Explain the the cryptoanalysis of Substitution cipher?
Answers
Answer:
The substitution cipher is more complicated than the Caesar and Affine ciphers. ... In those cases, the number of keys were 25 and 311 respectively. This allowed a brute force solution of trying all possible keys.
Answer:
it is possible to cryptanalyze simple substitution ciphers (both mono- and polyalphabetic) by using a fast algorithm based on a process where an initial key guess is refined through a number of iterations. In each step the plaintext corresponding to the current key is evaluated and the result used as a measure of how close we are in having discovered the correct key. It turns out that only knowledge of the digram distribution of the ciphertext and the expected digram distribution of the plaintext is necessary to solve the cipher. The algorithm needs to compute the distribution matrix only once and subsequent plaintext evaluation is done by manipulating this matrix only, and not by decrypting the ciphertext and reparsing the resulting plaintext in every iteration. The paper explains the algorithm and it shows some of the results obtained with an implementation in Pascal. A generalized version of the algorithm can be used for attacking other simple ciphers as well.