| Internet-Draft | MLS SemiPrivateMessage | October 2024 |
| Mahy | Expires 23 April 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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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].¶
Whenever a hash function is mentioned, it refers to the hash function defined in the cipher suite in use for the relevant MLS group.¶
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 required_capabilities
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 (key_and_nonces)
necessary to identify the sender leaf node and decrypt the
SemiPrivateMessage struct's ciphertext. Note that the
encrypted_sender_data cannot be decrypted by an external receiver,
but the sender_leaf_index is included with key_and_nonces and is
verified in another step. key_and_nonces is encrypted once for each
external receiver in the external_receivers extension.¶
As with a PrivateMessage, the sending client chooses an unused generation
in its own handshake ratchet and derives a key and nonce. It also
generates a fresh random four-byte reuse_guard.
The snippet below shows the syntax and encryption and decryption
construction of keys_and_nonces into encrypted_keys_and_nonces
for each external receiver.¶
struct {
opaque key<V>;
opaque nonce<V>;
opaque reuse_guard[4];
uint32 sender_leaf_index;
} PerMessageKeyAndNonces;
partial_context_hash = hash(sender_leaf_index || nonce)
struct {
opaque group_id<V>;
uint64 epoch;
opaque partial_context_hash<V>;
} SemiPrivateMessageContext;
PerMessageKeyAndNonces key_and_nonces;
SemiPrivateMessageContext semi_private_message_context;
encrypted_key_and_nonces = EncryptWithLabel(
external_receiver_public_key,
"SemiPrivateMessageReceiver",
semi_private_message_context, /* context */
keys_and_nonces)
key_and_nonces = DecryptWithLabel(
external_receiver_private_key,
"SemiPrivateMessageReceiver",
semi_private_message_context, /* 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_key_and_nonces.¶
ExternalReceiverRef = hash(ExternalReceiver)
struct {
ExternalReceiverRef external_receiver_ref;
HPKECiphertext encrypted_keys_and_nonces;
} KeyForExternalReceiver;
¶
The SemiPrivateMessage struct extends the PrivateMessage struct, adding
the keys_for_external_receivers list, the partial_context_hash needed
for its decryption context, and the hash of the FramedContentTBS to insure
that the sender cannot encrypt content to the external receivers that is
different from the other members, without detection.¶
The SemiPrivateContentAAD struct likewise extends the PrivateContentAAD
struct, adding the keys_for_external_receivers list, the
partial_context_hash and the framed_content_tbs_hash.¶
The SemiPrivateMessageContent struct is the same as
PrivateMessageContent except application messages are not included.¶
framed_content_tbs_hash = hash(FramedContentTBS)
struct {
opaque group_id<V>;
uint64 epoch;
ContentType content_type;
opaque authenticated_data<V>;
opaque partial_context_hash<V>;
KeyForExternalReceiver keys_for_external_receivers<V>;
opaque framed_content_tbs_hash<V>;
opaque encrypted_sender_data<V>;
opaque ciphertext<V>;
} SemiPrivateMessage;
struct {
select (SemiPrivateMessage.content_type) {
case proposal:
Proposal proposal;
case commit:
Commit commit;
};
FramedContentAuthData auth;
opaque padding[length_of_padding];
} SemiPrivateMessageContent;
struct {
opaque group_id<V>;
uint64 epoch;
ContentType content_type;
opaque authenticated_data<V>;
opaque partial_context_hash<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;
¶
Encryption of the ciphertext uses the cipher suite's AEAD algorithm using
the key, nonce xored with the reuse_guard, the
SemiPrivateMessageContent as the plaintext, and the
SemiPrivateContentAAD as the authenticated data.¶
Encryption of the encrypted_sender_data proceeds in the
same way for SemiPrivateMessage as for PrivateMessage.¶
Finally, as a safe wire format extension, the SemiPrivateMessage is
wrapped in an ExtensionContent struct.¶
After stripping off the the ExtensionContent struct, a member
receiver derives the sender_data_key and sender_data_nonce and decrypts the encrypted_sender_data, just as for a PrivateMessage.¶
The receiver uses the SenderData to lookup the key and nonce for
the correct generation in the (non-blank) sender's handshake ratchet.
The receiver verifies the partial_context_hash.¶
After xoring the nonce with the reuse_guard, the member decrypts the
ciphertext. It verifies the padding consists of the appropriate number of
zero bytes, and verifies that the framed_content_tbs_hash is correct.
Finally, it verifies that the signature in the FramedContentAuthData is
valid.¶
After stripping off the the ExtensionContent struct, an external receiver
looks in the keys_for_external_receivers field for its
external_receiver_ref. It calculates the semi_private_message_context
and uses HPKE to decrypt the encrypted_keys_and_nonces. Using the nonce
and sender_leaf_node it verifies the partial_context_hash.¶
After xoring the nonce with the reuse_guard, the member decrypts the
ciphertext. It verifies the padding consists of the appropriate number of
zero bytes, and verifies that the framed_content_tbs_hash is correct.
If the external receiver has a copy of the GroupContext, it verifies that
the signature in the FramedContentAuthData is valid.¶
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.¶
corrected a typo in SemiPrivateMessageContent¶
do not attempt to decrypt SenderData for external receivers; instead also encrypt the sender_leaf_index and reuse_guard.¶
make the encrypted_key_and_nonces context include the group_id, epoch, and a the hash of the sender_leaf_index and nonce. include that partial_context_hash in the AAD.¶
add a hash of the FramedContentTBS to the AAD to make sure the content encrypted to the external receiver is the same as that sent to members.¶
add explicit instructions about encryption and decryption.¶
TODO acknowledge.¶