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From: Anthony Towns <aj@erisian.com.au>
To: Christian Decker <decker.christian@gmail.com>,
	Bitcoin Protocol Discussion
	<bitcoin-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] Safer sighashes and more granular SIGHASH_NOINPUT
Date: Fri, 23 Nov 2018 16:04:04 +1000	[thread overview]
Message-ID: <20181123060404.fu4eyzcynbppmjcy@erisian.com.au> (raw)
In-Reply-To: <87k1l6d6lb.fsf@gmail.com>

On Wed, Nov 21, 2018 at 12:15:44PM +0100, Christian Decker via bitcoin-dev wrote:
> One minor thing that I noticed a while ago and that I meant
> to fix on BIP118 is that `hashSequence` does not need to be blanked for
> eltoo to work (since where it is needed we also use `sighash_single`),
> so I'm tempted to remove that redundant blanking. It may not make a lot
> of difference but it'd limit the ability to change the number of inputs
> to a NOINPUT transaction (this now being the only field that commits to
> the set of inputs).

Commiting to just the sequence numbers seems really weird to me; it
only really prevents you from adding inputs, since you could still
replace any input that was meant to be there by almost any arbitrary
other transaction...

I could see this *maybe* making sense if you at least committed to the
values of each input's outpoint; since that would be an actual constraint?

I don't think you can commit to anything else about the other inputs:

   -- txids of the other transactions wouldn't work if you had other
      NOINPUT txes, and would introduce O(N^2) validation cost if someone
      signed every input with NOINPUT but committed to the txids of
      every other input

   -- scriptPubKeys wouldn't really work for eltoo-like constructions
      that want to vary the scripts but apply the same sig, but might
      work sometimes?

   -- witness scripts for the other inputs could be unknown at your
      signing time, or arbitrarily large and thus a pain to have to send
      to a hardware wallet

Just treating NOINPUT as a subset of ANYONECANPAY seems simpler to
me though...

> As for your proposal, I really like the `sighash_scriptmask` proposal,
> and committing to the fees (with the `nofee` escape hatch) also works
> seems also a nice fix. My one concern is that introducing a new opcode
> to mask things in the sighash looks like a similar layering violation as
> `codeseparator` was, but that's just a minor issue imho.

I think OP_MASK is okay as far as layering goes, if you just think of it
as a (set of) multibyte "OP_MASKED_PUSH" opcode(s). So when you
pseudocode a script like:

    <n> OP_CSV OP_DROP <p> OP_CHECKSIG

and then decide <n> needs to be masked, you rewrite it as:

    [n] OP_CSV OP_DROP <p> OP_CHECKSIG

indicating n is masked, and don't worry about the exact bytes that will
encode the push, anymore than you currently worry about whether it's
OP_0, OP_1..16, <1..75>+1..75-bytes, PUSHDATA[1,2,3]+n+n-bytes.

As long as OP_MASK only applies to a PUSH and it's an error for OP_MASK
not to be immediately followed by that PUSH, I think that all works
out fine.

Cheers,
aj



  reply	other threads:[~2018-11-23  6:04 UTC|newest]

Thread overview: 55+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-11-19 22:37 [bitcoin-dev] Safer sighashes and more granular SIGHASH_NOINPUT 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 [this message]
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
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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