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From: Anthony Towns <aj@erisian.com.au>
To: Johnson Lau <jl2012@xbt.hk>
Cc: bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] Safer sighashes and more granular SIGHASH_NOINPUT
Date: Sun, 23 Dec 2018 14:26:59 +1000	[thread overview]
Message-ID: <20181223042659.munrqfe4l6nff2ug@erisian.com.au> (raw)
In-Reply-To: <73F32BC6-751E-4F35-BE6D-B31170FC0A54@xbt.hk>

On Sat, Dec 22, 2018 at 02:54:42AM +0800, Johnson Lau wrote:
> The question I would like to ask is: is OP_CODESEPARATOR useful under taproot? Generally speaking, CODESEPARATOR is useful only with conditional opcodes (OP_IF etc), and conditional opcodes are mostly replaced by merklized scripts. I am not sure how much usability is left with CODESEPARATOR

If you don't have conditionals, then I think committing to the (masked)
script gives you everything you could do with codeseparator.

If you don't commit to the (masked) script, don't have conditionals,
and don't have codeseparator, then I don't think you can make a signature
distinguish which alternative script it's intending to sign; but you can
just give each alternative script in the MAST a slight variation of the
key and that seems good enough.

OTOH, I think for (roughly) the example you gave:

  DEPTH 3 EQUAL
  IF <Bob> CHECKSIGVERIFY HASH160 <H> EQUALVERIFY CODESEP
  ELSE <n> CLTV DROP
  ENDIF
  <Alice> CHECKSIG

then compared to the taproot equivalent:

  P = muSig(Alice,Bob)
  S1 = <Alice1> CHECKSIGVERIFY <Bob> CHECKSIGVERIFY HASH160 <H> EQUAL
  S2 = <Alice2> CHECKSIGVERIFY <n> CLTV

the IF+CODESEP approach is actually cheaper (lighter weight) if you're
mostly (>2/3rds of the time) taking the S1 branch. This is because the
"DEPTH 3 EQUAL IF/ELSE/ENDIF CODESEP <n> CLTV DROP" overhead is less
than the 32B overhead to choose a merkle branch).

(That said, I'm not sure what Alice's signature in the S1 branch actually
achieves in that script; and without that in S1, the taproot approach is
cheaper all the time. Scriptless scripts would be cheaper still)

> If no one needs CODESEPARATOR, we might just disable it, and makes the validation code a bit simpler

Since it only affects the behaviour of the checkdls (checksig) operators,
even if it was disabled, it could be re-enabled fairly easily in a new
script subversion if needed (ie, it could be re-added when upgrading
witness version 1 from script version 0 to 1).

Cheers,
aj



  reply	other threads:[~2018-12-23  4:27 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
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 [this message]
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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