Simplified DES Cryptography and Network Security Reference: Sec 3.1 of Stallings Text

Components of DES Key generation Bit permutations S-boxes Bit Rotations and exclusive-or operations

Simplified DES Key: 10 bits Plaintext and ciphertext: 8 bits Generates two 8-bit keys Plaintext and ciphertext: 8 bits Encryption: ctxt = IP-1(fk(SW(fk(IP(ptxt),K1)),K2)) Decryption: ptxt = IP-1(fk(SW(fk(IP(ptxt),K2)),K1))

Key Generation Permute 10-bit key (using P10) Split into two 5-bit parts K1 Rotate both parts once, concatenate, then apply permutation P8 K2 Rotate both parts 3 times, concatenate, then apply permutation P8

Encryption Apply permutation IP to plaintext Apply a function f to update the left half of the result using K1 Switch left and right halves of the result Apply a function f to update the left half of the result using K2 Apply permutation IP-1 on the result to produce ciphertext

Decryption Apply permutation IP to the ciphertext Apply a function f to update the left half of the result using K2 Switch left and right halves of the result Apply a function f to update the left half of the result using K1 Apply permutation IP-1 on result to produce the plaintext

f(SK, input) Only the left four bits (L) of input will be affected; the right four bits (R) will be used but not affected L = L ^ F(R, SK) ^ means exclusive or F(R, SK) is a function that takes a 4-bit string and an 8-bit key

F(R,SK) Permute/expand R using EP to obtain R’ R’ = R’ ^ SK Becomes 8 bits R’ = R’ ^ SK Suppose R’ = b7b6b5b4b3b2b1b0 Use bit-pairs as indices for the S-boxes S0[b7b4][b6b5] S1[b3b0][b2b1] Permute using P4

Manipulating Bits in a Program Important Bit-wise operations: and &, or |, not ~, xor ^, shifts << >> Useful functions Retrieve a bit Set a bit Rotate/shift left

Exercises Encrypt by hand Key = 1011001001 Plaintext = 10111110 Use the posted SDES program to observe the effect on the ciphertext Of a bit change in the key Of a bit change in the plaintext