Two accounts each compute the same secret alone, and from it the same space id, guest list and read key: a one-to-one space with two writers, no invite slip, no referee. Approval is a local gate.

It is the substrate for direct messages and any two-party state.

Two accounts. Alice and Bob. Each has a public key and a private key.

From her private key and his public key, Alice computes a secret. From his private key and her public key, Bob computes the same one. Each works it out alone. Nothing is sent.

From that secret, both compute a space. Its id. Its guest list. Its read key. A one-to-one space, with exactly two writers.

The list is fixed. Two writers, forever. No invite slip. No accept. No referee needed.

Bob only has to be told. His device will not download the space until he says yes. Approval is a local gate, not a list operation.

This is the substrate for direct messages, and for any two-party shared state. Any two identities already share a space. They only have to open it.

Check yourself. What stops Bob's device from downloading a one-to-one space Alice computed?