Network Working Group R. Barnes
Internet-Draft Cisco
Intended status: Informational B. Beurdouche
Expires: 4 November 2022 Inria & Mozilla
R. Robert
J. Millican
Facebook
E. Omara
Google
K. Cohn-Gordon
University of Oxford
3 May 2022
The Messaging Layer Security (MLS) Protocol
draft-ietf-mls-protocol-14
Abstract
Messaging applications are increasingly making use of end-to-end
security mechanisms to ensure that messages are only accessible to
the communicating endpoints, and not to any servers involved in
delivering messages. Establishing keys to provide such protections
is challenging for group chat settings, in which more than two
clients need to agree on a key but may not be online at the same
time. In this document, we specify a key establishment protocol that
provides efficient asynchronous group key establishment with forward
secrecy and post-compromise security for groups in size ranging from
two to thousands.
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-protocol (https://github.com/mlswg/mls-
protocol).
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/.
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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This Internet-Draft will expire on 4 November 2022.
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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/
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Please review these documents carefully, as they describe your rights
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Change Log . . . . . . . . . . . . . . . . . . . . . . . 5
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.1. Presentation Langauge . . . . . . . . . . . . . . . . . . 12
2.1.1. Optional Value . . . . . . . . . . . . . . . . . . . 12
2.1.2. Variable-size Vector Headers . . . . . . . . . . . . 13
3. Operating Context . . . . . . . . . . . . . . . . . . . . . . 15
4. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 15
4.1. Cryptographic State and Evolution . . . . . . . . . . . . 16
4.2. Example Protocol Execution . . . . . . . . . . . . . . . 18
4.3. Relationships Between Epochs . . . . . . . . . . . . . . 22
5. Ratchet Tree Concepts . . . . . . . . . . . . . . . . . . . . 23
5.1. Ratchet Tree Terminology . . . . . . . . . . . . . . . . 24
5.2. Views of a Ratchet Tree . . . . . . . . . . . . . . . . . 25
5.3. Ratchet Tree Nodes . . . . . . . . . . . . . . . . . . . 27
6. Cryptographic Objects . . . . . . . . . . . . . . . . . . . . 29
6.1. Ciphersuites . . . . . . . . . . . . . . . . . . . . . . 29
6.2. Hash-Based Identifiers . . . . . . . . . . . . . . . . . 30
6.3. Credentials . . . . . . . . . . . . . . . . . . . . . . . 31
6.3.1. Uniquely Identifying Clients . . . . . . . . . . . . 32
7. Message Framing . . . . . . . . . . . . . . . . . . . . . . . 33
7.1. Content Authentication . . . . . . . . . . . . . . . . . 35
7.2. Encoding and Decoding a Plaintext . . . . . . . . . . . . 37
7.3. Encoding and Decoding a Ciphertext . . . . . . . . . . . 37
7.3.1. Content Encryption . . . . . . . . . . . . . . . . . 38
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7.3.2. Sender Data Encryption . . . . . . . . . . . . . . . 39
8. Ratchet Tree Operations . . . . . . . . . . . . . . . . . . . 40
8.1. Parent Node Contents . . . . . . . . . . . . . . . . . . 40
8.2. Leaf Node Contents . . . . . . . . . . . . . . . . . . . 41
8.3. Leaf Node Validation . . . . . . . . . . . . . . . . . . 44
8.4. Ratchet Tree Evolution . . . . . . . . . . . . . . . . . 45
8.5. Adding and Removing Leaves . . . . . . . . . . . . . . . 47
8.6. Synchronizing Views of the Tree . . . . . . . . . . . . . 49
8.7. Tree Hashes . . . . . . . . . . . . . . . . . . . . . . . 51
8.8. Parent Hash . . . . . . . . . . . . . . . . . . . . . . . 51
8.8.1. Using Parent Hashes . . . . . . . . . . . . . . . . . 54
8.8.2. Verifying Parent Hashes . . . . . . . . . . . . . . . 54
8.9. Update Paths . . . . . . . . . . . . . . . . . . . . . . 55
9. Key Schedule . . . . . . . . . . . . . . . . . . . . . . . . 56
9.1. Group Context . . . . . . . . . . . . . . . . . . . . . . 58
9.2. Transcript Hashes . . . . . . . . . . . . . . . . . . . . 60
9.3. External Initialization . . . . . . . . . . . . . . . . . 61
9.4. Pre-Shared Keys . . . . . . . . . . . . . . . . . . . . . 61
9.5. Exporters . . . . . . . . . . . . . . . . . . . . . . . . 64
9.6. Resumption PSK . . . . . . . . . . . . . . . . . . . . . 65
9.7. Epoch Authenticators . . . . . . . . . . . . . . . . . . 65
10. Secret Tree . . . . . . . . . . . . . . . . . . . . . . . . . 66
10.1. Encryption Keys . . . . . . . . . . . . . . . . . . . . 66
10.2. Deletion Schedule . . . . . . . . . . . . . . . . . . . 67
11. Key Packages . . . . . . . . . . . . . . . . . . . . . . . . 69
11.1. KeyPackage Validation . . . . . . . . . . . . . . . . . 71
11.2. KeyPackage Identifiers . . . . . . . . . . . . . . . . . 71
12. Group Creation . . . . . . . . . . . . . . . . . . . . . . . 71
12.1. Required Capabilities . . . . . . . . . . . . . . . . . 73
12.2. Reinitialization . . . . . . . . . . . . . . . . . . . . 73
12.3. Sub-group Branching . . . . . . . . . . . . . . . . . . 74
13. Group Evolution . . . . . . . . . . . . . . . . . . . . . . . 75
13.1. Proposals . . . . . . . . . . . . . . . . . . . . . . . 76
13.1.1. Add . . . . . . . . . . . . . . . . . . . . . . . . 76
13.1.2. Update . . . . . . . . . . . . . . . . . . . . . . . 77
13.1.3. Remove . . . . . . . . . . . . . . . . . . . . . . . 77
13.1.4. PreSharedKey . . . . . . . . . . . . . . . . . . . . 78
13.1.5. ReInit . . . . . . . . . . . . . . . . . . . . . . . 78
13.1.6. ExternalInit . . . . . . . . . . . . . . . . . . . . 79
13.1.7. AppAck . . . . . . . . . . . . . . . . . . . . . . . 79
13.1.8. GroupContextExtensions . . . . . . . . . . . . . . . 80
13.1.9. External Proposals . . . . . . . . . . . . . . . . . 81
13.2. Commit . . . . . . . . . . . . . . . . . . . . . . . . . 82
13.2.1. Creating a Commit . . . . . . . . . . . . . . . . . 85
13.2.2. Processing a Commit . . . . . . . . . . . . . . . . 88
13.2.3. Adding Members to the Group . . . . . . . . . . . . 90
13.3. Ratchet Tree Extension . . . . . . . . . . . . . . . . . 96
14. Extensibility . . . . . . . . . . . . . . . . . . . . . . . . 99
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14.1. Ciphersuites . . . . . . . . . . . . . . . . . . . . . . 99
14.2. Proposals . . . . . . . . . . . . . . . . . . . . . . . 100
14.3. Credential Extensibility . . . . . . . . . . . . . . . . 100
15. Sequencing of State Changes . . . . . . . . . . . . . . . . . 102
15.1. Server-Enforced Ordering . . . . . . . . . . . . . . . . 103
15.2. Client-Enforced Ordering . . . . . . . . . . . . . . . . 103
16. Application Messages . . . . . . . . . . . . . . . . . . . . 104
16.1. Message Encryption and Decryption . . . . . . . . . . . 104
16.2. Restrictions . . . . . . . . . . . . . . . . . . . . . . 105
16.3. Delayed and Reordered Application messages . . . . . . . 105
17. Security Considerations . . . . . . . . . . . . . . . . . . . 105
17.1. Confidentiality of the Group Secrets . . . . . . . . . . 106
17.2. Authentication . . . . . . . . . . . . . . . . . . . . . 106
17.3. Forward Secrecy and Post-Compromise Security . . . . . . 107
17.4. KeyPackage Reuse . . . . . . . . . . . . . . . . . . . . 107
17.5. Group Fragmentation by Malicious Insiders . . . . . . . 108
18. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 108
18.1. MLS Ciphersuites . . . . . . . . . . . . . . . . . . . . 109
18.2. MLS Extension Types . . . . . . . . . . . . . . . . . . 113
18.3. MLS Proposal Types . . . . . . . . . . . . . . . . . . . 114
18.4. MLS Credential Types . . . . . . . . . . . . . . . . . . 115
18.5. MLS Designated Expert Pool . . . . . . . . . . . . . . . 116
18.6. The "message/mls" MIME Type . . . . . . . . . . . . . . 117
19. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 117
20. References . . . . . . . . . . . . . . . . . . . . . . . . . 118
20.1. Normative References . . . . . . . . . . . . . . . . . . 118
20.2. Informative References . . . . . . . . . . . . . . . . . 119
Appendix A. Protocol Origins of Example Trees . . . . . . . . . 121
Appendix B. Array-Based Trees . . . . . . . . . . . . . . . . . 122
Appendix C. Link-Based Trees . . . . . . . . . . . . . . . . . . 126
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 128
1. Introduction
DISCLAIMER: This is a work-in-progress draft of MLS and has not yet
seen significant security analysis. It should not be used as a basis
for building production systems.
RFC EDITOR: PLEASE REMOVE THE FOLLOWING PARAGRAPH The source for this
draft is maintained in GitHub. Suggested changes should be submitted
as pull requests at https://github.com/mlswg/mls-protocol.
Instructions are on that page as well. Editorial changes can be
managed in GitHub, but any substantive change should be discussed on
the MLS mailing list.
A group of users who want to send each other encrypted messages needs
a way to derive shared symmetric encryption keys. For two parties,
this problem has been studied thoroughly, with the Double Ratchet
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emerging as a common solution [doubleratchet] [signal]. Channels
implementing the Double Ratchet enjoy fine-grained forward secrecy as
well as post-compromise security, but are nonetheless efficient
enough for heavy use over low-bandwidth networks.
For a group of size greater than two, a common strategy is to
unilaterally broadcast symmetric "sender" keys over existing shared
symmetric channels, and then for each member to send messages to the
group encrypted with their own sender key. Unfortunately, while this
improves efficiency over pairwise broadcast of individual messages
and provides forward secrecy (with the addition of a hash ratchet),
it is difficult to achieve post-compromise security with sender keys.
An adversary who learns a sender key can often indefinitely and
passively eavesdrop on that member's messages. Generating and
distributing a new sender key provides a form of post-compromise
security with regard to that sender. However, it requires
computation and communications resources that scale linearly with the
size of the group.
In this document, we describe a protocol based on tree structures
that enable asynchronous group keying with forward secrecy and post-
compromise security. Based on earlier work on "asynchronous
ratcheting trees" [art], the protocol presented here uses an
asynchronous key-encapsulation mechanism for tree structures. This
mechanism allows the members of the group to derive and update shared
keys with costs that scale as the log of the group size.
1.1. Change Log
RFC EDITOR PLEASE DELETE THIS SECTION.
draft-13
* TLS syntax updates (including variable-header-length vectors) (*)
* Stop generating redundant PKE key pairs. (*)
* Move validation of identity change to the AS
* Add message/mls MIME type registration
* Split LeafNode from KeyPackage (*)
* Remove endpoint_id (*)
* Reorganize to make section layout more sane
* Forbid proposals by reference in external commits (*)
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* Domain separation for KeyPackage and Proposal references (*)
* Downgrade MUST to SHOULD for commit senders including all valid
commits
* Stronger parent hashes for authenticated identities (*)
* Move wire_format to a separate tagged-union structure MLSMessage
* Generalize tree extend/truncate algorithms
* Add algorithms for link-based trees
* Forbid self-Update entirely (*)
* Consolidate resumption PSK cases (*)
* 384 Ciphersuite Addition
* Remove explicit version pin on HPKE (*)
* Remove the requirement for Add in external commit (*)
* Use smaller, fixed-size hash-based identifiers (*)
* Be explicit that Credentials can attest to multiple identities (*)
draft-12
* Use the GroupContext to derive the joiner_secret (*)
* Make PreSharedKeys non optional in GroupSecrets (*)
* Update name for this particular key (*)
* Truncate tree size on removal (*)
* Use HPKE draft-08 (*)
* Clarify requirements around identity in MLS groups (*)
* Signal the intended wire format for MLS messages (*)
* Inject GroupContext as HPKE info instead of AAD (*)
* Clarify extension handling and make extension updatable (*)
* Improve extensibility of Proposals (*)
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* Constrain proposal in External Commit (*)
* Remove the notion of a 'leaf index' (*)
* Add group_context_extensions proposal ID (*)
* Add RequiredCapabilities extension (*)
* Use cascaded KDF instead of concatenation to consolidate PSKs (*)
* Use key package hash to index clients in message structs (*)
* Don't require PublicGroupState for external init (*)
* Make ratchet tree section clearer.
* Handle non-member sender cases in MLSPlaintextTBS
* Clarify encoding of signatures with NIST curves
* Remove OPEN ISSUEs and TODOs
* Normalize the description of the zero vector
draft-11
* Include subtree keys in parent hash (*)
* Pin HPKE to draft-07 (*)
* Move joiner secret to the end of the first key schedule epoch (*)
* Add an AppAck proposal
* Make initializations of transcript hashes consistent
draft-10
* Allow new members to join via an external Commit (*)
* Enable proposals to be sent inline in a Commit (*)
* Re-enable constant-time Add (*)
* Change expiration extension to lifetime extension (*)
* Make the tree in the Welcome optional (*)
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* PSK injection, re-init, sub-group branching (*)
* Require the initial init_secret to be a random value (*)
* Remove explicit sender data nonce (*)
* Do not encrypt to joiners in UpdatePath generation (*)
* Move MLSPlaintext signature under the confirmation tag (*)
* Explicitly authenticate group membership with MLSPLaintext (*)
* Clarify X509Credential structure (*)
* Remove unneeded interim transcript hash from GroupInfo (*)
* IANA considerations
* Derive an authentication secret
* Use Extract/Expand from HPKE KDF
* Clarify that application messages MUST be encrypted
draft-09
* Remove blanking of nodes on Add (*)
* Change epoch numbers to uint64 (*)
* Add PSK inputs (*)
* Add key schedule exporter (*)
* Sign the updated direct path on Commit, using "parent hashes" and
one signature per leaf (*)
* Use structured types for external senders (*)
* Redesign Welcome to include confirmation and use derived keys (*)
* Remove ignored proposals (*)
* Always include an Update with a Commit (*)
* Add per-message entropy to guard against nonce reuse (*)
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* Use the same hash ratchet construct for both application and
handshake keys (*)
* Add more ciphersuites
* Use HKDF to derive key pairs (*)
* Mandate expiration of ClientInitKeys (*)
* Add extensions to GroupContext and flesh out the extensibility
story (*)
* Rename ClientInitKey to KeyPackage
draft-08
* Change ClientInitKeys so that they only refer to one ciphersuite
(*)
* Decompose group operations into Proposals and Commits (*)
* Enable Add and Remove proposals from outside the group (*)
* Replace Init messages with multi-recipient Welcome message (*)
* Add extensions to ClientInitKeys for expiration and downgrade
resistance (*)
* Allow multiple Proposals and a single Commit in one MLSPlaintext
(*)
draft-07
* Initial version of the Tree based Application Key Schedule (*)
* Initial definition of the Init message for group creation (*)
* Fix issue with the transcript used for newcomers (*)
* Clarifications on message framing and HPKE contexts (*)
draft-06
* Reorder blanking and update in the Remove operation (*)
* Rename the GroupState structure to GroupContext (*)
* Rename UserInitKey to ClientInitKey
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* Resolve the circular dependency that draft-05 introduced in the
confirmation MAC calculation (*)
* Cover the entire MLSPlaintext in the transcript hash (*)
draft-05
* Common framing for handshake and application messages (*)
* Handshake message encryption (*)
* Convert from literal state to a commitment via the "tree hash" (*)
* Add credentials to the tree and remove the "roster" concept (*)
* Remove the secret field from tree node values
draft-04
* Updating the language to be similar to the Architecture document
* ECIES is now renamed in favor of HPKE (*)
* Using a KDF instead of a Hash in TreeKEM (*)
draft-03
* Added ciphersuites and signature schemes (*)
* Re-ordered fields in UserInitKey to make parsing easier (*)
* Fixed inconsistencies between Welcome and GroupState (*)
* Added encryption of the Welcome message (*)
draft-02
* Removed ART (*)
* Allowed partial trees to avoid double-joins (*)
* Added explicit key confirmation (*)
draft-01
* Initial description of the Message Protection mechanism. (*)
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* Initial specification proposal for the Application Key Schedule
using the per-participant chaining of the Application Secret
design. (*)
* Initial specification proposal for an encryption mechanism to
protect Application Messages using an AEAD scheme. (*)
* Initial specification proposal for an authentication mechanism of
Application Messages using signatures. (*)
* Initial specification proposal for a padding mechanism to
improving protection of Application Messages against traffic
analysis. (*)
* Inversion of the Group Init Add and Application Secret derivations
in the Handshake Key Schedule to be ease chaining in case we
switch design. (*)
* Removal of the UserAdd construct and split of GroupAdd into Add
and Welcome messages (*)
* Initial proposal for authenticating handshake messages by signing
over group state and including group state in the key schedule (*)
* Added an appendix with example code for tree math
* Changed the ECIES mechanism used by TreeKEM so that it uses nonces
generated from the shared secret
draft-00
* Initial adoption of draft-barnes-mls-protocol-01 as a WG item.
2. Terminology
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.
Client: An agent that uses this protocol to establish shared
cryptographic state with other clients. A client is defined by
the cryptographic keys it holds.
Group: A group represents a logical collection of clents that share
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a common secret value at any given time. Its state is represented
as a linear sequence of epochs in which each epoch depends on its
predecessor.
Epoch: A state of a group in which a specific set of authenticated
clients hold shared cryptographic state.
Member: A client that is included in the shared state of a group,
hence has access to the group's secrets.
Key Package: A signed object describing a client's identity and
capabilities, and including a hybrid public-key encryption (HPKE
[RFC9180]) public key that can be used to encrypt to that client,
and which other clients can use to introduce the client to a new
group.
Signature Key: A signing key pair used to authenticate the sender of
a message.
Handshake Message: An MLSPlaintext or MLSCiphertext message carrying
an MLS Proposal or Commit object, as opposed to application data.
Application Message: An MLSCiphertext message carrying application
data.
Terminology specific to tree computations is described in
Section 5.1.
In general, symmetric values are referred to as "keys" or "secrets"
interchangeably. Either term denotes a value that MUST be kept
confidential to a Client. When labelling individual values, we
typically use "secret" to refer to a value that is used derive
further secret values, and "key" to refer to a value that is used
with an algorithm such as HMAC or an AEAD algorithm.
2.1. Presentation Langauge
We use the TLS presentation language [RFC8446] to describe the
structure of protocol messages. In addition to the base syntax, we
add two additional features, the ability for fields to be optional
and the ability for vectors to have variable-size length headers.
2.1.1. Optional Value
An optional value is encoded with a presence-signaling octet,
followed by the value itself if present. When decoding, a presence
octet with a value other than 0 or 1 MUST be rejected as malformed.
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struct {
uint8 present;
select (present) {
case 0: struct{};
case 1: T value;
}
} optional<T>;
2.1.2. Variable-size Vector Headers
In the TLS presentation language, vectors are encoded as a sequence
of encoded elements prefixed with a length. The length field has a
fixed size set by specifying the minimum and maximum lengths of the
encoded sequence of elements.
In MLS, there are several vectors whose sizes vary over significant
ranges. So instead of using a fixed-size length field, we use a
variable-size length using a variable-length integer encoding based
on the one in Section 16 of [RFC9000]. (They differ only in that the
one here requires a minimum-size encoding.) Instead of presenting
min and max values, the vector description simply includes a V. For
example:
struct {
uint32 fixed<0..255>;
opaque variable<V>;
} StructWithVectors;
Such a vector can represent values with length from 0 bytes to 2^30
bytes. The variable-length integer encoding reserves the two most
significant bits of the first byte to encode the base 2 logarithm of
the integer encoding length in bytes. The integer value is encoded
on the remaining bits, in network byte order. The encoded value MUST
use the smallest number of bits required to represent the value.
When decoding, values using more bits than necessary MUST be treated
as malformed.
This means that integers are encoded on 1, 2, or 4 bytes and can
encode 6-, 14-, or 30-bit values respectively.
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+========+=========+=============+=======+============+
| Prefix | Length | Usable Bits | Min | Max |
+========+=========+=============+=======+============+
| 00 | 1 | 6 | 0 | 63 |
+--------+---------+-------------+-------+------------+
| 01 | 2 | 14 | 64 | 16383 |
+--------+---------+-------------+-------+------------+
| 10 | 4 | 30 | 16384 | 1073741823 |
+--------+---------+-------------+-------+------------+
| 11 | invalid | - | - | - |
+--------+---------+-------------+-------+------------+
Table 1: Summary of Integer Encodings
Vectors that start with "11" prefix are invalid and MUST be rejected.
For example, the four byte sequence 0x9d7f3e7d decodes to 494878333;
the two byte sequence 0x7bbd decodes to 15293; and the single byte
0x25 decodes to 37.
