Five thousand people, one shared secret.
Communities bring the same end-to-end encryption to a crowd, and the server can't read a word of it.
Scale
Why a crowd breaks ordinary group crypto
A normal group encrypts every message once for each member. Twenty people means twenty copies in twenty queues. That's fine for a family thread and hopeless for a crowd. Somewhere in the thousands, the cost of sending a single message runs away from you.
Communities take the other road. A message is encrypted once and handed to everybody at the same time, so the five-thousandth member costs roughly what the fifth did.
- Small groups stop at 20 members for exactly this reason.
- Communities go to 5,000, and one message costs the same to send whether ten people or thousands are listening.
The shared key
How everyone lands on the same secret
Picture every member sitting at the tip of a branch on a tree. Climb from your own tip to the top and you pick up a key at each fork. Climb from somebody else's tip and you pick up different ones. Only the key at the very top belongs to everybody, and that top key is what all of the group's encryption is built from.
The shape is what saves the work. When the key has to change, whoever changes it redoes one path up to the top. Writing to every member by hand would take five thousand steps for five thousand people; the path takes about thirteen.
- Those keys are worked out on your own phone. The server never hands them to you and never sees them.
- The standard behind this is MLS, published as RFC 9420, and the tree itself is its TreeKEM design. It was built for groups exactly this size.
Re-keying
The key changes every time the group does
Every join and every departure pushes the group onto a new key that the old one can't be used to work out. The protocol calls each of those steps an epoch.
This is why leaving is final. Take somebody out and the group changes its key without them. A second before, they could read everything; a second after, they can't read a single new message. The same mechanism protects you: any member can refresh their own key, which locks out anyone who might have got hold of the old one.
- Somebody removed is shut out of everything sent after they go, with no grace window.
- Used-up keys are dropped as the group moves on, so a phone seized today won't open yesterday's messages.
Delivery
One ciphertext, five thousand readers
Your message is encrypted before it leaves your phone and posted once. Every member picks up that same encrypted block and opens it with the key they already hold. The server shuttles the bytes around.
It sees a block it can't open, a headcount and which member posted it. The words and the group's name are hidden from it.
Message content and the record of key changes are kept apart, and only the content expires. Go quiet for a month and your app replays the key changes it missed, one at a time, then picks up at today's messages. The messages sent while you were gone are lost, but you keep your access to the group.
- Group names are encrypted too. That key travels inside the invite and never passes through the server.
- Messages sit on the server for 7 days and then go, unless the founder turns on saved history. The key history outlives them, and that is what lets someone who's been away catch up and get back in.
Membership
Invite-only, and admission is a real event
Joining a community takes an invite from somebody and a deliberate step from you. Your phone introduces itself, a member already inside folds you in and changes the key, and only then can you read anything.
The invite also carries a fingerprint of whoever founded the group, and your app checks it before trusting a single message. That check stops a server from inventing a fake community and slipping you into it.
- Nobody joins silently. Every admission changes the key, so a newcomer starts from a clean slate.
- Leave, and the group changes its key again without you.
What communities are, and what they aren't yet
We'd rather tell you the rough edges up front.
Text, for now
Communities carry text only. Voice notes and attachments already work in one-to-one chats and small groups; bringing them to communities is still on the to-do list.
Up to 5,000
That's what one community holds. Need more people than that and you'd run more than one.
Only a headcount
The server sees the list of member IDs and moves encrypted blocks around. It can't read messages or names.
Their own lane
Communities are separate from the 20-person groups. Both exist side by side, and you pick per conversation.
End-to-end encryption for up to 5,000 people.
Communities are part of Aphotic, and one license covers everything.