Re: [bitcoin-dev] Safer sighashes and more granular SIGHASH_NOINPUT

[Johnson Lau <jl2012@xbt.hk>]

> On 23 Dec 2018, at 12:26 PM, Anthony Towns <aj@erisian.com.au> wrote:
> 
> On Sat, Dec 22, 2018 at 02:54:42AM +0800, Johnson Lau wrote:
>> The question I would like to ask is: is OP_CODESEPARATOR useful under taproot? Generally speaking, CODESEPARATOR is useful only with conditional opcodes (OP_IF etc), and conditional opcodes are mostly replaced by merklized scripts. I am not sure how much usability is left with CODESEPARATOR
> 
> If you don't have conditionals, then I think committing to the (masked)
> script gives you everything you could do with codeseparator.

I don’t think CODESEPARATOR is useful without conditionals. By useful I mean making a script more compact

> 
> If you don't commit to the (masked) script, don't have conditionals,
> and don't have codeseparator, then I don't think you can make a signature
> distinguish which alternative script it's intending to sign; but you can
> just give each alternative script in the MAST a slight variation of the
> key and that seems good enough.

You can and should always use a different in different branch. If this best practice is always followed, committing to masked script is not necessary

> 
> OTOH, I think for (roughly) the example you gave:
> 
>  DEPTH 3 EQUAL
>  IF <Bob> CHECKSIGVERIFY HASH160 <H> EQUALVERIFY CODESEP
>  ELSE <n> CLTV DROP
>  ENDIF
>  <Alice> CHECKSIG
> 
> then compared to the taproot equivalent:
> 
>  P = muSig(Alice,Bob)
>  S1 = <Alice1> CHECKSIGVERIFY <Bob> CHECKSIGVERIFY HASH160 <H> EQUAL
>  S2 = <Alice2> CHECKSIGVERIFY <n> CLTV
> 
> the IF+CODESEP approach is actually cheaper (lighter weight) if you're
> mostly (>2/3rds of the time) taking the S1 branch. This is because the
> "DEPTH 3 EQUAL IF/ELSE/ENDIF CODESEP <n> CLTV DROP" overhead is less
> than the 32B overhead to choose a merkle branch).
> 
> (That said, I'm not sure what Alice's signature in the S1 branch actually
> achieves in that script; and without that in S1, the taproot approach is
> cheaper all the time. Scriptless scripts would be cheaper still)
> 
>> If no one needs CODESEPARATOR, we might just disable it, and makes the validation code a bit simpler
> 
> Since it only affects the behaviour of the checkdls (checksig) operators,
> even if it was disabled, it could be re-enabled fairly easily in a new
> script subversion if needed (ie, it could be re-added when upgrading
> witness version 1 from script version 0 to 1).
> 
> Cheers,
> aj
> 

Yes, I don’t think it needs Alice signature in S1 at all. So the original example doesn’t even need CODESEPARATOR at all. 

Could anyone propose a better use case of CODESEPARATOR?