I'm a pure mathematics student(currently in a Msc program) looking to get into cryptography. I imagine most "regular" introductory books on cryptography would avoid being too heavy on mathematical details due to the target audience. So I'd like to ask, if someone is perhaps able to recommend good books that provide an introduction to cryptography for someone rather unfamiliar with it, but having all the neccessary mathematical background to understand full details and justifications of the algorithms etc.
Edit: I'm aware that I can't possibly know every theorem etc. involved, but I think it's safe to assume that I should be able to lookup any such additional details in reasonable time, while going through the book.