how to store additional data in a text file apart from it's content - C++

Question

I am doing this small university project, where I have to create a console-based text editor with some features, and making files password protected is one of them. As I said, it's a university project for an introductory OOP course, so it doesn't need to be the most secure thing on planet. I am planning to use a simple Caesar cipher to encrypt my file.

The only problem is the password. I'll use the password as the encryption key and it will work, but the problem is handling the case where the password is wrong. If no checks are placed then it would just show gibberish, but I want to make it so that it displays a message in case of a wrong password.

The idea I have come up with is to somehow store the hash of the unencrypted file in that text file (but it shouldn't show that hash when I open the file up with notepad) and after decrypting with the provided password, I can just hash the contents and check if it matches with the hidden hash stored in that file. Is it possible?

I am using Windows, by the way, and portability is not an issue.

Answer 1

In general, you can't theoretically design a data format where nothing but plain text is a valid subset of it, but there can also be metadata (hash or something else). Just think about it: how do you store something other than text (i. e. metadata) in a file where every single byte is to be interpreted as text?

That said, there are some tricks to hide the metadata in plain sight. With Unicode, the palette of tricks is wider. For example, you can use spacelike characters to encode metadata or indicate metadata presence in the way that the user won't notice. Consider Unicode BOM. It's the "zero-length space" character. Won't be seen in Notepad, serves as metadata. You could so something similar.

They already mentioned alternative data streams. While one of those could work to keep the metadata, alternative data stream don't survive archival, e-mailing, uploading to Google Drive/OneDrive/Dropbox, copying with a program that is not aware of them, or copying to a filesystem that doesn't support them (e. g. a CD or a flash drive with FAT).