Understanding DES Encryption: The Role Initial and Final Permutations

Data Encryption Standard (DES) is a symmetric-key algorithm for encrypting digital data. This means that the same key is used for both encryption and decryption.

Data Encryption Standards

Key Features Shown in the Diagram:

    1. Block Cipher: DES is a block cipher, meaning it operates on fixed-size blocks of data. The diagram shows that DES processes 64-bit blocks of plaintext at a time.
    2. Key Length: The encryption process uses a 56-bit key. While the actual key might be represented as 64 bits (including parity bits), the effective key length used in the algorithm is 56 bits.
    3. Encryption Process:
      • A 64-bit block of plaintext is fed into the DES cipher.
      • Using the 56-bit key, the DES algorithm performs a series of transformations (not detailed in this diagram) on the plaintext block.
      • The output of this process is a 64-bit block of ciphertext.
    4. Decryption Process:
      • To decrypt the ciphertext back to the original plaintext, the same 56-bit key is used, but the process is reversed.
      • A 64-bit block of ciphertext is fed into the DES reverse cipher.
      • Using the same 56-bit key, the DES algorithm in reverse order transforms the ciphertext block.
      • The output is the original 64-bit block of plaintext.

In summary, DES takes a 64-bit block of plaintext and a 56-bit key as input to produce a 64-bit block of ciphertext. The decryption process reverses this using the same key.

Data Encryption Standard - Initial and Final Permutation

DES (Initial and Final Permutations)

Initial Permutation: The First Shuffle

  1. Input: You start with your original 64-bit block of data (the plaintext).
  2. The Shuffle: The Initial Permutation is like taking these 64 bits and rearranging them in a specific, fixed order. It’s not about changing the bits themselves (like making a 0 into a 1), but just moving them around to different positions.
  3. Why Shuffle First? This initial shuffle helps to mix up the bits right at the beginning. It makes the relationship between the original data and the data that goes into the main rounds of DES a bit more complicated. This can make the encryption stronger against certain types of attacks.

Final Permutation: The Last Shuffle

  1. Input: After the data has gone through all the 16 rounds of the main DES process, you have a 64-bit block of data.
  2. The Shuffle: The Final Permutation is another fixed rearrangement of these 64 bits. It’s like taking the bits and putting them in a specific order to produce the final encrypted data (the ciphertext).
  3. Why Shuffle Last? The final shuffle is essentially the reverse of the initial shuffle (almost, not exactly the inverse). This means if you apply the final permutation and then the initial permutation (or vice versa), you almost get back to the original order. The reason for this final shuffle is partly historical and related to how DES was designed, and it doesn’t significantly add to the security of the algorithm itself.

Think of it like this: Imagine you have a deck of cards.

1. The Initial Permutation is like taking the deck and rearranging the cards in a specific pattern at the very beginning.
2. After playing the game (the 16 rounds of DES), the Final Permutation is like rearranging the deck again in another specific pattern to get the final result.

Example using 64 bits numbering input

Initial and Final Permutation in DES

Initial Permutation (Top Part of the Image):

    1. Top Row: This represents the positions of the bits in your original 64-bit data (though only some are explicitly labeled: 1, 2, 8, 25, 40, 58, 64, with “…” indicating the bits in between).
    2. Yellow Box labeled “Initial Permutation”: This is where the shuffling happens.
    3. Bottom Row: This represents the new positions of the bits after the Initial Permutation. The arrows show you where each original bit goes.
      • For example, there’s an arrow going from the bit at position 58 in the top row down to position 1 in the bottom row. This means the 58th bit of your original data becomes the 1st bit after the Initial Permutation.
      • Similarly, the bit at position 50 (which is somewhere in the “…” between 40 and 58) would go to position 2 (based on the actual IP table, though not explicitly shown here).
      • You can see another arrow going from position 1 in the top row to position 40 in the bottom row.

Final Permutation (Bottom Part of the Image):

    1. Top Row: This now represents the positions of the bits after the 16 rounds of the DES algorithm.
    2. Yellow Box labeled “Final Permutation”: This is the final shuffling.
    3. Bottom Row: This shows the positions of the bits in the final 64-bit ciphertext. Again, the arrows indicate the movement.
      • For instance, the bit at position 12 (which was the result after the rounds) moves to position 1 in the final output.
      • The bit at position 8 moves to position 2.
      • The bit at position 25 moves to position 8, and so on.

Related to how computers and the internet work, you might also be interested in understanding how IP addresses relate to domain names. You can learn more about this here: Mapping IP Address to Domain Name and Translating Domain Name to IP Address (Forward DNS).

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