It's a secret (14)

So we have finally arrived at a genuinely unbreakable cipher.  Using the Vigenère square with a keystream which simply consists of a series of random letters gives a would-be decipherer nothing to go on.  But before we get too excited, we need to take a big step backwards.  A is sending a message to B and all the encryption systems we've looked at so far require one fundamental thing - B needs to know what method of encryption A has used - known as the "key".  This means that at some point A has to contact B and pass the key.  And if this contact can be intercepted then it doesn't matter how good the system being used is, once the key is known it becomes useless. This has become known as the "key distribution problem" and is the weak point of any of the systems we've looked at so far.  So the new "holy grail" of cryptographers became to come up with a system which did not require the passing of a key.  At first glance this seems a contradiction in terms, but where there's a will...