Q11Information Security Systems

DES is a symmetric-key block cipher based entirely on Horst Feistel's architectural network. It violently shatters a 64-bit plaintext block into two 32-bit halves (Left $L_0$ and Right $R_0$) and subjects them to 16 identical, mathematically aggressive iterations (Rounds) of confusion and diffusion.

The Mathematical Mechanics of a Single Round ($i$)

Inside Round $i$, the Left half simply copies the previous Right half: $L_i=R_{i-1}$. The absolute cryptographic violence occurs in calculating the new Right half: $R_i=L_{i-1}\oplus F(R_{i-1},K_i)$, where $F$ is the highly complex Feistel function.

The Execution of the $F$-Function

• 1. Expansion Permutation (E-Box): The 32-bit $R_{i-1}$ is mathematically expanded to 48 bits by duplicating specific bits. This allows it to physically match the size of the 48-bit subkey ($K_i$).
• 2. Key Mixing (XOR): The 48-bit expanded block is violently XORed with the 48-bit subkey generated by the Key Schedule.
• 3. Substitution Boxes (S-Boxes): The absolute core of DES security. The 48-bit result is chopped into eight 6-bit chunks. Each chunk is fed into a specific non-linear S-Box. The S-Box mathematically crushes the 6 bits down to 4 bits using rigid look-up tables. This is the ONLY non-linear step in DES, providing absolute resistance against catastrophic differential cryptanalysis.
• 4. Permutation (P-Box): The resulting 32 bits from the S-Boxes are wildly rearranged (diffused) using a strict wire permutation to ensure the output violently affects completely different S-Boxes in the next round.