TECHNICAL_DOC // BLOCKCHAIN / 51-PERCENT-ATTACK
51%
ATTACK
ATTACK
A 51% attack occurs when a single entity controls more than half of Bitcoin's total
hashrate, enabling them to mine blocks faster than the honest network in expectation.
This allows double-spending confirmed transactions and selectively excluding transactions,
but cannot create coins from nothing, steal from unrelated addresses, or forge signatures.
WHAT_AN_ATTACKER_CAN_DO
ATTACK CAPABILITIES — WITH >50% HASHRATE
✓ Double-spend confirmed transactions:
1. Send tx to victim (gets N confirmations on public chain)
2. Secretly mine alternative chain excluding that tx
3. Release longer chain → public chain reorgs → original tx gone
4. Attacker keeps goods received + coins
✓ Block specific transactions:
Refuse to include targeted addresses in mined blocks
✓ Earn more block rewards:
By mining selfish mining strategies (see selfish mining)
✗ Cannot steal from arbitrary addresses (no signing key)
✗ Cannot create new bitcoins beyond coinbase reward
✗ Cannot change the total supply or protocol rules
✗ Cannot reverse transactions buried under many blocks
(probability decreases exponentially with depth)
Cost of Attack on Bitcoin (2024)
ECONOMIC SECURITY
The cost of acquiring 51% of Bitcoin's hashrate makes attacks economically irrational against the main chain — the attacker would lose more than they could steal.
Bitcoin hashrate (2024): ~600–700 EH/s
To attack: need ~300–350 EH/s of additional hardware
ASIC hardware cost (Antminer S21, ~200 TH/s):
~$5,000 per unit
To achieve 350 EH/s: 1.75 million units × $5,000
Hardware cost: ~$8.75 billion
Plus:
Power: ~35 GW = massive infrastructure cost
Opportunity cost: no block rewards while attacking
Market impact: Bitcoin price would likely collapse
Smaller PoW chains (ETC, BCH) have been successfully
51%-attacked at far lower cost — hashrate is security.
Confirmations as Security
PRACTICAL DEFENSE
Waiting for more block confirmations makes double-spend attacks exponentially more expensive. Satoshi's original paper quantified this probability.
Probability attacker can catch up from N blocks behind
(attacker has q fraction of hashrate):
p = 1-q (honest), q (attacker)
If q < p: probability of catching up → 0 as N grows
Satoshi's formula (q=0.1, 6 confirmations):
P(attacker succeeds) ≈ 0.00024 (0.024%)
Practical confirmation thresholds:
0 confirmations (0-conf): high risk, accept for small amounts only
1 confirmation: low risk for small amounts
6 confirmations: standard — extremely low risk
100 blocks: coinbase maturity requirement
TERMINOLOGY_INDEX
51% Attack
Control of majority hashrate enabling double-spend of confirmed transactions via private longer chain.
Double Spend
Spending the same coins twice by invalidating a confirmed transaction via a chain reorg.
Hashrate Security
The total network hashrate determines attack cost. Bitcoin's enormous hashrate makes attacks prohibitively expensive.
Confirmation Depth
Number of blocks mined after a transaction's block. Each additional block exponentially reduces reorg probability.