Encrypt file name

Question

I'm looking to encrypt (rename) mutiple files name based on a passphrase. Currently the passphrase would be numeric but I can change that to something else if required.

I would like to be able to get a encrypted file name in numeric and/or letter (avoid any special char since they are not allowed in filename) and to be able to decrypt the file name using a passphrase.

Is it possible to encrypt the filename with a function (I'm using C#) that would not allow a hacker to reverse engineer the function and then be able to decrypt the filename. Like a one way encryption but with the possibility for me to use a different decryption function on my own system to decrypt the filename.

Thanks for your ideas!

Answer 1

SOJPM already noted in the comments format-preserving encryption (Wiki Link), that should do the trick.

Simply speaking, FPE allows you a proper encryption method based on any arbitrary subset you want. In your case, that would be the common symbols in filenames, although you might want to be more restrictive to limit yourself to alphanumeric values.

FPE is a technique, which can be constructed in various ways, a few of them can be found in that wikipedia link at the beginning. I don't know if there are any available implementations of that, so you might have to do some of it on your own.

About your question with a passphrase:

You can do that, by using a well known Password-Based Key Derivation Function(PBKDF), e.g. scrypt or bcrypt to generate a key from your passphrase. And that key is then used in the FPE construction.