Question

Task 3: Encryption Mode – ECB vs. CBC
The file pic original.bmp can be downloaded from this lab’s website, and it contains a simple picture.
We would like to encrypt this picture, so people without the encryption keys cannot know what is in the
picture. Please encrypt the file using the ECB (Electronic Code Book) and CBC (Cipher Block Chaining)
modes, and then do the following:
1. Let us treat the encrypted picture as a picture, and use a picture viewing software to display it. However,
For the .bmp file, the first 54 bytes contain the header information about the picture, we have
to set it correctly, so the encrypted file can be treated as a legitimate .bmp file. We will replace the
header of the encrypted picture with that of the original picture. We can use the bless hex editor
tool (already installed on our VM) to directly modify binary files. We can also use the following
commands to get the header from p1.bmp, the data from p2.bmp (from offset 55 to the end of the
file), and then combine the header and data together into a new file.
$ head -c 54 p1.bmp > header
$ tail -c +55 p2.bmp > body
$ cat header body > new.bmp
2. Display the encrypted picture using a picture viewing program (we have installed an image viewer
program called eog on our VM). Can you derive any useful information about the original picture
from the encrypted picture? Please explain your observations.
Select a picture of your choice, repeat the experiment above, and report your observations.

Answer 1

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