Fidelity Digital Assets Maps Bitcoin Quantum Resistance Options
Key Takeaways
Signature Exposure: Bitcoin still authorizes spends with ECDSA and Schnorr, both tied to elliptic-curve assumptions a future CRQC could break.
SHRINCS Design: Blockstream’s hash-based BIP pairs a compact stateful spend path with a larger stateless recovery fallback.
Upgrade Constraint: A soft fork could add quantum-resistant rules, while social consensus remains the slowest step.
Fidelity Digital Assets published on September 1, 2026, the research note Bitcoin’s Path to Quantum Resistance by Senior Digital Assets Research Analyst Daniel Gray. The note maps Bitcoin quantum resistance options if a future cryptographically relevant quantum computer threatens private-key control of bitcoin, and it reviews larger hash-based signature designs, led by Blockstream‘s SHRINCS Bitcoin Improvement Proposal, as candidate defenses. Gray states cryptographically relevant quantum computers do not yet exist, while any move to quantum-resistant signatures would consume more block space, cut throughput, and still require a network-wide consensus change.
Signature Schemes and Hash-Based Designs
Gray writes private keys remain foundational to Bitcoin custody because they authorize transfers without an intermediary. Spends today use either the Elliptic Curve Digital Signature Algorithm or Schnorr signatures introduced in the 2021 Taproot upgrade. Both schemes rest on the elliptic curve discrete logarithm problem. The note says the assumption holds against classical computers and could fail against a sufficiently powerful cryptographically relevant quantum computer once a public key is revealed during a spend.
Size, not only algorithm choice, drives the engineering constraint. Fidelity cites default ECDSA public keys near 65 bytes and Schnorr public keys of 32 bytes, with most ECDSA keys stored in compressed form at roughly half the uncompressed length. Users pay for block space by the byte, so larger keys and signatures raise fees and reduce the number of transfers each block can carry. A future CRQC able to recover a private key from an exposed public key would place the associated coins at risk.
The note highlights SHRINCS, a Bitcoin Improvement Proposal from Blockstream formalized on August 26, 2026, as one concrete design under review. The scheme uses hash-based signatures built on one-way hash functions Fidelity describes as outside the reach of a CRQC and already central to Bitcoin mining and addressing. Blockstream’s earlier OP_CHECKSHRINCS write-up and the BIP pair two spend paths under one public key.
Reported construction parameters include:
- A compact stateful path with a signature budget limiting how many times a key may sign securely
- A requirement to track signing state across devices, a value a 12- or 24-word seed phrase does not store
- A stateless fallback reserved for lost-state recovery, at the cost of much larger signatures
- A claimed efficiency edge, in the recovery case, versus NIST SLH-DSA (FIPS 205)
Independent Blockstream materials associated with the BIP describe 48-byte public keys, stateful signatures starting near 548 bytes and growing with reuse toward about 4,619 bytes, and a stateless fallback near 5,776 bytes. Fidelity does not reprint those byte counts in the September 1 note. The research team still flags a material drop in transactions per second if quantum-resistant signatures replace today’s compact elliptic-curve signatures across the base layer.
Soft Fork Path and Market Implications
Formalizing SHRINCS as a BIP does not change consensus rules. Fidelity says a soft fork would still be required to recognize the new scheme. Because soft forks are backward compatible, the note argues they may face fewer adoption fights than some earlier protocol disputes. Holders worried about an earlier CRQC timeline could move coins to addresses able to use the new framework and keep spending with Schnorr under ordinary conditions, then switch to the quantum-resistant path if a credible threat appears.
Gray identifies Bitcoin’s social layer as the hardest implementation step. The Fidelity Digital Assets Research team writes urgency could still speed consensus if a credible CRQC threat materialized. The preferred end state in the note looks a lot like Bitcoin today. Multiple address types and signature schemes would coexist so a single-signature user and a collaborative or multi-signature custodian could pick different implementations.
The research posture sits alongside a separate institutional effort. On July 23, 2026, Fidelity Digital Assets joined Anchorage Digital, ARK Invest, BlackRock, Block, Blockstream, Coinbase, Galaxy, and Strategy as a founding member of the Bitcoin Security Consortium, an initiative backed by an aggregate $15 million in member pledges over three years for long-term Bitcoin security work, including post-quantum cryptography. Members direct funds independently and do not claim protocol control.
Fidelity’s investor conclusion is measured. The CRQC threat does not appear imminent. Open-source developers are already comparing candidate schemes. Throughput damage from larger signatures may matter less while the mempool stays relatively empty. Any deployed upgrade will still introduce new trade-offs. The research team believes Bitcoin can harden security and keep the core properties behind its value proposition.
Bottom Line
Fidelity sees an active Bitcoin quantum resistance path, with hash-based signatures feasible and consensus still the binding constraint.
Find out more here.
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Further articles, reports, and the latest quantum computing news may be found at The Qubit Report.
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