I'm looking for a toy encryption system that has a specific property that when decrypting it with a key that is close enough to the key used in the encryption, it will produce some of the original plain text.
The idea is to use it as a game/teaching system where kids can try to break a cyphertext but instead of using cryptanalisys on the cyphertext, I want them to try different keys and then the decrypted text would provide feedback as of how close the key really is, for example:
attack at dawn with key 1234
and let's say the cyphertext is
now, if I try to decrypt the cyphertext with key, for example, 1288, it should display some of the plaintext, in order to hint the breaker that the key is somewhat close to the real one
think of it as an encryption system a la Mastermind
A simple substitution cipher kind of does the trick, but I'm wondering if there are other ciphers that behave that way (preferrable with smaller key space)