1. In a method of communicating securely over an insecure communication channel of the type which communicates a message from a transmitter to a receiver by enciphering the message with a secret enciphering key at the transmitter, transmitting the enciphered message from the transmitter to the receiver, and deciphering the enciphered message with a secret deciphering key at the receiver, the improvement characterized by: generating the secret deciphering key as the multiplicative inverse, in modular arithmetic, of the secret enciphering key; generating the enciphered message by exponentiating, in modular arithmetic, the message with the secret enciphering key; deciphering the enciphered message by exponentiating, in modular arithmetic, the enciphered message with the secret deciphering key, wherein the step of: generating the secret deciphering key is performed by generating a secret deciphering key D, such that EQU D=K.sup.-1 (mod q-1) where 1.ltoreq.D.ltoreq.q-2, q is a prime number, and the secret enciphering key K is an independent random number chosen uniformly from the set of integers (1, 2,. .. q-2) which are relatively prime to q-1; generating the enciphered message is performed by generating an enciphered message C, such that EQU C=P.sup.K (mod q) where P is the message; and deciphering the enciphered message is performed by generating the message P, where EQU P=C.sup.D (mod q).