A Comparative Study of New Block Ciphers

1
A Comparative Study of New Block Ciphers

• Min Zhuo
• ECE 575
• Data Security and Cryptography
• Mar 11, 1999

2
Introduction

• Block Cipher and DES
• Advanced Encryption Standard Program
• 21 algorithms submitted to NIST
• Twofish - submitted by Counterpane Co., USA
• Frog - submitted by TecApro Co., Costa Rica
• Mars - submitted by IBM, USA
• RC6 - submitted by RSA, USA

3
Project Objectives

• Study of four Block Ciphers
• Algorithm
• Characteristics
• Comparison
• Structure
• Operation
• Security
• Efficiency

4
Evaluation Criteria

• Security
• Cryptoanalysis
• Mathematical basis
• Cost/Efficiency
• Computational Efficiency
• Algorithm Setup
• Key Setup and Change
• Encryption and Decryption
• Memory Requirement
• Algorithm characteristic
• Flexibility, Simplicity
• Hardware and Software Suitability

5
Twofish Overview

• 128-bit Block Feistal network
• 16 rounds
• Key length 128 bits, 192 bits, 256 bits
• Pre- and Post-whitening
• Key-dependent S-box
• Key can be precomputed for efficiency

6
Twofish Algorithm

• Whitening
• increases the difficulty of key search attacks.
• MDS(maximum distance separable) Matrix
• PHT ( pseudo-hadamard transform)

• Steps
• Pre-Whitening
• 16 round function
• Undo last swap
• Post-whitening

7
Twofish Round Function Block Diagram

• Sbox2

8
Frog Overview

• an unorthodox structure, 8 iterations
• block size 8-128 bytes, key size 5-125 bytes
• only byte level XORs and byte level substition
• a secret internal key, 2304 bits - the heart
  of FROG

9
Frog Algorithm

• Internal Key
• 8 records for 8 iterations
• record
• xorBuf, 16 bits
• SubstPermu 256 bits
• bombPermu16 bites

10
Mars Overview

• 128-bits block Festial-3 network
• A variable key size, from 128 to 1248 bits
• working with 32 bits words
• Strong 32 bits operations
• XORS, additions, subtraction, table lookups,
• multiplication, both fixed and data-dependent
  rotation
• a mixed structure

11
Mars Algorithm
Key addition
Eight rounds of unkeyed forward mixing
Eight rounds of keyed forward mixing
Eight rounds of keyed forward transformation
Eight rounds of unkeyed backwards transformation
Key subtraction
12
RC6 Overview

• 128-bit Block Feistal-2 network
• four 32-bits working registers for Input/Output
• 20 rounds
• key size 16,24,32 bytes
• key expansion
• Pre and Post whitening

13
RC6 Algorithm

• B B S 0
• D D S 1
• for i 1 to 20 do
• t ( B x ( 2B 1 ) ) ltltlt 5
• u ( D x ( 2D 1 ) ) ltltlt 5
• A ( ( A ? t ) ltltlt u ) S 2i
• C ( ( C ? u ) ltltlt t ) S 2i 1
  
• (A, B, C, D) (B, C, D, A)
• A A S 42
• C C S 43

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Comparison of Algorithm
the number of chosen plaintext the number of
known plaintext
15
Comparison of Operations
16
Comparison of Encryption Speed
source NIST Preliminary test report the
test platform IBM-compatible PC
17
Comparison of Key Setup Speed
18
Comparison of RAM Required
19
Conclusion

• Frog large memory required, has been broken.
• Twofish simple operations, limited resource
  required, can be used in some resource-limited
  environment, such as smartcard.
• Mars and RC6 high security and high efficiency
  in 32 bit processor, but are ill-suitable for
  smardcards.