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From: Johnson Lau <jl2012@xbt.hk>
In-Reply-To: <87ftv3xerx.fsf@rustcorp.com.au>
Date: Thu, 13 Dec 2018 04:00:50 +0800
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Message-Id: <DAAB7568-A004-4897-B5B3-0FBBC6895246@xbt.hk>
References: <CAPg+sBhuPG-2GXc+Bp0yv5ywry2fk56LPLT4AY0Kcs+YEoz4FA@mail.gmail.com>
	<87ftv3xerx.fsf@rustcorp.com.au>
To: Rusty Russell <rusty@rustcorp.com.au>,
	bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] Safer sighashes and more granular SIGHASH_NOINPUT
Precedence: list


> On 12 Dec 2018, at 5:42 PM, Rusty Russell via bitcoin-dev =
<bitcoin-dev@lists.linuxfoundation.org> wrote:
>=20
> Pieter Wuille via bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org> =
writes:
>> 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
> I'm asking on-list because I'm sure I'm not the only confused one.
>=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 =
signing?

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 =
unmasked script.

>=20
> 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 =
branch, not others (assuming you use the same pubkey in many branches, =
which is avoidable)

>=20
> I look forward to learning what I missed!
> Rusty.
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