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# MESSENGER — Room-based end-to-end messaging

The MESSENGER subsystem is responsible for secure end-to-end communication in groups of nodes in the GNUnet overlay network. MESSENGER builds on the CADET subsystem which provides a reliable and secure end-to-end communication between the nodes inside of these groups.

Additionally to the CADET security benefits, MESSENGER provides following properties designed for application level usage:

- MESSENGER provides integrity by signing the messages with the users provided ego

- MESSENGER adds (optional) forward secrecy by replacing the key pair of the used ego and signing the propagation of the new one with old one (chaining egos)

- MESSENGER provides verification of a original sender by checking against all used egos from a member which are currently in active use (active use depends on the state of a member session)

- MESSENGER offsers (optional) decentralized message forwarding between all nodes in a group to improve availability and prevent MITM-attacks

- MESSENGER handles new connections and disconnections from nodes in the group by reconnecting them preserving an efficient structure for message distribution (ensuring availability and accountablity)

- MESSENGER provides replay protection (messages can be uniquely identified via SHA-512, include a timestamp and the hash of the last message)

- MESSENGER allows detection for dropped messages by chaining them (messages refer to the last message by their hash) improving accountability

- MESSENGER allows requesting messages from other peers explicitly to ensure availability

- MESSENGER provides confidentiality by padding messages to few different sizes (512 bytes, 4096 bytes, 32768 bytes and maximal message size from CADET)

- MESSENGER adds (optional) confidentiality with ECDHE to exchange and use symmetric encryption, encrypting with both AES-256 and Twofish but allowing only selected members to decrypt (using the receivers ego for ECDHE)

Also MESSENGER provides multiple features with privacy in mind:

- MESSENGER allows deleting messages from all peers in the group by the original sender (uses the MESSENGER provided verification)

- MESSENGER allows using the publicly known anonymous ego instead of any unique identifying ego

- MESSENGER allows your node to decide between acting as host of the used messaging room (sharing your peer’s identity with all nodes in the group) or acting as guest (sharing your peer’s identity only with the nodes you explicitly open a connection to)

- MESSENGER handles members independently of the peer’s identity making forwarded messages indistinguishable from directly received ones ( complicating the tracking of messages and identifying its origin)

- MESSENGER allows names of members being not unique (also names are optional)

- MESSENGER does not include information about the selected receiver of an explicitly encrypted message in its header, complicating it for other members to draw conclusions from communication partners

libgnunetmessenger libgnunetmessenger ——————

The MESSENGER API (defined in `gnunet_messenger_service.h`) allows P2P applications built using GNUnet to communicate with specified kinds of messages in a group. It provides applications the ability to send and receive encrypted messages to any group of peers participating in GNUnet in a decentralized way ( without even knowing all peers’s identities).

MESSENGER delivers messages to other peers in "rooms". A room uses a variable amount of CADET "channels" which will all be used for message distribution. Each channel can represent an outgoing connection opened by entering a room with `GNUNET_MESSENGER_enter_room` or an incoming connection if the room was opened before via `GNUNET_MESSENGER_open_room`.

To enter a room you have to specify the "door" (peer’s identity of a peer which has opened the room) and the key of the room (which is identical to a CADET "port"). To open a room you have to specify only the key to use. When opening a room you automatically distribute a PEER-message sharing your peer’s identity in the room.

Entering or opening a room can also be combined in any order. In any case you will automatically get a unique member ID and send a JOIN-message notifying others about your entry and your public key from your selected ego.

The ego can be selected by name with the initial `GNUNET_MESSENGER_connect` besides setting a (identity-)callback for each change/confirmation of the used ego and a (message-)callback which gets called every time a message gets sent or received in the room. Once the identity-callback got called you can check your used ego with `GNUNET_MESSENGER_get_key` providing only its public key. The function returns NULL if the anonymous ego is used. If the ego should be replaced with a newly generated one, you can use `GNUNET_MESSENGER_update` to ensure proper chaining of used egos.

Also once the identity-callback got called you can check your used name with `GNUNET_MESSENGER_get_name` and potentially change or set a name via `GNUNET_MESSENGER_set_name`. A name is for example required to create a new ego with `GNUNET_MESSENGER_update`. Also any change in ego or name will automatically be distributed in the room with a NAME- or KEY-message respectively.

To send a message a message inside of a room you can use `GNUNET_MESSENGER_send_message`. If you specify a selected contact as receiver, the message gets encrypted automatically and will be sent as PRIVATE- message instead.

To request a potentially missed message or to get a specific message after its original call of the message-callback, you can use `GNUNET_MESSENGER_get_message`. Additionally once a message was distributed to application level and the message-callback got called, you can get the contact respresenting a message’s sender respectively with `GNUNET_MESSENGER_get_sender`. This allows getting name and the public key of any sender currently in use with `GNUNET_MESSENGER_contact_get_name` and `GNUNET_MESSENGER_contact_get_key`. It is also possible to iterate through all current members of a room with `GNUNET_MESSENGER_iterate_members` using a callback.

To leave a room you can use `GNUNET_MESSENGER_close_room` which will also close the rooms connections once all applications on the same peer have left the room. Leaving a room will also send a LEAVE-message closing a member session on all connected peers before any connection will be closed. Leaving a room is however not required for any application to keep your member session open between multiple sessions of the actual application.

Finally, when an application no longer wants to use CADET, it should call `GNUNET_MESSENGER_disconnect`. You don’t have to explicitly close the used rooms or leave them.

Here is a little summary to the kinds of messages you can send manually:

## MERGE-message

MERGE-messages will generally be sent automatically to reduce the amount of parallel chained messages. This is necessary to close a member session for example. You can also send MERGE-messages manually if required to merge two chains of messages.

## INVITE-message

INVITE-messages can be used to invite other members in a room to a different room, sharing one potential door and the required key to enter the room. This kind of message is typically sent as encrypted PRIVATE-message to selected members because it doesn’t make much sense to invite all members from one room to another considering a rooms key doesn’t specify its usage.

## TEXT-message

TEXT-messages can be used to send simple text-based messages and should be considered as being in readable form without complex decoding. The text has to end with a NULL-terminator character and should be in UTF-8 encoding for most compatibility.

## FILE-message

FILE-messages can be used to share files inside of a room. They do not contain the actual file being shared but its original hash, filename, URI to download the file and a symmetric key to decrypt the downloaded file.

It is recommended to use the FS subsystem and the FILE-messages in combination.

## DELETE-message

DELETE-messages can be used to delete messages selected with its hash. You can also select any custom delay relative to the time of sending the DELETE-message. Deletion will only be processed on each peer in a room if the sender is authorized.

The only information of a deleted message which being kept will be the chained hashes connecting the message graph for potential traversion. For example the check for completion of a member session requires this information.

### Member sessions

A member session is a triple of the room key, the member ID and the public key of the member’s ego. Member sessions allow that a member can change their ID or their ego once at a time without losing the ability to delete old messages or identifying the original sender of a message. On every change of ID or EGO a session will be marked as closed. So every session chain will only contain one open session with the current ID and public key.

If a session is marked as closed the MESSENGER service will check from the first message opening a session to its last one closing the session for completion. If a the service can confirm that there is no message still missing which was sent from the closed member session, it will be marked as completed.

A completed member session is not able to verify any incoming message to ensure forward secrecy preventing others from using old stolen egos.