Cryptography Fundamentals: Symmetric Ciphers and Block Design

Posted on Oct 9, 2025 in Information Consulting and Management

Cryptography and the Symmetric Cipher Model

Cryptography is the science of protecting information by transforming it into a secure format. It ensures data confidentiality, authenticity, integrity, and sometimes non-repudiation during communication or storage.

The Symmetric Cipher Model

In the symmetric cipher model, the same secret key is used for both encryption and decryption of messages.

Key Components of a Symmetric Cipher

  • Plaintext (P): The original readable message or data.
  • Encryption Algorithm: Takes the plaintext and a secret key to produce ciphertext.
  • Ciphertext (C): The scrambled, unreadable output of the encryption process.
  • Secret Key (K): Shared between sender and receiver. It must be kept confidential.
  • Decryption Algorithm: Takes the ciphertext and the same key to recover the original plaintext.

Common symmetric algorithms include AES, DES, and RC4.

Symmetric Cipher Working Principle

Sender: Plaintext → [Encryption Algorithm + Key] → Ciphertext

Receiver: Ciphertext → [Decryption Algorithm + Same Key] → Plaintext

Understanding the Caesar Cipher

The Caesar Cipher is one of the simplest and oldest encryption techniques. It is a type of substitution cipher, where each letter in the plaintext is shifted a fixed number of positions forward in the alphabet. This shifting wraps around the end of the alphabet.

For example, if the shift value = 3, then:

  • A → D
  • B → E
  • C → F
  • …
  • X → A
  • Y → B
  • Z → C

Caesar Cipher Example (Shift = 3)

Plaintext: HELLO

With a shift of 3, the encryption becomes:

  • H → K
  • E → H
  • L → O
  • L → O
  • O → R

Ciphertext: KHOOR

Working Mechanism of the DES Algorithm

DES (Data Encryption Standard) is a symmetric block cipher developed by IBM and adopted by the U.S. government in the 1970s. It encrypts data in 64-bit blocks using a 56-bit key.

Steps in the DES Algorithm

  1. Initial Permutation (IP): The 64-bit plaintext block is first rearranged using a fixed permutation table.
  2. Split into Two Halves: The result is divided into two 32-bit halves: Left (L0) and Right (R0).
  3. 16 Rounds of Encryption: For each round (1 to 16), a complex transformation is applied.
    • Apply a function f to the right half and the round key.
    • XOR the result with the left half.
    • Swap the halves.

    The core function f involves: Expansion (32 → 48 bits), XOR with the 48-bit round key, Substitution using S-boxes, and Permutation.

  4. Final Swap: After 16 rounds, the left and right halves are swapped back.
  5. Inverse Initial Permutation (IP⁻¹): The final 64-bit output is permuted again using the inverse of the initial permutation.

The result is the final ciphertext.

Essential Block Cipher Design Principles

Block ciphers encrypt data in fixed-size blocks. Designing a secure block cipher involves the following key principles:

  • Confusion: Hides the relationship between the key and the ciphertext, typically achieved using Substitution Boxes (S-boxes).
  • Diffusion: Spreads plaintext bits across many ciphertext bits to eliminate statistical patterns and relationships.
  • Key Size: Larger keys (e.g., 128 or 256 bits) offer better security against brute-force attacks.
  • Block Size: Typical sizes are 64 or 128 bits; larger blocks reduce repetition in the ciphertext.
  • Number of Rounds: More rounds increase security by applying repeated transformations, making cryptanalysis harder.
  • Round Function: The core logic used in each round, involving substitution and mixing operations.
  • Key Scheduling: The process that securely generates different round keys from the main secret key.
  • Attack Resistance: The cipher design must resist known attacks, including brute-force, linear, and differential cryptanalysis.

The One-Time Pad (OTP) Cryptosystem

The One-Time Pad is a symmetric encryption technique characterized by its perfect secrecy, provided it is used correctly. The key requirements are:

  • A random key (pad) is generated that is as long as the message.
  • Each bit or character of the plaintext is combined with the key using XOR (for binary) or modular addition (for letters).
  • The key is used only once and then discarded.

OTP Example

Plaintext: HELLO
Key: XMCKL (random letters of the same length)

Each character is encrypted using modular addition (A=0, B=1, …, Z=25).

Advantages of the One-Time Pad

  • Perfect Secrecy: If used correctly, it is theoretically unbreakable (information-theoretically secure).
  • Simple Algorithm: Encryption and decryption use basic operations (like XOR or modular addition).
  • No Patterns: Because the key is random and as long as the message, it is not vulnerable to frequency analysis.

Disadvantages of the One-Time Pad

  • Key Distribution Problem: The key must be securely shared in advance and must be as long as the message.
  • Key Management: The key must be used only once and kept perfectly secret.
  • Impractical for Long Messages: Requires generating and managing huge amounts of random keys for large data volumes.
  • Storage Issues: Both sender and receiver must store massive keys securely.