MLS A. Pham
Internet-Draft Google
Intended status: Informational M. Mularczyk
Expires: 24 April 2025 Amazon
R. Robert
Phoenix R&D
P. Slatala
Google
21 October 2024
MLS Wire Formats for PublicMessage and PrivateMessage without
authenticated_data
draft-pham-mls-additional-wire-formats-00
Abstract
This document describes an extension to support two new wire formats
for MLS messages: PublicMessageWithoutAAD and
PrivateMessageWithoutAAD.
About This Document
This note is to be removed before publishing as an RFC.
The latest revision of this draft can be found at
https://example.com/LATEST. Status information for this document may
be found at https://datatracker.ietf.org/doc/draft-pham-mls-
additional-wire-formats/.
Discussion of this document takes place on the mls Working Group
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Source for this draft and an issue tracker can be found at
https://github.com/anhph0/mls-additional-wire-formats.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Extension Definition . . . . . . . . . . . . . . . . . . . . 3
3. Message Framing . . . . . . . . . . . . . . . . . . . . . . . 3
4. Content Authentication . . . . . . . . . . . . . . . . . . . 4
5. PublicMessageWithoutAAD . . . . . . . . . . . . . . . . . . . 4
6. PrivateMessageWithoutAAD . . . . . . . . . . . . . . . . . . 5
7. Conventions and Definitions . . . . . . . . . . . . . . . . . 6
8. Security Considerations . . . . . . . . . . . . . . . . . . . 6
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
10. Normative References . . . . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6
1. Introduction
Sometimes it is desirable to have additional authenticated data to be
included in the computation of MLSMessage constructions, but not to
have it sent on the wire as part of these messages. A use-case is
applications that want to have some information available to the
server together with a MLSMessage and at the same time want to prove
the authenticity of the information to other clients.
An example of this is the case of delivery receipts where the server
needs to know that a message from Alice has been delivered to Bob,
but at the same time it wants Alice to be able to verify that the
delivery receipt indeed comes from Bob.
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This document proposes an extension to support new wire formats for
MLS PrivateMessage and PublicMessage to support such cases.
Applications will inject additional data as part of the MLSMessage
computation, but the additional data is not included in the
MLSMessage.
Note that it is the application's responsibility to know what needs
to be used as additional data when it processes messages with these
new wire formats.
2. Extension Definition
struct {
ExtensionType extension_type;
opaque extension_data<V>;
} ExtensionContent;
extension_type is a unique uint16 identifier registered in MLS
Extension Types IANA registry (see Section 17.3 of [RFC9420]). This
extension uses the mls_extension_message WireFormat as defined in
Section 2.1.7.1 of the Extensions Framework
(https://messaginglayersecurity.rocks/mls-extensions/draft-ietf-mls-
extensions.html#wire-formats), where the extension_data is TLS-
serialized MessageWithoutAAD.
enum {
PublicMessageWithoutAAD(0),
PrivateMessageWithoutAAD(1),
} MessageWithoutAAD;
3. Message Framing
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uint16 WireFormat;
struct {
opaque group_id<V>;
uint64 epoch;
Sender sender;
ContentType content_type;
select (FramedContent.content_type) {
case application:
opaque application_data<V>;
case proposal:
Proposal proposal;
case commit:
Commit commit;
};
} FramedContentWithoutAAD;
4. Content Authentication
FramedContentWithoutAAD is authenticated using the same procedure for
FramedContent described in Section 6.1 of [RFC9420]. A difference is
that in the FramedContentTBS definition, we have FramedContent with
authenticated_data being injected from the outside by the
application.
Moreover, the signature in the FramedContentAuthData is computed by
using SafeExtension.
SignWithLabel(SignatureKey, "LabeledExtensionContent", LabeledExtensionContent)
where LabeledExtensionContent is defined as:
label = Label
extension_type = ExtensionType
extension_data = FramedContent
with authenticated_data being injected to FramedContent by the
application.
5. PublicMessageWithoutAAD
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struct {
FramedContentWithoutAAD content;
FramedContentAuthData auth;
select (PublicMessageWithoutAAD.content.sender.sender_type) {
case member:
MAC membership_tag;
case external:
case new_member_commit:
case new_member_proposal:
struct{};
};
} PublicMessageWithoutAAD;
The membership_tag in the PublicMessageWithoutAAD authenticates the
sender's membership in the group. It is computed as follows:
membership_tag = MAC(membership_key, AuthenticatedContentTBM)
with AuthenticatedContentTBM and membership_key as defined as in the
[RFC9420]. authenticated_data in the FramedContent is injected by the
application.
6. PrivateMessageWithoutAAD
struct {
opaque group_id<V>;
uint64 epoch;
ContentType content_type;
opaque encrypted_sender_data<V>;
opaque ciphertext<V>;
} PrivateMessageWithoutAAD;
Similar to PrivateMessage, encrypted_sender_data and ciphertext are
encrypted using the AEAD function specified by the cipher suite in
use, using the SenderData and PrivateMessageContent structures as
input.
The SenderData is encrypted using the sender_data_secret of the
group.
The actual message content is encrypted using the key derived as
follows:
* Derive a secret as defined in Section 2.1.5 of the Extensions
Framework (https://messaginglayersecurity.rocks/mls-extensions/
draft-ietf-mls-extensions.html#name-exporting-secrets):
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DeriveExtensionSecret(Secret, Label) =
ExpandWithLabel(epoch_secret, "ExtensionExport " + ExtensionType +
" " + Label)
* Use the the secret in lieu of encryption_tree to seed the Secret
Tree (Section 9 of [RFC9420]).
* Follow the procedure of the Secret Tree to generate encryption
keys and nonces for the encryption of the message content.
7. Conventions and Definitions
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.
8. Security Considerations
No implications on the security of the base MLS protocol due to the
use of SafeExtension.
9. IANA Considerations
This document requests the addition of various new values under the
heading of "Messaging Layer Security".
10. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
[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,
July 2023, <https://www.rfc-editor.org/rfc/rfc9420>.
Authors' Addresses
Anh Pham
Google
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Email: anhph@google.com
Marta Mularczyk
Amazon
Email: mulmarta@amazon.com
Raphael Robert
Phoenix R&D
Email: ietf@raphaelrobert.com
Peter Slatala
Google
Email: psla@google.com
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