BOX 2 What Is Cryptography?

Historically, cryptography is rooted in the desire to keep information secret from those not authorized to see it. Modern cryptography is based on the operation of mathematical algorithms on digital data. The basic idea is that any message can be represented as a set of numbers (the plaintext). A mathematical algorithm, combined with a user-chosen number called an encrypting "key," is then used to transform the plaintext into a different set of numbers (the ciphertext). By undertaking other mathematical operations on the ciphertext in combination with a decrypting key, the meaningful plaintext can be recovered. The strength of cryptography rests on the fact that for certain choices of algorithm, going from the ciphertext to the plaintext without knowing the decrypting key is very difficult and takes an impractically long time if knowledge of the key is not available.

Today, cryptography has become an enabling tool in other areas. Cryptographic techniques continue to be used to ensure data confidentiality (keeping data private), but they are also applicable for purposes of maintaining data integrity (ensuring that data retrieved or received is identical to data originally stored or sent) and providing subject authentication (verification that the purported sender or author of a message is indeed its real sender or author).

These capabilities for protecting data confidentiality, data integrity, and user authentication are logically separate; that is, a given cryptographic product may implement functionality for any or all of these capabilities. For example, a "groupware" program for remote collaboration may implement cryptography for confidentiality (by encrypting messages sent between users) and cryptography for data integrity and user authentication (by appending a digital signature to all messages sent between users). Further, this program may be implemented in a way that these features can operate independently (either, both, or neither may be operative at the same time).

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Last Updated on 05/29/96

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