The following figure adapts the pseudocode provided in [RFC9000] to
add a check for minimum-length encoding:
ReadVarint(data):
// The length of variable-length integers is encoded in the
// first two bits of the first byte.
v = data.next_byte()
prefix = v >> 6
length = 1 << prefix
// Once the length is known, remove these bits and read any
// remaining bytes.
v = v & 0x3f
repeat length-1 times:
v = (v << 8) + data.next_byte()
return v
// Check that the encoder used the minimum bits required
if length > 1 && v < (1 << (length - 1)):
raise an exception
The use of variable-size integers for vector lengths allows vectors
to grow very large, up to 2^30 bytes. Implementations should take
care not to allow vectors to overflow available storage. To
facilitate debugging of potential interoperatbility problems,
implementations should provide a clear error when such an overflow
condition occurs.
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3. Operating Context
MLS is designed to operate in the context described in
[I-D.ietf-mls-architecture]. In particular, we assume that the
following services are provided:
* A Delivery Service that routes MLS messages among the participants
in the protocol. The following types of delivery are typically
required:
- Pre-publication of KeyPackage objects for clients
- Broadcast delivery of Proposal and Commit messages to members
of a group
- Unicast delivery of Welcome messages to new members of a group
* An Authentication Service that enables group members to
authenticate the credentials presented by other group members.
4. Protocol Overview
The core functionality of MLS is continuous group authenticated key
exchange (AKE). As with other authenticated key exchange protocols
(such as TLS), the participants in the protocol agree on a common
secret value, and each participant can verify the identity of the
other participants. MLS provides group AKE in the sense that there
can be more than two participants in the protocol, and continuous
group AKE in the sense that the set of participants in the protocol
can change over time.
The core organizing principles of MLS are _groups_ and _epochs_. A
group represents a logical collection of clients that share a common
secret value at any given time. The history of a group is divided
into a linear sequence of epochs. In each epoch, a set of
authenticated _members_ agree on an _epoch secret_ that is known only
to the members of the group in that epoch. The set of members
involved in the group can change from one epoch to the next, and MLS
ensures that only the members in the current epoch have access to the
epoch secret. From the epoch secret, members derive further shared
secrets for message encryption, group membership authentication, etc.
The creator of an MLS group creates the group's first epoch
unilaterally, with no protocol interactions. Thereafter, the members
of the group advance their shared cryptographic state from one epoch
to another by exchanging MLS messages:
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* A _KeyPackage_ object describes a client's capabilities and
provides keys that can be used to add the client to a group.
* A _Proposal_ message proposes a change to be made in the next
epoch, such as adding or removing a member
* A _Commit_ message initiates a new epoch by instructing members of
the group to implement a collection of proposals
* A _Welcome_ message provides a new member to the group with the
information to initialize their state for the epoch in which they
were added or in which they want to add themselves to the group
KeyPackage and Welcome messages are used to initiate a group or
introduce new members, so they are exchanged between group members
and clients not yet in the group.
Proposal and Commit messages are sent from one member of a group to
the others. MLS provides a common framing layer for sending messages
within a group: An _MLSPlaintext_ message provides sender
authentication for unencrypted Proposal and Commit messages. An
_MLSCiphertext_ message provides encryption and authentication for
both Proposal/Commit messages as well as any application data.
4.1. Cryptographic State and Evolution
The cryptographic state at the core of MLS is divided into three
areas of responsibility:
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.- ... -.
| |
| | |
| | | Key Schedule
| V |
| epoch_secret |
. | | | .
|\ Ratchet | | | Secret /|
| \ Tree | | | Tree / |
| \ | | | / |
| \ | V | / |
| +--> commit_secret --> epoch_secret --> encryption_secret -->+ |
| / | | | \ |
| / | | | \ |
| / | | | \ |
|/ | | | \|
' | V | '
| epoch_secret |
| | |
| | |
| V |
| |
'- ... -'
Figure 1: Overview of MLS group evolution
* A _ratchet tree_ that represents the membership of the group,
providing group members a way to authenticate each other and
efficiently encrypt messages to subsets of the group. Each epoch
has a distinct ratchet tree. It seeds the _key schedule_.
* A _key schedule_ that describes the chain of key derivations used
to progress from epoch to epoch (mainly using the _init_secret_
and _epoch_secret_); and to derive a variety of other secrets (see
Table 3) used during the current epoch. One of these (the
_encryption_secret_) is the root of _secret_tree_.
* A _secret tree_ derived from the key schedule that represents
shared secrets used by the members of the group to provide
confidentiality and forward secrecy for MLS messages. Each epoch
has a distinct secret tree.
Each member of the group maintains a view of these facets of the
group's state. MLS messages are used to initialize these views and
keep them in sync as the group transitions between epochs.
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Each new epoch is initiated with a Commit message. The Commit
instructs existing members of the group to update their views of the
ratchet tree by applying a set of Proposals, and uses the updated
ratchet tree to distribute fresh entropy to the group. This fresh
entropy is provided only to members in the new epoch, not to members
who have been removed, so it maintains the confidentiality of the
epoch secret (in other words, it provides post-compromise security
with respect to those members).
For each Commit that adds member(s) to the group, there is a single
corresponding Welcome message. The Welcome message provides all the
new members with the information they need to initialize their views
of the key schedule and ratchet tree, so that these views are
equivalent to the views held by other members of the group in this
epoch.
In addition to defining how one epoch secret leads to the next, the
key schedule also defines a collection of secrets that are derived
from the epoch secret. For example:
* An _encryption secret_ that is used to initialize the secret tree
for the epoch.
* A _confirmation key_ that is used to confirm that all members
agree on the shared state of the group.
* A _resumption secret_ that members can use to prove their
membership in the group, e.g., in the case of branching a
subgroup.
Finally, an _init secret_ is derived that is used to initialize the
next epoch.
4.2. Example Protocol Execution
There are three major operations in the lifecycle of a group:
* Adding a member, initiated by a current member;
* Updating the leaf secret of a member;
* Removing a member.
Each of these operations is "proposed" by sending a message of the
corresponding type (Add / Update / Remove). The state of the group
is not changed, however, until a Commit message is sent to provide
the group with fresh entropy. In this section, we show each proposal
being committed immediately, but in more advanced deployment cases an
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application might gather several proposals before committing them all
at once. In the illustrations below, we show the Proposal and Commit
messages directly, while in reality they would be sent encapsulated
in MLSPlaintext or MLSCiphertext objects.
Before the initialization of a group, clients publish KeyPackages to
a directory provided by the Service Provider.
Group
A B C Directory Channel
| | | | |
| KeyPackageA | | | |
+------------------------------------------------->| |
| | | | |
| | KeyPackageB | | |
| +-------------------------------->| |
| | | | |
| | | KeyPackageC | |
| | +--------------->| |
| | | | |
Figure 2: Clients A, B, and C publish KeyPackages to the directory
When a client A wants to establish a group with B and C, it first
initializes a group state containing only itself and downloads
KeyPackages for B and C. For each member, A generates an Add and
Commit message adding that member, and broadcasts them to the group.
It also generates a Welcome message and sends this directly to the
new member (there's no need to send it to the group). Only after A
has received its Commit message back from the server does it update
its state to reflect the new member's addition.
Upon receiving the Welcome message, the new member will be able to
read and send new messages to the group. However, messages sent
before they were added to the group will not be accessible.
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Group
A B C Directory Channel
| | | | |
| KeyPackageB, KeyPackageC | |
|<-------------------------------------------+ |
| | | | |
| | | | Add(A->AB) |
| | | | Commit(Add) |
+--------------------------------------------------------------->|
| | | | |
| Welcome(B) | | | |
+------------->| | | |
| | | | |
| | | | Add(A->AB) |
| | | | Commit(Add) |
|<---------------------------------------------------------------+
| | | | |
| | | | |
| | | | Add(AB->ABC) |
| | | | Commit(Add) |
+--------------------------------------------------------------->|
| | | | |
| | Welcome(C) | | |
+---------------------------->| | |
| | | | |
| | | | Add(AB->ABC) |
| | | | Commit(Add) |
|<---------------------------------------------------------------+
| |<------------------------------------------------+
| | | | |
Figure 3: Client A creates a group with clients B and C
Subsequent additions of group members proceed in the same way. Any
member of the group can download a KeyPackage for a new client and
broadcast Add and Commit messages that the current group will use to
update their state, and a Welcome message that the new client can use
to initialize its state and join the group.
To enforce the forward secrecy and post-compromise security of
messages, each member periodically updates the keys that represent
them to the group. A member does this by sending a Commit (possibly
with no proposals), or by sending an Update message that is committed
by another member. Once the other members of the group have
processed these messages, the group's secrets will be unknown to an
attacker that had compromised the sender's prior leaf secret.
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Update messages should be sent at regular intervals of time as long
as the group is active, and members that don't update should
eventually be removed from the group. It's left to the application
to determine an appropriate amount of time between Updates.
Group
A B ... Z Directory Channel
| | | | |
| | Update(B) | | |
| +------------------------------------------->|
| Commit(Upd) | | | |
+---------------------------------------------------------->|
| | | | |
| | | | Update(B) |
| | | | Commit(Upd) |
|<----------------------------------------------------------+
| |<-------------------------------------------+
| | |<----------------------------+
| | | | |
Figure 4: Client B proposes to update its key, and client A
commits the proposal. As a result, the keys for both B and A
updated, so the group has post-compromise security with respect
to both of them.
Members are removed from the group in a similar way. Any member of
the group can send a Remove proposal followed by a Commit message.
The Commit message provides new entropy to all members of the group
except the removed member. This new entropy is added to the epoch
secret for the new epoch, so that it is not known to the removed
member. Note that this does not necessarily imply that any member is
actually allowed to evict other members; groups can enforce access
control policies on top of these basic mechanism.
Group
A B ... Z Directory Channel
| | | | |
| | | Remove(B) | |
| | | Commit(Rem) | |
| | +---------------------------->|
| | | | |
| | | | Remove(B) |
| | | | Commit(Rem) |
|<----------------------------------------------------------+
| | |<----------------------------+
| | | | |
Figure 5: Client Z removes client B from the group
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4.3. Relationships Between Epochs
A group has a single linear sequence of epochs. Groups and epochs
are generally independent of one-another. However, it can sometimes
be useful to link epochs cryptographically, either within a group or
across groups. MLS derives a resumption pre-shared key (PSK) from
each epoch to allow entropy extracted from one epoch to be injected
into a future epoch. This link guarantees that members entering the
new epoch agree on a key if and only if they were members of the
group during the epoch from which the resumption key was extracted.
MLS supports two ways to tie a new group to an existing group. Re-
initialization closes one group and creates a new group comprising
the same members with different parameters. Branching starts a new
group with a subset of the original group's participants (with no
effect on the original group). In both cases, the new group is
linked to the old group via a resumption PSK.
epoch_A_[n-1]
|
|
|<-- ReInit
|
V
epoch_A_[n] epoch_B_[0]
. |
. PSK(usage=reinit) |
.....................>|
|
V
epoch_B_[1]
Figure 6: Reinitializing a group
epoch_A_[n] epoch_B_[0]
| |
| PSK(usage=branch) |
|....................>|
| |
V V
epoch_A_[n+1] epoch_B_[1]
Figure 7: Branching a group
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Applications may also choose to use resumption PSKs to link epochs in
other ways. For example, the following figure shows a case where a
resumption PSK from epoch n is injected into epoch n+k. This
demonstrates that the members of the group at epoch n+k were also
members at epoch n, irrespective of any changes to these members'
keys due to Updates or Commits.
epoch_A_[n]
|
| PSK(usage=application)
|.....................
| .
| .
... ...
| .
| .
V .
epoch_A_[n+k-1] .
| .
| .
|<....................
|
V
epoch_A_[n+k]
Figure 8: Reinjecting entropy from an earlier epoch
5. Ratchet Tree Concepts
The protocol uses "ratchet trees" for deriving shared secrets among a
group of clients. A ratchet tree is an arrangement of secrets and
key pairs among the members of a group in a way that allows for
secrets to be efficiently updated to reflect changes in the group.
Ratchet trees allow a group to efficiently remove any member by
encrypting new entropy to a subset of the group. A ratchet tree
assigns shared keys to subgroups of the overall group, so that, for
example, encrypting to all but one member of the group requires only
log(N) encryptions, instead of the N-1 encryptions that would be
needed to encrypt to each participant individually (where N is the
number of members in the group).
This remove operation allows MLS to efficiently achieve post-
compromise security. In an Update proposal or a full Commit message,
an old (possibly compromised) representation of a member is
efficiently removed from the group and replaced with a freshly
generated instance.
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5.1. Ratchet Tree Terminology
Trees consist of _nodes_. A node is a _leaf_ if it has no children,
and a _parent_ otherwise; note that all parents in our trees have
precisely two children, a _left_ child and a _right_ child. A node
is the _root_ of a tree if it has no parents, and _intermediate_ if
it has both children and parents. The _descendants_ of a node are
that node's children, and the descendants of its children, and we say
a tree _contains_ a node if that node is a descendant of the root of
the tree, or if the node itself is the root of the tree. Nodes are
_siblings_ if they share the same parent.
A _subtree_ of a tree is the tree given by any node (the _head_ of
the subtree) and its descendants. The _size_ of a tree or subtree is
the number of leaf nodes it contains. For a given parent node, its
_left subtree_ is the subtree with its left child as head
(respectively _right subtree_).
All trees used in this protocol are left-balanced binary trees. A
binary tree is _full_ (and _balanced_) if its size is a power of two
and for any parent node in the tree, its left and right subtrees have
the same size.
A binary tree is _left-balanced_ if for every parent, either the
parent is balanced, or the left subtree of that parent is the largest
full subtree that could be constructed from the leaves present in the
parent's own subtree. Given a list of n items, there is a unique
left-balanced binary tree structure with these elements as leaves.
(Note that left-balanced binary trees are the same structure that is
used for the Merkle trees in the Certificate Transparency protocol
[I-D.ietf-trans-rfc6962-bis].)
The _direct path_ of a root is the empty list, and of any other node
is the concatenation of that node's parent along with the parent's
direct path. The _copath_ of a node is the node's sibling
concatenated with the list of siblings of all the nodes in its direct
path, excluding the root.
For example, in the below tree:
* The direct path of C is (W, V, X)
* The copath of C is (D, U, Z)
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X = root
|
.-----+-----.
/ \
V Z
| |
.-+-. .-+
/ \ / \
U W Y +
/ \ / \ / \ |
A B C D E F G
0 1 2 3 4 5 6
Figure 9: A complete tree with seven members
A tree with n leaves has 2*n - 1 nodes. For example, the above tree
has 7 leaves (A, B, C, D, E, F, G) and 13 nodes.
Each leaf is given an _index_ (or _leaf index_), starting at 0 from
the left to n-1 at the right.
Finally, a node in the tree may also be _blank_, indicating that no
value is present at that node (i.e. no keying material). This is
often the case when a leaf was recently removed from the tree.
There are multiple ways that an implementation might represent a
ratchet tree in memory. For example, left-balanced binary trees can
be represented as an array of nodes, with node relationships computed
based on nodes' indices in the array. Or a more traditional
representation of linked node objects may be used. Appendix B and
Appendix C provide some details on how to implement the tree
operations required for MLS in these representations. MLS places no
requirements on implementations' internal representations of ratchet
trees. An implementation MAY use any tree representation and
associated algorithms, as long as they produce correct protocol
messages.
5.2. Views of a Ratchet Tree
We generally assume that each participant maintains a complete and
up-to-date view of the public state of the group's ratchet tree,
including the public keys for all nodes and the credentials
associated with the leaf nodes.
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No participant in an MLS group knows the private key associated with
every node in the tree. Instead, each member is assigned to a leaf
of the tree, which determines the subset of private keys it knows.
The credential stored at that leaf is one provided by the member.
In particular, MLS maintains the members' views of the tree in such a
way as to maintain the _tree invariant_:
The private key for a node in the tree is known to a member of
the group only if the node's subtree contains that member's leaf.
In other words, if a node is not blank, then it holds a public key.
The corresponding private key is known only to members occupying
leaves below that node.
The reverse implication is not true: A member may not know the
private keys of all the intermediate nodes they're below. Such a
member has an _unmerged_ leaf. Encrypting to an intermediate node
requires encrypting to the node's public key, as well as the public
keys of all the unmerged leaves below it. A leaf is unmerged when it
is first added, because the process of adding the leaf does not give
it access to all of the nodes above it in the tree. Leaves are
"merged" as they receive the private keys for nodes, as described in
Section 8.4.
For example, consider a four-member group (A, B, C, D) where the node
above the right two members is blank. (This is what it would look
like if A created a group with B, C, and D.) Then the public state
of the tree and the views of the private keys of the tree held by
each participant would be as follows, where _ represents a blank
node, ? represents an unknown private key, and pk(X) represents the
public key corresponding to the private key X:
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Public Tree
============================
pk(ABCD)
/ \
pk(AB) _
/ \ / \
pk(A) pk(B) pk(C) pk(D)
Private @ A Private @ B Private @ C Private @ D
============= ============= ============= =============
ABCD ABCD ABCD ABCD
/ \ / \ / \ / \
AB _ AB _ ? _ ? _
/ \ / \ / \ / \ / \ / \ / \ / \
A ? ? ? ? B ? ? ? ? C ? ? ? ? D
Note how the tree invariant applies: Each member knows only their own
leaf, and the private key AB is known only to A and B.
5.3. Ratchet Tree Nodes
A particular instance of a ratchet tree includes the same parameters
that define an instance of HPKE, namely:
* A Key Encapsulation Mechanism (KEM), including a DeriveKeyPair
function that creates a key pair for the KEM from a symmetric
secret
* A Key Derivation Function (KDF), including Extract and Expand
functions
* An AEAD encryption scheme
Each non-blank node in a ratchet tree contains up to five values:
* A public key
* A private key (only within the member's direct path, see below)
* A credential (only for leaf nodes)
* An ordered list of "unmerged" leaves (see Section 5.2)
* A hash of certain information about the node's parent, as of the
last time the node was changed (see Section 8.8).
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The _resolution_ of a node is an ordered list of non-blank nodes that
collectively cover all non-blank descendants of the node. The
resolution of a non-blank node with no unmerged leaves is just the
node itself. More generally, the resolution of a node is effectively
a depth-first, left-first enumeration of the nearest non-blank nodes
below the node:
* The resolution of a non-blank node comprises the node itself,
followed by its list of unmerged leaves, if any
* The resolution of a blank leaf node is the empty list
* The resolution of a blank intermediate node is the result of
concatenating the resolution of its left child with the resolution
of its right child, in that order
For example, consider the following subtree, where the _ character
represents a blank node and unmerged leaves are indicated in square
brackets:
...
/
_
|
.-+-.
/ \
_ Z[C]
/ \ / \
A _ C D
0 1 2 3
Figure 10: A tree with blanks and unmerged leaves
In this tree, we can see all of the above rules in play:
* The resolution of node Z is the list [Z, C]
* The resolution of leaf 1 is the empty list []
* The resolution of top node is the list [A, Z, C]
Every node, regardless of whether the node is blank or populated, has
a corresponding _hash_ that summarizes the contents of the subtree
below that node. The rules for computing these hashes are described
in Section 8.7.
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6. Cryptographic Objects
6.1. Ciphersuites
Each MLS session uses a single ciphersuite that specifies the
following primitives to be used in group key computations:
* HPKE parameters:
- A Key Encapsulation Mechanism (KEM)
- A Key Derivation Function (KDF)
- An AEAD encryption algorithm
* A hash algorithm
* A MAC algorithm
* A signature algorithm
MLS uses HPKE for public-key encryption [RFC9180]. The DeriveKeyPair
function associated to the KEM for the ciphersuite maps octet strings
to HPKE key pairs. As in HPKE, MLS assumes that an AEAD algorithm
produces a single ciphertext output from AEAD encryption (aligning
with [RFC5116]), as opposed to a separate ciphertext and tag.
Ciphersuites are represented with the CipherSuite type. HPKE public
keys are opaque values in a format defined by the underlying protocol
(see the Cryptographic Dependencies section of the HPKE specification
for more information).
opaque HPKEPublicKey<V>;
The signature algorithm specified in the ciphersuite is the mandatory
algorithm to be used for signatures in MLSMessageAuth and the tree
signatures. It MUST be the same as the signature algorithm specified
in the credentials in the leaves of the tree (including the leaf node
information in KeyPackages used to add new members).
Like HPKE public keys, signature public keys are represented as
opaque values in a format defined by the cipher suite's signature
scheme.
opaque SignaturePublicKey<V>;
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For ciphersuites using Ed25519 or Ed448 signature schemes, the public
key is in the format specified in [RFC8032]. For ciphersuites using
ECDSA with the NIST curves (P-256, P-384, or P-521), the public key
is the output of the uncompressed Elliptic-Curve-Point-to-Octet-
String conversion according to [SECG].
To disambiguate different signatures used in MLS, each signed value
is prefixed by a label as shown below:
SignWithLabel(SignatureKey, Label, Content) =
Signature.Sign(SignatureKey, SignContent)
VerifyWithLabel(VerificationKey, Label, Content) =
Signature.Verify(VerificationKey, SignContent)
Where SignContent is specified as:
struct {
opaque label<V> = "MLS 1.0 " + Label;
opaque content<V> = Content;
} SignContent;
Here, the functions Signature.Sign and Signature.Verify are defined
by the signature algorithm.
