From: Anthony Towns <aj@erisian•com.au>
To: Rusty Russell <rusty@rustcorp•com.au>,
Bitcoin Protocol Discussion
<bitcoin-dev@lists•linuxfoundation.org>
Subject: Re: [bitcoin-dev] Safer sighashes and more granular SIGHASH_NOINPUT
Date: Thu, 13 Dec 2018 10:24:38 +1000 [thread overview]
Message-ID: <20181213002438.r3bylrjcpwit5v2k@erisian.com.au> (raw)
In-Reply-To: <87ftv3xerx.fsf@rustcorp.com.au>
On Wed, Dec 12, 2018 at 08:12:10PM +1030, Rusty Russell via bitcoin-dev wrote:
> 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?
The way I'm thinking about it is there's four amounts of knowledge you
could have about the input you're spending at the time you generate a
signature:
ALL: you know everything about every input for this tx
SINGLE: you know everything about the input you're signing for, but
not necessarily the others
SCRIPTPUBKEY: you know the exact scriptPubKey you're trying to satisfy, but
don't know the txid
SCRIPTMASK: you don't know the txid, don't know the scriptPubKey, don't
know the other taproot branches, and maybe don't even know the masked
out terms in the script -- but you do know the structure of the
script, and the non-masked terms
There's no value to masking in any but the final case -- the txid and
scriptPubKey commit to the full scriptcode already, so also signing the
scriptcode is just belt-and-suspenders protection.
(It might be that the "SCRIPTPUBKEY" option isn't very useful in
practice; maybe you'll always either know the txid, or need to mask
something?)
> And I am struggling to understand the role of scriptmask in a taproot
> world, where the alternate script is both hidden and general?
In a taproot world, your scriptPubKey is a point P=Q+H(Q,S)*G, where S
is a merkle root of possibly many scripts, and is spendable either by:
sig(P)
Q, path(S,script), script, witness(script)
SCRIPTMASK lets you prepare a signature for one particular script in
advance, even before you've decided what the other scripts are (and even
what the base point Q is), let alone built an actual transaction.
At least, that's my current understanding; and I think it makes sense...
Cheers,
aj
next prev parent reply other threads:[~2018-12-13 0:24 UTC|newest]
Thread overview: 55+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-11-19 22:37 Pieter Wuille
2018-11-20 20:29 ` Anthony Towns
2018-11-21 11:20 ` Christian Decker
2018-11-21 17:55 ` Johnson Lau
2018-11-21 11:15 ` Christian Decker
2018-11-23 6:04 ` Anthony Towns
2018-11-23 9:40 ` Christian Decker
2018-11-24 8:13 ` Johnson Lau
2018-11-21 17:07 ` Russell O'Connor
2018-11-22 14:28 ` Johnson Lau
2018-11-22 16:23 ` Russell O'Connor
2018-11-22 20:52 ` Johnson Lau
2018-11-22 22:10 ` Russell O'Connor
2018-11-23 10:47 ` Johnson Lau
2018-11-23 5:03 ` Anthony Towns
2018-11-23 20:18 ` Russell O'Connor
2018-11-28 3:41 ` Pieter Wuille
2018-11-28 8:31 ` Johnson Lau
2018-11-29 17:00 ` Christian Decker
2018-11-29 18:29 ` Christian Decker
2018-12-06 16:57 ` Russell O'Connor
2018-12-09 19:13 ` Johnson Lau
2018-12-11 22:50 ` Russell O'Connor
2018-12-12 19:53 ` Johnson Lau
2018-12-13 16:50 ` Russell O'Connor
2018-12-13 0:05 ` Anthony Towns
2018-12-13 16:21 ` Russell O'Connor
2018-12-14 0:47 ` Anthony Towns
[not found] ` <CAAS2fgRma+Pw-rHJSOKRVBqoxqJ3AxHO9d696fWoa-sb17JEOQ@mail.gmail.com>
2018-12-13 16:34 ` Russell O'Connor
2018-12-09 22:41 ` David A. Harding
2018-12-11 15:36 ` Russell O'Connor
2018-12-11 17:47 ` David A. Harding
2018-12-12 9:42 ` Rusty Russell
2018-12-12 20:00 ` Johnson Lau
2018-12-12 23:49 ` Rusty Russell
2018-12-13 0:37 ` Rusty Russell
2018-12-14 9:30 ` Anthony Towns
2018-12-14 13:55 ` Johnson Lau
2018-12-17 3:10 ` Rusty Russell
2018-12-20 19:34 ` Johnson Lau
2018-12-20 23:17 ` Rusty Russell
2018-12-21 18:54 ` Johnson Lau
2018-12-23 4:26 ` Anthony Towns
2018-12-23 16:33 ` Johnson Lau
2018-12-24 12:01 ` ZmnSCPxj
2018-12-24 21:23 ` Johnson Lau
2018-12-16 6:55 ` Rusty Russell
2018-12-17 19:08 ` Johnson Lau
2018-12-18 4:22 ` Peter Todd
2018-12-19 0:39 ` Rusty Russell
2019-02-09 0:39 ` Pieter Wuille
2018-12-13 0:24 ` Anthony Towns [this message]
2018-11-28 0:54 Bob McElrath
2018-11-28 8:40 ` Johnson Lau
2018-11-28 14:04 ` Bob McElrath
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