It's a secret (12)

Of course, once it was realised that a repeating keyword allowed for the possibility of breaking a Vigenère cipher, cryptographers switched to using keyworms or keystreams - that is a series of words as long as the message itself - so, no repetitions.  There's a story - I don't know how true - of a British salesman in the mid-1930s who plied his trade in Europe.  As such he visited Germany often, and the Germans were suspicious that, although they accepted he was a genuine salesman, he was also keeping his eyes open for troop movements and the disposition of aircraft and tanks and such and sending details of these things back to the UK in his daily encrypted telegrams.  They made great efforts to decipher his telegrams but with no success.  On several occasions they searched the rooms he was staying in, in the hope of finding evidence that he was in fact a spy, but found nothing significant, although a couple of times, those searching reported back that he had a copy of "Alice in Wonderland" among his things.  But they put the idea of a grown man reading a book written for children down to the well known eccentricity of the English!  In fact he was doing exacly what they thought he was doing and the book was the basis of his cipher.  Hidden in his telegram would be a series of numbers, indicating a page number, a line number and a word number, and starting at that word he would write out as much of the book as necessary over his message and then encrypt it using a Vigenère square.  His oppo in the UK of course had an identical edition of "Alice" and would simply reverse the process.  Unbreakable?  Well nearly but not quite...