Re: [bitcoindev] Algorithm Agility for Bitcoin to maintain security in the face of quantum and classic breaks in the signature algorithms

Matt Corallo
2026-02-11T18:55:25Z

On 2/10/26 11:44 AM, Ethan Heilman wrote:
> > If Bitcoin disables Taproot key path spends before Q-day, then doing this via Taproot instead of
> BIP 360 would be preferable.
>
> I worry about making the transition to quantum-safe outputs depend on a contentious debate over a
> confiscatory soft fork. Uncertainty over whether the soft fork would be released and if released
> would be activated means that wallets and custodians are unlikely to have invested the resources
> into upgrading to support script only P2TR.

For what its worth I do not see a scenario where a decision ultimately made by the market will pick
the fork side with materially, say 5-10x higher, supply, over the side with lower supply...supply
and demand is king, especially with the "confiscatory" nature is basically nil as ~all wallets today
use seedphrases, which could still be spent with a ZK proof-of-seedphrase :).

> The benefit of BIP 360's P2MR (Pay-to-Merkle-Root) + SLH_DSA is that it avoids this controversy by
> being opt-in and non-confiscatory. This also means that BIP 360 + SLH_DSA is likely to activated
> early, allowing wallets and custodians ample time to build support after activation.

The drawback being that it will see zero relevant adoption until its way too late.

The only entities that would reasonably adopt something like this are large custodians, who aren't
worth worrying about as they'll easily migrate all their coins over the course of a few days or
weeks in an emergency scenario, and highly specialty wallets. The point of any PQ soft fork today is
if it can actually drive wallets to start making progress on PQ deployment. A new address type that
is 10x more expensive to transact with is going to see ~zero adoption in "consumer wallets" until
its urgent, at which point its obviously way, way too late.

Hell, *any* PQ soft fork is going to see limited adoption in "consumer wallets" until its urgent,
hence why I think the community will be basically forced to disable insecure spend paths and only
allow spends via ZK proof-of-seedphrase. But at least something that doesn't also 10x transaction
costs might reasonably be adopted by default by wallets that don't use seedphrases like Bitcoin Core.

> > We could define a new SegWit version that is a copy of Taproot. The new version number simply
> signals that the owner consents to a future deactivation of key path spends. Unlike BIP 360, this
> approach would still require actually disabling the key path before Q-day, but it is not
> confiscatory and allows using Taproot's benefits until then (with a privacy hit from having two
> versions of Taproot in parallel).
>
> Let's call this P2TRD (Pay-to-Tap-Root-Disablable). BIP 360 evolved from this P2TRD idea, to
> minimize the following hazards in P2TRD.
>
> 1.  P2TRD requires a soft fork that depends on accurately predicting Q-day or when EC Schnorr is
> classically broken. We must not only predict Q-day but also convince the community that the
> prediction is correct. If we mess up the timing, Bitcoin is significantly harmed. This means that
> people will constantly be yelling that we are messing up the timing. It will make quantum FUD worse
> not better.

No it doesn't - it requires a soft fork when the risk is imminent, but it happening somewhat before
that time is okay too.

> 2.  P2TRD (Pay-to-Tap-Root-Disablable) to be non-confiscatory users must create a script spend that
> replicates their key spend, But users and wallets are likely to screw this up and not create script
> spends. The is no way to see if a wallet is actually creating the script spend on the blockchain.

I mean people can create invalid addresses today in plenty of ways. How is this unique?

> 3. To be safe from long-exposure attacks P2TRD can't use the same public key for the script spend as
> the key spend. Since wallets will prefer the key spend to the script spend, a user might not realize
> if they lost the keying material for their script spend until after activation.

It would almost certainly just be a key derived from the seedphrase via another hash function, so
there's no real risk of this.

Matt

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