I'm looking to incorporate more crib usage in breaking ciphers in unknown enciphering schemes, or at least to gleam what information I may. This seems to be a big hurdle to me, and I'm looking for references on incorporating cribs into decryption schemes.
Now allow me to clarify. By "crib" I mean a word or a set of words that we highly suspect are in the message somewhere. For example, suppose I suspect that the word "parachute" - along with some other words, perhaps - are enciphered.
Some question why this is a good question. So I will try to give that some weight too. In the literature I have read, there is a big gap between elementary cryptanalysis and modern cryptanalysis. I feel that modern cryptanalysis focuses very heavily on some of the big-power, public-key schemes out there - why? because they're easy to use and very, very hard to break. But in the middle, there were many schemes.
In WWII, the Pacific US forces broke almost every Japanese naval encryption. Although I have learned many different classical encryption and decryption techniques, I am completely unfamiliar with how to approach getting information from a set of messages based solely on the fact that there are a few words that should appear in the message - the same sort of basic idea that went on for over 50 years in military intelligence. I have read that the US relied heavily on cribs, upon occasion even giving certain pieces of information to Japanese diplomats so that their messages would contain known words. I don't mean to alienate or demonize Japan or US-Japan relations - it just happens to be a big example. And I suspect many here have read the Codebreakers, and as this is at the beginning of the book, I suspect many are familiar with this idea.
To that end, I ask - can anyone refer me to any references on the use of cribs to extract information from a possibly unknown scheme? Helen Gaines's book, Cryptanalysis, uses many cribs throughout the book, but there is always the underlying idea that the scheme is known. Of course, historically, this is not the case.
FINAL NOTE: There is an aspect of Paulo Marques's answer that I find deeply troubling. To say that schemes should be built strong, even if the 'enemy' knows the scheme, does not mean that
- Every scheme is actually strong
- No information can ever be gotten from any scheme
- Since schemes are supposedly strong, that we should never attempt to break anything we don't know
This is simply not the case historically at all. In a recent meta post, Paulo Ebermann and I asserted our belief that this SE should not limit itself to modern cryptography. If we are suddenly in the minority, please let me know and I will search elsewhere.