[bitcoin-dev] Note on Sequence Lock Upgrades Defect

Jeremy jlrubin at mit.edu
Mon Sep 6 03:17:17 UTC 2021

BIP 68 says >= 2:
*This specification defines the meaning of sequence numbers for
transactions with an nVersion greater than or equal to 2 for which the rest
of this specification relies on.*
BIP-112 says not < 2
// Fail if the transaction's version number is not set high
// enough to trigger BIP 68 rules.
if (static_cast<uint32_t>(txTo->nVersion) < 2) return false;

A further proof that this needs fix: the flawed upgradable semantic exists
in script as well as in the transaction nSeqeunce. we can't really control
the transaction version an output will be spent with in the future, so it
would be weird/bad to change the semantic in transaction version 3.

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

On Sun, Sep 5, 2021 at 7:36 PM David A. Harding <dave at dtrt.org> wrote:

> On Fri, Sep 03, 2021 at 08:32:19PM -0700, Jeremy via bitcoin-dev wrote:
> > Hi Bitcoin Devs,
> >
> > I recently noticed a flaw in the Sequence lock implementation with
> respect
> > to upgradability. It might be the case that this is protected against by
> > some transaction level policy (didn't see any in policy.cpp, but if not,
> > I've put up a blogpost explaining the defect and patching it
> > https://rubin.io/bitcoin/2021/09/03/upgradable-nops-flaw/
> Isn't this why BIP68 requires using tx.version=2?  Wouldn't we just
> deploy any new nSequence rules with tx.version>2?
> -Dave
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