Symmetric Key Cryptography
Page 1: Introduction
Course: The Campus of Tomorrow, Higher Colleges of Technology CSF-3603
Topic: Cryptography and Data Protection
Focus: Symmetric Key Cryptography
Page 2: Course Learning Objectives (CLOs)
CLO2: Apply modern symmetric cryptography, including algorithmic structures and modes of operation.
Page 3: Lesson Objectives
Explain the mechanism of DES cipher
Differentiate between DES and 3-DES algorithms
Analyze the security of DES and 3-DES algorithms
Explain the mechanism of AES cipher
Analyze the security of AES
Explain modes of operation: ECB, CBC, CTR.
Page 4: DES Overview
Definition: Data Encryption Standard (DES) is a symmetric-key block cipher.
Type: Implementation of a Feistel Cipher.
Specifications:
Block size: 64 bits
Key size: 56 bits
Number of rounds: 16 (identical operations with different subkeys)
Page 5: DES Execution Steps
Initial Permutation (IP)
Divide message into left (L) and right (R) halves.
Feed right half (Ri) into function (F).
Output is XORed with left half (Li).
Repeat n rounds of encryption.
Each round uses a key K.
Generate new L and R for each round.
Produce a new subkey K each round.
Page 6: DES Round Structure
DES consists of 16 rounds that perform:
Split plaintext into 32-bit halves (Li and Ri).
Feed Ri into function f; XOR output with Li.
Swap left and right halves.
Page 7: Initial Permutation
Public knowledge before any rounds.
Permutation done using an initial permutation (IP) table.
Page 8: Round Function f
Core: Applies a 48-bit key to 32-bit input, producing a 32-bit output.
Page 9: Expansion Permutation Box
Expands right input (32 bits) to match round key (48 bits).
Page 10: XOR and S-boxes
XOR bits with round key (48 bits).
8 S-boxes for mixing (confusion) using 6-bit input to 4-bit output.
Page 11: S-box Rule
Details of how S-boxes operate and produce outputs.
Page 12: Combining S-box Outputs
Combines outputs from all 8 S-boxes into a 32-bit section.
Page 13: Straight Permutation
The 32-bit output from S-boxes undergoes straight permutation.
Page 14: Inverse Permutation (IP-1)
After 16 rounds, concatenate blocks 'L' and 'R', then permute using IP-1 table.
Page 15: DES Components
Visual representation:
P = Plaintext (64 bits)
DES = Ciphertext (64 bits)
K = Key (56 bits)
Page 16: DES Phase 1
Plaintext goes through an initial permutation (IP) to shuffle bits.
Page 17: DES Phase 2
Permuted bits and K are processed in a main module for 16 rounds using function F.
Page 18: DES Phase 3
Final ciphertext produced through inverse permutation (IP-1).
Page 19: Encryption Summary
P = Plaintext (64 bits), K = Key (56 bits), C = Ciphertext (64 bits)
Encryption mirrors Feistel cipher structure.
Page 20: DES Strengths
Properties:
Avalanche effect: small plaintext changes lead to large ciphertext changes.
Completeness: each ciphertext bit depends on many plaintext bits.
Page 21: DES Weaknesses
Key space too small (2^56), making it vulnerable to brute-force attacks.
Modern machines can crack DES within hours.
Page 22: Triple DES Overview
3DES improves DES by extending key size:
Runs DES algorithm three times with three different keys (K1, K2, K3).
Page 23: Triple DES Key Generation
Generates 3-DES key K consisting of K1, K2, K3.
Encryption process:
Encrypt with K1
Decrypt with K2
Encrypt with K3
Decryption follows reverse order.
Page 24: 3-DES Security Analysis
Benefits: 168-bit key length overcomes brute-force attacks.
Drawbacks: Slower than DES due to 3 times more rounds; 64-bit block size is less desirable.
Page 25: AES Introduction
AES: Advanced Encryption Standard, a widely used symmetric encryption algorithm.
Considered faster than Triple DES and offers larger key sizes.
Page 26: AES Specification
Characteristics:
Symmetric key, symmetric block cipher.
128-bit data with 128/192/256-bit keys.
Operates on a block of 4 columns of 4 bytes – iterative operation.
Page 27: AES Conceptual Scheme
Inputs: Plaintext (128 bits) and AES key (128-192-256 bits) lead to Ciphertext (128 bits).
Page 28: AES Rounds
Most rounds are identical; initial and final rounds vary.
Page 29: High-Level Description
Initial Round:
AddRoundKey: Combine state with round key.
Subsequent Rounds:
SubBytes, ShiftRows, MixColumns, AddRoundKey.
Final Round: SubBytes, ShiftRows, AddRoundKey (no MixColumns).
Page 30: Changing Plaintext to State Text
Data transformation example in hex format.
Page 31: Encryption Process Example
Sample 128-bit input state and corresponding hex representation.
Page 32: Sub Bytes Operation
Substitution of each byte, providing confusion using a single S-box of 16x16 bytes.
Page 33: Shift Row Operation
Circular shifts of bytes in the state matrix to the left.
Page 34: Mix Column Operation
Transforms each column of four bytes using mathematical functions.
Page 35: Add Round Key Operation
XORs the 16 bytes of the state with 128 bits of the round key.
Page 36: AES Security Analysis
More secure than DES due to larger key sizes.
No known practical cryptanalytic attacks.
Widely adopted in hardware and software.
Page 37: Modes of Operation
Various modes improve security by ensuring block ciphers can handle larger messages.
Page 38: Electronic Code Book (ECB)
Simplest and weakest mode.<br> - Identical plaintexts encrypt to identical ciphertexts.<br> - Susceptible to known-plaintext attacks, making it less secure.
Page 39: Cipher Block Chaining (CBC)
Stronger than ECB; each block produces different outputs.<br> - Utilizes an Initialization Vector (IV) for secure operation.
Page 40: Counter Mode (CTR)
Used in high-speed applications; generates keystream from counter.<br> - Not dependent on previous plaintext blocks; synchronous counters required.