The existence of good cryptosystems for key rates greater than the message redundancy (Corresp.)
Shyue-Ching Lu · IEEE Transactions on Information Theory · 1979
A general result related to Shannon's "random" cipher result is presented. By using the Fano inequality it is shown that the average probability of correct decryptment of a message digit is bounded away from one when the key rate is greater than the message redundancy for a given class of cryptosystems.