Re: [bitcoin-dev] Merkle branch verification & tail-call semantics for generalized MAST

[Johnson Lau <jl2012@xbt.hk>]

> On 12 Sep 2017, at 10:03 AM, Mark Friedenbach <mark@friedenbach.org> wrote:
> 
> My apologies for a delay in responding to emails on this list; I have
> been fighting a cold.
> 
> (Also my apologies to Johnson Lau, as I forgot to forward this to the list.)
> 
> On Sep 8, 2017, at 2:21 AM, Johnson Lau <jl2012@xbt.hk> wrote:
> 
>> Tail-call execution semantics require "unclean stake" , i.e. final
>> stake with more than one item. However, "unclean stake" is invalid
>> (not just non-standard) in BIP141, so you could only use it with
>> legacy P2SH (which is totally pointless....). A different design
>> like OP_EVAL might be needed, or you need a new witness script
>> version.
> 
> I believe you meant "unclean stack," and you are correct. This was
> also pointed out last tuesday by a participant at the in-person
> CoreDev meetup where the idea was presented.
> 
> This doesn't kill the idea, it just complicates the implementation
> slightly. A simple fix would be to allow tail-recursion to occur if
> the stack is not clean (as can happen with legacy P2SH as you point
> out, or yet to be defined version 1+ forms of segwit script), OR if
> there is a single item on the stack and the alt-stack is not empty.
> For segwit v0 scripts you then have to move any arguments to the alt
> stack before ending the redeem script, leaving just the policy script
> on the main stack.

This is ugly and actually broken, as different script path may require different number of stack items, so you don’t know how many OP_TOALTSTACK do you need. Easier to just use a new witness version

> 
>> I think you have also missed the sigOp counting of the executed
>> script. As you can't count it without executing the script, the
>> current static analysability is lost. This was one of the reasons
>> for OP_EVAL being rejected. Since sigOp is a per-block limit, any
>> OP_EVAL-like operation means block validity will depend on the
>> precise outcome of script execution (instead of just pass or fail),
>> which is a layer violation.
> 
> I disagree with this design requirement.
> 
> The SigOp counting method used by bitcoin is flawed. It incorrectly
> limits not the number of signature operations necessary to validate a
> block, but rather the number of CHECKSIGs potentially encountered in
> script execution, even if in an unexecuted branch. (Admitedly MAST
> makes this less of an issue, but there are still useful compact
> scripts that use if/else constructs to elide a CHECKSIG.) Nor will it
> account for aggregation when that feature is added, or properly
> differentiate between signature operations that can be batched and
> those that can not.
> 
> Additionally there are other resources used by script that should be
> globally limited, such as hash operations, which are not accounted for
> at this time and cannot be statically assessed, even by the flawed
> mechanism by which SigOps are counted. I have maintained for some time
> that bitcoin should move from having multiple separate global limits
> (weight and sigops, hashed bytes in XT/Classic/BCH) to a single linear
> metric that combines all of these factors with appropriate
> coefficients.
> 

I like the idea to have an unified global limit and suggested a way to do it (https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2017-January/013472.html). But I think this is off-topic here.



> A better way of handling this problem, which works for both SigOps and
> HashOps, is to have the witness commit to the maximum resources
> consumed by validation of the spend of the coin, to relay this data
> with the transaction and include it in the SigHash, and to use the
> committed maximum for block validation. This could be added in a
> future script version upgrade. This change would also resolve the
> issue that led to the clean stack rule in segwit, allowing future
> versions of script to use tail-call recursion without involving the
> alt-stack.
> 
> Nevertheless it is constructive feedback that the current draft of the
> BIP and its implementation do not count SigOps, at all. There are a
> couple of ways this can be fixed by evaluating the top-level script
> and then doing static analysis of the resulting policy script, or by
> running the script and counting operations actually performed.


In any case, I think maintaining static analysability for global limit(s) is very important. Ethereum had to give up their DAO softfork plan at the last minute, exactly due to the lack of this: http://hackingdistributed.com/2016/06/28/ethereum-soft-fork-dos-vector/

Otherwise, one could attack relay and mining nodes by sending many small size txs with many sigops, forcing them to validate, and discard due to insufficient fees.

Technically it might be ok if we commit the total validation cost (sigop + hashop + whatever) as the first witness stack item, but that’d take more space and I’m not sure if it is desirable. Anyway, giving up static analysability for scripts is a fundamental change to our existing model.

> 
> Additionally, it is possible that we take this time to re-evaluate
> whether we should be counting SigOps other than for legacy consensus
> rule compliance. The speed of verification in secp256k1 has made
> signature operations no longer the chief concern in block validation
> times.

Without the limit I think we would be DoS-ed to dead


>> Witness script versioning is by design fully compatible with P2SH
>> and BIP173, so there will be no hurdle for existing wallets to pay
>> to BIP114. Actually it should be completely transparent to them.
> 
> This is correct. Your feedback will be incorporated.
> 
>> For code complexity, the minimal BIP114 could be really simple, like
>> <30 lines of code? It looks complex now because it does much more
>> than simply hiding scripts in a hash.
> 
> Is there a repo that contains the latest implementation of BIP 114,
> for comparison purposes?


You can find it here: https://github.com/jl2012/bitcoin/commits/vault
https://github.com/jl2012/bitcoin/commit/f3f201d232d3995db38e09b171e4d1dea8d04ad2

But this does more than your proposal as it allows users adding extra scripts when spending a coin. The rationale is described in the revised BIP114:
https://github.com/jl2012/bips/blob/vault/bip-0114.mediawiki#Additional_scripts_in_witness

So to make it functionally comparable with your proposal, the IsMSV0Stack() function is not needed. The new 249-254 lines in interpreter.cpp could be removed. The new 1480-1519 lines could be replaced by a few lines copied from the existing P2WSH code. I can make a minimal version if you want to see how it looks like


> 
> Kind regards,
> Mark Friedenbach
>