Lecture 3 - Modern Ciphers

Question 1

What are Stream ciphers and give some examples?

Answer 1

• Stream ciphers: encode one bit/byte at a time
  
  • Examples: Rot-n, Vigenère, RC4
• Most stream ciphers are essentially pseudorandom bitstream generators
  
  • Random bitstream (keystream) XORed with plaintext to produce ciphertext
• Many stream ciphers / PRNGs are based on Linear Feedback Shift Registers (LFSR)
• RC4 (Rivest Cipher 4)
  
  • Not LFSR-based

Question 2

What is Data Encryption Standard (DES), the Potential weaknesses of DES and what is Triple DES (DESede)?

Answer 2

• A symmetric block cipher (NIST, 1977)
• Block size 64 bits, key size 56 bits
• 16 rounds of substitutions and transpositions
• Potential weaknesses:
  
  • 56-bit key is too short
  • The internal design criteria (“S-boxes”) were not public. There was fear that they were designed to be breakable
• Triple DES (DESede)
  
  • Apply DES three times (encrypt, decrypt, encrypt) with two/three keys
  • Increased key size

Question 3

RSA:

Part 1: Why all these n, p, q, thi?

Part 2: How to ensure Efficiency of key generation?

Part 3: What does RSA security depend on?

Answer 3

• Part 1:
  
  • Some deep mathematical knowledge is required to understand them…
• Part 2:
  
  • We need to pick large primes (p, q)
  • Randomly pick some large odd numbers and test for primality
  • Prime numbers are not as scarce as you may think!
• Part 3:
  
  • Security: depends on difficulty of factorization
    
    • If someone can factorize n into p and q, then they can compute (thi), and with e (public) they can compute d
    • It is believed that factorizing large numbers is computationally a very difficult problem

Question 4

What are block Ciphers?

Answer 4

• Block ciphers: encode fixed size blocks (e.g. 64 bits)
  
  • Examples: DES, RSA
  • Can mimic stream ciphers
  • Wider applicability