twister group chat draft
========================

group chat is a private and encrypted timeline shared among some users
which define a specific topic (or description).

it is tempting to use a new torrent where members would subscribe in
order to post their chat messages. however this idea does work as it is
impossible (very difficult?) to implement a distributed torrent with
multiple writers. (a clash occurs when two writers try to send the 
same piece number)

therefore, twister's group chat is a special case of direct messages
using a private key pair (outside of the blockchain). non-members do not
know about group chat creation as public key is never published.

members are invited to group using a special (bencoded) group_invite DM:

group_invite {
  desc: "free text description",
  key: "secret of the private key",
}

after sending the group_invite DM to a given user, another DM is sent
to the group itself to notify all existing members about the new
member list. this is another special DM:

group_members {
  [xxx,yyy,...]
}

it is true that, since every member knows both private/public keys of
the group, symmetric-key could have been used for group chat's messages.
however we reuse the same twister infrastructure of original DMs.
any group chats, invites and messages are indistinguible from normal 
DMs to non-members.

because all members have access to private key, there is no concept of
administrator. any member may invite new members and change group's 
description. it is not possible to exclude a member, so frontends may
offer an option to clone/create a new group excluding whoever they
want.

---

new RPCs for group chat:

- creategroup <description>
  returns groupalias (which is added to wallet).
  
  group aliases are prepended with "*" so they are not valid usernames
  and may not be propagated accidentaly to the network.
  
  group aliases are only valid locally, each twisterd instance will
  create its own alias upon receiving an invitation.

- listgroups
  [ groupalias1, groupalias2, ... ]
  
- getgroupinfo
  {alias:xxx,description:xxx,members:[xxx,yyy,...]}

- newgroupinvite <username> <k> <groupalias> [<newmember1>,<newmember2>,...]
  DM(group_invite) => newmember
  DM(group_members) => groupalias
  
- newgroupdescription <username> <k> <groupalias> <description>
  DM(group_invite) => groupalias
  
- leavegroup <username> <groupalias>
  not yet implemented
  
note: use getdirectmsgs/newdirectmsgto to obtain/post new messages 
      to group chats.