DES: How to find the Inverse of the Initial Permutation?
The inverse of the Initial Permutation (IP) of DES is the Final Permutation (FP) (in the Standard (NIST FIPS 46-3) FP is called "IP-1").  Number the 64 bits of the input to IP from 1 to 64.  Subject them to IP, so that the 1st 8 bits of the output of IP are bits { 58, 50, 42, 34, 26, 18, 10, 2 } etc. of the input.  Renumber (not reorder) the bits of the output of IP from 1 to 64.  Treat that as the input to FP.