CyberCode Academy

Course 17 - Computer Network Security Protocols And Techniques | Episode 3: Modern Ciphers: Structure, Standards (DES/AES)

Listen Later
In this lesson, you’ll learn about:
  • How modern cryptography differs from classical ciphers
  • The building blocks of bit-oriented encryption
  • How DES, 3DES, and AES work at a high level
  • Why block cipher modes of operation are necessary
Introduction This lesson provides a structured overview of modern cryptographic techniques, focusing on how today’s encryption systems operate at the bit level, how complex standards like DES and AES are constructed, and how modes of operation securely apply block ciphers to real-world data. Foundational Concepts of Modern Ciphers Modern cryptography is bit-oriented, meaning it works directly on bits rather than characters. This allows encryption of all digital data types, including text, audio, images, and video. Basic Cipher Components Complex modern ciphers are built by combining several simple operations:
  • XOR (Exclusive OR) Cipher
    • Performs a bitwise XOR between data and a key
    • Simple but essential for mixing key material with data
  • Rotation Cipher
    • Rotates bits left or right with wraparound
    • Helps spread bit influence across the data
  • Substitution Ciphers (S-Boxes)
    • Replace input bits with output bits using lookup tables
    • Variants include:
      • Equal size substitution (n = m)
      • Expansion (n < m)
      • Compression (n > m)
  • Transposition / Permutation Ciphers (P-Boxes or T-Boxes)
    • Reorder bits based on fixed permutation patterns
    • Can preserve size or perform expansion/reduction
    • Increase diffusion by spreading bit changes
Round Cipher Structure Most modern block ciphers use a round-based design:
  • Encryption is performed over multiple rounds
  • Each round applies substitution, permutation, and XOR
  • Each round uses a different subkey derived from a master key
  • Security increases with the number and complexity of rounds
Key Encryption Standards Data Encryption Standard (DES)
  • Early U.S. encryption standard
  • Operates on 64-bit blocks
  • Uses a 56-bit key (stored as 64 bits)
  • Consists of 16 rounds
DES Round Function Each round includes:
  • Splitting input into two 32-bit halves
  • Expansion P-box: 32 → 48 bits
  • XOR with a 48-bit round key
  • S-boxes: 48 → 32 bits
  • Straight permutation
  • Feistel structure swaps halves each round
Triple DES (3DES)
  • Designed to improve DES security
  • Applies DES three times in an Encrypt–Decrypt–Encrypt sequence
  • Key options:
    • Two-key version: 112-bit security
    • Three-key version: 168-bit security
  • More secure than DES, but slower and largely deprecated
Advanced Encryption Standard (AES)
  • Current global encryption standard
  • Replaced DES and 3DES
  • Operates on 128-bit blocks
  • Supports three key sizes:
    • 128-bit
    • 192-bit
    • 256-bit
  • More rounds are used as key size increases
  • Designed for high security and high performance
Modes of Operation for Block Ciphers Block ciphers encrypt fixed-size blocks, but real data streams require modes of operation to handle multiple blocks securely. 1. Electronic Code Book (ECB)
  • Each block encrypted independently
  • Identical plaintext blocks → identical ciphertext blocks
  • Leaks patterns and is insecure
  • Not recommended for real-world use
2. Cipher Block Chaining (CBC)
  • Each plaintext block is XORed with the previous ciphertext
  • Eliminates repeated ciphertext patterns
  • Requires an Initialization Vector (IV)
  • Suffers from error propagation across blocks
3. Cipher Feedback (CFB)
  • Converts block cipher into a stream-like cipher
  • Supports encrypting smaller data units (R bits)
  • Uses a shift register with feedback from ciphertext
  • Error propagation affects subsequent blocks
4. Output Feedback (OFB)
  • Similar to CFB but feeds back encrypted output instead of ciphertext
  • Encryption stream is independent of ciphertext
  • No error propagation
  • Requires careful IV synchronization
Initialization Vector (IV)
  • Required for CBC, CFB, and OFB modes
  • Ensures uniqueness of the first encryption block
  • Must be agreed upon by sender and receiver
  • Prevents pattern reuse across messages
Key Takeaways
  • Modern encryption operates at the bit level
  • Strong ciphers are built from simple operations combined over many rounds
  • DES introduced round-based block encryption but is no longer secure
  • 3DES improved security but is inefficient
  • AES is the modern standard due to strength and performance
  • Modes of operation are essential for securely encrypting large or streaming data
  • ECB is insecure, while CBC, CFB, and OFB address pattern leakage in different ways


You can listen and download our episodes for free on more than 10 different platforms:
https://linktr.ee/cybercode_academy
...more
View all episodesView all episodes
Download on the App Store
CyberCode AcademyBy CyberCode Academy