Cryptanalysis

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.

 

Method

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 ‚Äď The individual counts regardless of letter
Type ‚Äď The letter that the count is attributed to.
Order ‚Äď 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 ‚ÄėEnglish‚Äô a piece of text is.