The protection of a message by either of two encryption methods,
code or cipher, that transform plain text to cipher text. The first
of these involves a one for one substitution of a cipher text code
group for a plain text code group. The list of these substitutions
is called a codebook; the codebook must be kept secret in order to
protect the information.
A cipher process involves changing the plain text into cipher
text by means of a cryptographic transformation, usually in such a
way that each bit. character, or word of plain text is replaced by a
bit, character, or word of cipher text. The cryptographic
transformation is accomplished by a specific algorithm or device,
which is normally in two parts. The first part consists of a known
description of the algorithm or device; the second part is a key, a
set of data that is used toparticularize the algorithm or device for
a specific usage.
The transformation from cipher text to plain text is called