--- title: "Data Encryption Standard operation" subject: "Computer Networks and Security" chapter: 8 key: "semester_7/computer_networks_and_security:des-algorithm" source: https://www.bctengineer.com/q/semester_7/computer_networks_and_security/des-algorithm site: BCT Engineering --- # Data Encryption Standard operation Computer Networks and Security · Chapter 8: Network Security - Asked in: 2070 Chaitra, 5 marks - Video: [Introduction to Data Encryption Standard (DES)](https://www.youtube.com/watch?v=j53iXhTSi_s) — Neso Academy - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/computer_networks_and_security/des-algorithm - Whole chapter: https://www.bctengineer.com/md/semester_7/computer_networks_and_security/chapter-8 ## Question Explain the operation of the Data Encryption Standard (DES) algorithm. ## 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. ## Answer **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 ```text 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. DES block size and key size: A. 56-bit block, 64-bit key B. 128-bit, 128-bit C. 64-bit, 128-bit D. 64-bit block, 56-bit key Answer: D. 64-bit key includes 8 parity bits. (Key point: Block and key) 2. DES structure and rounds: A. Feistel, 8 rounds B. SPN, 10 rounds C. Feistel, 16 rounds D. Stream, 1 round Answer: C. Same algorithm decrypts with subkeys reversed. (Key point: Structure) 3. What does an S-box do? A. Rotates key B. 6 bits in, 4 bits out; gives non-linearity C. Expands 32 to 48 bits D. Permutes bits Answer: B. Eight S-boxes produce 32 bits. (Key point: S-boxes) 4. Why is DES obsolete? A. Block too large B. Needs public keys C. 56-bit key is too small and brute-forced D. It is too slow Answer: C. 3DES and AES replaced it. 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