If you really wanna learn crypto from square one, i highly suggest the coursera Cryptography I and II courses from Stanford (https://www.coursera.org/course/crypto
). This is a serious college level course on cryptography that can get a bit tough at times, but it teaches all the basics of crypto. It's entirely free and only really requires an email to sign up, but it can eat up quite a bit of time. I'd suggest first passing this course entirely, then proceeding on to other books/case studies/etc to expand your knowledge.