In this course, we have focused mostly on the truly quantum aspects of quantum computing and networking. In this lecture, we make an exception. Here, Tim Coopmans will introduce the one-time pad, an extremely simple, and very secure classical cryptographic protocol, which is not used in practice, due to the inconvenience of creating the shared randomness required to encrypt messages. We will see in the later lectures of this module exactly how this necessary shared randomness can be generated using quantum mechanics.

Main takeaways

• Cryptography is the study of methods to transmit messages through channels which an eavesdropper can access, in such a way that the eavesdropper cannot read them.
• There is a simple, classical scheme, called the one-time pad, which provides a sender and receiver (Alice and Bob) with a means of transmitting a secret message, given a string of random zeroes and ones which is the same size as the (binary-encoded) message.
• To use the one-time pad, Alice simply adds the random bits to the message, and transmits the sum to Bob, who subtracts his copy of the random bits (both of these operations are equivalent to XOR when the messages are binary).
• This cryptographic scheme can only fail if Eve gains access to the pad, or the same pad is used more than once.