Feb 21, 2009 at 3:48am
Hi everybody,
I am trying to write a code to multiply 2 unsigned characters within a certain field.
The multiplication works as follow:
Lets add unsigned char a and b
From the array L we get L[a] and L[b]!
add L[a] and L[b] using XOR
then look in the array E[L[a] xor L[b]]
That would be then the product of a and b in my defined field
for example: 0xb6*0x53
L[0xb6]=0xb1
L[0x53]=0x30
L[0xb6]+L[0x53]=0xe1
E[0x30]=0x36
For this I implemented the code below!
Trying to check whether my code is working I run the code and used printf but I got wiered answers at anytime!
Could you please help me to undestand the problem!
Best,
Dan
#include <stdlib.h>
#include <stdio.h>
unsigned char GFMUL(unsigned char , unsigned char);
int main()
{
unsigned char s; unsigned char l;unsigned char e;
s=0xb6;
l=0x53;
e= GFMUL(s,l);
printf("Result %x",e);
return 0;
}
unsigned char GFMUL(unsigned char , unsigned char )
{
unsigned char L[256]= {
0x00,0x19,0x01,0x32,0x02,0x1a,0xc6,0x4b,0xc7,0x1b,0x68,0x33,0xee,0xdf,0x03,
0x64,0x04,0xe0,0x0e,0x34,0x8d,0x81,0xef,0x4c,0x71,0x08,0xc8,0xf8,0x69,0x1c,0xc1,
0x7d,0xc2,0x1d,0xb5,0xf9,0xb9,0x27,0x6a,0x4d,0xe4,0xa6,0x72,0x9a,0xc9,0x09,0x78,
0x65,0x2f,0x8a,0x05,0x21,0x0f,0xe1,0x24,0x12,0xf0,0x82,0x45,0x35,0x93,0xda,0x8e,
0x96,0x8f,0xdb,0xbd,0x36,0xd0,0xce,0x94,0x13,0x5c,0xd2,0xf1,0x40,0x46,0x83,0x38,
0x66,0xdd,0xfd,0x30,0xbf,0x06,0x8b,0x62,0xb3,0x25,0xe2,0x98,0x22,0x88,0x91,0x10,
0x7e,0x6e,0x48,0xc3,0xa3,0xb6,0x1e,0x42,0x3a,0x6b,0x28,0x54,0xfa,0x85,0x3d,0xba,
0x2b,0x79,0x0a,0x15,0x9b,0x9f,0x5e,0xca,0x4e,0xd4,0xac,0xe5,0xf3,0x73,0xa7,0x57,
0xaf,0x58,0xa8,0x50,0xf4,0xea,0xd6,0x74,0x4f,0xae,0xe9,0xd5,0xe7,0xe6,0xad,0xe8,
0x2c,0xd7,0x75,0x7a,0xeb,0x16,0x0b,0xf5,0x59,0xcb,0x5f,0xb0,0x9c,0xa9,0x51,0xa0,
0x7f,0x0c,0xf6,0x6f,0x17,0xc4,0x49,0xec,0xd8,0x43,0x1f,0x2d,0xa4,0x76,0x7b,0xb7,
0xcc,0xbb,0x3e,0x5a,0xfb,0x60,0xb1,0x86,0x3b,0x52,0xa1,0x6c,0xaa,0x55,0x29,0x9d,
0x97,0xb2,0x87,0x90,0x61,0xbe,0xdc,0xfc,0xbc,0x95,0xcf,0xcd,0x37,0x3f,0x5b,0xd1,
0x53,0x39,0x84,0x3c,0x41,0xa2,0x6d,0x47,0x14,0x2a,0x9e,0x5d,0x56,0xf2,0xd3,0xab,
0x44,0x11,0x92,0xd9,0x23,0x20,0x2e,0x89,0xb4,0x7c,0xb8,0x26,0x77,0x99,0xe3,0xa5,
0x67,0x4a,0xed,0xde,0xc5,0x31,0xfe,0x18,0x0d,0x63,0x8c,0x80,0xc0,0xf7,0x70,0x07};
unsigned char E[256]={
0x01,0x03,0x05,0x0f,0x11,0x33,0x55,0xff,0x1a,0x2e,0x72,0x96,0xa1,0xf8,0x13,0x35,
0x5f,0xe1,0x38,0x48,0xd8,0x73,0x95,0xa4,0xf7,0x02,0x06,0x0a,0x1e,0x22,0x66,0xaa,
0xe5,0x34,0x5c,0xe4,0x37,0x59,0xeb,0x26,0x6a,0xbe,0xd9,0x70,0x90,0xab,0xe6,0x31,
0x53,0xf5,0x04,0x0c,0x14,0x3c,0x44,0xcc,0x4f,0xd1,0x68,0xb8,0xd3,0x6e,0xb2,0xcd,
0x4c,0xd4,0x67,0xa9,0xe0,0x3b,0x4d,0xd7,0x62,0xa6,0xf1,0x08,0x18,0x28,0x78,0x88,
0x83,0x9e,0xb9,0xd0,0x6b,0xbd,0xdc,0x7f,0x81,0x98,0xb3,0xce,0x49,0xdb,0x76,0x9a,
0xb5,0xc4,0x57,0xf9,0x10,0x30,0x50,0xf0,0x0b,0x1d,0x27,0x69,0xbb,0xd6,0x61,0xa3,
0xfe,0x19,0x2b,0x7d,0x87,0x92,0xad,0xec,0x2f,0x71,0x93,0xae,0xe9,0x20,0x60,0xa0,
0xfb,0x16,0x3a,0x4e,0xd2,0x6d,0xb7,0xc2,0x5d,0xe7,0x32,0x56,0xfa,0x15,0x3f,0x41,
0xc3,0x5e,0xe2,0x3d,0x47,0xc9,0x40,0xc0,0x5b,0xed,0x2c,0x74,0x9c,0xbf,0xda,0x75,
0x9f,0xba,0xd5,0x64,0xac,0xef,0x2a,0x7e,0x82,0x9d,0xbc,0xdf,0x7a,0x8e,0x89,0x80,
0x9b,0xb6,0xc1,0x58,0xe8,0x23,0x65,0xaf,0xea,0x25,0x6f,0xb1,0xc8,0x43,0xc5,0x54,
0xfc,0x1f,0x21,0x63,0xa5,0xf4,0x07,0x09,0x1b,0x2d,0x77,0x99,0xb0,0xcb,0x46,0xca,
0x45,0xcf,0x4a,0xde,0x79,0x8b,0x86,0x91,0xa8,0xe3,0x3e,0x42,0xc6,0x51,0xf3,0x0e,
0x12,0x36,0x5a,0xee,0x29,0x7b,0x8d,0x8c,0x8f,0x8a,0x85,0x94,0xa7,0xf2,0x0d,0x17,
0x39,0x4b,0xdd,0x7c,0x84,0x97,0xa2,0xfd,0x1c,0x24,0x6c,0xb4,0xc7,0x52,0xf6,0x01};
unsigned char a= 0x00,b=0x00;
int t = 0;
int i=0;
if (a == 0 || b == 0) return 0;
t = L[a] ^ L[b];
if (t > 255) t = t - 255;
return E[t];
}