[Rivest78] R. L. Rivest, A. Shamir, and L. Adleman, A Method for Obtaining Digital Signatures and Public-Key Cryptosystems, Comm. of the ACM (February 1978) 21, no. 2:120--126. [This famous paper introduced the best known and only unbroken class of public key cryptosystems. It forms the basis for almost all current public key systems.]