The Economics of a Reorg: Calculating the Cost of Attack

A Chain Reorganization is mathematically possible, but in Bitcoin, it is financially prohibitive for deep depths. Because nodes follow the Most Work chain, an attacker must expend more energy than the rest of the global mining network combined to reverse a confirmed transaction.

---

⚡ 1. The 51% Threshold

To guarantee a successful reorg of a specific depth, an attacker needs $>50\%$ of the total network hashrate. With this majority, they can mine blocks faster than the honest network, eventually creating a "shadow chain" with more cumulative work.

---

💸 2. Calculating Electricity Costs

The cost of a reorg can be estimated by looking at the current Network Hashrate and Mining Efficiency.

As of 2024, the Bitcoin network hashrate is approximately 600 EH/s (Exahashes per second). * Hardware: To match this, an attacker would need millions of state-of-the-art ASIC miners (e.g., Antminer S21). * Energy: The electricity required for a single hour of a 51% attack costs millions of dollars.

---

📉 3. The "Cost to Reverse" Equation

A simple way to visualize the security of a transaction is the Cost to Reverse (CtR):

$$\text{CtR} = \text{Time since transaction} \times \text{Network Hashrate} \times \text{Energy Price per Hash}$$

If a transaction has 6 confirmations (~1 hour): * An attacker must find 7 blocks in that same hour to overtake the chain. * They must pay for the electricity for all 7 blocks plus the hardware required to reach that hashrate.

---

🏛️ 4. The Opportunity Cost

Miners are profit-seekers. If a miner has 51% of the hashrate, they have two choices: 1. Attack: Attempt to reorg the chain to double-spend a payment. This risks crashing the price of Bitcoin and making their expensive hardware worthless. 2. Mine Honestly: Simply mine the next blocks on the main chain and collect the Block Rewards and Fees.

For almost all rational actors, Option 2 is vastly more profitable. This is the "Game Theory" defense that makes deep reorgs practically non-existent in Bitcoin's history.

---

🛡️ 5. Deep Reorg History

While 1 or 2 block reorgs happen occasionally due to latency, Bitcoin has never suffered a successful, deep malicious reorg (e.g., 50+ blocks). The "Most Work" rule, combined with the sheer scale of the global hashrate, creates a "Work Barrier" that becomes insurmountable as time passes.