The ciphersuites are defined in section Section 18.1.
6.2. Hash-Based Identifiers
Some MLS messages refer to other MLS objects by hash. For example,
Welcome messages refer to KeyPackages for the members being welcomed,
and Commits refer to Proposals they cover. These identifiers are
computed as follows:
opaque HashReference[16];
HashReference KeyPackageRef;
HashReference LeafNodeRef;
HashReference ProposalRef;
MakeKeyPackageRef(value) = KDF.expand(
KDF.extract("", value), "MLS 1.0 KeyPackage Reference", 16)
MakeLeafNodeRef(value) = KDF.expand(
KDF.extract("", value), "MLS 1.0 Leaf Node Reference", 16)
MakeProposalRef(value) = KDF.expand(
KDF.extract("", value), "MLS 1.0 Proposal Reference", 16)
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For a KeyPackageRef, the value input is the encoded KeyPackage, and
the ciphersuite specified in the KeyPackage determines the KDF used.
For a LeafNodeRef, the value input is the LeafNode object for the
leaf node in question. For a ProposalRef, the value input is the
MLSMessageContentAuth carrying the proposal. In the latter two
cases, the KDF is determined by the group's ciphersuite.
6.3. Credentials
Each member of a group presents a credential that associates an
identity with the member's key material. This information is
verified according to the Authentication Service in use for a group.
A Credential can provide multiple identifiers for the client. It is
up to the application to decide which identifier or identifiers to
use at the application level. For example, a certificate in an
X509Credential may attest to several domain names or email addresses
in its subjectAltName extension. An application may decide to
present all of these to a user, or if it knows a "desired" domain
name or email address, it can check that the desired identifier is
among those attested. Using the terminology from [RFC6125], a
Credential provides "presented identifiers", and it is up to the
application to supply a "reference identifier" for the authenticated
client, if any.
Credentials MAY also include information that allows a relying party
to verify the identity / signing key binding, such as a signature
from a trusted authority.
// See IANA registry for registered values
uint16 CredentialType;
struct {
opaque cert_data<V>;
} Certificate;
struct {
CredentialType credential_type;
select (credential_type) {
case basic:
opaque identity<V>;
case x509:
Certificate chain<V>;
};
} Credential;
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A BasicCredential is a bare assertion of an identity, without any
additional information. The format of the encoded identity is
defined by the application.
For an X.509 credential, each entry in the chain represents a single
DER-encoded X.509 certificate. The chain is ordered such that the
first entry (chain[0]) is the end-entity certificate and each
subsequent certificate in the chain MUST be the issuer of the
previous certificate. The public key encoded in the
subjectPublicKeyInfo of the end-entity certificate MUST be identical
to the signature_key in the LeafNode containing this credential.
The signatures used in this document are encoded as specified in
[RFC8446]. In particular, ECDSA signatures are DER-encoded and EdDSA
signatures are defined as the concatenation of r and s as specified
in [RFC8032].
Each new credential that has not already been validated by the
application MUST be validated against the Authentication Service.
Applications SHOULD require that a client present the same set of
identifiers throughout its presence in the group, even if its
Credential is changed in a Commit or Update. If an application
allows clients to change identifiers over time, then each time the
client presents a new credential, the application MUST verify that
the set of identifiers in the credential is acceptable to the
application for this client.
6.3.1. Uniquely Identifying Clients
MLS implementations will presumably provide applications with a way
to request protocol operations with regard to other clients (e.g.,
removing clients). Such functions will need to refer to the other
clients using some identifier. MLS clients have a few types of
identifiers, with different operational properties.
The Credentials presented by the clients in a group authenticate
application-level identifiers for the clients. These identifiers may
not uniquely identify clients. For example, if a user has multiple
devices that are all present in an MLS group, then those devices'
clients could all present the user's application-layer identifiers.
Internally to the protocol, group members are uniquely identified by
their leaves, expressed as LeafNodeRef objects. These identifiers
are unstable: They change whenever the member sends a Commit, or
whenever an Update proposal from the member is committed.
MLS provides two unique client identifiers that are stable across
epochs:
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* The index of a client among the leaves of the tree
* The epoch_id field in the key package
The application may also provide application-specific unique
identifiers in the extensions field of KeyPackage or LeafNode
objects.
7. Message Framing
Handshake and application messages use a common framing structure.
This framing provides encryption to ensure confidentiality within the
group, as well as signing to authenticate the sender within the
group.
The main structure is MLSMessageContent, which contains the content
of the message. This structure is authenticated using MLSMessageAuth
(see Section 7.1). The two structures are combined in
MLSMessageContentAuth, which can then be encoded/decoded from/to
MLSPlaintext or MLSCiphertext, which are then included in the
MLSMessage structure.
MLSCiphertext represents a signed and encrypted message, with
protections for both the content of the message and related metadata.
MLSPlaintext represents a message that is only signed, and not
encrypted. Applications MUST use MLSCiphertext to encrypt
application messages and SHOULD use MLSCiphertext to encode handshake
messages, but MAY transmit handshake messages encoded as MLSPlaintext
objects in cases where it is necessary for the Delivery Service to
examine such messages.
enum {
reserved(0),
mls10(1),
(255)
} ProtocolVersion;
enum {
reserved(0),
application(1),
proposal(2),
commit(3),
(255)
} ContentType;
enum {
reserved(0),
member(1),
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external(2),
new_member(3),
(255)
} SenderType;
struct {
SenderType sender_type;
switch (sender_type) {
case member:
LeafNodeRef member_ref;
case external:
uint32 sender_index;
case new_member:
struct{};
}
} Sender;
enum {
reserved(0),
mls_plaintext(1),
mls_ciphertext(2),
mls_welcome(3),
mls_group_info(4),
mls_key_package(5),
(255)
} WireFormat;
struct {
opaque group_id<V>;
uint64 epoch;
Sender sender;
opaque authenticated_data<V>;
ContentType content_type;
select (MLSMessageContent.content_type) {
case application:
opaque application_data<V>;
case proposal:
Proposal proposal;
case commit:
Commit commit;
}
} MLSMessageContent;
struct {
ProtocolVersion version = mls10;
WireFormat wire_format;
select (MLSMessage.wire_format) {
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case mls_plaintext:
MLSPlaintext plaintext;
case mls_ciphertext:
MLSCiphertext ciphertext;
case mls_welcome:
Welcome welcome;
case mls_group_info:
GroupInfo group_info;
case mls_key_package:
KeyPackage key_package;
}
} MLSMessage;
External sender types are sent as MLSPlaintext, see Section 13.1.9
for their use.
The following structure is used to fully describe the data
transmitted in plaintexts or ciphertexts.
struct {
WireFormat wire_format;
MLSMessageContent content;
MLSMessageAuth auth;
} MLSMessageContentAuth;
7.1. Content Authentication
MLSMessageContent is authenticated using the MLSMessageAuth
structure.
struct {
opaque mac_value<V>;
} MAC;
struct {
// SignWithLabel(., "MLSMessageContentTBS", MLSMessageContentTBS)
opaque signature<V>;
select (MLSMessageContent.content_type) {
case commit:
// MAC(confirmation_key,
// GroupContext.confirmed_transcript_hash)
MAC confirmation_tag;
case application:
case proposal:
struct{};
}
} MLSMessageAuth;
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The signature is computed using SignWithLabel with label
"MLSMessageContentTBS" and with a content that covers the message
content and the wire format that will be used for this message. If
the sender's sender_type is Member, the content also covers the
GroupContext for the current epoch, so that signatures are specific
to a given group and epoch.
The sender MUST use the private key corresponding to the following
signature key depending on the sender's sender_type:
* member: The signature key contained in the Credential at the leaf
with the sender's LeafNodeRef
* external: The signature key contained in the Credential at the
index indicated by the sender_index in the external_senders group
context extension (see Section Section 13.1.9.1). In this case,
the content_type of the message MUST NOT be commit, since only
members of the group or new joiners can send Commit messages.
* new_member: The signature key depends on the content_type:
- proposal: The signature key in the credential contained in
KeyPackage in the Add proposal (see Section Section 13.1.9).
- commit: The signature key in the credential contained in the
KeyPackage in the Commit's path (see Section Section 9.3).
struct {
ProtocolVersion version = mls10;
WireFormat wire_format;
MLSMessageContent content;
select (MLSMessageContentTBS.content.sender.sender_type) {
case member:
case new_member:
GroupContext context;
case external:
struct{};
}
} MLSMessageContentTBS;
Recipients of an MLSMessage MUST verify the signature with the key
depending on the sender_type of the sender as described above.
The confirmation tag value confirms that the members of the group
have arrived at the same state of the group.
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A MLSMessageAuth is said to be valid when both the signature and
confirmation_tag fields are valid.
7.2. Encoding and Decoding a Plaintext
Plaintexts are encoded using the MLSPlaintext structure.
struct {
MLSMessageContent content;
MLSMessageAuth auth;
select(MLSPlaintext.content.sender.sender_type) {
case member:
MAC membership_tag;
case external:
case new_member:
struct{};
}
} MLSPlaintext;
The membership_tag field in the MLSPlaintext object authenticates the
sender's membership in the group. For messages sent by members, it
MUST be set to the following value:
struct {
MLSMessageContentTBS content_tbs;
MLSMessageAuth auth;
} MLSMessageContentTBM;
membership_tag = MAC(membership_key, MLSMessageContentTBM)
When decoding a MLSPlaintext into a MLSMessageContentAuth, the
application MUST check membership_tag, and MUST check that the
MLSMessageAuth is valid.
7.3. Encoding and Decoding a Ciphertext
Ciphertexts are encoded using the MLSCiphertext structure.
struct {
opaque group_id<V>;
uint64 epoch;
ContentType content_type;
opaque authenticated_data<V>;
opaque encrypted_sender_data<V>;
opaque ciphertext<V>;
} MLSCiphertext;
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encrypted_sender_data and ciphertext are encrypted using the AEAD
function specified by the ciphersuite in use, using as input the
structures MLSSenderData and MLSCiphertextContent.
7.3.1. Content Encryption
The ciphertext content is encoded using the MLSCiphertextContent
structure.
struct {
select (MLSCiphertext.content_type) {
case application:
opaque application_data<V>;
case proposal:
Proposal proposal;
case commit:
Commit commit;
}
MLSMessageAuth auth;
opaque padding<V>;
} MLSCiphertextContent;
In the MLS key schedule, the sender creates two distinct key ratchets
for handshake and application messages for each member of the group.
When encrypting a message, the sender looks at the ratchets it
derived for its own member and chooses an unused generation from
either the handshake or application ratchet depending on the content
type of the message. This generation of the ratchet is used to
derive a provisional nonce and key.
Before use in the encryption operation, the nonce is XORed with a
fresh random value to guard against reuse. Because the key schedule
generates nonces deterministically, a client must keep persistent
state as to where in the key schedule it is; if this persistent state
is lost or corrupted, a client might reuse a generation that has
already been used, causing reuse of a key/nonce pair.
To avoid this situation, the sender of a message MUST generate a
fresh random 4-byte "reuse guard" value and XOR it with the first
four bytes of the nonce from the key schedule before using the nonce
for encryption. The sender MUST include the reuse guard in the
reuse_guard field of the sender data object, so that the recipient of
the message can use it to compute the nonce to be used for
decryption.
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+-+-+-+-+---------...---+
| Key Schedule Nonce |
+-+-+-+-+---------...---+
XOR
+-+-+-+-+---------...---+
| Guard | 0 |
+-+-+-+-+---------...---+
===
+-+-+-+-+---------...---+
| Encrypt/Decrypt Nonce |
+-+-+-+-+---------...---+
The Additional Authenticated Data (AAD) input to the encryption
contains an object of the following form, with the values used to
identify the key and nonce:
struct {
opaque group_id<V>;
uint64 epoch;
ContentType content_type;
opaque authenticated_data<V>;
} MLSCiphertextContentAAD;
When decoding a MLSCiphertextContent, the application MUST check that
the MLSMessageAuth is valid.
7.3.2. Sender Data Encryption
The "sender data" used to look up the key for the content encryption
is encrypted with the ciphersuite's AEAD with a key and nonce derived
from both the sender_data_secret and a sample of the encrypted
content. Before being encrypted, the sender data is encoded as an
object of the following form:
struct {
LeafNodeRef sender;
uint32 generation;
opaque reuse_guard[4];
} MLSSenderData;
MLSSenderData.sender is assumed to be a member sender type. When
constructing an MLSSenderData from a Sender object, the sender MUST
verify Sender.sender_type is member and use Sender.sender for
MLSSenderData.sender.
The reuse_guard field contains a fresh random value used to avoid
nonce reuse in the case of state loss or corruption, as described in
Section 7.3.1.
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The key and nonce provided to the AEAD are computed as the KDF of the
first KDF.Nh bytes of the ciphertext generated in the previous
section. If the length of the ciphertext is less than KDF.Nh, the
whole ciphertext is used without padding. In pseudocode, the key and
nonce are derived as:
ciphertext_sample = ciphertext[0..KDF.Nh-1]
sender_data_key = ExpandWithLabel(sender_data_secret, "key",
ciphertext_sample, AEAD.Nk)
sender_data_nonce = ExpandWithLabel(sender_data_secret, "nonce",
ciphertext_sample, AEAD.Nn)
The Additional Authenticated Data (AAD) for the SenderData ciphertext
is the first three fields of MLSCiphertext:
struct {
opaque group_id<V>;
uint64 epoch;
ContentType content_type;
} MLSSenderDataAAD;
When parsing a SenderData struct as part of message decryption, the
recipient MUST verify that the LeafNodeRef indicated in the sender
field identifies a member of the group.
8. Ratchet Tree Operations
The ratchet tree for an epoch describes the membership of a group in
that epoch, providing public-key encryption (HPKE) keys that can be
used to encrypt to subsets of the group as well as information to
authenticate the members. In order to reflect changes to the
membership of the group from one epoch to the next, corresponding
changes are made to the ratchet tree. In this section, we describe
the content of the tree and the required operations.
8.1. Parent Node Contents
As discussed in Section 5.3, the nodes of a ratchet tree contain
several types of data describing individual members (for leaf nodes)
or subgroups of the group (for parent nodes). Parent nodes are
simpler:
struct {
HPKEPublicKey encryption_key;
opaque parent_hash<V>;
uint32 unmerged_leaves<V>;
} ParentNode;
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The encryption_key field contains an HPKE public key whose private
key is held only by the members at the leaves among its descendants.
The parent_hash field contains a hash of this node's parent node, as
described in Section 8.8. The unmerged_leaves field lists the leaves
under this parent node that are unmerged, according to their indices
among all the leaves in the tree.
8.2. Leaf Node Contents
A leaf node in the tree describes all the details of an individual
client's appearance in the group, signed by that client. It is also
used in client KeyPackage objects to store the information that will
be needed to add a client to a group.
enum {
reserved(0),
key_package(1),
update(2),
commit(3),
(255)
} LeafNodeSource;
struct {
ProtocolVersion versions<V>;
CipherSuite ciphersuites<V>;
ExtensionType extensions<V>;
ProposalType proposals<V>;
CredentialType credentials<V>;
} Capabilities;
struct {
uint64 not_before;
uint64 not_after;
} Lifetime;
// See IANA registry for registered values
uint16 ExtensionType;
struct {
ExtensionType extension_type;
opaque extension_data<V>;
} Extension;
struct {
HPKEPublicKey encryption_key;
SignaturePublicKey signature_key;
Credential credential;
Capabilities capabilities;
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LeafNodeSource leaf_node_source;
select (leaf_node_source) {
case key_package:
Lifetime lifetime;
case update:
struct {}
case commit:
opaque parent_hash<V>;
}
Extension extensions<V>;
// SignWithLabel(., "LeafNodeTBS", LeafNodeTBS)
opaque signature<V>;
} LeafNode;
struct {
HPKEPublicKey encryption_key;
SignaturePublicKey signature_key;
Credential credential;
Capabilities capabilities;
LeafNodeSource leaf_node_source;
select (leaf_node_source) {
case key_package:
Lifetime lifetime;
case update:
struct{};
case commit:
opaque parent_hash<V>;
}
Extension extensions<V>;
select (leaf_node_source) {
case key_package:
struct{};
case update:
opaque group_id<V>;
case commit:
opaque group_id<V>;
}
} LeafNodeTBS;
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The encryption_key field conains an HPKE public key whose private key
is held only by the member occupying this leaf (or in the case of a
LeafNode in a KeyPackage object, the issuer of the KeyPackage). The
credential contains authentication information for this member, as
described in Section 6.3.
The capabilities field indicates what protocol versions,
ciphersuites, extensions, credential types, and non-default proposal
types are supported by a client. Proposal and extension types
defined in this document are considered "default" and thus need not
be listed, while any credential types the application wishes to use
MUST be listed. Extensions that appear in the extensions field of a
LeafNode MUST be included in the extensions field of the capabilities
field, and the credential type used in the LeafNode MUST be included
in the credentials field of the capabilities field.
The leaf_node_source field indicates how this LeafNode came to be
added to the tree. This signal tells other members of the group
whether the leaf node is required to have a lifetime or parent_hash,
and whether the group_id is added as context to the signature.
Whether these fields can be computed by the client represented by the
LeafNode depends on when the LeafNode was created. For example, a
KeyPackage is created before the client knows which group it will be
used with, so its signature can't bind to a group_id.
In the case where the leaf was added to the tree based on a pre-
published KeyPackage, the lifetime field represents the times between
which clients will consider a LeafNode valid. These times are
represented as absolute times, measured in seconds since the Unix
epoch (1970-01-01T00:00:00Z). Applications MUST define a maximum
total lifetime that is acceptable for a LeafNode, and reject any
LeafNode where the total lifetime is longer than this duration.
In the case where the leaf node was inserted into the tree via a
Commit message, the parent_hash field contains the parent hash for
this leaf node (see Section 8.8).
The LeafNodeTBS structure covers the fields above the signature in
the LeafNode. In addition, when the leaf node was created in the
context of a group (the update and commit cases), the group ID of the
group is added as context to the signature.
LeafNode objects stored in the group's ratchet tree are updated
according to the evolution of the tree. Each modification of
LeafNode content MUST be reflected by a change in its signature.
This allows other members to verify the validity of the LeafNode at
any time, particularly in the case of a newcomer joining the group.
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8.3. Leaf Node Validation
The validity of a LeafNode needs to be verified at a few stages:
* When a LeafNode is downloaded in a KeyPackage, before it is used
to add the client to the group
* When a LeafNode is received by a group member in an Add, Update,
or Commit message
* When a client joining a group receives LeafNode objects for the
other members of the group in the group's ratchet tree
The client verifies the validity of a LeafNode using the following
steps:
* Verify that the credential in the LeafNode is valid according to
the authentication service and the client's local policy. These
actions MUST be the same regardless of at what point in the
protocol the LeafNode is being verified with the following
exception: If the LeafNode is an update to another LeafNode, the
authentication service MUST additionally validate that the set of
identities attested by the credential in the new LeafNode is
acceptable relative to the identities attested by the old
credential.
* Verify that the signature on the LeafNode is valid using the
public key in the LeafNode's credential
* Verify that the LeafNode is compatible with the group's
parameters. If the GroupContext has a required_capabilities
extension, then the required extensions, proposals, and credential
types MUST be listed in the LeafNode's capabilities field.
* Verify that the credential type is supported by all members of the
group, as specified by the capabilities field of each member's
LeafNode, and that the capabilities field of this LeafNode
indicates support for all the credential types currently in use by
other members.
* Verify the lifetime field:
- When validating a LeafNode in a KeyPackage before sending an
Add proposal, the current time MUST be within the lifetime
range. A KeyPackage containing a LeafNode that is expired or
not yet valid MUST NOT be sent in an Add proposal.
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- When receiving an Add or validating a tree, checking the
lifetime is RECOMMENDED, if it is feasible in a given
application context. Because of the asynchronous nature of
MLS, the lifetime may have been valid when the leaf node was
proposed for addition, even if it is expired at these later
points in the protocol.
* Verify that the leaf_node_source field has the appropriate value
for the context in which the LeafNode is being validated (as
defined in Section 8.2).
* Verify that the following fields in the LeafNode are unique among
the members of the group (including any other members added in the
same Commit):
- encryption_key
- signature_key
* Verify that the extensions in the leaf node are supported. The ID
for each extension in the extensions field MUST be listed in the
field capabilities.extensions of the LeafNode.
8.4. Ratchet Tree Evolution
In order to provide forward secrecy and post-compromise security,
whenever a member initiates an epoch change (i.e., commits; see
Section 13.2), they refresh the key pairs of their leaf and of all
nodes on their leaf's direct path (all nodes for which they know the
secret key).
The member initiating the epoch change generates the fresh key pairs
using the following procedure. The procedure is designed in a way
that allows group members to efficiently communicate the fresh secret
keys to other group members, as described in Section 8.9.
Recall the definition of resolution from Section 5.3. To begin with,
the generator of the UpdatePath updates its leaf and its leaf's
_filtered direct path_ with new key pairs. The filtered direct path
of a node is obtained from the node's direct path by removing all
nodes whose child on the nodes's copath has an empty resolution (any
unmerged leaves of the copath child count towards its resolution).
