Internet-Draft | MIMI+MLS Protocol | October 2024 |
Barnes, et al. | Expires 24 April 2025 | [Page] |
This document specifies the More Instant Messaging Interoperability (MIMI) transport protocol, which allows users of different messaging providers to interoperate in group chats (rooms), including to send and receive messages, share room policy, and add participants to and remove participants from rooms. MIMI describes messages between providers, leaving most aspects of the provider-internal client-server communication up to the provider. MIMI integrates the Messaging Layer Security (MLS) protocol to provide end-to-end security assurances, including authentication of protocol participants, confidentiality of messages exchanged within a room, and agreement on the state of the room.¶
This note is to be removed before publishing as an RFC.¶
The latest revision of this draft can be found at https://bifurcation.github.io/ietf-mimi-protocol/draft-ralston-mimi-protocol.html. Status information for this document may be found at https://datatracker.ietf.org/doc/draft-ietf-mimi-protocol/.¶
Discussion of this document takes place on the More Instant Messaging Interoperability Working Group mailing list (mailto:mimi@ietf.org), which is archived at https://mailarchive.ietf.org/arch/browse/mimi/. Subscribe at https://www.ietf.org/mailman/listinfo/mimi/.¶
Source for this draft and an issue tracker can be found at https://github.com/bifurcation/ietf-mimi-protocol.¶
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The More Instant Messaging Interoperability (MIMI) transport protocol enables providers of end-to-end encrypted instant messaging to interoperate. As described in the MIMI architecture [I-D.barnes-mimi-arch], group chats and direct messages are described in terms of "rooms". Each MIMI protocol room is hosted at a single provider (the "hub" provider"), but allows users from different providers to become participants in the room. The hub provider is responsible for ordering and distributing messages, enforcing policy, and authorizing messages. It also keeps a copy of the room state, which includes the room policy and participant list, which it can provide to new joiners. Each provider also stores initial keying material for its own users (who may be offline).¶
This document describes the communication among different providers necessary to support messaging application functionality, for example:¶
In support of these functions, the protocol also has primitives to fetch initial keying material and fetch the current state of the underlying end-to-end encryption protocol for the room.¶
Messages sent inside each room are end-to-end encrypted using the Messaging Layer Security (MLS) protocol [RFC9420], and each room is associated with an MLS group. MLS also ensures that clients in a room agree on the room policy and participation. MLS is integrated into MIMI in such a way as to ensure that a client is joined to a room's MLS group only if the client's user is a participant in the room, and that all clients in the group agree on the state of the room (including, for example, the room's participant list).¶
In this version of the document, we have tried to capture enough concrete functionality to enable basic application functionality, while defining enough of a protocol framework to indicate how to add other necessary functionality. The following functions are likely to be needed by the complete protocol, but are not covered here:¶
In this document, we introduce a notional concept of roles for participants, and permissions for roles. Actual messaging systems have more complex and well-specified authorization policies about which clients can take which actions in a room.¶
In this document, all adds / removes / joins / leaves are initiated from within the group, or by a new joiner who already has permission to join, as this aligns well with MLS. Messaging applications support a variety of other flows, some of which this protocol will need to support.¶
Certain entities in the MIMI system need to be identified in the protocol. In this document, we define a notional syntax for identifiers, but a more concrete one should be defined.¶
There is no mechanism in this document for reporting abusive behavior to a messaging provider.¶
In some cases, the identifier used to initiate communications with a user might be different from the identifier that should be used internally. For example, a user-visible handle might need to be mapped to a durable internal identifier. This document provides no mechanism for such resolution.¶
While MLS provides basic message authentication, users should also be able to (cryptographically) tie the identity of other users to their respective providers. Further authentication such as tying clients to their users (or the user's other clients) may also be desirable.¶
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
Terms and definitions are inherited from [I-D.barnes-mimi-arch]. We also make use of terms from the MLS protocol [RFC9420].¶
Throughout this document, the examples use the TLS Presentation Language [RFC8446] and the semantics of HTTP [RFC7231] respectively as placeholder a set of binary encoding mechanism and transport semantics.¶
The protocol layering of the MIMI transport protocol is as follows:¶
An application layer that enables messaging functionality¶
A security layer that provides end-to-end security guarantees:¶
A transport layer that provides secure delivery of protocol objects between servers.¶
MIMI uses MLS for end-to-end security, using the MLS AppSync proposal type to efficiently synchronize room state across the clients involved in a room [RFC9420] [I-D.barnes-mls-appsync]. The MIMI transport is based on HTTPS over mutually-authenticated TLS.¶
This section walks through a basic scenario that illustrates how a room works in the MIMI protocol. The scenario involves the following actors:¶
Service providers a.example
, b.example
, and c.example
represented by
servers ServerA
, ServerB
, and ServerC
respectively¶
Users Alice (alice
), Bob (bob
) and Cathy (cathy
) of the service providers a.example
, b.example
, and c.example
respectively.¶
Clients ClientA1
, ClientA2
, ClientB1
, etc. belonging to these users¶
A room clubhouse
hosted by hub provider a.example
where the three users interact.¶
Inside the protocol, each provider is represented by a domain name in the
host
production of the authority
of a MIMI URI [RFC3986]. Specific
hosts or servers are represented by domain names, but not by MIMI URIs.
Examples of different types of identifiers represented in a MIMI URI are
shown in the table below:¶
Identifier type | Example URI |
---|---|
Provider |
mimi://a.example
|
User |
mimi://a.example/u/alice
|
Client |
mimi://a.example/d/ClientA1
|
Room |
mimi://a.example/r/clubhouse
|
MLS group |
mimi://a.example/g/clubhouse
|
As noted in [I-D.barnes-mimi-arch], the MIMI protocol only defines interactions
between service providers' servers. Interactions between clients and servers
within a service provider domain are shown here for completeness, but
surrounded by [[ double brackets ]]
.¶
The first step in the lifetime of a MIMI room is its creation on the hub server. This operation is local to the service provider, and does not entail any MIMI protocol operations. However, it must establish the initial state of the room, which is then the basis for protocol operations related to the room.¶
For authorization purposes, MIMI uses permissions based on room-defined roles.
