My code

… was written in C++11 and can be compiled with G++, Clang++, Visual C++. You can download it, as well as the input data, too. Or just jump to my GitHub repository.

The code contains #ifdefs to switch between the original problem and the Hackerrank version.
Enable #ifdef ORIGINAL to produce the result for the original problem (default setting for most problems).

       
#include <vector>
       
#include <iostream>
       
 
       
int main()
       
{
       
  // read encrypted message from STDIN
       
  std::vector<unsigned char> encrypted;
       
 
       
//#define ORIGINAL

       
#ifdef ORIGINAL
       
  // read numbers in CSV format, e.g. 74,2,999,1,...
       
  while (true)
       
  {
       
    unsigned int current;
       
    std::cin >> current;
       
 
       
    // no more data
       
    if (!std::cin)
       
      break;
       
 
       
    encrypted.push_back(current);
       
 
       
    // skip comma
       
    std::cin.get();
       
  }
       
 
       
#else
       
 
       
  // text length
       
  unsigned int size;
       
  std::cin >> size;
       
  encrypted.clear();
       
  // read text from STDIN
       
  while (size--)
       
  {
       
    unsigned int current;
       
    std::cin >> current;
       
    encrypted.push_back(current);
       
  }
       
#endif
       
 
       
  // try all possible keys
       
  for (unsigned char i = 'a'; i <= 'z'; i++)
       
    for (unsigned char j = 'a'; j <= 'z'; j++)
       
      for (unsigned char k = 'a'; k <= 'z'; k++)
       
      {
       
        // current key
       
        const unsigned char key[] = { i, j, k };
       
 
       
        // apply XOR to all bytes
       
        std::vector<unsigned char> decoded;
       
        for (size_t pos = 0; pos < encrypted.size(); pos++)
       
          decoded.push_back(encrypted[pos] ^ key[pos % 3]);
       
 
       
        // classify result
       
        bool valid = true;
       
        for (auto d : decoded)
       
        {
       
          // only allow:
       
          // 0-9 a-z A-Z ;:,.'?-!() and space
       
          valid  = (d >= '0' && d <= '9');
       
          valid |= (d >= 'a' && d <= 'z');
       
          valid |= (d >= 'A' && d <= 'Z');
       
          valid |= (d == ' ' || d == ',' || d == '.' || d == '?' || d == '!');
       
          valid |= (d == ';' || d == '-' || d == '\'');
       
          valid |= (d == '(' || d == ')');
       
 
       
          // reject any invalid character
       
          if (!valid)
       
            break;
       
        }
       
 
       
        // try next key
       
        if (!valid)
       
          continue;
       
 
       
        // bonus feature: show decoded text
       
//#define SHOW_DECODED

       
#ifdef  SHOW_DECODED
       
        for (auto d : decoded)
       
          std::cout << d;
       
        std::cout << std::endl;
       
#endif
       
 
       
#ifdef ORIGINAL
       
        // we found the key, now add all ASCII codes of the decrypted text
       
        // add ASCII codes
       
        unsigned int asciiSum = 0;
       
        for (auto d : decoded)
       
          asciiSum += d;
       
        std::cout << asciiSum << std::endl;
       
        return 0;
       
#else
       
        // print key
       
        std::cout << i << j << k << std::endl;
       
#endif
       
      }
       
 
       
  return 0;
       
}

This solution contains 15 empty lines, 20 comments and 10 preprocessor commands.