Internet-Draft | MLS | October 2024 |
Robert | Expires 24 April 2025 | [Page] |
- Workgroup:
- Network Working Group
- Internet-Draft:
- draft-ietf-mls-extensions-05
- Published:
- Intended Status:
- Informational
- Expires:
The Messaging Layer Security (MLS) Extensions
Abstract
This document describes extensions to the Messaging Layer Security (MLS) protocol.¶
Discussion Venues
This note is to be removed before publishing as an RFC.¶
Source for this draft and an issue tracker can be found at https://github.com/mlswg/mls-extensions.¶
Status of This Memo
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.¶
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.¶
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."¶
This Internet-Draft will expire on 24 April 2025.¶
Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the document authors. All rights reserved.¶
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.¶
1. Introduction
This document describes extensions to [mls-protocol] that are not part of the main protocol specification. The protocol specification includes a set of core extensions that are likely to be useful to many applications. The extensions described in this document are intended to be used by applications that need to extend the MLS protocol.¶
1.1. Change Log
RFC EDITOR PLEASE DELETE THIS SECTION.¶
draft-05¶
-
Include definition of ExtensionState extension¶
-
Add safe use of AAD to Safe Extensions framework¶
-
Clarify how capabilities negotiation works in Safe Extensions framework¶
draft-04¶
-
No changes (prevent expiration)¶
draft-03¶
draft-02¶
-
No changes (prevent expiration)¶
draft-01¶
-
Add Content Advertisement extensions¶
draft-00¶
2. Safe Extensions
The MLS specification is extensible in a variety of ways (see Section 13 of [RFC9420]) and describes the negotiation and other handling of extensions and their data within the protocol. However, it does not provide guidance on how extensions can or should safely interact with the base MLS protocol. The goal of this section is to simplify the task of developing MLS extensions.¶
More concretely, this section defines the Safe Extension API, a library of extension components which simplifies development and security analysis of extensions, provides general guidance on using the built-in functionality of the base MLS protocol to build extensions, defines specific examples of extensions built on top of the Safe Extension API alongside the built-in mechanisms of the base MLS protocol, defines a number of labels registered in IANA which can be safely used by extensions, so that the only value an extension developer must add to the IANA registry itself is a unique ExtensionType.¶
2.1. Safe Extension API
The Safe Extension API is a library that defines a number of components from which extensions can be built. In particular, these components provide extensions the ability to:¶
-
Make use of selected private and public key material from the MLS specification, e.g. to encrypt, decrypt, sign, verify and derive fresh key material.¶
-
Inject key material via PSKs in a safe way to facilitate state agreement without the use of a group context extension.¶
-
Export secrets from MLS in a way that, in contrast to the built-in export functionality of MLS, preserves forward secrecy of the exported secrets within an epoch.¶
-
Define new WireFormat, Proposal, Credential, GroupContext, GroupInfo, KeyPackage, and LeafNode extensions which can interact safely with arbitrary sets of other current or future Safe Extensions.¶
-
Anchor extension-specific state in an MLS group to ensure agreement and manage state acces authorization across extensions.¶
The Safe Extension API is not an extension itself, it only defines components from which other extensions can be built. Some of these components modify the MLS protocol and, therefore, so do the extensions built from them.¶
Where possible, the API makes use of mechanisms defined in the MLS
specification. For example, part of the safe API is the use of the
SignWithLabel
function described in Section 5.1.2 of [RFC9420].¶
2.1.1. Security
An extension is called safe if it does not modify the base MLS protocol or other MLS extensions beyond using components of the Safe Extension API. The Safe Extension API provides the following security guarantee: If an application uses MLS and only safe MLS extensions, then the security guarantees of the base MLS protocol and the security guarantees of safe extensions, each analyzed in isolation, still hold for the composed extended MLS protocol. In other words, the Safe Extension API protects applications from careless extension developers. As long as all used extensions are safe, it is not possible that a combination of extensions (the developers of which did not know about each other) impedes the security of the base MLS protocol or any used extension. No further analysis of the combination is necessary. This also means that any security vulnerabilities introduced by one extension do not spread to other extensions or the base MLS.¶
2.1.2. Core Struct Extensions
Every type of MLS extension can have data associated with it. The "MLS
Extensions Types" registry historically represented extensibility of four
core structs (GroupContext
, GroupInfo
, KeyPackage
, and LeafNode
)
that have far reaching effects on the use of the protocol. The majority of
MLS extensions registered at the time of this writing extend one or more
of these core structs.¶
-
GroupContext Extensions: Any data in a group context extension is agreed-upon by all members of the group in the same way as the rest of the group state. As part of the GroupContext, it is also sent encrypted to new joiners via Welcome messages and (depending on the architecture of the application) may be available to external joiners. Note that in some scenarios, the GroupContext may also be visible to components that implement the delivery service. While MLS extensions can define arbitrary GroupContext extensions, it is recommended to make use of
ExtensionState
extensions to store state in the group's GroupContext.¶ -
GroupInfo Extensions: GroupInfo extensions are included in the GroupInfo struct and thus sent encrypted and authenticated by the signer of the GroupInfo to new joiners as part of Welcome messages. It can thus be used as a confidential and authenticated channel from the inviting group member to new joiners. Just like GroupContext extensions, they may also be visible to external joiners or even parts of the delivery service. Unlike GroupContext extensions, the GroupInfo struct is not part of the group state that all group members agree on.¶
-
KeyPackage Extensions: KeyPackages (and the extensions they include) are pre-published by individual clients for asynchronous group joining. They are included in Add proposals and become part of the group state once the Add proposal is committed. They are, however, removed from the group state when the owner of the KeyPackage does the first commit with a path. As such, KeyPackage extensions can be used to communicate data to anyone who wants to invite the owner to a group, as well as the other members of the group the owner is added to. Note that KeyPackage extensions are visible to the server that provides the KeyPackages for download, as well as any part of the delivery service that can see the public group state.¶
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LeafNode Extensions: LeafNodes are a part of every KeyPackage and thus follow the same lifecycle. However, they are also part of any commit that includes an UpdatePath and clients generally have a leaf node in each group they are a member of. Leaf node extensions can thus be used to include member-specific data in a group state that can be updated by the owner at any time.¶
2.1.3. Common Data Structures
The Safe Extension API reuses the ExtensionType
and the "MLS Extension
Types" IANA registry used for these core structs (see Section 17.3 of
[RFC9420]), even for safe extensions with no core struct changes.
