CSE 5/7353 – January 25 th 2006

1. CSE 5/7353 – January 25th 2006 Cryptography

2. Conventional Encryption • Shared Key • Substitution • Transposition

3. 5 Types Cryptanalysis

4. Strength of Cipher • Unconditionally Secure • Computationally Secure

5. Steganography • List Types

6. General Cipher Characteristics • Key Size • Transposition / Substitution • Block / Stream • Avalanche Effect • Surviving Plain Text Structure – Attacks • Historical Uses • Practical Observations

7. Caesar Cipher

8. Caesar Cipher Characteristics • Key Size • Transposition / Substitution • Block / Stream • Avalanche Effect • Surviving Plain Text Structure – Attacks • Historical Uses • Practical Observations

9. Letter Substitution

10. Cipher Characteristics • Key Size • Transposition / Substitution • Block / Stream • Avalanche Effect • Surviving Plain Text Structure – Attacks • Historical Uses • Practical Observations

11. Play Fair Cipher

12. Play Fair Cipher Characteristics • Key Size • Transposition / Substitution • Block / Stream • Avalanche Effect • Surviving Plain Text Structure – Attacks • Historical Uses • Practical Observations

13. Vigenere Cipher

14. Cipher Characteristics • Key Size • Transposition / Substitution • Block / Stream • Avalanche Effect • Surviving Plain Text Structure – Attacks • Historical Uses • Practical Observations

15. Vernam Cipher

16. Vernam Cipher Characteristics • Key Size • Transposition / Substitution • Block / Stream • Avalanche Effect • Surviving Plain Text Structure – Attacks • Historical Uses • Practical Observations

17. Transposition Ciphers

18. Transposition Cipher Characteristics • Key Size • Transposition / Substitution • Block / Stream • Avalanche Effect • Surviving Plain Text Structure – Attacks • Historical Uses • Practical Observations

19. Rotor Machines

20. Rotor Cipher Characteristics • Key Size • Transposition / Substitution • Block / Stream • Avalanche Effect • Surviving Plain Text Structure – Attacks • Historical Uses • Practical Observations

21. Shannon

22. Shannon • Diffusion • Plain Text “Smearing” • Not Permutation • Confusion • Key Obfuscation

23. Feistel Cipher

24. Fiestel Cipher Characteristics • Key Size • Transposition / Substitution • Block / Stream • Avalanche Effect • Surviving Plain Text Structure – Attacks • Historical Uses • Practical Observations

25. Modern Ciphers

26. DES • Currently the most widely used block cipher in the world • IBM’s LUCIFER was the precursor • One of the largest users of the DES is the banking industry, particularly with EFT • Although the standard is public, the design criteria used are classified

27. DES Security • Recent analysis has shown that DES is well designed (diffusion & confusion) • Rapid advances in computing speed though have rendered the 56 bit key susceptible to exhaustive key search • 1999 in 22hrs! • 3 DES • DES also theoretically broken using Differential or Linear Cryptanalysis • In practice, unlikely to be a problem yet

28. Overview of DES Encryption • Basic process consists of: • An initial permutation (IP) • 16 rounds of a complex key dependent calculation F • A final permutation, being the inverse of IP

29. Initial permutation Round 1 L R i – 1 i – 1 Round 2 K F 56-bit i key … + Round 16 L R i i Final permutation • 64-bit key (56-bits + 8-bit parity) • 16 rounds • Each Round

30. DES Cipher Characteristics • Key Size • Transposition / Substitution • Block / Stream • Avalanche Effect • Surviving Plain Text Structure – Attacks • Historical Uses • Practical Observations

31. Advanced Encryption Standard AES

32. Origins of AES • In 1999, NIST issued a new standard that said 3DES should be used • 168-bit key length • Algorithm is the same as DES • 3DES had drawbacks • Algorithm is sluggish in software • Only uses 64-bit block size

33. Origins of AES (Cont’d) • In 1997, NIST issued a CFP for AES • security strength >= 3DES • improved efficiency • must be a symmetric block cipher (128-bit) • key lengths of 128, 192, and 256 bits

34. Origins of AES (cont’d) • First round of evaluation • 15 proposed algorithms accepted • Second round • 5 proposed algorithms accepted • Rijndael, Serpent, 2fish, RC6, and MARS • Final Standard - November 2001 • Rijndael selected as AES algorithm

35. The AES Cipher • Block length is 128 bits • Key length is 128, 192, or 256 bits • NOT a Feistel structure • Processes entire block in parallel during each round using substitutions and permutations • The key that is provided as input is expanded • Array of forty-four 32-bit words (w[i]) • Four distinct words serve as round key (128 bits)

36. Decryption • Not identical to encryption • Equivalent structure exists • May need different implementations if encryption and decryption are needed • Quite often only encryption needed • Digest

37. AES Cipher Characteristics • Key Size • Transposition / Substitution • Block / Stream • Avalanche Effect • Surviving Plain Text Structure – Attacks • Historical Uses • Practical Observations