Cryptography Reference
In-Depth Information
2.2.1
Symmetric Encryption Systems
When one talks about cryptography, one is often referring to confidentiality protec-
tion using symmetric encryption systems (to encrypt and decrypt data).
Encryption
is the process that turns a
plaintext message
into a
ciphertext
,and
decryption
is the
reverse process (i.e., the process that turns a ciphertext into a plaintext message).
As suggested in Definition 2.6, a
symmetric encryption system
consists of a set
of possible plaintext messages (i.e., the plaintext message space), a set of possible
ciphertexts (i.e., the ciphertext space), a set of possible keys (i.e., the key space), as
well as two families of encryption and decryption functions (or algorithms) that are
inverse to each other.
!
Figure 2.4
The working principle of a symmetric encryption system.
Definition 2.6 (Symmetric encryption system)
A
symmetric encryption system
or
cipher
consists of the following five components:
;
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•
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
.
, the functions
D
k
and
E
k
must be inverse to each other (i.e.,
D
k
(
E
k
(
m
)) =
E
k
(
D
k
(
m
)) =
m
).
For every key
k
∈K
and every message
m
∈M
In some literature, the plaintext message space is denoted by
P
. In this topic, however, we
conventionally use the letter “P” to refer to a probability distribution.
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