Security, cost and alternatives (proof of stake)

Proof of work secures Bitcoin, but at a cost. This lesson examines the 51% attack, the energy cost, then the most widespread alternative: proof of stake.

The 51% attack

The security of proof of work rests on one assumption: the majority of computing power is honest. If an attacker controlled more than 50% of the total power (the hashrate), they could mine faster than all the rest of the network combined.

Réseau honnête : --[N]--[N+1]--[N+2]          (chaîne publique)
Attaquant (51%): --[N]--[N+1']-[N+2']-[N+3']  (chaîne secrète, plus longue)

En publiant sa chaîne plus longue, l'attaquant impose sa version
=> il peut annuler des transactions récentes (double dépense).

They could not, however, steal other people's coins or create money at will: signatures remain verified by everyone. The main threat is the double spending of their own recent transactions. On a large network, gathering 51% of the power costs a fortune, which deters the attack.

The energy cost

The downside of proof of work is that its security comes from the expenditure of energy: the more the network consumes, the more expensive it is to attack. At Bitcoin's scale, this consumption reaches that of a medium-sized country. This is a major environmental criticism.

The alternative: proof of stake

Proof of stake replaces the expenditure of energy with a stake of tokens. Instead of "burning" computation, a participant (the validator) locks up an amount of cryptocurrency as a guarantee. The protocol then chooses, weighted by the stake, who validates the next block.

A dishonest validator sees their stake confiscated (slashing). The attack becomes costly no longer in electricity, but in capital at risk. Ethereum switched to proof of stake in 2022 (The Merge), reducing its consumption by about 99%.

Comparison table

Criterion	Proof of work	Proof of stake
Resource committed	Computing power	Locked tokens (stake)
Main cost	Energy (electricity)	Locked capital
Who produces a block	The miner who finds the nonce	A randomly drawn validator
Penalty for a cheater	Work wasted for nothing	Stake confiscated (slashing)
Energy consumption	Very high	Low
Example	Bitcoin	Ethereum (since 2022)

What to remember about the trade-offs

No system is perfect: proof of work is battle-tested but energy-hungry; proof of stake is frugal but debated. The goal remains the same: to make the attack more costly than the expected gain.

In summary

• The 51% attack consists in controlling the majority of the power (or of the stake) to rewrite recent transactions; it allows double spending but not theft.
• Proof of work draws its security from an enormous energy cost, its main criticism.
• Proof of stake replaces energy with a confiscable stake of tokens; Ethereum adopted it in 2022 (−99% consumption).
• In all cases, the goal is to make the attack more expensive than the expected benefit.