CMM7XX

Symmetric Cryptography

Session 3: Stream and Block Ciphers, DES

James Williams

jwilliams@staff.newman.ac.uk

Learning Objectives

  • Differentiate the architectures of Stream and Block Ciphers.
  • Analyze vulnerabilities of legacy stream ciphers (RC4) and architecture of modern equivalents (ChaCha20).
  • Deconstruct the Feistel Network structure used in DES.
  • Evaluate fundamental Block Cipher Modes of Operation (ECB, CBC, GCM).
  • Understand attack vectors like Padding Oracles.

Task 1: Feistel Network Simulation

Timing: 45 minutes

Using Python or pen-and-paper for small 8-bit blocks:

  1. Define an 8-bit plaintext block and split it into $L_0$ (4 bits) and $R_0$ (4 bits).
  2. Define a dummy, completely non-invertible, lossy function for $f$. (e.g., $f(x) = 0$ or $f(x) = x \land K$).
  3. Execute one encryption round to find $L_1$ and $R_1$.
  4. Execute the decryption round using $L_1$ and $R_1$.
  5. Prove mathematically and programmatically that $L_0, R_0$ are recovered perfectly, demonstrating the power of the XOR swap mechanism.

15 Minute Break

Please return promptly for Part 2.

Part 2: Modes of Operation & Cryptanalysis

Duration: 1.5 Hours

Practical Resource: NIST SP 800-38A: Block Cipher Modes of Operation

Advanced Cryptanalysis against Block Ciphers

DES S-Boxes were designed by IBM to specifically resist these attacks:

  • Differential Cryptanalysis: Analyzes how differences in input plaintexts affect differences at the output. Requires Chosen-Plaintext.
  • Linear Cryptanalysis: Finds affine approximations to the action of a cipher. Developed in 1993, breaking DES faster than brute force ($2^{43}$ known plaintexts).
Practical Resource: Saltzer & Schroeder: Protection of Information in Computer Systems

Modes of Operation (1)

How do we encrypt data larger than a single block?

  • ECB (Electronic Codebook): Identical plaintext blocks produce identical ciphertext blocks. Preserves structural patterns. Never use.
  • CBC (Cipher Block Chaining): Introduces an IV. $C_i = E_K(P_i \oplus C_{i-1})$. Provides randomization. Must be sequential.
Practical Resource: NIST SP 800-38A: Block Cipher Modes of Operation

Modes of Operation (2)

  • CTR (Counter Mode): Turns a block cipher into a stream cipher. Encrypts a counter value to generate a keystream. Highly parallelizable.
  • GCM (Galois/Counter Mode): Authenticated Encryption. Provides confidentiality (CTR) and integrity (universal hashing over a Galois field). Modern standard in TLS 1.3.
Practical Resource: NIST SP 800-38A: Block Cipher Modes of Operation

Introduction to Task 2

Visualizing cryptographic failure is the best way to understand modes of operation. ECB mode fails to provide "Diffusion" across multiple blocks.

We will generate the famous "ECB Penguin" to see what happens when structural data is encrypted without block chaining.
Practical Resource: NIST SP 800-38A: Block Cipher Modes of Operation

Task 2: ECB vs CBC & Padding Oracles

Timing: 50 minutes

  1. Download the provided uncompressed .bmp image of the Linux Tux penguin.
  2. Use the OpenSSL CLI to encrypt the image using aes-128-ecb.
  3. Manipulate the ciphertext file header (copy the first 54 bytes from the original) so it renders as a valid image. Observe the ciphertext visually.
  4. Repeat the process using aes-128-cbc. Compare the outputs.
  5. Discussion: Discuss how CBC mode requires padding (PKCS#7) and how this padding can lead to a Padding Oracle Attack (Chosen-Ciphertext Attack).
Practical Resource: CyberChef: The Cyber Swiss Army Knife

Summary & Next Steps

  • Block ciphers require secure Modes of Operation to handle arbitrary data lengths safely.
  • Authenticated Encryption (GCM) is essential to prevent malleability attacks against modes like CBC and CTR.
Next Week: Asymmetric Cryptography (RSA and Key Management)
Practical Resource: NIST SP 800-38A: Block Cipher Modes of Operation