For example, a room might have a role named "admin", which has canAddUser
,
canRemoveUser
, and canSetUserRole
permisions.¶
Here, we assume that Alice uses ClientA1 to create a room with the following base policy properties:¶
Room Identifier: mimi://a.example/r/clubhouse
¶
Roles: admin = [canAddUser, canRemoveUser, canSetUserRole]
¶
And the following participant list:¶
Participants: [[mimi://a.example/u/alice, "admin"]]
¶
ClientA1 also creates an MLS group with group ID mimi://a.example/g/clubhouse
and
ensures via provider-local operations that Alice's other clients are members of
this MLS group.¶
Adding Bob to the room entails operations at two levels. First, Bob's user identity must be added to the room's participant list. Second, Bob's clients must be added to the room's MLS group.¶
The process of adding Bob to the room thus begins by Alice fetching key material for Bob's clients. Alice then updates the room by sending an MLS Commit over the following proposals:¶
An AppSync proposal updating the room state by adding Bob to the participant list¶
Add proposals for Bob's clients¶
The MIMI protocol interactions are between Alice's server ServerA and Bob's server ServerB. ServerB stores KeyPackages on behalf of Bob's devices. ServerA performs the key material fetch on Alice's behalf, and delivers the resulting KeyPackages to Alice's clients. Both ServerA and ServerB remember the sources of the KeyPackages they handle, so that they can route a Welcome message for those KeyPackages to the proper recipients -- ServerA to ServerB, and ServerB to Bob's clients.¶
NOTE: In the protocol, it is necessary to have consent (see Section 7) and access control on these operations. We have elided that step here in the interest of simplicity.¶
The process of adding Bob was a bit abbreviated because Alice is a user of the hub service provider. When Bob adds Cathy, we see the full process, involving the same two steps (KeyPackage fetch followed by Add), but this time indirected via the hub server ServerA. Also, now that there are users on ServerB involved in the room, the hub ServerA will have to distribute the Commit adding Cathy and Cathy's clients to ServerB as well as forwarding the Welcome to ServerC.¶
Now that Alice, Bob, and Cathy are all in the room, Cathy wants to say hello to everyone. Cathy's client encapsulates the message in an MLS PrivateMessage and sends it to ServerC, who forwards it to the hub ServerA on Cathy's behalf. Assuming Cathy is allowed to speak in the room, ServerA will forward Cathy's message to the other servers involved in the room, who distribute it to their clients.¶
A user removing another user follows the same flow as adding the user. The user performing the removal creates an MLS commit covering Remove proposals for all of the removed user's devices, and an AppSync proposal updating the room state to remove the removed user from the room's participant list.¶
One's own user leaving is slightly more complicated than removing another user, because the leaving user cannot remove all of their devices from the MLS group. Instead, the leave happens in three steps:¶
The leaving client constructs MLS Remove proposals for all of the user's devices (including the leaving client), and an AppSync proposal that removes its user from the participant list.¶
The leaving client sends these proposals to the hub. The hub caches the proposals.¶
The next time a client attempts to commit, the hub requires the client to include the cached proposals.¶
The hub thus guarantees the leaving client that they will be removed as soon as possible.¶
Many users have multiple clients often running on different devices (for example a phone, a tablet, and a computer). When a user creates a new client, that client needs to be able to join all the MLS groups associated with the rooms in which the user is a participant.¶
In MLS in order to initiate joining a group the joining client needs to get the current GroupInfo
and ratchet_tree
, and then send an External Commit to the hub. In MIMI,
the hub keeps or reconstructs a copy of the GroupInfo, assuming that other
clients may not be available to assist the client with joining.¶
For Cathy's new client (ClientC3) to join the MLS group and therefore fully participate in the room with Alice, ClientC3 needs to fetch the MLS GroupInfo, and then generate an External Commit adding ClientC3.¶
Cathy's new client sends the External Commit to the room's MLS group by sending an /update to the room.¶
MIMI servers communicate using HTTPS. The HTTP request MUST identify the source and target providers for the request, in the following way:¶
The target provider is indicated using a Host header [RFC9110]. If the provider is using a non-standard port, then the port component of the Host header is ignored.¶
The source provider is indicated using a From header [RFC9110]. The
mailbox
production in the From header MUST use the addr-spec
variant, and
the local-part
of the address MUST contain the fixed string mimi
. Thus,
the content of the From header will be mimi@a.example
, where a.example
is
the domain name of the source provider.¶
NOTE: The use of the From header field here is not really well-aligned with its intended use. The WG should consider whether this is correct, or whether a new header field would be better. Perhaps something like "From-Host" to match Host?¶
The TLS connection underlying the HTTPS connection MUST be mutually authenticated. The certificates presented in the TLS handshake MUST authenticate the source and target provider domains, according to [RFC6125].¶
The bodies of HTTP requests and responses are defined by the individual endpoints defined in Section 4.3.¶
Every MIMI room has an MLS group associated to it, which provides end-to-end security guarantees. The clients participating in the room manage the MLS-level membership by sending Commit messages covering Add and Remove proposals.¶
Every application message sent within a room is authenticated and confidentiality-protected by virtue of being encapsulated in an MLS PrivateMessage object.¶
MIMI uses the MLS application state synchronization mechanism ([I-D.barnes-mls-appsync]) to ensure that the clients involved in a MIMI room agree on the state of the room. Each MIMI message that changes the state of the room is encapsulated in an AppSync proposal and transmitted inside an MLS PublicMessage object.¶
The PublicMessage encapsulation provides sender authentication, including the ability for actors outside the group (e.g., servers involved in the room) to originate AppSync proposals. Encoding room state changes in MLS proposals ensures that a client will not process a commit that confirms a state change before processing the state change itself.¶
TODO: A little more needs to be said here about how MLS is used. For example: What types of credential are required / allowed? If servers are going to be allowed to introduce room changes, how are their keys provisioned as external signers? Need to maintain the membership and the list of queued proposals.¶
Servers in MIMI provide a few functions that enable messaging applications. All servers act as publication points for key material used to add their users to rooms. The hub server for a room tracks the state of the room, and controls how the room's state evolves, e.g., by ensuring that changes are compliant with the room's policy. Non-hub servers facilitate interactions between their clients and the hub server.¶
In this section, we describe the state that servers keep. The following top level section describes the HTTP endpoints exposed to enable these functions.¶
Every MIMI server is a publication point for users' key material, via the
keyMaterial
endpoint discussed in Section 5.2. To support this
endpoint, the server stores a set of KeyPackages, where each KeyPackage belongs
to a specific user and device.¶
