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I have a question. I read that the Κ test of Riedman was something like this:
Friedman’s Κ test attempts to figure out the period of the cipher by calculating several values of Κ(C,C n)n, where C n represents the cipher text with n shifts to the left, thus determining the most likely value of the cipher.
But deep down, what does Κ do? superimpose the two texts and see indications of coincidence? Can someone explain this method better?
Do you have any reference to the material you consulted, where you got this information from? I don’t understand exactly what you’re asking. In fact, statistical tests like this are used in Criptanalysis, but for me at least it doesn’t make sense to "try to figure out the period of a cipher" using real data - at least when applied in modern cryptography (in the old one, maybe it makes more sense, I don’t know, I’m not an expert). Crossbow example: if you encrypt random data by any algorithm however bad, the end result will be random, and it will not be periodic.
– mgibsonbr
By date (1920), it most likely does not apply to what we call cryptography today :) https://en.wikipedia.org/wiki/Vigenère_cipher#Friedman_test - When it comes to "cipher", it’s like the "Caesar cipher", for example.
– Bacco
a Power cipher for example, in this wikipedia link I don’t quite understand if it’s really what I say in my post, is it? It really works like this ? see two text match index and calculate various values? but how do you know the period of the cipher? @Bacco
– PRVS
That is, yes, if someone can really summarize me in.wikipedia.org/wiki/Vigenère_cipher#Friedman_test this in a simpler way, because there really is something that is failing me. @mgibsonbr
– PRVS
Ah, I understand, in the question you didn’t mention that it was the Vigenère cipher. This is a figure that I consider "historical" - because nowadays it offers no security - and in the case of it it makes sense to speak of "period". Because the key is repeated N times and combined directly with the plaintext. And in fact you can recover the key using statistical methods (I have studied this, but not with Friedman’s test, using a simpler method). I’m gonna read that stuff and see if I can give an answer.
– mgibsonbr