Fundamentals of Information Encryption - Multilevel Security for Relational Databases - page 71

Table 4.2 Types of Attacks on Encrypted Information

TYPE OF ATTACK PREREQUISITES FOR THE CRYPTANALYST

Ciphertext only • The algorithm of encryption

• Ciphertext to be decrypted

Known plaintext • The algorithm of encryption

• Ciphertext to be decrypted
• Plaintext message and ciphertext generated
with the secret key

Chosen ciphertext • The algorithm of encryption

• Ciphertext to be decrypted
• Ciphertext chosen by the cryptanalyst with
its plaintext generated with the decryption
algorithm and the decryption key

Chosen plaintext • Encryption algorithm

• Ciphertext to be decoded
• Plaintext chosen by the cryptanalyst with
its ciphertext generated with the encryption
algorithm and the encryption key

4.5 Conventional Symmetric Block Ciphers

This section gives a brief overview of the construction of some popular
conventional encryption algorithms. Each of the following encryption
algorithms is a symmetric block cipher algorithm. Symmetric means
that the key used for encryption and decryption is the same, while
block means that the data (information) to be encrypted is divided
into blocks of equal length [51,52].

4.5.1 Data Encryption Standard (DES)

The DES is the most well-known symmetric key block cipher and it
has enjoyed widespread use internationally [53].
The DES is a block cipher, which encrypts data in 64-bit blocks.
A 64-bit block of the plaintext comes at one end of the algorithm and
a 64-bit block of ciphertext goes out at the other end of the algorithm.
The same algorithm and the same key with 56 bits are used in the
encryption and decryption processes except for minor differences in
the key schedule. The key is a 64-bit number. In every 8 bits, 1 bit
(the least significant bit) is used for parity checking and can be ignored.
The DES is based on four basic operations: expansion, permutation,
XOR, and substitution. The data to be encrypted are first divided into