Each KeyPackage includes a list of its MLS client's capabilities (MLS
protocol versions, cipher suites, extensions, proposal types, and credential
types). When claiming KeyPackages, the requester includes the list of
RequiredCapabilites
to ensure the new joiner is compatible with and
capable of participating in the corresponding room.¶
The hub server for the room stores the state of the room, comprising:¶
The base policy of the room, which does not depend on the specific participants in the room. For example, this includes the room roles and their permissions.¶
The participant list: a list of the users who are participants of the room, and each user's role in the room.¶
TODO: We need a more full description of the room, room state syntax.¶
When a client requests key material via the hub, the hub records the KeyPackageRef values for the returned KeyPackages, and the identity of the provider from which they were received. This information is then used to route Welcome message to the proper provider.¶
The participant list can be changed by adding or removing users, or changing a user's role. These changes are described without a specific syntax as a list of adds, removes, and role changes:¶
To put these changes into effect, a client or server encodes them in an AppSync
proposal, signs the proposal as a PublicMessage, and submits them to the
update
endpoint on the hub.¶
This section describes the specific endpoints necessary to provide the functionality in the example flow. The framing for each endpoint includes a protocol so that different variations of the end-to-end encryption can be used.¶
TODO: Determine the what needs to be included in the protocol. MIMI version, e2e protocol version, etc.¶
The syntax of the MIMI protocol messages are described using the TLS presentation language format (Section 3 of [RFC8446]).¶
enum { reserved(0), mls10(1), (255) } Protocol;¶
Like the ACME protocol (See Section 7.1.1 of [RFC8555]), the MIMI protocol uses a directory document to convey the HTTPS URLs used to reach certain endpoints (as opposed to hard coding the endpoints).¶
The directory URL is discovered using the mimi-protocol-directory
well-known
URI. The response is a JSON document with URIs for each type of endpoint.¶
GET /.well-known/mimi-protocol-directory¶
{ "keyMaterial": "https://mimi.example.com/v1/keyMaterial/{targetUser}", "update": "https://mimi.example.com/v1/update{roomId}", "notify": "https://mimi.example.com/v1/notify/{roomId}", "submitMessage": "https://mimi.example.com/v1/submitMessage/{roomId}", "groupInfo": "https://mimi.example.com/v1/groupInfo/{roomId}", "requestConsent": "https://mimi.example.com/v1/requestConsent/{targetUser}", "updateConsent": "https://mimi.example.com/v1/updateConsent/{requesterUser}", "identifierQuery": "https://mimi.example.com/v1/identifierQuery/{domain}", "reportAbuse": "https://mimi.example.com/v1/reportAbuse/{roomId}" }¶
This action attempts to claim initial keying material for all the clients of a single user at a specific provider. The keying material is designed for use in a single room and may not be reused. It uses the HTTP POST method.¶
POST /keyMaterial/{targetUser}¶
The target user's URI is listed in the request path. KeyPackages requested using this primitive MUST be sent via the hub provider of whatever room they will be used in. (If this is not the case, the hub provider will be unable to forward a Welcome message to the target provider).¶
The path includes the target user. The request body includes the protocol (currently just MLS 1.0), and the requesting user. When the request is being made in the context of adding the target user to a room, the request MUST include the room ID for which the KeyPackage is intended, as the target may have only granted consent for a specific room.¶
For MLS, the request includes a non-empty list of acceptable MLS ciphersuites,
and an MLS RequiredCapabilities
object (which contains credential types,
non-default proposal types, and extensions) required by the requesting provider
(these lists can be an empty).¶
The request body has the following form.¶
struct { opaque uri<V>; } IdentifierUri; struct { Protocol protocol; IdentifierUri requestingUser; IdentifierUri targetUser; IdentifierUri roomId; select (protocol) { case mls10: CipherSuite acceptableCiphersuites<V>; RequiredCapabilities requiredCapabilities; }; } KeyMaterialRequest;¶
The response contains a user status code that indicates keying material was
returned for all the user's clients (success
), that keying material was
returned for some of their clients (partialSuccess
), or a specific user code
indicating failure. If the user code is success or partialSuccess, each client
is enumerated in the response. Then for each client with a client success
code, the structure includes initial keying material (a KeyPackage for MLS 1.0).
If the client's code is nothingCompatible
, the client's capabilities are
optionally included (The client's capabilities could be omitted for privacy
reasons.)¶
If the user code is noCompatibleMaterial
, the provider MAY populate the
clients
list. For any other user code, the provider MUST NOT populate the
clients
list.¶
Keying material provided from one response MUST NOT be provided in any other
response.
The target provider MUST NOT provide expired keying material (ex: an MLS
KeyPackage containing a LeafNode with a notAfter
time past the current date
and time).¶
enum { success(0); partialSuccess(1); incompatibleProtocol(2); noCompatibleMaterial(3); userUnknown(4); noConsent(5); noConsentForThisRoom(6); userDeleted(7); (255) } KeyMaterialUserCode; enum { success(0); keyMaterialExhausted(1), nothingCompatible(2), (255) } KeyMaterialClientCode; struct { KeyMaterialClientCode clientStatus; IdentifierUri clientUri; select (protocol) { case mls10: select (clientStatus) { case success: KeyPackage keyPackage; case nothingCompatible: optional<Capabilities> clientCapabilities; }; }; } ClientKeyMaterial; struct { Protocol protocol; KeyMaterialUserCode userStatus; IdentifierUri userUri; ClientKeyMaterial clients<V>; } KeyMaterialResponse;¶
The semantics of the KeyMaterialUserCode
are as follows:¶
success
indicates that key material was provided for every client of the
target user.¶
partialSuccess
indicates that key material was provided for at least one
client of the target user.¶
incompatibleProtocol
indicates that either one of providers supports the
protocol requested, or none of the clients of the target user support the
protocol requested.¶
noCompatibleMaterial
indicates that none of the clients was able to
supply key material compatible with the requiredCapabilities
field in the
request.¶
userUnknown
indicates that the target user is not known to the target
provider.¶
noConsent
indicates that the requester does not have consent to fetch
key material for the target user. The target provider can use this response
as a catch all and in place of other status codes such as userUnknown
if
desired to preserve the privacy of its users.¶
noConsentForThisRoom
indicates that the target user might have allowed
a request for another room, but does not for this room. If the provider
does not wish to make this distinction, it can return noConsent
instead.¶
userDeleted
indicates that the target provider wishes the requester to
know that the target user was previously a valid user of the system and has
been deleted. A target provider can of course use userUnknown
if the
provider does wish to keep or specify this distinction.¶
The semantics of the KeyMaterialClientCode
are as follows:¶
success
indicates that key material was provided for the specified
client.¶
keyMaterialExhausted
indicates that there was no keying material
available for the specified client.¶
nothingCompatible
indicates that the specified clients had no key
material compatible with the requiredCapabilities
field in the request.¶
