The Data Encryption Standard (DES) has a function consists of four steps.…
2019
The Data Encryption Standard (DES) has a function consists of four steps. Which of the following is correct order of these four steps?
- A.
an expansion permutation, 𝑆-boxes, an XOR operation, a straight permutation
- B.
an expansion permutation, an XOR operation, 𝑆-boxes, a straight permutation
- C.
a straight permutation, 𝑆-boxes, an XOR operation, an expansion permutation
- D.
a straight permutation, an XOR operation, 𝑆-boxes, an expansion permutation
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Correct answer: B
Correct sequence of the DES round function's four internal steps:
Expansion permutation (E): Expands the 32-bit input to 48 bits so it can be combined with the 48-bit round subkey.
XOR with the round subkey: The expanded 48-bit data is XORed with the 48-bit subkey for that round.
S-box substitution: The 48-bit result is split into eight 6-bit blocks, each fed to an S-box to produce 32 bits in total; this introduces nonlinearity.
Straight (P) permutation: Permutes the 32-bit output from the S-boxes to produce the final function output for the round.
Why the order matters: XOR must come after expansion so the subkey aligns with the expanded data; S-boxes require the post-XOR 6-bit inputs; the straight permutation reorganizes the S-box output as the final step. In the full DES round (Feistel structure), this function's output is then XORed with the left half.
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