Such a removed node does not need a key pair, since after blanking
it, its resolution consists of a single node on the filtered direct
path. Using the key pair of the node in the resolution is equivalent
to using the key pair of the removed node.
* Blank all the nodes on the direct path from the leaf to the root.
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* Generate a fresh HPKE key pair for the leaf.
* Generate a sequence of path secrets, one for each node on the
leaf's filtered direct path, as follows. In this setting,
path_secret[0] refers to the first parent node in the filtered
direct path, path_secret[1] to the second parent node, and so on.
path_secret[0] is sampled at random
path_secret[n] = DeriveSecret(path_secret[n-1], "path")
* Compute the sequence of HPKE key pairs (node_priv,node_pub), one
for each node on the leaf's direct path, as follows.
node_secret[n] = DeriveSecret(path_secret[n], "node")
node_priv[n], node_pub[n] = KEM.DeriveKeyPair(node_secret[n])
The node secret is derived as a temporary intermediate secret so that
each secret is only used with one algorithm: The path secret is used
as an input to DeriveSecret and the node secret is used as an input
to DeriveKeyPair.
For example, suppose there is a group with four members, with C an
unmerged leaf at Z:
Y
|
.-+-.
/ \
X Z[C]
/ \ / \
A B C D
0 1 2 3
Figure 11: A full tree with one unmerged leaf
If member B subsequently generates an UpdatePath based on a secret
"leaf_secret", then it would generate the following sequence of path
secrets:
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path_secret[1] ---> node_secret[1] -------> node_priv[1], node_pub[1]
^
|
|
path_secret[0] ---> node_secret[0] -------> node_priv[0], node_pub[0]
^
|
|
leaf_secret ------> leaf_node_secret --+--> leaf_priv, leaf_pub
| |
'-------. .-------'
|
leaf_node
After applying the UpdatePath, the tree will have the following
structure, where lp and np[i] represent the leaf_priv and node_priv
values generated as described above:
node_priv[1] --------> Y'
|
.-+-.
/ \
node_priv[0] ----> X' Z[C]
/ \ / \
A B C D
^
leaf_priv -----------+
0 1 2 3
8.5. Adding and Removing Leaves
In addition to the path-based updates to the tree described above, it
is also necessary to add and remove leaves of the tree in order to
reflect changes to the membership of the group (see Section 13.1.1
and Section 13.1.3). Leaves are always added and removed at the
right edge of the tree: Either a new rightmost leaf is added, or the
rightmost leaf is removed. Nodes' parent/child node relationships
are then updated to maintain the tree's left-balanced structure.
These operations are also known as _extending_ and _truncating_ the
tree.
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To add a new leaf: Add leaf L as the new rightmost leaf of the tree.
Add a blank parent node P whose right child is L. P is attached to
the tree as the right child of the only appropriate node to make the
updated tree left-balanced (or set it as a new root). The former
right child of P's parent becomes P's left child (or the old root
becomes P's left child if P is the new root).
_ <-- new parent _
__|_ __|__
/ \ / \
X ===> X | ===> X _ <-- new parent
/ \ / \ | / \ / \
A B A B C <-- new leaf A B C D <-- new leaf
To remove the rightmost leaf: Remove the rightmost leaf node L and
its parent node P. If P was the root of the tree, P's left child is
now the root of the tree. Otherwise, set the right child of P's
parent to be P's left child.
Y Y
__|__ __|_
/ \ / \
X Z <-- remove parent ===> X | <-- reassign child
/ \ / \ / \ |
A B C D <-- remove leaf A B C
Y <-- remove parent
__|_
/ \
X | ===> X <-- reassign root
/ \ | / \
A B C <-- remove leaf A B
Note that in the rest of the protocol, the rightmost leaf will only
be removed when it is blank.
Concrete algorithms for these operations on array-based and link-
based trees are provided in Appendix B and Appendix C. The concrete
algorithms are non-normative. An implementation MAY use any
algorithm that produces the correct tree in its internal
representation.
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8.6. Synchronizing Views of the Tree
After generating fresh key material and applying it to ratchet
forward their local tree state as described in the Section 8.4, the
generator must broadcast this update to other members of the group in
a Commit message, who apply it to keep their local views of the tree
in sync with the sender's. More specifically, when a member commits
a change to the tree (e.g., to add or remove a member), it transmits
an UpdatePath containing a set of public keys and encrypted path
secrets for intermediate nodes in the filtered direct path of its
leaf. The other members of the group use these values to update
their view of the tree, aligning their copy of the tree to the
sender's.
An UpdatePath contains the following information for each node in the
filtered direct path of the sender's leaf, including the root:
* The public key for the node
* Zero or more encrypted copies of the path secret corresponding to
the node
The path secret value for a given node is encrypted for the subtree
corresponding to the parent's non-updated child, that is, the child
on the copath of the sender's leaf node. There is one encryption of
the path secret to each public key in the resolution of the non-
updated child.
The recipient of an UpdatePath processes it with the following steps:
1. Compute the updated path secrets.
* Identify a node in the filtered direct path for which the
recipient is in the subtree of the non-updated child.
* Identify a node in the resolution of the copath node for which
the recipient has a private key.
* Decrypt the path secret for the parent of the copath node
using the private key from the resolution node.
* Derive path secrets for ancestors of that node using the
algorithm described above.
* The recipient SHOULD verify that the received public keys
agree with the public keys derived from the new path_secret
values.
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2. Merge the updated path secrets into the tree.
* Blank all nodes on the direct path of the sender's leaf.
* For all nodes on the filtered direct path of the sender's
leaf,
- Set the public key to the received public key.
- Set the list of unmerged leaves to the empty list.
- Store the updated hash of the next node on the filtered
direct path (represented as a ParentNode struct), going
from root to leaf, so that each hash incorporates all the
non-blank nodes above it. The root node always has a zero-
length hash for this value.
* For nodes where a path secret was recovered in step 1
("Compute the updated path secrets"), compute and store the
node's updated private key.
For example, in order to communicate the example update described in
the previous section, the sender would transmit the following values:
+=============+====================================================+
| Public Key | Ciphertext(s) |
+=============+====================================================+
| node_pub[1] | E(pk(Z), path_secret[1]), E(pk(C), path_secret[1]) |
+-------------+----------------------------------------------------+
| node_pub[0] | E(pk(A), path_secret[0]) |
+-------------+----------------------------------------------------+
Table 2
In this table, the value node_pub[i] represents the public key
derived from node_secret[i], pk(X) represents the current public key
of node X, and E(K, S) represents the public-key encryption of the
path secret S to the public key K (using HPKE).
After processing the update, each recipient MUST delete outdated key
material, specifically:
* The path secrets used to derive each updated node key pair.
* Each outdated node key pair that was replaced by the update.
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8.7. Tree Hashes
To allow group members to verify that they agree on the public
cryptographic state of the group, this section defines a scheme for
generating a hash value (called the "tree hash") that represents the
contents of the group's ratchet tree and the members' leaf nodes.
The tree hash of a tree is the tree hash of its root node, which we
define recursively, starting with the leaves.
The tree hash of a leaf node is the hash of leaf's LeafNodeHashInput
object which might include a LeafNode object depending on whether or
not it is blank.
struct {
uint32 leaf_index;
optional<LeafNode> leaf_node;
} LeafNodeHashInput;
Now the tree hash of any non-leaf node is recursively defined to be
the hash of its ParentNodeHashInput. This includes an optional
ParentNode object depending on whether the node is blank or not.
struct {
optional<ParentNode> parent_node;
opaque left_hash<V>;
opaque right_hash<V>;
} ParentNodeHashInput;
The left_hash and right_hash fields hold the tree hashes of the
node's left and right children, respectively.
8.8. Parent Hash
The parent_hash field in ratchet tree nodes carries information to
authenticate the information in the ratchet tree. Parent hashes
chain together so that the signature on a leaf node, by covering the
leaf node's parent hash, indirectly includes information about the
structure of the tree at the time the leaf node was last updated.
Consider a ratchet tree with a non-blank parent node P and children V
and S.
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...
/
P
__|__
/ \
V S
/ \ / \
... ... ... ...
The parent hash of P changes whenever an UpdatePath object is applied
to the ratchet tree along a path from a leaf U traversing node V (and
hence also P). The new "Parent hash of P (with copath child S)" is
obtained by hashing P's ParentHashInput struct.
struct {
HPKEPublicKey encryption_key;
opaque parent_hash<V>;
opaque original_sibling_tree_hash<V>;
} ParentHashInput;
The field encryption_key contains the HPKE public key of P. If P is
the root, then the parent_hash field is set to a zero-length octet
string. Otherwise, parent_hash is the Parent Hash of the next node
after P on the filtered direct path of U. This way, P's Parent Hash
fixes the new HPKE public key of each node V on the path from P to
the root. Note that the path from P to the root may contain some
blank nodes that are not fixed by P's Parent Hash. However, for each
node that has an HPKE key, this key is fixed by P's Parent Hash.
Finally, original_sibling_tree_hash is the tree hash of S in the
ratchet tree modified as follows:
* Extend the subtree of S by adding blank leaves until it is full,
i.e., until its number of leaves is a power of 2 (see
Section 8.5).
* For each leaf L in P.unmerged_leaves, blank L and remove it from
the unmerged_leaves sets of all parent nodes.
Observe that original_sibling_tree_hash does not change between
updates of P. This property is crucial for the correctness of the
protocol.
For example, in the following tree:
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W [F]
______|_____
/ \
U Y [F]
__|__ __|__
/ \ / \
T _ _ |
/ \ / \ / \ |
A B C D E F G
Figure 12: A tree illustrating parent hash computations.
With P = W and S = Y, original_sibling_tree_hash is the tree hash of
the following tree:
Y
__|__
/ \
_ _
/ \ / \
E _ G _
Because W.unmerged_leaves includes F, F is blanked and removed from
Y.unmerged_leaves.
Note that no recomputation is needed if the tree hash of S is
unchanged since the last time P was updated. This is the case for
computing or processing a Commit whose UpdatePath traverses P, since
the Commit itself resets P. (In other words, it is only necessary to
recompute the original sibling tree hash when validating group's tree
on joining.) More generally, if none of the entries in
P.unmerged_leaves is in the subtree under S (and thus no nodes were
truncated), then the original tree hash at S is the tree hash of S in
the current tree.
If it is necessary to recompute the original tree hash of a node, the
efficiency of recomputation can be improved by caching intermediate
tree hashes, to avoid recomputing over the subtree when the subtree
is included in multiple parent hashes. A subtree hash can be reused
as long as the intersection of the parent's unmerged leaves with the
subtree is the same as in the earlier computation.
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8.8.1. Using Parent Hashes
The Parent Hash of P appears in three types of structs. If V is
itself a parent node then P's Parent Hash is stored in the
parent_hash field of the structs ParentHashInput and ParentNode of
the node before P on the filtered direct path of U. (The ParentNode
struct is used to encapsulate all public information about that node
that must be conveyed to a new member joining the group as well as to
define its Tree Hash.)
If, on the other hand, V is the leaf U and its LeafNode has
leaf_node_source set to commit, then the Parent Hash of P (with V's
sibling as copath child) is stored in the parent_hash field. This is
true in particular of the LeafNode object sent in the leaf_node field
of an UpdatePath. The signature of such a LeafNode thus also attests
to which keys the group member introduced into the ratchet tree and
to whom the corresponding secret keys were sent. This helps prevent
malicious insiders from constructing artificial ratchet trees with a
node V whose HPKE secret key is known to the insider yet where the
insider isn't assigned a leaf in the subtree rooted at V. Indeed,
such a ratchet tree would violate the tree invariant.
8.8.2. Verifying Parent Hashes
Parent hashes are verified at two points in the protocol: When
joining a group and when processing a Commit.
The parent hash in a node U is valid with respect to a parent node P
if the following criteria hold:
* U is a descendant of P in the tree
* The nodes between U and P in the tree are all blank
* The parent_hash field of U is equal to the parent hash of P with
copath child S, where S is the child of P that is not on the path
from U to P.
A parent node P is "parent-hash valid" if it can be chained back to a
leaf node in this way. That is, if there is leaf node L and a
sequence of parent nodes P_1, ..., P_N such that P_N = P and each
step in the chain is authenticated by a parent hash: L's parent hash
is valid with respect to P_1, P_1's parent hash is valid with respect
to P_2, and so on.
When joining a group, the new member MUST authenticate that each non-
blank parent node P is parent-hash valid. This can be done "bottom
up" by building chains up from leaves and verifying that all non-
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blank parent nodes are covered by exactly one such chain, or "top
down" by verifying that there is exactly one descendant of each non-
blank parent node for which the parent node is parent-hash valid.
When processing a Commit message that includes an UpdatePath, clients
MUST recompute the expected value of parent_hash for the committer's
new leaf and verify that it matches the parent_hash value in the
supplied leaf_node. After being merged into the tree, the nodes in
the UpdatePath form a parent-hash chain from the committer's leaf to
the root.
8.9. Update Paths
As described in Section 13.2, each MLS Commit message may optionally
transmit a LeafNode and parent node values along its direct path.
The path contains a public key and encrypted secret value for all
intermediate nodes in the filtered direct path from the leaf to the
root. The path is ordered from the closest node to the leaf to the
root; each node MUST be the parent of its predecessor.
struct {
opaque kem_output<V>;
opaque ciphertext<V>;
} HPKECiphertext;
struct {
HPKEPublicKey encryption_key;
HPKECiphertext encrypted_path_secret<V>;
} UpdatePathNode;
struct {
LeafNode leaf_node;
UpdatePathNode nodes<V>;
} UpdatePath;
For each UpdatePathNode, the resolution of the corresponding copath
node MUST be filtered by removing all new leaf nodes added as part of
this MLS Commit message. The number of ciphertexts in the
encrypted_path_secret vector MUST be equal to the length of the
filtered resolution, with each ciphertext being the encryption to the
respective resolution node.
The HPKECiphertext values are computed as
kem_output, context = SetupBaseS(node_public_key, group_context)
ciphertext = context.Seal("", path_secret)
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where node_public_key is the public key of the node that the path
secret is being encrypted for, group_context is the current
GroupContext object for the group, and the functions SetupBaseS and
Seal are defined according to [RFC9180].
Decryption is performed in the corresponding way, using the private
key of the resolution node.
9. Key Schedule
Group keys are derived using the Extract and Expand functions from
the KDF for the group's ciphersuite, as well as the functions defined
below:
ExpandWithLabel(Secret, Label, Context, Length) =
KDF.Expand(Secret, KDFLabel, Length)
DeriveSecret(Secret, Label) =
ExpandWithLabel(Secret, Label, "", KDF.Nh)
Where KDFLabel is specified as:
struct {
uint16 length = Length;
opaque label<V> = "MLS 1.0 " + Label;
opaque context<V> = Context;
} KDFLabel;
The value KDF.Nh is the size of an output from KDF.Extract, in bytes.
In the below diagram:
* KDF.Extract takes its salt argument from the top and its Input Key
Material (IKM) argument from the left
* DeriveSecret takes its Secret argument from the incoming arrow
* 0 represents an all-zero byte string of length KDF.Nh.
When processing a handshake message, a client combines the following
information to derive new epoch secrets:
* The init secret from the previous epoch
* The commit secret for the current epoch
* The GroupContext object for current epoch
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Given these inputs, the derivation of secrets for an epoch proceeds
as shown in the following diagram:
init_secret_[n-1]
|
|
V
commit_secret --> KDF.Extract
|
|
V
ExpandWithLabel(., "joiner", GroupContext_[n], KDF.Nh)
|
|
V
joiner_secret
|
|
V
psk_secret (or 0) --> KDF.Extract
|
|
+--> DeriveSecret(., "welcome")
| = welcome_secret
|
V
ExpandWithLabel(., "epoch", GroupContext_[n], KDF.Nh)
|
|
V
epoch_secret
|
|
+--> DeriveSecret(., <label>)
| = <secret>
|
V
DeriveSecret(., "init")
|
|
V
init_secret_[n]
A number of values are derived from the epoch secret for different
purposes:
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+==================+=====================+=======================+
| Label | Secret | Purpose |
+==================+=====================+=======================+
| "sender data" | sender_data_secret | Deriving keys to |
| | | encrypt sender data |
+------------------+---------------------+-----------------------+
| "encryption" | encryption_secret | Deriving message |
| | | encryption keys (via |
| | | the secret tree) |
+------------------+---------------------+-----------------------+
| "exporter" | exporter_secret | Deriving exported |
| | | secrets |
+------------------+---------------------+-----------------------+
| "external" | external_secret | Deriving the external |
| | | init key |
+------------------+---------------------+-----------------------+
| "confirm" | confirmation_key | Computing the |
| | | confirmation MAC for |
| | | an epoch |
+------------------+---------------------+-----------------------+
| "membership" | membership_key | Computing the |
| | | membership MAC for an |
| | | MLSPlaintext |
+------------------+---------------------+-----------------------+
| "resumption" | resumption_psk | Proving membership in |
| | | this epoch (via a PSK |
| | | injected later) |
+------------------+---------------------+-----------------------+
| "authentication" | epoch_authenticator | Confirming that two |
| | | clients have the same |
| | | view of the group |
+------------------+---------------------+-----------------------+
Table 3: Epoch-derived secrets
The external_secret is used to derive an HPKE key pair whose private
key is held by the entire group:
external_priv, external_pub = KEM.DeriveKeyPair(external_secret)
The public key external_pub can be published as part of the GroupInfo
struct in order to allow non-members to join the group using an
external commit.
9.1. Group Context
Each member of the group maintains a GroupContext object that
summarizes the state of the group:
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struct {
opaque group_id<V>;
uint64 epoch;
opaque tree_hash<V>;
opaque confirmed_transcript_hash<V>;
Extension extensions<V>;
} GroupContext;
The fields in this state have the following semantics:
* The group_id field is an application-defined identifier for the
group.
* The epoch field represents the current version of the group.
* The tree_hash field contains a commitment to the contents of the
group's ratchet tree and the credentials for the members of the
group, as described in Section 8.7.
* The confirmed_transcript_hash field contains a running hash over
the messages that led to this state.
* The extensions field contains the details of any protocol
extensions that apply to the group.
When a new member is added to the group, an existing member of the
group provides the new member with a Welcome message. The Welcome
message provides the information the new member needs to initialize
its GroupContext.
Different changes to the group will have different effects on the
group state. These effects are described in their respective
subsections of Section 13.1. The following general rules apply:
* The group_id field is constant.
* The epoch field increments by one for each Commit message that is
processed.
* The tree_hash is updated to represent the current tree and
credentials.
* The confirmed_transcript_hash field is updated with the data for
an MLSPlaintext message encoding a Commit message as described
below.
* The extensions field changes when a GroupContextExtensions
proposal is committed.
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9.2. Transcript Hashes
The transcript hashes computed in MLS represent a running hash over
all Proposal and Commit messages that have ever been sent in a group.
Commit messages are included directly. Proposal messages are
indirectly included via the Commit that applied them. Both types of
message are included by hashing the MLSPlaintext in which they were
sent.
The confirmed_transcript_hash is updated with an MLSMessageContent
and MLSMessageAuth containing a Commit in two steps:
struct {
WireFormat wire_format;
MLSMessageContent content; //with content.content_type == commit
opaque signature<V>;
} MLSMessageCommitContent;
struct {
MAC confirmation_tag;
} MLSMessageCommitAuthData;
interim_transcript_hash_[0] = ""; // zero-length octet string
confirmed_transcript_hash_[n] =
Hash(interim_transcript_hash_[n] ||
MLSMessageCommitContent_[n]);
interim_transcript_hash_[n+1] =
Hash(confirmed_transcript_hash_[n] ||
MLSMessageCommitAuthData_[n]);
Thus the confirmed_transcript_hash field in a GroupContext object
represents a transcript over the whole history of MLSMessage Commit
messages, up to the confirmation_tag field of the most recent Commit.
The confirmation tag is then included in the transcript for the next
epoch. The interim transcript hash is computed by new members using
the confirmation_tag of the GroupInfo struct, while existing members
can compute it directly.
As shown above, when a new group is created, the
interim_transcript_hash field is set to the zero-length octet string.
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9.3. External Initialization
In addition to initializing a new epoch via KDF invocations as
described above, an MLS group can also initialize a new epoch via an
asymmetric interaction using the external key pair for the previous
epoch. This is done when an new member is joining via an external
commit.
In this process, the joiner sends a new init_secret value to the
group using the HPKE export method. The joiner then uses that
init_secret with information provided in the GroupInfo and an
external Commit to initialize their copy of the key schedule for the
new epoch.
kem_output, context = SetupBaseS(external_pub, "")
init_secret = context.export("MLS 1.0 external init secret", KDF.Nh)
Members of the group receive the kem_output in an ExternalInit
proposal and preform the corresponding calculation to retrieve the
init_secret value.
context = SetupBaseR(kem_output, external_priv, "")
init_secret = context.export("MLS 1.0 external init secret", KDF.Nh)
In both cases, the info input to HPKE is set to the GroupInfo for the
previous epoch, encoded using the TLS serialization.
9.4. Pre-Shared Keys
Groups which already have an out-of-band mechanism to generate shared
group secrets can inject those into the MLS key schedule to seed the
MLS group secrets computations by this external entropy.
Injecting an external PSK can improve security in the case where
having a full run of updates across members is too expensive, or if
the external group key establishment mechanism provides stronger
security against classical or quantum adversaries.
