simplex-chat/simplex-messaging.md

Simplex messaging protocol (SMP)

Table of contents

• Abstract
• Introduction
• SMP Model
• Out-of-band messages
• Simplex connection
• SMP procedure
• SMP elements
• SMP qualities and features
• Cryptographic algorithms
• Simplex connection IDs
• Server privacy requirements
• SMP commands
  • Correlating responses with commands
  • Command authentication
  • Recipient commands
    • Create connection command
    • Subscribe to connection
    • Secure connection command
    • Delete message command
    • Suspend connection
    • Delete connection
  • Sender commands
    • Send message command
  • Server messages
    • New connection response
    • Deliver connection message
    • Error responses
    • OK response
• Appendices
  • Appendix A. Transport connection with the SMP server
  • Appendix B. Sending out-of-band message

Abstract

Simplex messaging protocol is a transport agnostic client-server protocol for asynchronous distributed secure unidirectional message transmission.

It's designed with the focus on communication security and integrity, under the assumption that any part of the message transmission network can be compromised.

It addresses the problems of existing communication protocols that undermine communication security and privacy:

• Identity related problems:

  • visibility of user contacts to anybody observing messages
  • unsolicited messages (spam and abuse)
  • trademark issues (when usernames are used)
  • privacy issues (when phone numbers are used)
  • Participants' identities are known to the network. Depending on the identity type (e.g., phone number, DNS-based, username, uuid, public key, etc.) it creates different problems, but in all cases it exposes participants and their contacts graph to the network and also allows for unsolicited messages (spam and abuse).

• MITM attack. Any mechanism of the encryption key exchange via the same network is prone to this type of attack when the public keys of the participants are substituted with the public keys of the attacker intercepting communication. While some solutions have been proposed that complicate MITM attack (social millionaire, OTR), if the attacker understands the protocol and has intercepted and can substitute all information exchanged between the participants, it is still possible to substitute encryption keys. It means that the existing E2EE implementations in messaging protocols and platforms can be compromised by the attacked who compromised the server or communication channel.

Introduction

The objective of Simplex Messaging Protocol (SMP) is to facilitate the secure and private unidirectional transfer of messages from senders to recipients.

SMP is independent of the particular transmission system and requires only a reliable ordered data stream channel. While this document describes transport over TCP, other transports are also possible.

The protocol describes the set of commands that recipient and sender can exchange with the SMP server to create and to operate a unidirectional "connection" (a data abstraction identifying one of many communication channels managed by the server) and to send messages from the sender to the recipient via the SMP server.

More complex communication scenarios can be designed using multiple connections - for example, a duplex communication channel can be made of 2 simplex connections.

Protocol is designed with the focus on privacy and security, to some extent deprioritizing reliability by requiring that SMP servers only store messages until they are delivered to the recipients and, in any case, for a limited period of time. For communication scenarios requiring more reliable transmission the users should use several SMP servers to pass each message and implement some additional protocol (e.g., based on blockchain) to ensure that messages are not removed, inserted or re-ordered - this is out of scope of this document.

SMP removes the need for participants' identities and provides E2EE without the possibility of MITM attack attack relying on two pre-requisites:

• the users can establish a secure encrypted transport connection with the SMP server. Appendix A has a possible simple protocol of such transport connection over TCP, but any other transport connection encryption protocol can be used.
• the recipient can pass a single message to the sender via pre-existing secure and private communication channel (out-of-band message) - the information in this message is used to establish the connection via SMP server that the sender will use to send the encrypted messages to the recipient.

SMP Model

The SMP model has three communication participants: the recipient, the message broker (SMP server) that is chosen and, possibly, controlled by the recipient, and the sender.

SMP server manages multiple "simplex connections" - data records on the server that identify communication channels from the senders to the recipients. The same communicating party that is the sender in one connection, can be the recipient in another - without exposing this fact to the server.

The connection record consists of 2 unique random IDs generated by the server, one for the recipient and another for the sender, and 2 keys to authenticate the recipient and the sender respectively. The users of SMP protocol must use a unique key for each connection, to avoid the possibility of aggregating and analysing their connections in case SMP server is compromised.

Creating and using this connection requires sending commands to the SMP server from the recipient and the sender - they are described in detail in SMP commands section.