This is because many extensions modify a core struct, either primarily or
to store state (related to the group or a client) associated with another
aspect of that extension.¶
Most Safe Extension API components also use the following data structure, which
provides domain separation by extension_type
of various extension_data
.¶
struct { ExtensionType extension_type; opaque extension_data<V>; } ExtensionContent;¶
Where extension_type
is set to the type of the extension to which the
extension_data
belongs.¶
When a label is required for an extension, the following data structure is used.¶
struct { opaque label; ExtensionContent extension_content; } LabeledExtensionContent;¶
2.1.4. Negotiating Support for Safe Extensions
MLS defines a Capabilities
struct for LeafNodes (in turn used in
KeyPackages), which describes which extensions are supported by the
associated node.
However, that struct (defined in Section 7.2 of [RFC9420]) only has
fields for a subset of the extensions possible in MLS, as reproduced below.¶
... struct { ProtocolVersion versions<V>; CipherSuite cipher_suites<V>; ExtensionType extensions<V>; ProposalType proposals<V>; CredentialType credentials<V>; } Capabilities; ...¶
Therefore, all safe extensions MUST be represented by their extension_type
in the extensions
vector (originally intended for core struct extensions),
regardless of their type.¶
If the LeafNode supports any safe extension Credentials, the credentials
vector will contain any non-safe credentials supported, plus the extension_credential
defined in {extension-credential}.¶
If the LeafNode supports any safe extension Proposals, then proposals
will
contain any non-default non-safe extensions, and whichever safe extension
proposal types defined in {mls-proposal-types} are relevant to the supported
safe proposals.¶
Likewise, the required_capabilities
GroupContext extension (defined
in Section 11.1 of [RFC9420] and reproduced below) contains all
mandatory to support non-default non-safe, and safe extensions in its
extension_types
vector. Its credential_types
vector contains any
mandatory non-safe credential types, plus extensions_credential
if any
safe credential is required. Its proposal_types
vector contains any
mandatory to support non-default non-safe Proposals, and the relevant safe
proposal type or types corresponding to any required safe proposals.¶
struct { ExtensionType extension_types<V>; ProposalType proposal_types<V>; CredentialType credential_types<V>; } RequiredCapabilities;¶
2.1.5. Hybrid Public Key Encryption (HPKE)
This component of the Safe Extension API allows extensions to make use of all
HPKE key pairs generated by MLS. An extension identified by an ExtensionType can
use any HPKE key pair for any operation defined in [RFC9180], such as
encryption, exporting keys and the PSK mode, as long as the info
input to
Setup<MODE>S
and Setup<MODE>R
is set to LabeledExtensionContent with
extension_type
set to ExtensionType. The extension_data
can be set to an
arbitrary Context specified by the extension designer (and can be empty if not
needed). For example, an extension can use a key pair PublicKey, PrivateKey to
encrypt data as follows:¶
SafeEncryptWithContext(ExtensionType, PublicKey, Context, Plaintext) = SealBase(PublicKey, LabeledExtensionContent, "", Plaintext) SafeDecryptWithContext(ExtensionType, PrivateKey, Context, KEMOutput, Ciphertext) = OpenBase(KEMOutput, PrivateKey, LabeledExtensionContent, "", Ciphertext)¶
Where the fields of LabeledExtensionContent are set to¶
label = "MLS 1.0 ExtensionData" extension_type = ExtensionType extension_data = Context¶
For operations involving the secret key, ExtensionType MUST be set to the ExtensionType of the implemented extension, and not to the type of any other extension. In particular, this means that an extension cannot decrypt data meant for another extension, while extensions can encrypt data to other extensions.¶
In general, a ciphertext encrypted with a PublicKey can be decrypted by any entity who has the corresponding PrivateKey at a given point in time according to the MLS protocol (or extension). For convenience, the following list summarizes lifetimes of MLS key pairs.¶
-
The key pair of a non-blank ratchet tree node. The PrivateKey of such a key pair is known to all members in the node’s subtree. In particular, a PrivateKey of a leaf node is known only to the member in that leaf. A member in the subtree stores the PrivateKey for a number of epochs, as long as the PublicKey does not change. The key pair of the root node SHOULD NOT be used, since the external key pair recalled below gives better security.¶
-
The external_priv, external_pub key pair used for external initialization. The external_priv key is known to all group members in the current epoch. A member stores external_priv only for the current epoch. Using this key pair gives better security guarantees than using the key pair of the root of the ratchet tree and should always be preferred.¶
-
The init_key in a KeyPackage and the corresponding secret key. The secret key is known only to the owner of the KeyPackage and is deleted immediately after it is used to join a group.¶
2.1.6. Signature Keys
MLS session states contain a number of signature keys including the ones in the LeafNode structs. Extensions can safely sign content and verify signatures using these keys via the SafeSignWithLabel and SafeVerifyWithLabel functions, respectively, much like how the basic MLS protocol uses SignWithLabel and VerifyWithLabel.¶
In more detail, an extension identified by ExtensionType should sign and verify using:¶
SafeSignWithLabel(ExtensionType, SignatureKey, Label, Content) = SignWithLabel(SignatureKey, "LabeledExtensionContent", LabeledExtensionContent) SafeVerifyWithLabel(ExtensionType, VerificationKey, Label, Content, SignatureValue) = VerifyWithLabel(VerificationKey, "LabeledExtensionContent", LabeledExtensionContent, SignatureValue)¶
Where the fields of LabeledExtensionContent are set to¶
label = Label extension_type = ExtensionType extension_data = Content¶
