Subject: Re: [bitcoindev] Pre-emptive commit/reveal for quantum-safe migration (poison-pill)

From: Nagaev Boris <[email-masked]>
Date: Tue, 3 Jun 2025 18:49:04 -0300

Hi Leo,

Thanks for the clarifications, much appreciated!
I have a couple of questions:

1. How is a weak announcement stored in the blockchain and in the UTXO set?
I assume it must be a transaction, correct? And it should somehow mark
the UTXO as planned to be spent for 144 blocks?
How would older (non-upgraded) nodes interpret a transaction
containing a weak announcement? Would they just skip over it without
any special processing?
If so, is there a problem for nodes that upgrade after the fork: would
they have to reprocess all blocks since the fork to find and index all
missed weak announcements?

2. In the case of reclaiming a UTXO after a weak announcement by an
attacker: why would the legitimate owner wait for a weak announcement
at all?
If the EC public key was already leaked, it seems they should publish
a strong announcement themselves rather than wait. If the EC public
key wasn't leaked, there's nothing to worry about even if someone
publishes a weak announcement: they are most likely bluffing, since
they wouldn't have the actual public key.

Best,
Boris

On Tue, Jun 3, 2025 at 3:29 PM Leo Wandersleb <[email-masked]> wrote:
>
> Hi conduition,
>
> Thanks for your careful analysis - excellent catches.
>
> You're absolutely right about the txid vulnerability. The commitment must be to the complete transaction including witness data (wTXID or equivalent) to prevent an attacker from pre-committing to unsigned transactions. This is essential - otherwise an attacker could indeed enumerate the UTXO set and create commitments without knowing the private keys.
>
> Regarding updates: You're correct that frequent updates would be needed as wallets receive new UTXOs. However, I don't see this as a major issue - users could batch their commitments periodically (say, monthly) rather than after every transaction. The scheme is particularly important for existing UTXOs that already have exposed pubkeys (old P2PK, reused addresses, etc.). For new UTXOs, wallets should ideally migrate to quantum-safe addresses once available. OpenTimestamps aggregation would indeed help with scaling and provide plausible deniability about the number of UTXOs being protected.
>
> The time delay serves a different purpose than you might expect. It's not about preventing commitment forgery after pubkey exposure, but rather about allowing priority based on commitment age when multiple parties claim the same UTXO:
>
> 1. Weak announcement starts the 144-block window
> 2. During this window, anyone with a strong commitment can reveal it
> 3. The oldest valid commitment wins
>
> This creates the "poison pill" effect: an attacker might crack a key and try to spend a UTXO, but if the original owner has an older commitment, they can reclaim it during the window. The uncertainty about which UTXOs have poison pills makes attacking large "lost" UTXOs risky - hence less disruptive to the network.
>
> The delay essentially allows a "commitment priority contest" where age determines the winner, protecting users who prepared early while still allowing these users to not move their funds.
>
> Best,
>
> Leo
>
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-- 
Best regards,
Boris Nagaev

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