It's a secret (21)

So ironically we have now arrived at a situation where the ultimate cryptography method is a method which is eminently crackable, it's just that the time it would take to do that makes it unfeasible to try. And this raises an interesting point, which is that secrets have a shelf-life - a sell-by or use-by date - in other words there comes a point when a secret isn't a secret any more.  Take D-Day - perhaps the most top top-secret of the war.  And yet, by early morning on the 6th June 1944 it wasn't a secret any more - the troops were landing on the beaches.  So this is something to bear in mind when choosing an encryption method - you only need one which will hold the "enemy" up for long enough that the secret you are seeking to conceal ceases to be a secret.  Don't use your best method unless its use is justified.  One of the reasons that the geniuses of Bletchley Park managed to defeat the German Enigma machine was that, rather than using Enigma just for the most important messages, the Germans used it for everything - including mundane non-secret things like daily weather reports, which gave the decrypters a way in.  Anyway - is PPK the end of the story?  Not quite...