TL;DR After enigma was cracked, the British created their own unbreakable code called Typex. This code fixed the fatal flaw of enigma; a letter could never be itself.

The way the enigma machine worked was through a series of rotors. In the factory which made enigma machines, the person building it was told to pick three rotors out of a box of five (giving 60 possibilities as to what the code may be). Each rotor has 26 starting places on it (26x26x26=17576. (with the 3 out of 5 rotors, that makes 1054560 possibilities)). Military machines had a plugboard, with 10 wires to connect 10 pairs of letters (26!/6!/10!/2^10=150738274937250 (with the previous two factors, that equates to 158962555217826360000)).

With that number of possibilities, the code seemed unbreakable. But the way it was broken was that, each letter contained a weather report (Wetterbericht) and/or a salute to their leader (Heil Hitler). Because of the way the enigma machine was built, and the way it was wired, meant that the W in Wetterbericht could never be a W (the same thing applies to the e, t, etc.). After they cracked it, the British took the code for enigma and made sure that the W in Wetterbericht could sometimes be a W. This was called Typex.