[bitcoin-dev] Let's deploy BIP65 CHECKLOCKTIMEVERIFY!

Peter Todd pete at petertodd.org
Sun Sep 27 18:50:31 UTC 2015


Summary
-------

It's time to deploy BIP65 CHECKLOCKTIMEVERIFY.

I've backported the CLTV op-code and a IsSuperMajority() soft-fork to
the v0.10 and v0.11 branches, pull-reqs #6706 and #6707 respectively. A
pull-req for git HEAD for the soft-fork deployment has been open since
June 28th, #6351 - the opcode implementation itself was merged two
months ago.

We should release a v0.10.3 and v0.11.1 with CLTV and get the ball
rolling on miner adoption. We have consensus that we need CLTV, we have
a well tested implementation, and we have a well-tested deployment
mechanism. We also don't need to wait for other soft-fork proposals to
catch up - starting the CLTV deployment process isn't going to delay
future soft-forks, or for that matter, hard-forks.

I think it's possible to safely get CLTV live on mainnet before the end
of the year. It's time we get this over with and done.


Detailed Rational
-----------------

1) There is a clear need for CLTV

Escrow and payment channels both benefit greatly from CLTV. In
particular, payment channel implementations are made significantly
simpler with CLTV, as well as more secure by removing the malleability
vulnerability.

Why are payment channels important? There's a lot of BTC out there
vulnerable to theft that doesn't have to be. For example, just the other
day I was talking with Nick Sullivan about ChangeTip's vulnerability to
theft, as well as regulatory uncertainty about whether or not they're a
custodian of their users' funds. With payment channels ChangeTip would
only be able to spend as much of a deposit as a user had spent, keeping
the rest safe from theft. Similarly, in the other direction - ChangeTip
to their users - in many cases it is feasible to also use payment
channels to immediately give users control of their funds as they
receive them, again protecting users and helping make the case that
they're not a custodian. In the future I'm sure we'll see fancy
bi-directional payment channels serving this role, but lets not let
perfect be the enemy of good.


2) We have consensus on the semantics of the CLTV opcode

Pull-req #6124 - the implementation of the opcode itself - was merged
nearly three months ago after significant peer review and discussion.
Part of that review process included myself(1) and mruddy(2) writing
actual demos of CLTV. The chance of the CLTV semantics changing now is
near-zero.


3) We have consensus that Bitcoin should adopt CLTV

The broad peer review and discussion that got #6124 merged is a clear
sign that we expect CLTV to be eventually adopted. The question isn't if
CLTV should be added to the Bitcoin protocol, but rather when.


4) The CLTV opcode and IsSuperMajority() deployment code has been
   thoroughly tested and reviewed

The opcode implementation is very simple, yet got significant review,
and it has solid test coverage by a suite of tx-(in)valid.json tests.
The tests themselves have been reviewed by others, resulting in Esteban
Ordano's pull-req #6368 by Esteban Ordano which added a few more cases.

As for the deployment code, both the actual IsSuperMajority() deployment
code and associated unit-tests tests were copied nearly line-by-line
from the succesful BIP66. I did this deliberately to make all the peer
review and testing of the deployment mechanism used in BIP66 be equally
valid for CLTV.


5) We can safely deploy CLTV with IsSuperMajority()

We've done two soft-forks so far with the IsSuperMajority() mechanism,
BIP34 and BIP66. In both cases the IsSuperMajority() mechanism itself
worked flawlessly. As is well-known BIP66 in combination with a large %
of the hashing power running non-validating "SPV" mining operations did
lead to a temporary fork, however the root cause of this issue is
unavoidable and not unique to IsSuperMajority() soft-forks.

Pragmatically speaking, now that miners are well aware of the issue it
will be easy for them to avoid a repeat of that fork by simply adding
IsSuperMajority() rules to their "SPV" mining code. Equally turning off
SPV mining (temporarily) is perfectly feasable.


6) We have the necessary consensus to deploy CLTV via IsSuperMajority()

The various "nVersion bits" proposals - which I am a co-author of - have
the primary advantage of being able to cleanly deal with the case where
a soft-fork fails to get adopted. However, we do have broad consensus,
including across all sides of the blocksize debate, that CLTV should be
adopted. The risk of CLTV failing to get miner adoption, and thus
blocking other soft-forks, is very low.


