Assume the transmission of binary ones and zeroes and let them be represented by either rectilinearly (i.e., vertical or horizontal) or diagonally (i.e., left-diagonal or right diagonal) polarised photons. Alice (transmitter) randomly swaps between the rectilinear and diagonal schemes, known as “bases” in quantum cryptography terminology, during transmission. Bob (receiver) measures the photons in one of the two bases chosen at random. The choices of basis made by Alice and Bob thus comprise two independent random sequences. Therefore, about half of the basis choices will be the same and are called the “compatible” bases, and the other half will be different and are called the “incompatible” bases. After transmission and measurement, Alice and Bob compare publicly, making use for this purpose of a classical communications channel, the two independent random sets of polarization bases that were used, without revealing the polarization states that they observed. The bit values of those polarization states measured in the compatible bases yield the “sifted key.” Any adversary Eve can intercept both the quantum and the public classical channel, but thanks to the non-cloning theorem of quantum physics, she will not be able to measure the polarizations without revealing her presence, since she cannot know the random basis choices in advance.