Frequency-Analysis Attack

Tags

cryptography

A form of [see page 11, attack] on substitution ciphers which tries to exploit the frequency of certain characters in the ciphertext to reveal information about the plaintext. For example the most common character in English words is e so if the character that occurs the most in the ciphertext is an z then it's highly likely that an input of z can be decrypted to a plaintext e.

More complex applications of frequency-analysis use the most-common Bigrams (two letter sequences) or (Trigrams) three letter sequences to help you guess the correspondences between the plaintext to ciphertext mappings.

Frequency analysis isn't as effective against poly-alphabetic ciphers but because the frequencies still occur for every $n$th character (when we change the underlying cipher used) we can still use frequency-analysis to attack the substitution cipher if we can determine when the underlying cipher we use changes.