Re: [bitcoin-dev] [Lightning-dev] More thoughts on NOINPUT safety

[Anthony Towns <aj@erisian.com.au>]

On Thu, Mar 21, 2019 at 10:05:09AM +0000, ZmnSCPxj wrote:
> > IF OP_CODESEPARATOR <i> OP_CHECKLOCKTIMEVERIFY OP_DROP ENDIF
> > <muSig(A_u,B_u)> OP_CHECKDLSVERIFY <Q> OP_CHECKDLS
> > Signing with NOINPUT,NOSCRIPT and codeseparatorpos=1 enforces CLTV
> > and allows binding to any prior update tx -- so works for an update tx
> > spending previous update txs; while signing with codeseparatorpos=-1
> > and NOINPUT but committing to the script code and nSequence (for the
> > CSV delay) allows binding to only that update tx -- so works for the
> > settlement tx. That's two pubkeys, two sigs, and the taproot point
> > reveal.
> 
> Actually, the shared keys are different in the two branches above.

Yes, if you're not committing to the script code you need the separate
keys as otherwise any settlement transaction could be used with any
update transaction. 

If you are committing to the script code, though, then each settlement
sig is already only usable with the corresponding update tx, so you
don't need to roll the keys. But you do need to make it so that the
update sig requires the CLTV; one way to do that is using codeseparator
to distinguish between the two cases.

> Also, I cannot understand `OP_CODESEPARATOR`, please no.

If codeseparator is too scary, you could probably also just always
require the locktime (ie for settlmenet txs as well as update txs), ie:

  OP_CHECKLOCKTIMEVERIFY OP_DROP
  <muSig(A_u,B_u)> OP_CHECKDLSVERIFY <Q> OP_CHECKDLS

and have update txs set their timelock; and settlement txs set a absolute
timelock, relative timelock via sequence, and commit to the script code.

(Note that both those approaches (with and without codesep) assume there's
some flag that allows you to commit to the scriptcode even though you're
not committing to your input tx (and possibly not committing to the
scriptpubkey). BIP118 doesn't have that flexibility, so the A_s_i and
B_s_i key rolling is necessary)

Cheers,
aj