Blockstream, a leading Bitcoin infrastructure company, has advanced the discussion on Bitcoin’s quantum resistance by publishing a new Bitcoin Improvement Proposal (BIP) for its post-quantum signature scheme called SHRINCS. The move highlights Blockstream’s active role in preparing the network for potential future threats posed by quantum computers, despite CEO Adam Back maintaining that practical quantum attacks may not materialize for decades.
SHRINCS: A new approach to quantum-resistant signatures
Blockstream’s SHRINCS, or State Hardened Randomized INcremental Chain Signature, was introduced to address concerns that quantum computing could eventually break current cryptographic schemes and undermine Bitcoin’s security. The company tested SHRINCS in production on its Liquid sidechain in March, and researchers Jonas Nick and Mikhail Kudinov formally proposed the opcode in May.
Jonas Nick described SHRINCS as “the first concrete proposal for a post-quantum signature scheme designed specifically for Bitcoin.” He cautioned, however, that it is not meant as the final signature solution, but as a strong candidate among currently available options.
Blockstream Research’s Jonas Nick argued that while SHRINCS is not optimal in every regard, it “is a very good trade-off among the options we have now.”
Marin Ivezic, founder of Applied Quantum and an authority on post-quantum cryptography, commented that SHRINCS is “the most Bitcoin-native post-quantum signature design anyone has produced.” He noted its compatibility with BIP-39 seed recovery and reliance on SHA-256, the same hash function integral to Bitcoin mining.
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Mini dictionary: Blockstream, founded in 2014, is a Canadian company known for its work on Bitcoin infrastructure, the Liquid sidechain, and cryptographic research.
Efficiency, implementation, and technical trade-offs
SHRINCS signatures are significantly smaller than most post-quantum signatures, starting at 548 bytes plus a 48-byte public key, but remain around nine times larger than Bitcoin’s current Schnorr (64 bytes) or ECDSA (70 bytes) signatures. In comparison, NIST-endorsed post-quantum signatures can be up to 123 times larger. Efficient signature size is essential to preserving Bitcoin’s network throughput and decentralization.
The proliferation of larger signatures could drastically reduce transaction throughput. With the NIST lattice-based ML-DSA scheme, throughput could fall to 0.5 transactions per second (TPS), while SPHINCS+ would allow only 0.36 TPS. In contrast, researchers estimate Bitcoin could still achieve around 3 TPS with widespread SHRINCS implementation, remaining close to today’s transaction rates.
| Signature Scheme | Signature Size | Estimated TPS |
|---|---|---|
| Schnorr | 64 bytes | 6.5 |
| ML-DSA (NIST lattice-based) | ~123x current | 0.5 |
| SPHINCS+ (NIST hash-based) | ~123x current | 0.36 |
| SHRINCS | 548–4,619 bytes | 3 |
The design of SHRINCS also makes use of Bitcoin’s Segregated Witness (SegWit), which allows signature data to occupy less blockspace by allocating it to a separate area, further mitigating the impact of increased signature size.
SHRINCS differs from SPHINCS+ by taking a stateful approach, storing used keys locally instead of including a multi-layer hash tree to remain stateless. Each time a key is used, the resulting signature grows by 16 bytes. However, if a device is lost, the fallback stateless transaction required for key recovery could reach 5,777 bytes.
Yoon Auh, founder of BOLTS Technologies, pointed out that SHRINCS includes “statefulness, compact signing paths, fallbacks, assumptions about how many times a seed is initialized, and rules for when devices must switch to larger stateless signatures.”
Marin Ivezic emphasized, “The binding constraint in Bitcoin’s quantum migration isn’t cryptography, it’s governance,” highlighting the challenge of gaining community consensus for any quantum upgrade.
Other research directions and future prospects
Blockstream is also experimenting with lattice-based signatures, considered less reliable by some researchers, and exploring the possibility of aggregating signatures with zero-knowledge (ZK) proofs. With ZK proof aggregation, Bitcoin’s throughput might reach up to 6.7 TPS if used together with SHRINCS. However, the introduction of ZK proofs would be a radical protocol change and would likely face significant community debate before activation.
The latest BIP provides only a first step, as a formal security proof for SHRINCS remains outstanding. Its eventual adoption will depend not only on technical performance but also on the ability of the Bitcoin community to find consensus on security upgrades.
Blockstream’s ongoing research includes continued optimization of the SHRINCS protocol and its support on mainstream hardware wallets. In March, the company also published SHRIMPS, intended to operate alongside SHRINCS for device backup and signature flexibility, though it has since been integrated as a stateless path under the same public key.
Every upgrade related to quantum security in Bitcoin is likely to generate significant debate, as the implications touch the core of the network’s design and operation. Researchers caution that the challenge is not only cryptographic but also political, as any upgrade demands broad support and careful governance.
