[bitcoin-dev] Version 1 witness programs (first draft)

[Luke Dashjr <luke@dashjr.org>]

I've put together a first draft for what I hope to be a good next step for 
Segwit and Bitcoin scripting:
    https://github.com/luke-jr/bips/blob/witnessv1/bip-witnessv1.mediawiki

This introduces 5 key changes:

1. Minor versions for witnesses, inside the witness itself. Essentially the 
witness [major] version 1 simply indicates the witness commitment is SHA256d, 
and nothing more.

The remaining two are witness version 1.0 (major 1, minor 0):

2. As previously discussed, undefined opcodes immediately cause the script to 
exit with success, making future opcode softforks a lot more flexible.

3. If the final stack element is not exactly true or false, it is interpreted 
as a tail-call Script and executed. (Credit to Mark Friedenbach)

4. A new shorter fixed-length signature format, eliminating the need to guess 
the signature size in advance. All signatures are 65 bytes, unless a condition 
script is included (see #5).

5. The ability for signatures to commit to additional conditions, expressed in 
the form of a serialized Script in the signature itself. This would be useful 
in combination with OP_CHECKBLOCKATHEIGHT (BIP 115), hopefully ending the 
whole replay protection argument by introducing it early to Bitcoin before any 
further splits.

This last part is a big ugly right now: the signature must commit to the 
script interpreter flags and internal "sigversion", which basically serve the 
same purpose. The reason for this, is that otherwise someone could move the 
signature to a different context in an attempt to exploit differences in the 
various Script interpretation modes. I don't consider the BIP deployable 
without this getting resolved, but I'm not sure what the best approach would 
be. Maybe it should be replaced with a witness [major] version and witness 
stack?

There is also draft code implementing [the consensus side of] this:
    https://github.com/bitcoin/bitcoin/compare/master...luke-jr:witnessv1

Thoughts? Anything I've overlooked / left missing that would be 
uncontroversial and desirable? (Is any of this unexpectedly controversial for 
some reason?)

Luke