Out-of-band messages

The out-of band invitation message is sent via some trusted alternative channel by the recipient to the sender. This message is used to share the encryption (a.k.a. "public") key that the sender will use to encrypt the messages (to be decrypted by the recipient), sender connection ID, server address and any other information necessary to establish secure encrypted connection with SMP server (see Appendix A for a simple transport protocol example).

The syntax of the message defined with ABNF is:

outOfBandMsg = "(" connId "," serverHost "," transportInfo ")"
connId = encoded ; defined below
serverHost = DQUOTE hostname DQUOTE ; RFC 1123, section 2.1
transportInfo = JSON ; for example, TCP port number and encryption key fingerprint

Defining the approach to out-of-band message passing is out of scope of the simplex messaging protocol. See Appendix B for one of the possible practical approaches to passing out-of-band message.

Simplex connection

The simplex connection is the main unit of SMP protocol. It is used by:

• the sender of the connection (who received out-of-band message) to send messages to the server using connection ID, signed by sender's key.
• the recepient of the connection (who created the connection and who sent out-of-band message) will use it to retrieve messages from the server, signing the commands by the recepient key.
• participant identities are not shared with the server - new unique keys and connection IDs are used for each connection.

This simplex connection can serve as a building block for more complex communication network. For example, two (or more, for redundancy) simplex connections can be used to create a duplex communication channel. Higher level primitives that are only known to system participants in their client applications can be created as well - user profiles, contacts, conversations, groups and broadcasts. Simplex messaging servers have have the information about the low-level simplex connections. In this way a high level of privacy and security of the conversations is provided. Application level primitives are not in scope of the simplex messaging protocol.

This approach is based on the concept of unidirectional networks that are used for applications with high level of information security.

Access to each connection is controlled with unique (not shared with other connections) assymetric key pairs, separate for sender and the recipient. The sender and the receiver have private keys, and the server has associated public keys to authnticate participants' commands by verifying the signatures.

The messages sent into the connection are encrypted and decrypted using another key pair - the recepient has the private key and the sender has the associated public key.

Simplex connection diagram:

Connection is defined by recipient ID RID unique for the server. It also has a different unique sender ID SID. Sender key (SK) is used by the server to verify sender's commands (identified by SID) to send messages. Recipient key (RK) is used by the server to verify recipient's commands (identified by SID) to retrieve messages.

The protocol uses different IDs for sender and recipient in order to provide an additional connection privacy by complicating correlation of senders and recipients commands sent over the network - even though they are encrypted using server's public key, in case this key is compromised it would still be difficult to correlate senders and recipients, it would require access to connections records on the server.

SMP procedure

The SMP procedure of creating a simplex connection on SMP server is explained using participants Alice (the recipient) who wants to receive the messages from Bob (the sender).

To create a simpelex connection Alice and Bob follow these steps:

1. Alice creates a simplex connection on the server:
   1. decides which SMP server to use (can be the same or different server that Alice uses for other connections) and opens secure encrypted transport connection to the chosen SMP server (see Appendix A).
   2. generates a new random public/private key pair (encryption key - EK) that she did not use before for Bob to encrypt the messages.
   3. generates another new random public/private key pair (recepient key - RK) that she did not use before for her to sign commands and to decrypt the transmissions received from the server.
   4. sends the command to the server to create a simplex connection (see create in Create connection command). This command can either be anonymous or the server can be configured to use the signature field to authenticate the users who are allowed to create connections. This connection command contains previouisly generated uniqie "public" key RK that will be used to sign the following commands related to the same connection, for example to subscribe to the messages received to this connection or to update the connection, e.g. by setting the key required to send the messages (initially Alice creates the connection that accepts unsigned commands to send messages, so anybody could send the message via this connection if they knew the connection ID and server address).
   5. The server responds with connection IDs (connResp):
      • recipient ID RID for Alice to manage the connection and to receive the messages.
      • sender ID SID for Bob to send messages to the connection.
2. Alice sends an out-of-band message to Bob via the alternative channel that both Alice and Bob trust (see Simplex messaging protocol abstract and Appendix B). The message must include:
   • the unique "public" key (EK) that Bob must use to encrypt messages.
   • SMP server address and information to open secure encrypted transport connection (see Appendix A)
   • the sender connection ID SID for Bob to use.
3. Bob, having received the out-of-band message from Alice, accepts the connection:
   1. generates a new random public/private key pair (sender key - SK) that he did not use before for him to sign commands to Alice's server to send the messages.
   2. prepares the confirmation message for Alice to secure the connection. This message includes:
      • previously generated "public" key SK that will be used by Alice's server to authenticate Bob's commands to send messages, once the connection is secured.
      • optionally, any additional information (application specific, e.g. Bob's profile name and details).
   3. encrypts the confirmation body with the "public" key EK (that Alice provided via the out-of-band message).
   4. sends the encrypted message to the server with connection ID SID (see send in Send message command) to confirm the connection. This message to confirm the connection must not be signed - signed messages will be rejected until Alice secures the connection (below).
4. Alice receives Bob's message from the server using recipient connection ID RID (possibly, via the same transport connection she already has opened - see message in Deliver connection message):
   1. she decrypts received message with "private" key EK.
   2. even though anybody could have sent the message to the connection with ID SID before it is secured (e.g. if communication is compromised), Alice would ignore all messages until the decryption succeeds (i.e. the result contains the expected message format). Optionally, in the client application, she also may identify Bob using the information provided, but it is out of scope of SMP protocol.
5. Alice secures the connection RID so only Bob can send messages to it (see secure in Secure connection command):
   1. she sends the command with RID signed with "private" key RK to update the connection to only accept requests signed by "private" key SK provided by Bob.
   2. From this moment the server will accept only signed commands to SID, so only Bob will be able to send messages to the connection SID (corresponding to RID that Alice has).
   3. Once connection is secured, Alice deletes SID and SK - even if Alice's client is compromosed in the future, the attacker would not be able to send messages pretending to be Bob.
6. The simplex connection RID is now established on the server.

This flow is shown on the sequence diagram below.

Creating simplex connection from Bob to Alice:

Bob now can securely send messages to Alice:

1. Bob sends the message:
   1. he encrypts the message to Alice with "public" key EK (provided by Alice, only known to Alice and Bob, used only for one simplex connection).
   2. he signs the command to the server connection SID using the "private" key SK (that only he knows, used only for this connection).
   3. he sends the command to the server (see send in Send message command), that the server will authenticate using the "public" key SK (that Alice earlier provided to the server).
2. Alice receives the message(s):
   1. she signs the command to the server to subscribe to the connection RID with the "private" key RK (see subscribeCmd in Subscribe to connection).
   2. the server, having authenticated Alice's command with the "public" key RK that she provided, delivers Bob's message(s) (see message in Deliver connection message).
   3. she decrypts Bob's message(s) with the "private" key EK (that only she has).

This flow is show on sequence diagram below.

Sending messages from Bob to Alice via simplex connection:

Simplex connection operation:

Sequence diagram does not show E2EE - connection itself knows nothing about encryption between sender and receiver.

A higher level protocol application protocol should define the semantics that allow to use two simplex connections (or two sets of connections for redundancy) for the bi-directional chat and for any other communication scenarios.

The SMP is intentionally unidirectional - it provides no answer to how Bob will know that the transmission succeeded, and whether Alice received any messages. There may be a situation when Alice wants to securely receive the messages from Bob, but she does not want Bob to have any proof that she received any messages - this low-level simplex messaging protocol can be used in this scenario, as all Bob knows as a fact is that he was able to send one unsigned message to the server that Alice provided, and now can only send messages signed with the key SK that he sent to the server - it does not prove that any message was received by Alice.

For practical purposes of bi-directional conversation, now that Bob can securely send encrypted messages to Alice, Bob can establish the second simplex connection that will allow Alice to send messages to Bob in the same way. If both Alice and Bob have their respective uniqie "public" keys (Alice's and Bob's EKs of two separate connections), the conversation can be both encrypted and signed.

The established connections can also be used to change the encryption keys providing forward secrecy.

This protocol also can be used for off-the-record messaging, as Alice and Bob can have multiple connections established between them and only information they pass to each other allows proving their identity, so if they want to share anything off-the-record they can initiate a new connection without linking it to any other information they exchanged. As a result, this protocol provides better anonymity and better protection from MITM than OTR protocol.