At minimum, as each MLS KeyPackage is returned to a requesting provider (on
behalf of a requesting IM client), the target provider needs to associate its
KeyPackageRef
with the target client and the hub provider needs to associate
its KeyPackageRef
with the target provider. This ensures that Welcome messages
can be correctly routed to the target provider and client. These associations
can be deleted after a Welcome message is forwarded or after the KeyPackage
leaf_node.lifetime.not_after
time has passed.¶
Adds, removes, and policy changes to the room are all forms of updating the room state. They are accomplished using the update transaction which is used to update the room base policy, participation list, or its underlying MLS group. It uses the HTTP POST method.¶
POST /update/{roomId}¶
Any change to the participant list or room policy (including
authorization policy) is communicated via an AppSync
proposal type
with the applicationId
of mimiParticipantList
or mimiRoomPolicy
respectively. When adding a user, the proposal containing the participant list
change MUST be committed either before or simultaneously with the corresponding
MLS operation.¶
Removing an active user from a participant list or banning an active participant likewise also happen simultaneously with any MLS changes made to the commit removing the participant.¶
A hub provider which observes that an active participant has been removed or
banned from the room, MUST prevent any of its clients from sending or
receiving any additional application messages in the corresponding MLS group;
MUST prevent any of those clients from sending Commit messages in that group;
and MUST prevent it from sending any proposals except for Remove
and
SelfRemove
[I-D.ietf-mls-extensions] proposals for its users in that group.¶
The update request body is described below, using the
RatchetTreeOption
and PartialGroupInfo
structs defined in
[I-D.mahy-mls-ratchet-tree-options]:¶
struct { /* A Proposal or Commit which is either a PublicMessage; */ /* or a SemiPrivateMessage */ MLSMessage proposalOrCommit; select (proposalOrCommit.content.content_type) { case commit: /* Both the Welcome and GroupInfo omit the ratchet_tree */ optional<Welcome> welcome; GroupInfoOption groupInfoOption; RatchetTreeOption ratchetTreeOption; case proposal: /* a list of additional proposals, each represented */ /* as either PublicMessage or SemiPrivateMessage */ MLSMessage moreProposals<V>; } HandshakeBundle; enum { reserved(0), full(1), partial(2), (255) } GroupInfoRepresentation; struct { GroupInfoRepresentation representation; select (representation) { case full: GroupInfo groupInfo; case partial: PartialGroupInfo partialGroupInfo; } } GroupInfoOption; struct { select (room.protocol) { case mls10: HandshakeBundle bundle; }; } UpdateRequest;¶
The semantics of GroupInfoRepresentation
are as follows:¶
full
means that the entire GroupInfo will be included.¶
partial
means that a PartialGroupInfo
struct will be shared and
that the Distribution Service is expected to reconstruct the GroupInfo
as described in [I-D.mahy-mls-ratchet-tree-options].¶
For example, in the first use case described in the Protocol Overview, Alice creates a Commit
containing an AppSync proposal adding Bob (mimi://b.example/b/bob
), and Add proposals for all
Bob's MLS clients. Alice includes the Welcome message which will be sent for
Bob, a GroupInfo object for the hub provider, and complete ratchet_tree
extension.¶
A handshake message could be sent by the client as an MLS
PublicMessage
(which is visible to all providers), or as an MLS
SemiPrivateMessage
[I-D.mahy-mls-semiprivatemessage] encrypted
for the members and the hub provider as the sole external_receiver
.
(The contents and sender of a SemiPrivateMessage
would not be visible to
other providers). The use of SemiPrivateMessage
allows the Hub to
accomplish its policy enforcement responsibilities without the other
providers being aware of the membership of non-local users.¶
The response body is described below:¶
enum { success(0), wrongEpoch(1), notAllowed(2), invalidProposal(3), (255) } UpdateResponseCode; struct { UpdateResponseCode responseCode; string errorDescription; select (responseCode) { case success: /* the hub acceptance time (in milliseconds from the UNIX epoch) */ uint64 acceptedTimestamp; case wrongEpoch: /* current MLS epoch for the MLS group */ uint64 currentEpoch; case invalidProposal: ProposalRef invalidProposals<V>; }; } UpdateRoomResponse¶
The semantics of the UpdatedResponseCode
values are as follows:¶
success
indicates the UpdateRequest
was accepted and will be distributed.¶
wrongEpoch
indicates that the hub provider is using a different epoch. The
currentEpoch
is provided in the response.¶
notAllowed
indicates that some type of policy or authorization prevented the
hub provider from accepting the UpdateRequest
.¶
invalidProposal
indicates that at least one proposal is invalid. A list of
invalidProposals is provided in the response.¶
End-to-end encrypted (application) messages are submitted to the hub for authorization and eventual fanout using an HTTP POST request.¶
POST /submitMessage/{roomId}¶
The request body is as follows:¶
struct { Protocol protocol; select(protocol) { case mls10: /* PrivateMessage containing an application message */ MLSMessage appMessage; IdentifierURI sendingUri; }; } SubmitMessageRequest;¶
If the protocol is MLS 1.0, the request body (appMessage
) is an MLSMessage
with a WireFormat of PrivateMessage, and a content_type
of application
.
The sendingUri
is a valid URI of the sender and is an active participant
in the room.¶
The response indicates if the message was accepted by the hub provider. If a
frankingTag
was included in the FrankAAD
extension in the PrivateMessage
Additional Authenticated Data (AAD) in the request, the server attempts to
frank the message and includes the serverFrank
in a successful response
(see the next subsection).¶
enum { accepted(0), notAllowed(1), epochTooOld(2), (255) } SubmitResponseCode; struct { Protocol protocol; select(protocol) { case mls10: SubmitResponseCode statusCode; select (statusCode) { case success: /* the hub acceptance time (in milliseconds from the UNIX epoch) */ uint64 acceptedTimestamp; optional uint8[32] serverFrank; case epochTooOld: /* current MLS epoch for the MLS group */ uint64 currentEpoch; }; }; } SubmitMessageResponse;¶
The semantics of the SubmitResponseCode
values are as follows:¶
success
indicates the SubmitMessageRequest
was accepted and will be distributed.¶
notAllowed
indicates that some type of policy or authorization prevented the
hub provider from accepting the UpdateRequest
. This could include
nonsensical inputs such as an MLS epoch more recent than the hub's.¶
epochTooOld
indicates that the hub provider is using a new MLS epoch
for the group. The currentEpoch
is provided in the response.¶
ISSUE: Do we want to offer a distinction between regular application messages and ephemeral applications messages (for example "is typing" notifications), which do not need to be queued at the target provider.¶
Franking is the placing of a cryptographic "stamp" on a message. In the MIMI context, the Hub is able to mark that it received a message without learning the message content. A receiver that decrypts the message can use a valid frank to prove it was received by the Hub and that the content was sent by a specific sender. Outsiders (including follower providers) never learn the content of the message, nor the sender.¶
Franking was popularized by Facebook and described in their whitepaper [SecretConversations] about their end-to-end encryption system. This franking mechanism is largely motivated by that solution with two significant changes as discussed in the final paragraph of this section.¶
When ready to send an application message with the MIMI content format,
the sender generates a new cryptographically random 256-bit franking_key
.
