This might sound pretty weird, but I was thinking of a game a person could play alone with a pen and paper.
I got this idea where the person would write a random four letter string on a paper, and would have to decode it. There would be a mean to verify if the person is getting closer to the goal or not. So, I decided to ask here.
Do you think it would be possible to create a simple game where one would have to decrypt a simple string (a bit like the BitCoin miners do, I guess?), where there would be a logical next step from every point during the decryption and where it would be possible to verify if the contemplated logical next step brings the player closer to the solution or not.
Similar Question: Toy encryption system that provides "hints"
Interesting, but not-quite-it option: Vigenère cipher (Wikipedia article)