An appraisal is given of the current status, both technical and nontechnical, of cryptologic research. The principal concepts of both secret-key and public-key cryptography are described. C.E. Shanon's theory of secrecy (1949) and G.J. Simon's theory authenticity (1984) are reviewed for the insight that they give into practical cryptographic systems. Public-key concepts are illustrated through consideration of the Diffie-Hellman public-key-distribution system and the Rivest-Shamir-Adleman public-key cryptosystem. The subtleties of cryptographic protocols are shown through consideration of some such specific protocols. >