An example mechanism to generate the franking_key
safely is discussed in
Section 8.1.1.¶
Next the sender attaches to the message the franking_key
and any other
fields the sender wishes to commit that are not otherwise represented in the
content. For a MIMI content object, the sender creates a CBOR "FrankingAssertion" map containing the franking_key
, sender URI, and room
URI. It adds this FrankingAssertion to the extensions map at the top level
of the MIMI content using the integer key TBD1.¶
/ FrankingAssertion map / { / FrankingKey / 1: h'9c8af7674941aa95f8df37bd36ea89f2 a3ab433aa5baa8e5e465f08a7e8e3b57', / SenderURI / 2: "mimi://b.example/u/alice", / RoomURI / 3: "mimi://hub.example/r/Rl33FWLCYWOwxHrYnpWDQg", }¶
Note that this assertion does not vouch for the validity of these values, it just means that the sender is claiming it sent the values in the content, and cannot later deny to a receiver that it sent them.¶
Then the client calculates the franking_tag
, as the HMAC SHA256 of the
application_data
(which includes the FrankingAssertion extension), using the franking_key
:¶
franking_tag = HMAC_SHA256( franking_key, application_data)¶
The client includes the franking_tag
in the Additional Authenticated Data
of the MLS PrivateMessage using the Safe Extension FrankAAD
. The client
uses the MIMI submitMessage to send its message, and also asserts a sender
identity to the Hub, which could be a valid pseudonym, and needs to match
the sender URI value embedded in the message. If the message is accepted,
the response includes the accepted timestamp and the serverFrank (generated
by the server).¶
The Hub relies on a per-epoch secret shared among the members of the group
and itself to obfuscate the message metadata (the context
) the Hub uses
while franking. It derives the franking_context_secret
(with the label
"franking_context") from the ap_exporter_secret
in the Associated Party
Key Schedule [I-D.kohbrok-mls-associated-parties].¶
When the Hub receives an acceptable application message with the FrankAAD
AAD extension and a valid sender identity, it calculates a server frank for
the message as follows:¶
context = senderURI || roomURI || acceptedTimestamp serverFrank = HMAC_SHA256(HUBkey, franking_tag || context ) franking_context_hash = SHA256(franking_context_secret || context)¶
HUBkey
is a secret symmetric key used on the Hub which the Hub can use to verify its own tags.¶
The Hub fans out the encrypted message (which includes the franking_tag
),
the serverFrank
, the acceptedTimestamp
, the room URI, and the
franking_context_hash
. Note that the senderURI
is not included in the
application message, so the sender can remain anonymous with respect to
follower providers.¶
When a client receives and decrypts an otherwise valid application message
from a hub provider, the client looks for the existence of a frank
(consisting of the franking_tag
in the AAD, the serverFrank
and the
franking_context_hash
. If so, it derives the franking_context_secret
from the ap_exporter_secret
in the Associated Party Key Schedule
[I-D.kohbrok-mls-associated-parties]; then it verifies the construction of
the franking_tag
from the content of the message, and the construction of
the franking_context_hash
from the sender URI, room ID,
acceptedTimestamp
, and franking_context_secret
.¶
The receiving client receives a sender identifier in three different locations. The receiver verifies that they are all the same:¶
the sender's identity in its credential in its MLS LeafNode¶
the sender's identity asserted in the FrankingAssertion map inside the MIMI Content¶
the (hidden) sender's identity in the context used to create the serverFrank
. The client hashes the concatenation of the sender's identity, the room ID, and the acceptedTimestamp. If this hash matches the context_validation hash, then the identity used by the server was correct.¶
The receiver needs to store the frank with the decoded message so it can be used later.¶
Unlike in the Facebook franking scheme [SecretConversations], the sender
"commits to" its franking_tag
as Additional Authenticated Data (AAD) inside the end-to-end encrypted message, and the hub only sends a hash of
its context. This first change insures that the client cannot come up with
another franking_key
and message that has the same franking_tag
[Grubbs2017][InvisibleSalamanders]. According to [Grubbs2017],
"... [Facebook's] franking scheme does not bind [the franking tag] to [the
ciphertext] by including [the franking tag] in the associated data during
encryption".
The second change allows receivers to validate the sender URI in the hub's
context, without revealing the sender URI to follower providers.¶
If the hub provider accepts an application or handshake message (proposal or commit) message, it forwards that message to all other providers with active participants in the room and all local clients which are active members. This is described as fanning the message out. One can think of fanning a message out as presenting an ordered list of MLS-protected events to the next "hop" toward the receiving client.¶
An MLS Welcome message is sent to the providers and local users associated with
the KeyPackageRef
values in the secrets
array of the Welcome. In the case
of a Welcome message, a RatchetTreeOption
(see Section 3 of [I-D.mahy-mls-ratchet-tree-options]) is also included
in the FanoutMessage.¶
The hub provider also fans out any messages which originate from itself (ex: MLS External Proposals).¶
The hub can include multiple concatenated FanoutMessage
objects relevant to
the same room. This endpoint uses the HTTP POST method.¶
POST /notify/{roomId}¶
struct { uint8[32] franking_tag; uint8[32] serverFrank; uint8[32] franking_context_hash; } Frank; struct { /* the hub acceptance time (in milliseconds from the UNIX epoch) */ uint64 timestamp; select (protocol) { case mls10: /* A PrivateMessage containing an application message, a PublicMessage containing a proposal or commit, or a Welcome message. */ MLSMessage message; select (message.wire_format) { case application: optional Frank frank; case welcome: RatchetTreeOption ratchetTreeOption; }; }; } FanoutMessage;¶
NOTE: Correctly fanning out Welcome messages relies on the hub and target
providers storing the KeyPackageRef
of claimed KeyPackages.¶
A client which receives a success
to either an UpdateRoomResponse
or a
SubmitMessageResponse
can view this as a commitment from the hub provider that
the message will eventually be distributed to the group. The hub is not
expected to forward the client's own message to the client or its provider.
