 Olaoluwa Osuntokun wrote:

> > Second, why fail if the passed scalar is greater than the curve order vs
> > just reducing modulo the order and using that value?
>
> Thinking about it a bit more, I think this is totally the right decision 
> and
> makes a lot of sense. 
>
> The probability that a random sha256 output will be greater than the order
> of the curve is ~1/2^128, and is something that any key generation code or
> other related application needs to deal with. 
>
> Eliminating all sources of malleability in newly proposed op codes is
> important as it serves to increase the binding of transactions w.r.t 
> blocks. 
>
> Stronger binding of transactions to blocks post segwit (otherwise possible
> to malleate transactions, causing the witness commitment validation to fail
> for an otherwise valid block), helps to mitigate a class of active
> relay impediment attacks and minimizes front-running/extractable value.
>
> -- Laolu
>
> On Wed, Sep 3, 2025 at 7:38 PM Olaoluwa Osuntokun <lao...@gmail.com> 
> wrote:
>
>>
>> Hi Jeremy,
>>
>> Cool idea, I had a similar one myself a while back. Shows that great chefs
>> think alike ;). Here're some questions that came to mind as I was 
>> reviewing
>> the doc.
>>
>> First, why accept only x-only keys? 
>>
>> From the PoV of Bitcoin Script today, they aren't used anywhere within the
>> execution environment. They also add some complexity to protocols that 
>> need
>> to accept them as input for further manipulation. They are indeed used for
>> Taproot output public keys, but those keys don't ever make their way down
>> into Script as an op code argument.
>>
>> The musig2 BIP originally accepted x-only keys as input, but was switched 
>> to
>> instead accept normal compressed public keys in version v0.8.0 [1]. The
>> switch over enabled some simplifications in the BIP, as it enabled
>> eliminating one of the accumulator variables. For more details, see the
>> discussion that led to this change [2].
>>
>> This comment from Tim resonates with my experience wrangling with bugs
>> introduced by improper/implicit handling of x-only keys over the years: 
>>
>> > Sigh yeah, x-only keys save a byte on chain but it seems the price we 
>> pay
>> > is a high engineering complexity. I think it's fair to say that noone 
>> had
>> > really anticipated this [1].
>>
>> Second, why fail if the passed scalar is greater than the curve order vs
>> just reducing modulo the order and using that value? 
>>
>> This would mean that in some cases, the direct value of a hash can't be 
>> used
>> as the scalar tweak. The probability of this happening for sha256 outputs 
>> is
>> very low, but it presents developers with yet another thing to keep in 
>> mind
>> in order to use the op code safely.
>>
>> You bring up the point that this allows the side stepping of a new source 
>> of
>> witness malleability, but in the year of Satoshi 16 (2025), aside from 
>> relay
>> nuisance shenanigans, is this something developers still need to care 
>> about?
>>
>> Third, why allocate a cost of 50 op cost vs something lower to better
>> account for the difference in operation vs normal `OP_CHECKSIG`? 
>>
>> Validating a BIP-340 signature requires an extra scalar base mult
>> (ignoring the Strauss-Shamir trick for double scalar mult for a sec) vs
>> `OP_TWEAKADD` as it's defined in your draft BIP. As a result, one could
>> argue that the op code should have a lower cost vs `OP_CHECKSIG`.
>>
>> -- Laolu
>>
>> [1]: https://github.com/jonasnick/bips/pull/37 
>> [2]: https://github.com/jonasnick/bips/issues/32
>>
>> On Sat, Aug 23, 2025 at 10:36 AM jeremy <jeremy....@gmail.com> 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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>>>
>>

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