e tradition has it to call the originator Alice and the destinee Bob. The malicious interceptor, who eavesdrops, is Eve. From this point, I will use these three nicknames. For Alice, the encryption task is like putting a message into a locked steel box and sending it to Bob through the mail. If Bob does not have a copy of Alice's key to open the lock, then he can't retrieve Alice's message. Alice must then provide Bob a copy of her key, for instance by meeting regularly in some agreed time and place. But this way of proceeding amounts to the same thing as passing on and hiding key books, notwithstanding the time and effort required. This intractable problem of key exchange caused cryptographers to search for new directions in the way message "boxes" could be locked and unlocked. They found that one simple solution could be to use two locks instead of one! To visualize how this can work, assume that both Alice and Bob have different keys of their own. Alice locks her message box and sends it to Bob (step 1). Bob cannot open it, since he does not have Alice's key. What he does is to put a second lock on the box, lock it with his own key, and send the box back to Alice (step 2). Then Alice opens the first lock with her key. She then sends back the box to Bob (step 3). Now Bob can open the box, since he has the key of the remaining lock (step 4). Such a process is referred to as double-key encryption. The remarkable result is that the message is safely communicated without Alice and Bob exchanging any key whatsoever. Consider now a practical illustration. Table 25.4 shows the different steps of double-key encryption, using the principle of adding (XOR) a same-length key (since the purpose is just to prove the point through successive XOR additions, I have chosen random keys for Alice and Bob). We see from the above example that at the end of this double-key encryption process, Bob is able to retrieve the plaintext unaltered (step 4). We note again that in the different 25.3 Double-key encryption Table 25.4 Double-key