Avoiding key distribution seems logically
impossible — surely, if Alice wants to lock something in a box
so that only Bob can open it, she must give him a copy of the
key. Or, in terms of cryptography, if Alice wants to encipher a
message so that only Bob can decipher it, she must give him a
copy of the key. Key exchange is an inevitable part of enci-
pherment — or is it?

Now picture the following situation. As before, Alice wants
to send an intensely personal message to Bob. Again, she puts
her secret message in an iron box, padlocks it and sends it to
Bob. When the box arrives, Bob adds his own padlock and
sends the box back to Alice. When Alice receives the box, it is
now secured by two padlocks. She removes her own padlock,
leaving just Bob's padlock to secure the box. Finally she sends
the box back to Bob. And here is the crucial difference: Bob
can now open the box, because it is secured only with his own
padlock, to which he alone has the key.

The implications of this little story are enormous. It demon-
strates that a secret message can be securely exchanged be-
tween two people without necessarily exchanging a key. For the
first time there is some hope that key exchange might not be
an inevitable part of cryptography. We can reinterpret the story
in terms of encryption. Alice uses her own key to encrypt a
message to Bob, who encrypts it again with his own key and
returns it. When Alice receives the doubly encrypted message,
she removes her own encryption and returns it to Bob, who can
then remove his own encryption and read the message.