Subject: Re: [bitcoin-dev] BIP Proposal: The Great Consensus Cleanup

From: Matt Corallo <[email-masked]>
Date: Thu, 7 Mar 2019 19:44:23 +0000

Replies inline.

On 3/7/19 10:44 AM, Luke Dashjr wrote:
> On Wednesday 06 March 2019 21:39:15 Matt Corallo wrote:
>> I'd like to ask the BIP editor to assign a BIP number.
> 
> Needs a Backward Compatibility section, and should have a bips repo PR opened
> after discussion on the ML.

Oops, I guess most of the "Discussion" section can just be moved into a 
"Backwards Compatibility" section. Will do before PR'ing.

>>    * The 4th change (making non-standard signature hash types invalid)
>> may be worth discussing. In order to limit the number of potential
>> signature hashes which could be used per-input (allowing us to cache
>> them to avoid re-calculation), we can disable non-standard sighash
>> types. Alternatively, however, most of the same effect could be achieved
>> by caching the just-before-the-last-byte sighash midstate and hashing
>> only the last byte when a checking signatures. Still, them having been
>> non-standard for many years makes me doubt there is much risk involved
>> in disabling them, and I don't see much potential use-case for keeping
>> them around so I'd like to just remove them.
> 
> I don't understand what is being removed here.

This refers to the following spec change:

If the sighash type byte (ie last byte in a signature being evaluated 
during the execution of OP_CHECKSIG[VERIFY] or OP_CHECKMULTISIG[VERIFY]) 
is anything other than 1, 2, 3, 0x81, 0x82, or 0x83, the script 
execution fails. This does not apply to 0-length signature stack elements.

>> As for why the timewarp vulnerability should (IMO rather obviously) be
>> fixed, it seems rather clear that the only potential use for exploiting
>> it would be either to inflate the currency supply maliciously by miners
>> or to fork in what amounts to extension blocks. As for why extension
>> blocks are almost certainly not the right approach to such changes, its
>> likely worth reading this old post:
>> https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2017-January/013510
>> .html
> 
> While I agree that extension blocks are typically a bad choice, I'm not sure
> the argument really applies to forward blocks. (That being said, I find
> forward blocks overcomplicated and probably not a reason to avoid this.)

I agree they are somewhat separate ideas, but the arguments in that 
thread apply equally to timewarp-based inter-block-time reductions. If 
you want to discuss it further, I'd suggest a new thread.

>> * Transactions smaller than 65 bytes when serialized without witness
>> data are invalid.
> 
> Rationale should include the reason(s) why the size doesn't count the witness
> here.

Will add.

>> ** Note that miners today only enforce increasing timestamps against the
>> median-timestamp-of-last-11-blocks, so miners who do not upgrade may
>> mine a block which violates this rule at the beginning of a difficulty
>> window if the last block in a difficulty window has a timestamp in the
>> future. Thus, it is strongly recommended that SPV clients enforce the
>> new nTime rules to avoid following any potential forks which occur.
> 
> This should probably be moved outside Discussion. (Perhaps to the missing
> Backward Compatibility section?)
> 
>> * There are several early-stage proposals which may affect the execution
>> of scripts, including proposals such as Schnorr signatures, Taproot,
>> Graftroot, and MAST. These proposals are not expected to have any
>> interaction with the changes in this BIP, as they are likely to only
>> apply to SegWit scripts, which are not covered by any of the new rules
>> except for the sighash type byte rule. Thus, the sighash type byte rule
>> defined above only applies to *current* signature-checking opcodes, as
>> any new signature-checking is likely to be implemented via the
>> introduction of new opcodes.
> 
> It's not clear that new opcodes will necessarily always be used. Probably
> would be good to clarify the "non-Segwit or witness v0 only" rule in the
> Specification section.

Note that you inherently have to use a new opcode for such things - the 
non-standard type bytes *are* defined and define a sighash/signature, 
they can't be simply redefined to a new sighash/signature type in a soft 
fork.