How did a 1993 trick against junk email end up securing Bitcoin?

Proof of work started as a fix for junk email. In 1993, Cynthia Dwork and Moni Naor proposed making the sender's computer solve a small, deliberately annoying math problem, like computing modular square roots, before a message could go through: challenging to solve, yet straightforward to verify. Bitcoin, which went online in 2009, kept exactly that asymmetry. Its miners compete to add each new block, and the energy they spend in that competition is what gives the network its security and resistance to attacks.

The idea evolved step by step. Adam Back described a proof-of-work system called Hashcash as a protection against email spam in 1997: one Hashcash stamp can represent about 2⁵² hash computations to create, yet it is checked with a single computation, by confirming that its SHA-1 hash begins with 52 binary zeros. The term "proof of work" itself was coined and formalized in a 1999 paper, and in 2004 Hal Finney adapted the idea to digital tokens with "reusable proof of work" built on SHA-1.

Bitcoin then popularized proof of work as the foundation for consensus in a permissionless decentralized network. Miners compete to append blocks and mine new currency, and each miner's chance of success is proportional to the computing effort they spend. The core asymmetry is still Dwork and Naor's: moderately hard, yet feasible, for whoever does the work, but easy for everyone else to check.

That same design is where the controversy begins. Because miners have to use a lot of energy to add each block, proof-of-work systems have been criticized by some environmentalists, who argue that they generate externalities not reflected in market electricity prices. Mining itself moved from ordinary processors to graphics cards, then to specialized chips called ASICs. An idea born to make spamming a little more annoying became the engine that secures Bitcoin.