However, the client and its provider need to be prepared to receive the
client's (effectively duplicate) message. This situation can occur during
failover in high availability recovery scenarios.¶
Clients that are being removed SHOULD receive the corresponding Commit message, so they can recognize that they have been removed and clean up their internal state. A removed client might not receive a commit if it was removed as a malicious or abusive client, or if it obviously deleted.¶
The response to a FanoutMessage contains no body. The HTTP response code indicates if the messages in the request were accepted (201 response code), or if there was an error. The hub need not wait for a response before sending the next fanout message.¶
If the hub server does not contain an HTTP 201 response code, then it SHOULD
retry the request, respecting any guidance provided by the server in HTTP header
fields such as Retry-After. If a follower server receives a duplicate request
to the /notify
endpoint, in the sense of a request from the same hub server
with the same request body as a previous /notify
request, then the follower
server MUST return a 201 Accepted response. In such cases, the follower server
SHOULD process only the first request; subsequent duplicate requests SHOULD be
ignored (despite the success response).¶
NOTE: These deduplication provisions require follower servers to track which request bodies they have received from which hub servers. Since the matching here is byte-exact, it can be done by keeping a rolling list of hashes of recent messages.¶
This byte-exact replay criterion might not be the right deduplication strategy. There might be situations where it is valid for the same hub server to send the same payload multiple times, e.g., due to accidental collisions.¶
If this is a concern, then an explicit transaction ID could be introduced. The follower server would still have to keep a list of recently seen transaction IDs, but deduplication could be done irrespective of the content of request bodies.¶
When a client joins an MLS group without an existing member adding the client to the MLS group, that is called an external join. This is useful a) when a new client of an existing user needs to join the groups of all the user's rooms. It can also be used b) when a client did not have key packages available but their user is already in the participation list for the corresponding room, c) when joining an open room, or d) when joining using an external authentication joining code. In MIMI, external joins are accomplished by fetching the MLS GroupInfo for a room's MLS group, and then sending an external commit incorporating the GroupInfo.¶
The GroupInfoRequest uses the HTTP POST method.¶
POST /groupInfo/{roomId}¶
The request provides an MLS credential proving the requesting client's real or pseudonymous identity. This user identity is used by the hub to correlate this request with the subsequent external commit. The credential may constitute sufficient permission to authorize providing the GroupInfo and later joining the group. Alternatively, the request can include an optional opaque joining code, which the requester could use to prove permission to fetch the GroupInfo, even if it is not yet a participant.¶
The request also provides a signature public key corresponding to the requester's credential. It also specifies a CipherSuite which merely needs to be one ciphersuite in common with the hub. It is needed only to specify the algorithms used to sign the GroupInfoRequest and GroupInfoResponse.¶
struct { Protocol protocol; select (protocol) { case mls10: CipherSuite cipher_suite; SignaturePublicKey requestingSignatureKey; Credential requestingCredential; HPKEPublicKey groupInfoPublicKey; optional opaque joiningCode<V>; }; } GroupInfoRequestTBS; struct { Protocol protocol; select (protocol) { case mls10: CipherSuite cipher_suite; SignaturePublicKey requestingSignatureKey; Credential requestingCredential; HPKEPublicKey groupInfoPublicKey; opaque joiningCode<V>; /* SignWithLabel(., "GroupInfoRequestTBS", GroupInfoRequestTBS) */ opaque signature<V>; }; } GroupInfoRequest;¶
If successful, the response body contains the GroupInfo and a way
to get the ratchet_tree, both encrypted with the groupInfoPublcKey
passed in the request.¶
enum { reserved(0), success(1), notAuthorized(2), noSuchRoom(3), (255) } GroupInfoCode; struct { GroupInfo groupInfo; /* without embedded ratchet_tree */ RatchetTreeOption ratchetTreeOption; } GroupInfoRatchetTreeTBE; GroupInfoRatchetTreeTBE group_info_ratchet_tree_tbe; encrypted_groupinfo_and_tree = EncryptWithLabel( groupInfoPublicKey, "GroupInfo and ratchet_tree encryption", room_id, /* context */ group_info_ratchet_tree_tbe) struct { Protocol version; GroupInfoCode status; select (protocol) { case mls10: CipherSuite cipher_suite; opaque room_id<V>; ExternalSender hub_sender; opaque encrypted_groupinfo_and_tree<V>; }; } GroupInfoResponseTBS; struct { Protocol version; GroupInfoCode status; select (protocol) { case mls10: CipherSuite cipher_suite; opaque room_id<V>; ExternalSender hub_sender; opaque encrypted_groupinfo_and_tree<V>; /* SignWithLabel(., "GroupInfoResponseTBS", GroupInfoResponseTBS) */ opaque signature<V>; }; } GroupInfoResponse;¶
The semantics of the GroupInfoCode
are as follows:¶
success
indicates that GroupInfo and ratchet tree was provided as
requested.¶
notAuthorized
indicates that the requester was not authorized to access
the GroupInfo.¶
noSuchRoom
indicates that the requested room does not exist. If the hub
does not want to reveal if a room ID does not exist it can use
notAuthorized
instead.¶
TODO: Consider adding additional failure codes/semantics for joining codes (ex: code expired, already used, invalid)¶
ISSUE: What security properties are needed to protect a GroupInfo object in the MIMI context are still under discussion. It is possible that the requester only needs to prove possession of their private key. The GroupInfo in another context might be sufficiently sensitive that it should be encrypted from the end client to the hub provider (unreadable by the local provider).¶
As discussed in Section 7, there are many ways that a provider could implicitly determine consent. This section describes a mechanism by which providers can explicitly request consent from a user of another provider, cancel such a request, convey that consent was granted, or convey that consent was revoked or preemptively denied.¶
Since they are not necessarily in the context of a room, consent requests are sent directly from the provider of the user requesting consent, to the provider of the target user. (There is no concept of a hub outside of the context of a room.)¶
POST /requestConsent/{targetDomain} POST /updateConsent/{requesterDomain}¶
A requestConsent
request is used by one provider to request explicit
consent from a target user at another provider to fetch the target's
KeyPackages (which is a prerequisite for adding the target to a group); or
to cancel that request.
