Cryptography Tutorials - Herong's Tutorial Notes
Dr. Herong Yang, Version 4.00
 This site Web Cipher - DES Algorithm Part:   1  2  3  4  (Continued from previous part...) DES algorithm: ```Input: T: 64 bits of clear text k1, k2, ..., k16: 16 round keys IP: Initial permutation FP: Final permutation f(): Round function Output: C: 64 bits of cipher text Algorithm: T' = IP(T), applying initial permutation (L0, R0) = T', dividing T' into two 32-bit parts (L1, R1) = (R0, L0 ^ f(R0, k1)) (L2, R2) = (R1, L1 ^ f(R1, k2)) ...... C' = (R16, L16), swapping the two parts C = FP(C'), applying final permutation ``` where ^ is the XOR operation. The round function f(R,k) is defined as: ```Input: R: 32-bit input data k: 48-bit round key E: Expansion permutation P: Round permutation s(): S boxes function Output R' = f(R,k): 32-bit output data Algorithm X = E(R), applying expansion permutation and returning 48-bit data X' = X ^ k, XOR with the round key X" = s(X'), applying S boxes function and returning 32-bit data R' = P(X"), applying the round permutation ``` The S boxes function s(X) is defined as: ```Input: X: 48-bit input data S1, S2, ..., S8: 8 S boxes - 4 x 16 tables Output: X' = s(X): 32-bit output data Algorithm: (X1, X2, ..., X8) = X, dividing X into 8 6-bit parts X' = (S1(X1), S2(X2), ..., S8(X8)) where Si(Xi) is the value at row r and column c of S box i with r = 2*b1 + b6 c = 8*b2 + 4*b3 + 2*b3 + b4 b1, b2, b3, b4, b5, b6 are the 6 bits of the Xi ``` DES cipher algorithm supporting tables: Initial Permutation - 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 ``` Final Permutation - FP: ```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 ``` Expansion permutation - E: ```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 ``` Round permutation - 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 ``` (Continued on next part...) Part:   1  2  3  4

Dr. Herong Yang, updated in 2007
Cryptography Tutorials - Herong's Tutorial Notes - Cipher - DES Algorithm