Database Reference
In-Depth Information
K
e
K
d
Plaintext
Recovered
Ciphertext
Public channel
Encryption
Decryption
Figure 4.1
General encryption and decryption of a cipher.
where
K
e
is the encryption key and
E
is the encryption function.
Similarly, the decryption procedure is defined as
()
PD C
(4.2)
K
d
where
K
d
is the decryption key and
D
is the decryption function.
The security of a cipher should only rely on the decryption key,
K
d
,
since an adversary can recover the plaintext from the observed cipher-
text once he gets
K
d
.
Figure 4.1 shows a block diagram for encryp-
tion/decryption of a cipher.
4.3 Classification of Encryption Algorithms
Encryption algorithms are classified in various ways: according to
their structures of the algorithms, according to keys, or according to
their percentage of encrypted data [39,40].
4.3.1 Classification according to Encryption Structure
Encryption algorithms are classified as block ciphers and stream
ciphers.
A
block cipher
is a kind of symmetric-key encryption algorithm that
converts a fixed-length block of plaintext data to a block of ciphertext
data of the same length. The fixed length is called the block size.
For several block ciphers, the block size is 64 or 128 bits. The larger
the block size is, the more secure is the cipher, but the more complex
are the encryption and decryption algorithms and devices. Modern
block ciphers have the following features [41]:
1. Variable key size
2. Mixed arithmetic operations, which can provide nonlinearity
3. Data-dependent rotations and key-dependent rotations
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