# Private Groups | 2019-11-07

## simplest thing

mix sends notification to group: bbq is cancelled. pure broadcasts, no tangles.

`sbot.privateGroups.add({key, id})`

`sbot.backlinks.read(query)`

things we're not doing:
- optimisations like "only try keys from people I'm in groups with"
- possible future: `sbot.privateGroups.add({key, feeds: [id...]})`
- indexing past messages

content: {
  type: 'gathering',
  name: 'private group call',
  startTime: {
    epoch: 1573074104361
  },
  
  // user doesn't need to know this
  tangles: { 
    group: { root, previous },
    gathering: { root, previous }
  }
}

what features does adding someone to a key look like

content: {
  type: 'group/update',
  
  writers: { 
    <@dominicFeed>: 1,
    <@keksFeed>: 1
  },
  
  tangles: {
    group: { root, previous },
    membership: { root, previous }
  }
}

// agree ^this is more sugary than needed immediately

content: {
  type: 'entrust',
  key: <groupKey.base64>
  tangles: {
    group: {root, previous}
  },
  recps: [mixFeed, group.id]  // << NOTE group.id
}

// ^this one is a minimal entrust message for a group.

**Properties**:
- `tangles.group` is present so as a new person I can traverse (up the tangle) the history of things I need to decrypt
- `recps` includes the person being added and the group, meaning
- mix receives the messages about being added to the group
- the group received a message seeing that mix has read-access
- // this is polite / transparent :heart:

- `ssb-server` now needs to know when it sees `group.id` to encrypt to that group as well
- what `group.id` is still needs determining:
- could be a hash of the group's root %messageId ?
- could be a hash of the group's private key?
- some past prototyping https://github.com/dominictarr/ssb-private-groups/blob/master/index.js#L65
- if id is @feedId, just use that
- if it's a @messageId (or unique group.id), check if it's a valid group id and then look up / derive the encryption key for that group and use that for encryption

publishing fails if:
- any recps are duds
- if recps === []

Q? How how does encryption work when there's a feedId + a group.id ?
- plan to change the format to box2
- box2 is specified? yes
- it will use a slot

Q? does this work for multiFeed.id just the same?
- mix : we would want asymmetric keys for this

### Parts of the system

High

-

- backlinks index

Low

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## simple thing (simplest ++ / v2)

how to solve the "I received the key later" problem

- could do this in backlinks.read or in the backlinks indexing

- could tangle all messages for a group