Blockstream Proposes SHRINCS to Shield Bitcoin From Quantum Attacks

Blockstream has published a Bitcoin Improvement Proposal (BIP) for SHRINCS, a post-quantum signature scheme designed to protect the network from future quantum computer attacks. The proposal aims to secure Bitcoin's ledger without crippling its transaction throughput.

SHRINCS is a hash-based signature scheme that relies on SHA-256 assumptions, the same cryptographic foundation used in Bitcoin mining. According to the proposal, the scheme features a 48-byte public key and a minimum signature size of 548 bytes, though signatures can grow up to 4,619 bytes. The system has already undergone production testing on the Liquid sidechain and supports full BIP-39 seed recovery.

The Quantum Threat

Quantum computers pose a theoretical risk to Bitcoin because they could potentially reverse engineer private keys from public keys, allowing attackers to steal funds. To counter this, the industry has looked toward NIST-approved post-quantum signatures. However, these standard schemes are significantly larger than the current ECDSA or Schnorr signatures, which would severely degrade Bitcoin's block economics.

According to data cited in the proposal, adopting NIST-approved schemes could drop Bitcoin's transaction speed to between 0.36 and 0.5 transactions per second (TPS). In contrast, SHRINCS would allow the blockchain to run at approximately 3 TPS. While this is a significant improvement over NIST standards, it remains below the estimated 6.5 TPS currently possible with Taproot's Schnorr signatures.

The Trade-off

To achieve this efficiency, SHRINCS introduces "statefulness." This means that user devices must store a record of used keys to prevent them from being reused, which would compromise security. As a result of this design, signatures increase in size by 16 bytes each time they are used.

Jonas Nick of Blockstream Research described the design as a "very good trade-off among the options we have now." Marin Ivezic, founder of Applied Quantum, added that SHRINCS is the "most Bitcoin-native post-quantum signature design anyone has produced."

Industry Implications

Transitioning to quantum security is viewed as a critical long-term survival requirement for Bitcoin. SHRINCS represents a middle ground that attempts to solve the "size problem" without requiring radical changes to the network, such as massive increases in block size. By balancing security with block economics, the proposal offers a path toward quantum resistance that maintains a functional level of throughput.

Next Steps

While the technical framework is established, the path to implementation remains uncertain. Marin Ivezic noted that the primary constraint in Bitcoin’s quantum migration is not the cryptography itself, but rather the network's governance. The community must now determine if the added complexity of stateful signatures is an acceptable price for quantum security.