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From: Johnson Lau <jl2012@xbt.hk>
To: Russell O'Connor <roconnor@blockstream.io>
Cc: bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] Safer sighashes and more granular SIGHASH_NOINPUT
Date: Fri, 23 Nov 2018 18:47:10 +0800	[thread overview]
Message-ID: <D8767C89-06AF-4FA7-B640-E99FE8A443C5@xbt.hk> (raw)
In-Reply-To: <CAMZUoKkdjHYd8BR6PCae-UG_QRoujW36kYf8s4Gk2FVBeSJnrw@mail.gmail.com>

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>Even still, each call to OP_CODESEPARATOR / OP_CHECKSIG pair requires recomputing a new #5. scriptCode from BIP 143, and hence computes a new transaction digest.

In the existing sighash (i.e. legacy and BIP143), there are 6 canonical SIGHASH types: 1, 2, 3, 0x81, 0x82, 0x83. In consensus, however, all 256 types are valid and distinct. An adversarial miner could use non-standard sighash types to nullify any attempt to cache sighash values (i.e. you have to compute a new tx digest for every OP_CHECKSIG, even without using OP_CODESEPARATOR).

The only way to prevent this is reject OP_CODESEPARATOR, FindAndDelete(), and non-standard SIGHASH with a softfork. However, this doesn’t work in the next-generation SIGHASH, as tens of standard sighash types will exist. And, more importantly, sighash cache is no longer necessary in segwit, with the legacy O(n^2) hash bug being fixed.

In summary, sighash cache is not necessary nor efficient in the next-generation SIGHASH, and is not a sufficient reason to remove OP_CODESEPARATOR, especially when people find OP_CODESEPARATOR useful in some way.

But just to be clear, I think OP_CODESEPARATOR should be deprecated in legacy scripts. There is a general negative sentiment against OP_CODESEPARATOR but I think we need to evaluate case by case.

> On 23 Nov 2018, at 6:10 AM, Russell O'Connor <roconnor@blockstream.io> wrote:
> 
> 
> 
> On Thu, Nov 22, 2018 at 3:53 PM Johnson Lau <jl2012@xbt.hk <mailto:jl2012@xbt.hk>> wrote:
> Assuming a script size of 128 bytes (including SHA256 padding), 2^20 scripts is 134MB. Double it to 268MB for the merkle branch hashes. With roughly 100MB/s, this should take 2.5s (or 42min for 30 levels). However, memory use is not considered.
> 
> >each call to this operation effectively takes O(script-size) time
> I’m not sure if this is correct. Actually, CTransactionSignatureSerializer() scans every script for OP_CODESEPARATOR. Scripts with and without OP_CODESEPARATOR should take exactly the same O(script-size) time (see https://github.com/bitcoin/bitcoin/pull/14786 <https://github.com/bitcoin/bitcoin/pull/14786>)
> Also, this is no longer a concern under segwit (BIP143), which CTransactionSignatureSerializer() is not used. Actually, OP_CODESEPARATOR under segwit is way simpler than the proposed OP_MASK. If one finds OP_MASK acceptable, there should be no reason to reject OP_CODESEPARATOR.
> 
> Even still, each call to OP_CODESEPARATOR / OP_CHECKSIG pair requires recomputing a new #5. scriptCode from BIP 143, and hence computes a new transaction digest.  I understood that this issue was the main motivation for wanting to deprecate OP_CODESEPARATOR and remove it from later versions of script.
> 
> However, given that we are looking at a combinatorial explosion in SIGHASH flag combinations already, coupled with existing SigOp limitations, maybe the cost of recomputing scriptCode with OP_CODESEPARATOR isn't such a big deal.
> 
> And even if we choose remove the behavior of OP_CODESEPARATOR in new versions of Script, it seems more than 30 layers of sequential OP_IFs can be MASTified, so there is no need to use OP_CODESEPARATOR within that limit.
> 
> >One suggestion I heard (I think I heard it from Pieter) to achieve the above is to add an internal counter that increments on every control flow operator,……...
> If I have to choose among OP_CODESEPARATOR and “flow operator counting”, I’d rather choose OP_CODESEPARATOR. At least we don’t need to add more lines to the consensus code, just for something that is mostly archivable with MAST.
> 


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  reply	other threads:[~2018-11-23 10:47 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 [this message]
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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