DES 구조

DES ( data encryption standard ) is a symetric encryption.

 This means that the encryption and decryption is done with the same key.

permutations

Permutations are just bit pattern replacements.
Here are the 10 bit permutations.

premutation	old pattern	new pattern
p10	9876543210	7583609124
p8	9876543210	47362501
p4	3210	2013
ip	76543210	62574031
ip_inv	76543210	47531602
ep	3210	03212103
sw	76543210	32107654

functions

Des has two functions f_k and F. These are shown graphically below.

f_k(L,R,SK) = ( L ^ F(R,SK),R )

s-boxes

These are simple two dimensional arrays of values.

We use data at runtime to determine what row and column to get. ex :

 static const uval S0[4][4] = 

{ { 1, 0, 3, 2 }, { 3, 2, 1, 0 }, { 0, 2, 1, 3 }, { 3, 1, 0, 3 } };  

simple DES

I am doing simple DES ( S-DES or 10 bit DES ).

I am working on 8 bit data blocks, with a 10 bit key. ( click on the images for larger versions )

Here is how generating subkeys works.

Here is how encryption works.

Here is how f_k works. The function F,  is embedded inside this

simple.c simple DES source code

real DES

The above example is S-DES, which is a scaled down version of DES. DES usually has a 56-bit key, and 16 48-bit subkeys are generated. It works on 64-bit input blocks. F acts on 32 bits not 4. The s-boxes are 4 by 16, not 4 by 4 ( using the first and last bit for row ).

The images were created using Graphviz

 

 

 

#include<stdio.h> #include<stdlib.h>  /*    Simple DES    I am implimenting what is called