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Note that, as a PSK may have a different lifetime than an update, it
does not necessarily provide the same Forward Secrecy (FS) or Post-
Compromise Security (PCS) guarantees as a Commit message. Unlike the
key pairs populated in the tree by an Update or Commit, which are
always freshly generated, PSKs may be pre-distributed and stored.
This creates the risk that a PSK may be compromised in the process of
distribution and storage. The security that the group gets from
injecting a PSK thus depends on both the entropy of the PSK and the
risk of compromise. These factors are outside of the scope of this
document, but should be considered by application designers relying
on PSKs.
Each PSK in MLS has a type that designates how it was provisioned.
External PSKs are provided by the application, while resumption PSKs
are derived from the MLS key schedule and used in cases where it is
necessary to authenticate a member's participation in a prior epoch.
The injection of one or more PSKs into the key schedule is signaled
in two ways: Existing members are informed via PreSharedKey proposals
covered by a Commit, and new members added in the Commit are informed
via GroupSecrets object in the Welcome message corresponding to the
Commit. To ensure that existing and new members compute the same PSK
input to the key schedule, the Commit and GroupSecrets objects MUST
indicate the same set of PSKs, in the same order.
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enum {
reserved(0),
external(1),
resumption(2),
(255)
} PSKType;
enum {
reserved(0),
application(1),
reinit(2),
branch(3),
} ResumptionPSKUsage;
struct {
PSKType psktype;
select (PreSharedKeyID.psktype) {
case external:
opaque psk_id<V>;
case resumption:
ResumptionPSKUsage usage;
opaque psk_group_id<V>;
uint64 psk_epoch;
}
opaque psk_nonce<V>;
} PreSharedKeyID;
struct {
PreSharedKeyID psks<V>;
} PreSharedKeys;
On receiving a Commit with a PreSharedKey proposal or a GroupSecrets
object with the psks field set, the receiving Client includes them in
the key schedule in the order listed in the Commit, or in the psks
field respectively. For resumption PSKs, the PSK is defined as the
resumption_psk of the group and epoch specified in the PreSharedKeyID
object. Specifically, psk_secret is computed as follows:
struct {
PreSharedKeyID id;
uint16 index;
uint16 count;
} PSKLabel;
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psk_extracted_[i] = KDF.Extract(0, psk_[i])
psk_input_[i] = ExpandWithLabel(psk_extracted_[i], "derived psk",
PSKLabel, KDF.Nh)
psk_secret_[0] = 0
psk_secret_[i] = KDF.Extract(psk_input_[i-1], psk_secret_[i-1])
psk_secret = psk_secret_[n]
Here 0 represents the all-zero vector of length KDF.Nh. The index
field in PSKLabel corresponds to the index of the PSK in the psk
array, while the count field contains the total number of PSKs. In
other words, the PSKs are chained together with KDF.Extract
invocations (labelled "Extract" for brevity in the diagram), as
follows:
0 0 = psk_secret_[0]
| |
V V
psk_[0] --> Extract --> ExpandWithLabel --> Extract = psk_secret_[1]
|
0 |
| |
V V
psk_[1] --> Extract --> ExpandWithLabel --> Extract = psk_secret_[2]
|
0 ...
| |
V V
psk_[n-1] --> Extract --> ExpandWithLabel --> Extract = psk_secret_[n]
In particular, if there are no PreSharedKey proposals in a given
Commit, then the resulting psk_secret is psk_secret_[0], the all-zero
vector.
9.5. Exporters
The main MLS key schedule provides an exporter_secret which can be
used by an application as the basis to derive new secrets called
exported_value outside the MLS layer.
MLS-Exporter(Label, Context, key_length) =
ExpandWithLabel(DeriveSecret(exporter_secret, Label),
"exporter", Hash(Context), key_length)
Each application SHOULD provide a unique label to MLS-Exporter that
identifies its use case. This is to prevent two exported outputs
from being generated with the same values and used for different
functionalities.
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The exported values are bound to the group epoch from which the
exporter_secret is derived, hence reflects a particular state of the
group.
It is RECOMMENDED for the application generating exported values to
refresh those values after a Commit is processed.
9.6. Resumption PSK
The main MLS key schedule provides a resumption_psk that is used as a
PSK to inject entropy from one epoch into another. This
functionality is used in the reinitialization and branching processes
described in Section 12.2 and Section 12.3, but may be used by
applications for other purposes.
Some uses of resumption PSKs might call for the use of PSKs from
historical epochs. The application SHOULD specify an upper limit on
the number of past epochs for which the resumption_psk may be stored.
9.7. Epoch Authenticators
The main MLS key schedule provides a per-epoch epoch_authenticator.
If one member of the group is being impersonated by an active
attacker, the epoch_authenticator computed by their client will
differ from those computed by the other group members.
This property can be used to construct defenses against impersonation
attacks that are effective even if members' signature keys are
compromised. As a trivial example, if the users of the clients in an
MLS group were to meet in person and reliably confirm that their
epoch authenticator values were equal (using some suitable user
interface), then each user would be assured that the others were not
being impersonated in the current epoch. As soon as the epoch
changed, though, they would need to re-do this confirmation. The
state of the group would have changed, possibly introducing an
attacker.
More generally, in order for the members of an MLS group to obtain
concrete authentication protections using the epoch_authenticator,
they will need to use it in some secondary protocol (such as the
face-to-face protocol above). The details of that protocol will then
determine the specific authentication protections provided to the MLS
group.
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10. Secret Tree
For the generation of encryption keys and nonces, the key schedule
begins with the encryption_secret at the root and derives a tree of
secrets with the same structure as the group's ratchet tree. Each
leaf in the Secret Tree is associated with the same group member as
the corresponding leaf in the ratchet tree.
If N is a parent node in the Secret Tree then the secrets of the
children of N are defined as follows (where left(N) and right(N)
denote the children of N):
tree_node_[N]_secret
|
|
+--> ExpandWithLabel(., "tree", "left", KDF.Nh)
| = tree_node_[left(N)]_secret
|
+--> ExpandWithLabel(., "tree", "right", KDF.Nh)
= tree_node_[right(N)]_secret
The secret in the leaf of the Secret Tree is used to initiate two
symmetric hash ratchets, from which a sequence of single-use keys and
nonces are derived, as described in Section 10.1. The root of each
ratchet is computed as:
tree_node_[N]_secret
|
|
+--> ExpandWithLabel(., "handshake", "", KDF.Nh)
| = handshake_ratchet_secret_[N]_[0]
|
+--> ExpandWithLabel(., "application", "", KDF.Nh)
= application_ratchet_secret_[N]_[0]
10.1. Encryption Keys
As described in Section 7, MLS encrypts three different types of
information:
* Metadata (sender information)
* Handshake messages (Proposal and Commit)
* Application messages
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The sender information used to look up the key for content encryption
is encrypted with an AEAD where the key and nonce are derived from
both sender_data_secret and a sample of the encrypted message
content.
For handshake and application messages, a sequence of keys is derived
via a "sender ratchet". Each sender has their own sender ratchet,
and each step along the ratchet is called a "generation".
A sender ratchet starts from a per-sender base secret derived from a
Secret Tree, as described in Section 10. The base secret initiates a
symmetric hash ratchet which generates a sequence of keys and nonces.
The sender uses the j-th key/nonce pair in the sequence to encrypt
(using the AEAD) the j-th message they send during that epoch. Each
key/nonce pair MUST NOT be used to encrypt more than one message.
Keys, nonces, and the secrets in ratchets are derived using
DeriveTreeSecret. The context in a given call consists of the
current position in the ratchet.
DeriveTreeSecret(Secret, Label, Generation, Length) =
ExpandWithLabel(Secret, Label, Generation, Length)
Where Generation is encoded as a uint32.
ratchet_secret_[N]_[j]
|
+--> DeriveTreeSecret(., "nonce", j, AEAD.Nn)
| = ratchet_nonce_[N]_[j]
|
+--> DeriveTreeSecret(., "key", j, AEAD.Nk)
| = ratchet_key_[N]_[j]
|
V
DeriveTreeSecret(., "secret", j, KDF.Nh)
= ratchet_secret_[N]_[j+1]
Here, AEAD.Nn and AEAD.Nk denote the lengths in bytes of the nonce
and key for the AEAD scheme defined by the ciphersuite.
10.2. Deletion Schedule
It is important to delete all security-sensitive values as soon as
they are _consumed_. A sensitive value S is said to be _consumed_ if
* S was used to encrypt or (successfully) decrypt a message, or if
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* a key, nonce, or secret derived from S has been consumed. (This
goes for values derived via DeriveSecret as well as
ExpandWithLabel.)
Here, S may be the init_secret, commit_secret, epoch_secret,
encryption_secret as well as any secret in a Secret Tree or one of
the ratchets.
As soon as a group member consumes a value they MUST immediately
delete (all representations of) that value. This is crucial to
ensuring forward secrecy for past messages. Members MAY keep
unconsumed values around for some reasonable amount of time to handle
out-of-order message delivery.
For example, suppose a group member encrypts or (successfully)
decrypts an application message using the j-th key and nonce in the
ratchet of leaf node L in some epoch n. Then, for that member, at
least the following values have been consumed and MUST be deleted:
* the commit_secret, joiner_secret, epoch_secret, encryption_secret
of that epoch n as well as the init_secret of the previous epoch
n-1,
* all node secrets in the Secret Tree on the path from the root to
the leaf with node L,
* the first j secrets in the application data ratchet of node L and
* application_ratchet_nonce_[L]_[j] and
application_ratchet_key_[L]_[j].
Concretely, suppose we have the following Secret Tree and ratchet for
participant D:
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G
|
.-+-.
/ \
E F
/ \ / \
A B C D
/ \
HR0 AR0--+--K0
| |
| +--N0
|
AR1--+--K1
| |
| +--N1
|
AR2
Then if a client uses key K1 and nonce N1 during epoch n then it must
consume (at least) values G, F, D, AR0, AR1, K1, N1 as well as the
key schedule secrets used to derive G (the encryption_secret), namely
init_secret of epoch n-1 and commit_secret, joiner_secret,
epoch_secret of epoch n. The client MAY retain (not consume) the
values K0 and N0 to allow for out-of-order delivery, and SHOULD
retain AR2 for processing future messages.
11. Key Packages
In order to facilitate the asynchronous addition of clients to a
group, key packages are pre-published that provide some public
information about a user. A KeyPackage object specifies:
1. A protocol version and ciphersuite that the client supports,
2. a public key that others can use to encrypt a Welcome message to
this client, (an "init key") and
3. the content of the leaf node that should be added to the tree to
represent this client.
KeyPackages are intended to be used only once and SHOULD NOT be
reused except in case of last resort. (See Section 17.4). Clients
MAY generate and publish multiple KeyPackages to support multiple
ciphersuites.
The value for init_key MUST be a public key for the asymmetric
encryption scheme defined by cipher_suite, and it MUST be unique
among the set of KeyPackages created by this client. Likewise, the
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leaf_node field MUST be valid for the ciphersuite, including both the
encryption_key and signature_key fields. The whole structure is
signed using the client's signature key. A KeyPackage object with an
invalid signature field MUST be considered malformed.
The signature is computed by the function SignWithLabel with a label
KeyPackage and a content comprising of all of the fields except for
the signature field.
struct {
ProtocolVersion version;
CipherSuite cipher_suite;
HPKEPublicKey init_key;
LeafNode leaf_node;
Extension extensions<V>;
// SignWithLabel(., "KeyPackageTBS", KeyPackageTBS)
opaque signature<V>;
} KeyPackage;
struct {
ProtocolVersion version;
CipherSuite cipher_suite;
HPKEPublicKey init_key;
LeafNode leaf_node;
Extension extensions<V>;
} KeyPackageTBS;
If a client receives a KeyPackage carried within an MLSMessage
object, then it MUST verify that the version field of the KeyPackage
has the same value as the version field of the MLSMessage. The
version field in the KeyPackage provides an explicit signal of the
intended version to the other members of group when they receive the
KeyPackage in an Add proposal.
The field leaf_node.capabilities indicates what protocol versions,
ciphersuites, credential types, and non-default proposal/extension
types are supported by the client. (Proposal and extension types
defined in this document are considered "default" and not listed.)
This information allows MLS session establishment to be safe from
downgrade attacks on the parameters described (as discussed in
Section 12), while still only advertising one version / ciphersuite
per KeyPackage.
The field leaf_node.leaf_node_source of the LeafNode in a KeyPackage
MUST be set to key_package.
Extension included in the extensions or leaf_node.extensions fields
MUST be included in the leaf_node.capabilities field.
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11.1. KeyPackage Validation
The validity of a KeyPackage needs to be verified at a few stages:
* When a KeyPackage is downloaded by a group member, before it is
used to add the client to the group
* When a KeyPackage is received by a group member in an Add message
The client verifies the validity of a KeyPackage using the following
steps:
* Verify that the ciphersuite and protocol version of the KeyPackage
match those in use in the group.
* Verify the leaf_node field of the KeyPackage according to the
process defined in Section 8.3.
* Verify that the signature on the KeyPackage is valid using the
public key in leaf_node.credential.
* Verify that the value of leaf_node.encryption_key is different
from the value of the init_key field.
11.2. KeyPackage Identifiers
Within MLS, a KeyPackage is identified by its hash (see, e.g.,
Section 13.2.3.2). The external_key_id extension allows applications
to add an explicit, application-defined identifier to a KeyPackage.
opaque external_key_id<V>;
12. Group Creation
A group is always created with a single member, the "creator". The
other members are added when the creator effectively sends itself an
Add proposal and commits it, then sends the corresponding Welcome
message to the new participants. These processes are described in
detail in Section 13.1.1, Section 13.2, and Section 13.2.3.2.
The creator of a group MUST take the following steps to initialize
the group:
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* Fetch KeyPackages for the members to be added, and select a
version and ciphersuite according to the capabilities of the
members. To protect against downgrade attacks, the creator MUST
use the capabilities information in these KeyPackages to verify
that the chosen version and ciphersuite is the best option
supported by all members.
* Initialize a one-member group with the following initial values:
- Ratchet tree: A tree with a single node, a leaf containing an
HPKE public key and credential for the creator
- Group ID: A value set by the creator
- Epoch: 0
- Tree hash: The root hash of the above ratchet tree
- Confirmed transcript hash: The zero-length octet string
- Interim transcript hash: The zero-length octet string
- Init secret: A fresh random value of size KDF.Nh
- Extensions: Any values of the creator's choosing
* For each member, construct an Add proposal from the KeyPackage for
that member (see Section 13.1.1)
* Construct a Commit message that commits all of the Add proposals,
in any order chosen by the creator (see Section 13.2)
* Process the Commit message to obtain a new group state (for the
epoch in which the new members are added) and a Welcome message
* Transmit the Welcome message to the other new members
The recipient of a Welcome message processes it as described in
Section 13.2.3.2. If application context informs the recipient that
the Welcome should reflect the creation of a new group (for example,
due to a branch or reinitialization), then the recipient MUST verify
that the epoch value in the GroupInfo is equal to 1.
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In principle, the above process could be streamlined by having the
creator directly create a tree and choose a random value for first
epoch's epoch secret. We follow the steps above because it removes
unnecessary choices, by which, for example, bad randomness could be
introduced. The only choices the creator makes here are its own
KeyPackage and the leaf secret from which the Commit is built.
12.1. Required Capabilities
The configuration of a group imposes certain requirements on clients
in the group. At a minimum, all members of the group need to support
the ciphersuite and protocol version in use. Additional requirements
can be imposed by including a required_capabilities extension in the
GroupContext.
struct {
ExtensionType extension_types<V>;
ProposalType proposal_types<V>;
CredentialType credential_types<V>;
} RequiredCapabilities;
This extension lists the extensions, proposals, and credential types
that must be supported by all members of the group. The "default"
proposal and extension types defined in this document are assumed to
be implemented by all clients, and need not be listed in
RequiredCapabilities in order to be safely used.
For new members, support for required capabilities is enforced by
existing members during the application of Add commits. Existing
members should of course be in compliance already. In order to
ensure this continues to be the case even as the group's extensions
can be updated, a GroupContextExtensions proposal is invalid if it
contains a required_capabilities extension that requires non-default
capabilities not supported by all current members.
12.2. Reinitialization
A group may be reinitialized by creating a new group with the same
membership and different parameters, and linking it to the old group
via a resumption PSK. The members of a group reinitialize it using
the following steps:
1. A member of the old group sends a ReInit proposal (see
Section 13.1.5)
2. A member of the old group sends a Commit covering the ReInit
proposal
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3. A member of the old group sends a Welcome message for the new
group that matches the ReInit
* The group_id, version, and cipher_suite fields in the Welcome
message MUST be the same as the corresponding fields in the
ReInit proposal.
* The epoch in the Welcome message MUST be 1
* The Welcome MUST specify a PreSharedKey of type resumption
with usage reinit. The group_id must match the old group, and
the epoch must indicate the epoch after the Commit covering
the ReInit.
* The psk_nonce included in the PreSharedKeyID of the resumption
PSK MUST be a randomly sampled nonce of length KDF.Nh, for the
KDF defined by the new group's ciphersuite.
Note that these three steps may be done by the same group member or
different members. For example, if a group member sends a commit
with an inline ReInit proposal (steps 1 and 2), but then goes
offline, another group member may send the corresponding Welcome.
This flexibility avoids situations where a group gets stuck between
steps 2 and 3.
Resumption PSKs with usage reinit MUST NOT be used in other contexts.
A PreSharedKey proposal with type resumption and usage reinit MUST be
considered invalid.
12.3. Sub-group Branching
A new group can be formed from a subset of an existing group's
members, using the same parameters as the old group.
A member can create a sub-group by performing the following steps:
1. Determine a subset of existing members that should be a part of
the sub-group.
2. Create a new tree for the sub-group by fetching a new KeyPackage
for each existing member that should be included in the sub-
group.
3. Create a Welcome message that includes a PreSharedKey of type
resumption with usage branch.
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A client receiving a Welcome including a PreSharedKey of type
resumption with usage branch MUST verify that the new group reflects
a subgroup branched from the referenced group.
* The version and ciphersuite values in the Welcome MUST be the same
as those used by the old group.
* Each LeafNode in a new subgroup MUST match some LeafNode in the
original group. In this context, a pair of LeafNodes is said to
"match" if the identifiers presented by their respective
credentials are considered equivalent by the application.
In addition, to avoid key re-use, the psk_nonce included in the
PreSharedKeyID object MUST be a randomly sampled nonce of length
KDF.Nh.
Resumption PSKs with usage branch MUST NOT be used in other contexts.
A PreSharedKey proposal with type resumption and usage branch MUST be
considered invalid.
13. Group Evolution
Over the lifetime of a group, its membership can change, and existing
members might want to change their keys in order to achieve post-
compromise security. In MLS, each such change is accomplished by a
two-step process:
1. A proposal to make the change is broadcast to the group in a
Proposal message
2. A member of the group or a new member broadcasts a Commit message
that causes one or more proposed changes to enter into effect
In cases where the Proposal and Commit are sent by the same member,
these two steps can be combined by sending the proposals in the
commit.
The group thus evolves from one cryptographic state to another each
time a Commit message is sent and processed. These states are
referred to as "epochs" and are uniquely identified among states of
the group by eight-octet epoch values. When a new group is
initialized, its initial state epoch is 0x0000000000000000. Each
time a state transition occurs, the epoch number is incremented by
one.
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13.1. Proposals
Proposals are included in an MLSMessageContent by way of a Proposal
structure that indicates their type:
// See IANA registry for registered values
uint16 ProposalType;
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 app_ack: AppAck;
case group_context_extensions: GroupContextExtensions;
};
} Proposal;
On receiving an MLSMessageContent containing a Proposal, a client
MUST verify the signature inside MLSMessageAuth. If the signature
verifies successfully, then the Proposal should be cached in such a
way that it can be retrieved by hash (as a ProposalOrRef object) in a
later Commit message.
13.1.1. Add
An Add proposal requests that a client with a specified KeyPackage be
added to the group. The proposer of the Add MUST verify the validity
of the KeyPackage, as specified in Section 11.1.
struct {
KeyPackage key_package;
} Add;
An Add is applied after being included in a Commit message. The
position of the Add in the list of proposals determines the leaf node
where the new member will be added. For the first Add in the Commit,
the corresponding new member will be placed in the leftmost empty
leaf in the tree, for the second Add, the next empty leaf to the
right, etc. If no empty leaf exists, the tree is extended to the
right.
* Validate the KeyPackage as specified in Section 11.1. The
leaf_node_source field in the LeafNode MUST be set to key_package.
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* Identify the leaf L for the new member: if there are empty leaves
in the tree, L is the leftmost empty leaf. Otherwise, the tree is
extended to the right by one leaf node and L is the new leaf.
* For each non-blank intermediate node along the path from the leaf
L to the root, add L's leaf index to the unmerged_leaves list for
the node.
* Set the leaf node L to a new node containing the LeafNode object
carried in the leaf_node field of the KeyPackage in the Add.
13.1.2. Update
An Update proposal is a similar mechanism to Add with the distinction
that it replaces the sender's LeafNode in the tree instead of adding
a new leaf to the tree.
struct {
LeafNode leaf_node;
} Update;
A member of the group applies an Update message by taking the
following steps:
* Validate the LeafNode as specified in Section 8.3. The
leaf_node_source field MUST be set to update.
