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

From: Mark Friedenbach <[email-masked]>
Date: Mon, 11 Sep 2017 19:03:42 -0700

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 <[email-masked]> 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.

> 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.

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.

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.

> 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?

Kind regards,
Mark Friedenbach