There is no theory explained here. You are assumed to have a sufficient background knowledge of cryptography in order to use the tools and algorithms provided in this API. We may give some guidance or make recommendations in this manual and in the example code. These are made in good faith but may be out of date or incorrect. Use any guidance or recommendations at your own risk and make whatever checks you deem necessary before relying on them.
For a good introduction to the principles of cryptography refer to Bruce Schneier's Applied Cryptography [SCHN96] or William Stallings Cryptography and Network Security [STAL05]. For a more advanced treatment, see Handbook of Applied Cryptography by Menezes, van Oorschot and Vanstone [MENE97].