DES: How to find the Inverse of the Initial Permutation? I am doing a DES problem by hand and I want to know how you calculate the inverse of the initial permutation. I know what the permutation is, but how do you find it based off the given 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.