From: Jonas Nick <jonasdnick@gmail.com>
To: Johnson Lau <jl2012@xbt.hk>, Jonas Nick <jonasdnick@gmail.com>,
	bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org>
References: <871sewirni.fsf@gmail.com>
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Message-ID: <7bb51255-bfe5-8f11-55ae-ddabebe76941@gmail.com>
Date: Wed, 26 Sep 2018 20:40:02 +0000
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Subject: Re: [bitcoin-dev] BIP sighash_noinput
Precedence: list

Oh, I missed that that's still the case with NOINPUT - thanks for pointing it
out. In that case there's no reason to sign the other inputs' sequence and
that's even better because the current NOINPUT proposal already enables
taprootifiability of eltoo unilateral closings.

On 9/26/18 7:45 PM, Johnson Lau wrote:
> In BIP143, the nSequence of the same input is always signed, with any hashtype. Why do you need to sign the sequence of other inputs?
> 
>> On 26 Sep 2018, at 5:36 PM, Jonas Nick via bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org> wrote:
>>
>>> At the risk of bikeshedding, shouldn't NOINPUT also zero out the
>>> hashSequence so that its behaviour is consistent with ANYONECANPAY?
>>
>> There is a good reason for not doing that. If NOINPUT would sign the
>> hashSequence then it would be possible to get rid of OP_CSV in eltoo update
>> scripts. As a result update scripts could be taprootified because the more
>> common branch (settlement) would be just a 2-of-2 multisig. Applying taproot
>> would then make unilateral settlement look like a single pubkey spend and avoid
>> having to reveal the unexecuted (update) branch.
>>
>> Eltoo update transaction outputs consist of two branches, update and
>> settlement, where the update branch can be spend by a more recent update
>> transaction if an obsolete update transaction ends up spending the funding
>> output. The settlement branch is a 2-of-2 multisig with a relative timelock
>> using OP_CSV. Removing OP_CSV is possible because both parties signature is
>> required to spend the update transaction. They will only sign if the input has
>> the right sequence numbers which is sufficient to enforce the timeout (BIP68) -
>> assuming they are covered by the signature.
>>
>> There's a catch: hashSequence includes the sequence numbers of all transaction
>> inputs. That's not a problem for eltoo because settlement transactions only
>> have one input. The update mechanism with update transactions relies on being
>> able to bump the fee by unilaterally adding inputs and and change outputs to
>> the transaction. That's also not a problem because update spends do not use
>> relative timelocks and they are signed with SINGLE. So whenever NOINPUT is
>> combined SINGLE the hashSequence should be zeroed. This is in fact what a
>> minimal change to the current NOINPUT implementation would naturally do (see
>> below). However, that's error-prone when using NOINPUT in other contexts so in
>> general it would be better if NOINPUT would only sign the sequence number of
>> the corresponding input.
>>
>> Another downside of this approach is that you can never rebind to an output
>> with an OP_CSV that requires a larger sequence number, unless you also sign
>> with SIGHASH_SINGLE. It's difficult to imagine application where this would be
>> an issue.
>>
>> This is the modification to the NOINPUT implementation
>> (https://github.com/cdecker/bitcoin/commits/noinput) which makes eltoo
>> unilateral closes taprootifiable:
>> +++ b/src/script/interpreter.cpp
>> @@ -1223,7 +1223,7 @@ uint256 SignatureHash(const CScript& scriptCode, const CTransaction& txTo, unsig
>>             hashPrevouts = cacheready ? cache->hashPrevouts : GetPrevoutHash(txTo);
>>         }
>>
>> -        if (!(nHashType & SIGHASH_ANYONECANPAY) && (nHashType & 0x1f) != SIGHASH_SINGLE && (nHashType & 0x1f) != SIGHASH_NONE && !noinput) {
>> +        if (!(nHashType & SIGHASH_ANYONECANPAY) && (nHashType & 0x1f) != SIGHASH_SINGLE && (nHashType & 0x1f) != SIGHASH_NONE) {
>>             hashSequence = cacheready ? cache->hashSequence : GetSequenceHash(txTo);
>>         }
>>
>> On 5/1/18 4:58 PM, Russell O'Connor via bitcoin-dev wrote:
>>> At the risk of bikeshedding, shouldn't NOINPUT also zero out the
>>> hashSequence so that its behaviour is consistent with ANYONECANPAY?
>>>
>>
> 
>