DES question

[akimp3]

Hi,

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

[mike]

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.

[akimp3]

Hi

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.

akimp3

[mike]

You're welcome. That's what I'm here for.