Cryptography Reference
In-Depth Information
return) in the form of seven-bit binary numbers—i.e.,
as a string of seven 1s and 0s. In ASCII a lowercase a is
always 1100001, an uppercase
A
always 1000001, and so
on. Acronyms are also widely known and used codes, as,
for example, FAQ (for “frequently asked question”) and
COD (meaning “cash on delivery”). Occasionally such a
code word achieves an independent existence (and mean-
ing) while the original equivalent phrase is forgotten or
at least no longer has the precise meaning attributed to
the code word—e.g., modem (originally standing for
“modulator-demodulator”).
Ciphers, as in the case of codes, also replace a piece
of information (an element of the plaintext that may con-
sist of a letter, word, or string of symbols) with another
object. The difference is that the replacement is made
according to a rule defined by a secret key known only to
the transmitter and legitimate receiver in the expectation
that an outsider, ignorant of the key, will not be able to
invert the replacement to decrypt the cipher. In the past,
the blurring of the distinction between codes and ciphers
was relatively unimportant. In contemporary communi-
cations, however, information is frequently both encoded
and encrypted so that it is important to understand the
difference. A satellite communications link, for exam-
ple, may encode information in ASCII characters if it is
textual, or pulse-code modulate and digitize it in binary-
coded decimal (BCD) form if it is an analog signal such as
speech. The resulting coded data is then encrypted into
ciphers by using the Data Encryption Standard or the
Advanced Encryption Standard (DES or AES; described
in the chapter "Modern Electronic Cryptography").
Finally, the resulting cipher stream itself is encoded again,
using error-correcting codes for transmission from the
ground station to the orbiting satellite and thence back
