ctf_seminar/tasks/reverse/server/khabalov.c

#include <time.h>
#include <stdio.h>
#include <stdlib.h>
#define _GNU_SOURCE
#include <string.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;
}


unsigned char *make_message(const unsigned char* s) {
    int len = strlen(s);
    unsigned char* message = (char*) calloc(1, len + 11);

    message[8] = len + 7;
    message[10] = len;

    message[9] = 0x2A;

    memcpy(message + 11, s, len);

    *((unsigned int*)message + 1) = time(0LL);
    
    *(unsigned int*) message = MakeChecksum(message + 4, len + 7);

    return message;
}

int main() {
    unsigned char message[] = "g1mm3_th3_k3y";

    size_t len = strlen(message);

    for (int i = 0; i < len; i++) {
        message[i] ^= 42;
    }

    fwrite(make_message(message), 1, strlen(message) + 11, stdout);

    return 0;
}