Subject: [bitcoindev] A Post Quantum Migration Proposal

From: Jameson Lopp <[email-masked]>
Date: Sat, 12 Jul 2025 17:36:31 -0400

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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