Keychat also supports shared-key groups, where all group members share a public-private key pair. When a member sends a group message, they use this shared public-private key pair to send a Nostr Encrypted Direct Message (NIP4) to this shared public-private key pair. To outsiders, it appears as if they are sending messages to themselves. Other group members receive and decrypt the message using the shared key pair.
When a group member is removed by the group admin, the shared key is updated. The group admin notifies the remaining group members of the new group key by sending them a Nostr DM NIP4 message.
This is an end-to-end encrypted group chat, but it lacks both forward secrecy and backward secrecy. In terms of metadata privacy, outsiders can only see that someone is continually sending messages to themselves.
The overall security of a shared-key group is not as robust as that of a pairwise group. It might be better to consider it as a semi-public or public group.
If the shared-key group has many members, most of whom are strangers, we recommend participating in the shared-key group chat with a new ID, rather than using your primary ID.
Additionally, we are exploring sender key groups and Message Layer Security (MLS).
By the way, the security of all encrypted group chats currently does not match the security of one-on-one chats.