SKN | Fidelity Warns Bitcoin Faces a Long-Term Quantum Computing Threat

Key Points:

• Fidelity Digital Assets says Bitcoin’s current signature schemes could eventually be vulnerable to sufficiently powerful quantum computers, although no cryptographically relevant quantum computer exists today.
• Bitcoin currently relies on ECDSA and Schnorr signatures, both based on the elliptic curve discrete logarithm problem, which quantum computing could potentially solve.
• A recently formalized Bitcoin Improvement Proposal called SHRINCS offers one potential quantum-resistant path, but larger signatures could reduce Bitcoin’s transaction throughput.

Bitcoin’s security is widely associated with its decentralized architecture and cryptographic design, but Fidelity Digital Assets is highlighting a long-term technological risk that could eventually challenge one of the network’s most fundamental assumptions. In a September 1 research report, Fidelity examined the potential impact of cryptographically relevant quantum computers and the technical changes Bitcoin could require to remain secure in a post-quantum environment.

Bitcoin’s Existing Cryptography Is Not Designed for Quantum Machines

Every Bitcoin transaction requires a digital signature to demonstrate that the person authorizing a transaction controls the associated funds. Bitcoin currently uses ECDSA and Schnorr signatures, with Schnorr signatures introduced through the Taproot upgrade.

Both rely on the elliptic curve discrete logarithm problem, a mathematical challenge considered impractical for today’s conventional computers to reverse. A sufficiently powerful quantum computer, however, could potentially derive a private key from information exposed when a transaction is signed.

That distinction is important. Fidelity is not warning that Bitcoin can currently be broken by quantum computers. The report explicitly notes that cryptographically relevant quantum computers do not yet exist. The concern is instead that developing a solution could take years, making preparation a network-level planning issue rather than a response to an immediate attack.

Larger Quantum-Safe Signatures Create a Scalability Trade-Off

The principal technical problem is that quantum-resistant cryptographic schemes generally require substantially larger signatures than Bitcoin’s existing systems. Fidelity notes that an ECDSA public key is roughly 65 bytes, while a Schnorr public key is approximately 32 bytes, although commonly used compressed ECDSA keys are smaller.

That difference matters because Bitcoin users ultimately pay for block space on a per-byte basis. Larger cryptographic data would consume more block capacity, potentially increasing transaction costs and reducing the number of transactions that can fit into each block.

Fidelity therefore frames quantum resistance as a balancing exercise between security, usability and network throughput. The impact could be less severe while Bitcoin’s mempool remains near empty, but the trade-off could become more significant during periods of heavy network demand.

SHRINCS Offers One Possible Path Forward

One proposal highlighted by Fidelity is SHRINCS, a recently formalized Bitcoin Improvement Proposal developed by Blockstream. The approach combines stateful and stateless hash-based signature schemes designed to provide protection against quantum attacks while addressing some of the efficiency challenges associated with larger signatures.

The stateful component can offer greater efficiency but requires users to maintain a signing “state” across their devices. Losing that state could create security and recovery complications. The stateless alternative eliminates that requirement but produces significantly larger signatures, making it more suitable as a recovery mechanism.

Bitcoin’s Quantum Upgrade Would Require Network Consensus

SHRINCS is not an immediate Bitcoin protocol upgrade. Fidelity notes that implementing the proposal would require a soft fork, meaning Bitcoin participants would need to reach sufficient consensus to introduce support for the new signature framework.

This governance dimension may ultimately prove as challenging as the cryptography itself. Bitcoin’s decentralized structure means there is no central authority capable of unilaterally selecting a quantum-resistant standard. If a credible quantum threat emerges, however, Fidelity believes the resulting urgency could accelerate agreement around an upgrade.

For institutional investors, the key issue is therefore preparation rather than immediate disruption. Quantum computing is not currently an operational threat to Bitcoin, but the network’s developers are already evaluating potential defenses. Future developments in quantum computing, SHRINCS adoption, wallet compatibility and Bitcoin governance will determine how effectively the ecosystem can transition before the technology becomes capable of challenging existing cryptographic protections.