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Authentication allows the recipient of information to determine its
origin—that is, to confirm the sender's identity.
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Nonrepudiation prevents the sender of information from claiming at a later
date that the information was never sent.
The sections that follow introduce the concepts of public-key cryptography that
underlie these capabilities.
Encryption and Decryption
Encryption is the process of transforming information so it is unintelligible to
anyone but the intended recipient. Decryption is the process of transforming
encrypted information so that it is intelligible again. A cryptographic algorithm,
also called a cipher, is a mathematical function used for encryption or decryption.
In most cases, two related functions are employed, one for encryption and the
other for decryption.
With most modern cryptography, the ability to keep encrypted information secret
is based not on the cryptographic algorithm, which is widely known, but on a
number called a key that must be used with the algorithm to produce an encrypted
result or to decrypt previously encrypted information. Decryption with the correct
key is simple. Decryption without the correct key is very difficult, and in some
cases impossible for all practical purposes.
The sections that follow introduce the use of keys for encryption and decryption.
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Symmetric-Key Encryption
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Public-Key Encryption
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Key Length and Encryption Strength
Appendix J
Introduction to Public-Key Cryptography
Encryption and Decryption
765
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