Except they weren’t really deciphering messages one letter at a time. If you’ve got the right key, you get the whole message practically all at once. If you don’t have the right key, then you’ve got gobbledygook.
It’s an enjoyable movie, but it goes so far beyond the usual composite characters/simplified details/collapsed timeframe that the disclaimer should say “sort of tangentially related to a true story.”
The way I understand it, the machines actually are doing it one letter at a time until they reach a contradiction. My understanding comes from the videos linked in this thread along with the BBC version of the play that The Immitation Game is based on.
In a strict sense, it’s one letter at a time, but you go from one letter to the next so quickly that you’d never have a chance to autocomplete “hit” into “hitler”.
As far as I know the code talkers were used for tactical communication not strategic. Unless you had people who could both intercept communications many of which were done by wire and decrypt on the fly it might as well have been an unbreakable code. It doesn’t matter if someone somewhere can decrypt the message if the message is “take Hill 45, time now.”
Apparently, the fact that the code talkers used a code based on their native language, rather than their native language alone, was useful: the Japanese actually captured a Navajo speaker at Bataan, and knew enough to torture him to make him attempt to decipher the code - but he couldn’t, even if he had wanted to.
The guy certainly had it rough!
Tortured because the Japanese thought he was Japanese - then, after convincing the Japanese he was Navajo, tortured again to make him decode a code based, improbably enough, on his language - which he couldn’t; then, just to round things out - shipped off to Nagasaki to be nuked!
He survived it all, though, to give the above interview at the age of 72.
So someone back in Berlin or Tokyo spends an afternoon cracking the code, and then they give their field officers a sheet of paper with the cracked code on it, letting them decode any future transmissions nearly as quickly as the code talkers could.
From the accounts, it seems it wasn’t a letter-for-letter substitution - more the use of invented words for modern military terms and suchlike.
From the above link:
This made it generally uncrackable - even though the Japanese had a native speaker to torture. Though admittedly, had that speaker been working with a modern cryptographer, it would be no more difficult to crack than any other ‘book code’.