Date: Tue, 09 Aug 2022 13:09:13 +0000
To: bitcoin-dev@lists.linuxfoundation.org
From: Ali Sherief <ali@notatether.com>
Reply-To: Ali Sherief <ali@notatether.com>
Message-ID: <20220809130908.yykum37tre7um4j3@artanis>
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Subject: [bitcoin-dev] Regarding BIP322 edge cases
Precedence: list

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 w=
ith them.

So in an older copy of the draft at https://github.com/bitcoin/bips/blob/b6=
b0126e2d04793ba52a40f05d24538fa3f2c9ad/bip-0322.mediawiki , I found the som=
e 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 Transacti=
ons"?
> TODO: Some invalid opcode to allow only in various proof types?
> TODO: A way for the initial signer to delegate to another scriptPubKey; n=
eeded for better privacy and CoinJoin/Lightning compatibility

So to start with, I believe it will be very helpful to limit what opcodes s=
criptPubKeys to be elligible to sign from them. The specification already d=
oes so to a point, but in order for these to be recognizable, it's my opini=
on 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 inf=
er from context where the proof is coming from. After all, since they canno=
t be broadcasted, they can't be mined in blocks, so will never be encounter=
ed 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-obtain=
ed 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 in=
put.

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 h=
as the original signer's scriptPubKey, the second branch contains the deleg=
ated signer's scriptPubKey.

- Ali