Hi Johnson,

Thanks for the review. I do agree that OP_COSHV (note the pluralization --
it would also be possible to do a OP_COHV <index> <hash> to do specific

I think the point of OP_COSHV is that something like ANYPREVOUT is much
more controversial. OP_COSHV is a subset by design. The IF on ANYPREVOUT is
substantial, discussion I've seen shows that the safety of ANYPREVOUT is
far from fully agreed. (I'll respond to your other email on the subject
too). OP_COSHV is also proposed specifically as a congestion control
mechanism, and so keeping it very easy to verify and minimal data
(optimizations allow reducing it to just OP_COSHV with no 32 byte argument)
suggest this approach is preferable.

In an earlier version, rather than have it be the first input restriction,
I had implemented it an an only one input restriction. This makes it easier
to work with SIGHASH_SINGLE. This works by having the PrecomputedData have
a atomic test_flag. However I felt that the statefulness between
verifications was not great and so I simplified it.

There actually is a reason to require minimal push -- maybe we can change
the rule to be non-minimal pushes are ignored, because we can later extend
it with a different rule. This seems a little error prone. There's also no
reason to not just treat OP_COSHV as a pushdata 32 itself, and drop the
extra byte if we don't care about versioning later.

Requiring a signature actually makes COSHV less useful. So I'm against that
-- such a signature prevents using OP_COSHV for non-interactive
setups/uncoordinated setups where the txids are unstable. It also makes
building the trees more expensive. If you want this feature, a better thing
to do would be to always tweak leaf nodes of the tx tree entropy so that
it's unique per key and doesn't impose extra data at every node, only the
leafs of the expansion tree.

@JeremyRubin <https://twitter.com/JeremyRubin>

On Fri, May 24, 2019 at 12:13 PM Johnson Lau <jl2...@xbt.hk> wrote:

> Functionally, COHV is a proper subset of ANYPREVOUT (NOINPUT). The only
> justification to do both is better space efficiency when making covenant.
> With eltoo as a clear usecase of ANYPREVOUT, I’m not sure if we really
> want a very restricted opcode like COHV. But these are my comments, anyway:
> 1. The “one input” rule could be relaxed to “first input” rule. This
> allows adding more inputs as fees, as an alternative to CPFP. In case the
> value is insufficient to pay the required outputs, it is also possible to
> rescue the UTXO by adding more inputs.
> 2. While there is no reason to use non-minimal push, there is neither a
> reason to require minimal push. Since minimal push is never a consensus
> rule, COHV shouldn’t be a special case.
> 3. As I suggested in a different post (
> https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2019-May/016963.html),
> the argument for requiring a prevout binding signature may also be
> applicable to COHV
> On 21 May 2019, at 4:58 AM, Jeremy via bitcoin-dev <
> bitcoin-dev@lists.linuxfoundation.org> wrote:
> Hello bitcoin-devs,
> Below is a link to a BIP Draft for a new opcode,
> OP_CHECKOUTPUTSHASHVERIFY. This opcode enables an easy-to-use trustless
> congestion control techniques via a rudimentary, limited form of covenant
> which does not bear the same technical and social risks of prior covenant
> designs.
> Congestion control allows Bitcoin users to confirm payments to many users
> in a single transaction without creating the UTXO on-chain until a later
> time. This therefore improves the throughput of confirmed payments, at the
> expense of latency on spendability and increased average block space
> utilization. The BIP covers this use case in detail, and a few other use
> cases lightly.
> The BIP draft is here:
> https://github.com/JeremyRubin/bips/blob/op-checkoutputshashverify/bip-coshv.mediawiki
> The BIP proposes to deploy the change simultaneously with Taproot as an
> OPSUCCESS, but it could be deployed separately if needed.
> An initial reference implementation of the consensus changes and  tests
> which demonstrate how to use it for basic congestion control is available
> at https://github.com/JeremyRubin/bitcoin/tree/congestion-control.  The
> changes are about 74 lines of code on top of sipa's Taproot reference
> implementation.
> Best regards,
> Jeremy Rubin
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