The Messaging Layer Security (MLS) Extensions
draft-ietf-mls-extensions-00
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft whose latest revision state is "Active".
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|
|---|---|---|---|
| Author | Raphael Robert | ||
| Last updated | 2022-11-25 | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
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| Additional resources | Mailing list discussion | ||
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draft-ietf-mls-extensions-00
Network Working Group R. Robert
Internet-Draft Phoenix R&D
Intended status: Informational 25 November 2022
Expires: 29 May 2023
The Messaging Layer Security (MLS) Extensions
draft-ietf-mls-extensions-00
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
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on 29 May 2023.
Copyright Notice
Copyright (c) 2022 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
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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.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Change Log . . . . . . . . . . . . . . . . . . . . . . . 2
2. Extensions . . . . . . . . . . . . . . . . . . . . . . . . . 2
2.1. AppAck . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1.1. Description . . . . . . . . . . . . . . . . . . . . . 3
2.2. Targeted messages . . . . . . . . . . . . . . . . . . . . 4
2.2.1. Description . . . . . . . . . . . . . . . . . . . . . 4
2.2.2. Format . . . . . . . . . . . . . . . . . . . . . . . 4
2.2.3. Encryption . . . . . . . . . . . . . . . . . . . . . 6
2.2.4. Authentication . . . . . . . . . . . . . . . . . . . 7
2.2.5. Guidance on authentication schemes . . . . . . . . . 8
2.2.6. Security considerations . . . . . . . . . . . . . . . 9
3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
3.1. MLS Extension Types . . . . . . . . . . . . . . . . . . . 9
3.1.1. targeted_messages_capability MLS Extension . . . . . 9
3.1.2. targeted_messages MLS Extension . . . . . . . . . . . 10
3.2. MLS Proposal Types . . . . . . . . . . . . . . . . . . . 10
3.2.1. AppAck Proposal . . . . . . . . . . . . . . . . . . . 10
4. Informative References . . . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 11
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-00
* Initial adoption of draft-robert-mls-protocol-00 as a WG item.
* Add Targeted Messages extension (*)
2. Extensions
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2.1. AppAck
Type: Proposal
2.1.1. 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.
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* 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.
2.2. Targeted messages
2.2.1. 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 reuse mechanisms from [mls-protocol], in particular
[hpke].
2.2.2. Format
This extensions introduces a new message type to the MLS protocol,
TargetedMessage in WireFormat and MLSMessage:
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enum {
...
mls_targeted_message(6),
...
(255)
} WireFormat;
struct {
ProtocolVersion version = mls10;
WireFormat wire_format;
select (MLSMessage.wire_format) {
...
case mls_targeted_message:
TargetedMessage targeted_message;
}
} MLSMessage;
The TargetedMessage message type is defined as follows:
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;
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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.
2.2.3. Encryption
Targeted messages use HPKE to encrypt the message content between two
leaves. The HPKE keys of the LeafNode are used to that effect,
namely the encryption_key field.
In addition, TargetedMessageSenderAuthData is encrypted in a similar
way to MLSSenderData as described in section 7.3.2 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:
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``` sender_auth_data_secret = MLS-Exporter("targeted message sender
auth data", "", KDF.Nh)
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;
2.2.3.1. Padding
The TargetedMessage structure does not include a padding field. It
is the responsibility of the sender to add padding to the message as
used in the next section.
2.2.4. 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 2.2.5 for more information.
For the PSK part of the authentication, clients export a dedicated
secret:
targeted_message_psk = MLS-Exporter("targeted message psk", "",
KDF.Nh)
Th functions SealAuth and OpenAuth are defined in [hpke]. Other
functions are defined in [mls-protocol].
2.2.4.1. Authentication with HPKE
The sender MUST set the authentication scheme to
TargetedMessageAuthScheme.HPKEAuthPsk.
The sender then computes the following:
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(kem_output, hpke_ciphertext) = SealAuthPSK(receiver_node_public_key,
group_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,
group_context, targeted_message_tbm, hpke_ciphertext,
targeted_message_psk, psk_id, sender_node_public_key)
2.2.4.2. 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:
``` (kem_output, hpke_ciphertext) = SealPSK(receiver_node_public_key,
group_context, targeted_message_tbm, message, targeted_message_psk,
epoch)
signature = SignWithLabel(., "TargetedMessageTBS",
targeted_message_tbs) ```
The recipient computes the following:
message = OpenPSK(kem_output, receiver_node_private_key,
group_context, targeted_message_tbm, hpke_ciphertext,
targeted_message_psk, epoch)
The recipient MUST verify the message authentication:
VerifyWithLabel.verify(sender_leaf_node.signature_key,
"TargetedMessageTBS", targeted_message_tbs, signature)
2.2.5. 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.
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2.2.6. Security considerations
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.
3. 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
3.1. MLS Extension Types
3.1.1. 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.
Template:
* Value: 0x0006
* Name: targeted_messages_capability
* Message(s): LN: This extension may appear in LeafNode objects
* Recommended: Y
* Reference: RFC XXXX
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3.1.2. 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.
Template:
* Value: 0x0007
* Name: targeted_messages
* Message(s): GC: This extension may appear in GroupContext objects
* Recommended: Y
* Reference: RFC XXXX
3.2. MLS Proposal Types
3.2.1. AppAck Proposal
Template:
* Value: 0x0008
* Name: app_ack
* Recommended: Y
* Path Required: Y
* Reference: [RFC XXXX]
4. 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>.
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[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/>.
Author's Address
Raphael Robert
Phoenix R&D
Email: ietf@raphaelrobert.com
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