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Data Encryption Standard operation

Short answer

DES is a symmetric 64-bit block cipher with a 56-bit key using a 16-round Feistel structure. Each round expands the 32-bit right half to 48 bits, XORs with a subkey, applies eight S-boxes for non-linear substitution, and permutes the result. The key schedule generates sixteen 48-bit subkeys via rotations and permutations. The 56-bit key is now insecure; Triple DES and AES have replaced it.

Explain the operation of the Data Encryption Standard (DES) algorithm.

DES (Data Encryption Standard) is a symmetric block cipher adopted in 1977 by the US National Bureau of Standards (NBS, now NIST). It encrypts a 64-bit block of plaintext with a 56-bit key (64 bits with 8 parity bits) into a 64-bit ciphertext block, using a Feistel structure of 16 rounds. The same algorithm decrypts if the 16 subkeys are used in reverse order.

Steps

64-bit plaintext
      |
 Initial Permutation (IP)
      |
 split: L0 (32) | R0 (32)
      |
 16 rounds:  Li = R(i-1)
             Ri = L(i-1) XOR f(R(i-1), Ki)
      |
 swap L16, R16
      |
 Final Permutation (IP^-1)
      |
64-bit ciphertext

Round function f(R, K)

  1. Expansion (E): R (32 bits) is expanded to 48 bits by repeating some bits.
  2. Key mixing: XOR with the 48-bit round key Ki.
  3. Substitution: the 48 bits are divided into eight 6-bit groups; each goes through an S-box that gives 4 bits (8 S-boxes: 6 bits in, 4 bits out), total 32 bits. The S-boxes give the non-linearity.
  4. Permutation (P): the 32 bits are shuffled.

Key schedule

The 56-bit key is obtained through Permuted Choice 1 (PC-1) and split into two 28-bit halves, C and D. In each round both halves are rotated left by 1 or 2 bits, and Permuted Choice 2 (PC-2) selects 48 bits as the subkey Ki.

Remarks

  • The 56-bit key is too small: DES was broken by brute force in 1998 (in days; now in hours). Triple DES (3DES) applies DES three times, and AES has replaced DES.

✓ Answer checked by Claude Opus 5.5 (high effort)

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