* Verify that the encryption_key value in the LeafNode is different
from the corresponding field in the LeafNode being replaced.
* Replace the sender's LeafNode with the one contained in the Update
proposal
* Blank the intermediate nodes along the path from the sender's leaf
to the root
13.1.3. Remove
A Remove proposal requests that the member with LeafNodeRef removed
be removed from the group.
struct {
LeafNodeRef removed;
} Remove;
A member of the group applies a Remove message by taking the
following steps:
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* Identify a leaf node matching removed. This lookup MUST be done
on the tree before any non-Remove proposals have been applied (the
"old" tree in the terminology of Section 13.2), since proposals
such as Update can change the LeafNode stored at a leaf. Let L be
this leaf node.
* Replace the leaf node L with a blank node
* Blank the intermediate nodes along the path from L to the root
* Truncate the tree by removing leaves from the right side of the
tree until the rightmost leaf node is not blank.
13.1.4. PreSharedKey
A PreSharedKey proposal can be used to request that a pre-shared key
be injected into the key schedule in the process of advancing the
epoch.
struct {
PreSharedKeyID psk;
} PreSharedKey;
The psktype of the pre-shared key MUST be external and the psk_nonce
MUST be a randomly sampled nonce of length KDF.Nh. When processing a
Commit message that includes one or more PreSharedKey proposals,
group members derive psk_secret as described in Section 9.4, where
the order of the PSKs corresponds to the order of the PreSharedKey
proposals in the Commit.
13.1.5. ReInit
A ReInit proposal represents a request to reinitialize the group with
different parameters, for example, to increase the version number or
to change the ciphersuite. The reinitialization is done by creating
a completely new group and shutting down the old one.
struct {
opaque group_id<V>;
ProtocolVersion version;
CipherSuite cipher_suite;
Extension extensions<V>;
} ReInit;
A member of the group applies a ReInit proposal by waiting for the
committer to send the Welcome message that matches the ReInit,
according to the criteria in Section 12.2.
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If a ReInit proposal is included in a Commit, it MUST be the only
proposal referenced by the Commit. If other non-ReInit proposals
have been sent during the epoch, the committer SHOULD prefer them
over the ReInit proposal, allowing the ReInit to be resent and
applied in a subsequent epoch. The version field in the ReInit
proposal MUST be no less than the version for the current group.
13.1.6. ExternalInit
An ExternalInit proposal is used by new members that want to join a
group by using an external commit. This proposal can only be used in
that context.
struct {
opaque kem_output<V>;
} ExternalInit;
A member of the group applies an ExternalInit message by initializing
the next epoch using an init secret computed as described in
Section 9.3. The kem_output field contains the required KEM output.
13.1.7. AppAck
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 {
LeafNodeRef 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
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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.
13.1.8. GroupContextExtensions
A GroupContextExtensions proposal is used to update the list of
extensions in the GroupContext for the group.
struct { Extension extensions<V>; } GroupContextExtensions;
A member of the group applies a GroupContextExtensions proposal with
the following steps:
* If the new extensions include a required_capabilities extension,
verify that all members of the group support the required
capabilities (including those added in the same commit, and
excluding those removed).
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* Remove all of the existing extensions from the GroupContext object
for the group and replacing them with the list of extensions in
the proposal. (This is a wholesale replacement, not a merge. An
extension is only carried over if the sender of the proposal
includes it in the new list.)
Note that once the GroupContext is updated, its inclusion in the
confirmation_tag by way of the key schedule will confirm that all
members of the group agree on the extensions in use.
13.1.9. External Proposals
Add and Remove proposals can be constructed and sent to the group by
a party that is outside the group. For example, a Delivery Service
might propose to remove a member of a group who has been inactive for
a long time, or propose adding a newly-hired staff member to a group
representing a real-world team. Proposals originating outside the
group are identified by a external or new_member SenderType in
MLSPlaintext.
ReInit proposals can also be sent to the group by a external sender,
for example to enforce a changed policy regarding MLS version or
ciphersuite.
The new_member SenderType is used for clients proposing that they
themselves be added. For this ID type the sender value MUST be zero
and the Proposal type MUST be Add. The MLSPlaintext MUST be signed
with the private key corresponding to the KeyPackage in the Add
message. Recipients MUST verify that the MLSPlaintext carrying the
Proposal message is validly signed with this key.
The external SenderType is reserved for signers that are pre-
provisioned to the clients within a group. It can only be used if
the external_senders extension is present in the group's group
context extensions.
An external proposal MUST be sent as an MLSPlaintext object, since
the sender will not have the keys necessary to construct an
MLSCiphertext object.
13.1.9.1. External Senders Extension
The external_senders extension is a group context extension that
contains credentials of senders that are permitted to send external
proposals to the group.
Credential external_senders<V>;
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13.2. Commit
A Commit message initiates a new epoch for the group, based on a
collection of Proposals. It instructs group members to update their
representation of the state of the group by applying the proposals
and advancing the key schedule.
Each proposal covered by the Commit is included by a ProposalOrRef
value, which identifies the proposal to be applied by value or by
reference. Commits that refer to new Proposals from the committer
can be included by value. Commits for previously sent proposals from
anyone (including the committer) can be sent by reference. Proposals
sent by reference are specified by including the hash of the
MLSPlaintext in which the proposal was sent (see Section 6.2).
enum {
reserved(0),
proposal(1)
reference(2),
(255)
} ProposalOrRefType;
struct {
ProposalOrRefType type;
select (ProposalOrRef.type) {
case proposal: Proposal proposal;
case reference: ProposalRef reference;
}
} ProposalOrRef;
struct {
ProposalOrRef proposals<V>;
optional<UpdatePath> path;
} Commit;
A group member that has observed one or more valid proposals within
an epoch MUST send a Commit message before sending application data.
This ensures, for example, that any members whose removal was
proposed during the epoch are actually removed before any application
data is transmitted.
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The sender of a Commit MUST include all valid proposals that it has
received during the current epoch. Invalid proposals include, for
example, proposals with an invalid signature or proposals that are
semantically invalid, such as an Add when the sender does not have
the application-level permission to add new users. Proposals with a
non-default proposal type MUST NOT be included in a commit unless the
proposal type is supported by all the members of the group that will
process the Commit (i.e., not including any members being added or
removed by the Commit).
If there are multiple proposals that apply to the same leaf, or
multiple PreSharedKey proposals that reference the same
PreSharedKeyID, the committer MUST choose one and include only that
one in the Commit, considering the rest invalid. The committer MUST
NOT include any Update proposals generated by the committer, since
they would be duplicative with the path field in the Commit. The
committer MUST prefer any Remove received, or the most recent Update
for the leaf if there are no Removes. If there are multiple Add
proposals containing KeyPackages that the committer considers to
represent the same client or a client already in the group (for
example, identical KeyPackages or KeyPackages sharing the same
Credential), the committer again chooses one to include and considers
the rest invalid. The committer MUST consider invalid any Add or
Update proposal if the Credential in the contained KeyPackage shares
the same signature key with a Credential in any leaf of the group, or
if the LeafNode in the KeyPackage shares the same encryption_key with
another LeafNode in the group.
The Commit MUST NOT combine proposals sent within different epochs.
Due to the asynchronous nature of proposals, receivers of a Commit
SHOULD NOT enforce that all valid proposals sent within the current
epoch are referenced by the next Commit. In the event that a valid
proposal is omitted from the next Commit, and that proposal is still
valid in the current epoch, the sender of the proposal MAY resend it
after updating it to reflect the current epoch.
A member of the group MAY send a Commit that references no proposals
at all, which would thus have an empty proposals vector. Such a
Commit resets the sender's leaf and the nodes along its direct path,
and provides forward secrecy and post-compromise security with regard
to the sender of the Commit. An Update proposal can be regarded as a
"lazy" version of this operation, where only the leaf changes and
intermediate nodes are blanked out.
By default, the path field of a Commit MUST be populated. The path
field MAY be omitted if (a) it covers at least one proposal and (b)
none of the proposals covered by the Commit are of "path required"
types. A proposal type requires a path if it cannot change the group
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membership in a way that requires the forward secrecy and post-
compromise security guarantees that an UpdatePath provides. The only
proposal types defined in this document that do not require a path
are:
* add
* psk
* app_ack
* reinit
New proposal types MUST state whether they require a path. If any
instance of a proposal type requires a path, then the proposal type
requires a path. This attribute of a proposal type is reflected in
the "Path Required" field of the proposal type registry defined in
Section 18.3.
Update and Remove proposals are the clearest examples of proposals
that require a path. An UpdatePath is required to evict the removed
member or the old appearance of the updated member.
In pseudocode, the logic for validating the path field of a Commit is
as follows:
pathRequiredTypes = [
update,
remove,
external_init,
group_context_extensions
]
pathRequired = false
for i, id in commit.proposals:
proposal = proposalCache[id]
assert(proposal != null)
pathRequired = pathRequired ||
(proposal.msg_type in pathRequiredTypes)
if len(commit.proposals) == 0 || pathRequired:
assert(commit.path != null)
To summarize, a Commit can have three different configurations, with
different uses:
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1. An "empty" Commit that references no proposals, which updates the
committer's contribution to the group and provides PCS with
regard to the committer.
2. A "partial" Commit that references proposals that do not require
a path, and where the path is empty. Such a commit doesn't
provide PCS with regard to the committer.
3. A "full" Commit that references proposals of any type, which
provides FS with regard to any removed members and PCS for the
committer and any updated members.
13.2.1. Creating a Commit
When creating or processing a Commit, three different ratchet trees
and their associated GroupContexts are used:
1. "Old" refers to the ratchet tree and GroupContext for the epoch
before the commit. The old GroupContext is used when signing the
MLSPlainText so that existing group members can verify the
signature before processing the commit.
2. "Provisional" refers to the ratchet tree and GroupContext
constructed after applying the proposals that are referenced by
the Commit. The provisional GroupContext uses the epoch number
for the new epoch, and the old confirmed transcript hash. This
is used when creating the UpdatePath, if the UpdatePath is
needed.
3. "New" refers to the ratchet tree and GroupContext constructed
after applying the proposals and the UpdatePath (if any). The
new GroupContext uses the epoch number for the new epoch, and the
new confirmed transcript hash. This is used when deriving the
new epoch secrets, and is the only GroupContext that newly-added
members will have.
A member of the group creates a Commit message and the corresponding
Welcome message at the same time, by taking the following steps:
* Construct an initial Commit object with the proposals field
populated from Proposals received during the current epoch, and an
empty path field.
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* Generate the provisional ratchet tree and GroupContext by applying
the proposals referenced in the initial Commit object, as
described in Section 13.1. Update proposals are applied first,
followed by Remove proposals, and then finally Add proposals. Add
proposals are applied in the order listed in the proposals vector,
and always to the leftmost unoccupied leaf in the tree, or the
right edge of the tree if all leaves are occupied.
- Note that the order in which different types of proposals are
applied should be updated by the implementation to include any
new proposals added by negotiated group extensions.
- PreSharedKey proposals are processed later when deriving the
psk_secret for the Key Schedule.
* Decide whether to populate the path field: If the path field is
required based on the proposals that are in the commit (see
above), then it MUST be populated. Otherwise, the sender MAY omit
the path field at its discretion.
* If populating the path field: Create an UpdatePath using the
provisional ratchet tree and GroupContext. Any new member (from
an add proposal) MUST be excluded from the resolution during the
computation of the UpdatePath. The leaf_node for this UpdatePath
MUST have leaf_node_source set to commit. Note that the LeafNode
in the UpdatePath effectively updates an existing LeafNode in the
group and thus MUST adhere to the same restrictions as LeafNodes
used in Update proposals (aside from leaf_node_source).
- Assign this UpdatePath to the path field in the Commit.
- Apply the UpdatePath to the tree, as described in Section 8.6,
creating the new ratchet tree. Define commit_secret as the
value path_secret[n+1] derived from the path_secret[n] value
assigned to the root node.
* If not populating the path field: Set the path field in the Commit
to the null optional. Define commit_secret as the all-zero vector
of length KDF.Nh (the same length as a path_secret value would
be). In this case, the new ratchet tree is the same as the
provisional ratchet tree.
* Derive the psk_secret as specified in Section 9.4, where the order
of PSKs in the derivation corresponds to the order of PreSharedKey
proposals in the proposals vector.
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* Construct an MLSMessageContent object containing the Commit
object. Sign the MLSMessageContent using the old GroupContext as
context.
- Use the MLSMessageContent to update the confirmed transcript
hash and generate the new GroupContext.
- Use the init_secret from the previous epoch, the commit_secret
and the psk_secret as defined in the previous steps, and the
new GroupContext to compute the new joiner_secret,
welcome_secret, epoch_secret, and derived secrets for the new
epoch.
- Use the confirmation_key for the new epoch to compute the
confirmation_tag value, and the membership_key for the old
epoch to compute the membership_tag value in the MLSPlaintext.
- Calculate the interim transcript hash using the new confirmed
transcript hash and the confirmation_tag from the
MLSMessageAuth.
* Construct a GroupInfo reflecting the new state:
- Group ID, epoch, tree, confirmed transcript hash, interim
transcript hash, and group context extensions from the new
state
- The confirmation_tag from the MLSMessageAuth object
- Other extensions as defined by the application
- Optionally derive an external keypair as described in Section 9
(required for External Commits, see Section 13.2.3.1)
- Sign the GroupInfo using the member's private signing key
- Encrypt the GroupInfo using the key and nonce derived from the
joiner_secret for the new epoch (see Section 13.2.3.2)
* For each new member in the group:
- Identify the lowest common ancestor in the tree of the new
member's leaf node and the member sending the Commit
- If the path field was populated above: Compute the path secret
corresponding to the common ancestor node
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- Compute an EncryptedGroupSecrets object that encapsulates the
init_secret for the current epoch and the path secret (if
present).
* Construct a Welcome message from the encrypted GroupInfo object,
the encrypted key packages, and any PSKs for which a proposal was
included in the Commit. The order of the psks MUST be the same as
the order of PreSharedKey proposals in the proposals vector.
* If a ReInit proposal was part of the Commit, the committer MUST
create a new group with the parameters specified in the ReInit
proposal, and with the same members as the original group. The
Welcome message MUST include a PreSharedKeyID with the following
parameters:
- psktype: resumption
- usage: reinit
- group_id: The group ID for the current group
- epoch: The epoch that the group will be in after this Commit
13.2.2. Processing a Commit
A member of the group applies a Commit message by taking the
following steps:
* Verify that the epoch field of the enclosing MLSMessageContent is
equal to the epoch field of the current GroupContext object
* Verify that the signature on the MLSMessageContent message as
described in Section Section 7.1.
* Verify that all PreSharedKey proposals in the proposals vector
have unique PreSharedKeyIDs and are available.
* Generate the provisional ratchet tree and GroupContext by applying
the proposals referenced in the initial Commit object, as
described in Section 13.1. Update proposals are applied first,
followed by Remove proposals, and then finally Add proposals. Add
proposals are applied in the order listed in the proposals vector,
and always to the leftmost unoccupied leaf in the tree, or the
right edge of the tree if all leaves are occupied.
- Note that the order in which different types of proposals are
applied should be updated by the implementation to include any
new proposals added by negotiated group extensions.
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* Verify that the path value is populated if the proposals vector
contains any Update or Remove proposals, or if it's empty.
Otherwise, the path value MAY be omitted.
* If the path value is populated: Process the path value using the
provisional ratchet tree and GroupContext, to generate the new
ratchet tree and the commit_secret:
- Validate the LeafNode as specified in Section 8.3. The
leaf_node_source field MUST be set to commit.
- Verify that the encryption_key value in the LeafNode is
different from the committer's current leaf node.
- Apply the UpdatePath to the tree, as described in Section 8.6,
and store leaf_node at the committer's leaf.
- Verify that the LeafNode has a parent_hash field and that its
value matches the new parent of the sender's leaf node.
- Define commit_secret as the value path_secret[n+1] derived from
the path_secret[n] value assigned to the root node.
* If the path value is not populated: Define commit_secret as the
all-zero vector of length KDF.Nh (the same length as a path_secret
value would be).
* Update the confirmed and interim transcript hashes using the new
Commit, and generate the new GroupContext.
* Derive the psk_secret as specified in Section 9.4, where the order
of PSKs in the derivation corresponds to the order of PreSharedKey
proposals in the proposals vector.
* Use the init_secret from the previous epoch, the commit_secret and
the psk_secret as defined in the previous steps, and the new
GroupContext to compute the new joiner_secret, welcome_secret,
epoch_secret, and derived secrets for the new epoch.
* Use the confirmation_key for the new epoch to compute the
confirmation tag for this message, as described below, and verify
that it is the same as the confirmation_tag field in the
MLSMessageAuth object.
* If the above checks are successful, consider the new GroupContext
object as the current state of the group.
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* If the Commit included a ReInit proposal, the client MUST NOT use
the group to send messages anymore. Instead, it MUST wait for a
Welcome message from the committer meeting the requirements of
Section 12.2.
13.2.3. Adding Members to the Group
New members can join the group in two ways. Either by being added by
a group member, or by adding themselves through an external Commit.
In both cases, the new members need information to bootstrap their
local group state.
struct {
CipherSuite cipher_suite;
opaque group_id<V>;
uint64 epoch;
opaque tree_hash<V>;
opaque confirmed_transcript_hash<V>;
Extension group_context_extensions<V>;
Extension other_extensions<V>;
MAC confirmation_tag;
LeafNodeRef signer;
// SignWithLabel(., "GroupInfoTBS", GroupInfoTBS)
opaque signature<V>;
} GroupInfo;
New members MUST verify the signature using the public key taken from
the credential in the leaf node of the member with LeafNodeRef
signer. The signature covers the following structure, comprising all
the fields in the GroupInfo above signature:
struct {
CipherSuite cipher_suite;
opaque group_id<V>;
uint64 epoch;
opaque tree_hash<V>;
opaque confirmed_transcript_hash<V>;
Extension group_context_extensions<V>;
Extension other_extensions<V>;
MAC confirmation_tag;
LeafNodeRef signer;
} GroupInfoTBS;
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13.2.3.1. Joining via External Commits
External Commits are a mechanism for new members (external parties
that want to become members of the group) to add themselves to a
group, without requiring that an existing member has to come online
to issue a Commit that references an Add Proposal.
Whether existing members of the group will accept or reject an
External Commit follows the same rules that are applied to other
handshake messages.
New members can create and issue an External Commit if they have
access to the following information for the group's current epoch:
* group ID
* epoch ID
* ciphersuite
* public tree hash
* confirmed transcript hash
* confirmation tag of the most recent Commit
* group extensions
* external public key
In other words, to join a group via an External Commit, a new member
needs a GroupInfo with an ExternalPub extension present in the
other_extensions.
struct {
HPKEPublicKey external_pub;
} ExternalPub;
Thus, a member of the group can enable new clients to join by making
a GroupInfo object available to them. Note that because a GroupInfo
object is specific to an epoch, it will need to be updated as the
group advances. In particular, each GroupInfo object can be used for
one external join, since that external join will cause the epoch to
change.
Note that the tree_hash field is used the same way as in the Welcome
message. The full tree can be included via the ratchet_tree
extension Section 13.3.
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The information in a GroupInfo is not generally public information,
but applications can choose to make it available to new members in
order to allow External Commits.
In principle, External Commits work like regular Commits. However,
their content has to meet a specific set of requirements:
* External Commits MUST contain a path field (and is therefore a
"full" Commit). The joiner is added at the leftmost free leaf
node (just as if they were added with an Add proposal), and the
path is calculated relative to that leaf node.
* The Commit MUST NOT include any proposals by reference, since an
external joiner cannot determine the validity of proposals sent
within the group
* The proposals included by value in an External Commit MUST meet
the following conditions:
- There MUST be a single ExternalInit proposal.
- There MAY be a single Remove proposal, where the LeafNode in
the path field MUST meet the same criteria as the LeafNode in
an Update for the removed leaf (see Section 13.1.2). In
particular, the credential in the LeafNode MUST present a set
of identifiers that is acceptable to the application for the
removed participant.
- There MAY be one or more PreSharedKey proposals.
- There MUST NOT be any other proposals.
* External Commits MUST be signed by the new member. In particular,
the signature on the enclosing MLSPlaintext MUST verify using the
public key for the credential in the leaf_node of the path field.
* When processing a Commit, both existing and new members MUST use
the external init secret as described in Section 9.3.
* The sender type for the MLSPlaintext encapsulating the External
Commit MUST be new_member
External Commits come in two "flavors" -- a "join" commit that adds
the sender to the group or a "resync" commit that replaces a member's
prior appearance with a new one.
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Note that the "resync" operation allows an attacker that has
compromised a member's signature private key to introduce themselves
into the group and remove the prior, legitimate member in a single
Commit. Without resync, this can still be done, but requires two
operations, the external Commit to join and a second Commit to remove
the old appearance. Applications for whom this distinction is
salient can choose to disallow external commits that contain a
Remove, or to allow such resync commits only if they contain a
"reinit" PSK proposal that demonstrates the joining member's presence
in a prior epoch of the group. With the latter approach, the
attacker would need to compromise the PSK as well as the signing key,
but the application will need to ensure that continuing, non-
resynchronizing members have the required PSK.
