TL;DR:
- Blockstream published a technical proposal called SHRINCS to protect Bitcoin transactions against quantum computers.
- SHRINCS would process around three transactions per second, compared to the 0.36 allowed by the post-quantum scheme standardized by NIST.
- The Blockstream proposal was designed by Jonas Nick and Mikhail Kudinov, and was already tested in March on Liquid, Blockstream’s sidechain.
Blockstream, a Bitcoin infrastructure firm whose engineers have contributed to protocol development for more than a decade, published a technical proposal called SHRINCS, a new signature scheme for Bitcoin transactions designed to resist attacks from quantum computers without severely compromising the network’s processing capacity.
The underlying problem in the industry is that Bitcoin proves ownership of funds through digital signatures based on elliptic curve cryptography. Theoretically speaking, a sufficiently powerful quantum computer, running Shor’s algorithm developed in 1994, could calculate the private key from the public key visible on the blockchain, which would allow an attacker to forge signatures and spend other people’s coins. More than 1.1 million BTC belonging to the network’s creator, Satoshi Nakamoto, sit in addresses with exposed public keys.
SHRINCS: The Size Problem
Replacing the current signature system is a genuine technical obstacle: post-quantum signatures standardized by the National Institute of Standards and Technology (NIST) can be dozens of times larger than the Schnorr signatures Bitcoin uses today. Blockstream estimated that the network could process around 6.5 transactions per second with Schnorr, a figure that drops to just 0.36 with SLH-DSA, the hash scheme standardized by NIST.
SHRINCS raises that capacity to approximately three transactions per second. Its signatures start at 324 bytes compared to Schnorr’s 64, but the SegWit upgrade of 2017 grants a discount to the space allocated for signatures within each block, which reduces the actual impact on network capacity. The system is built on SHA-256, the same hash function Bitcoin already uses in its mining mechanism, meaning it introduces no new mathematical assumptions into the protocol’s security foundation.
Blockstream’s Proposal Is Neither the Best Nor the Definitive One
Jonas Nick, who designed SHRINCS alongside his colleague Mikhail Kudinov, acknowledged that it is the first post-quantum signature proposal conceived specifically for Bitcoin, while clarifying that it does not aim to be the definitive system or the best in every respect.
Among its limitations, the scheme requires each wallet to use a single-use key per transaction, which makes it necessary to keep a precise record of keys already used. Restoring an old wallet backup or losing that record could complicate access to funds.
The proposal still lacks a formal security proof and its reference software is not ready for production use. Incorporating SHRINCS into Bitcoin would require a soft fork with sufficient network backing. The initiative coincides with a similar direction that Ethereum researchers began exploring this week.
