DES Algorithm

Module 03 / Lesson 01

Visual Explanation

How DES Works

The Data Encryption Standard (DES) is a symmetric-key block cipher. It encrypts data in 64-bit blocks using a 56-bit key. The algorithm uses a complex structure known as the Feistel Network, which consists of 16 rounds of processing.

Key Specifications:

Block Size: 64 bits
Key Size: 56 bits (effectively)
Structure: Feistel Network
Rounds: 16 identical rounds

Main Operations:

Initial Permutation (IP)
Expansion (32 to 48 bits)
S-Box Substitution (Confusion)
P-Box Permutation (Diffusion)

Feistel Logic:

Each round, the block is split into Left (L) and Right (R) halves. The R half undergoes a function with the round key and is then XORed with the L half.

Python Implementation (Conceptual XOR)

This code illustrates the core Feistel step in DES rounds:

def feistel_round(left, right, key):
    # Simplified Feistel Round logic
    # 1. Store original right side to become new left
    new_left = right
    
    # 2. Right side function: XOR(Right, Key)
    # (In real DES, 'right' goes through expansion and S-Boxes)
    function_result = int(right, 2) ^ int(key, 2)
    
    # 3. New Right = Left XOR Function_Result
    new_right = bin(int(left, 2) ^ function_result)[2:].zfill(32)
    
    return new_left, new_right

# Example simulation of one round
L = "11010010110100101101001011010010"
R = "00110011001100110011001100110011"
K = "10101010101010101010101010101010"

next_L, next_R = feistel_round(L, R, K)
print(f"Next Left:  {next_L}")
print(f"Next Right: {next_R}")

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