Overview
=========

What is a key?
It is a gpg symmetrically encrypted, ascii-armored file.
The encryption key is a function (see below, on KDF section) of your tomb
passphrase.

Layout
======

Before coming to the gpg part, there could be some "header" lines specifying
metatada. They're done like this:
_FIELD_params_params_and_more_params_

where FIELD should be the description for the header.
Pay much attention to the fact that there should ONLY be ASCII characters there,
to avoid encoding issues and whatever. Needs something more? Use base64encode.
(Of course, you're free to pack params into a single field, base64encoding
whatever you want).
And every header field should be in only one line.

KDF
===

Key Derivation Functions, are functions which will make your key stronger
spending some CPU time: the basic idea is that you have to compute that function
just once in a while, but an attacker that wants to bruteforce has to compute it
for every passphrase he's checking. This will make the bruteforce much more
expensive.

The header line format is _KDF_$method_$params_$params_... where $method is the
method we are using (ie: scrypt) and params is something that it needs (ie:
salt).