Add FindAndDelete for Signature hash calculation

[sanket1729]

Add support sighash for scripts with OP_CODESEPARATOR.

[sanket1729]

I will do it at some point in the future but is up for grabs if anyone wants to do it, it's up for grabs

[apoelstra]

lol nobody wants to do this, I'm afraid it's gonna be on you :)

[vss96]

I would like to give this a try but I might be out of my depth here 😅

[apoelstra]

@vss96 that would be welcome! But be aware that our signature hash computation has changed a bit since this issue was opened. In particular, since #628 we have a new structure, util::sighash::SigHashCache which is responsible for computing sighashes.

To implement CODESEPARATOR, you need to change the legacy sighash algorithm to implement FindAndDelete (see the Bitcoin Core source code for what this is); for the segwit and taproot sighashes, the CODESEPARAOR behavior is much simpler and can be found in their respective BIPs (141 for segwit and 341 for taproot, I believe)

[sanket1729]

@vss96 you can reach me on IRC(libera) or on Twitter for more assistance

[tcharding]

@vss96 its been a while since you posted so I had a go at this one. Hope you don't mind. I've only done part of it so far, holla at me if you are working on it still.

[vss96]

@tcharding Please go ahead and work on it, I never got the time to work on this 😓 .

[tcharding]

Its late at night but I think I've got this, correct me if I'm wrong

• I believe FindAndDelete is only called during script evaluation so we don't need to worry about it
• For legacy sighash OP_CODESEPARATOR is handled by Do not serialize OP_CODESEPARATOR #879
• For segwit sighash OP_CODESEPARATOR is not explicitly checked for, the script is serialized directly (see https://github.com/bitcoin/bitcoin/blob/8435d7f11a89bb3f93306646f62cc2179693e072/src/script/interpreter.cpp#L1630)
• For taproot sighash the only mention of OP_CODESEPARATOR I see is the code_separator_pos (m_codeseparator_pos in bitcoin-core) and we handle that already

So unless I'm mistaken #879 fully closes this issue. Some hand-wavy supporting evidence; all the tests in bitcoin/bitcoin/src/test/data/sighash.json appear to be in libbtc and going off number of LOC (about 500) #879 adds all of them and they all pass.

[tcharding]

Shall we close this one then @sanket1729?

ref: #777 (comment)

[tcharding]

We already have an implementation of FindAndDelete, its script_find_and_remove - and we are removing it :)