Question on C code, how to make this code more efficient

Hi Guys,

I'm helping a friend making their C code more efficient for  encryption program. Do you guys have any ideas on what i can do/modify to make it more efficient and improve on the current code?

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#include <sys/time.h>

typedef int int32;
typedef unsigned char int8;

static void combine_it(int8  *output, int8  *input_1, const int8  *input_2, int32 length)
{
   int32 i = 0;

   for(i=0; i<length; i++)
   {
       output[i] = (int8)(input_1[i] ^ input_2[i]);
   }

   return;
}

static void itoa( int32 num, int8  *alpha, int32 radix )
{
   if( radix == 10 )
   {
       sprintf(alpha, "%i", num);
   }
   else if( radix == 16 )
   {
       sprintf(alpha, "%X", num);
   }
}

int8 *create_new_key(int8 modifier, const int8  *input_1, int32 length)
{
   int8  leading[3];
   int32 i_leading;
   int8 * temp_string = NULL;
   int8 * ret;
   int32 i = 0;

   itoa(modifier/2, leading, 10);
   i_leading = atoi(leading);
   temp_string = (int8 *) malloc(8);
   ret = (int8 *) malloc(length);
   memset(temp_string, 0, 8);
   temp_string[0] = 0; 

   if( (modifier+1)%2 == 0 ) {
       temp_string[0] = (int8)((i_leading<<4) + 8);
   }   
   else {
       temp_string[0] = (int8)(i_leading<<4);
   }   

   for(i=0; i<(length>>3); i++)
   {
       combine_it(ret+i*8, temp_string, input_1+i*8, 8);
   }   
   free(temp_string);
   return ret;
}   

// Do not change anything in main
// main only contains example input
// Change anything else
int main(int argc, char **argv) {

   // Example input data
   int8 data[32] = { 1, 2, 3, 4, 5, 6, 7, 8, 9,10,11,12,13,14,15,16,
                    17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32};
   int8 *resp = create_new_key(0x10, data, sizeof(data));
   free(resp);

   return 0;
}

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