Frequency analysis can be very useful in cryptanalysts when trying to break encryption.
By utilizing the class of substitution ciphers there is a fairly high possibility that an attacker can detect a pattern amongst the characters that have been substituted. Thus when the attacker is presented with a large enough sample of the cipher-text we find that the attacker can decrypt the content and render it into plaintext.
As we are aware in the English alphabet the letter E is the most frequent letter occurring at 12.7%, followed by the letters T at 9.1%, and A at 8.2%, with the letters O and I occurring in descending rank, whereas the least frequent letters are Q, Z and X.
Frequency patterns also exist for the consonants “th” and vowels “ea”, as well as “of”, “to”, “in”… Frequency analysis also shows us patterns of repeated letters such as “ss”, “ee”, “oo”...