For signing operations, the ExtensionType MUST be set to the ExtensionType of the implemented extension, and not to the type of any other extension. In particular, this means that an extension cannot produce signatures in place of other extensions. However, extensions can verify signatures computed by other extensions. Note that domain separation is ensured by explicitly including the ExtensionType with every operation.¶
2.1.7. Exporting Secrets
An extension can use MLS as a group key agreement protocol by exporting symmetric keys. Such keys can be exported (i.e. derived from MLS key material) in two phases per epoch: Either at the start of the epoch, or during the epoch. Derivation at the start of the epoch has the added advantage that the source key material is deleted after use, allowing the derived key material to be deleted later even during the same MLS epoch to achieve forward secrecy. The following protocol secrets can be used to derive key from for use by extensions:¶
The extension_secret is an additional secret derived from the epoch_secret at the beginning of the epoch in the same way as the other secrets listed in Table 4 of [RFC9420] using the label "extension".¶
Any derivation performed by an extension either from the epoch_secret or the extension_secret has to use the following function:¶
DeriveExtensionSecret(Secret, Label) = ExpandWithLabel(Secret, "ExtensionExport " + ExtensionType + " " + Label)¶
Where ExpandWithLabel is defined in Section 8 of [RFC9420] and where ExtensionType MUST be set to the ExtensionType of the implemented extension.¶
2.1.9. Extension Designer Tools
The safe extension API allows extension designers to sign and encrypt payloads without the need to register their own IANA labels. Following the same pattern, this document also provides ways for extension designers to define their own wire formats, proposals, credentials, and for structured data in the Additional Authenticated Data.¶
2.1.9.1. Core Struct Extensions
Each extension of the GroupContext, GroupInfo, KeyPackage, and/or LeafNode
structs is required to define the format of its data. These types of
extensions SHOULD NOT use the ExtensionContent
struct since the extension_type
is already in the parent data structure.¶
2.1.9.2. Wire Formats
Extensions can define their own MLS messages by using the mls_extension_message MLS Wire Format. The mls_extension_message Wire Format is IANA registered specifically for this purpose and extends the select statement in the MLSMessage struct as follows:¶
case mls_extension_message: ExtensionContent extension_content;¶
The extension_type in extension_content
MUST be set to the type of the
extension in question.
Processing of self-defined wire formats has to be defined by the extension.¶
2.1.9.3. Proposals
Similar to wire formats, extensions can define their own proposals by using one of three dedicated extension proposal types: extension_proposal, extension_path_proposal and extension_external_propsal. Each type contains the same ExtensionContent struct, but is validated differently: extension_proposal requires no UpdatePath and can not be sent by an external sender extension_path_proposal requires an UpdatePath and can not be sent by an external sender extensions_external_proposal requires no UpdatePath and can be sent by an external sender.¶
Each of the three proposal types is IANA registered and extends the select statement in the Proposal struct as follows:¶
case extension_proposal: ExtensionContent extension_content; case extension_path_proposal: ExtensionContent extension_content; case extension_external_proposal: ExtensionContent extension_content;¶
The extension_type MUST be set to the type of the extension in question.¶
Processing and validation of self-defined proposals has to be defined by the extension. However, validation rules can lead to a previously valid commit to become invalid, not the other way around. This is with the exception of proposal validation for external commits, where self-defined proposals can be declared valid for use in external commits. More concretely, if an external commit is invalid, only because the self-defined proposal is part of it (the last rule in external commit proposal validation in Section 12.2 of [RFC9420]), then the self-defined validation rules may rule that the commit is instead valid.¶
2.1.9.4. Credentials
Extension designers can also define their own credential types via the IANA registered extension_credential credential type. The extension_credential extends the select statement in the Credential struct as follows:¶
case extension_credential: ExtensionContent extension_content;¶
The extension_type in the extension_content must be set to that of the extension in question with the extension_data containing all other relevant data. Note that any credential defined in this way has to meet the requirements detailed in Section 5.3 of the MLS specification.¶
2.1.9.5. Additional Authenticated Data (AAD)
The PrivateContentAAD
struct in MLS can contain arbitrary additional
application-specific AAD in its authenticated_data
field. This framework
defines a framing used to allow multiple extensions to add AAD safely
without conflicts or ambiguity.¶
When any AAD safe extension is included in the authenticated_data
field,
the "safe" AAD items MUST come before any non-safe data in the
authenticated_data
field. Safe AAD items are framed using the SafeAAD
struct and are sorted in increasing numerical order of the ExtensionType
as described below:¶
struct { ExtensionType extension_type; opaque aad_item_data<V>; } SafeAADItem; struct { SafeAADItem aad_items<V>; } SafeAAD;¶
If the SafeAAD
is present or not is determined by the presence of the
extension_aad
GroupContext extension in the required_capabilities
of the
group. If extension_aad
is present in required_capabilities
but no
"safe" AAD items are present, the aad_items
is a zero-length vector.¶
Each extension which include a SafeAADItem
needs to advertise its
ExtensionType
in its LeafNode capabilities.extensions
. Extensions MAY
require an ExtensionType
to be included in required_capabilities
, but
members which encounter a SafeAADItem
they do not recognize can safely
ignore it.¶
2.1.10. Extension state: anchoring, storage and agreement
The safe extension framework can help an MLS extension ensure that all group
