[bitcoin-dev] Regarding BIP322 edge cases

[Ali Sherief <ali@notatether.com>]

Although there is a Github issue/PR at https://github.com/bitcoin/bips/pull/1347 for addressing all the TODO items of BIP322, I decided to throw it in the mailing list again to see if anyone else has suggestions for dealing with them.

So in an older copy of the draft at https://github.com/bitcoin/bips/blob/b6b0126e2d04793ba52a40f05d24538fa3f2c9ad/bip-0322.mediawiki , I found the some TODO items, and I will copy-paste the ones in the Specification section (for full proofs) here:

> TODO: How does this interact with as-of-yet-unspecified "Silent Transactions"?
> TODO: Some invalid opcode to allow only in various proof types?
> TODO: A way for the initial signer to delegate to another scriptPubKey; needed for better privacy and CoinJoin/Lightning compatibility

So to start with, I believe it will be very helpful to limit what opcodes scriptPubKeys to be elligible to sign from them. The specification already does so to a point, but in order for these to be recognizable, it's my opinion that one of the NOPs should be placed at the beginning of the script to activate proof parsing mode.

Of course, an opcode is not necessary at all, if the program is able to infer from context where the proof is coming from. After all, since they cannot be broadcasted, they can't be mined in blocks, so will never be encountered in a full node's usual verifier. I'm not sure what is to be gained from adding an opcode - the only source for real transactions is from P2P-obtained blocks, so when a human inputs a signature to be verified, it can check that a real transaction is not being inserted by looking for the invalid input.

For Silent Transactions, I have already given my suggestion in the PR, that some subsection can be made saying that it can operate with them by using its scriptPubKey (and other stuff that may be necessary - I am not excatly sure what goes inside the Witness stack of message_signature).

In the case of the last TODO, related to delegation to another scriptPubKey, I am not quite sure at the moment what to do about it - perhaps you guys can place a MAST (two Merkle branches, to be specific) - the first branch has the original signer's scriptPubKey, the second branch contains the delegated signer's scriptPubKey.

- Ali