There's a fun (toy) crypto challenge I do every year for undergrad students, you can see the past problems here: http://www.cwu.edu/math/previous-challenges htt

There's a fun (toy) crypto challenge I do every year for undergrad students, you can see the past problems here: http://www.cwu.edu/math/previous-challenges http://www.cwu.edu/math/previous-challenges (I wasn't a big fan of the 2016 problems)