Side Channel: DPA (Part 2)

Table of Contents

Instructions

In this lab you will use the round 1 key you obtained from your DPA attack last lab in order to obtain the full DES key. The round 1 key is 48 bits in size; whereas the full DES key is 64-bits. You will need to figure out how to obtain the full 64-bit key from the 48-bit round key. Once you have the original 64-bit key, you can test it on the provided plaintext/ciphertext pairs in the dataset to verify you have the correct key.

Some important points to get you started:

  • Although the DES key is 64 bits, only 56 bits are actually used by the DES algorithm. The other 8 bits are parity bits. If you use the des.py code provided in the lab6/ directory to verify your key, the parity bits are ignored, so just set them to 0.
  • The key schedule algorithm takes the 56-bit key and generates a 48-bit key for each round. You have the 48-bit round 1 key obtained from your DPA attack; however, since you don’t know the key in other rounds, you will need to figure out how to obtain these other 8 missing bits.
  • I suggest you start by reading through this page: https://ritul-patidar.medium.com/key-expansion-function-and-key-schedule-of-des-data-encryption-standard-algorithm-1bfc7476157. It has a great description of the DES key schedule algorithm. With some simple Python code, you can work backwards through the key schedule algorithm to go from the round 1 key back to the original key.

What to Submit

Make sure your submission tag on Github includes the following files in your lab7/ directory:

  1. A text file, key.txt that contains the 64-bit DES key. Again, the parity bits are ignored, so for simplicity and consistency, please set them to 0. The key should be an ASCII hex string, for example: “0123456789ABCDEF”.
  2. Your code that computes the 64-bit key. This should do the following:
    • Have your round 1 key hardcoded in the beginning of the file.
    • Compute the 64-bit key using the round 1 key.
    • Verify the key by checking several plaintext/ciphertext pairs in the dataset (these can be hardcoded in the file; you do not need to load the full dataset in your script).

Helpful Tips

DES Data Structures

The des.py file provided to you last lab has several data structures that may be helpful in this lab. For example, the pc1_table is used to permute the 64-bit key to a 56-bit key. By making use of these data structures, you can implement this lab with a small amount of Python code.

When I implemented this lab, I stored the key as a Python string (or list) of 1s and 0s. This made it very easy to manipulate the key bits and make use of the data structures provided in des.py.