How simplex connections are used by the participants is not in scope of this low level simplex messaging protocol.

SMP qualities and features

The simplex messaging protocol:

• defines only message-passing protocol:
  • transport agnostic - the protocol does not define how clients connect to the servers and does not require persistent connections. While a generic term "command" is used, it can be implemented in various ways - TCP connection, HTTP requests, messages over (web)sockets, etc..
  • not semantic - the protocol does not assign any meaning to connections and messages. While on the application level the connections and messages can have different meaning (e.g., for messages: text or image chat message, message acknowledgement, participant profile information, status updates, changing "public" key to encrypt messages, changing servers, etc.), on the simplex messaging protocol level all the messages are binary and their meaning can only be interpreted by client applications and not by the servers - this interpretation is in scope of application level protocol and out of scope of this simplex messaging protocol.
• client-server architecture:
  • multiple servers, that can be deployed by the system users, can be used to send and retrieve messages.
  • servers do not communicate with each other and do not even "know" about other servers.
  • clients only communicate with servers (excluding the initial out-of-band message), so the message passing is asynchronous.
  • for each connection, the message recipient defines the server through which the sender should send messages.
  • while multiple servers and multiple connections can be used to pass each message, it is in scope of application level protocol(s), and out of scope of this simplex messaging protocol.
  • servers store messages only until they are retrieved by the recipients, and in any case, for a limited time.
  • servers are required to NOT store any message history or delivery log, but even if the server is compromised, it does not allow to decrypt the messages or to determine the list of connections established by any participant - this information is only stored on client devices.
• the only element provided by SMP servers is simplex connections:
  • each connection is created and managed by the connection recipient.
  • assymetric encryption is used to sign and verify the requests to send and receive the messages.
  • one unique "public" key is used for the servers to authenticate requests to send the messages into the connection, and another unique "public" key - to retrieve the messages from the connection. "Unique" here means that each "public" key is used only for one connection and is not used for any other context - effectively this key is not public and does not represent any participant identity.
  • both "public" keys are provided to the server by the connection recepient when the connection is established.
  • the "public" keys known to the server and used to authenticate commands from the participants are unrelated to the keys used to encrypt and decrypt the messages - the latter keys are also unique per each connection but they are only known to participants, not to the servers.
  • messaging graph can be asymmetric: Bob's ability to send messages to Alice does not automatically lead to the Alice's ability to send messages to Bob.
  • connections are identified by sender and recipient server IDs.

Cryptographic algorithms

Simplex messaging clients need to cryptographically sign commands:

• with the recipient's key RK (server to verify):
  • to subscribe to connection.
  • to secure the connection.
  • to delete received messages.
  • to suspend the connection.
  • to delete the connection.
• with the sender's key SK:
  • to send messages (server to verify).

To sign and verify commands, clients and servers MUST use RSA-PSS algorythm defined in RFC3447.

To optinally sign and verify messages, clients SHOULD use RSA-PSS algorythm.

To encrypt and decrypt messages, clients and servers SHOULD use RSA-OAEP algorythm defined in RFC3447.

The reasons to use these algorithms:

• they are supported by WebCrypto API.
• they are newer versions than RSA-PKCS1-v1_5 encryption and signature schemes.
• they are more widely supported than ECC algorithms

Future versions of the protocol may allow different algorithms.

Simplex connection IDs

Simplex messaging servers MUST generate 2 different IDs for each new connection - for recipient (that created the connection) and for sender. It is REQUIRED that:

• these IDs are different and unique within the server.
• based on 128-bit integers generated with cryptographically strong pseudo-random number generator.

Server privacy requirements

Simplex messaging server implementations MUST NOT create, store or send to any other servers:

• logs of the client commands and transport connections in the production environment.
• history of deleted connections, retrieved or removed messages.
• snapshots of the database they use to store connections and messages (instead simplex messaging clients must manage redundancy by using more than one simplex messaging server.
• any other information that may compromise privacy or forward secrecy of communication between clients using simplex messaging servers.

SMP commands

Commands syntax below is provided using ABNF.

