Re: WWII Code, and Carrier PIgeons,
If it is a cipher from a random one use key pad it will not be cracked. Im not sure what kind of codes they used, I doubt it was enigma because the pigeons were carrying messages to locations behind enemy lines and you wouldn't want to have an enigma cipher machine anywhere close to enemy lines. So my guess is that they would pre-coordinate one use pads. A randomly generated grid of letters, numbers and symbols, with a substitution cypher, destroyed after decoding and never used again. Unless you have the pad, you will never decode the message. With a randomly generated pad there is no way to crack the code because normal methods like linking repeating symbols/letters with the most commonly used letters doesn't work if the pad was random. The same letter could be encoded with several different characters from the pad. It is unbreakable if the pad is really random.
That does not mean that it cannot be compromised, there is no free lunch. The risk in this method is that the pads themselves could be compromised or recovered, like the enigma machine was.
If it was any other kind of cipher then the NSA will likely crack it if they wanted to. One other though that would probably be tough even for the NSA would be a book cipher...where each side has the same book or series of books, and you use a series of numbers to select a letter from the book. For instance, each side has the same second print edition of Charles Dicken's David Copperfield. The code is 034041065...which means page 34, 4th paragraph, 10th line of the 4th paragraph, 65th letter of that line. Different ways to do it, but that is the jist of it. There are lots of books, and the code depends on how the book is printed...in this case even knowing the code was based on David Copperfield would not be enough, you would need the same print edition.
Codes are interesting, and the WWII code work done by the mathematicians at Bletchley Park is nothing short of amazing.