13.2.3.2. Joining via Welcome Message
The sender of a Commit message is responsible for sending a single
Welcome message to all the new members added via Add proposals. The
Welcome message provides the new members with the current state of
the group, after the application of the Commit message. The new
members will not be able to decrypt or verify the Commit message, but
will have the secrets they need to participate in the epoch initiated
by the Commit message.
In order to allow the same Welcome message to be sent to all new
members, information describing the group is encrypted with a
symmetric key and nonce derived from the joiner_secret for the new
epoch. The joiner_secret is then encrypted to each new member using
HPKE. In the same encrypted package, the committer transmits the
path secret for the lowest (closest to the leaf) node which is
contained in the direct paths of both the committer and the new
member. This allows the new member to compute private keys for nodes
in its direct path that are being reset by the corresponding Commit.
If the sender of the Welcome message wants the receiving member to
include a PSK in the derivation of the epoch_secret, they can
populate the psks field indicating which PSK to use.
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struct {
opaque path_secret<V>;
} PathSecret;
struct {
opaque joiner_secret<V>;
optional<PathSecret> path_secret;
PreSharedKeys psks;
} GroupSecrets;
struct {
KeyPackageRef new_member;
HPKECiphertext encrypted_group_secrets;
} EncryptedGroupSecrets;
struct {
CipherSuite cipher_suite;
EncryptedGroupSecrets secrets<V>;
opaque encrypted_group_info<V>;
} Welcome;
The client processing a Welcome message will need to have a copy of
the group's ratchet tree. The tree can be provided in the Welcome
message, in an extension of type ratchet_tree. If it is sent
otherwise (e.g., provided by a caching service on the Delivery
Service), then the client MUST download the tree before processing
the Welcome.
On receiving a Welcome message, a client processes it using the
following steps:
* Identify an entry in the secrets array where the new_member value
corresponds to one of this client's KeyPackages, using the hash
indicated by the cipher_suite field. If no such field exists, or
if the ciphersuite indicated in the KeyPackage does not match the
one in the Welcome message, return an error.
* Decrypt the encrypted_group_secrets using HPKE with the algorithms
indicated by the ciphersuite and the HPKE private key
corresponding to the GroupSecrets. If a PreSharedKeyID is part of
the GroupSecrets and the client is not in possession of the
corresponding PSK, return an error.
* From the joiner_secret in the decrypted GroupSecrets object and
the PSKs specified in the GroupSecrets, derive the welcome_secret
and using that the welcome_key and welcome_nonce. Use the key and
nonce to decrypt the encrypted_group_info field.
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welcome_nonce = KDF.Expand(welcome_secret, "nonce", AEAD.Nn)
welcome_key = KDF.Expand(welcome_secret, "key", AEAD.Nk)
* Verify the signature on the GroupInfo object. The signature input
comprises all of the fields in the GroupInfo object except the
signature field. The public key and algorithm are taken from the
credential in the leaf node of the member with LeafNodeRef signer.
If there is no matching leaf node, or if signature verification
fails, return an error.
* Verify the integrity of the ratchet tree.
- Verify that the tree hash of the ratchet tree matches the
tree_hash field in the GroupInfo.
- For each non-empty parent node, verify that exactly one of the
node's children are non-empty and have the hash of this node
set as their parent_hash value (if the child is another parent)
or has a parent_hash field in the LeafNode containing the same
value (if the child is a leaf). If either of the node's
children is empty, and in particular does not have a parent
hash, then its respective children's parent_hash values have to
be considered instead.
- For each non-empty leaf node, validate the LeafNode as
described in Section 8.3.
* Identify a leaf in the tree array (any even-numbered node) whose
LeafNode is identical to the one in the KeyPackage. If no such
field exists, return an error. Let my_leaf represent this leaf in
the tree.
* Construct a new group state using the information in the GroupInfo
object.
- The GroupContext contains the group_id, epoch, tree_hash,
confirmed_transcript_hash, and group_context_extensions fields
from the GroupInfo object.
- The new member's position in the tree is at the leaf my_leaf,
as defined above.
- Update the leaf my_leaf with the private key corresponding to
the public key in the node.
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- If the path_secret value is set in the GroupSecrets object:
Identify the lowest common ancestor of the leaf node my_leaf
and of the node of the member with LeafNodeRef
GroupInfo.signer. Set the private key for this node to the
private key derived from the path_secret.
- For each parent of the common ancestor, up to the root of the
tree, derive a new path secret and set the private key for the
node to the private key derived from the path secret. The
private key MUST be the private key that corresponds to the
public key in the node.
* Use the joiner_secret from the GroupSecrets object to generate the
epoch secret and other derived secrets for the current epoch.
* Set the confirmed transcript hash in the new state to the value of
the confirmed_transcript_hash in the GroupInfo.
* Verify the confirmation tag in the GroupInfo using the derived
confirmation key and the confirmed_transcript_hash from the
GroupInfo.
* Use the confirmed transcript hash and confirmation tag to compute
the interim transcript hash in the new state.
13.3. Ratchet Tree Extension
By default, a GroupInfo message only provides the joiner with a
commitment to the group's ratchet tree. In order to process or
generate handshake messages, the joiner will need to get a copy of
the ratchet tree from some other source. (For example, the DS might
provide a cached copy.) The inclusion of the tree hash in the
GroupInfo message means that the source of the ratchet tree need not
be trusted to maintain the integrity of tree.
In cases where the application does not wish to provide such an
external source, the whole public state of the ratchet tree can be
provided in an extension of type ratchet_tree, containing a
ratchet_tree object of the following form:
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enum {
reserved(0),
leaf(1),
parent(2),
(255)
} NodeType;
struct {
NodeType node_type;
select (Node.node_type) {
case leaf: LeafNode leaf_node;
case parent: ParentNode parent_node;
};
} Node;
optional<Node> ratchet_tree<V>;
The nodes are listed in the order specified by a left-to-right in-
order traversal of the rachet tree. Each node is listed between its
left subtree and its right subtree. (This is the same ordering as
specified for the array-based trees outlined in Appendix B.)
The leaves of the tree are stored in even-numbered entries in the
array (the leaf with index L in array position 2*L). The root node
of the tree is at position 2^k - 1 of the array, where k is the
largest number such that 2^k is smaller than the length of the array.
Intermediate parent nodes can be identified by performing the same
calculation to the subarrays to the left and right of the root,
following something like the following algorithm:
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# Assuming a class Node that has left and right members
def subtree_root(nodes):
# If there is only one node in the array return it
if len(nodes) == 1:
return Node(nodes[0])
# Otherwise, the length of the array MUST be odd
if len(nodes) % 2 == 0:
raise Exception("Malformed node array {}", len(nodes))
# Identify the root of the subtree
k = 0
while (2**(k+1)) < len(nodes):
k += 1
R = 2**k - 1
root = Node(nodes[R])
root.left = subtree_root(nodes[:R])
root.right = subtree_root(nodes[(R+1):])
return root
(Note that this is the same ordering of nodes as in the array-based
tree representation described in Appendix B. The algorithms in that
section may be used to simplify decoding this extension into other
representations.)
The example tree in Section 5.1 would be represented as an array of
nodes in the following form, where R represents the "subtree root"
for a given subarray of the node array:
P
|
.-----+-----.
/ \
N R
| |
.-+-. .-+
/ \ / \
M O Q |
/ \ / \ / \ |
A B C D E F G
1 1 1
0 1 2 3 4 5 6 7 8 9 0 1 2
<-----------> R <------->
<---> R <---> <---> R -
- R - - R - - R -
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The presence of a ratchet_tree extension in a GroupInfo message does
not result in any changes to the GroupContext extensions for the
group. The ratchet tree provided is simply stored by the client and
used for MLS operations.
If this extension is not provided in a Welcome message, then the
client will need to fetch the ratchet tree over some other channel
before it can generate or process Commit messages. Applications
should ensure that this out-of-band channel is provided with security
protections equivalent to the protections that are afforded to
Proposal and Commit messages. For example, an application that
encrypts Proposal and Commit messages might distribute ratchet trees
encrypted using a key exchanged over the MLS channel.
Regardless of how the client obtains the tree, the client MUST verify
that the root hash of the ratchet tree matches the tree_hash of the
GroupContext before using the tree for MLS operations.
14. Extensibility
The base MLS protocol can be extended in a few ways. New
ciphersuites can be added to enable the use of new cryptographic
algorithms. New types of proposals can be used to perform new
actions within an epoch. Extension fields can be used to add
additional information to the protocol. In this section, we discuss
some constraints on these extensibility mechanisms that are necessary
to ensure broad interoperability.
14.1. Ciphersuites
As discussed in Section 6.1, MLS allows the participants in a group
to negotiate the cryptographic algorithms used within the group.
This extensibility is important for maintaining the security of the
protocol over time [RFC7696]. It also creates a risk of
interoperability failure due to clients not supporting a common
ciphersuite.
The ciphersuite registry defined in Section 18.1 attempts to strike a
balance on this point. On the one hand, the base policy for the
registry is Specification Required, a fairly low bar designed to
avoid the need for standards work in cases where different ciphers
are needed for niche applications. There is a higher bar (Standards
Action) for ciphers to set the Recommended field in the registry.
This higher bar is there in part to ensure that the interoperability
implications of new ciphersuites are considered.
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MLS ciphersuites are defined independent of MLS versions, so that in
principle the same ciphersuite can be used across versions.
Standards work defining new versions of MLS should consider whether
it is desirable for the new version to be compatible with existing
ciphersuites, or whether the new version should rule out some
ciphersuites. For example, a new version could follow the example of
HTTP/2, which restricted the set of allowed TLS ciphers (see
Section 9.2.2 of [RFC7540].
14.2. Proposals
Commit messages do not have an extension field because the set of
protocols is extensible. As discussed in Section 13.2, Proposals
with a non-default proposal type MUST NOT be included in a commit
unless the proposal type is supported by all the members of the group
that will process the Commit.
14.3. Credential Extensibility
In order to ensure that MLS provides meaningful authentication it is
important that each member is able to authenticate some identity
information for each other member. Identity information is encoded
in Credentials, so this property is assured by ensuring that members
use compatible credential types.
The types of credential that may be used in a group is restricted to
what all members of the group support, as specified by the
capabilities field of each LeafNode in the RatchetTree. An
application can introduce new credential types by choosing an
unallocated identifier from the registry in Section 18.4 and
indicating support for the credential type in published LeafNodes,
whether in Update proposals to existing groups or KeyPackages that
are added to new groups. Once all members in a group indicate
support for the credential type, members can start using LeafNodes
with the new credential. Application may enforce that certain
credential types always remain supported by adding a
required_capabilities extension to the group's GroupContext, which
would prevent any member from being added to the group that doesn't
support them.
In future extensions to MLS, it may be useful to allow a member to
present more than one credential. For example, such credentials
might present different attributes attested by different authorities.
To be consistent with the general principle stated at the beginning
of this section, such an extension would need to ensure that each
member can authenticate some identity for each other member. For
each pair of members (Alice, Bob), Alice would need to present at
least one credential of a type that Bob supports. ## Extensions
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This protocol includes a mechanism for negotiating extension
parameters similar to the one in TLS [RFC8446]. In TLS, extension
negotiation is one-to-one: The client offers extensions in its
ClientHello message, and the server expresses its choices for the
session with extensions in its ServerHello and EncryptedExtensions
messages. In MLS, extensions appear in the following places:
* In KeyPackages, to describe additional information related to the
client
* In LeafNodes, to describe additional information about the client
or its participation in the group (once in the ratchet tree)
* In the GroupInfo, to tell new members of a group what parameters
are being used by the group, and to provide any additional details
required to join the group
* In the GroupContext object, to ensure that all members of the
group have the same view of the parameters in use
In other words, an application can use GroupContext extensions to
ensure that all members of the group agree on a set of parameters.
Clients indicate their support for parameters in the capabilities
field of their LeafNode. New members of a group are informed of the
group's GroupContext extensions via the group_context_extensions
field in the GroupInfo object. The other_extensions field in a
GroupInfo object can be used to provide additional parameters to new
joiners that are used to join the group.
This extension mechanism is designed to allow for secure and forward-
compatible negotiation of extensions. For this to work,
implementations MUST correctly handle extensible fields:
* A client that posts a KeyPackage MUST support all parameters
advertised in it. Otherwise, another client might fail to
interoperate by selecting one of those parameters.
* A client initiating a group MUST ignore all unrecognized
ciphersuites, extensions, and other parameters. Otherwise, it may
fail to interoperate with newer clients.
* A client adding a new member to a group MUST verify that the
LeafNode for the new member is compatible with the group's
extensions. The capabilities field MUST indicate support for each
extension in the GroupContext.
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* If any extension in a GroupInfo message is unrecognized (i.e., not
contained in the capabilities of the corresponding KeyPackage),
then the client MUST reject the Welcome message and not join the
group.
* The extensions populated into a GroupContext object are drawn from
those in the GroupInfo object, according to the definitions of
those extensions.
* Any field containing a list of extensions MUST NOT have more than
one extension of any given type.
Note that the latter two requirements mean that all MLS extensions
are mandatory, in the sense that an extension in use by the group
MUST be supported by all members of the group.
This document does not define any way for the parameters of the group
to change once it has been created; such a behavior could be
implemented as an extension.
15. Sequencing of State Changes
Each Commit message is premised on a given starting state, indicated
by the epoch field of the enclosing MLSMessageContent. If the
changes implied by a Commit messages are made starting from a
different state, the results will be incorrect.
This need for sequencing is not a problem as long as each time a
group member sends a Commit message, it is based on the most current
state of the group. In practice, however, there is a risk that two
members will generate Commit messages simultaneously, based on the
same state.
When this happens, there is a need for the members of the group to
deconflict the simultaneous Commit messages. There are two general
approaches:
* Have the Delivery Service enforce a total order
* Have a signal in the message that clients can use to break ties
As long as Commit messages cannot be merged, there is a risk of
starvation. In a sufficiently busy group, a given member may never
be able to send a Commit message, because he always loses to other
members. The degree to which this is a practical problem will depend
on the dynamics of the application.
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It might be possible, because of the non-contributivity of
intermediate nodes, that Commit messages could be applied one after
the other without the Delivery Service having to reject any Commit
message, which would make MLS more resilient regarding the
concurrency of Commit messages. The Messaging system can decide to
choose the order for applying the state changes. Note that there are
certain cases (if no total ordering is applied by the Delivery
Service) where the ordering is important for security, ie. all
updates must be executed before removes.
Regardless of how messages are kept in sequence, implementations MUST
only update their cryptographic state when valid Commit messages are
received. Generation of Commit messages MUST NOT modify a client's
state, since the endpoint doesn't know at that time whether the
changes implied by the Commit message will succeed or not.
15.1. Server-Enforced Ordering
With this approach, the Delivery Service ensures that incoming
messages are added to an ordered queue and outgoing messages are
dispatched in the same order. The server is trusted to break ties
when two members send a Commit message at the same time.
Messages should have a counter field sent in clear-text that can be
checked by the server and used for tie-breaking. The counter starts
at 0 and is incremented for every new incoming message. If two group
members send a message with the same counter, the first message to
arrive will be accepted by the server and the second one will be
rejected. The rejected message needs to be sent again with the
correct counter number.
To prevent counter manipulation by the server, the counter's
integrity can be ensured by including the counter in a signed message
envelope.
This applies to all messages, not only state changing messages.
15.2. Client-Enforced Ordering
Order enforcement can be implemented on the client as well, one way
to achieve it is to use a two step update protocol: the first client
sends a proposal to update and the proposal is accepted when it gets
50%+ approval from the rest of the group, then it sends the approved
update. Clients which didn't get their proposal accepted, will wait
for the winner to send their update before retrying new proposals.
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While this seems safer as it doesn't rely on the server, it is more
complex and harder to implement. It also could cause starvation for
some clients if they keep failing to get their proposal accepted.
16. Application Messages
The primary purpose of the Handshake protocol is to provide an
authenticated group key exchange to clients. In order to protect
Application messages sent among the members of a group, the
Application secret provided by the Handshake key schedule is used to
derive nonces and encryption keys for the Message Protection Layer
according to the Application Key Schedule. That is, each epoch is
equipped with a fresh Application Key Schedule which consist of a
tree of Application Secrets as well as one symmetric ratchet per
group member.
Each client maintains their own local copy of the Application Key
Schedule for each epoch during which they are a group member. They
derive new keys, nonces and secrets as needed while deleting old ones
as soon as they have been used.
Application messages MUST be protected with the Authenticated-
Encryption with Associated-Data (AEAD) encryption scheme associated
with the MLS ciphersuite using the common framing mechanism. Note
that "Authenticated" in this context does not mean messages are known
to be sent by a specific client but only from a legitimate member of
the group. To authenticate a message from a particular member,
signatures are required. Handshake messages MUST use asymmetric
signatures to strongly authenticate the sender of a message.
16.1. Message Encryption and Decryption
The group members MUST use the AEAD algorithm associated with the
negotiated MLS ciphersuite to AEAD encrypt and decrypt their
Application messages according to the Message Framing section.
The group identifier and epoch allow a recipient to know which group
secrets should be used and from which Epoch secret to start computing
other secrets and keys. The sender identifier is used to identify
the member's symmetric ratchet from the initial group Application
secret. The application generation field is used to determine how
far into the ratchet to iterate in order to reproduce the required
AEAD keys and nonce for performing decryption.
Application messages SHOULD be padded to provide some resistance
against traffic analysis techniques over encrypted traffic. [CLINIC]
[HCJ16] While MLS might deliver the same payload less frequently
across a lot of ciphertexts than traditional web servers, it might
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still provide the attacker enough information to mount an attack. If
Alice asks Bob: "When are we going to the movie ?" the answer
"Wednesday" might be leaked to an adversary by the ciphertext length.
An attacker expecting Alice to answer Bob with a day of the week
might find out the plaintext by correlation between the question and
the length.
The content and length of the padding field in MLSCiphertextContent
can be chosen at the time of message encryption by the sender. It is
recommended that padding data is comprised of zero-valued bytes and
follows an established deterministic padding scheme.
16.2. Restrictions
During each epoch senders MUST NOT encrypt more data than permitted
by the security bounds of the AEAD scheme used.
Note that each change to the Group through a Handshake message will
also set a new encryption_secret. Hence this change MUST be applied
before encrypting any new application message. This is required both
to ensure that any users removed from the group can no longer receive
messages and to (potentially) recover confidentiality and
authenticity for future messages despite a past state compromise.
16.3. Delayed and Reordered Application messages
Since each Application message contains the group identifier, the
epoch and a message counter, a client can receive messages out of
order. If they are able to retrieve or recompute the correct AEAD
decryption key from currently stored cryptographic material clients
can decrypt these messages.
For usability, MLS clients might be required to keep the AEAD key and
nonce for a certain amount of time to retain the ability to decrypt
delayed or out of order messages, possibly still in transit while a
decryption is being done.
17. Security Considerations
The security goals of MLS are described in
[I-D.ietf-mls-architecture]. We describe here how the protocol
achieves its goals at a high level, though a complete security
analysis is outside of the scope of this document.
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17.1. Confidentiality of the Group Secrets
Group secrets are partly derived from the output of a ratchet tree.
Ratchet trees work by assigning each member of the group to a leaf in
the tree and maintaining the following property: the private key of a
node in the tree is known only to members of the group that are
assigned a leaf in the node's subtree. This is called the _ratchet
tree invariant_ and it makes it possible to encrypt to all group
members except one, with a number of ciphertexts that's logarithmic
in the number of group members.
The ability to efficiently encrypt to all members except one allows
members to be securely removed from a group. It also allows a member
to rotate their keypair such that the old private key can no longer
be used to decrypt new messages.
17.2. Authentication
The first form of authentication we provide is that group members can
verify a message originated from one of the members of the group.
For encrypted messages, this is guaranteed because messages are
encrypted with an AEAD under a key derived from the group secrets.
For plaintext messages, this is guaranteed by the use of a
membership_tag which constitutes a MAC over the message, under a key
derived from the group secrets.
The second form of authentication is that group members can verify a
message originated from a particular member of the group. This is
guaranteed by a digital signature on each message from the sender's
signature key.
The signature keys held by group members are critical to the security
of MLS against active attacks. If a member's signature key is
compromised, then an attacker can create LeafNodes and KeyPackages
impersonating the member; depending on the application, this can then
allow the attacker to join the group with the compromised member's
identity. For example, if a group has enabled external parties to
join via external commits, then an attacker that has compromised a
member's signature key could use an external commit to insert
themselves into the group -- even using a "resync"-style external
commit to replace the compromised member in the group.
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Applications can mitigate the risks of signature key compromise using
pre-shared keys. If a group requires joiners to know a PSK in
addition to authenticating with a credential, then in order to mount
an impersonation attack, the attacker would need to compromise the
relevant PSK as well as the victim's signature key. The cost of this
mitigation is that the application needs some external arrangement
that ensures that the legitimate members of the group to have the
required PSKs.
17.3. Forward Secrecy and Post-Compromise Security
Post-compromise security is provided between epochs by members
regularly updating their leaf key in the ratchet tree. Updating
their leaf key prevents group secrets from continuing to be encrypted
to previously compromised public keys.
Forward-secrecy between epochs is provided by deleting private keys
from past version of the ratchet tree, as this prevents old group
secrets from being re-derived. Forward secrecy _within_ an epoch is
provided by deleting message encryption keys once they've been used
to encrypt or decrypt a message.
