20 November 1973
A NOTE ON 'NON-SECRET ENCRYPTION'
by C C Cocks
A possible implementation is suggested of J H Ellis's proposed method of encryption involving no sharing
of secret information (key lists, machine set-ups, pluggings etc) between sender and receiver.
Note on "Non-Secret Encryption"
1. In [1] J H Ellis describes a theoretical method of encryption which does not necessitate the sharing of secret information between the sender and receiver. The following describes a possible implementation of this. a.The receiver picks 2 primes P, Q satisfying the conditions i.P does not divide Q-1. ii.Q does not divide P-1. He then transmits N = PQ to the sender. b.The sender has a message, consisting of numbers C1, C2, ... Crwith 0 < Ci< N He sends each, encoded as Diwhere Di= CiNreduced modulo N. c.To decode, the receiver finds, by Euclid's Algorithm, numbers P', Q' satisfying i.P P' = 1 (mod Q - 1) ii.Q Q' = 1 (mod P - 1) Then Ci= DiP'(mod Q) and Ci= DiQ'(mod P) and so Cican be calculated. Processes Involved