Each transmission between the client and the server must have this format/syntax (after the decryption):

transmission = signed CRLF signature CRLF
signed = connId CRLF msg
msg = recipientCmd / send / serverMsg
recipientCmd = create / subscribe / secure / deleteMsg / suspend / delete
serverMsg = conn / ok / error / message
connId = (encoded " ") / "" ; empty connection ID is used with "create" command
signature = encoded / "" ; empty signature can be used with "create" and "send" commands
encoded = base64

base64 encoding should be used with padding, as defined in section 4 of RFC 4648

The syntax of specific commands and responses is defined below.

Correlating responses with commands

The server must send conn, error and ok responses in the same order within each connection ID as the commands received in the transport connection, so that they can be correlated by the clients.

If the transport connection is closed before some responses are sent, these responses should be discarded.

Command authentication

The SMP servers must athenticate all transmissions (excluding create and send commands sent with empty signatures) by verifying the provided signatures. Signature should be the hash of the first part signed (including CRLF characters) of transmission, encrypted with the key associated with the connection ID (sender's or recepient's, depending on which connection ID is used).

Recipient commands

Sending any of the commands in this section (other than create, that is sent without connection ID) is only allowed with recipient's ID (RID). If sender's ID is used the server must respond with "ERROR AUTH" response (see Error responses).

Create connection command

This command is sent by the recipient to the SMP server to create the new connection. The syntax is:

create = %s"CREATE " recipientKey
recipientKey = encoded

If the connection is created successfully, the server must send conn response with the recipient's and sender's connection IDs:

conn = %s"CONN " recipientId " " senderId
recipientId = encoded
senderId = encoded

Once the connection is created, the recipient gets automatically subscribed to receive the messages from that connection, until the transport connection is closed. The subscribe command is needed only to start receiving the messages from the existing connection when the new transport connection is opened.

signature part of transmission should an empty string; SMP servers can also use it to authenticate users who are allowed to create simplex connections.

Subscribe to connection

When the simplex connection was not created in the current transport connection, the recipient must use this command to start receiving messages from it:

subscribe = %s"SUB"

If subscription is successful (ok response) the recipient will be receiving the messages from this connection until the transport connection is closed - there is no command to unsubscribe.

Secure connection command

This command is sent by the recipient to the server to add sender's key to the connection:

secure = %s"SECURE " senderKey
senderKey = encoded

senderKey is received from the sender as part of the first message - see Send Message Command.

Once the connection is secured only signed messages can be sent to it.

Delete message command

The recipient should send this command once the message was stored in the client, to notify the server that the message should be deleted:

deleteMsg = %s"DELMSG " msgId
msgId = encoded

Even if this command is not sent by the recipient, the servers should limit the time of message storage, whether it was delivered to the recipient or not.

Suspend connection

The recipient can suspend connection prior to deleting it to make sure no messages are lost:

suspendCmd = %s"SUSPEND"

The server must respond with "ERROR AUTH" to any messages sent after the connection was suspended (see Error responses).

The server must respond ok to this command only after all messages related to the connection were delivered.

This command can be sent multiple times (in case transport connection was interrupted and the response was not delivered), the server should still respond ok even if the connection is already suspended.

There is no command to unsuspend the connection. Servers must delete suspended connections that were not deleted after some period of time.

Delete connection

The recipient can delete the connection, whether it was suspended or not.

All undelivered messages will not be delivered - they will be deleted as soon as command is received, before the response is sent.

deleteCmd = %s"DELETE"

Sender commands

Currently SMP defines only one command that can be used by sender - send message. This command must be used with sender's ID, if recipient's ID is used the server must respond with "ERROR AUTH" response (see Error responses).

Send message command

This command is sent to the server by the sender both to confirm the connection after the sender received out-of-band message from the recipient and to send messages after the connection is secured:

send = %s"SEND " msgBody
msgBody = stringMsg | binaryMsg
stringMsg = ":" string ; until CRLF in the transmission
string = *(%x01-09 / %x0B-0C / %x0E-FF %) ; any characters other than NUL, CR and LF
binaryMsg = size CRLF msgBody CRLF ; the last CRLF is in addition to CRLF
                        ; in the transmission
size = 1*DIGIT ; size in bytes
msgBody = *OCTET ; any content of specified size - safe for binary

stringMsg is allowed primarily to test SMP servers, e.g. via telnet.

