Re: [bitcoindev] Re: A Post Quantum Migration Proposal

[Boris Nagaev <bnagaev@gmail.com>]

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On Monday, July 14, 2025 at 4:06:17 PM UTC-3 Ethan Heilman wrote:

I want to clarify two points:


> Even if go all to upgrade to lattices-based schemes, we have no certainty 
that novels flaws won't be found, one can just go to see the modifications 
of the NIST-approved schemes in between their rounds of selection that 
we'll never reach something like "self-sovereign peace of mind"...

The informational proposal for post-quantum signatures in BIP-360 has one 
lattice-based scheme and one hash-based scheme (SLH-DSA SPHINCS+). The 
intention of including a hash-based scheme is to ensure that there will 
always be at least one signature scheme in Bitcoin that is secure. 
Cryptographic hashes are considered one of the safest assumptions possible 
and are used throughout Bitcoin (merkle tree, PoW, TXID, etc...).

Using P2QRH + SLH_DSA, you can have:
- a tapleaf for SLH-DSA 
- and a tapleaf for a more efficient signature scheme (ML-DSA, Schnorr, 
whatever)

Then no matter what happens to any of the other signature schemes, you can 
use that SLH-DSA tapleaf to spend safely. This strategy isn't just about 
quantum resistance but protecting against unexpected cryptanalytic 
breakthroughs. If I wanted to store Bitcoins in cold storage for 100 years, 
this is how I would do it.

>  This is especially worrying as if I'm understanding you correctly you're 
justifying this position as that somehow we should protect the price of the 
currency as an end in itself (i.e "Beyond its impact on price, ..."). It's 
unclear the price of bitcoin versus what fiat or hard asset (e.g oil) you 
have in mind. [...] To put it simply, even if a quantum attacker can 
tomorrow starts to steal vulnerable coins, 1 BTC will be always equal to 1 
BTC. Full stop.

I can't speak for Jameson, but let me put forward my own concern. If miners 
can buy much less electricity for 1-BTC this is a major problem for 
Bitcoin. If the price of electricity denominated in Bitcoin goes way up, 
miners will have to mine at a massive loss. Many will stop mining, then the 
block rate will go down and Bitcoin will appear to be less valuable (high 
fees, slow confirmation, panic), which makes mining even more of a loss, 
and so on. This also invites miners who have nothing left to lose to engage 
in mining attacks.

One reason I believe a soft fork to freeze quantum vulnerable coins is 
likely, is that miners will be incentivized to mine on such a soft fork. 
The non-frozen chain will simply not be affordable to mine on and will be 
abandoned. In the moment of crisis, all someone has to do is create a 
client that does a soft fork freeze of quantum vulnerable coins and 
the miner will have no choice but to adopt it or stop mining. The worst 
time to do a soft fork like this would be in a moment of crisis.


The original chain, where P2PK coins are not frozen, could actually 
generate significant fees for miners -- especially if multiple parties with 
quantum computers are competing to claim the same outputs. While the price 
of 1 BTC on that chain might be lower, miners' profits (measured in dollars 
or in electricity) could still be higher. As a result, miners may be 
incentivized to switch to that chain in order to collect a cut of P2PK 
coins.

However, the more important factor is where holders choose to go. There 
will be a market to trade coins between the original chain and the fork 
that implements freezing. Everyone who held satoshis prior to the fork will 
have an equal number of satoshis on both chains, and each individual will 
decide which to keep and which to convert on the market. The market will 
ultimately answer the question: what matters more -- preventing private 
theft via quantum computers, or preventing organized confiscation through 
consensus rules?
 

Note that such a death spiral and the incentives for a soft fork are 
possible prior to quantum attacks on Bitcoin. Merely the threat of quantum 
attacks and the widespread belief that Bitcoin will not freeze unspendable 
coins and thereby inflate the supply of spendable bitcoin.


Eventually, facts, not fears, are what matter. If quantum attacks do not 
materialize, someone could buy discounted bitcoins and profit later when, 
after several years, the attacks still haven't occurred.
 

On Mon, Jul 14, 2025 at 10:09 AM Antoine Riard <antoin...@gmail.com> wrote:

Hi Jameson,

Thanks for your thoughts on this complex subject.

