Computer Security Lecture 21

1
Computer Security Lecture 21

• Phillip G. Bradford
• Computer Science
• University of Alabama

2
Outline

• Review last lecture
• DES
• IDEA
• BlowFish

3
Objectives

• Complete DES
• Look at other important Block Ciphers
• Variations on S-Boxes
• Applications

4
International Data Encryption Algorithm (IDEA)

• Lei Massey
• Block Cipher using 128-bit key for 64 bit blocks
• Design goals
• Block Length deter statistical analysis
• Key Length deter exhaustive search
• Confusion complicated method
• Diffusion All plaintext bits influence all
  ciphertext bits

5
IDEA Implementation

• For Software
• Sub blocks powers of 2
• 16 bit sub blocks here
• Elementary operations
• Used in complex ways
• For Hardware
• Encryption Decryption use same hardware
• Regular structure for hardware

6
IDEA Confusion

• More on Confusion
• Mix The Three Operations
• XOR
• Addition mod 216
• Multiplication mod 216 1
• Each pair of these operations is
• Not distributive
• Not Associative

7
IDEA Confusion

• The M/A Structure
• 216 1 is Prime!
• So, the multiplication function for the IDEAis
  invertible
• We already know how to invert XOR
• Recall Addition mod a composite number is
  invertible

8
IDEA M/A
F1
F2
M
A
Z5
M
Z6
A
G2
G1
9
IDEA

• 8 Rounds, Then One Output Transform

Round i
K1
K4
K2
K5
K6
K3
10
IDEA Round i
M/A
11
IDEA Key Schedule

• Start with a key of 128 bits
• 8 Rounds
• 6 sub-keys per round
• Rotate 25 bits after each round
• k1,k6
• k7,,k12
• k13,,k18
• k49,,k52

12
IDEA Decryption

• Use same block-round machine
• Input ciphertext
• Output plaintext
• Different schedule for the keys

13
BlowFish

• Schneier
• Objectives
• Fast
• Compact runs in less than 5K of memory
• Simple to implement and analyze its strength
• Variable secure can give it larger keys

14
BlowFish

• Keys
• Four S-Boxes
• vary depending on the (sub) keys
• A form of auto-keying for sub-keys too
• Stallings book says Perhaps the most
  formidable conventional encryption algorithm in
  the book