[bitcoin-dev] Note on Sequence Lock Upgrades Defect

darosior darosior at protonmail.com
Mon Sep 6 06:16:44 UTC 2021

Hi Jeremy,

I think it would be nice to have and suggested something similar (enforce minimality) in the context of
Miniscript a few months ago [0].

However your code:

const bool seq_is_reserved = (txin.nSequence < CTxIn::SEQUENCE_FINAL-2) && (
// when sequence is set to disabled, it is reserved for future use
((txin.nSequence & CTxIn::SEQUENCE_LOCKTIME_DISABLE_FLAG) != 0) ||
// when sequence has bits set outside of the type flag and locktime mask,
// it is reserved for future use.
txin.nSequence) != 0)

Would effectively prevent Lightning Network commitment transactions from relaying. The protocol uses
a hack encoding the commitment transaction numbering in the part of nSequence (and nLockTime)
without consensus meaning. This both sets the LOCKTIME_DISABLE_FLAG and uses bits outside of
the mask.

[0] https://github.com/rust-bitcoin/rust-miniscript/pull/246#issue-671512626
[1] https://github.com/lightningnetwork/lightning-rfc/blob/master/03-transactions.md#commitment-transaction
‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐
Le lundi 6 septembre 2021 à 5:17 AM, Jeremy via bitcoin-dev <bitcoin-dev at lists.linuxfoundation.org> a écrit :

> 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)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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