Popular demonstration of cryptographic tools I'm looking for an activity for highschoolers/college-freshmen that will demonstrate crypto topics (e.g., encryption, signatures, zero-knowledge), and will be fun and motivating. It is supposed consist of small tasks that the audience can do by themselves (=no complicated math).

I vaguely remember seeing a website with suggestions for such activities, but I cannot find any information now. Any suggestions / ideas?
There are some engaging lessons on encryption in Unit 4 of the AP Computer Science Principles curriculum from Code.org.  Lessons 5, 6 &amp; 7 take students from the concept of encryption using a caesar cypher, to a vigenere cypher, and then through the concepts of public key encryption.  Students usually have fun with the widgets that interactively demonstrate the caesar cypher (https://studio.code.org/s/csp4/stage/5/puzzle/2 ) and vigenere cypher (https://studio.code.org/s/csp4/stage/6/puzzle/2 ).  There are lesson plans with student worksheets and videos that support and extend the ideas the students are exposed to when they play with the widgets (https://curriculum.code.org/csp/unit4/ ). The materials are free to access.