members agree on a piece of extension-specific state by using the
ExtensionState
GroupContext extension. The ownership of an ExtensionState
extension in the context of the safe extension framework is determined by the
extension_type
field. The extension with a matching extension_type
is called
the owning extension.¶
enum { reserved(0), read(1), none(2), (255) } Permissions; enum { reserved(0), hash(1), data(2), } HashOrData; struct { HashOrData hash_or_data; select(hash_or_data) { case hash: HashReference state_hash; case data: opaque state<V>; } } ExtensionPayload; struct { extensionType extension_type; Permissions read; ExtensionPayload payload; } ExtensionState;¶
The ExtensionState
GroupContext extension contains data either directly (if
hash_or_data = data
) or inditectly via a hash (if hash_or_data = hash
).¶
The owning extension can read and write the state stored in an ExtensionState
extension using an extension-defined proposal (see Section 2.1.9.3). The semantics
of the proposal determines how the state is changed.¶
The read
variable determines the permissions that other MLS extensions have
w.r.t. the data stored within. read
allows other MLS extensions to read that
data via their own proposals, while none
marks the data as private to the
owning MLS extension.¶
Other extensions may never write to the ExtensionState
of the owning MLS
extension.¶
2.1.10.1. Direct vs. hash-based storage
Storing the data directly in the ExtensionState
means the data becomes part of
the group state. Depending on the application design, this can be advantageous,
because it is distributed via Welcome messages. However, it could also mean that
the data is visible to the delivery service. Additionally, if the application
makes use of GroupContextExtension proposals, it may be necessary to send all of
the data with each such extension.¶
Including the data by hash only allows group members to agree on the data indirectly, relying on the collision resistance of the associated hash function. The data itself, however, may have to be transmitted out-of-band to new joiners.¶
2.1.10.2. GroupContextExtensions
MLS allows applications to modify GroupContext extensions via the GroupContextExtension proposal. However, control via that proposal involves including all GroupContext extensions in each such proposal. This makes data management more costly than via extension-specific proposals, which can, for example, include only the data to be changed for a given GroupContext extension, or define semantics that allow modification based on local data only.¶
2.2. Extension Design Guidance
While extensions can modify the protocol flow of MLS and the associated properties in arbitrary ways, the base MLS protocol already enables a number of functionalities that extensions can use without modifying MLS itself. Extension authors should consider using these built-in mechanisms before employing more intrusive changes to the protocol.¶
3. Extensions
3.1. AppAck
Type: Proposal¶
3.1.1. Description
An AppAck proposal is used to acknowledge receipt of application messages. Though this information implies no change to the group, it is structured as a Proposal message so that it is included in the group's transcript by being included in Commit messages.¶
struct { uint32 sender; uint32 first_generation; uint32 last_generation; } MessageRange; struct { MessageRange received_ranges<V>; } AppAck;¶
An AppAck proposal represents a set of messages received by the sender in the
current epoch. Messages are represented by the sender
and generation
values
in the MLSCiphertext for the message. Each MessageRange represents receipt of a
span of messages whose generation
values form a continuous range from
first_generation
to last_generation
, inclusive.¶
AppAck proposals are sent as a guard against the Delivery Service dropping
application messages. The sequential nature of the generation
field provides
a degree of loss detection, since gaps in the generation
sequence indicate
dropped messages. AppAck completes this story by addressing the scenario where
the Delivery Service drops all messages after a certain point, so that a later
generation is never observed. Obviously, there is a risk that AppAck messages
could be suppressed as well, but their inclusion in the transcript means that if
they are suppressed then the group cannot advance at all.¶
The schedule on which sending AppAck proposals are sent is up to the application, and determines which cases of loss/suppression are detected. For example:¶
-
The application might have the committer include an AppAck proposal whenever a Commit is sent, so that other members could know when one of their messages did not reach the committer.¶
-
The application could have a client send an AppAck whenever an application message is sent, covering all messages received since its last AppAck. This would provide a complete view of any losses experienced by active members.¶
-
The application could simply have clients send AppAck proposals on a timer, so that all participants' state would be known.¶
An application using AppAck proposals to guard against loss/suppression of application messages also needs to ensure that AppAck messages and the Commits that reference them are not dropped. One way to do this is to always encrypt Proposal and Commit messages, to make it more difficult for the Delivery Service to recognize which messages contain AppAcks. The application can also have clients enforce an AppAck schedule, reporting loss if an AppAck is not received at the expected time.¶
3.2. Targeted messages
3.2.1. Description
MLS application messages make sending encrypted messages to all group members easy and efficient. Sometimes application protocols mandate that messages are only sent to specific group members, either for privacy or for efficiency reasons.¶
Targeted messages are a way to achieve this without having to create a new group with the sender and the specific recipients – which might not be possible or desired. Instead, targeted messages define the format and encryption of a message that is sent from a member of an existing group to another member of that group.¶
