1. Introduction to Computer Security ©2004 Matt Bishop Information Security Principles Assistant Professor Dr. Sana’a Wafa Al-Sayegh 1 st Semester 2008-2009 ITGD 2202 University of Palestine

2. Introduction to Computer Security ©2004 Matt Bishop Data Encryption Standard Overview of the DES Generation of Round Keys Encipherment DES Modes

3. Introduction to Computer Security ©2004 Matt Bishop Overview of the DES A block cipher: –encrypts blocks of 64 bits using a 64 bit key –outputs 64 bits of ciphertext A product cipher –basic unit is the bit –performs both substitution and transposition (permutation) on the bits Cipher consists of 16 rounds (iterations) each with a round key generated from the user-supplied key

4. Introduction to Computer Security ©2004 Matt Bishop Generation of Round Keys Round keys are 48 bits each

5. Introduction to Computer Security ©2004 Matt Bishop PC-1 PC-2

6. Introduction to Computer Security ©2004 Matt Bishop Key ASCII 12345678 A6501100101 910111213141516 B6601100110 1718192021222324 C6701100111 2526272829303132 D6801101000 3334353637383940 E6901101001 4142434445464748 F7001110000 4950515253545556 G7101110001 5758596061626364 H7201110010 C0 0000000 0111111 1111111 1111110 D0 1000011 0000001 1100011 0000000 PC-1

7. Introduction to Computer Security ©2004 Matt Bishop LSH

8. Introduction to Computer Security ©2004 Matt Bishop LSH Before After C0 0000000 0111111 1111111 1111110 D0 1000011 0000001 1100011 0000000 C0 0000000 1111111 1111111 1111100 D0 0000110 0000011 1000110 0000001

9. Introduction to Computer Security ©2004 Matt Bishop PC-2 C0 0000000 1111111 1111111 1111110 D0 0000110 0000011 1000110 0000001 100 00 00 00

10. Introduction to Computer Security ©2004 Matt Bishop Encipherment

11. Introduction to Computer Security ©2004 Matt Bishop IP 58 50 42 34 26 18 10 2 60 52 44 36 28 20 12 4 62 54 46 38 30 22 14 6 64 56 48 40 32 24 16 8 57 49 41 33 25 17 9 1 59 51 43 35 27 19 11 3 61 53 45 37 29 21 13 5 63 55 47 39 31 23 15 7 IP-1 40 8 48 16 56 24 64 32 39 7 47 15 55 23 63 31 38 6 46 14 54 22 62 30 37 5 45 13 53 21 61 29 36 4 44 12 52 20 60 28 35 3 43 11 51 19 59 27 34 2 42 10 50 18 58 26 33 1 41 9 49 17 57 25

12. Introduction to Computer Security ©2004 Matt Bishop The f Function

13. Introduction to Computer Security ©2004 Matt Bishop E BIT-SELECTION TABLE 32 1 2 3 4 5 4 5 6 7 8 9 8 9 10 11 12 13 12 13 14 15 16 17 16 17 18 19 20 21 20 21 22 23 24 25 24 25 26 27 28 29 28 29 30 31 32 1 P 16 7 20 21 29 12 28 17 1 15 23 26 5 18 31 10 2 8 24 14 32 27 3 9 19 13 30 6 22 11 4 25 S1 Column Number Row No. 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 0 14 4 13 1 2 15 11 8 3 10 6 12 5 9 0 7 1 0 15 7 4 14 2 13 1 10 6 12 11 9 5 3 8 2 4 1 14 8 13 6 2 11 15 12 9 7 3 10 5 0 3 15 12 8 2 4 9 1 7 5 11 3 14 10 0 6 13

14. Introduction to Computer Security ©2004 Matt Bishop DES Modes Electronic Code Book Mode (ECB) –Encipher each block independently Cipher Block Chaining Mode (CBC) –Xor each block with previous ciphertext block –Requires an initialization vector for the first one Encrypt-Decrypt-Encrypt Mode (2 keys: k, k) –c = DES k (DES k –1 (DES k (m))) Encrypt-Encrypt-Encrypt Mode (3 keys: k, k, k  ) –c = DES k (DES k (DES k  (m)))

15. Introduction to Computer Security ©2004 Matt Bishop CBC Mode Encryption  init. vector m1m1 DES c1c1  m2m2 c2c2 sent … … …

16. Introduction to Computer Security ©2004 Matt Bishop CBC Mode Decryption  init. vector c1c1 DES m1m1 … … …  c2c2 m2m2