View Full Version : DES question

March 15th, 2005, 17:39

anyone knows why the decryption function of DES is the same as the encryption function exept the key order?
I have to find the response (mathematical) of this question. I have looked in Handbook of Applied Cryptography, Applied Cryptography,Cryptography theory and practice and the Internet but i hav'nt found the response.

Thanks in advance

March 15th, 2005, 18:47
It's a Feistel cipher. The Feistel construction splits a message into two pieces, computes a keyed nonlinear function on one half and xors the result to the other half, then switches them. That's called a "round".

If you xor the same thing in twice, it cancels out. So they just reverse the key order to cancel each step out, and you get the plaintext back.

March 16th, 2005, 14:04

Thank you very much. You always help me to learn new things in cryptography. I just want to thank you for all the things you have done for me.


March 19th, 2005, 03:07
You're welcome. That's what I'm here for.