Q18Information System Security

Adopted in 1977, DES is a symmetric-key block cipher that became the absolute global standard for encryption. It is mathematically based entirely on the Feistel Network architecture. DES takes a rigidly fixed 64-bit plaintext block and a 56-bit secret key, and violently crushes them through 16 highly aggressive iterations (rounds) of mathematical confusion and diffusion to output a 64-bit ciphertext.

Before the core mathematics begin, the 64-bit plaintext is fed into the Initial Permutation matrix. This step violently rearranges the bits (e.g., bit 58 becomes bit 1, bit 50 becomes bit 2) strictly according to a hardcoded table. It provides zero cryptographic security but was designed to optimize data loading into 1970s hardware.

The permuted 64-bit block is violently shattered into two distinct 32-bit halves: Left ($L_0$) and Right ($R_0$). The algorithm then executes 16 identical mathematical rounds. For round $i$:

• The Left Pass: The new Left half is simply a direct copy of the previous Right half: $$L_i=R_{i-1}$$
• The Right Calculation: The absolute core of the cryptography. The previous Right half is fed into a complex function $F$ along with a 48-bit Subkey ($K_i$). The output is then aggressively XORed with the previous Left half: $$R_i=L_{i-1}\oplus F(R_{i-1},K_i)$$

The function $F$ takes the 32-bit $R_{i-1}$ and executes four violent operations:

• Expansion (E-Box): The 32 bits are mathematically expanded to 48 bits by duplicating 16 specific bits. This is required to match the 48-bit subkey.
• Key Mixing: The 48-bit expanded block is violently XORed with the 48-bit Subkey $K_i$.
• Substitution (S-Boxes): THE ABSOLUTE CRITICAL STEP. The 48-bit result is chopped into eight 6-bit chunks. Each chunk is fed into a distinct S-Box. The S-Box uses rigid look-up tables to mathematically crush the 6 bits down to 4 bits. This is the ONLY non-linear operation in DES. It violently introduces "Confusion", making it mathematically impossible for hackers to trace equations back to the key.
• Permutation (P-Box): The resulting 32 bits are wildly rearranged to introduce "Diffusion", ensuring that one bit change here will violently affect multiple S-Boxes in the next round.

The original key is 64 bits, but 8 bits are discarded for parity, leaving a 56-bit active key. The algorithm must mathematically generate sixteen distinct 48-bit subkeys ($K_1$ to $K_{16}$).

• The 56-bit key is split into two 28-bit halves.
• In each round, both halves are violently subjected to a Circular Left Shift (by 1 or 2 bits depending on the round).
• After the shift, 48 bits are extracted using a Permuted Choice (PC-2) table to form the subkey $K_i$.