The goal is to provide a one-shot messaging mechanism that provides confidentiality and authentication.¶
Targeted Messages makes use the Safe Extension API as defined in Section 2.1. reuse mechanisms from [mls-protocol], in particular [hpke].¶
3.2.2. Format
This extension uses the mls_extension_message
WireFormat as defined in Section
Section 2.1.9.2, where the content is a TargetedMessage
.¶
struct { opaque group_id<V>; uint64 epoch; uint32 recipient_leaf_index; opaque authenticated_data<V>; opaque encrypted_sender_auth_data<V>; opaque hpke_ciphertext<V>; } TargetedMessage; enum { hpke_auth_psk(0), signature_hpke_psk(1), } TargetedMessageAuthScheme; struct { uint32 sender_leaf_index; TargetedMessageAuthScheme authentication_scheme; select (authentication_scheme) { case HPKEAuthPsk: case SignatureHPKEPsk: opaque signature<V>; } opaque kem_output<V>; } TargetedMessageSenderAuthData; struct { opaque group_id<V>; uint64 epoch; uint32 recipient_leaf_index; opaque authenticated_data<V>; TargetedMessageSenderAuthData sender_auth_data; } TargetedMessageTBM; struct { opaque group_id<V>; uint64 epoch; uint32 recipient_leaf_index; opaque authenticated_data<V>; uint32 sender_leaf_index; TargetedMessageAuthScheme authentication_scheme; opaque kem_output<V>; opaque hpke_ciphertext<V>; } TargetedMessageTBS; struct { opaque group_id<V>; uint64 epoch; opaque label<V> = "MLS 1.0 targeted message psk"; } PSKId;¶
Note that TargetedMessageTBS
is only used with the
TargetedMessageAuthScheme.SignatureHPKEPsk
authentication mode.¶
3.2.3. Encryption
Targeted messages uses HPKE to encrypt the message content between two leaves.¶
3.2.3.1. Sender data encryption
In addition, TargetedMessageSenderAuthData
is encrypted in a similar way to
MLSSenderData
as described in section 6.3.2 in [mls-protocol]. The
TargetedMessageSenderAuthData.sender_leaf_index
field is the leaf index of the
sender. The TargetedMessageSenderAuthData.authentication_scheme
field is the
authentication scheme used to authenticate the sender. The
TargetedMessageSenderAuthData.signature
field is the signature of the
TargetedMessageTBS
structure. The TargetedMessageSenderAuthData.kem_output
field is the KEM output of the HPKE encryption.¶
The key and nonce provided to the AEAD are computed as the KDF of the first
KDF.Nh bytes of the hpke_ciphertext
generated in the following section. If the
length of the hpke_ciphertext is less than KDF.Nh, the whole hpke_ciphertext is
used. In pseudocode, the key and nonce are derived as:¶
sender_auth_data_secret = DeriveExtensionSecret(extension_secret, "targeted message sender auth data") ciphertext_sample = hpke_ciphertext[0..KDF.Nh-1] sender_data_key = ExpandWithLabel(sender_auth_data_secret, "key", ciphertext_sample, AEAD.Nk) sender_data_nonce = ExpandWithLabel(sender_auth_data_secret, "nonce", ciphertext_sample, AEAD.Nn)¶
The Additional Authenticated Data (AAD) for the SenderAuthData
ciphertext is
the first three fields of TargetedMessage
:¶
struct { opaque group_id<V>; uint64 epoch; uint32 recipient_leaf_index; } SenderAuthDataAAD;¶
3.2.4. Authentication
For ciphersuites that support it, HPKE mode_auth_psk
is used for
authentication. For other ciphersuites, HPKE mode_psk
is used along with a
signature. The authentication scheme is indicated by the authentication_scheme
field in TargetedMessageContent
. See Section 3.2.5
for more information.¶
For the PSK part of the authentication, clients export a dedicated secret:¶
targeted_message_psk = DeriveExtensionSecret(extension_secret, "targeted message psk")¶
The functions SealAuth
and OpenAuth
defined in [hpke] are used as
described in Section 2.1.5 with an empty context. Other functions are defined in
[mls-protocol].¶
3.2.4.1. Authentication with HPKE
The sender MUST set the authentication scheme to
TargetedMessageAuthScheme.HPKEAuthPsk
.¶
As described in Section 2.1.5 the hpke_context
is a LabeledExtensionContent struct
with the following content, where group_context
is the serialized context of
the group.¶
label = "MLS 1.0 ExtensionData" extension_type = ExtensionType extension_data = group_context¶
The sender then computes the following:¶
(kem_output, hpke_ciphertext) = SealAuthPSK(receiver_node_public_key, hpke_context, targeted_message_tbm, message, targeted_message_psk, psk_id, sender_node_private_key)¶
The recipient computes the following:¶
message = OpenAuthPSK(kem_output, receiver_node_private_key, hpke_context, targeted_message_tbm, hpke_ciphertext, targeted_message_psk, psk_id, sender_node_public_key)¶
3.2.4.2. Authentication with signatures
The sender MUST set the authentication scheme to
TargetedMessageAuthScheme.SignatureHPKEPsk
. The signature is done using the
signature_key
of the sender's LeafNode
and the corresponding signature
scheme used in the group.¶
The sender then computes the following with hpke_context
defined as in
Section 3.2.4.1:¶
(kem_output, hpke_ciphertext) = SealPSK(receiver_node_public_key, hpke_context, targeted_message_tbm, message, targeted_message_psk, epoch)¶
The signature is computed as follows, where the extension_type
is the type of
this extension (see Section 4).¶
signature = SafeSignWithLabel(extension_type, ., "TargetedMessageTBS", targeted_message_tbs)¶
The recipient computes the following:¶
message = OpenPSK(kem_output, receiver_node_private_key, hpke_context, targeted_message_tbm, hpke_ciphertext, targeted_message_psk, epoch)¶
The recipient MUST verify the message authentication:¶
SafeVerifyWithLabel.verify(extension_type, sender_leaf_node.signature_key, "TargetedMessageTBS", targeted_message_tbs, signature)¶
3.2.5. Guidance on authentication schemes
If the group’s ciphersuite does not support HPKE mode_auth_psk
,
implementations MUST choose TargetedMessageAuthScheme.SignatureHPKEPsk
.¶
If the group’s ciphersuite does support HPKE mode_auth_psk
, implementations
CAN choose TargetedMessageAuthScheme.HPKEAuthPsk
if better efficiency and/or
repudiability is desired. Implementations SHOULD consult
[hpke-security-considerations] beforehand.¶
3.3. Content Advertisement
3.3.1. Description
This section describes two extensions to MLS. The first allows MLS clients
to advertise their support for specific formats inside MLS application_data
.
