How To Encrypt A Directory With Application-Specific Keys?

Question

I'm working on a C++ application that stores (and frequently accesses) its data across many files within a single directory via numerous classes, database libraries, etc. I would like to start encrypting all this data on disk using a key managed by the application.

On windows, programmatically enabling EFS for the directory would be perfect if the application could set the encryption key directly rather than using one based on the logged-in user's password. This does not seem possible.

Because there are numerous places in the code that read/write files, some with full random-access, a library that exports something akin to the OS file operation API would be easiest in the absence of direct OS support.

Windows is the biggest user base but something usable on Mac and Linux would be a big plus.

Any suggestions?

Answer 1

Have you considered storing the files in an encrypted archive, such as a zip file? Not knowing what language you're working in makes it difficult to give a more specific answer.

Answer 2

I am not an expert but I have a few suggestions,
('user' is considered Male here)

In Windows, when an user is logged in, he will have all the rights to manipulate a file or folder which belongs to him.
But, if he tries to manipulate files or folders which belong to other users, he is not allowed to do so. So, he can't manipulate other user's files.
If the user is administrator, he will have all the rights to manipulate the file.
In your case, I believe the user is not administrator.

So, I suggest to you to:
Create a user for this application, say Bob(Bob is not the user who is going to use this application).
Then you create a folder to store your files and give all permission only to Bob(and the admin).
For all others, there no permission to do anything to the folder.
Your application must be installed by administrator.

When you start, Your application asks Bob's password from current user.
If the password is correct,
Using your application allow full access folder permission to the current user. Then application can work as you wish.
If the user quits the application,
Remove all permission for current user so that after quitting this application others can't access that content.
But Administrator can still access this content.

This is similar like Linux user management.
When you are getting a shell you can change user using su username when finishes the work and he quits the shell then others can't use those files. But root user can do anything in Linux.

But there is one problem.
Administrator can get full content and access since he can modify the permissions.
To disallow this, save the files by zipping using any technique and by using a password. That password is only known by the user who uses it. Before saving a file to the folder, zip it using a password. Before using the file, unzip it using the password.

If the zipping password is same as user password, then there is a problem.
The administrator can reset user password to something and open his account.
To prevent this you can use the following technique:
Use zip password as reverse of user password, so that if his password is 1234 the zipping password is 4321, or use hash value of user password as zipping password.
If the zip password is hash value like MD5 hash or SHA-1 then brute force on zip file won't work due to the password length.
I don't know if this is a working solution or not or if this can be implemented or not.
But you can take some ideas out of this, if you think there is some good ideas in this.

Answer 3

Unless I've missed something in your question, TrueCrypt seems to be an ideal solution for you.

It will allow to:

1. Utilize existing OS file API (as the mounted volume will behave just like regular volume)
2. Programmatically manage access key (password)
3. Limit access to the mounted volume (by OS mechanics)
4. Utilize same principles on all the platforms (Windows, Mac, Linux)

Or, if you want, you can encrypt/decrypt individual files with it.

There's a number of examples on how to use it around.

Answer 4

Configure the application to run as a service account, intended only for it. Then, configure EFS. Since the application's service account will own the files, its user-specific keys should be the only valid EFS keys for it, so only the application (which will be the only thing running under this new service account) will be able to access the files.