Data Packing

I was doing some random experiments using assembly and C. This is a simple example in packing 2 numbers inside a register in assembly. Let’s assume

$latex al < 2^{5}&bg=232225&fg=bbbbbb&s=4$
$latex bl < 2^{3}&bg=232225&fg=bbbbbb&s=4$

We want to store these 2 values inside the dl register. The dl register is of 8 bits, so 5 + 3 = 8 bits

Packing

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; al < 2 ^ 5, bl < 2 ^ 3
mov al, bl
shl dl, 3
or  dl, bl  ; xor would also work fine

Unpacking

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mov cl, dl   ; Make a copy of dl
and dl, 111b ; Extract lowest 3 bits (b)
mov bl, dl
shr cl, 3    ; Extract highest 5 bits (a)
mov al, cl

After experimenting with this I thought of coding a high level version of this using C. In here we pack and unpack a DWORD and a short inside a DWORD64 variable and build a string value and again unpack the DWORD64 value back to their original variables with the original values.

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#include <stdio.h>
#include <windows.h>
/*
* Author : @OsandaMalith
* Website: https://osandamalith.com
* Description: Simple experiment of packing and unpacking a DWORD and a short inside a DWORD64 variable and making a string value.
*/
int main() {
 
DWORD a = 1633971809;  // a < 2 ^ 32
short b =  29519;     // b < 2 ^ 16;
DWORD64 pack = 0, temp = 0;
printf("Value of a = %d\n",a);
printf("Value of b = %d\n\n",b);
puts("\t-=[Packing]=-\n");
pack = a;
pack <<= 0x10; // shift 2 bytes left
pack |= b;
printf("Packed data: 0x%I64X\n\n",pack);
printf("As a string: %s\n\n",&pack);
// clear the variables
a ^= a;
b ^= b;
 
puts("\t-=[Unpacking]=-\n");
temp = pack;
pack &= 0xffff; // Extract the lowest 16 bits
b = pack;
temp >>= 0x10; // Extract the highest 32 bits
a = temp;
printf("Value of a = %d\n",a);
printf("Value of b = %d\n\n",b);
return 0;
}

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