Permutation

[candexis]

Hi, well I need to permute a variable of 64 bits according to the following table, so could you please tell me how to accomplish that:

PC-1

57 49 41 33 25 17 9
1 58 50 42 34 26 18
10 2 59 51 43 35 27
19 11 3 60 52 44 36
63 55 47 39 31 23 15
7 62 54 46 38 30 22
14 6 61 53 45 37 29
21 13 5 28 20 12 4

you know this works as follow: if the variable that is goig to be permuted is
K = 00010011 00110100 01010111 01111001 10011011 10111100 11011111 11110001

Since the first entry in the table is "57", this means that the 57th bit of the original variable K becomes the first bit of the permuted variable K+. The 49th bit of the original variable becomes the second bit of the permuted variable. The 4th bit of the original variable is the last bit of the permuted variable, and this will be the permuted variable K+
K+ = 1111000 0110011 0010101 0101111 0101010 1011001 1001111 0001111

Thanks for your help.

[Banfa]

To start with your array of numbers only has 56 entries but assuming it has 64 then start with a 64 bit variable set to 0, for each number in turn locate the bit and coipy it to the right place

for instance the 3rd number (index 2) is 41

long long K = ....;
long long result = 0;

if ( K & (1<<(41-1)) != 0 )
{
result |= 1<<(3-1);
}

[candexis]

So, I have to write the code that you gave me 56 times, or there is a way to put this code in a repetitive way. Thanks for your help. Edwin Alvear

[candexis]

I mean do I have to do it like this:


if ( K & (d<<(57-1)) != 0 )
{
nuevo |= d<<(1-1);
}

if ( K & (d<<(49-1)) != 0 )
{
nuevo |= d<<(2-1);
}

if ( K & (d<<(41-1)) != 0 )
{
nuevo |= d<<(3-1);
}
. . . . . . . . . . . . . . . . ....... .

[Banfa]

More or less although it isn't clear where you got d from.

However you would be sensible to define an array of your bit positions as an array

char table[256] = {
    57 49 41 33 25 17 9
    /* ... more data */
}
 

The you can use a for loop to iterate through the table

nuevo = 0;
 
for(ix=0; ix<256; ix++)
{
    if ( K & (1<<(table[ix]-1)) != 0 )
    {
        nuevo |= 1<<ix;
    }
 

[candexis]

I will like to know whay the table must have 256 datas if the original one, only 56 datas, and also why the sentence for goes to 256. thanks

[Banfa]

Mistake :D

The loop has the same number of iterations as the table has entries and should really be

for(ix=0; ix<sizeof table/sizeof table[0]; ix++)
{
}
 

The table should have the number of entries you require (56 it appears).