First of all, substitution does not change the letters' frequencies, so if you have a decent amount of enciphered text and you know the language it was written in, you can try frequency analysis. However, the simple substitution cipher is considered a weak cipher because it is vulnerable to cryptoanalysis. Because of this, if you want to decipher the text without knowing the key, the brute force approach is out of the question. Thus, for the English alphabet, the number of keys is 26! (factorial of 26), which is about. 1įor a simple substitution cipher, the set of all possible keys is the set of all possible permutations. It is a cipher key, and it is also called a substitution alphabet. Substitution of single letters separately - simple substitution - can be demonstrated by writing out the alphabet in some order to represent the substitution. The receiver deciphers the text by performing the inverse substitution. In cryptography, a substitution cipher is a method of encrypting by which units of plaintext are replaced with ciphertext, according to a fixed system the "units" may be single letters (the most common), pairs of letters, triplets of letters, mixtures of the above, and so forth.
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