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

From: Christian Decker <[email-masked]>
Date: Wed, 21 Nov 2018 12:15:44 +0100

Hi Pieter,

great proposal, I think this may address some of the (perceived)
downsides of BIP118, by committing to the script when possible
(always?). One minor thing that I noticed a while ago and that I meant
to fix on BIP118 is that `hashSequence` does not need to be blanked for
eltoo to work (since where it is needed we also use `sighash_single`),
so I'm tempted to remove that redundant blanking. It may not make a lot
of difference but it'd limit the ability to change the number of inputs
to a NOINPUT transaction (this now being the only field that commits to
the set of inputs).

As for your proposal, I really like the `sighash_scriptmask` proposal,
and committing to the fees (with the `nofee` escape hatch) also works
seems also a nice fix. My one concern is that introducing a new opcode
to mask things in the sighash looks like a similar layering violation as
`codeseparator` was, but that's just a minor issue imho.

Cheers,
Christian

Pieter Wuille via bitcoin-dev <[email-masked]>
writes:
> Hello everyone,
>
> For future segwit versions, I think it would be good add a few things
> to the sighash by default that were overlooked in BIP143:
> * Committing to the absolute transaction fee (in addition to just the
> amount being spent in each input) would categorically remove concerns
> about wallets lying about fees to HW devices or airgapped signers.
> * Committing to the scriptPubKey (in addition to the scriptCode) would
> prevent lying to devices about the type of output being spent, even
> when the scriptCode is correct. As a reminder, the scriptCode is the
> actually executed script (which is the redeemscript in non-segwit
> P2SH, and the witnesscript in P2WSH/P2WPKH).
>
> As this implies additional information that may not be desirable to
> commit to in all circumstances, it makes sense to make these optional.
> This obviously interacts with SIGHASH_NOINPUT, which really adds two
> different ways of rebinding signatures to inputs:
> * Changing the prevout (so that the txid doesn't need to be known when
> the signature is created)
> * Changing the script (so that the exact scriptPubKey/redeemScript/...
> doesn't need to be known when the signature is created)
>
> Of course, the second implies the first, but do all use cases require
> both being able to change the prevout and (arbitrarily) changing the
> scriptPubKey? While BIP118 correctly points out this is secure if the
> same keys are only used in scripts with which binding is to be
> permitted, I feel it would be preferable if signatures/scripts would
> explicitly state what can change. One way to accomplish this is by
> indicating exactly what in a script is subject to change.
>
> Here is a combined proposal:
> * Three new sighash flags are added: SIGHASH_NOINPUT, SIGHASH_NOFEE,
> and SIGHASH_SCRIPTMASK.
> * A new opcode OP_MASK is added, which acts as a NOP during execution.
> * The sighash is computed like in BIP143, but:
>   * If SIGHASH_SCRIPTMASK is present, for every OP_MASK in scriptCode
> the subsequent opcode/push is removed.
>   * The scriptPubKey being spent is added to the sighash, unless
> SIGHASH_SCRIPTMASK is set.
>   * The transaction fee is added to the sighash, unless SIGHASH_NOFEE is set.
>   * hashPrevouts, hashSequence, and outpoint are set to null when
> SIGHASH_NOINPUT is set (like BIP118, but not for scriptCode).
>
> So my question is whether anyone can see ways in which this introduces
> redundant flexibility, or misses obvious use cases?
>
> Cheers,
>
> -- 
> Pieter
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