Implementing-cryptography-using-python

Chapter 5 ■ Stream Ciphers and Block Ciphers 159
Plaintext Plaintext Plaintext
Key
block cipher
encryption
Key
block cipher
encryption
Key
block cipher
encryption
Ciphertext Ciphertext Ciphertext
Figure 5.5: ECB mode encryption
Ciphertext Ciphertext Ciphertext
Key
block cipher
decryption
Key
block cipher
decryption
Key
block cipher
decryption
Plaintext Plaintext Plaintext
Figure 5.6: ECB mode decryption
In Figure 5.6, you will see that ciphertext enters the block cipher decryption
mode, which accepts a key and produces the decryption.
CBC Mode
The CBC mode, formally named Cipher Block Chaining, was created in 1976
by Ehrsam, Meyer, Smith, and Tuchman. When using CBC mode, each block
of plaintext is XOR’d with the previous ciphertext block. This ensures that each
ciphertext block depends on all plaintext blocks processed up to that point. As
shown in Figure 5.4, the IV is used on the initial block and then the produced
ciphertext is used to encrypt the plaintext of the next block. Figure 5.7 shows
the inverse operation.
CBC is the dominant mode of operation. Its major drawback is that encryption
must be performed sequentially as opposed to encrypting in parallel. A
second drawback is that the message must be padded to a multiple of the cipher
block size.