Try the Vigenère cipher. It’s easy if you know Babbage’s method (AKA Kasiski examination), but otherwise should be pretty challenging. Another advantage is that it has a lot of factors that could be manipulated to change the difficulty of the task, e.g. the key size or single letters of the plaintext.
Try the Vigenère cipher. It’s easy if you know Babbage’s method (AKA Kasiski examination), but otherwise should be pretty challenging. Another advantage is that it has a lot of factors that could be manipulated to change the difficulty of the task, e.g. the key size or single letters of the plaintext.