The request body is a ConsentEntry
, with a consentOperation
of request
or cancel
respectively. It includes the URI of requesting user in the
requesterUri
and the target user URI in the targetUri
. If consent is only
requested for a single room, the requester includes the roomId
. The
combination of the requesterUri
, targetUri
, and optional roomId
represents the ConsentScope
. A cancel
MUST use the same ConsentScope
as a previous request
.¶
For a requestContent
, the targetUri
needs to be in one of the domains of
the receiving provider, and the requesterUri
needs to be in one of the
domains of the sending provider.¶
The response to a requestConsent
request is usually a 201 Accepted
(indicating the requestConsent
was received), optionally a 404 Not Found
(indicating the targetUri
is unknown), or a 500-class response. The
201 response code merely indicates that the request was received. A provider
that does not wish to reveal if a user is not found can respond with a 201
Accepted. Likewise in response to a cancel
which has no request
matching the
ConsentScope
, a 201 Accepted is sent and no further action is taken.¶
enum { cancel(0), request(1), grant(2), revoke(3), (255) } ConsentOperation; struct { ConsentOperation consentOperation; IdentifierUri requesterUri; IdentifierUri targetUri; optional<RoomId> roomId; select (consentOperation) { case grant: KeyPackage clientKeyPackages<V>; }; } ConsentEntry; struct { IdentifierUri requesterUri; IdentifierUri targetUri; optional<RoomId> roomId; } ConsentScope;¶
An updateConsent
request is used by one provider to provide explicit
notice from a target user at one provider that consent for a specific
"requester" was granted, revoked, or preemptively denied. In this context,
the requester is the party that will later request KeyPackages for the target. The request body is
a ConsentEntry
, with a consentOperation
of grant
(for a grant), or
revoke
for revocation or denial. Like a request, it includes the URI of the
"requesting user" in the requesterUri
and the target user URI in the
targetUri
. If consent is only granted or denied for a single room, the request includes the optional roomId
.¶
A grant
or revoke
does not need to be in response to an explicit request, nor does the ConsentScope
need to match a previous request
for the same targetUri
and requesterUri
pair.¶
For example, in some systems there is a notion of a bilateral connection
request. The party that initiates the connection request (for example Alice)
would send a requestConsent
for the target (ex: Bob), and send an
unsolicited updateConsent
with Bob as the "requestor" and itself (Alice)
as the target.¶
In a grant
, the sender includes a list of clientKeyPackages
for the
target user, which can be empty. For the case of a bilateral connection,
a grant of consent with a matching ConsentScope
often results in an
immediate Add to a group. If the list is non-empty this reduces the
number of messages which need to be sent.¶
For updateConsent
the requesterUri
needs to be in one of the domains of
the receiving provider, and the targetUri
needs to be in one of the
domains of the sending provider.¶
The response to an updateConsent
is usually a 201 Accepted (indicating
the updateConsent
was received), optionally a 404 Not Found (indicating the
requesterUri
is unknown), or a 500-class response. The response code
merely indicates that the request was received. A provider that does not
wish to reveal if a user is not found can respond with a 201 Accepted.¶
NOTE: Revoking consent for a user might be privacy sensitive. If this
is the case the target provider does not need to send a revoke
to inform
the requester provider.¶
The identifier query is to find the internal URI for a specific user on a specific provider. It is only sent from the local provider to the target provider (it does not transit a hub).¶
Note that this POST request is idempotent and safe in the sense defined by Section 9.2.2 of [RFC9110].¶
POST /identifierQuery/{domain}¶
Consider three users Xavier, Yolanda, and Zach all with accounts on provider XYZ. Xavier is a sales person and wants to be contactable easily by potential clients on the XYZ provider. He configures his profile on XYZ so that searching for his first or last name or handle will find his profile and allow Alice to send him a consent request (it is out of scope how Alice verifies she has found the intended Xavier and not a different Xavier or an impostor). Yolanda has her XYZ handle on her business cards and the email signature she uses with clients. She configures her profile so that a query for her exact handle will find her profile and allow Alice to send her a consent request. Zach does not wish to be queryable at all. He has configured his account so even an exact handle search returns no results. He could still send a join link out-of-band to Alice for her to join a room of Zach's choosing.¶
The request body is described as:¶
enum { reserved(0), handle(1), nick(2), email(3), phone(4), partialName(5), wholeProfile(6), oidcStdClaim(7), vcardField(8), (255) } SearchIdentifierType; struct { SearchIdentifierType searchType; opaque searchValue<V>; /* a UTF8 string */ select(type) { case oidcStdClaim: opaque claimName<V>; case vcardField: opaque fieldName<V>; }; } IdentifierRequest;¶
The response body is described as an IdentifierResponse
. It can contain
multiple matches depending on the type of query and the policy of the target
provider.¶
The response contains a code indicating the status of the query. success
means that at least one result matched the query. notFound
means that
while the request was acceptable, no results matched the query.
