frequency analysis cipher decryption tool

Frequency analysis of the first character of 4-char blocks in ciphertext I J 0.06966 C D 0.02782 P Q 0.01929 Z a 0.00074Īnd with a kilobytes of ciphertext, it is likely that the most frequent character in the fraction of the ciphertext kept corresponds to E in the plaintext, thus F in the Base64. The mapping, ordered by decreasing expected letter frequency in English plaintext (according to this source) is: Plaintext Plaintext Plaintext Plaintext If among the characters kept the last one is different from all the others, it likely represents = in Base64 all the other characters kept should be among 26 characters (else the assumption made above is disproved). It follows that we can perform single-letter frequency analysis on the ciphertext restricted to the last character of every block of four characters. We see there is a straight mapping from the last of every three characters in the plaintext to the last of every four characters in the Base64 encoding (notice that the characters in Base64 codes the 5 lower bits of the character in the plaintext, on the right of the above picture, which are characteristic of the letter). Frequency analysis of the last character of 4-char blocks in ciphertext It is useful to visualize how 3 consecutive bytes of plaintext map to 4 consecutive Base64 characters.ġ. This implies the high 3 bits of each byte of plaintext are 010. I'll assume that the plaintext consists entirely of capital ASCII letters as in the example. My question mostly pertains to longer messages, as my example may be too short for any sort of real analysis. Since the bytes are basically split up resulting in different character units that may or may not correspond, is frequency analysis still feasible? Does encoding plaintext this way before encryption add any form of security? Let's also assume that the character set in subsequent messages is in the same order(ie the key is re-used).

YhAc5bTOWeYvBXCpVuoMJLdg8P1jHN3ZIsi67SrK4E_D9-x2qRQmnlkwUaFt0G=zfĪbcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789-_= When the character set is shuffled, the final cryptext can look something like this: What if I wanted to do frequency analysis?įor example, 'RUNFORYOURLIFE' when encoded in Base64 is: The shuffled alphabet is, in essence, the key. The characters in the encoded plaintext are substituted with a randomly shuffled character set(A-z, 0-9, -, _, =).Let's imagine a cipher that works like the following:

Frequency analysis cipher decryption tool