I have been studying ways to amend a simple substitution cipher, and one of the toy suggestions was to use CBC in the following way:
- identify each letter with a number from $0\ldots 25$
- start with a random IV, i.e. just a random letter
- add IV to the first letter in the plaintext, modulo 26, and then encrypt according to the old substitution rules
(then continue with CBC in this way, using modulo 26 arithmetic instead of XOR)
The resource then goes on to say that while this clearly can't be cracked by simple frequency analysis on letters, the ciphertext can be split into 26 sets, that is, the letters that follow each ciphertext letter, and then frequency counting can be done. Later on, the author reiterates that by partitioning the message into 26 groups that follow each ciphertext letter, frequency analysis can be done.
I've been trying to figure out what they meant by this. My main problem is trying to interpret the "26 groups that follow each ciphertext letter" part, and I didn't get anywhere with it. (Perhaps they meant grouping sets of 26 adjacent ciphertext letters together, and then modular arithmetic would ensure some repetition somehow? I don't really see how that would work, but I also don't see any other interpretations)
Would greatly appreciate any suggestions!