Fundamentals of Information Encryption - Multilevel Security for Relational Databases - page 86

C 0 = IV E K I 1 E K I 2 E K I 3

P 1 P 2 P 3

C 1 C 2 C 3

Figure 4.15 Using a block cipher in the CFB mode.

bits of the encrypted IV and the corresponding bits of the first block
of the image. The result is the encrypted version of the first block.
For the encryption of each of the next plaintext blocks, the previous
ciphertext block is encrypted and the output is XORed with the cur-
rent plaintext block to create the current ciphertext block. The XOR
operation conceals plaintext patterns.
Common to the CBC mode, changing the IV to the same plaintext
block results in different outputs. Though the IV need not be secret,
some applications would see this as desirable [36,37]. Figure 4.15
shows the CFB mode. The encryption algorithm is

C j = P j ⊕ I j (4.10)

and the decryption algorithm is

P j = C j ⊕ I j (4.11)

I j = E K ( C j −1 ), j = 1, 2, 3, … (4.12)

C 0 = IV (4.13)

4.6.4 he OFB Mode

This mode is similar to the CFB mode. It begins by encrypting
the IV. The bits of the encrypted IV are XORed with the corre-
sponding bits of the first plaintext block to get the corresponding
ciphertext block. Also, the output of the encryption algorithm is
used as an input to the next encryption step instead of the IV. This
process continues until the last block. Changing the IV for the same