Encryption has a remarkably long and varied history. Spies have been using it to convey secret messages ever since there were secret messages to convey. One powerful method of encryption uses random digits. Two documents are prepared with the same random sequence of numbers. The spy is sent out with one and the spy master retains the other. The digits are used as follows. Suppose that the word to be encrypted is SPY and the random digits are $352.$ Then $S$ becomes $V($three letters after $S),P$ becomes $U($five letters after $P),$ and $Y$ becomes $A($two letters after $Y,$ restarting at A after $Z).$ The spy would encrypt a message and then destroy the document used to encrypt it$.$ This is an early example of

asymmetric key encryption.

symmetric key encryption.

a hashing algorithm.

public key encryption.