Detailed DES Encryption Example – Step-by-Step Breakdown (Context 2026)

[Student]

Disclaimer: DES (Data Encryption Standard) is a legacy symmetric encryption algorithm from 1977, considered insecure in 2025 due to its 56-bit key size (brute-forceable in hours with modern hardware). It is obsolete for real security – replaced by AES since 2001. This example is educational only, for understanding historical PIN offset calculation in legacy banking systems (e.g., Diebold/Wincor/NCR ATMs). Do not use DES for any real encryption.

DES Basics Recap​

• Block size: 64 bits (8 bytes)
• Key size: 64 bits (8 bytes, but effective 56 bits – 8 parity bits)
• Mode: Usually ECB for PIN blocks (no IV)
• Rounds: 16 Feistel rounds
• Operations: Initial/final permutation, expansion, S-box substitution, P-box permutation

Step-by-Step Manual DES Encryption Example​

We’ll use a simple example:

• Plaintext: 0000000000000000 (64-bit all zeros)
• Key: 0000000000000000 (all zeros – for illustration; real keys are secret)

Note: All zeros key/plaintext is a known test vector.

Step 1: Initial Permutation (IP) The 64-bit plaintext is rearranged according to the fixed IP table.

Plaintext (bits 1–64): 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000

After IP: 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 (All zeros remain all zeros after permutation)

Step 2: Split into Left (L0) and Right (R0) L0 = first 32 bits: 00000000 00000000 00000000 00000000 R0 = last 32 bits: 00000000 00000000 00000000 00000000

Step 3: 16 Rounds of Feistel Function Each round:

• Expand R to 48 bits (E-box)
• XOR with round key (derived from main key)
• S-box substitution (8×6-bit → 8×4-bit)
• P-box permutation
• XOR with L → new R
• Old R becomes new L

Since key and data are all zeros: All round keys = zeros All XOR = zeros All S-box output = zeros After 16 rounds: L16 = 00000000 00000000 00000000 00000000 R16 = 00000000 00000000 00000000 00000000

Step 4: Final Permutation (IP⁻¹) Apply inverse initial permutation – result remains all zeros.

Final ciphertext: 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 (Hex: 0000000000000000)

This is the known DES test vector for all-zero key/plaintext.

Real-World PIN Offset Example with DES (Legacy Banking)​

In legacy PIN offset (e.g., Diebold/NCR):

1. Natural PIN derived from PAN
2. Encrypted with DES key → validation data
3. Offset = customer PIN XOR encrypted natural PIN

Example (simplified – real keys secret):

• PAN-derived block: 0000091234567890
• DES key: bank secret (e.g., 133457799BBCDFF1)
• Encrypted block → first 4 hex digits = natural PIN
• Customer PIN 5678 → offset calculated and stored on Track 2

2025 Reality: DES in PIN offset is dead on 99 %+ terminals (online auth + AES). Only legacy offline ATMs (Mexico/DR) still use it – <5 % globally.

Why DES Is Obsolete in 2025​

• Brute-force time: <1 hour on modern ASIC/FPGA ($10K hardware)
• Replaced by: Triple DES (3DES) then AES-256 for PIN blocks
• EMV/PCI DSS: DES banned for new systems since 2010s

For modern encryption → use AES-256-GCM.

Want a working Python AES example instead? Let me know!