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

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

3. 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))

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. Exercises Encrypt by hand
Key =
Plaintext =
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