If you have broken the enemies crypto AND he doesn't know you have - then you can also send fake messages which they are going to believe.
So the Brits could have sent a message ordering a few panzer armies from Normandy to the Pas de Calais. It doesn't seem that this was ever done - presumably because it gave the game away.
Even more interestingly - the Brits kept secret that they had cracked enigma into the 1980s because after the war they distributed the 'uncrackable' German code machine to lots of rather gullible allies
It's an interesting bit of military psychology that while every side in WWII broke their enemy's codes - all of them believed all their own codes to be unbreakable!
It wasn't a nonce as such - it was a key exchange, slightly more like a salt in modern terms.
One of the main failings of the Germans was their love of formal introductions and sign offs in official messages, especially by the navy which otherwise had rather better crypto than the other branches. However if you are already stuck with a language that has a very fixed word and sentence structure you don't help by signing all your messages with the same 40 character long formal greeting.
Supposedly Buran's launch pad is also due to this.
Because it's pad was built on top of an earlier Apollo pad the shuttle launches in the wrong orientation and has to make a roll just after takeoff to head into orbit.
The Russians copied this non-optimal arrangement for Buran believing that there must be some advantage that they didn't know about but Nasa had discovered,
Amateurs! One modern leader of an impoverished 3rd world country managed to fake an entire nuclear, biological, chemical warfare capability and the ICBMs necessary to attack Britain within 45mins.
And he did it so successfully we were forced to invade.
All crypto systems fail when faced with human operators!
There was one story of an Afrika corp operator who sent HIT - LER as the 2 'secret' triplets every day for the entire North Africa campaign. Then disastrously he was captured!
It's an interesting question as to whether at least the best (4-wheel naval) system could be cracked without operator error or deliberate plain text seeding.
The only flaw of the enigma is that a letter cannot be encrypted to itself - I have heard different accounts of if this was enough to always make it crackable.
So the Brits could have sent a message ordering a few panzer armies from Normandy to the Pas de Calais. It doesn't seem that this was ever done - presumably because it gave the game away.
Even more interestingly - the Brits kept secret that they had cracked enigma into the 1980s because after the war they distributed the 'uncrackable' German code machine to lots of rather gullible allies