Encrypting using Caesar Cipher C++

Question

So I have run a problem while trying to convert text from a file into a Caesar Cipher code. The content seem to be converted properly but near the end of the file it seem to dump some error at the end.

Question: What is the error causing to add the \FF to the end of the file?

Content of the Original File:

After Encryption :

as you can see from the picture I get the following added to the file \FF

this is the following code for Caesar Cipher...

void file_encription()
{
    std::string filename = "alpha.dat";
    ifstream inputFile( filename.c_str() );
    string plaintext;

    do
    {
      plaintext += inputFile.get();
    }
    while(inputFile);

            //string with the whole file saved...
    //string for the file plaintext

    std::string  &ciphertext = plaintext;

    std::int decipher;

    //Caesar Cipher code...
    std::cout << "please enter a number...";
        cin >> decipher;

        int shift = decipher % 26 

    for(int i = 0; i < ciphertext.size(); ++i)
    {
        if (islower(ciphertext[i]))
    {
            ciphertext[i] = (ciphertext[i] - 'a' + shift) % 26 + 'a';
    }
        else if (isupper(ciphertext[i]))
     {
            ciphertext[i] = (ciphertext[i] - 'A' + shift) % 26 + 'A';
         }
    }

        std::cout <<"your file was converted and saved as: decrypted_text.txt" << endl;
        ofstream finale("decrypted_text.txt");

            finale << ciphertext << endl;
        finale.close();
cin.get();
return BegginingPage();
}

Answer 1

You are also reading eof (-1 / 0xFF) with this here:

do
{
  plaintext += inputFile.get();
}
while(inputFile);

use:

std::ifstream t("file.txt");
std::string str((std::istreambuf_iterator<char>(t)),
             std::istreambuf_iterator<char>());

or

std::ifstream t("file.txt");
std::stringstream buffer;
buffer << t.rdbuf();
std::string str = buffer.str();

and btw you should call the file after encryption encrypted_text not decrypted_text ;)