A basic Caesar cypher consists of shifting the letters in a message by a fixed offset. For an offset of 1, for example, a will become b, b will become c, and z will become a. If the offset is 13, we have the ROT13 algorithm.
Lists and binaries can both be stringlike. Write a Caesar protocol that applies to both. It would include two functions: encrypt(string, shift) and rot13(string).
Using a list of words in your language, write a program to look for words where the result of calling rot13(word) is also a word in the list. (For various English word lists, look at http://wordlist.sourceforge.net/. The SCOWL collection looks promising, as it already has words divided by size.)