L0CK

* Index of Coincidence (IC) was introduced in 1920s by William Friedman
* measures variation of frequencies of letters in ciphertext
o period = 1 => simple subs => variation is high, IC high
o period > 1 => poly subs => variation is reduced, IC low
o first define a measure of roughness (MR) giving variation of frequencies of individual characters relative to a uniform distribution