Introduction to Symmetric Block Cipher

1
Introduction to Symmetric Block Cipher

• Jing Deng
• Based on
• Prof. Rick Hans Lecture Slides
• Dr. Andreas Steffens Security Tutorial

2
Cryptography
plaintext
ciphertext
plaintext

• Encryption algorithm also called a cipher
• Cryptography has evolved so that modern
  encryption and decryption use secret keys
• Cryptographic algorithms can be openly published
• Only have to protect the keys

3
Symmetric-Key Cryptography
Secure Key Distribution

• Both sender and receiver keys are the same KAKB
• The keys must be kept secret and securely
  distributed
• Thus, also called Secret Key Cryptography
• Data Encryption Standard (DES)

4
Cryptanalysis

• Brute force try every key
• Ciphertext-only attack
• Attacker knows ciphertext of several messages
  encrypted with same key (but doesnt know
  plaintext).
• Possible to recover plaintext (also possible to
  deduce key) by looking at frequency of ciphertext
  letters
• Known-plaintext attack
• Attackers observes pairs of plaintext/ciphertext
  encrypted with same key.
• Possible to deduce key and/or devise algorithm to
  decrypt ciphertext.

5
Cryptanalysis (2)

• Chosen-plaintext attack
• Attacker can choose the plaintext and look at the
  paired ciphertext
• Attacker has more control than known-plaintext
  attack and may be able to gain more info about
  key
• Adaptive Chosen-Plaintext attack
• Attacker chooses a series of plaintexts, basing
  the next plaintext on the result of previous
  encryption
• Examples
• Differential cryptanalysis DES is resistant it
• Linear cryptanalysis
• Cryptanalysis attacks often exploit the
  redundancy of natural language
• Lossless compression before encryption removes
  redundancy

6
Examples

• Simple and non-secure ciphers
• Shift Cipher Caesar Cipher
• Affine Cipher
• Vigenere Cipher
• Hill Cipher
• Information-secure cipher
• One-Time Pad

7
Confusion and Diffusion

• Terms courtesy of Claude Shannon, father of
  Information Theory
• Confusion Substitution
• a -gt b
• Caesar cipher
• Diffusion Transposition or Permutation
• abcd -gt dacb
• DES

8
Confusion and Diffusion (2)

• Confusion a classical Substitution Cipher

Courtesy Andreas Steffen

• Modern substitution ciphers take in N bits and
  substitute N bits using lookup table called
  S-Boxes

9
Confusion and Diffusion (3)

• Diffusion a classical Transposition cipher

Courtesy Andreas Steffen

• modern Transposition ciphers take in N bits and
  permute using lookup table called P-Boxes

10
Block Cipher

• Divide input bit stream into n-bit sections,
  encrypt only that section, no dependency/history
  between sections

Courtesy Andreas Steffen

• In a good block cipher, each output bit is a
  function of all n input bits and all k key bits

11
Example DES

• Data Encryption Standard (DES)
• Encodes plaintext in 64-bit chunks using a 64-bit
  key (56 bits 8 bits parity)
• Uses a combination of diffusion and confusion to
  achieve security
• Was cracked in 1997
• Parallel attack exhaustively search key space
• Decryption in DES its symmetric! Use KA again
  as input and then the same keys except in reverse
  order

12
Example DES (2)

• DES
• 64-bit input is permuted
• 16 stages of identical operation
• differ in the 48-bit key extracted from 56-bit
  key - complex
• R2 R1 is encrypted with K1 and XORd with L1
• L2R1,
• Final inverse permutation stage

13
Example DES (3)
14
Beyond DES

• Triple-DES put the output of DES back as input
  into DES again with a different key, loop again
  356 168 bit key
• Advanced Encryption Standard (AES)
• Requirements
• shall be designed so that the key length may be
  increased as needed.
• block size n 128 bits, key size k 128, 192,
  256 bits
• Candidates MARS, twofish, RC6, Serpent, Rijndael
• successor (Rijndael)

15
Encryption Mode (ECB)

• Electronic Code Book (ECB) mode for block
  ciphers of a long digital sequence

• Vulnerable to replay attacks if an attacker
  thinks block C2 corresponds to amount, then
  substitute another Ck
• Attacker can also build a codebook of ltCk,
  guessed Pkgt pairs

16
Encryption Mode (CBC)

• Cipher Block Chaining (CBC) mode for block
  ciphers

• Inhibits replay attacks and codebook building
  identical input plaintext Pi Pk wont result in
  same output code due to memory-based chaining
• IV Initialization Vector use only once

17
Stream Cipher

• Stream ciphers

• Rather than divide bit stream into discrete
  blocks, as block ciphers do, XOR each bit of your
  plaintext continuous stream with a bit from a
  pseudo-random sequence
• At receiver, use same symmetric key, XOR again to
  extract plaintext

18
Encryption Mode (OFB)