First and foremost, I think your following statement: "Never before has 
Bitcoin faced
an existential threat to its cryptographic primitives" is very myopic, 
given that
cryptanalysts and number theorists are making progress every year in their 
works, and
each bitcoin cryptographic primitive has been and is constantly analyzed to 
uncover
potential weaknesses.

So in my view the quantum threat is a bit less specific that the image 
you're painting
of it. Even if go all to upgrade to lattices-based schemes, we have no 
certainty that
novels flaws won't be found, one can just go to see the modifications of 
the NIST-approved
schemes in between their rounds of selection that we'll never reach 
something like
"self-sovereign peace of mind"...Unless we start to forbid people of 
practicing the
art of mathematics, practice which has been ongoing since Euclide and 
Pythagore...

I do concede that quantum is a bit different, as after all new physics 
paradigm
do not happen often (Heisenberg published in the 20s iirc), though that's 
in my
view the flaw of your reasoning as you're assuming some "post-quantum" 
upgraded
state where bitcoin, as a community and a network, would be definitely safe 
from
advances in applied science. At minima, in my understanding, you're arguing 
this
time is different to justify extra-ordinary technical measures never seen 
before,
namely the freezing of "vulnerable" coins.

I'm worried this is opening a Pandora box, where we would introduce a 
precedent
that it is legitimate as a community to technicaly confiscate some coins of 
users, 
without their _consents_, for extra-ordinary reasons. That's opening a 
worms of
shenanigans in the future...There is no guarantee that this precedent won't
be leveraged in the future by any group of entities to justify future 
upgrades
eroding one of the "fundamental property" you're yourself deeming as 
valuable.

This is especially worrying as if I'm understanding you correctly you're 
justifying
this position as that somehow we should protect the price of the currency 
as an end
in itself (i.e "Beyond its impact on price, ..."). It's unclear the price 
of bitcoin
versus what fiat or hard asset (e.g oil) you have in mind. And in anyway, 
as far
as I know, none of the bitcoin devs is seating on the board of the FED, the 
ECB
or the BoJ...

To put it simply, even if a quantum attacker can tomorrow starts to steal
vulnerable coins, 1 BTC will be always equal to 1 BTC. Full stop. In my 
humble
opinion, let's not introduce the idea that, we, as a community of 
stakeholders
and developers we have a positive "fiduciary" duty to act to maintain the 
price
of bitcoin in some "monetary snake" with another class of assets...

That's also the problem with game theory, all the matrices of analysis are
based on some scale of utilitarism. See Von Neuman's Theory of Games, the
section on "The Notion of Utility". My subjective appreciation of the value
of my coins might not be your subjective appreication of the value of your
coins.

Now I do understand the perspective of the institutional holders, the 
exchanges,
the custodians or any other industry providers, who might be in the full 
uncertainty
about their business responsibilities in case of a quantum threat affecting 
their
custodied coins. But, first legally speaking there is something call "force 
majeure"
and in view of the quantum threat, which is a risk discussed far beyond the 
bitcoin
industry, they should be able to shield themselves behind that. Secondly, 
if there
is any futute upgrade "opt-in" only path a la BIP360, you can move your 
funds or
the ones under custody  under a PQC scheme like Dilthium or Falcon and be 
good
without caring about what the others users are doing. Thirdly, if you're an 
actor
in the industry like Coinbase and you're deeply concerned about how 
extended maelstrom
on the price might affect the viability of your operations, it is unclear 
to me why
you don't call MunichRe or any other company like that tomorrow to craft 
and be
covered by specific insurance on quantum threats...

To be frank, all those considerations on how "I cannot see how the currency 
can
maintain any value at all in such a setting", is a strong red flag of low 
time
preferences. It's not like we're used to strong volatility in bitcoin with 
the
almost 2 decades of operations of the network. In my view, it's more a hint 
of
very high-exposition by some to a single class of asset, i.e bitcoin, 
rather than wise
diversification... And a push to sacrify a "fundamental property" i.e 
"conservatism"
in view of short-term concerns (i.e the stability of the currency price 
along
a period of few years).

Do not get me wrong, I'm certainly not of the school "let's reward quantum
attackers". Leveraging techical superiority and employing CRQRC to steal
vulnerable coins would be clearly a theft. But ethically, the best we can 
do is
to have an opt-in upgrade path and be pro-active, by education and outreach,
to have the maximum of coin owners upgrading to non-vulnerable addresses 
types.
Then show the level of "fortitude" or "endurance" as a community in face of 
price
fluctuations for a while, while seeing regularly old P2PK coins hacked. 
Marcus
Aurelius can be bought for few bucks in most of decent libraries...

