Cryptography I by Dan Boneh. https://www.coursera.org/learn/crypto (note even though it says crypto it is NOT related to blockchain crypto).
It is related, but only in the sense that crypto currencies are built on cryptography primitives.
[0] Since 2021 this phrase has been expanded to "roll your own crypto scam" and it's only used as an ethical statement. Don't "roll your own crypto scam" because it's wrong. (Okay, maybe this part is just a joke and people don't really say that. It's hard to say for sure.)