- [Communication between SMP agents](#communication-between-smp-agents)
- [Message syntax](#messages-between-smp-agents)
- [SMP agent commands](#smp-agent-commands)
- [Connection invitation](#connection-invitation)
## Abstract
The purpose of SMP agent protocol is to define the syntax and the semantics of communications between the client and the agent that connects to [SMP](./simplex-messaging.md) servers.
It provides:
- convenient protocol to create and manage a bi-directional (duplex) connection to the users of SMP agents consisting of two separate unidirectional (simplex) SMP queues, abstracting away multiple steps required to establish bi-directional connections.
- management of E2E encryption between SMP agents, generating ephemeral RSA keys for each connection.
- SMP command authentication on SMP servers, generating ephemeral RSA keys for each SMP queue.
- TCP transport handshake and encryption with SMP servers.
- validation of message integrity.
SMP agent protocols provides no encryption or any security on the client side - it is assumed that the agent is executed in the trusted and secure environment.
The future versions of this protocol could provide:
- managing redundant SMP queues with more than 1 queue in each direction.
- managing simple symmetric groups as a foundation for chat groups and device synchronization.
- agent cluster - synchronizing states of multiple agents.
- secure "synchronous" streams with symmetric message encryption and connection-level authentication (requires extending SMP protocol) - it can be used, e.g., for file transfers.
## SMP agent
SMP agent is a client-side process or library that communicates via SMP servers using [simplex messaging protocol (SMP)](./simplex-messaging.md) with other SMP agents according to the commands received from its users. This protocol is a middle layer in SMP protocols stack (above SMP protocol but below any application level protocol) - it is intended to be used by client-side applications that need secure asynchronous bi-directional communication channels ("connections").
The agent must have a persistent storage to manage the states of known connections and of the client-side information of two SMP queues that each connection consists of, and also the buffer of the most recent messages. The number of the messages that should be stored is implementation specific, depending on the error management approach that the agent implements; at the very least the agent must store the hash and id of the last received message.
## SMP agent protocol components
SMP agent protocol has 3 main parts:
- the syntax and semantics of messages that SMP agents exchange between each other in order to:
- negotiate establishing unidirectional (simplex) encrypted queues on SMP server(s)
- exchange client messages and delivery notifications, providing sequential message IDs and message integrity (by including the hash of the previous message).
- the syntax and semantics of the commands (a higher level interface than SMP protocol) that are sent over TCP or other sequential protocol by agent clients to the agents. This protocol allows to create and manage multiple connections, each consisting of two simplex SMP queues.
- the syntax and semantics of the message that the clients of SMP agents should send out-of-band (as pre-shared "invitation" including SMP server, queue ID and encryption key) to ensure [E2E encryption][1] the integrity of SMP queues and protection against active attacks ([MITM attacks][2]).
The procedure of establishing a duplex connection is explained on the example of Alice and Bob creating a bi-directional connection comprised of two unidirectional (simplex) queues, using SMP agents (A and B) to facilitate it, and two different SMP servers (which could be the same server). It is shown on the diagram above and has these steps:
1. Alice requests the new connection from the SMP agent A using `NEW` command.
2. Agent A creates an SMP queue on the server (using [SMP protocol](./simplex-messaging.md)) and responds to Alice with the invitation that contains queue information and the encryption key Bob's agent B should use. The invitation format is described in [Connection invitation](#connection-invitation).
3. Alice sends the invitation to Bob via any secure channel they have (out-of-band message).
4. Bob sends `JOIN` command with the invitation as a parameter to agent B to accept the connection.
- Agent B sends unauthenticated message to SMP queue with ephemeral key that will be used to authenticate commands to the queue, as described in SMP protocol.
- Agent A receives the KEY and secures the queue.
- Agent B tries sending authenticated SMP SEND command with agent `HELLO` message until it succeeds. Once it succeeds, Bob's agent "knows" the queue is secured.
6. Agent B creates a new SMP queue on the server.
7. Establish Bob's SMP queue:
- Agent B sends `REPLY` message with the invitation to this 2nd queue to Alice's agent (via the 1st queue).
- Agent A having received this `REPLY` message sends unauthenticated message to SMP queue with Alice agent's ephemeral key that will be used to authenticate commands to the queue, as described in SMP protocol.
- Bob's agent receives the key and secures the queue.
- Alice's agent keeps sending `HELLO` message until it succeeds.
8. Agents A and B notify Alice and Bob that connection is established.
- Once sending `HELLO` succeeds, Alice's agent sends to Alice `CON` notification that confirms that now both parties can communicate.
- Once Bob's agent receives `HELLO` from Alice's agent, it sends to Bob `CON` notification as well.
At this point the duplex connection between Alice and Bob is established, they can use `SEND` command to send messages. The diagram also shows how the connection status changes for both parties, where the first part is the status of the SMP queue to receive messages, and the second part - the status of the queue to send messages.
The most communication happens between the agents and servers, from the point of view of Alice and Bob they have only 3 steps to do:
1. Alice requests a new connection with `NEW` command and receives the invitation.
2. Alice passes invitation out-of-band to Bob.
3. Bob accepts the connection by sending `JOIN` command with the invitation to his agent.
SMP agents communicate via SMP servers managing creation, deletion and operations of SMP queues.
Agents can use SMP message client body (the part of the SMP message after header - see SMP protocol) to transmit agent client messages and exchange messages between each other.
Each SMP message client body, once decrypted, contains 3 parts (one of them may include binary message body), as defined by `decryptedSmpMessageBody` syntax:
-`agentMsgHeader` - agent message header that contains sequential agent message ID for a particular SMP queue, agent timestamp (ISO8601) and the hash of the previous message.
-`agentMessage` - a command/message to the other SMP agent:
- to establish the connection with two SMP queues (`helloMsg`, `replyQueueMsg`)
- to send and to acknowledge user messages (`clientMsg`, `acknowledgeMsg`)
- to notify another agent about queue deletion (`deleteQueueMsg`)
-`msgPadding` - an optional message padding to make all SMP messages have consistent size as an additional privacy protection measure.
ackProhibitedMsg = %"PROHIBITED" ; e.g. "HELLO" or "REPLY"
ackSyntaxErr = %"SYNTAX" SP syntaxErrCode
syntaxErrCode = 1*DIGIT ; TODO
```
## SMP agent commands
This part describes the transmissions between users and client-side SMP agents: commands that the users send to create and operate duplex connections and SMP agent responses and messages they deliver.
Commands syntax below is provided using [ABNF][1].
Each transmission between the user and SMP agent must have this format/syntax:
badMsgIdErr = %"ID" SP previousMsgId ; ID is lower than the previous
badHashErr = %"HASH"
acknowledge = %s"ACK" SP agentMsgId ; ID assigned by receiving agent (in MSG "R")
received = %s"RCVD" SP agentMsgId ; ID assigned by sending agent (in SENT response)
ok = %s"OK"
error = %s"ERR" SP errorType
encoded = base64
```
## Connection invitation
Connection invitation `qInfo` is generated by SMP agent in response to `newCmd` command (`"NEW"`), used by another party user with `joinCmd` command (`"JOIN"`), and then another invitation is sent by the agent in `replyQueueMsg` and used by the first party agent to connect to the reply queue (the second part of the process is invisible to the users).
Connection invitation is a text with the following syntax:
