decodescript could infer miniscript from given script in "segwit" context #27007

I was playing around with a probably-too-cute script for LN channel smart contract, and was looking for something to decide for me if miniscript can infer the structure of the script.

f.e.

decodescript 82012088a914ffffffffffffffffffffffffffffffffffffffff8820ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffad51b2

{
  "asm": "OP_SIZE 32 OP_EQUALVERIFY OP_HASH160 ffffffffffffffffffffffffffffffffffffffff OP_EQUALVERIFY ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff OP_CHECKSIGVERIFY 1 OP_CHECKSEQUENCEVERIFY",
  "desc": "raw(82012088a914ffffffffffffffffffffffffffffffffffffffff8820ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffad51b2)#5guaahuv",
  "type": "nonstandard",
  "p2sh": "33YyjgxsJYwSi8Wbp7TaCSFAuJBoUfYWDQ",
  "segwit": {
    "asm": "0 99f5d0b69f9828fea9363198cf812e95d7fc5e94c7881092b8e3f43984c1bd5c",
    "desc": "addr(bc1qn86apd5lnq50a2fkxxvvlqfwjhtlch55c7yppy4cu06rnpxph4wqrar3md)#wtlf0gwk",
    "hex": "002099f5d0b69f9828fea9363198cf812e95d7fc5e94c7881092b8e3f43984c1bd5c",
    "address": "bc1qn86apd5lnq50a2fkxxvvlqfwjhtlch55c7yppy4cu06rnpxph4wqrar3md",
    "type": "witness_v0_scripthash",
    "p2sh-segwit": "3MRJN2wTnD2BVm31HLcBwueAYpbbximrtk"
  }
}

Since it's evaluated at "top level for desc it cannot get any meaningful structure from it. Perhaps in segwit it could try infering the miniscript in the segwit context

The issue here is that in decodescript we do try to parse a wsh descriptor from a constructed P2WSH scriptPubKey: https://github.com/bitcoin/bitcoin/blob/2b211b41e36f914b8d0487e698b619039cc3c8e2/src/rpc/rawtransaction.cpp#L519-L530

But inferScript won't be able to analyze the witness script itself since we are passing a dummy signature provider: https://github.com/bitcoin/bitcoin/blob/2b211b41e36f914b8d0487e698b619039cc3c8e2/src/core_write.cpp#L150-L156

Therefore GetCScript won't return anything: https://github.com/bitcoin/bitcoin/blob/2b211b41e36f914b8d0487e698b619039cc3c8e2/src/script/descriptor.cpp#L1622-L1629

I would think that all we need to do is add the RPC argument script to the signature provider used for inference.

Yes, it's that simple (well, example I gave above was xonly key, so added 0x02 prefix):

decodescript 82012088a914ffffffffffffffffffffffffffffffffffffffff882102ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffad51b2
{
  "asm": "OP_SIZE 32 OP_EQUALVERIFY OP_HASH160 ffffffffffffffffffffffffffffffffffffffff OP_EQUALVERIFY 02ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff OP_CHECKSIGVERIFY 1 OP_CHECKSEQUENCEVERIFY",
  "desc": "raw(82012088a914ffffffffffffffffffffffffffffffffffffffff882102ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffad51b2)#x8xuy2m2",
  "type": "nonstandard",
  "p2sh": "2MzccBLNZbDP7tXZkrKKp5Zi7wDX4XpvAjQ",
  "segwit": {
    "asm": "0 ef65a26f417cee177ce157608612a95ff6a45a052dc440883cf2d069fca8fc82",
    "desc": "wsh(and_v(and_v(v:hash160(ffffffffffffffffffffffffffffffffffffffff),v:pk(02ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff)),older(1)))#twc36s59",
    "hex": "0020ef65a26f417cee177ce157608612a95ff6a45a052dc440883cf2d069fca8fc82",
    "address": "bcrt1qaaj6ym6p0nhpwl8p2asgvy4ftlm2gks99hzypzpu7tgxnl9gljpq8j9ewh",
    "type": "witness_v0_scripthash",
    "p2sh-segwit": "2MxgwFJU1Ei4eYqbShbVSX2ecYYooEm8zxy"
  }
}

@instagibbs do you intend to PR this? Otherwise i can do it.

I didn't love the interface change I did for it. You give it a shot?

On Wed, Feb 1, 2023, 8:47 AM Antoine Poinsot @.***> wrote:

@instagibbs https://github.com/instagibbs do you intend to PR this? Otherwise i can do it.

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