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 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. > > -- > You received this message because you are subscribed to the Google Groups > "Bitcoin Development Mailing List" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to bitcoindev+unsubscribe@googlegroups.com. > To view this discussion visit > https://groups.google.com/d/msgid/bitcoindev/bc9ff794-b11e-47bc-8840-55b2bae22cf0n%40googlegroups.com > > . > -- You received this message because you are subscribed to the Google Groups "Bitcoin Development Mailing List" group. To unsubscribe from this group and stop receiving emails from it, send an email to bitcoindev+unsubscribe@googlegroups.com. To view this discussion visit https://groups.google.com/d/msgid/bitcoindev/CAO3Pvs-TMwQuxa2JJq8MY%3D%3DG0nFsqrTis6sPHLayxZOqPuvBtQ%40mail.gmail.com.