These are expressed using the extensive IANA Media Types registry (formerly
called MIME Types). The accepted_media_types
LeafNode extension lists the
formats a client supports inside application_data
. The second, the
required_media_types
GroupContext extension specifies which media types
need to be supported by all members of a particular MLS group.
These allow clients to confirm that all members of a group can communicate.
Note that when the membership of a group changes, or when the policy of the
group changes, it is responsibility of the committer to insure that the membership
and policies are compatible.¶
Finally, this document defines a minimal framing format so MLS clients can signal which media type is being sent when multiple formats are permitted in the same group. As clients are upgraded to support new formats they can use these extensions to detect when all members support a new or more efficient encoding, or select the relevant format or formats to send.¶
Note that the usage of IANA media types in general does not imply the usage of MIME
Headers [RFC2045] for framing. Vendor-specific media subtypes starting with
vnd.
can be registered with IANA without standards action as described in
[RFC6838]. Implementations which wish to send multiple formats in a single
application message, may be interested in the multipart/alternative
media type
defined in [RFC2046] or may use or define another type with similar semantics
(for example using TLS Presentation Language syntax [RFC8446]).¶
3.3.2. Syntax
MediaType is a TLS encoding of a single IANA media type (including top-level
type and subtype) and any of its parameters. Even if the parameter_value
would have required formatting as a quoted-string
in a text encoding, only
the contents inside the quoted-string
are included in parameter_value
.
MediaTypeList is an ordered list of MediaType objects.¶
struct { opaque parameter_name<V>; /* Note: parameter_value never includes the quotation marks of an * RFC 2045 quoted-string */ opaque parameter_value<V>; } Parameter; struct { /* media_type is an IANA top-level media type, a "/" character, * and the IANA media subtype */ opaque media_type<V>; /* a list of zero or more parameters defined for the subtype */ Parameter parameters<V>; } MediaType; struct { MediaType media_types<V>; } MediaTypeList; MediaTypeList accepted_media_types; MediaTypeList required_media_types;¶
Example IANA media types with optional parameters:¶
image/png text/plain ;charset="UTF-8" application/json application/vnd.example.msgbus+cbor¶
For the example media type for text/plain
, the media_type
field
would be text/plain
, parameters
would contain a single Parameter
with a parameter_name
of charset
and a parameter_value
of UTF-8
.¶
3.3.3. Expected Behavior
An MLS client which implements this section SHOULD include the
accepted_media_types
extension in its LeafNodes, listing
all the media types it can receive. As usual, the
client also includes accepted_media_types
in its capabilities
field in
its LeafNodes (including LeafNodes inside its KeyPackages).¶
When creating a new MLS group for an application using this specification,
the group MAY include a required_media_type
extension in the GroupContext
Extensions. As usual, the client also includes
required_media_types
in its capabilities
field in its LeafNodes
(including LeafNodes inside its KeyPackages). When used in a group, the client
MUST include the required_media_types
and accepted_media_types
extensions
in the list of extensions in RequiredCapabilities.¶
MLS clients SHOULD NOT add an MLS client to an MLS group with required_media_types
unless the MLS client advertises it can support all of the required MediaTypes.
As an exception, a client could be preconfigured to know that certain clients
support the requried types. Likewise, an MLS client is already forbidden from
issuing or committing a GroupContextExtensions Proposal which introduces required
extensions which are not supported by all members in the resulting epoch.¶
3.3.4. Framing of application_data
When an MLS group contains the required_media_types
GroupContext extension,
the application_data
sent in that group is interpreted as ApplicationFraming
as defined below:¶
struct { MediaType media_type; opaque<V> application_content; } ApplicationFraming;¶
The media_type
MAY be zero length, in which case, the media type of the
application_content
is interpreted as the first MediaType specified in
required_media_types
.¶
3.4. SelfRemove Proposal
The design of the MLS protocol prevents a member of an MLS group from removing itself immediately from the group. (To cause an immediate change in the group, a member must send a Commit message. However the sender of a Commit message knows the keying material of the new epoch and therefore needs to be part of the group.) Instead a member wishing to remove itself can send a Remove Proposal and wait for another member to Commit its Proposal.¶
Unfortunately, MLS clients that join via an External Commit ignore pending, but otherwise valid, Remove Proposals. The member trying to remove itself has to monitor the group and send a new Remove Proposal in every new epoch until the member is removed. In a group with a burst of external joiners, a member connected over a high-latency link (or one that is merely unlucky) might have to wait several epochs to remove itself. A real-world situation in which this happens is a member trying to remove itself from a conference call as several dozen new participants are trying to join (often on the hour).¶
This section describes a new SelfRemove
Proposal extension type. It is
designed to be included in External Commits.¶
3.4.1. Extension Description
This document specifies a new MLS Proposal type called SelfRemove
. Its syntax
is described using the TLS Presentation Language [@!RFC8446] below (its content
is an empty struct). It is allowed in External Commits and requires an UpdatePath.
