 Suppose Alice and Bob are separated and want to communicate a secret message, without revealing any information to Eve, an eavesdropper. They can do this in a classical world if they share a ‘one-time pad,’ a cryptographic key represented by a sequence of random bits at least as long as the number of bits required to communicate the message. In fact, this is the only secure way to achieve perfect security in a classical world. To send a message to Bob, Alice communicates which bits in the key Bob should flip. The resulting sequence of bits is the message. In addition, they would need to have some way of encoding messages as sequences of bits, by representing letters of the alphabet and spaces and punctuation symbols as binary numbers, which could be done by some standard, publicly available scheme.