Encrypted file-size approximation with multiple recipients

[Tim Chase]

I've been trying to find a good explanation on how something like

  gpg -r DEADBEEF -r CAFEBABE -r 8BADFOOD -o output.gpg -e input.txt

works.  The best I've been able to find is this:

http://lists.gnupg.org/pipermail/gnupg-users/2007-October/031938.html

I'm mostly interested in the overhead, so I set up 4 distinct
homedirs for testing.  It looks like each additional recipient adds
about 271 bytes (though one of them only has an extra 270 bytes), and
there's a per-file overhead of about 66 or 67 bytes.

So from my experimentation, the final file-size ends up being
something like

  input_file_size + 67 + (271 * recipient_count)

but I'm not sure how much that might change based on conditions I'm
not taking into consideration (all my test GPG users were just
gpg1 at example.com, gpg2 at example.com, etc), all with 2048-bit keys.

Is there a more formal formula that can be used to approximate the
overhead of multi-recipient encryption?

Thanks,

-tkc