Substantially identical numerical sequences known only to stations A and B are generated in a manner not subject to duplication by an eavesdropper and not subject to cryptanalytic attack because they are not derived using a mathematical function (such, as for example, factoring). The sequences are independently derived utilizing a physical phenomena that can only be “measured” precisely the same at stations A and B. Signals are simultaneously transmitted from each station toward the other through a communication channel having a characteristic physical property capable of modifying the signals in a non-deterministic way, such as causing a phase shift. Each signal is “reflected” by the opposite station back toward its station of origin. The effect of the communication channel is “measured” by comparing original and reflected signals. Measured differences are quantized and expressed as numbers.