Re: [bitcoin-dev] BIP sighash_noinput

From: Jonas Nick <jonasdnick@gmail•com>
To: Russell O'Connor <roconnor@blockstream•io>,
	Bitcoin Protocol Discussion
	<bitcoin-dev@lists•linuxfoundation.org>,
	Christian Decker <decker.christian@gmail•com>
Subject: Re: [bitcoin-dev] BIP sighash_noinput
Date: Wed, 26 Sep 2018 09:36:57 +0000	[thread overview]
Message-ID: <feff454d-2cc2-2104-5c6c-69630506bd56@gmail.com> (raw)
In-Reply-To: <CAMZUoK=V9Dii7ja6n6mpW_tWt00PuT=-9Z8XoyKOY3y0_AwK5w@mail.gmail.com>

> 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?
> 
> On Mon, Apr 30, 2018 at 12:29 PM, Christian Decker via bitcoin-dev <
> bitcoin-dev@lists•linuxfoundation.org> wrote:
> 
>> Hi all,
>>
>> I'd like to pick up the discussion from a few months ago, and propose a new
>> sighash flag, `SIGHASH_NOINPUT`, that removes the commitment to the
>> previous
>> output. This was previously mentioned on the list by Joseph Poon [1], but
>> was
>> never formally proposed, so I wrote a proposal [2].
>>
>> We have long known that `SIGHASH_NOINPUT` would be a great fit for
>> Lightning.
>> They enable simple watch-towers, i.e., outsource the need to watch the
>> blockchain for channel closures, and react appropriately if our
>> counterparty
>> misbehaves. In addition to this we just released the eltoo [3,4] paper
>> which
>> describes a simplified update mechanism that can be used in Lightning, and
>> other
>> off-chain contracts, with any number of participants.
>>
>> By not committing to the previous output being spent by the transaction,
>> we can
>> rebind an input to point to any outpoint with a matching output script and
>> value. The binding therefore is no longer explicit through a reference, but
>> through script compatibility, and the transaction ID reference in the
>> input is a
>> hint to validators. The sighash flag is meant to enable some off-chain
>> use-cases
>> and should not be used unless the tradeoffs are well-known. In particular
>> we
>> suggest using contract specific key-pairs, in order to avoid having any
>> unwanted
>> rebinding opportunities.
>>
>> The proposal is very minimalistic, and simple. However, there are a few
>> things
>> where we'd like to hear the input of the wider community with regards to
>> the
>> implementation details though. We had some discussions internally on
>> whether to
>> use a separate opcode or a sighash flag, some feeling that the sighash flag
>> could lead to some confusion with existing wallets, but given that we have
>> `SIGHASH_NONE`, and that existing wallets will not sign things with unknown
>> flags, we decided to go the sighash way. Another thing is that we still
>> commit
>> to the amount of the outpoint being spent. The rationale behind this is
>> that,
>> while rebinding to outpoints with the same value maintains the value
>> relationship between input and output, we will probably not want to bind to
>> something with a different value and suddenly pay a gigantic fee.
>>
>> The deployment part of the proposal is left vague on purpose in order not
>> to
>> collide with any other proposals. It should be possible to introduce it by
>> bumping the segwit script version and adding the new behavior.
>>
>> I hope the proposal is well received, and I'm looking forward to discussing
>> variants and tradeoffs here. I think the applications we proposed so far
>> are
>> quite interesting, and I'm sure there are many more we can enable with this
>> change.
>>
>> Cheers,
>> Christian
>>
>> [1] https://lists.linuxfoundation.org/pipermail/bitcoin-dev/
>> 2016-February/012460.html
>> [2] https://github.com/cdecker/bips/blob/noinput/bip-xyz.mediawiki
>> [3] https://blockstream.com/2018/04/30/eltoo-next-lightning.html
>> [4] https://blockstream.com/eltoo.pdf
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> 
> 
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