Optional per-peer state
=======================

An Autocrypt-capable MUA must store some specific state about each
of its peers.

This document attempts to describe additional optional state that can
improve the user experience in some corner cases.

Unlike the standard Autocrypt level 1 state management, some of these
rules depend on a MUA being able to keep track of whether it has
seen a given message before or not, and these guidelines may cause
non-deterministic results depending on the order that messages are
encountered.

Additional state
----------------

An agent MAY store additional per-peer metadata about observed
Autocrypt messages. This can be used to provide more helpful
information when user intervention is required.

* ``counting_since``: The UTC timestamp of when we started counting
* ``count_have_ach``: A count of parsed AutoCrypt headers
* ``count_no_ach``: A count of messages without AutoCrypt headers
* ``bad_user_agent``: The apparent user-agent (if known) of the last
  message seen without AutoCrypt headers.

The theory is that a message of the form "The recipient may not be
able to read encrypted mail" could be augmented with reasons such as
"The last 5 messages we saw from them all came from a non-AutoCrypt
capable e-mail application", or "Their most recent message was sent on
April 5th using Apple Mail on an iPad."

Managing additional state
-------------------------

When processing a message from the peer:

 - If ``counting_since`` is unset, set it to the current time.
   Otherwise, if ``effective_date`` is greater than ``counting_since``:

   - If no ``Autocrypt`` header is available, increment ``count_no_ac``.
   - If an ``Autocrypt`` header is available, increment ``count_have_ac``.


When processing a message without an autocrypt header from a peer who
has send an header in the past and thus has a ``autocrypt_timestamp``:

 - set ``bad_user_agent`` to the apparent user-agent of the message

 - If ``counting_since`` is older than ``autocrypt_timestamp`` and more
   than 35 days older than ``effective_date``:

   - set ``counting_since`` to ``last_seen``
   - set ``count_have_ach`` to zero
   - set ``count_no_ach`` to one


Using additional state
----------------------

During message composition, if the Autocrypt recommendation is
``discourage`` this state can be used to craft a more-informative
warning message for the user.