. SDES Mangler Function

In f_K the rightmost 4 bits are passed through unchanged, and the leftmost 4 bits are "mangled" by the non-invertible function F:

fK(L,R) = L XOR F(R,Ki), R    -- encrypt or decrypt

E/P = { 4, 1, 2, 3, 2, 3, 4, 1}

P4 = { 2, 4, 3, 1}

S0 =  1 0 3 2     S1 =  0 1 2 3
      3 2 1 0           2 0 1 3
      0 2 1 3           3 0 1 0
      3 1 3 2           2 1 0 3

n1n2n3n4  ->  Si[n1n4][n2n3]

Example:

R =  1010

E/P  0101 0101

K1 = 1010 0100

XOR  1111 0001

S0[11][11] -> 10    S1[01][00] -> 10

     1010

P4   0011