Blockstream, led by co-founder and CEO Adam Back, has published a Bitcoin Improvement Proposal (BIP) for SHRINCS, a post-quantum signature scheme designed to enhance Bitcoin’s resistance to quantum computing threats. The proposal follows Blockstream’s experimental deployment of SHRINCS on its Liquid sidechain, marking a practical step toward preparing Bitcoin for potential future cryptographic vulnerabilities.
The scheme, developed by Blockstream Research’s Jonas Nick and Mikhail Kudinov, is described as the first concrete proposal for a post-quantum signature scheme tailored specifically for Bitcoin. While SHRINCS is not positioned as Bitcoin’s final solution, its advocates argue it represents a viable trade-off among current options. Unlike traditional Bitcoin signatures such as ECDSA or Schnorr, which are 64-70 bytes, SHRINCS signatures range from 548 to 4,619 bytes, depending on usage. This expansion is significantly smaller than other post-quantum alternatives endorsed by NIST, which can be 38 to 123 times larger.
The proposal highlights a key trade-off: SHRINCS reduces signature size compared to other post-quantum schemes but introduces statefulness, where each signature uses a new one-time key stored on the device. This approach avoids the multi-layer hash tree structures of schemes like SPHINCS+, which contribute to their larger size. However, SHRINCS signatures grow by 16 bytes with each use, and users risk fund loss if a device is lost without access to a stateless fallback transaction, estimated at 5,777 bytes.
Blockstream has tested SHRINCS in production on the Liquid sidechain, including embedding a copy of Bitcoin’s white paper in a transaction. Under Segregated Witness (SegWit), where signature bytes occupy less blockspace, Blockstream estimates Bitcoin could maintain a throughput of approximately 3 transactions per second using SHRINCS. This compares to 6.5 TPS with current Schnorr signatures, 0.5 TPS with NIST’s lattice-based ML-DSA, and 0.36 TPS with NIST’s hash-based SPHINCS+.
The BIP acknowledges unresolved risks, including the lack of a completed security proof and potential incompatibilities between implementations using hypertree pruning. Blockstream has also incorporated a companion scheme, initially named SHRIMPS, into SHRINCS as a stateless fallback path under the same 48-byte public key, reducing its size by 26% compared to standard parameters. The company continues to explore lattice-based signatures and zero-knowledge proof aggregation, which could further improve throughput to 6.7 TPS if combined with SHRINCS.
Critics note that SHRINCS introduces additional complexity and maintenance obligations, as every new consensus rule in Bitcoin becomes a permanent requirement. The scheme’s statefulness and fallback mechanisms also introduce potential user failure modes, raising questions about long-term viability and adoption.
