Implementing-cryptography-using-python

182 Chapter 6 ■ Using Cryptography with Images
The next step is to file read the plane.bmp image; we will store the binary file
into a variable named byteblock:
>>> with open("plane.bmp", "rb") as f:
>>> byteblock = f.read()
The byteblock will need to be in multiples of 16 bytes. If not, it will result in
an error that states: Input strings must be a multiple of 16 in length. The byteblock
value will depend on the size of the image. To examine the byteblock, you can
examine the length of the variable:
>>> print (len(byteblock))
261654
Since the input string must be a multiple of 16, we will need to examine how
many bytes are left over when we take the modulo. In this case, it is 6:
>>> print (len(byteblock)%16)
6
Our goal is now to move the bytes that are multiples of 16 into a variable that
we can use to isolate the block while subtracting the overflow. Here, we will
store the value in byteblock_trimmed:
byteblock_trimmed = byteblock[64:-6]
>>> print (len(byteblock_trimmed))
261584
>>> print (len(byteblock_trimmed)%16)
0
To ensure that the image can be encrypted and decrypted without a data loss,
you will need to combine the two sets of byte blocks:
# byteblock_trimmed must not have extra bytes or an error will occur
ciphertext = cipher.encrypt(byteblock_trimmed)
ciphertext = byteblock[0:64] + ciphertext + byteblock[-6:]
Now, all that is needed for the image is to save it using the ciphertext:
with open("plane_ecb.jpg", "w") as f:
f.write(ciphertext)