Bitcoin SHRINCS Draft Sets 5,777-Byte Signature for Counter Loss

SHRINCS, a draft quantum-resistant signature system for Bitcoin (BTC), would let a wallet keep signing when it cannot establish its correct state counter, but doing so would require a 5,777-byte signature.

The design offers a smaller signing method, with signatures ranging from 548 to 4,619 bytes, when the signer can establish its current counter. It also provides a stateless method that does not depend on that record. The 5,777-byte fallback is about 10.5 times the smallest compact signature, a comparison between signatures rather than complete Bitcoin transactions.

That distinction matters when a wallet is restored from a static backup. The backup may recover the key without establishing the counter’s current value. SHRINCS requires the signer to refuse the compact method if the counter is lost, corrupted or uncertain; it can then use the stateless method without relying on an outdated record.

Each compact signature uses a signing slot, and reusing a slot for a different message can permit forgery. The counter must be saved before a signature is released. Those requirements also complicate custody systems that sign across independent servers, which must coordinate their use of signing slots and preserve an accurate counter through failures.

Bitcoin’s deployed Schnorr specification uses 64-byte signatures, so even SHRINCS’s smallest signature would take more space. Signatures contribute to transaction weight, which Bitcoin calculates as three times the size without witness data plus the total transaction size. Larger signatures would therefore use more block space, but the effect would depend on the rules chosen for deployment and how often signers use each method.

The 10.5-times signature ratio does not imply a 10.5-fold increase in transaction size, node storage or fees. Transactions contain data beyond signatures, and SHRINCS specifies neither deployment rules nor an additional witness discount. Its eventual effect on fees and capacity cannot be determined from signature size alone.

SHRINCS is part of a broader Bitcoin quantum-upgrade discussion.

SHRINCS remains a draft. Its security proof and fully optimized implementation are unfinished, and it sets no rules for deployment on Bitcoin.