THE POSTCARD PROTOCOL

The postcard protocol is a small client-server protocol built on udp for short
one-off messages.  Messages must be contained in one packet (512 bytes); with
metadata taking up around 100 bytes that means messages can be up to around 400
bytes in length, which is roughly postcard-sized.

A postcard message looks like this (sans leading indentation):

    <recipient><newline>
    <ed25519 public key of sender><newline>
    <message>

A postcard server ("Post office") is a persistent process somewhere online that exists to
receive postcards and deliver them to the proper user.  It could be fun to have
a block of ports, each of which serves as one user's "P.O. box" on that server.
If an entire block isn't available, one port could be used and delivery made
using the first line of the postcard.

Spam mitigation has two layers:

1. The sender must know the recipient's server (and possibly port number) and
   user name.  Post offices must keep a list of user names and their associated
   port numbers, and if the recipient's name and port fail to align, or if the
   office doesn't have a user, the message is rejected.

2. Sender identity is verified using ED25519 public key cryptography (chosen
   because of its short length---a mere 32 bytes).  Post offices should keep
   lists of friends and fuckers, letting the first deliver to their users and
   telling the second to shove it.  When receiving a message from an unknown
   recipient, the post office should hold the message for a given amount of time
   and allow a moderator ("postmaster") to decide whether the sender is a
   friend or fucker.

UDP was chosen as a protocol precisely for its lossy and unpredictable nature:
postcards themselves can get lost in the mail or damaged by rain or machinery,
after all.  However, if a postmaster wants to provide a greater level of
service for senders to their office, they may respond with one of the following
codes upon receipt of message:

(1) HM --- message received but waiting for moderation
(2) OK --- message received and delivered
(3) FU --- fuck off, fucker!

Of course, a postmaster can decide to not send HM or simply not to reply to
fuckers.  They can just pretend the message was lost in the mail.