Frequency analysis

The frequency analysis is a method of cryptanalysis. This statistical characteristics of the encrypted text to be exploited in order to draw conclusions on the unencrypted message. The frequency analysis is used since the 7th century to decipher texts that have been encrypted using monoalphabetic encryption without transposition.

The frequency analysis was discovered by an Arab scholar, until much later, this knowledge came to Europe.

Technology

The frequency analysis is used to decrypt ciphertexts without known plaintext. The individual letters are counted here and noted their frequency, usually in percent, relative to the total number of characters (letters frequency ).

Now, due to the specific frequency of specific letters in a language that the E, for example, comes in the German language with about 17 %, by far the most frequently before, closed on his alphabet. Comes in a message say for example the otherwise quite rare letter Q with about 17 % before, so this suggests that Q is in this encryption for e. When more than one assignment exist, the same procedure can additionally also on bigrams, ie letter pairs, apply.

Since the accuracy of the frequency increases with the length of a message, a long message is much easier to decipher than a short. This is true for most cryptanalytic methods.

Frequency analysis and frequency analysis also refers to a method of quantitative content analysis in empirical social research.