Attacks on the simple substitution cipher with the calculation of their reliability

Abstract

The historical simple substitution cipher is widely known. In educational literature, the study of ciphers and attacks on them begins with it. Calculation of the complexity of attacks on a simple substitution cipher is usually ignored due to the simplicity of the implementation of these attacks. The question why there are no results on the assessment of the reliability of such attacks (the probability of achieving the goal of the task) remains open. From the practice of carrying out attacks on this cipher, it is believed that knowledge of 25–27 ciphertext characters is sufficient for the English and Russian languages. Claude Shannon proved this fact using the entropy per message letter he calculated. This article proposes a modernization of the ideas embodied in well-known attacks, which makes it possible to calculate the probability of correctly determining the plaintext of a simple substitution cipher using a known ciphertext.