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From: Rusty Russell <rusty@rustcorp.com.au>
To: Johnson Lau <jl2012@xbt.hk>,
	bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] Safer sighashes and more granular SIGHASH_NOINPUT
Date: Thu, 13 Dec 2018 11:07:28 +1030	[thread overview]
Message-ID: <87h8fiqn1z.fsf@rustcorp.com.au> (raw)
In-Reply-To: <87pnu6s3v5.fsf@rustcorp.com.au>

Rusty Russell <rusty@rustcorp.com.au> writes:
>> However, I’m not sure if there is any useful NOINPUT case with unmasked script.
>
> This is *not* true of Eltoo; the script itself need not change for the
> rebinding (Christian, did something change?).

This is wrong, sorry.  I re-checked the paper, and the constant for the
timelock comparison changes on each new update.

(The alternative was a new opcode like OP_TIMELOCKGREATERVERIFY which
required remembering the nLocktime for the UTXO).

So now my opinion is closer to yours: what's the use for NOINPUT &&
!NOMASK?

And is it worthwhile doing the mask complexity, rather than just
removing the commitment to script with NOINPUT?  It *feels* safer to
restrict what scripts we can sign, but is it?

Note that NOINPUT is only useful when you can't just re-sign the tx, and
you need to be able to create a new tx even if this input is spent once
(an attacker can do this with SIGHASH_MASK or not!).  ie. any other
inputs need to be signed NOINPUT or this one
SIGHASH_SINGLE|ANYONECANPAY.

You already need both key-reuse and amount-reuse to be exploited.
SIGHASH_MASK only prevents you from reusing this input for a "normal"
output; if you used this key for multiple scripts of the same form,
you're vulnerable[1].  Which, given the lightning software will be using
the One True Script, is more likely that your normal wallet using the
same keys.

So I don't think it's worth it.  SIGHASH_NOINPUT is simply dangerous
with key-reuse, and Don't Do That.

Cheers,
Rusty.
[1] Attacker can basically clone channel state to another channel.


  reply	other threads:[~2018-12-13  0:37 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
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 [this message]
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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