If the real original filename is inside the file, then I agree a one-way mapping is OK, since the feature of "restore this directory" can be implemented by opening each backup file and checking the original name

Is crypt going to generate a unique one-way mapping? If so, I don't see why that's any better than just using a 2-way function. If not, then it still has the problem of needing to open the file before overcopying to make sure it's the right one, giving disambiguating suffixes to the name, and searching for a base name with the right suffix.

—John