[bitcoindev] CHECKSIGFROMSTACK(VERIFY/ADD)

[Brandon Black <freedom@reardencode.com>]

Hi list,

As we're working toward numbering and merge for the CHECKSIGFROMSTACK
(CSFS) BIP, there are 2 open questions[1] that may be worth resolving
before it is merged as a draft:

* Should CHECKSIGFROMSTACKVERIFY (CSFSV) be added to pre-tapscript?

The proposed opcode always evaluates BIP340 Schnorr signatures
regardless of script version, so making it available in earlier script
versions makes Schnorr signatures available on those script versions for
certain use cases. 

My personal thinking in initially including CSFSV in earlier script
versions was basically that it's compatible with NOP forking, so why
not. Because LNHANCE includes CTV which is designed as a NOP compatible
upgrade, also including CSFSV fits well with CTV.

The other side of the argument is that we shouldn't include
compatibility with earlier script versions unless there's a concrete
benefit to doing so. For CTV, the possibility of bare CTV is a
compelling reason to add it to earlier script versions, but there's not
a similarly compelling reason to include CSFSV.

Using a scarce NOP to provide Schnorr signed commitments to earlier
scripts may not be worthwhile.


* Should we include CHECKSIGFROMSTACKADD?

Obviously, if script multisig is going to be a common use case for
checking signatures on stack data CHECKSIGFROMSTACKADD simplifies the
corresponding scripts by a few WU per key. As MuSig2 and FROST are
progressing in standardization and implementation, I do not expect
script multisig to be a dominant use for these opcodes, so I did not
include CSFSA initially.

Here the argument is somewhat the inverse of CSFSV on legacy: We have
many OP_SUCCESSes available, so the cost of allocating one for CSFSA is
low, and the benefit is that making script multisigs with CSFSA (such as
those produced by miniscript) is simpler and less error prone.

--

I would love to hear thoughts about both of these questions from the
list, and will update the BIP and implementations of CSFS(V/A) based on
your feedback.

Thanks much!

--Brandon

[1]: https://github.com/bitcoin/bips/pull/1535#issuecomment-2111195930

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