Moni Naor

In 1992 Naor and Cynthia Dwork published Pricing via Processing or Combatting Junk Mail: every message would carry the solution to a moderately annoying computational puzzle. Trivial for someone sending one letter, ruinous for someone sending a million. The idea became known as proof of work and turned up sixteen years later in Bitcoin, where it now burns electricity on the scale of small countries. Spam survived.

With Moti Yung he showed in 1990 how to build encryption that resists chosen-ciphertext attacks, and in 1994 he and Adi Shamir invented visual cryptography: an image is split across two transparencies that each look like noise but reveal the secret when stacked. Decryption requires no computer, just an overhead projector sheet.