I'm definitely on the "no old coins confiscation" position you're 
underlighting:

"I don't see why old coins should be confiscated. The better option is to 
let
those with quantum computers free up old coins. While this might have an
inflationary impact on bitcoin's price, to use a turn of phrase, the 
inflation
is transitory. Those with low time preference should support returning lost
coins to circulation".

Notwhitstanding that I disagree with your position, one can only appreciate
the breadth and depth with which you're gathering and articulating all the
elements on this complex problem.

Best,
Antoine
OTS hash: c064b43047bf3036faf098b5ac8e74930df63d25629f590a4195222979402826
Le lundi 14 juillet 2025 à 00:53:34 UTC+1, Tadge Dryja a écrit :

Hi   

While I generally agree that "freeze" beats "steal", and that a lot of lead 
time is good, I don't think this plan is viable.
To me the biggest problem is that it ties activation of a PQ output type to 
*de*activation of EC output types.  That would mean that someone who wants 
to keep using all the great stuff in libsecp256k1 should try to prevent 
BIP360 from being activated.

Sure, there can be risks from CRQCs.  But this proposal would go the other 
direction, disabling important functionality and even destroying coins 
preemptively, in anticipation of something that may never happen.

Also, how do you define "quantum-vulnerable UTXO"?  Would any P2PKH, or 
P2WPKH output count?  Or only P2PKH / P2WPKH outputs where the public key 
is already known?  I can understand disabling spends from known-pubkey 
outputs, but for addresses where the public key has never been revealed, 
commit/reveal schemes (like the one I posted about & am working on a 
follow-up post for) should safely let people spend from those outputs 
indefinitely.

With no evidence of a QRQC, I can see how there would be people who'd say 
"We might never really know if a CRQC exists, so we need to disable EC 
spends out of caution" and others who'd say "Don't disable EC spends, since 
that's destroying coins", and that could be a persistent disagreement.  But 
I hope if we did in fact have a proof that a CRQC has broken secp256k1, 
there would be significant agreement on freezing known-pubkey EC outputs.

-Tadge
On Saturday, July 12, 2025 at 8:46:09 PM UTC-4 Jameson Lopp wrote:

Building upon my earlier essay against allowing quantum recovery of bitcoin 
<https://groups.google.com/g/bitcoindev/c/uUK6py0Yjq0/m/6peEaa90AQAJ> I 
wish to formalize a proposal after several months of discussions.

This proposal does not delve into the multitude of issues regarding post 
quantum cryptography and trade-offs of different schemes, but rather is 
meant to specifically address the issues of incentivizing adoption and 
migration of funds *after* consensus is established that it is prudent to 
do so.

As such, this proposal requires P2QRH as described in BIP-360 or potential 
future proposals.
Abstract

This proposal follows the implementation of post-quantum (PQ) output type 
(P2QRH) and introduces a pre-announced sunset of legacy ECDSA/Schnorr 
signatures. It turns quantum security into a private incentive: fail to 
upgrade and you will certainly lose access to your funds, creating a 
certainty where none previously existed. 

   - 
   
   Phase A: Disallows sending of any funds to quantum-vulnerable addresses, 
   hastening the adoption of P2QRH address types.
   - 
   
   Phase B: Renders ECDSA/Schnorr spends invalid, preventing all spending 
   of funds in quantum-vulnerable UTXOs. This is triggered by a 
   well-publicized flag-day roughly five years after activation.
   - 
   
   Phase C (optional): Pending further research and demand, a separate BIP 
   proposing a fork to allow recovery of legacy UTXOs through ZK proof of 
   possession of BIP-39 seed phrase.  
   
Motivation

We seek to secure the value of the UTXO set and minimize incentives for 
quantum attacks. This proposal is radically different from any in Bitcoin’s 
history just as the threat posed by quantum computing is radically 
different from any other threat in Bitcoin’s history.  Never before has 
Bitcoin faced an existential threat to its cryptographic primitives. A 
successful quantum attack on Bitcoin would result in significant economic 
disruption and damage across the entire ecosystem. Beyond its impact on 
price, the ability of miners to provide network security may be 
significantly impacted.  

