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Cryptography and shared secret keys can be used to secure electronic files
and communications. Cryptographic assurances are categorized as follows.
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Authentication is assurance of the identity of the person at the
other end of the line. Because Bob can't send the shared secret, Alice
challenges Bob to correctly encrypt a previously unused random number
with their shared secret key. Only the shared secret key will correctly
encrypt the random number.
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Integrity, or message authentication, is assurance that a
file has not been changed during transit. A message and a shared secret
key make a unique message authentication code (MAC), or message fingerprint.
Only someone with a copy of the shared secret key can correctly reproduce
the fingerprint.
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Nonrepudiation is assurance that the sender cannot deny that a file
was sent. This cannot be done using a secret key alone; it requires
a mutually trusted third party or public key technology.
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