Side Channel: DPA (Part I)

Table of Contents

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.

Preliminary

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 lab_dpa_ii/ 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.

Instructions

  1. Write a Python program that does the following:
    • Has your round 1 key hardcoded in the beginning of the file.
    • Computes the 64-bit key using the round 1 key.
    • Verifies 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).

  2. Save your key in a text file, lab_dpa_ii/key.txt, as an ASCII hex string. For example, if your key is 0x0123456789ABCDEF, then the contents of key.txt should be:

     0123456789ABCDEF

    Again, the parity bits are ignored, so for simplicity and consistency, please set them to 0.

What to Submit

Tag your submission as lab_dpa_ii and make sure to commit:

  1. Your Python program that computes the 64-bit key (place it in lab_dpa_ii/).
  2. Your text file containing the 64-bit key (lab_dpa_ii/key.txt).

Helpful Tips

DES Data Structures

The des.py file provided to you 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.