Crypto can extend lowercase alphabet characterset to ASCII plaintext space all strings of ASCII chars key space ASCII chars map plaintext chars to ciphertext chars using XOR c m k where is XOR c k m k k m k k m 0 m size of key space is only 256 easily brute forcible or find the most frequent character and XOR with e to deduce the key Substitution cipher key space permutations over 1 25 Enc pi m 1 m l output c 1 c l where c i pi m i Dec pi c 1 c l output m 1 m l where m i pi c i key space 26 2 88 let p i freq of ith char in English text 0 i 26 c i freq of ith char of cipertext If k is the key c i k 26 p i sum i 0 25 p i c i k 26 sum i 0 25 p i 2 0 065 find j such that that sum is closest to 0 065 Vigenere cipher key of up to t max characters match up the key repeated with the message and shift up the message chars with the corresponding key chars e g if key is abc helloworld abcabcabca hfnlpyosnd defeats frequency analysis keyspace 26 tmax 2 4 5 tmax 2 60