Cryptography Reference
In-Depth Information
Chapter 10
Symmetric Encryption Systems
In this chapter, we elaborate on symmetric encryption systems. More specifically, we
introduce the topic in Section 10.1, address block ciphers, stream ciphers, and per-
fectly (i.e., information-theoretically) secure encryption systems in Sections 10.2-
10.4, and conclude with some final remarks in Section 10.5. Note that symmetric
encryption systems are the most widely deployed cryptographic systems in use to-
day and that many topics on cryptography elaborate only on these systems. Conse-
quently, this chapter is important and rather long.
10.1
INTRODUCTION
According to Definition 2.6, a
symmetric encryption system
(or
cipher
) consists of
the following five components:
•
A plaintext message space
M
;
•
A ciphertext space
C
;
•
A key space
K
;
•
A family
E
=
{
E
k
:
k
∈K}
of encryption functions
E
k
:
M−→C
;
•
A family
D
=
{
D
k
:
K
∈K}
of decryption functions
D
k
:
C−→M
.
In every practically relevant symmetric encryption system
D
k
(
E
k
(
m
)) =
m