SelfRemove proposals are only allowed in a Commit by reference. SelfRemove
cannot be sent as an external proposal.¶
struct {} SelfRemove; struct { ProposalType msg_type; select (Proposal.msg_type) { case add: Add; case update: Update; case remove: Remove; case psk: PreSharedKey; case reinit: ReInit; case external_init: ExternalInit; case group_context_extensions: GroupContextExtensions; case self_remove: SelfRemove; }; } Proposal;¶
The description of behavior below only applies if all the members of a group support this extension in their capabilities; such a group is a "self-remove-capable group".¶
An MLS client which supports this extension can send a SelfRemove Proposal whenever it would like to remove itself from a self-remove-capable group. Because the point of a SelfRemove Proposal is to be available to external joiners (which are not yet members), these proposals MUST be sent in an MLS PublicMessage.¶
Whenever a member receives a SelfRemove Proposal, it includes it along with any other pending Propsals when sending a Commit. It already MUST send a Commit of pending Proposals before sending new application messages.¶
When a member receives a Commit referencing one or more SelfRemove Proposals,
it treats the proposal like a Remove Proposal, except the leaf node to remove
is determined by looking in the Sender leaf_index
of the original Proposal.
The member is able to verify that the Sender was a member.¶
Whenever a new joiner is about to join a self-remove-capable group with an
External Commit, the new joiner MUST fetch any pending SelfRemove Proposals
along with the GroupInfo object, and include the SelfRemove Proposals
in its External Commit by reference. (An ExternalCommit can contain zero or
more SelfRemove proposals). The new joiner MUST validate the SelfRemove
Proposal before including it by reference, except that it skips the validation
of the membership_tag
because a non-member cannot verify membership.¶
During validation, SelfRemove proposals are processed after Update proposals and before Remove proposals. If there is a pending SelfRemove proposal for a specific leaf node and a pending Remove proposal for the same leaf node, the Remove proposal is invalid. A client MUST NOT issue more than one SelfRemove proposal per epoch.¶
The MLS Delivery Service (DS) needs to validate SelfRemove Proposals it
receives (except that it cannot validate the membership_tag
). If the DS
provides a GroupInfo object to an external joiner, the DS SHOULD attach any
SelfRemove proposals known to the DS to the GroupInfo object.¶
As with Remove proposals, clients need to be able to receive a Commit message which removes them from the group via a SelfRemove. If the DS does not forward a Commit to a removed client, it needs to inform the removed client out-of-band.¶
3.5. Last resort KeyPackages
Type: KeyPackage extension¶
3.5.1. Description
Section 10 of [RFC9420] details that clients are required to pre-publish KeyPackages s.t. other clients can add them to groups asynchronously. It also states that they should not be re-used:¶
-
KeyPackages are intended to be used only once and SHOULD NOT be reused except in the case of a "last resort" KeyPackage (see Section 16.8). Clients MAY generate and publish multiple KeyPackages to support multiple cipher suites.¶
Section 16.8 of [RFC9420] then introduces the notion of last-resort KeyPackages as follows:¶
-
An application MAY allow for reuse of a "last resort" KeyPackage in order to prevent denial-of-service attacks.¶
However, [RFC9420] does not specify how to distinguish regular KeyPackages from last-resort ones. The last_resort_key_package KeyPackage extension defined in this section fills this gap and allows clients to specifically mark KeyPackages as KeyPackages of last resort that MAY be used more than once in scenarios where all other KeyPackages have already been used.¶
The extension allows clients that pre-publish KeyPackages to signal to the Delivery Service which KeyPackage(s) are meant to be used as last resort KeyPackages.¶
An additional benefit of using an extension rather than communicating the information out-of-band is that the extension is still present in Add proposals. Clients processing such Add proposals can authenticate that a KeyPackage is a last-resort KeyPackage and MAY make policy decisions based on that information.¶
4. IANA Considerations
This document requests the addition of various new values under the heading of "Messaging Layer Security". Each registration is organized under the relevant registry Type.¶
RFC EDITOR: Please replace XXXX throughout with the RFC number assigned to this document¶
4.2. MLS Extension Types
This document updates the MLS Extension Types registry to insert a new column ("Safe") between the "Recommended" column and the "Reference" column. The value of the "Safe" column for the first (0x0000) and last (0xF000-0xFFFF) rows is "-" while the value of all other existing rows is "N".¶
-
Safe: Whether the extension is a Safe Extension as defined in Section 2 of RFC XXXX. Valid values are:¶
This document also extends the list of allowable values for the "Message(s)" column, such that the list may be empty (represented by "-") if the extension is a Safe Extension.¶
4.2.1. targeted_messages_capability MLS Extension
The targeted_messages_capability
MLS Extension Type is used in the
capabilities field of LeafNodes to indicate the support for the Targeted
Messages Extension. The extension does not carry any payload.¶
4.2.2. targeted_messages MLS Extension
The targeted_messages
MLS Extension Type is used inside GroupContext objects. It
indicates that the group supports the Targeted Messages Extension.¶
4.2.3. accepted_media_types MLS Extension
The accepted_media_types
MLS Extension Type is used inside LeafNode objects. It
contains a MediaTypeList representing all the media types supported by the
MLS client referred to by the LeafNode.¶
4.2.4. required_media_types MLS Extension
The required_media_types MLS Extension Type is used inside GroupContext objects. It contains a MediaTypeList representing the media types which are mandatory for all MLS members of the group to support.¶
4.2.5. last_resort_key_package MLS Extension
The last_resort_key_package MLS Extension Type is used inside KeyPackage objects. It marks the KeyPackage for usage in last resort scenarios and contains no additional data.¶
4.2.6. extension_aad MLS Extension
The extension_aad MLS Extension Type is used to signal support for SafeAAD
in LeafNode capabilities, and in GroupContext required_capabilities
. It contains no additional data.¶
4.2.7. safe_extensions MLS Extension
The safe_extensions
MLS Extension Type is used to signal support for the
Safe Extensions Framework in LeafNode capabilities, and in GroupContext
required_capabilities
. It contains no additional data.¶
4.2.8. core_struct_extensions MLS Extension
The core_struct_extensions
MLS Extension Type is used to signal support
for one or more Core Struct Extensions using the Safe Extensions Framework.
