Blockstream Proposes SHRINCS to Shield Bitcoin From Quantum Attacks

Researchers at Blockstream have proposed a new digital signature scheme called SHRINCS, designed to protect the Bitcoin network from future quantum computer attacks. The proposal seeks to resolve a critical tension between quantum security and the limited space available in Bitcoin blocks.

SHRINCS signatures start at 324 bytes, making them significantly smaller than the NIST-standardized SLH-DSA signatures, which average approximately 7,856 bytes. While still larger than the 64-byte Schnorr signatures currently in use, the reduced size is intended to prevent the "block bloat" that typically accompanies quantum-resistant upgrades. To ensure compatibility with existing infrastructure, the system is built using SHA-256, the same hash function already utilized by Bitcoin's core systems. According to Blockstream researcher Jonas Nick, this is the first concrete post-quantum signature proposal designed specifically for Bitcoin.

The Quantum Threat

Bitcoin currently relies on elliptic-curve cryptography, a system that is vulnerable to Shor's algorithm. If a sufficiently powerful quantum computer were developed, it could derive private keys from public keys visible on the blockchain. This vulnerability is especially acute for "exposed" addresses, including the estimated 1.1 million bitcoin attributed to Satoshi Nakamoto. Because these public keys are already known to the network, they would be the first targets for a quantum-enabled attacker.

Balancing Security and Scale

The primary technical hurdle in migrating to quantum-resistant signatures is the impact on network utility. Larger signatures take up more block space, which drastically reduces the number of transactions the network can process per second. Industry analysis suggests that adopting a standard like SLH-DSA could drop Bitcoin's throughput from 6.5 to 0.36 transactions per second. In contrast, estimates indicate that SHRINCS could maintain a throughput of roughly 3 transactions per second, preserving a higher level of scalability while providing essential security.

The Path to Implementation

Integrating SHRINCS into the Bitcoin mainnet would not be a simple update; it would require a soft fork and network-wide consensus. Specifically, the implementation would likely involve the introduction of a new opcode, such as OP_CHECKSHRINCS, to allow the network to validate the new signature type. While the proposal offers a viable path forward, it currently lacks a completed security proof, meaning further academic and community vetting is required before it can be considered for deployment.