Use OpenSSL to encrypt file contents in c++

Question

I am creating an application which encrypts some text and then it writes it to a file. The problem I am seeing is that when I run the encryption more than once, I am only able to decrypt the text I added firstly.

This is the code:

encrypt and decrypt functions:

std::vector<char> CryptCTR::encrypt(const char* textToEncrypt)
{
  size_t sizeToEncrypt = strlen(textToEncrypt);
  std::vector< char > encryptedText ( sizeToEncrypt );
  unsigned long long lengthProcessed = 0;
  do {
     auto maxCipherLen = sizeToEncrypt > std::numeric_limits<int>::max() ? std::numeric_limits<int>::max() : sizeToEncrypt; 
     encryptCTR(textToEncrypt, 
                &encryptedText[lengthProcessed], 
                maxCipherLen);
     lengthProcessed += maxCipherLen;
     sizeToEncrypt -= maxCipherLen;
  } while (sizeToEncrypt > 0);
  return encryptedText;
}
std::vector<char> CryptCTR::decrypt(const char* textToDecrypt/*, SecurityKey& key*/) 
{
  size_t sizeToDecrypt = strlen(textToDecrypt);
  unsigned long long lengthProcessed = 0;
  std::vector< char > recoveredText ( sizeToDecrypt );
  do
  {
     int maxPlainLen = sizeToDecrypt > std::numeric_limits<int>::max() ? std::numeric_limits<int>::max() : sizeToDecrypt;
     decryptCTR(textToDecrypt,
             &recoveredText[lengthProcessed],
             sizeToDecrypt/*, &key*/);
     lengthProcessed += maxPlainLen;
     sizeToDecrypt -= maxPlainLen;
  } while(sizeToDecrypt > 0);
  return recoveredText;
}

void CryptCTR::encryptCTR ( const char* pvBufferIn, char* pvBufferOut, unsigned long dwLength )
 {
    CRYPTO_ctr128_encrypt(
     (const unsigned char*)pvBufferIn,
     (unsigned char*)pvBufferOut,
     dwLength,
     (AES_KEY*)&m_aesSecurityKey.getKey().front(),
     state.ivec,
     state.ecount,
     &state.num,
     (block128_f)AES_encrypt);
}

void CryptCTR::decryptCTR ( const char* pvBufferIn, char* pvBufferOut, unsigned long dwLength)
{
  CRYPTO_ctr128_encrypt((const unsigned char*)pvBufferIn,
  (unsigned char*)pvBufferOut,
  dwLength,
  (AES_KEY*)&m_aesSecurityKey.getKey().front(),
  state.ivec,
  state.ecount,
  &state.num,
  (block128_f)AES_encrypt);
}

If I encrypt decrypt and then decrypt once it works. But the problem starts when I try to do this:

// ENCODING //
{
   for (int i=0;i<kStrVector.size();++i)
   {
      CryptCTR cryptCtr;
      //SecurityKey aesExampleKey ( std::vector< char > (1) );
      cryptCtr.initializeAesKey(/*&aesExampleKey*/);
      const char* textToEncrypt = kStrVector.at(i).c_str();
      auto output = cryptCtr.encrypt(textToEncrypt/*, aesExampleKey*/);
      std::ofstream out("encoded-append.dat", std::ios::app);
      for (const auto &e : output)
         out << e;
      out.close();
   }
}

// DECODING //
{
   CryptCTR cryptCtr;
   std::ifstream t("encoded-append.dat");
   std::stringstream buffer;
   buffer << t.rdbuf();
   std::string ss = buffer.str();
   //SecurityKey aesExampleKey2 ( std::vector< char > (1) );
   cryptCtr.initializeAesKey(/*&aesExampleKey2*/);
   //initializeAesKey ( (const unsigned char*)CRYPT_AES_KEY, sizeof(CRYPT_AES_KEY) * 8, &aesExmapleKey2 );

   auto output = cryptCtr.decrypt(buffer.str().c_str()/*, aesExampleKey2*/);
   std::string s2(output.begin(), output.end());
   std::ofstream out("decoded-append.dat");
   out << s2;
   out.close();
}

Then the program is able to decrypt only the text written in the first iteration.

Could you please tell me what I have wrong in my code?

Thanks in advance and regards

Answer 1

Shot in the dark guess, but encryption is typically binary. As such your encrypted output will have null char bytes inside it. So if you treat the encrypted data as a null terminated string, you risk losing data. Hence, your decrypt function should taking in a vector instead of a null terminated string.

Also, don't forget to pass ios_base::binary into the constructor of your ifstream and ofstream constructors for reading/writing to your .dat file. This will be important if you want your code to run reliably on Windows.