From: Rusty Russell <rusty@rustcorp.com.au>
To: Pieter Wuille <pieter.wuille@gmail.com>,
	Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>,
	Bitcoin Dev <bitcoin-dev@lists.linuxfoundation.org>
In-Reply-To: <CAPg+sBhuPG-2GXc+Bp0yv5ywry2fk56LPLT4AY0Kcs+YEoz4FA@mail.gmail.com>
References: <CAPg+sBhuPG-2GXc+Bp0yv5ywry2fk56LPLT4AY0Kcs+YEoz4FA@mail.gmail.com>
Date: Wed, 12 Dec 2018 20:12:10 +1030
Message-ID: <87ftv3xerx.fsf@rustcorp.com.au>
MIME-Version: 1.0
Content-Type: text/plain
Subject: Re: [bitcoin-dev] Safer sighashes and more granular SIGHASH_NOINPUT
Precedence: list

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.

I'm asking on-list because I'm sure I'm not the only confused one.

Having the SIGHASH_SCRIPTMASK flag is redundant AFAICT: why not always
perform mask-removal for signing?

If you're signing arbitrary scripts, you're surely in trouble already?

And I am struggling to understand the role of scriptmask in a taproot
world, where the alternate script is both hidden and general?

I look forward to learning what I missed!
Rusty.