Implementing-cryptography-using-python

Chapter 6 ■ Using Cryptography with Images 185
Figure 6.10: ECB security issue
Exploring a Simple CBC Mode Example
Now that you understand how to encrypt and decrypt using ECB block mode,
we will examine the CBC mode. One important difference between the two
modes is the use of an initialization vector (IV) and the specification of the block
mode, which in this case is AES.MODE _ CBC:
from Crypto.Cipher import AES
iv = "1111222233334444"
key = "aaaabbbbccccdddd"
cipher = AES.new(key, AES.MODE_CBC, iv)
# encrypt using CBC mode
with open("plane.bmp", "rb") as f:
byteblock = f.read()
pad = len(byteblock)%16 * -1
byteblock_trimmed = byteblock[64:pad]
ciphertext = cipher.encrypt(byteblock_trimmed)
ciphertext = byteblock[0:64] + ciphertext + byteblock[pad:]
with open("plane_cbc.bmp", "w") as f:
f.write(ciphertext)
# decrypt using the reverse process