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

[Johnson Lau <jl2012@xbt.hk>]

Some comments with the tail-call execution semantics BIP:

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 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.

(An alternative is to make sigOp a per-input limit instead of per-block limit, just like the 201 nOp limit. But this is a very different security model)

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.

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.



> On 7 Sep 2017, at 8:38 AM, Mark Friedenbach via bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org> wrote:
> 
> I would like to propose two new script features to be added to the
> bitcoin protocol by means of soft-fork activation. These features are
> a new opcode, MERKLE-BRANCH-VERIFY (MBV) and tail-call execution
> semantics.
> 
> In brief summary, MERKLE-BRANCH-VERIFY allows script authors to force
> redemption to use values selected from a pre-determined set committed
> to in the scriptPubKey, but without requiring revelation of unused
> elements in the set for both enhanced privacy and smaller script
> sizes. Tail-call execution semantics allows a single level of
> recursion into a subscript, providing properties similar to P2SH while
> at the same time more flexible.
> 
> These two features together are enough to enable a range of
> applications such as tree signatures (minus Schnorr aggregation) as
> described by Pieter Wuille [1], and a generalized MAST useful for
> constructing private smart contracts. It also brings privacy and
> fungibility improvements to users of counter-signing wallet/vault
> services as unique redemption policies need only be revealed if/when
> exceptional circumstances demand it, leaving most transactions looking
> the same as any other MAST-enabled multi-sig script.
> 
> I believe that the implementation of these features is simple enough,
> and the use cases compelling enough that we could BIP 8/9 rollout of
> these features in relatively short order, perhaps before the end of
> the year.
> 
> I have written three BIPs to describe these features, and their
> associated implementation, for which I now invite public review and
> discussion:
> 
> Fast Merkle Trees
> BIP: https://gist.github.com/maaku/41b0054de0731321d23e9da90ba4ee0a
> Code: https://github.com/maaku/bitcoin/tree/fast-merkle-tree
> 
> MERKLEBRANCHVERIFY
> BIP: https://gist.github.com/maaku/bcf63a208880bbf8135e453994c0e431
> Code: https://github.com/maaku/bitcoin/tree/merkle-branch-verify
> 
> Tail-call execution semantics
> BIP: https://gist.github.com/maaku/f7b2e710c53f601279549aa74eeb5368
> Code: https://github.com/maaku/bitcoin/tree/tail-call-semantics
> 
> Note: I have circulated this idea privately among a few people, and I
> will note that there is one piece of feedback which I agree with but
> is not incorporated yet: there should be a multi-element MBV opcode
> that allows verifying multiple items are extracted from a single
> tree. It is not obvious how MBV could be modified to support this
> without sacrificing important properties, or whether should be a
> separate multi-MBV opcode instead.
> 
> Kind regards,
> Mark Friedenbach
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