Group Id

We want to be able to mention a group without revealing info about it. We’ll also use this group_id in the recps field, and when we go to publish a message, map that group_id into that groups symmetric key (group_key) which we then use for envelope encryption

Properties we want - leaks nothing about the group e.g. who started it, where it started - can be used by those who know about the group to look up the associated key


The group_id is defined as the cloaked message id (see envelope-spec/cloaked_id/ for the group/init message which started the group.



Using group_id

Because what this id is referencing is obfuscated, it’s safe to use in public (e.g. refernece or link to it from a public post)

We also use this in the content.recps field of classic messages.

var content = {
  recps: [
    '%g/JTmMEjG4JP2aQAO0LM8tIoRtNkTq07Se6h1qwnQKb=.cloaked', // << a cloaked group_id

This provides a little obfuscation about the group for any recipients of the message, but primarily the primary intention is to make this the canonical (safe) reference handle for a group. This make it less likely for programmers to be tempted to reference the root message id and copy that to an unsafe (public) context.

Mapping a group_id to a group_key

If we’re using group_id in recps, then we need a way to map this to the key we’ll use for boxing.

When you’re entrusted with access to a private group, you will receive: - the location of the initial message for that group - the group_key for that group

This will allow you to derive the group_id for that group (using definition above). You will need to keep an index which will allow you map group_id to group_key so that when you see a recps field containing the cloaked group_id you’ll know which shared key to include in envelope recp_keys

Design Considerations

  1. the parts you could identify a group uniquely by are:
    • it’s initialisation message
      • the public id of this message is knowable by all
      • the msg_key for this message is only know to people in the group
      • the read_key is known to anyone who can read the message (in group or been given permission)
    • it’s private group_key for decrypting all messages
  2. the above defintion uses the more general “cloaked message id” spec, and has the advantages:
    • it is not derived from the group_key, so this could later be cycled (potentially)
    • anyone who can read the init message can reference the group, without being able to read the whole group history (you only need the read capability for the group init messsage)
  3. this means the group_id is bound to the feed
    • it can’t be set / determined until a group initialisation message is published