🧠 Long-Range vs Short-Range Quantum Attacks on Bitcoin🧠 Long-Range vs Short-Range Quantum Attacks on Bitcoin

Quantum computers will change the security assumptions of Bitcoin. But how exactly?

Let’s break it down.👇

Bitcoin relies on ECC to protect your private keys.
But quantum computers break ECC.

Quantum computers open two attack windows:

• Short-range: pubkey is revealed before confirmation
• Long-range: pubkey was exposed long ago and still sits on-chain

🔐 1- Short-Range Attacks (a.k.a. Transaction Hijacking)🔐 1- Short-Range Attacks (a.k.a. Transaction Hijacking)

Let’s start with a P2PKH transaction.
It looks secure… until you broadcast it.

👀 When Does the Public Key Appear?👀 When Does the Public Key Appear?

In P2PKH, your public key stays hidden until you spend.
The moment you sign a transaction, your pubkey becomes visible.

A Window for AttackA Window for Attack

From that moment, there’s a race:
Can a quantum attacker compute your private key before your transaction gets mined?

⏳ Mempool = Danger Zone⏳ Mempool = Danger Zone

Here’s how it plays out:

1. You sign and broadcast your tx
2. It enters the mempool
3. Miner hasn’t mined it yet
   → That’s the attack window

Next step: they broadcast a conflicting transaction that spends the same coins.Next step: they broadcast a conflicting transaction that spends the same coins.

Miner Chooses the Higher FeeMiner Chooses the Higher Fee

🕵️ 10. Transaction Hijacking🕵️ 10. Transaction Hijacking

Also known as a front-running attack.
It’s specific to the time window between broadcast and mining.

🧱 2- Long-Range Attacks🧱 2- Long-Range Attacks

These are scarier.
They target coins that already have public keys exposed — like old P2PK outputs.

These UTXOs are permanently vulnerable.
A quantum attacker can sweep them at any time.

Once a public key is on-chain, the attacker doesn’t need perfect timing.
They can break it at any moment in the future.

Examples of script types vulnerable to long-range attacks:

• P2PK
• P2MS
• P2TR (script-path)
  All reveal the pubkey before spend.

📊 P2PK Holds 8.68% of All BTC📊 P2PK Holds 8.68% of All BTC

That’s ~1.72 million BTC sitting in publicly-exposed outputs.
Mostly Satoshi-era coins.

(Source Chaincode last report on "Bitcoin and Quantum Computing")

Even P2PKH isn't totally safe.
If a user reuses the address, their public key becomes exposed and vulnerable to future long-range attacks.

🔜 What’s Next?🔜 What’s Next?

In upcoming posts, we’ll cover:

• Bitcoin Mining and Grover’s Algorithm
• The “Burn vs Steal” Dilemma
• Post-Quantum Proposals: Lamport, OP_CAT, P2QRH...

Stay tuned.

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so you don’t miss what’s coming next!