The signature with this command must be empty, otherwise it must result in ERROR SYNTAX response.

The first message is sent to confirm the connection - it should contain sender's server key (see decrypted message syntax below) - this message must be sent without signature.

Once connection is secured (see Secure connection command), messages must be sent with signature.

The server must respond with "ERROR AUTH" response in the following cases:

• connection does not exist or suspended,
• connection is secured but the transmission does NOT have a signature,
• connection is NOT secured but the transmission has a signature.

Until the connection is secured, the server should accept any number of unsigned messages - it both enables the legimate sender to resend the confirmation in case of failure and also allows the simplex messaging client to ignore any confirmation messages that may be sent by the attackers (assuming they could have intercepted the connection ID in the server response, but do not have a correct encryption key passed to sender in out-of-band message).

The body should be encrypted with the recipient's "public" key (EK); once decrypted it must have this format:

decryptedBody = reserved CRLF clientBody CRLF
reserved = senderKeyMsg / *VCHAR
senderKeyMsg = "KEY " senderKey
senderKey = encoded
clientBody = *OCTET

reserved for the initial unsigned message is used to transmit sender's server key and can be used in future revision of the protocol for other purposes.

Server messages

New connection response

Server must respond with this message when the new connection is created.

See its syntax in Create connection command

Deliver connection message

The server must deliver messages to all subscribed simplex connections on the currently open transport connection. The syntax for the message delivery is:

message = %s"MSG " msgId " " timestamp " " msgBody
msgId = encoded
timestamp = date-time; RFC3339

msgId - a unique within the server message ID based on 128-bit cryptographically random integer.

timestamp - the UTC time when the server received the message from the sender, must be in date-time format defined by RFC 3339

msgBody - string or binary message, see syntax in Send message command

Error responses

The server can respond with an error response in the following cases:

• unknown command name ("CMD"),
• incorrect command or transmission syntax ("SYNTAX") - wrong format, number of parameters or incorrect format of parameters ,
• authentication error ("AUTH") - incorrect signature, unknown or suspended connection, sender's ID is used in place of recipient's and vice versa, and some other cases (see Send message command)
• internal server error ("INTERNAL").

The syntax for error responses:

error = %s"ERROR " errorType
errorType = %s"CMD" / %s"SYNTAX" / %s"AUTH" / %s"INTERNAL"

Server implementations must aim to respond within the same time for each command in all cases when "ERROR AUTH" response is required to prevent timing attacks (e.g., the server should execute signature verification even when the connection does not exist on the server).

OK response

When the command is successfully executed by the server, it should respond with OK response:

ok = %s"OK"

Appendices

Appendix A.

Secure encrypted transport connection with the SMP server.

Both the recipient and the sender can use TCP or some other (possibly higher level) transport protocol to communicate with the server.

Some protocol should be used to ecrypt the connection traffic - one simple option that does not require any cetralized certificate authority is below.

When the connection is established, the server sends the binary encryption key that the client should match with key or fingerprint available to them - if they do not match, they should terminate the connection.

The client should respond with the symmetric key that will be used by both the client and the server to encrypt all traffic in the connection - this key should be encrypted with the public key initially sent by the server.

After the symmetric key is sent to the server, all communication should happen in encrypted binary chunks having a fixed size of 4096 bytes irrespective of the size of the command/message that should be sent. Smaller messages should be padded, multiple commands/messages can be packed into a single chunk. If the application using SMP needs to transmit a file or a larger message, it should be broken down into fragments. The format of application level messages within SMP commands is out of scope of this protocol.

Appendix B.

Sending out-of-band message.

SMP does not prescribe the channel to pass out-of-band message - it should be agreed by the client applications.

For practical purposes various solutions can be used, e.g. one of the versions or the analogues of QR code (or their sequence) that is read via the camera, either directly from the participant's device or via the video call. Although a video call still allows for a highly sophisticated MITM attack, it would require that in addition to compromising simplex connection to intercept messages, the attacker also identifies and compromises the video connection in another channel and substitutes the video in real time.