Cloud Computing Standards and Security - Deploying and Managing a Cloud Infrastructure - page 337

Ciphers

A cipher is a cryptographic algorithm used to perform encryption or decryption through a
series of mathematical steps. When data is converted from plain text to ciphertext or code,
the procedure is known as enciphering or encoding . Similarly, when cipher or code is con-
verted to plain text, the procedure is called deciphering or decoding.
Ciphers can be categorized in a couple of ways:
Block or Stream Ciphers Ciphers that work on blocks of data or on a continuous stream
of data.
Symmetric or Asymmetric Key Algorithms Ciphers that use a key for enciphering and
deciphering. Cipher algorithms that use the same key for both encryption and decryption
are called symmetric key algorithms. Similarly, algorithms that use different keys for both
procedures are known as asymmetric key algorithms.
Symmetric key algorithms require that both sender and recipient share the key. This
could be a security hazard because the key has to be shared. Asymmetric key algorithms
prevent loss of confidentiality due to the public/private key property of such ciphers.
Table 11.1 shows some ciphers with their respective properties and aspects.

TABLE 11.1 Ciphers and their properties and aspects

Cipher Category Key size (bits) Block size (bits)

Advanced Encryption Stan-
dard (AES), 2001

Block cipher, symmetric
key algorithm

128, 192, 256 128

Data Encryption Standard
(DES), 1970s

Block cipher, symmetric
key algorithm

56 (+8 parity) 64

Triple Data Encryption Algo-
rithm (3DES/TDEA), 1998

Block cipher, symmetric
key algorithm

16 8, 112 64

Ron Rivest, Adi Shamir, and
Leonard Adleman (RSA),
1977

Asymmetric key algorithm 1,024 to 4,096

RC4 Stream cipher 40 to 2,048

RC5 Block cipher, symmetric

key algorithm

0 to 2,040 32, 64, 128