#include <stdio.h>
#include <string.h>
#include <time.h>
#include <stdlib.h>

unsigned int MakeChecksum(unsigned char *buffer, int len)
{
  unsigned int result;

  result = 0;
  for (int i = 0; i < len; i = i + 1) {
    result = (result << 3 | result >> 0x1d) ^ buffer[i];
  }
  return result;
}

//0: [CHECKSUM] - 4 bytes
//[
//4:    [TIME] - 4 bytes
//8:    [PAYLOAD + HEADER LEN] - 1 byte 
//9:    [TYPE] - 1 byte
//10:   [PAYLOAD LEN] - 1 byte
//11:   [PAYLOAD]
//]

int main() { 
   unsigned char type = 0x2A;
   unsigned char payload[] = "g1mm3_th3_k3y";
   unsigned char payload_len = strlen(payload);
   
   unsigned char *flag_payload = malloc(payload_len);
   for (int i = 0; i < payload_len; ++i)
           flag_payload[i] = payload[i] ^ 42;
   
   unsigned char packet_len = payload_len + 7;
   time_t create_time = time(NULL);

   char *packet = calloc(payload_len + 0xb, sizeof(char));

   memcpy(&packet[11], flag_payload, payload_len);
   memcpy(&packet[10], &payload_len, 1);
   memcpy(&packet[9], &type, 1);
   memcpy(&packet[8], &packet_len, 1);
   memcpy(&packet[4], &create_time, 4);

   int crc = MakeChecksum(packet + 4, packet_len);
   memcpy(packet, &crc, 4);
   
   // Output
   fwrite(packet, 1, payload_len + 0xb, stdout);

   free(packet);
   free(flag_payload);
}