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

From: Boris Nagaev <[email-masked]>
Date: Thu, 5 Jun 2025 07:54:25 -0700 (PDT)

Hi Leo, hi list,

You raise a valid point that a wTXID coming from a quantum attacker could 
be invalid, and the scheme must be robust against this. In this scenario, 
the attacker is not motivated by profit but rather by the desire to harm 
the legitimate owner.

I think we do not need to include a full transaction in the announcement to 
address this.

I propose the following scheme. There is just one announcement type, which 
was defined as a weak proof in my previous message. An announcement 
includes the UTXO, wTXID, and a proof (tagged_hash(EC public key || 
wTXID)). This tuple can be stored in an OP_RETURN output. The rule of a 
hypothetical soft fork would be: to spend an EC output, there must be an 
announcement with the corresponding wTXID and a valid proof published at 
least X blocks earlier.

This design prevents an attacker from maliciously blocking someone else's 
coin. There is no "earliest announcement wins" rule. Any valid announcement 
wins once it is sufficiently buried. If an attacker publishes an invalid 
announcement, the legitimate owner can simply publish their own and wait X 
blocks before sweeping the funds. If they have already published an 
announcement (as they should), they can use it without delay.

The scheme also preserves Satoshi's privacy: anyone can publish an 
announcement for Satoshi's coins, but without the actual spending 
transaction, nobody can tell whether the announcement is valid.

A downside of the scheme is that all announcements must be stored by nodes 
until the corresponding UTXO is spent, and there can be multiple 
announcements for a single coin. Another downside is that announcements 
cannot be packed into a Merkle root, because the UTXO must be visible for 
Satoshi to identify his coins, and he must be able to verify the proof. 
Perhaps there are ways to improve the scheme to address these efficiency 
issues.

Best,
Boris

On Thursday, June 5, 2025 at 9:55:34 AM UTC-3 Leo Wandersleb wrote:

Hi Boris, hi list,

I think the weak announcement is a bad idea once EC crypto is broken to the 
point where an attacker can break the key before the transaction gets mined 
but the strong announcements should still hold as they have less urgency. 
If the attacker sees the transaction in a strong announcement with a full 
transaction, he cannot win even if he gets into a block first, as the 
strong announcement proves a prior commitment to that transaction and would 
win even if it gets mined only some blocks later.

A scheme where the announcement does not contain the full transaction is 
problematic as the transaction might then turn out to not be valid. Then 
nodes would wait for the "winning" wtxid blocking the UTXO forever.

So the scheme is:

After activation at block height X:

1. **Vulnerable UTXOs cannot be spent directly** - they require a prior 
announcement 
2. **Commitment** to a wTXID that spends the vulnerable UTXO. Multiple 
wTXIDs can be stored in a hash tree in an OP_RETURN
3. **Reveal** full transaction with proof of prior commitment but not as a 
normal transaction yet
4. **Counter Reveal**: For 144 blocks, others can reveal older commitments. 
This protects exposed pubkeys!
5. **After 144 blocks**: The UTXO can be spent according to the strongest 
announcement (oldest commitment of valid transaction wins).

As (5) is just the normal transaction, the scheme is a soft fork and 
compatible with pre-recorded transactions where the keys were lost. It 
would at least double the on-chain costs for these vulnerable UTXOs as they 
would have to store the full transaction twice. We can make the 
announcements prunable again though.

Best,

Leo
On Wednesday, 4 June 2025 at 20:40:32 UTC+2 Boris Nagaev wrote:

Hi Leo,

I think it is possible to provide privacy for Satoshi and also reduce the 
size of a weak announcement (strong announcements can already be small: 
just a txid or a Merkle root of many txids).

Importantly, we cannot include the whole signed transaction in the weak 
announcement. Doing so would leak the EC public key immediately, allowing 
an attacker to create their own valid weak announcement. We must avoid 
revealing the public key until the actual spending transaction is broadcast.

We need a scheme where the EC public key is not leaked in a weak 
announcement, but the legitimate owner can verify it, while no one else 
can. Also, once the EC public key is revealed, anyone should be able to 
verify a past weak announcement (to validate the transaction when it is 
broadcast). This reduces to the following requirement: we need a proof of 
knowledge of the EC public key that can be verified if the public key is 
known and provides no information otherwise.

I think this is called a zero-knowledge proof. One simple approach could be 
to apply a tagged hash function to the concatenation of the EC public key 
and the future wTXID, and include this in the weak announcement. The 
structure would be:

   -     UTXO (previous TXID and output index)
   -     future spending wTXID
   -     proof := tagged_hash(EC public key || wTXID)

The wTXID is included in the concatenation to bind the proof to a 
particular future transaction. Otherwise, someone could copy a weak 
announcement and substitute their own wTXID.

Satoshi could publish a strong announcement now and then monitor all weak 
announcements involving his UTXOs. If someone publishes a weak announcement 
for one of his coins, he could verify the "proof" field. If it is valid, it 
would mean someone has cracked his key with a quantum computer, and he 
would need to use his strong announcement immediately to reclaim the funds 
before the attacker does.

Best,
Boris

On Wednesday, June 4, 2025 at 2:40:53 PM UTC-3 Leo Wandersleb wrote:

Hi Boris,

the announcements, weak and strong would have to not be transactions yet to 
be compatible with legacy nodes and thus keep it a soft-fork. They could be 
OP_RETURN data. Only after the 144 blocks, the upgraded full nodes would 
allow the inclusion of the actual transaction. This would mean the 
transaction would be both in full in the OP_RETURN strong announcement and 
without the witness part later, so it would be a bit expensive this way but 
maybe we can do better?

A node that gets updated would have to re-index all the blockchain to find 
announcements if we don't introduce a time frame for actually using the 
announcements. We could also say that any announcement has to be used 
within another 1000 blocks. Then the upgrading node would have to re-index 
the last 1000 blocks.

The legitimate owner of a UTXO might wait for an attack for privacy 
reasons. My proposal would allow Satoshi himself to make all his UTXOs 
quantum safe without any of us learning about him being active. He could 
add one 64B OP_RETURN in 2027 and when QC becomes an issue, we would learn 
about him having been active in 2027 in 2040 when actually somebody tried 
to attack and not in 2027 when people started to panic because of imminent 
quantum breakthroughs.
Hmm ... a problem is the weak announcement doesn't require keys, so anybody 
could provoke Satoshi to come forward. Maybe we have to add key ownership 
as a requirement for the "weak" announcement, too. So it should also 
contain a serialized transaction.

Best,

Leo

On Wednesday, 4 June 2025 at 04:15:59 UTC+2 Nagaev Boris wrote:

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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