4003-482-01/4005-705-01 Cryptography
Chapter 3. Data Encryption Standard
Lecture Notes

Block Cipher Operation

Cipher	Block Size	Key Size
DES	64 bits	56 bits
Triple-DES	64 bits	168 bits
	64 bits	112 bits
AES	128 bits	128 bits
	128 bits	192 bits
	128 bits	256 bits

Block Cipher Architecture

Feistel Networks

• A Feistel network is a way to turn a one-way (noninvertible) function F into a two-way (invertible) function

  Forward direction (e.g., encryption):		Reverse direction (e.g., decryption):
  L1 = R0 R1 = L0 xor F(R0)		R0 = L1 L0 = R1 xor F(R0)

The Data Encryption Standard (DES)

• Data Encryption Standard (DES). Federal Information Processing Standards Publication 46-3, October 25, 1999. http://csrc.nist.gov/publications/fips/fips46-3/fips46-3.pdf

• Based on an iterated Feistel network

• Principal concepts
  • 16 rounds
  • The round function F contains:
    • Permutations
    • Substitutions -- 8 different S-boxes
    • Round key addition
  • Round keys derived from block cipher key using a key schedule
  • Initial and final permutations
  • Decryption the same as encryption with the round keys in reverse order

Triple-DES

• Defined in FIPS PUB 46-3

• Let E_K(P) denote DES encryption of plaintext P with key K

• Let D_K(C) denote DES decryption of ciphertext C with key K

• Keying Option 1, 168-bit key consisting of three 56-bit keys K1, K2, K3
  • Encryption: C = E_K3(D_K2(E_K1(P)))
  • Decryption: P = D_K3(E_K2(D_K1(C)))

• Keying Option 2, 112-bit key consisting of two 56-bit keys K1, K2
  • Encryption: C = E_K1(D_K2(E_K1(P)))
  • Decryption: P = D_K1(E_K2(D_K1(C)))

• Keying Option 3, 56-bit key K1
  • Encryption: C = E_K1(D_K1(E_K1(P)))
  • Decryption: P = D_K1(E_K1(D_K1(C)))
  • For backwards compatibility with regular DES

PRESENT

• A. Bogdanov, L. Knudsen, G. Leander, C. Paar, A. Poschmann, M. Robshaw, Y. Seurin, and C. Vikkelsoe. PRESENT: an ultra-lightweight block cipher. In P. Paillier and I. Verbauwhede, eds. 9th International Workshop on Cryptographic Hardware and Embedded Systems (CHES 2007), Springer LNCS 4727, 2007.

• An "ultralightweight" block cipher
  • Intended for implementation in hardware on small devices (smart cards, RFID chips)
  • 64-bit block
  • 80-bit or 128-bit key

• Based on a substitution-permutation network instead of a Feistel network

Threefish

• The Skein hash function, a SHA-3 finalist candidate, is built on top of the Threefish block cipher

• Threefish versions
  • Threefish-256: 256-bit block, 256-bit key
  • Threefish-512: 512-bit block, 512-bit key
  • Threefish-1024: 1024-bit block, 1024-bit key

• Threefish's key schedule has an additional 128-bit "tweak" input, as well as the key

• Skein/Threefish specification: http://www.schneier.com/skein.pdf

Cryptography • 4003-482-01/4005-705-01 • Spring Quarter 2013

Course Page

Alan Kaminsky • Department of Computer Science • Rochester Institute of Technology • 4486 + 2220 = 6706

Home Page

Copyright © 2012 Alan Kaminsky. All rights reserved. Last updated 22-Mar-2012. Please send comments to ark­@­cs.rit.edu.