DES Algorithm

1. DES Algorithm Provides Confidentiality

2. Conventional Encryption Algorithms • Data Encryption Standard (DES) • The most widely used encryption scheme • The algorithm is reffered to the Data Encryption Algorithm (DEA) • DES is a block cipher • The plaintext is processed in 64-bit blocks • The key is 56-bits in length

3. Data Encryption Standard (DES) • The algorithm has 16 rounds. Each round has the following architecture: Li and Ri are each 32-bit long strings

4. DES • The overall processing at each iteration: • Li= Ri-1 • Ri = Li-1 F(Ri-1, Ki) • Concerns about: • The algorithm and the key length (56-bits)

7. X=IP(M) Upper Lower

8. DES • Before any rounds, the plaintext bits are permuted using an initial permutation. • Hence, at the end of the 16 rounds the inverse permutation is applied. • The initial permutation is public knowledge

9. DES • The key argument (J) is 48 bits. • The text argument needs to be expanded to 48 bits.

10. DES: Expansion Function Added • The 32-bits of R_I are permuted and 16 of them are repeated twice to obtain a 48 bit string.

11. DES Round Structure column

12. DES: S Blocks. • S blocks takes in as input 6-bit arguments and outputs four bits. • This is the substitution part of the cipher. • Each S block has a different functionality as defined by the corresponding tables.

15. DES • After substitution, the function output is now 32 bits and it goes through a fixed permutation. • Thus we perform “confusion” and “diffusion” steps in each round.

16. DES: Key generation for each round • The parity bits are stripped away. • The bits are permuted by PC-1 • LS_I represents cyclic shift by one position if I=1, 2, 9, or 16; otherwise shift by 2

17. Key Generation (Discard each 8th bit) Discard these

18. DES Permuted Choice Two (PC-21) Permuted Choice One (PC-1) • PC-2 converts 56 bits into 48 bits

19. DES • The overall effect is to pick at each round 48 of the 56 bits and permute the selected bits. Thus for each round one can perform a table look up to calculate K_I.

20. Substitution Boxes S • have eight S-boxes which map 6 to 4 bits • each S-box is actually 4 little 4 bit boxes • outer bits 1 & 6 (row bits) select one rows • inner bits 2-5 (col bits) are substituted • result is 8 lots of 4 bits, or 32 bits • row selection depends on both data & key • feature known as autoclaving (autokeying) • example:S(18 09 12 3d 11 17 38 39) = 5fd25e03

21. DES Key Schedule • forms subkeys used in each round • consists of: • initial permutation of the key (PC1) which selects 56-bits in two 28-bit halves • 16 stages consisting of: • selecting 24-bits from each half • permuting them by PC2 for use in function f, • rotating each half separately either 1 or 2 places depending on the key rotation schedule K

22. DES Decryption • decrypt must unwind steps of data computation • with Feistel design, do encryption steps again • using subkeys in reverse order (SK16 … SK1) • note that IP undoes final FP step of encryption • 1st round with SK16 undoes 16th encrypt round • …. • 16th round with SK1 undoes 1st encrypt round • then final FP undoes initial encryption IP • thus recovering original data value

23. Time to break a code (106 decryptions/µs)

24. Triple DEA • Use three keys and three executions of the DES algorithm (encrypt-decrypt-encrypt) • C = ciphertext • P = Plaintext • EK[X] = encryption of X using key K • DK[Y] = decryption of Y using key K • Effective key length of 168 bits C = EK3[DK2[EK1[P]]]

25. Triple DEA

26. Other Symmetric Block Ciphers • International Data Encryption Algorithm (IDEA) • 128-bit key • Used in PGP • Blowfish • Easy to implement • High execution speed • Run in less than 5K of memory

27. Other Symmetric Block Ciphers • RC5 • Suitable for hardware and software • Fast, simple • Adaptable to processors of different word lengths • Variable number of rounds • Variable-length key • Low memory requirement • High security • Data-dependent rotations • Cast-128 • Key size from 40 to 128 bits • The round function differs from round to round

28. Cipher Block Modes of Operation • Cipher Block Chaining Mode (CBC) • The input to the encryption algorithm is the XOR of the current plaintext block and the preceding ciphertext block. • Repeating pattern of 64-bits are not exposed

30. Location of Encryption Device • Link encryption: • A lot of encryption devices • High level of security • Decrypt each packet at every switch • End-to-end encryption • The source encrypt and the receiver decrypts • Payload encrypted • Header in the clear • High Security: Both link and end-to-end encryption are needed (see Figure 2.9)

32. Key Distribution • A key could be selected by A and physically delivered to B. • A third party could select the key and physically deliver it to A and B. • If A and B have previously used a key, one party could transmit the new key to the other, encrypted using the old key. • If A and B each have an encrypted connection to a third party C, C could deliver a key on the encrypted links to A and B.

33. Key Distribution (See Figure 2.10) • Session key: • Data encrypted with a one-time session key.At the conclusion of the session the key is destroyed • Permanent key: • Used between entities for the purpose of distributing session keys

35. Recommended Reading • Stallings, W. Cryptography and Network Security: Principles and Practice, 2nd edition. Prentice Hall, 1999 • Scneier, B. Applied Cryptography, New York: Wiley, 1996 • Mel, H.X. Baker, D. Cryptography Decrypted. Addison Wesley, 2001