Re: [bitcoin-dev] cleanstack alt stack & softfork improvements (Was: Merkle branch verification & tail-call semantics for generalized MAST)

[Mark Friedenbach <mark@friedenbach.org>]

> On Sep 19, 2017, at 10:13 PM, Johnson Lau <jl2012@xbt.hk> wrote:
> 
> If we don’t want this ugliness, we could use a new script version for every new op code we add. In the new BIP114 (see link above), I suggest to move the script version to the witness, which is cheaper.

To be clear, I don’t think it is so much that the version should be moved to the witness, but rather that there are two separate version values here — one in the scriptPubKey which specifies the format and structure of the segwit commitment itself, and another in the witness which gates functionality in script or whatever else is used by that witness type. Segwit just unfortunately didn’t include the latter, an oversight that should be corrected on the on the next upgrade opportunity.

The address-visible “script version” field should probably be renamed “witness type” as it will only be used in the future to encode how to check the witness commitment in the scriptPubKey against the data provided in the witness. Upgrades and improvements to the features supported by those witness types won’t require new top-level witness types to be defined. Defining a new opcode, even one with modifies the stack, doesn’t change the hashing scheme used by the witness type.

v0,32-bytes is presently defined to calculate the double-SHA256 hash of the top-most serialized item on the stack, and compare that against the 32-byte commitment value. Arguably it probably should have hashed the top two values, one of which would have been the real script version. This could be fixed however, even without introducing a new witness type. Do a soft-fork upgrade that checks if the witness redeem script is push-only, and if so then pop the last push off as the script version (>= 1), and concatenate the rest to form the actual redeem script. We inherit a little technical debt from having to deal with push limits, but we avoid burning v0 in an upgrade to v1 that does little more than add a script version.

v1,32-bytes would then be used for a template version of MAST, or whatever other idea comes along that fundamentally changes the way the witness commitment is calculated.

Mark