3.3 Task 3: Corrupted Cipher Text

Lab Question:

Display the encrypted picture using any picture viewing software and include the screenshot in your report. Can you derive any useful information about the original picture from the encrypted picture? Please explain your observations.

To understand the properties of ECB and CBC, we would like to do the following exercise:

1. Create .txt file that contains the plaintext AAAAAAAABBBBBBBBCCCCCCCCDDDDDDDDEEEEEEEEFFFFFFFF

2. Encrypt the file using -des-ecb and -des-cbc modes, respectively. Use the key 0011223344556677 and the IV 7766554433221100.

3. Unfortunately, the 30th byte in the encrypted file got corrupted and changes to FF. You can achieve this corruption using a hex editor.

4. Decrypt the corrupted file using the correct key and IV.

Lab Question:

(1) What is the plaintext after decrypting the corrupted file, if the encryption mode is ECB or CBC, respectively? Open the files in GHex and show the screenshot in your report. (2) Please explain why.