Cryptography

Two-Time Pad, or All-Nonced-Up known text file name: known_b01e4e62.txt (File link: https://www86.zippyshare.com/v/HHY3xZRb/file.html

decrypted KAP file name: 0ddd5a3c.bin (File link: https://www86.zippyshare.com/v/3zeDbGAy/file.html)

This assignment is centered on cracking a cipher in the case that a nonce is misused. You will see two subdirectories: KPA (known plaintext attack) and COA (ciphertext only attack).

In each of these directories are 58 books from Project Gutenberg, encrypted and with their titles masked.

You have been assigned to decrypt one file from KPA and one file from COA (please see the mapping file in this directory).

In the KPA directory, all files were encrypted using AES in CTR mode using the same key and the same IV. Furthermore you have been given a piece of plaintext for one of these files, namely b01e4e62 which happens to be the Declaration of Independence. This should allow you to use the two-time-pad trick to decrypt any file in the KPA directory.

The COA directory is similar. All files were encrypted using AES in CTR mode using the same key and the same IV. However the key is not the same as for the KPA directory. In this case you also want to crack your file. But in this case there is no known plaintext, so you must do a COA attack.

The COA problem is quite a bit harder than the KPA problem. Most students will not be able to solve this problem. If you cannot solve it it does not mean you will fail the project. Be ready to explain the progress you are able to make.

Here are a couple of hints.

What happens when you XOR the code for an alphabetical letter with the code for "space" in the ASCII standard?

Can you think of a way to find the locations of all the spaces in all of the files?

If you know where all the spaces are, how can you use this to find the keystream?

Using the keystream, how can you decrypt your target?

Please proper and workable code