From: Rusty Russell <rusty@rustcorp.com.au>
To: Johnson Lau <jl2012@xbt.hk>
In-Reply-To: <2302A26C-FB9C-47D2-AF6C-4D2EF02FFAC0@xbt.hk>
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Date: Fri, 21 Dec 2018 09:47:15 +1030
Message-ID: <87y38jn5z8.fsf@rustcorp.com.au>
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Cc: bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] Safer sighashes and more granular SIGHASH_NOINPUT
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Johnson Lau <jl2012@xbt.hk> writes:

>> On 17 Dec 2018, at 11:10 AM, Rusty Russell <rusty@rustcorp.com.au> wrote:
>> My anti-complexity argument leads me to ask why we'd support
>> OP_CODESEPARATOR at all?  Though my argument is weaker here: no wallet
>> need support it.
>
> Because it could make scripts more compact in some cases?
>
> This is an example: https://github.com/bitcoin/bitcoin/pull/11423#issueco=
mment-333441321 <https://github.com/bitcoin/bitcoin/pull/11423#issuecomment=
-333441321>
>
> But this is probably not a good example for taproot, as it could be more =
efficient by making the 2 branches as different script merkle leaves.

Thanks, I hadn't seen this before!  That's also the first time I've seen
SIGHASH_NONE used.

>> But I don't see how OP_CODESEPARATOR changes anything here, wrt NOINPUT?
>> Remember, anyone can create an output which can be spent by any NOINPUT,
>> whether we go for OP_MASK or simply not commiting to the input script.
>>=20
>
> Let me elaborate more. Currently, scriptCode is truncated at the last exe=
cuted CODESEPARATOR. If we have a very big script with many CODESEPARATORs =
and CHECKSIGs, there will be a lot of hashing to do.
>
> To fix this problem, it is proposed that the new sighash will always comm=
it to the same H(script), instead of the truncated scriptCode. So we only n=
eed to do the H(script) once, even if the script is very big

Yes, I read this as proposed, it is clever.  Not sure we'd be
introducing it if OP_CODESEPARATOR didn't already exist, but at least
it's a simplfication.

> In the case of NOINPUT with MASKEDSCRIPT, it will commit to the H(masked_=
script) instead of H(script).
>
> To make CODESEPARATOR works as before, the sighash will also commit to th=
e position of the last executed CODESEPARATOR. So the semantics doesn=E2=80=
=99t change. For scripts without CODESEPARATOR, the committed value is a co=
nstant.
>
> IF NOINPUT does not commit to H(masked_script), technically it could stil=
l commit to the position of the last executed CODESEPARATOR. But since the =
wallet is not aware of the actual content of the script, it has to guess th=
e meaning of such committed positions, like =E2=80=9Cwith the HD key path m=
/x/y/z, I assume the script template is blah blah blah because I never use =
this path for another script template, and the meaning of signing the 3rd C=
ODESEPARATOR is blah blah blah=E2=80=9D. It still works if the assumptions =
hold, but sounds quite unreliable to me.

My question is more fundamental.  If NOINPUT doesn't commit to the input
at all, no script, no code separator, nothing.  I'm struggling to
understand your original comment was "without signing the script or
masked script, OP_CODESEPARATOR becomes unusable or insecure with
NOINPUT."

I mean, non-useful, sure.  Its purpose is to alter signature behavior,
and from the script POV there's no signature with this form of NOINPUT.
But other than the already-established "I reused keys for multiple
outputs" oops, I don't see any new dangers?

Thanks,
Rusty.