Charles H. Bennett, a physicist at the IBM Thomas J. Watson Research Center, and Gilles Brassard, a computer scientist at the University of Montreal, met at a computer science conference in Puerto Rico in 1979 and, inspired by Stephen Wiesner's earlier, unpublished ideas on 'quantum money', developed over the following years the first practical quantum cryptography protocol. They presented it in December 1984 at a conference in Bangalore, India, under the title 'Quantum cryptography: Public key distribution and coin tossing' — hence the name BB84, from the authors' initials and the year. The protocol solves a problem classical cryptography can only address through computational assumptions (such as the difficulty of factoring large numbers): it allows two parties, conventionally called 'Alice' and 'Bob', to establish a shared secret key whose security does not depend on any computational limitation of the adversary, but on the laws of physics themselves. The idea exploits two fundamental properties of quantum mechanics: Heisenberg's uncertainty principle and the no-cloning theorem, which prevent measuring or copying an unknown quantum state without detectably altering it. Alice sends Bob a sequence of individual photons randomly encoded in one of four possible polarizations; any attempt by an eavesdropper to intercept and measure those photons inevitably introduces detectable disturbances, revealing their presence before any information can be compromised. The result is a form of communication whose security is guaranteed by the laws of physics, not by the difficulty of a mathematical problem — even against an adversary with an unlimited-power quantum computer. BB84 becomes the most studied and implemented quantum key distribution protocol in the world, laying the scientific foundations of quantum cryptography as a discipline and giving rise, decades later, to operational quantum communication networks and a multi-billion-dollar quantum cybersecurity industry.