Blockstream’s SHRINCS BIP Raises Bitcoin’s Quantum Security Stakes Amid Significant Challenges

A new Bitcoin Improvement Proposal is live. It covers SHRINCS, an experimental post-quantum signature scheme that Blockstream has been quietly developing, and it’s already drawing attention from developers who’ve been watching the quantum threat debate simmer for years.

Blockstream, led by CEO Adam Back — who’s publicly skeptical about how close quantum computing actually is to threatening Bitcoin — pushed the proposal forward anyway. Smart move, probably. The research side of the house clearly isn’t waiting for the threat to materialize before mapping out a response. Jonas Nick, part of Blockstream Research, calls SHRINCS the first post-quantum signature proposal built specifically for Bitcoin. Not the final answer, he’s careful to say. Just the first serious one.

What SHRINCS Actually Does

The scheme is already running on Blockstream’s Liquid sidechain. It got tested there in March, which gives it at least some real-world footing — though the cryptographic validation work is still early and pretty thin by most standards. It’s not like this thing has been stress-tested at scale. Not yet.

Here’s the core tension: current NIST-endorsed post-quantum schemes are massive compared to what Bitcoin uses today. Bitcoin’s Schnorr signatures sit at 64 bytes. SHRINCS signatures land somewhere between 548 and 4,619 bytes depending on usage. That’s a huge jump. The kind of jump that would normally crush transaction throughput and bloat the blockchain fast.

But there’s a partial fix baked in. Under Bitcoin’s Segregated Witness protocol, the weight of those larger signatures gets partially absorbed. The result, per Blockstream’s numbers, is a transaction speed of roughly 3 TPS — which is basically where Bitcoin sits right now. So the throughput hit, at least under SegWit, seems manageable.

That said, each SHRINCS signature grows by 16 bytes with each use. Scalability questions don’t disappear just because SegWit softens the blow.

The Key Management Problem

SHRINCS uses one-time keys. They’re stored on devices. And that’s where things get complicated fast — because key reuse is a serious risk with stateful signature schemes like this one. If a key gets reused, the security guarantees fall apart. The BIP itself flags the complexity of deploying SHRINCS on hardware wallets, which aren’t exactly built for this kind of state management.

Lose your device data? You’re looking at a large fallback transaction to recover. The mechanics work, but they add a layer of responsibility that most Bitcoin users probably aren’t used to dealing with. It’s a real usability gap, and it’s not clear yet how the community would handle it at scale.

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Compare that to something like SPHINCS+, a stateless scheme that uses multi-layer hash trees to sidestep the key-reuse problem entirely. No state to manage, no device dependency. But stateless schemes tend to be even larger, so there’s a different trade-off sitting on that side of the ledger.

Blockstream also introduced SHRIMPS in March — a companion scheme meant to support backup devices that can co-sign transactions from the same seed. Whether SHRIMPS makes it into the current BIP is unclear. No firm details on that yet.

Lattice Schemes and the ZK Proof Question

Blockstream’s research isn’t locked onto SHRINCS alone. The team has been looking at lattice-based signature schemes too. They’re smaller than hash-based options, which is genuinely appealing. But they’re considered less proven cryptographically, and that matters a lot when you’re talking about securing a network worth what Bitcoin’s worth.

The more interesting number in the research is 6.7 TPS. That’s the transaction speed Blockstream thinks might be achievable if SHRINCS gets combined with zero-knowledge proof aggregation. More than double the current rate. But ZK proof integration into Bitcoin isn’t a simple toggle — it’s a governance conversation that would touch block size, consensus rules, and probably drag on for years.

And that’s kind of the whole problem here. The technical side of post-quantum Bitcoin is hard, but the governance side might be harder. Blockstream has deliberately separated the signature scheme question from broader network changes like block size increases or ZK proof adoption. Smart framing — it keeps the SHRINCS discussion from getting swallowed by every other Bitcoin upgrade debate. But it also means the path to any real implementation runs through a community consensus process that has never been fast or easy.

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Any solution that moves forward needs to be technically solid and broadly acceptable to Bitcoin stakeholders. Those two things don’t always overlap. The community has seen proposals stall for years over smaller disagreements than this one.

What’s clear right now is that Blockstream is doing the work — the Liquid sidechain tests, the SHRIMPS companion scheme, the lattice research, the ZK proof modeling. What’s murky is which of these threads, if any, actually gets woven into Bitcoin’s base layer, and when.

Each SHRINCS signature grows by 16 bytes per use.

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