It appears in LeafNode capabilities, and in GroupContext
required_capabilities
. It contains no additional data.¶
4.3. MLS Proposal Types
4.3.5. SelfRemove Proposal
The self_remove
MLS Proposal Type is used for a member to remove itself
from a group more efficiently than using a remove
proposal type, as the
self_remove
type is permitted in External Commits.¶
4.6. MLS Extension Types
This document modifies the rules of the "MLS Extension Types" registry to add a new Message type as follows:¶
-
AD: Authenticated Additional Data¶
The AD
Message type refers to an ExtensionType
used inside the
SafeAADItem
structure defined in Section 2.1.9.5.¶
5. Security considerations
5.2. Targeted Messages
In addition to the sender authentication, Targeted Messages are authenticated by using a preshared key (PSK) between the sender and the recipient. The PSK is exported from the group key schedule using the label "targeted message psk". This ensures that the PSK is only valid for a specific group and epoch, and the Forward Secrecy and Post-Compromise Security guarantees of the group key schedule apply to the targeted messages as well. The PSK also ensures that an attacker needs access to the private group state in addition to the HPKE/signature's private keys. This improves confidentiality guarantees against passive attackers and authentication guarantees against active attackers.¶
5.3. Content Advertisement
Use of the accepted_media_types
and rejected_media_types
extensions
could leak some private information visible in KeyPackages and inside an MLS group.
They could be used to infer a specific implementation, platform, or even version.
Clients should consider carefully the privacy implications in their environment of
making a list of acceptable media types available.¶
5.4. SelfRemove
An external recipient of a SelfRemove Proposal cannot verify the
membership_tag
. However, an external joiner also has no way to
completely validate a GroupInfo object that it receives. An insider
can prevent an External Join by providing either an invalid GroupInfo object
or an invalid SelfRemove Proposal. The security properties of external joins
does not change with the addition of this proposal type.¶
6. References
6.1. Normative References
- [RFC8446]
- Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", RFC 8446, DOI 10.17487/RFC8446, , <https://www.rfc-editor.org/rfc/rfc8446>.
- [RFC9180]
- Barnes, R., Bhargavan, K., Lipp, B., and C. Wood, "Hybrid Public Key Encryption", RFC 9180, DOI 10.17487/RFC9180, , <https://www.rfc-editor.org/rfc/rfc9180>.
- [RFC9420]
- Barnes, R., Beurdouche, B., Robert, R., Millican, J., Omara, E., and K. Cohn-Gordon, "The Messaging Layer Security (MLS) Protocol", RFC 9420, DOI 10.17487/RFC9420, , <https://www.rfc-editor.org/rfc/rfc9420>.
6.2. Informative References
- [hpke]
- "Hybrid Public Key Encryption", n.d., <https://www.rfc-editor.org/rfc/rfc9180.html](https://www.rfc-editor.org/rfc/rfc9180.html>.
- [hpke-security-considerations]
- "HPKE Security Considerations", n.d., <https://www.rfc-editor.org/rfc/rfc9180.html#name-key-compromise-impersonatio](https://www.rfc-editor.org/rfc/rfc9180.html#name-key-compromise-impersonatio>.
- [mls-protocol]
- "The Messaging Layer Security (MLS) Protocol", n.d., <https://datatracker.ietf.org/doc/draft-ietf-mls-protocol/](https://datatracker.ietf.org/doc/draft-ietf-mls-protocol/>.
- [RFC2045]
- Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies", RFC 2045, DOI 10.17487/RFC2045, , <https://www.rfc-editor.org/rfc/rfc2045>.
- [RFC2046]
- Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types", RFC 2046, DOI 10.17487/RFC2046, , <https://www.rfc-editor.org/rfc/rfc2046>.
- [RFC6838]
- Freed, N., Klensin, J., and T. Hansen, "Media Type Specifications and Registration Procedures", BCP 13, RFC 6838, DOI 10.17487/RFC6838, , <https://www.rfc-editor.org/rfc/rfc6838>.