Bitcoin transactions are protected by Schnorr and ECDSA signatures, both built on elliptic-curve cryptography. A sufficiently capable quantum computer could recover a private key from an exposed public key and spend the funds it protects. Nobody knows when or whether such a machine might exist or not, but migrating Bitcoin to new cryptography would take years, so the groundwork has to happen well before the threat does.

For a broader look at what quantum computing means for Bitcoin, start with Blockstream's quantum page.

SHRINCS is a hash-based signature scheme proposed by Blockstream Research as a practical post-quantum option for Bitcoin. It gives a single public key two signing paths: (1) a compact stateful path for everyday use, where signatures start at 324 bytes and grow by roughly 16 bytes with each additional signature, and (2) a stateless fallback that still works if the signer ever loses its state but at the cost of larger signature size. Its security rests only on the SHA-256 hash function, which Bitcoin already relies on for mining and addresses. Blockstream’s Director of Research Jonas Nick introduced the design in depth in his OP_CHECKSHRINCS post.

Each hash-based signature scheme has a set of parameters that define its shape, size, and the cost of its underlying functions (key generation, signature generation, and verification), and every parameter choice trades one metric against the others. On Bitcoin, where signature bytes compete for block space, where signatures need to be verified by all nodes of the network, and where signatures as well as keys need to be generated on resource-constrained devices (such as hardware wallets), those choices decide whether a post-quantum signature stays practical. The design choice behind SHRINCS was to find the shortest signature while keeping it practical for Bitcoin use cases.

In this blog post, we show how we swept over 25,935 candidates and arrived at a stateless fallback roughly 25% smaller than SLH-DSA and slightly cheaper from the signature generation and verification perspective. We also built an open-source explorer, so anyone can verify our choices.

...read more at blog.blockstream.com