simple text file encryption based on a key

Question

I am trying to implement a simple text file encryption technique and I am using the following code to do so. The code is not written by me, I just googled and got it. The encryption technique seems to be pretty simple, concise and easy to implement. I see that it has only one function that can do the encryption and the decryption on the fly. Just pass the key it will do the trick. However, I just wanted to know, is it possible for me to check if the key is passed by the user is correct or not. Currently it will just encrypt / decrypt the text file based on the passed key. But there is no mechanism to check if we are decrypting with correct key or not. Whatever the key we pass, it will get decrypted, but it will not be readable. Any idea how to tackle this problem..?

procedure TEnDeCrypt.EnDecryptFile(pathin, pathout: string; Chave: Word);
var
  InMS, OutMS: TMemoryStream;
  cnt: Integer;
  C: byte;
begin
  InMS  := TMemoryStream.Create;
  OutMS := TMemoryStream.Create;
  try
    InMS.LoadFromFile(pathin);
    InMS.Position := 0;
    for cnt := 0 to InMS.Size - 1 DO
      begin
        InMS.Read(C, 1);
        C := (C xor not (ord(chave shr cnt)));
        OutMS.Write(C, 1);
      end;
    OutMS.SaveToFile(pathout);
  finally
    InMS.Free;
    OutMS.Free;
  end;
end;

Answer 1

Generate a checksum on the plain text using a hashing algorithm and store it at the beginning of the encrypted file.

You can verify the key by hashing the decrypted text and ensure that the checksum matches.

If you use a strong hashing algorithm such as SHA256 to generate the checksum, it will be difficult for the user to automate a brute force attack because it will be computationally expensive.

To ensure that the file is intact, you may also wish to store a checksum on the encrypted file and store it in the file header as well. Otherwise, there will be no way to differentiate an invalid password from a truncated file.

I typically use the Blowfish encryption algorithm, which is available for Delphi from multiple sources. Blowfish has no known weaknesses and is fairly compact and fast.