ambiguous
means that a field (ex: handle) or combination of fields
(ex: first and last name) need to match exactly for the provider to return
any responses. forbidden
means that use of this endpoint is not allowed
by the provider or that an unspecified field or combination of fields is
not allowed in an identifier query. unsupportedField
means that the
provider does not support queries on one of the fields queried.¶
enum { success(0), notFound(1), ambiguous(2), forbidden(3), unsupportedField(4), (255) } IdentifierQueryCode; enum { reserved(0), oidcStdClaim(7), vcardField(8), (255) } FieldSource; struct { FieldSource fieldSource; string fieldName; opaque fieldValue<V>; } ProfileField; struct { IdentifierUri stableUri; ProfileField fields<V>; } UserProfile; struct { IdentifierQueryCode responseCode; IdentifierUri uri<V>; UserProfile foundProfiles<V>; } IdentifierResponse;¶
TODO: The format of specific identifiers is discussed in [I-D.mahy-mimi-identity]. Any specific conventions which are needed should be merged into this document.¶
Abuse reports are only sent to the hub provider. They are sent as an HTTP POST request.¶
POST /reportAbuse/{roomId}¶
The reportingUser
optionally contains the identity of the user sending the
abuseReport
, while the allegedAbuserUri
contains the URI of the alleged
sender of abusive messages. The reasonCode
is reserved to identify the type of
abuse, and the note
is a UTF8 human-readable string, which can be empty.¶
TODO: Find a standard taxonomy of reason codes to reference for
the AbuseType
. The IANA Messaging Abuse Report Format parameters are
insufficient.¶
Finally, abuse reports can optionally contain a handful of allegedly
AbusiveMessage
s, each of which contains an allegedly abusive message, its franks, and its timestamp.¶
struct { /* the MIMI Content message containing */ /* alleged abusive content */ opaque mimi_content<V>; Frank frank; uint64 acceptedTimestamp; } AbusiveMessage; enum { reserved(0), (255) } AbuseType; struct { IdentifierUri reportingUser; IdentifierUri allegedAbuserUri; AbuseType reasonCode; opaque note<V>; AbusiveMessage messages<V>; } AbuseReport;¶
There is no response body. The response code only indicates if the abuse report was accepted, not if any specific automated or human action was taken.¶
The state of a room consists of its room ID, its base policy, its participant list (including the role and participation state of each participant), and the associated end-to-end protocol state (its MLS group state) that anchors the room state cryptographically.¶
While all parties involved in a room agree on the room's state during a specific epoch, the Hub is the arbiter that decides if a state change is valid, consistent with the room's then-current policy. All state-changing events are sent to the Hub and checked for their validity and policy conformance, before they are forwarded to any follower servers or local clients.¶
As soon as the Hub accepts an event that changes the room state, its effect is applied to the room state and future events are validated in the context of that new state.¶
The room state is thus changed based on events, even if the MLS proposal implementing the event was not yet committed by a client. Note that this only applies to events changing the room state.¶
Each room is represented cryptographically by an MLS group. The Hub that manages the room also manages the list of group members, i.e. the list of clients belonging to users currently in the room.¶
The MLS protocol follows a proposal-commit paradigm. Any party involved in a room (follower server, Hub or clients) can send proposals (e.g. to add/remove/update clients of a user or to re-initialize the group with different parameters). However, only clients can send commits, which contain all valid previously sent proposals and apply them to the MLS group state.¶
The MIMI usage of MLS ensures that the Hub, all follower servers and the clients of all active participants agree on the group state, which includes the client list and the key material used for message encryption (although the latter is only available to clients). Since the group state also includes a copy of the room state at the time of the most recent commit, it is also covered by the agreement.¶
MLS requires that MLS proposals from the Hub and
from follower servers (external senders in MLS terminology) be authenticated
using key material contained in the external_senders
extension of the MLS
group. Each MLS group associated with a MIMI room MUST therefore contain an
external_senders
extension. That extension MUST contain at least the
Certificate of the Hub.¶
When a user from a follower server becomes a participant in the room, the
Certificate of the follower server MAY be added to the extension. When the last
participant belonging to a follower server leaves the room, the certificate of
that user MUST be removed from the list. Changes to the external_senders
extension only take effect when the MLS proposal containing the event is
committed by a MIMI commit.¶
Most instant messaging systems have some notion of how a user consents to be added to a room, and how they manipulate this consent.¶
In the connection-oriented model, once two users are connected, either user can add the other to any number of rooms. In other systems (often with many large and/or public rooms), a user needs to consent individually to be added to a room.¶
The MIMI consent mechanism supports both models and allows them to coexist. It allows a user to request consent, grant consent, revoke consent, and cancel a request for consent. Each of these consent operations can indicate a specific room, or indicate any room.¶
A connection grant or revoke does not need to specify a room if a connection request did, or vice versa. A connection grant or revoke does not even need to follow a connection request.¶
For example, Alice could ask for consent to add Bob to a specific room. Bob could send a connection grant for Alice to add him to any room, or a connection revoke preventing Alice from adding him to any room. Similarly, Alice might have sent a connection request to add Bob for any room (as a connection request), which Bob ignored or did not see. Later, Bob wants to join a specific room administered by Alice. Bob sends a connection grant for the specific room for Alice and sends a Knock request to Alice asking to be added. Finally, Cathy could send a connection grant for Bob (even if Bob did not initiate a connection request to Cathy), and Alice could recognize Cathy on the system and send a connection revoke for her preemptively.¶
NOTE: Many providers use additional factors to apply default consent within their service such as a user belonging to a specific workgroup or employer, participating in a related room (ex: WhatsApp "communities"), or presence of a user in the other user's contact list. MIMI does not need to provide a way to replicate or describe these supplemental mechanisms, since they are strongly linked to specific provider policies.¶
Consent requests have sensitive privacy implications. The sender of a consent request should receive an acknowledgement that the request was received by the provider of the target user. For privacy reasons, the requestor should not know if the target user received or viewed the request. The original requestor will obviously find out if the target grants consent, but a consent revocation/rejection is typically not communicated to the revoked/rejected user (again for privacy reasons).¶
Consent operations are only sent directly between the acting provider (sending the request, grant, revoke, or cancel) and the target provider (the object of the consent). In other words, the two providers must have a direct peering relationship.¶
In our example, Alice requests consent from Bob for any room. Later, Bob sends a grants consent to Alice to add him to any room. At the same time as sending the consent request, Alice grants consent to Bob to add her to any room.¶
The MIMI protocol incorporates several layers of security.¶
Individual protocol actions are protected against network attackers with mutually-authenticated TLS, where the TLS certificates authenticate the identities that the protocol actors assert at the application layer.¶
Messages and room state changes are protected end-to-end using MLS. The protection is "end-to-end" in the sense that messages sent within the group are confidentiality-protected against all servers involved in the delivery of those messages, and in the sense that the authenticity of room state changes is verified by the end clients involved in the room. The usage of MLS ensures that the servers facilitating the exchange cannot read messages in the room or falsify room state changes, even though they can read the room state change messages.¶
Each room has an authorization policy that dictates which protocol actors can perform which actions in the room. This policy is enforced by the hub server for the room. The actors for whom the policy is being evaluated authenticate their identities to the hub server using the MLS PublicMessage signed object format, together with the identity credentials presented in MLS. This design means that the hub is trusted to correctly enforce the room's policy, but this cost is offset by the simplicity of not having multiple policy enforcement points.¶
TBD.¶
To ensure a strong source of entropy for the franking_key
included in each
message, the client can export a secret from the MLS key schedule, for
example with the label franking_base_secret
and calculate the
franking_key
as the HMAC of a locally generated nonce and the
franking_base_secret
.¶
franking_key = HMAC_SHA256( franking_base_secret, nonce )¶
TODO:¶