Foundations of Cryptography  
   Oded Goldreich. A public domain book available on-line. Only partially completed.
   http://theory.lcs.mit.edu/~oded/frag.html

Paul Garrett: Crypto and Number Theory  
   Lecture notes for a course on cryptography and number theory.
   http://www.math.umn.edu/~garrett/crypto/

Chaffing and Winnowing  
   An interesting paper by Ron Rivest about providing confidentiality without encryption.
   http://theory.lcs.mit.edu/~rivest/chaffing.txt

Index of Cryptography Papers Available Online  
   A research tool for the cryptography community maintained by Counterpane Systems. Over 400 papers online.
   http://www.counterpane.com/biblio/

Boolean Function Nonlinearity  
   A detailed discussion of cryptographic nonlinearity, what it means and how it is computed, with active JavaScript panels to perform the computation.
   http://www.ciphersbyritter.com/JAVASCRP/NONLMEAS.HTM

Bibliography on Secret Sharing Schemes  
   List maintained by Douglas Stinson and Ruizhong Wei.
   http://www.cacr.math.uwaterloo.ca/~dstinson/ssbib.html