# lecture 6 DES part1.pdf

1 de Jun de 2023
1 de 20

### lecture 6 DES part1.pdf

• 1. Computer Security Prepared by: Assistant Prof. Eman Sanad Faculty of computers and Artificial Intelligence Cairo University 1
• 2. Lecture 6 • Feistel cipher • DES • DES encryption overview • DES Round structure Dr. Eman Sanad, faculty of computers and Artificial Intelligence , Cairo University 2
• 3. Feistel cipher cont… Example: The fifteenth round of encryption, corresponding to the second round of decryption. “prove that” • Suppose that the blocks at each stage are 32 bits (two 16-bit halves) • the key size is 24 bits. • at the end of encryption round fourteen, the value of the intermediate block (in hexadecimal) is DE7F03A6. LE 14 = DE7F and RE 14 = 03A6. • Also assume that the value of K 15 is 12DE52. • After round 15, we have LE 15 = 03A6 RE 15 = F(03A6, 12DE52) ⊕ DE7F. Dr. Eman Sanad, faculty of computers and Artificial Intelligence , Cairo University 3
• 4. Feisel cipher cont… Dr. Eman Sanad, faculty of computers and Artificial Intelligence , Cairo University 4
• 5. Feisel cipher cont… • assume that : LD 1 = RE 15 , RD 1 = LE 15 • prove that: LD 2 = RE 14 , RD 2 = LE 14 • start with: LD 1 = F(03A6, 12DE52) ⊕ DE7F RD 1 = 03A6 LD 2 = 03A6 = RE 14 , RD 2 = F(03A6, 12DE52) ⊕ [F(03A6, 12DE52) ⊕ DE7F] = DE7F = LE14 Dr. Eman Sanad, faculty of computers and Artificial Intelligence , Cairo University 5
• 6. Feisel cipher cont… Dr. Eman Sanad, faculty of computers and Artificial Intelligence , Cairo University 6
• 7. Data Encryption Standard (DES) • Most widely used block cipher in world • Adopted in 1977 by NBS (now NIST) • Encrypts 64-bit data using 56-bit key • Has widespread use • has been considerable controversy over its security Dr. Eman Sanad, faculty of computers and Artificial Intelligence , Cairo University 7
• 8. DES Encryption Overview Dr. Eman Sanad, faculty of computers and Artificial Intelligence , Cairo University 8
• 9. Initial Permutation IP • first step of the data computation • IP reorders the input data bits • quite regular in structure (easy in h/w) Example: IP(675a6967 5e5a6b5a) = (ffb2194d 004df6fb) Dr. Eman Sanad, faculty of computers and Artificial Intelligence , Cairo University 9
• 10. Dr. Eman Sanad, faculty of computers and Artificial Intelligence , Cairo University 10
• 11. Initial Permutation (IP) Dr. Eman Sanad, faculty of computers and Artificial Intelligence , Cairo University 11
• 12. DES Round Structure • uses two 32-bit L & R halves • As for any Feistel cipher can describe as: L i = R i–1 R i = L i–1 ⊕ F(R i–1 , K i ) • F takes 32-bit R half and 48-bit subkey: • expands R to 48-bits using perm E • adds to subkey using XOR • passes through 8 (S-boxes) to get 32-bit result • finally permutes using 32-bit perm P Dr. Eman Sanad, faculty of computers and Artificial Intelligence , Cairo University 12
• 13. Dr. Eman Sanad, faculty of computers and Artificial Intelligence , Cairo University 13
• 14. Single round of DES algorithm Dr. Eman Sanad, faculty of computers and Artificial Intelligence , Cairo University 14
• 15. 8 S-Boxes Dr. Eman Sanad, faculty of computers and Artificial Intelligence , Cairo University 15
• 16. Substitution Boxes S • have eight S-boxes which map 6 to 4 bits o each S-box is actually 4 little 4 bit boxes o outer bits 1 & 6 (row bits) select one row of 4 o inner bits 2-5 (col bits) are substituted o 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 Dr. Eman Sanad, faculty of computers and Artificial Intelligence , Cairo University 16
• 17. Substitution Boxes S Example: Dr. Eman Sanad, faculty of computers and Artificial Intelligence , Cairo University 17
• 18. Substitution Boxes S • The S-boxes are the core of DES in terms of cryptographic strength. They are the only nonlinear element in the algorithm and provide confusion. Dr. Eman Sanad, faculty of computers and Artificial Intelligence , Cairo University 18
• 19. Substitution Boxes S Dr. Eman Sanad, faculty of computers and Artificial Intelligence , Cairo University 19
• 20. Dr. Eman Sanad, faculty of computers and Artificial Intelligence , Cairo University 20 Thank you