   - 
   
   Accelerating quantum progress. 
   - 
      
      NIST ratified three production-grade PQ signature schemes in 2024; 
      academic road-maps now estimate a cryptographically-relevant quantum 
      computer as early as 2027-2030. [McKinsey 
      <https://www.mckinsey.com/~/media/mckinsey/business%20functions/mckinsey%20digital/our%20insights/the%20year%20of%20quantum%20from%20concept%20to%20reality%20in%202025/quantum-monitor-2025.pdf?shouldIndex=false>
      ]
      - 
   
   Quantum algorithms are rapidly improving
   - 
      
      The safety envelope is shrinking by dramatic increases in algorithms 
      even if the pace of hardware improvements is slower. Algorithms are improving 
      up to 20X 
      <https://security.googleblog.com/2025/05/tracking-cost-of-quantum-factori.html>, 
      lowering the theoretical hardware requirements for breaking classical 
      encryption.
      - 
   
   Bitcoin’s exposed public keys. 
   - 
      
      Roughly 25% of all bitcoin have revealed a public key on-chain; those 
      UTXOs could be stolen with sufficient quantum power.  
      - 
   
   We may not know the attack is underway. 
   - 
      
      Quantum attackers could compute the private key for known public keys 
      then transfer all funds weeks or months later, in a covert bleed to not 
      alert chain watchers. Q-Day may be only known much later if the attack 
      withholds broadcasting transactions in order to postpone revealing their 
      capabilities.
      - 
   
   Private keys become public. 
   - 
      
      Assuming that quantum computers are able to maintain their current 
      trajectories and overcome existing engineering obstacles, there is a near 
      certain chance that all P2PK (and other outputs with exposed pubkeys) 
      private keys will be found and used to steal the funds.
      - 
   
   Impossible to know motivations. 
   - 
      
      Prior to a quantum attack, it is impossible to know the motivations 
      of the attacker.  An economically motivated attacker will try to remain 
      undetected for as long as possible, while a malicious attacker will attempt 
      to destroy as much value as possible.  
      - 
   
   Upgrade inertia. 
   - 
      
      Coordinating wallets, exchanges, miners and custodians historically 
      takes years.
      - 
      
      The longer we postpone migration, the harder it becomes to coordinate 
      wallets, exchanges, miners, and custodians. A clear, time-boxed pathway is 
      the only credible defense.
      - 
      
      Coordinating distributed groups is more prone to delay, even if 
      everyone has similar motivations. Historically, Bitcoin has been slow to 
      adopt code changes, often taking multiple years to be approved.
      
Benefits at a Glance
   
   - 
   
   Resilience: Bitcoin protocol remains secure for the foreseeable future 
   without waiting for a last-minute emergency.
   - 
   
   Certainty: Bitcoin users and stakeholders gain certainty that a plan is 
   both in place and being implemented to effectively deal with the threat of 
   quantum theft of bitcoin.  
   - 
   
   Clarity: A single, publicized timeline aligns the entire ecosystem 
   (wallets, exchanges, hardware vendors).
   - 
   
   Supply Discipline: Abandoned keys that never migrate become unspendable, 
   reducing supply, as Satoshi described 
   <https://bitcointalk.org/index.php?topic=198.msg1647#msg1647>.  
   
Specification

Phase

What Happens

Who Must Act

Time Horizon

Phase A - Disallow spends to legacy script types

Permitted sends are from legacy scripts to P2QRH scripts

Everyone holding or accepting BTC.

3 years after BIP-360 implementation

Phase B – Disallow spends from quantum vulnerable outputs

At a preset block-height, nodes reject transactions that rely on 
ECDSA/Schnorr keys. 

Everyone holding or accepting BTC.

2 years after Phase A activation.

Phase C – Re-enable spends from quantum vulnerable outputs via ZK Proof

Users with frozen quantum vulnerable funds and a HD wallet seed phrase can 
construct a quantum safe ZK proof to recover funds.

Users who failed to migrate funds before Phase B.

