Bernard has been doing some cryptography reading and highlights this from Simon Singh's "The Code Book":
"Imagine that Alice and Bob live in a country where the postal system is completely corrupt. Alice wants to send an intensely personal message to Bob. She puts it inside a strongbox, closes it, and secures it with a padlock and key. She puts the padlock box in the post and keeps the key. However, when the box reaches Bob he is unable to open it because he does not have the key."
'Is there anyway for Alice to send the key to Bob securely? No, avoiding key distribution seems logically impossible.'
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 encipherment - or is it?"
"Now picture the following scenario: Alice puts her secret message in a strongbox, 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."