7) Using IsSuperMajority() to deploy CLTV doesn't limit or delay other upgrades

It _is_ possible for multiple IsSuperMajority() soft-forks to coexist,
in the sense that if one soft-fork is "in flight" that doesn't prevent
another soft-fork from also being deployed simultaneously.

In particular, if we deploy CLTV via IsSuperMajority() that does _not_
impact the adoption schedule for other future soft-forks, including
soft-forks using a future nVersion bits deployment mechanism.

For instance, suppose we start deployment of CLTV right now with
nVersion=4 blocks. In three months we have 25% miner support, and start
deploying CHECKSEQUENCEVERIFY with nVersion=5 blocks. For miners
supporting only OP_CLTV, the nVersion=5 blocks still trigger OP_CLTV;
miners creating nVersion=5 blocks are simply stating that they support
both soft-forks. Equally, if in three months we finish a nVersion bits
proposal, those miners will be advertising nVersion=(1 << 29) blocks,
which also advertise OP_CLTV support.


8) BIP101 miners have not proved to be a problem for CLTV deployment

While there was concern that BIP101's use of nVersion would cause
issues with a IsSuperMajority() softfork, the % of blocks with BIP101
nVersion's never reached more than 1%, and currently is hovering at
around 0.1%

As Gavin Andresen has stated that he is happy to add CLTV to BIP101, and
thus Bitcoin XT, I believe we can expect those miners to safely support
CLTV well before soft-fork enforcement happens. Secondly, the 95%
enforcement threshold means we can tolerate a fairly high % of miners
running pre-CLTV BIP101 implementations without fatal effects in the
unlikely event that those miners don't upgrade.


9) Doing another IsSuperMajority() soft-fork doesn't "burn a bit"

This is a common myth! All nVersion bits proposals involve permanently
setting a high-order bit to 1, which results in nVersion >= all prior
IsSuperMajority() soft-forks. In short, we can do a nearly unlimited
number of IsSuperMajority() soft-forks without affecting future nVersion
bits soft-forks at all.


10) Waiting for nVersion bits and CHECKSEQUENCEVERIFY will significantly
    delay deployment of CLTV

It's been proposed multiple times that we wait until we can do a single
soft-fork with CSV using the nVersion bits mechanism.

nVersion bits doesn't even have an implementation yet, nor has solid
consensus been reached on the exact semantics of how nVersion bits
should work. The stateful nature of nVersion bits soft-forks requires a
significant amount of new code compared to IsSuperMajority() soft-forks,
which in turn will require a significant amount of testing. (again I'll
point out I'm a co-author to all the nVersion bits proposals)

CSV has an implementation, but there is still debate going on about what
the exact semantics of it should be. Getting the semantics right is
especially important as part of CSV includes changing the meaning of
nSequence, restricting future uses of that field. There have been many
proposals to use nSequence, e.g. for proof-of-stake blocksize voting,
and it has the unique capability of being a field that is both unused,
and signed by scriptSigs. We shouldn't take potentially restricting
future uses of it lightly.

CSV is also significantly more complex and invasive than CLTV in terms
of code changes. A large % of the mining power is running forks
of Bitcoin Core with custom changes - modifying these forks with new
features is a labor intensive and slow process.

If CLTV is ready now, why delay it - potentially for 6-12 months - for
other proposals to catch up? Equally if they do catch up, great! As
explained above an in-flight CLTV soft-fork won't delay future upgrades.


11) Even if CLTV is broken/obsoleted there is very little carrying cost
    to having it

Suppose we decide in two years that CLTV was botched and we need to fix
it. What's the "carrying cost" of having implemented CLTV in the first
place?

We'll have used up one of our ten soft-forkable NOPs, but if we ever
"run out" it's easy to use extension NOPs(3). Similarly, future script
improvements like OP_MAST - or even a hard-fork - can easily expand the
range of NOPs to the point where this is a non-issue.

If you don't use OP_CLTV in your scripts there is zero effect on your
transactions; we're not limiting future improvements to Bitcoin in any
way other than using up a NOP by implementing CLTV.


References
----------

1) https://github.com/petertodd/checklocktimeverify-demos
2) https://github.com/mruddy/bip65-demos
3) https://github.com/bitcoin/bitcoin/pull/5496#issuecomment-101293403
4) https://github.com/bitcoin/bips/blob/master/bip-0112.mediawiki

-- 
'peter'[:-1]@petertodd.org
000000000000000006a257845da185433cbde54a74be889b1c046a267dcf4ab2
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