[Bitcoin-development] CLTV opcode allocation; long-term plans?

[Peter Todd <pete@petertodd.org>]

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Matt Corallo brought up¹ the issue of OP_NOP scarcity on the mempool
only CLTV pull-req²:

    "I like merging this, but doing both CLTV things in one swoop would be
    really nice. Certainly if we're gonna use one of the precious few
    OP_NOPs we have we might as well make it more flexible."

I have two lines of thought on this:

1) We're going to end up with a Script v2.0 reasonably soon, probably
   based on Russel O'Connor and Pieter Wuille's Merkelized Abstract Syntax
   Tree³ idea. This needs at most a single OP_NOPx to implement and mostly
   removes the scarcity of upgradable NOP's.

2) Similarly in script v1.0 even if we do use up all ten OP_NOPx's, the
   logical thing to do is implement an <actual opcode #> OP_EXTENDED.

3) It's not clear what form a relative CLTV will actually take; the BIP
   itself proposes a OP_PREVOUT_HEIGHT_VERIFY/OP_PREVOUT_DATA along with
   OP_ADD, with any opcode accessing non-reorg-safe prevout info being made
   unavailable until the coinbase maturity period has passed for
   soft-fork safeness.

That said, if people have strong feelings about this, I would be willing
to make OP_CLTV work as follows:

    <nLockTime> 1 OP_CLTV

Where the 1 selects absolute mode, and all others act as OP_NOP's. A
future relative CLTV could then be a future soft-fork implemented as
follows:

    <relative nLockTime> 2 OP_CLTV

On the bad side it'd be two or three days of work to rewrite all the
existing tests and example code and update the BIP, and (slightly) gets
us away from the well-tested existing implementation. It also may
complicate the codebase compared to sticking with just doing a Script
v2.0, with the additional execution environment data required for v2.0
scripts cleanly separated out. But all in all, the above isn't too big
of a deal.

Interested in your thoughts.

1) https://github.com/bitcoin/bitcoin/pull/5496#issuecomment-98568239
2) https://github.com/bitcoin/bitcoin/pull/5496
3) http://css.csail.mit.edu/6.858/2014/projects/jlrubin-mnaik-nityas.pdf

-- 
'peter'[:-1]@petertodd.org
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