Classical Cipher - Cryptanalysis of Classical Ciphers

Cryptanalysis of Classical Ciphers

Classical ciphers are commonly quite easy to break. Many of the classical ciphers can be broken even if the attacker only knows sufficient ciphertext and hence they are susceptible to a ciphertext-only attack. Some classical ciphers (e.g. the Caesar cipher) have a small key space. These ciphers can be broken with a brute force attack, that is by simply trying out all keys. Substitution ciphers can have a large key space, but are often susceptible to a frequency analysis, because for example frequent letters in the plaintext language correspond to frequent letters in the ciphertexts. Polyalphabetic ciphers such as the Vigenère cipher prevent a simple frequency analysis by using multiple substitutions. However, more advanced techniques such as the Kasiski examination can still be used to break these ciphers.

On the other hand, modern ciphers are designed to withstand much stronger attacks than ciphertext-only attacks. A good modern cipher must be secure against a wide range of potential attacks including known-plaintext attacks and chosen-plaintext attacks as well as chosen-ciphertext attacks. For these ciphers an attacker should not be able to find the key even if he knows any amount of plaintext and corresponding ciphertext and even if he could select plaintext or ciphertext himself. Classical ciphers do not satisfy these much stronger criteria and hence are no longer of interest for serious applications.