From: Rusty Russell <rusty@rustcorp.com.au>
To: Johnson Lau <jl2012@xbt.hk>,
	bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org>
In-Reply-To: <DAAB7568-A004-4897-B5B3-0FBBC6895246@xbt.hk>
References: <CAPg+sBhuPG-2GXc+Bp0yv5ywry2fk56LPLT4AY0Kcs+YEoz4FA@mail.gmail.com>
	<87ftv3xerx.fsf@rustcorp.com.au>
	<DAAB7568-A004-4897-B5B3-0FBBC6895246@xbt.hk>
Date: Thu, 13 Dec 2018 10:19:02 +1030
Message-ID: <87pnu6s3v5.fsf@rustcorp.com.au>
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Subject: Re: [bitcoin-dev] Safer sighashes and more granular SIGHASH_NOINPUT
Precedence: list

Johnson Lau <jl2012@xbt.hk> writes:
>> On 12 Dec 2018, at 5:42 PM, Rusty Russell via bitcoin-dev <bitcoin-dev@l=
ists.linuxfoundation.org> wrote:
>>=20
>> Pieter Wuille via bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org> wr=
ites:
>>> Here is a combined proposal:
>>> * Three new sighash flags are added: SIGHASH_NOINPUT, SIGHASH_NOFEE,
>>> and SIGHASH_SCRIPTMASK.
>>> * A new opcode OP_MASK is added, which acts as a NOP during execution.
>>> * The sighash is computed like in BIP143, but:
>>>  * If SIGHASH_SCRIPTMASK is present, for every OP_MASK in scriptCode
>>> the subsequent opcode/push is removed.
>>=20
>> Having the SIGHASH_SCRIPTMASK flag is redundant AFAICT: why not always
>> perform mask-removal for signing?
>
> Because a hardware wallet may want to know what exact script it is signin=
g?

OK, removing OP_MASKs unconditionally would introduce a hole without
some explicit flag to say they've been removed (the "real script" could
be something different with OP_MASKs).  We could have the signature
commit to the outputscript, but that's a bit meh.

> Masked script has reduced security, but this is a tradeoff with
> functionality (e.g. eltoo can=E2=80=99t work without masking part of the
> script). So when you don=E2=80=99t need that extra functionality, you go =
back
> to better security
>
> However, I=E2=80=99m not sure if there is any useful NOINPUT case with un=
masked script.

This is *not* true of Eltoo; the script itself need not change for the
rebinding (Christian, did something change?).

So, can we find an example where OP_MASK is useful?

>> If you're signing arbitrary scripts, you're surely in trouble already?
>>=20
>> And I am struggling to understand the role of scriptmask in a taproot
>> world, where the alternate script is both hidden and general?
>
> It makes sure that your signature is applicable to a specific script bran=
ch, not others (assuming you use the same pubkey in many branches, which is=
 avoidable)

If I'm using SIGHASH_NOINPUT, I'm already required to take care with key
reuse.

Without a concrete taproot proposal it's hard to make assertions, but
if the signature flags that it's using the taproot script, it's
no less safe, and more general AFAICT.

Thanks!
Rusty.