The hunt for getting a DES with plain BSD license w/o advertisement clause
==========================================================================

$Id$

This all feels very silly given that DES is about 30 years old and now
is deprecated.

Helpful documents on the way:

Schider's crypto wasn't that useful since it only told how to do DES,
not how to do des fast or how to not use DES. I find this to be a
common thread in the book, it explain each tool in great detail, but
not its limitations.

Dag Arne Osvik: Efficient Implementation of the Data Encryption Standard

Some threads on sci.crypto was also useful.

PC1 transformations
===================


Getting the PC1 bit mangling working was hard, I never got it to work.

Printning out the bit usage made me realize a lookup table could be
used since only 12 bits are used from the first half and 16 from the
second.

01110000 01110000 01110000 01110000 01111000 01111000 01111000 01111000 
00001111 00001111 00001111 00001111 00000111 00000111 00000111 00000111 

The pattern is getting more obvious if it's printed out where the bits
are coming from.

 8 16 24  -  -  -  -  - 
 7 15 23  -  -  -  -  - 
 6 14 22  -  -  -  -  - 
 5 13 21  -  -  -  -  - 
 4 12 20 28  -  -  -  - 
 3 11 19 27  -  -  -  - 
 2 10 18 26  -  -  -  - 
 1  9 17 25  -  -  -  - 

 -  -  - 60 56 48 40  - 
 -  -  - 59 55 47 39  - 
 -  -  - 58 54 46 38  - 
 -  -  - 57 53 45 37  - 
 -  -  -  - 52 44 36  - 
 -  -  -  - 51 43 35  - 
 -  -  -  - 50 42 34  - 
 -  -  -  - 49 41 33  - 

Only 3 bits-table is needed for the first half and 4 bits for the
second half because they are on diffrent shift offsets.

So to get the bitpattern bit-pattern

gen_pattern("pc1_c_3", 7,  [ 5, 13, 21 ], 0, 0x1000000);
gen_pattern("pc1_c_4", 15, [ 1, 9, 17, 25 ], 0, 0x1000000);
gen_pattern("pc1_d_3", 7, [ 49, 41, 33 ], 32, 0x1000000);
gen_pattern("pc1_d_4", 15, [ 57, 53, 45, 37 ], 32, 0x1000000);

PC2 transformations
===================

PC2 is also a table lookup, since it's a 24 bit field, I use 4 6-bit
lookup tables. Printing the reverse of the PC2 table reveal that some
of the bits are not used, namely (9, 18, 22, 25) from c and (7, 10,
15, 26) from d.

pc2 from c
----------

 5 24  7 16  6 10 20 
18  - 12  3 15 23  1 
 9 19  2  - 14 22 11 
 - 13  4  - 17 21  8 

pc2 from d
----------

51 35 31 52 39 45  - 
50 32  - 43 36 29 48 
 - 41 38 47 33 40 42 
49 37 30 46  - 34 44 

So we generate tables for that too.

gen_pattern("pc2_c_1", 63, [  5, 24,  7, 16,  6, 10 ], 0, 0x800000);
gen_pattern("pc2_c_2", 63, [ 20, 18, 12,  3, 15, 23 ], 0, 0x800000);
gen_pattern("pc2_c_3", 63, [  1,  9, 19,  2, 14, 22 ], 0, 0x800000);
gen_pattern("pc2_c_4", 63, [ 11, 13,  4, 17, 21,  8 ], 0, 0x800000);

gen_pattern("pc2_d_1", 63, [ 51, 35, 31, 52, 39, 45 ], 28, 0x800000);
gen_pattern("pc2_d_2", 63, [ 50, 32, 43, 36, 29, 48 ], 28, 0x800000);
gen_pattern("pc2_d_3", 63, [ 41, 38, 47, 33, 40, 42 ], 28, 0x800000);
gen_pattern("pc2_d_4", 63, [ 49, 37, 30, 46, 34, 44 ], 28, 0x800000);


SBOX transformations
====================

The SBOX transformations are 6 bit to 4 bit transformations.

Here I grew tired and used Richard Outerbridge SBOXes. Thank you
Richard.