1. What is the flaw in the Enigma Machine?
In an Enigma coded message, a letter cannot appear as itself, nor the same corresponding letter twice (unless it appears 26+ times). There where also certain words or phrases (eg. Heil Hitler) that would appear often in a message. By recognising these phrases, codebreakers could eliminate/investigate the possibilities of certain letters.
2. What did the Allies do to crack it?
They created a device – a ‘Bombe’ – that would crunch through every Enigma possibility until it found a setting in which no letter would appear the same twice. When a letter was entered, it as aligned with 25 letters – all except for itself. Alan Turing was the mastermind behind this operation, and many German messages were decrypted using his ‘Bombe’.