TBD pending research, demand, and consensus.
Rationale
   
   - 
   
   Even if Bitcoin is not a primary initial target of a cryptographically 
   relevant quantum computer, widespread knowledge that such a computer exists 
   and is capable of breaking Bitcoin’s cryptography will damage faith in the 
   network . 
   - 
   
   An attack on Bitcoin may not be economically motivated - an attacker may 
   be politically or maliciously motivated and may attempt to destroy value 
   and trust in Bitcoin rather than extract value.  There is no way to know in 
   advance how, when, or why an attack may occur.  A defensive position must 
   be taken well in advance of any attack.  
   - 
   
   Bitcoin’s current signatures (ECDSA/Schnorr) will be a tantalizing 
   target: any UTXO that has ever exposed its public key on-chain (roughly 25 
   % of all bitcoin) could be stolen by a cryptographically relevant quantum 
   computer.
   - 
   
   Existing Proposals are Insufficient.  
   1. 
      
      Any proposal that allows for the quantum theft of “lost” bitcoin is 
      creating a redistribution dilemma. There are 3 types of proposals:
      1. 
         
         Allow anyone to steal vulnerable coins, benefitting those who 
         reach quantum capability earliest.
         2. 
         
         Allow throttled theft of coins, which leads to RBF battles and 
         ultimately miners subsidizing their revenue from lost coins.
         3. 
         
         Allow no one to steal vulnerable coins.
         - 
   
   Minimizes attack surface
   1. 
      
      By disallowing new spends to quantum vulnerable script types, we 
      minimize the attack surface with each new UTXO.  
      2. 
      
      Upgrades to Bitcoin have historically taken many years; this will 
      hasten and speed up the adoption of new quantum resistant script types. 
      3. 
      
      With a clear deadline, industry stakeholders will more readily 
      upgrade existing infrastructure to ensure continuity of services.  
      - 
   
   Minimizes loss of access to funds 
   1. 
      
      If there is sufficient demand and research proves possible, 
      submitting a ZK proof of knowledge of a BIP-39 seed phrase corresponding to 
      a public key hash or script hash would provide a trustless means for legacy 
      outputs to be spent in a quantum resistant manner, even after the sunset.  
      

Stakeholder

Incentive to Upgrade

Miners

• Larger size PQ signatures along with incentive for users to migrate will 
create more demand for block space and thus higher fees collected by miners.

• Post-Phase B, non-upgraded miners produce invalid blocks.

• A quantum attack on Bitcoin will significantly devalue both their 
hardware and Bitcoin as a whole. 

Institutional Holders

• Fiduciary duty: failing to act to prevent a quantum attack on Bitcoin 
would violate the fiduciary duty to shareholders.  

• Demonstrating Bitcoin’s ability to effectively mitigate emerging threats 
will prove Bitcoin to be an investment grade asset.

Exchanges & Custodians

• Concentrated risk: a quantum hack could bankrupt them overnight.

• Early migration is cheap relative to potential losses, potential lawsuits 
over improper custody and reputational damage.

Everyday Users

• Self-sovereign peace of mind.

• Sunset date creates a clear deadline and incentive to improve their 
security rather than an open-ended “some day” that invites procrastination.

Attackers

• Economic incentive diminishes as sunset nears, stolen coins cannot be 
spent after Q-day.

Key Insight: As mentioned earlier, the proposal turns quantum security into 
a private incentive to upgrade.  

This is not an offensive attack, rather, it is defensive: our thesis is 
that the Bitcoin ecosystem wishes to defend itself and its interests 
against those who would prefer to do nothing and allow a malicious actor to 
destroy both value and trust.  


"Lost coins only make everyone else's coins worth slightly more. Think of 
it as a donation to everyone." - Satoshi Nakamoto


If true, the corollary is:


"Quantum recovered coins only make everyone else's coins worth less. Think 
of it as a theft from everyone."


The timelines that we are proposing are meant to find the best balance 
between giving ample ability for account owners to migrate while 
maintaining the integrity of the overall ecosystem to avoid catastrophic 
attacks.  

Backward Compatibility

As a series of soft forks, older nodes will continue to operate without 
modification. Non-upgraded nodes, however, will consider all post-quantum 
witness programs as anyone-can-spend scripts. They are strongly encouraged 
to upgrade in order to fully validate the new programs.

Non-upgraded wallets can receive and send bitcoin from non-upgraded and 
upgraded wallets until Phase A. After Phase A, they can no longer receive 
from any other wallets and can only send to upgraded wallets.  After Phase 
B, both senders and receivers will require upgraded wallets. Phase C would 
likely require a loosening of consensus rules (a hard fork) to allow 
vulnerable funds recovery via ZK proofs.

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