| Internet-Draft | MLS SemiPrivateMessage | August 2024 |
| Mahy | Expires 2 February 2025 | [Page] |
This document defines a SemiPrivateMessage for the Messaging Layer Security (MLS) protocol. It allows members to share otherwise private commits and proposals with a designated list of external receivers rather than send these handshakes in a PublicMessage.¶
This note is to be removed before publishing as an RFC.¶
The latest revision of this draft can be found at https://rohanmahy.github.io/mls-semiprivatemessage/draft-mahy-mls-semiprivatemessage.html. Status information for this document may be found at https://datatracker.ietf.org/doc/draft-mahy-mls-semiprivatemessage/.¶
Discussion of this document takes place on the Messaging Layer Security Working Group mailing list (mailto:mls@ietf.org), which is archived at https://mailarchive.ietf.org/arch/browse/mls/. Subscribe at https://www.ietf.org/mailman/listinfo/mls/.¶
Source for this draft and an issue tracker can be found at https://github.com/rohanmahy/mls-semiprivatemessage.¶
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Copyright (c) 2024 IETF Trust and the persons identified as the document authors. All rights reserved.¶
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This document defines two extensions of MLS [RFC9420]. The first is the
SemiPrivateMessage wire format Safe Extension (see Section 2.1.7.1 of [I-D.ietf-mls-extensions], which allows an otherwise PrivateMessage
to be shared with a predefined list of external receivers. It is restricted
for use only with commits or proposals. The second is the
external_receivers GroupContext extension that contains the list of
external receivers and allows members to agree on that list.¶
SemiPrivateMessages are expected to be useful in federated environments where messages routinely cross multiple administrative domains, but the MLS Distribution Service needs to see the content of commits and proposals where group members would otherwise send handshakes using PublicMessage.¶
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.¶
This document uses terminology extensively from MLS [RFC9420] and the Safe Extensions framework, defined in Section 2 of [I-D.ietf-mls-extensions].¶
The external_receivers GroupContext extension is used for all members
to agree on the list of external receivers in the current epoch. Its
construction mirrors the syntax of the external_senders extension in
[RFC9420].¶
struct {
HPKEPublicKey external_receiver_public_key;
Credential credential;
} ExternalReceiver;
¶
The SemiPrivateMessage wire format Safe Extension also has an
extension type which is carried in the GroupContext to indicate use
of the wire format in a group (and in the Capabilities of LeafNodes).
SemiPrivateMessage substantially reuses the construction of PrivateMessage,
but like a Welcome message also contains information (keys_and_nonces)
necessary to decrypt the SemiPrivateMessage struct's ciphertext and
encrypted_sender_data, encrypted once for each external receiver in the
external_receivers extension.¶
The snippet below shows the syntax and encryption and decryption
construction of keys_and_nonces into encrypted_keys_and_nonces¶
struct {
opaque sender_data_key<V>;
opaque sender_data_nonce<V>;
opaque key<V>;
opaque nonce<V>;
} PerMessageKeysAndNonces;
PerMessageKeysAndNonces keys_and_nonces;
encrypted_keys_and_nonces = EncryptWithLabel(
external_receiver_public_key,
"SemiPrivateMessageReceiver",
SemiPrivateMessage.ciphertext, /* context */
keys_and_nonces)
keys_and_nonces = DecryptWithLabel(
external_receiver_private_key,
"SemiPrivateMessageReceiver",
SemiPrivateMessage.ciphertext, /* context */
encrypted_keys_and_nonces.kem_output,
encrypted_keys_and_nonces.ciphertext)
¶
The KeyForExternalReceiver structure contains a hash of the
ExternalReceiver as a reference and the encrypted_keys_and_nonces.¶
/* Using the hash function of the group ciphersuite */
ExternalReceiverRef = hash(ExternalReceiver)
struct {
ExternalReceiverRef external_receiver_ref;
HPKECiphertext encrypted_keys_and_nonces;
} KeyForExternalReceiver;
¶
The SemiPrivateMessage struct mirrors the PrivateMessage struct and adds
the keys_for_external_receivers list. The SemiPrivateContentAAD struct
mirrors the PrivateContentAAD struct. It likewise adds the
keys_for_external_receivers list, and also adds a hash of the
FramedContentTBS struct to insure that the content encrypted to an
external receiver is that same as that provided to members.¶
The SemiPrivateMessageContent struct is the same as
PrivateMessageContent except for the addition of
keys_for_external_receivers, and that application messages are
not included.¶
Encryption of the ciphertext and encrypted_sender_data proceed in the
same way for SemiPrivateMessage as for PrivateMessage. Finally, as a safe wire format extension, the SemiPrivateMessage is wrapped in an
ExtensionContent struct.¶
struct {
opaque group_id<V>;
uint64 epoch;
ContentType content_type;
opaque authenticated_data<V>;
KeyForExternalReceiver keys_for_external_receivers<V>;
opaque encrypted_sender_data<V>;
opaque ciphertext<V>;
} SemiPrivateMessage;
struct {
select (PrivateMessage.content_type) {
case proposal:
Proposal proposal;
case commit:
Commit commit;
};
KeyForExternalReceiver keys_for_external_receivers<V>;
FramedContentAuthData auth;
opaque padding[length_of_padding];
} SemiPrivateMessageContent;
struct {
opaque group_id<V>;
uint64 epoch;
ContentType content_type;
opaque authenticated_data<V>;
KeyForExternalReceiver keys_for_external_receivers<V>;
opaque framed_content_tbs_hash<V>;
} SemiPrivateContentAAD;
/* IANA-registered value for semi_private_message */
extension_type = TBD2
SemiPrivateMessage extension_data;
¶
These two extensions provide a privacy improvement over sending handshake messages using PublicMessage. The handshake is shared with a specific list of receivers, and that list is visible as part of the GroupContext.¶
TODO More Security.¶
The semi_private_message MLS Extension Type is used to signal support
for the SemiPrivateMessage Wire Format (a Safe Extension).¶
The external_receivers extension contains a list of external receivers
targeted in a SemiPrivateMessage.¶
TODO acknowledge.¶