Dan Boneh's "Cryptography I". https://www.coursera.org/learn/crypto Was one of the best basic lecture I have seen on the crypto topic. I have enjoyed several lectures in my studies but some things I just understood after the good and interesting explanations in this course. Really amazing. Still waiting for the the Crypto 2 course.

If you understand german I recommend you math lessons by Christian Spannagel https://www.youtube.com/@pharithmetik. Makes some less interesting topics enjoyable.