You can do that with Pack:

`pack -i ./test.pack --include=/a/file.txt`

or a couple files and folders at once:

`pack -i ./test.pack --include=/a/file.txt --include=/a/folder/`

Use `--list` to get a list of all files:

`pack -i ./test.pack --list`

Such random access using `--include` is very fast. As an example, if I want to extract just a .c file from the whole codebase of Linux, it can be done (on my machine) in 30 ms, compared to near 500 ms for WinRAR or 2500 ms for tar.gz. And it will just worsen when you count encryption. For now, Pack encryption is not public, but when it is, you can access a file in a locked Pack file in a matter of milliseconds rather than seconds.