Cryptanalysis “is the study of ciphers, ciphertext, or cryptosystems with a view to finding weaknesses in them that will permit retrieval of the plaintext from the ciphertext, without necessarily knowing the key or the algorithm”.

There are a couple of methods of quantifying how close to English text is.



Depends on property
Frequency Type Order
Index of Coincidence True False False
Chi-Squared Statistic True True False
N-Grams Probability True True True

Frequency – The individual counts regardless of letter
Type – The letter that the count is attributed to.
Order – The letters that proceed and follow this letter.

The best method is normal using N-Grams Probability as it takes into account all three properties, however this is not always possible when quantifying how ‘English’ a piece of text is.