ABSTRACT
We begin our discussion by defining some terms that will
be used throughout the entry. The first term is encryption.
In simplest terms, encryption is the process of making
information unreadable by unauthorized persons. The pro-
cess may be manual, mechanical, or electronic, and the
core of this entry is to describe the many ways that the
encryption process takes place. Encryption is to be distin-
guished from message-hiding. Invisible inks, microdots,
and the like are the stuff of spy novels and are used in the
trade; however, we will not spend any time discussing
these techniques for hiding information. Fig. 1 shows a
conceptual version of an encryption system. It consists of a
sender and a receiver, a message (called the “plain text”),
the encrypted message (called the “cipher text”), and an
item called a “key.” The encryption process, which trans-
forms the plain text into the cipher text, may be thought of
as a “black box.” It takes inputs (the plain text and key) and
produces output (the cipher text). The messages may be
handwritten characters, electromechanical representations
as in a Teletype, strings of 1s and 0s as in a computer or
computer network, or even analog speech. The black box
will be provided with whatever input/output devices it
needs to operate; the insides, or cryptographic algorithm
will, generally, operate independently of the external
representation of the information.
