Hexadecimal Multiplication in C (simple)

// function for multiplying two big number in big endian

// big endian means, the number in a first index is the most significant byte.

static void multiply(const unsigned char *a, int la, const unsigned char *b, int lb, unsigned char *mul, int *lmul)

{

unsigned char *c = NULL;

unsigned char *temp = NULL;

int i,j,k=0,x=0,y;

long int r=0;

long sum = 0;

c = (unsigned char*)malloc(la+lb);

temp = (unsigned char*)malloc(la*lb*2);

la--;

lb--;

for(i=lb;i>=0;i--){

r=0;

for(j=la;j>=0;j--){

temp[k++] = (b[i]*a[j] + r)%256;

r = (b[i]*a[j]+r)/256;

}

temp[k++] = r;

x++;

for(y = 0;y<x;y++){

temp[k++] = 0;

}

}

k=0;

r=0;

for(i=0;i<la+lb+2;i++){

sum =0;

y=0;

for(j=1;j<=lb+1;j++){

if(i <= la+j){

sum = sum + temp[y+i];

}

y += j + la + 1;

}

c[k++] = (sum+r) %256;

r = (sum+r)/256;

}

c[k] = r;

j=0;

while(c[k-1]==0)

k--;

for(i=k-1;i>=0;i--){

mul[j++]=c[i];

}

*lmul = j;

free(c);

free(temp);

}