Whatever the moral standing of the participants, there is an age-old approach that Alice and Bob may employto cut Eve out of the conversation even if Eve intercepts messages that pass between them. They can encrypt the data using a cipher key that is known onlyto Alice and Bob. What they may arrange to do is to meet in a secure environment where they exchange with one another a secret number(let us say 57)and then return home.When the time comes, Alice will want to send a message to Bob and, just to illustrate the point, suppose that message can be represented by a single digit between 1 and 9. On the big day, Alice wants to send the message‘8’to Bob. She takes her message and adds the secret ingredient, that is to say she masks its true value by adding 57 and so sends the message to Bob, across an insecure channel, of 8+57=65. Bob receives this message and subtracts the secret number to retrieve Alice’s plaintext 65-57=8.