OK, for some reason, the CRC press let the University of Waterloo give away all the chapters in the Handbook of Applied Cryptography for free. You can still buy the book on Amazon for $85 though, so if you like the book and find it useful you should go pay for it so the author can get some cash.

This is a great resource for anyone curious about cryptography, especially those that intend on getting a career in information security. Subject covered in the book include cryptographic mathematics, public key infrastructure, hash functions and data integrity, digital signatures and much more. Really interesting (and informative) stuff, I suggest you go check it out.