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To: Ali Sherief <ali@notatether.com>
Date: Wed, 10 Aug 2022 17:05:59 +0200
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Subject: Re: [bitcoin-dev] Regarding BIP322 edge cases
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> I suppose in the case of legacy P2PKH signing, a hypothetical OP_CHECKDAT=
ASIG can take <signature> <pubkeyhash> off the stack and perform an ECDSA p=
ublic key recovery

You can always perform key recovery for legacy ECDSA: "<signature> OP_SWAP =
OP_CHECKSIG" is always spendable, for any valid <r,s> DER-encoded pair. Her=
e, if "<signature> <messageHash> <pubkey> OP_CHECKDATASIG" works for signat=
ure verification BIP, then you only need to provide "<signature>" as an inp=
ut, the rest could be placed in some output, and your program could easily =
compute that. Or you can use "OP_DUP OP_HASH160 <address> OP_EQUALVERIFY <m=
essageHash> OP_SWAP OP_CHECKDATASIG" as your output script, then the signer=
 has to provide a valid signature, and a valid public key explicitly, but i=
f you only have some signature, then you can always recover that key by you=
rself.

Because if OP_CHECKDATASIG is not handled, then still, signing by scripts c=
an handle that, but the Script would be unnecessarily complex. And because =
there is no need to introduce OP_CHECKDATASIG on-chain, it can be safely do=
ne inside this BIP.

> I'm not sure how an OP_RESERVED in an unexcuted OP_IF is going to help im=
plement an ECDSA pubkey recovery + DUP/HASH160/EQUALVERIFY hybrid instructi=
on.

Because if you want to support any script, and you want to add a new opcode=
, you have to make it somehow compatible with existing schemes. You have to=
 assign some byte for OP_CHECKDATASIG or for any new opcodes you want to ad=
d. And you don't want to assign just the next free opcode, because you will=
 get OP_CHECKSIGADD or other opcodes from TapScript, so your script decoder=
 will no longer handle two script versions at once. Unless you want to make=
 it also compatible with altcoins like BCH, but I guess it is not needed.


On 2022-08-10 15:53:31 user Ali Sherief <ali@notatether.com> wrote:
> > Backward compatibility. If we don't have OP_CHECKDATASIG, then it has t=
o be somehow introduced to make it compatible with "Bitcoin Message".

I suppose in the case of legacy P2PKH signing, a hypothetical OP_CHECKDATAS=
IG can take <signature> <pubkeyhash> off the stack and perform an ECDSA pub=
lic key recovery, followed by SHA256/RIPEMD160, kind of like a hybrid betwe=
en OP_DUP/OP_HASH160/OP_EQUALVERIFY and OP_CHECKSIG.

But the implementations would have to decode the Base58 address into "0x00"=
 plus the address hash. As the only supported invoice type for the Legacy s=
igning methods, this should be straight forward to do.

> And we have opcodes like OP_RESERVED, that can be wrapped in OP_IF, then =
it is "conditionally valid transaction".

I'm not sure how an OP_RESERVED in an unexcuted OP_IF is going to help impl=
ement an ECDSA pubkey recovery + DUP/HASH160/EQUALVERIFY hybrid instruction.

- Ali

On Wed, 10 Aug 2022 04:59:46 +0200, vjudeu@gazeta.pl wrote:
> > I'm not sure what is to be gained from adding an opcode
>
> Backward compatibility. If we don't have OP_CHECKDATASIG, then it has to =
be somehow introduced to make it compatible with "Bitcoin Message". And we =
have opcodes like OP_RESERVED, that can be wrapped in OP_IF, then it is "co=
nditionally valid transaction". It is also possible to assign some unused o=
pcode, but then it will be more complex, because in Script, those opcodes m=
ake transaction invalid, but inside TapScript, those opcodes are defined as=
 OP_SUCCESS, and make things automatically valid.
>
>
> On 2022-08-09 22:53:34 user Ali Sherief via bitcoin-dev <bitcoin-dev@list=
s.linuxfoundation.org> wrote:
> > 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 i=
t in the mailing list again to see if anyone else has suggestions for deali=
ng with them.
>
> So in an older copy of the draft at https://github.com/bitcoin/bips/blob/=
b6b0126e2d04793ba52a40f05d24538fa3f2c9ad/bip-0322.mediawiki , I found the s=
ome 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 Transac=
tions"?
> > 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 opi=
nion that one of the NOPs should be placed at the beginning of the script t=
o activate proof parsing mode.
>
> Of course, an opcode is not necessary at all, if the program is able to i=
nfer from context where the proof is coming from. After all, since they can=
not be broadcasted, they can't be mined in blocks, so will never be encount=
ered in a full node's usual verifier. I'm not sure what is to be gained fro=
m adding an opcode - the only source for real transactions is from P2P-obta=
ined blocks, so when a human inputs a signature to be verified, it can chec=
k 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, th=
at some subsection can be made saying that it can operate with them by usin=
g its scriptPubKey (and other stuff that may be necessary - I am not excatl=
y sure what goes inside the Witness stack of message_signature).
>
> In the case of the last TODO, related to delegation to another scriptPubK=
ey, I am not quite sure at the moment what to do about it - perhaps you guy=
s can place a MAST (two Merkle branches, to be specific) - the first branch=
 has the original signer's scriptPubKey, the second branch contains the del=
egated signer's scriptPubKey.
>
> - Ali