QCNEWS
Fidelity's research note frames quantum resistance as a three axis tradeoff between cryptography, block space, and consensus, and the slowest axis is the people.
Fidelity Digital Assets published "Bitcoin's Path to Quantum Resistance" on September 1, 2026, written by Senior Digital Assets Research Analyst Daniel Gray (The Qubit Report). The note is a contingency map, not a crisis report. It lays out the engineering choices a future Bitcoin upgrade would have if a cryptographically relevant quantum computer (CRQC) ever arrives.
Bitcoin today authorizes every spend with one of two signature schemes. The older one, ECDSA, anchors most existing wallets. Taproot, the 2021 upgrade, added Schnorr signatures, which produce smaller, cheaper transactions. Both schemes rest on the same math: a class of curve-based problems that classical computers find intractable. A sufficiently powerful quantum computer, by contrast, is widely expected to crack that math and recover a private key from any public key it has seen. Default ECDSA public keys sit near 65 bytes; Schnorr keys are 32 bytes, and most ECDSA keys are stored in compressed form at roughly half the uncompressed length. The exposure window is the moment of spend. Until a wallet broadcasts a transaction, its public key often stays hidden inside the address. After the spend, the key is permanently recorded on the blockchain. That is the moment a future quantum computer would have a target.
Hash-based signatures are built from one-way hash functions, the same kind of one-way scrambling Bitcoin already uses for mining and address generation, and are widely considered outside the reach of a CRQC. The cost is size. A typical hash-based signature runs several kilobytes, compared with 64 bytes for a Schnorr signature. Bitcoin pays for block space by the byte. Bigger signatures mean fewer transfers per block, higher fees for every user, and a permanent tax on throughput. Fidelity's note frames this as a tradeoff the network has to price, not a switch it can flip.
Blockstream's SHRINCS, formalized as a Bitcoin Improvement Proposal on August 26, 2026, is the most concrete candidate design on the table. It is a hash-based signature scheme that pairs a compact stateful spend path with a larger stateless recovery fallback (SHRINCS BIP). It is one proposal in a family that includes XMSS, LMS, and SPHINCS+; Fidelity does not endorse it as the only path, but it is the design the note treats as most developed. The "stateful" leg requires users to track which one-time keys they have spent, an operational burden Bitcoin wallets have never carried.
A soft fork could in principle add quantum-resistant rules to Bitcoin without splitting the chain. The cryptography is converging faster than the politics. Fidelity's research note is explicit on which step is slowest: social consensus, the network-wide agreement needed to change the rules for everyone, is the binding constraint (Fidelity Digital Assets). The threat itself remains hypothetical. CRQCs do not yet exist. The value of mapping the constraint stack now is that if one ever does, the design choices have already been debated, the size penalties have already been priced, and the consensus path has already been walked.
If consensus does not land in time, the exposure is concentrated in legacy coins. The early Bitcoin addresses whose public keys were revealed years ago are often described as belonging to Satoshi-era holders whose keys may no longer be provably spendable (news.bitcoin.com, Crypto Briefing). The engineering questions are now mapped. SHRINCS was formalized as a BIP on August 26, 2026. The next concrete step is whether it, or a competing hash-based proposal, gathers enough community review to enter a soft-fork activation process.