a "perfect" encryption system that could be made impenetrable, even in principle, to an adversary with unlimited resources. Based on this work, secret-key cryptography (defined below) blossomed, with the most public work in this area being the Data Encryption Standard promulgated in 1975 by the National Bureau of Standards (now the National Institute of Standards and Technology). The second major revolution occurred in 1976 with the first discussion in the open literature of asymmetric cryptography, inspired by a landmark paper of Whitfield Diffie and Martin Hellman.1
Cryptography can help to ensure the integrity of data (i.e., that data retrieved or received are identical to data originally stored or sent), to authenticate specific parties (i.e., that the purported sender or author of a message is indeed its real sender or author), to facilitate nonrepudiation, and to preserve the confidentiality of information that may have come improperly into the possession of unauthorized parties.
To understand how cryptographic methods span a range of communication and storage needs, consider the general problem of sending a private message from Party A to Party B. Centuries ago, such a process was accomplished by Party A writing a letter containing his or her signature (authentication). The letter was sealed inside a container to prevent accidental disclosure (confidential transmission). If Party B received the container with a broken seal, it meant that the letter had been disclosed or altered and Party B would take appropriate actions (data integrity). Otherwise, Party B would verify Party A's signature and read the message. In the information era, each of the steps remains essentially the same, except that automated tools perform most of the work and are explained below.
Digital information is transmitted (or stored) so that it can be received (or retrieved). For two reasons, it is possible that the information received or retrieved might differ from the original information transmitted or stored:
1. A technical problem may inadvertently alter one or more of the bits of information in question. No digital transmission-receiving or stor-
1 Whitfield Diffie and Martin Hellman, "New Directions in Cryptography," IEEE Transactions on Information Theory, Volume IT-22, IEEE Press, New York, 1976, pp. 644-654.