title: How to know the length of a key and the key itself in the context of Friedman test and Vigenere cipher
body: <p>The title says it all... I can't seem to get the idea... I know it uses the index of coincidence but I'm not sure with whom or what am I supposed to compare it with... How is the formula used?... It would help me a lot if it were exposed in an algorithmic representation...</p>