Post-compromise security is also provided for new groups by members
regularly generating new KeyPackages and uploading them to the
Delivery Service, such that compromised key material won't be used
when the member is added to a new group.
17.4. KeyPackage Reuse
KeyPackages are intended to be used only once. That is, once a
KeyPackage has been used to introduce the corresponding client to a
group, it SHOULD be deleted from the KeyPackage publication system.
Reuse of KeyPackages can lead to replay attacks.
An application MAY allow for reuse of a "last resort" KeyPackage in
order to prevent denial of service attacks. Since a KeyPackage is
needed to add a client to a new group, an attacker could prevent a
client being added to new groups by exhausting all available
KeyPackages. To prevent such a denial of service attack, the
KeyPackage publication system SHOULD rate limit KeyPackage requests,
especially if not authenticated.
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17.5. Group Fragmentation by Malicious Insiders
It is possible for a malicious member of a group to "fragment" the
group by crafting an invalid UpdatePath. Recall that an UpdatePath
encrypts a sequence of path secrets to different subtrees of the
group's ratchet trees. These path secrets should be derived in a
sequence as described in Section 8.4, but the UpdatePath syntax
allows the sender to encrypt arbitrary, unrelated secrets. The
syntax also does not guarantee that the encrypted path secret
encrypted for a given node corresponds to the public key provided for
that node.
Both of these types of corruption will cause processing of a Commit
to fail for some members of the group. If the public key for a node
does not match the path secret, then the members that decrypt that
path secret will reject the commit based on this mismatch. If the
path secret sequence is incorrect at some point, then members that
can decrypt nodes before that point will compute a different public
key for the mismatched node than the one in the UpdatePath, which
also causes the Commit to fail. Applications SHOULD provide
mechanisms for failed commits to be reported, so that group members
who were not able to recognize the error themselves can reject the
commit and roll back to a previous state if necessary.
Even with such an error reporting mechanism in place, however, it is
still possible for members to get locked out of the group by a
malformed commit. Since malformed Commits can only be recognized by
certain members of the group, in an asynchronous application, it may
be the case that all members that could detect a fault in a Commit
are offline. In such a case, the Commit will be accepted by the
group, and the resulting state possibly used as the basis for further
Commits. When the affected members come back online, they will
reject the first commit, and thus be unable to catch up with the
group.
Applications can address this risk by requiring certain members of
the group to acknowledge successful processing of a Commit before the
group regards the Commit as accepted. The minimum set of
acknowledgements necessary to verify that a Commit is well-formed
comprises an acknowledgement from one member per node in the
UpdatePath, that is, one member from each subtree rooted in the
copath node corresponding to the node in the UpdatePath.
18. IANA Considerations
This document requests the creation of the following new IANA
registries:
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* MLS Ciphersuites (Section 18.1)
* MLS Extension Types (Section 18.2)
* MLS Proposal Types (Section 18.3)
* MLS Credential Types (Section 18.4)
All of these registries should be under a heading of "Messaging Layer
Security", and assignments are made via the Specification Required
policy [RFC8126]. See Section 18.5 for additional information about
the MLS Designated Experts (DEs).
RFC EDITOR: Please replace XXXX throughout with the RFC number
assigned to this document
18.1. MLS Ciphersuites
A ciphersuite is a combination of a protocol version and the set of
cryptographic algorithms that should be used.
Ciphersuite names follow the naming convention:
CipherSuite MLS_LVL_KEM_AEAD_HASH_SIG = VALUE;
Where VALUE is represented as a sixteen-bit integer:
uint16 CipherSuite;
+===========+==================================+
| Component | Contents |
+===========+==================================+
| LVL | The security level |
+-----------+----------------------------------+
| KEM | The KEM algorithm used for HPKE |
| | in ratchet tree operations |
+-----------+----------------------------------+
| AEAD | The AEAD algorithm used for HPKE |
| | and message protection |
+-----------+----------------------------------+
| HASH | The hash algorithm used for HPKE |
| | and the MLS transcript hash |
+-----------+----------------------------------+
| SIG | The Signature algorithm used for |
| | message authentication |
+-----------+----------------------------------+
Table 4
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The columns in the registry are as follows:
* Value: The numeric value of the ciphersuite
* Name: The name of the ciphersuite
* Recommended: Whether support for this ciphersuite is recommended
by the IETF MLS WG. Valid values are "Y" and "N". The
"Recommended" column is assigned a value of "N" unless explicitly
requested, and adding a value with a "Recommended" value of "Y"
requires Standards Action [RFC8126]. IESG Approval is REQUIRED
for a Y->N transition.
* Reference: The document where this ciphersuite is defined
Initial contents:
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+======+=====================================================+=+====+
|Value | Name |R|Ref |
+======+=====================================================+=+====+
|0x0000| RESERVED |-|RFC |
| | | |XXXX|
+------+-----------------------------------------------------+-+----+
|0x0001| MLS_128_DHKEMX25519_AES128GCM_SHA256_Ed25519 |Y|RFC |
| | | |XXXX|
+------+-----------------------------------------------------+-+----+
|0x0002| MLS_128_DHKEMP256_AES128GCM_SHA256_P256 |Y|RFC |
| | | |XXXX|
+------+-----------------------------------------------------+-+----+
|0x0003| MLS_128_DHKEMX25519_CHACHA20POLY1305_SHA256_Ed25519 |Y|RFC |
| | | |XXXX|
+------+-----------------------------------------------------+-+----+
|0x0004| MLS_256_DHKEMX448_AES256GCM_SHA512_Ed448 |Y|RFC |
| | | |XXXX|
+------+-----------------------------------------------------+-+----+
|0x0005| MLS_256_DHKEMP521_AES256GCM_SHA512_P521 |Y|RFC |
| | | |XXXX|
+------+-----------------------------------------------------+-+----+
|0x0006| MLS_256_DHKEMX448_CHACHA20POLY1305_SHA512_Ed448 |Y|RFC |
| | | |XXXX|
+------+-----------------------------------------------------+-+----+
|0x0007| MLS_256_DHKEMP384_AES256GCM_SHA384_P384. |Y|RFC |
| | | |XXXX|
+------+-----------------------------------------------------+-+----+
|0xff00| Reserved for Private Use |-|RFC |
|- | | |XXXX|
|0xffff| | | |
+------+-----------------------------------------------------+-+----+
Table 5
All of these ciphersuites use HMAC [RFC2104] as their MAC function,
with different hashes per ciphersuite. The mapping of ciphersuites
to HPKE primitives, HMAC hash functions, and TLS signature schemes is
as follows [RFC9180] [RFC8446]:
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+======+========+========+========+========+========================+
|Value | KEM | KDF | AEAD | Hash | Signature |
+======+========+========+========+========+========================+
|0x0001| 0x0020 | 0x0001 | 0x0001 | SHA256 | ed25519 |
+------+--------+--------+--------+--------+------------------------+
|0x0002| 0x0010 | 0x0001 | 0x0001 | SHA256 | ecdsa_secp256r1_sha256 |
+------+--------+--------+--------+--------+------------------------+
|0x0003| 0x0020 | 0x0001 | 0x0003 | SHA256 | ed25519 |
+------+--------+--------+--------+--------+------------------------+
|0x0004| 0x0021 | 0x0003 | 0x0002 | SHA512 | ed448 |
+------+--------+--------+--------+--------+------------------------+
|0x0005| 0x0012 | 0x0003 | 0x0002 | SHA512 | ecdsa_secp521r1_sha512 |
+------+--------+--------+--------+--------+------------------------+
|0x0006| 0x0021 | 0x0003 | 0x0003 | SHA512 | ed448 |
+------+--------+--------+--------+--------+------------------------+
|0x0007| 0x0011 | 0x0002 | 0x0002 | SHA384 | ecdsa_secp384r1_sha384 |
+------+--------+--------+--------+--------+------------------------+
Table 6
The hash used for the MLS transcript hash is the one referenced in
the ciphersuite name. In the ciphersuites defined above, "SHA256",
"SHA384", and "SHA512" refer to the SHA-256, SHA-384, and SHA-512
functions defined in [SHS].
It is advisable to keep the number of ciphersuites low to increase
the chances clients can interoperate in a federated environment,
therefore the ciphersuites only include modern, yet well-established
algorithms. Depending on their requirements, clients can choose
between two security levels (roughly 128-bit and 256-bit). Within
the security levels clients can choose between faster X25519/X448
curves and FIPS 140-2 compliant curves for Diffie-Hellman key
negotiations. Additionally clients that run predominantly on mobile
processors can choose ChaCha20Poly1305 over AES-GCM for performance
reasons. Since ChaCha20Poly1305 is not listed by FIPS 140-2 it is
not paired with FIPS 140-2 compliant curves. The security level of
symmetric encryption algorithms and hash functions is paired with the
security level of the curves.
The mandatory-to-implement ciphersuite for MLS 1.0 is
MLS_128_DHKEMX25519_AES128GCM_SHA256_Ed25519 which uses Curve25519
for key exchange, AES-128-GCM for HPKE, HKDF over SHA2-256, and
Ed25519 for signatures.
Values with the first byte 255 (decimal) are reserved for Private
Use.
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New ciphersuite values are assigned by IANA as described in
Section 18.
18.2. MLS Extension Types
This registry lists identifiers for extensions to the MLS protocol.
The extension type field is two bytes wide, so valid extension type
values are in the range 0x0000 to 0xffff.
Template:
* Value: The numeric value of the extension type
* Name: The name of the extension type
* Message(s): The messages in which the extension may appear, drawn
from the following list:
- KP: KeyPackage objects
- LN: LeafNode objects
- GC: GroupContext objects (and the group_context_extensions
field of GroupInfo objects)
- GI: The other_extensions field of GroupInfo objects
* Recommended: Whether support for this extension is recommended by
the IETF MLS WG. Valid values are "Y" and "N". The "Recommended"
column is assigned a value of "N" unless explicitly requested, and
adding a value with a "Recommended" value of "Y" requires
Standards Action [RFC8126]. IESG Approval is REQUIRED for a Y->N
transition.
* Reference: The document where this extension is defined
Initial contents:
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+======+=======================+============+=============+=========+
|Value | Name | Message(s) | Recommended |Reference|
+======+=======================+============+=============+=========+
|0x0000| RESERVED | N/A | N/A |RFC XXXX |
+------+-----------------------+------------+-------------+---------+
|0x0001| external_key_id | KP | Y |RFC XXXX |
+------+-----------------------+------------+-------------+---------+
|0x0002| ratchet_tree | GI | Y |RFC XXXX |
+------+-----------------------+------------+-------------+---------+
|0x0003| required_capabilities | GC | Y |RFC XXXX |
+------+-----------------------+------------+-------------+---------+
|0x0004| external_pub | GI | Y |RFC XXXX |
+------+-----------------------+------------+-------------+---------+
|0xff00| Reserved for Private | N/A | N/A |RFC XXXX |
|- | Use | | | |
|0xffff| | | | |
+------+-----------------------+------------+-------------+---------+
Table 7
18.3. MLS Proposal Types
This registry lists identifiers for types of proposals that can be
made for changes to an MLS group. The extension type field is two
bytes wide, so valid extension type values are in the range 0x0000 to
0xffff.
Template:
* Value: The numeric value of the proposal type
* Name: The name of the proposal type
* Recommended: Whether support for this extension is recommended by
the IETF MLS WG. Valid values are "Y" and "N". The "Recommended"
column is assigned a value of "N" unless explicitly requested, and
adding a value with a "Recommended" value of "Y" requires
Standards Action [RFC8126]. IESG Approval is REQUIRED for a Y->N
transition.
* Path Required: Whether a Commit covering a proposal of this type
is required to have its path field populated (see Section 13.2).
* Reference: The document where this extension is defined
Initial contents:
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+======+==========================+=============+========+==========+
|Value | Name | Recommended |Path |Reference |
| | | |Required| |
+======+==========================+=============+========+==========+
|0x0000| RESERVED | N/A |N/A |RFC XXXX |
+------+--------------------------+-------------+--------+----------+
|0x0001| add | Y |N |RFC XXXX |
+------+--------------------------+-------------+--------+----------+
|0x0002| update | Y |Y |RFC XXXX |
+------+--------------------------+-------------+--------+----------+
|0x0003| remove | Y |Y |RFC XXXX |
+------+--------------------------+-------------+--------+----------+
|0x0004| psk | Y |N |RFC XXXX |
+------+--------------------------+-------------+--------+----------+
|0x0005| reinit | Y |N |RFC XXXX |
+------+--------------------------+-------------+--------+----------+
|0x0006| external_init | Y |Y |RFC XXXX |
+------+--------------------------+-------------+--------+----------+
|0x0007| app_ack | Y |N |RFC XXXX |
+------+--------------------------+-------------+--------+----------+
|0x0008| group_context_extensions | Y |Y |RFC XXXX |
+------+--------------------------+-------------+--------+----------+
|0xff00| Reserved for Private Use | N/A |N/A |RFC XXXX |
|- | | | | |
|0xffff| | | | |
+------+--------------------------+-------------+--------+----------+
Table 8
18.4. MLS Credential Types
This registry lists identifiers for types of credentials that can be
used for authentication in the MLS protocol. The credential type
field is two bytes wide, so valid credential type values are in the
range 0x0000 to 0xffff.
Template:
* Value: The numeric value of the credential type
* Name: The name of the credential type
* Recommended: Whether support for this credential is recommended by
the IETF MLS WG. Valid values are "Y" and "N". The "Recommended"
column is assigned a value of "N" unless explicitly requested, and
adding a value with a "Recommended" value of "Y" requires
Standards Action [RFC8126]. IESG Approval is REQUIRED for a Y->N
transition.
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* Reference: The document where this credential is defined
Initial contents:
+=================+==============+=============+===========+
| Value | Name | Recommended | Reference |
+=================+==============+=============+===========+
| 0x0000 | RESERVED | N/A | RFC XXXX |
+-----------------+--------------+-------------+-----------+
| 0x0001 | basic | Y | RFC XXXX |
+-----------------+--------------+-------------+-----------+
| 0x0002 | x509 | Y | RFC XXXX |
+-----------------+--------------+-------------+-----------+
| 0xff00 - 0xffff | Reserved for | N/A | RFC XXXX |
| | Private Use | | |
+-----------------+--------------+-------------+-----------+
Table 9
18.5. MLS Designated Expert Pool
Specification Required [RFC8126] registry requests are registered
after a three-week review period on the MLS DEs' mailing list: mls-
reg-review@ietf.org (mailto:mls-reg-review@ietf.org), on the advice
of one or more of the MLS DEs. However, to allow for the allocation
of values prior to publication, the MLS DEs may approve registration
once they are satisfied that such a specification will be published.
Registration requests sent to the MLS DEs mailing list for review
SHOULD use an appropriate subject (e.g., "Request to register value
in MLS Bar registry").
Within the review period, the MLS DEs will either approve or deny the
registration request, communicating this decision to the MLS DEs
mailing list and IANA. Denials SHOULD include an explanation and, if
applicable, suggestions as to how to make the request successful.
Registration requests that are undetermined for a period longer than
21 days can be brought to the IESG's attention for resolution using
the iesg@ietf.org (mailto:iesg@ietf.org) mailing list.
Criteria that SHOULD be applied by the MLS DEs includes determining
whether the proposed registration duplicates existing functionality,
whether it is likely to be of general applicability or useful only
for a single application, and whether the registration description is
clear. For example, the MLS DEs will apply the ciphersuite-related
advisory found in Section 6.1.
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IANA MUST only accept registry updates from the MLS DEs and SHOULD
direct all requests for registration to the MLS DEs' mailing list.
It is suggested that multiple MLS DEs be appointed who are able to
represent the perspectives of different applications using this
specification, in order to enable broadly informed review of
registration decisions. In cases where a registration decision could
be perceived as creating a conflict of interest for a particular MLS
DE, that MLS DE SHOULD defer to the judgment of the other MLS DEs.
18.6. The "message/mls" MIME Type
This document registers the "message/mls" MIME media type in order to
allow other protocols (ex: HTTP [RFC7540]) to convey MLS messages.
Media type name: message
Media subtype name: mls
Required parameters: none
Optional parameters: version
version: The MLS protocol version expressed as
a string <major>.<minor>. If omitted the version is "1.0",
which corresponds to MLS ProtocolVersion mls10. If for some
reason the version number in the MIME type parameter differs
from the ProtocolVersion embedded in the protocol, the protocol
takes precedence.
Encoding scheme: MLS messages are represented using the TLS
presentation language [RFC8446]. Therefore MLS messages need to
be treated as binary data.
Security considerations: MLS is an encrypted messaging layer
designed to be transmitted over arbitrary lower layer protocols.
The security considerations in this document (RFC XXXX) also
apply.
19. Contributors
* Joel Alwen
Wickr
joel.alwen@wickr.com
* Karthikeyan Bhargavan
INRIA
karthikeyan.bhargavan@inria.fr
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* Cas Cremers
University of Oxford
cremers@cispa.de
* Alan Duric
Wire
alan@wire.com
* Britta Hale
Naval Postgraduate School
britta.hale@nps.edu
* Srinivas Inguva
Twitter
singuva@twitter.com
* Konrad Kohbrok
Aalto University
konrad.kohbrok@datashrine.de
* Albert Kwon
MIT
kwonal@mit.edu
* Brendan McMillion
Cloudflare
brendan@cloudflare.com
* Eric Rescorla
Mozilla
ekr@rtfm.com
* Michael Rosenberg
Trail of Bits
michael.rosenberg@trailofbits.com
* Thyla van der Merwe
Royal Holloway, University of London
thyla.van.der@merwe.tech
20. References
20.1. Normative References
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
DOI 10.17487/RFC2104, February 1997,
<https://www.rfc-editor.org/rfc/rfc2104>.
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[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>.
[RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
DOI 10.17487/RFC7540, May 2015,
<https://www.rfc-editor.org/rfc/rfc7540>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/rfc/rfc8126>.
[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>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<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,
February 2022, <https://www.rfc-editor.org/rfc/rfc9180>.
20.2. Informative References
[art] Cohn-Gordon, K., Cremers, C., Garratt, L., Millican, J.,
and K. Milner, "On Ends-to-Ends Encryption: Asynchronous
Group Messaging with Strong Security Guarantees", 18
January 2018, <https://eprint.iacr.org/2017/666.pdf>.
[CLINIC] Miller, B., Huang, L., Joseph, A., and J. Tygar, "I Know
Why You Went to the Clinic: Risks and Realization of HTTPS
Traffic Analysis", Privacy Enhancing Technologies pp.
143-163, DOI 10.1007/978-3-319-08506-7_8, 2014,
<https://doi.org/10.1007/978-3-319-08506-7_8>.
[doubleratchet]
Cohn-Gordon, K., Cremers, C., Dowling, B., Garratt, L.,
and D. Stebila, "A Formal Security Analysis of the Signal
Messaging Protocol", 2017 IEEE European Symposium on
Security and Privacy (EuroS&P),
DOI 10.1109/eurosp.2017.27, April 2017,
<https://doi.org/10.1109/eurosp.2017.27>.
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[HCJ16] Husák, M., Čermák, M., Jirsík, T., and P. Čeleda, "HTTPS
traffic analysis and client identification using passive
SSL/TLS fingerprinting", EURASIP Journal on Information
Security Vol. 2016, DOI 10.1186/s13635-016-0030-7,
February 2016,
<https://doi.org/10.1186/s13635-016-0030-7>.
[I-D.ietf-mls-architecture]
Beurdouche, B., Rescorla, E., Omara, E., Inguva, S., Kwon,
A., and A. Duric, "The Messaging Layer Security (MLS)
Architecture", Work in Progress, Internet-Draft, draft-
ietf-mls-architecture-07, 4 October 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-mls-
architecture-07>.
[I-D.ietf-trans-rfc6962-bis]
Laurie, B., Langley, A., Kasper, E., Messeri, E., and R.
Stradling, "Certificate Transparency Version 2.0", Work in
Progress, Internet-Draft, draft-ietf-trans-rfc6962-bis-42,
31 August 2021, <https://datatracker.ietf.org/doc/html/
draft-ietf-trans-rfc6962-bis-42>.
[RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated
Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
<https://www.rfc-editor.org/rfc/rfc5116>.
[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
2011, <https://www.rfc-editor.org/rfc/rfc6125>.
[RFC7696] Housley, R., "Guidelines for Cryptographic Algorithm
Agility and Selecting Mandatory-to-Implement Algorithms",
BCP 201, RFC 7696, DOI 10.17487/RFC7696, November 2015,
<https://www.rfc-editor.org/rfc/rfc7696>.
[RFC8032] Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
Signature Algorithm (EdDSA)", RFC 8032,
DOI 10.17487/RFC8032, January 2017,
<https://www.rfc-editor.org/rfc/rfc8032>.
[RFC9000] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", RFC 9000,
DOI 10.17487/RFC9000, May 2021,
<https://www.rfc-editor.org/rfc/rfc9000>.
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[SECG] "Elliptic Curve Cryptography, Standards for Efficient
Cryptography Group, ver. 2", 2009,
<https://secg.org/sec1-v2.pdf>.
[SHS] Dang, Q., "Secure Hash Standard", National Institute of
Standards and Technology report,
DOI 10.6028/nist.fips.180-4, July 2015,
<https://doi.org/10.6028/nist.fips.180-4>.
[signal] Perrin(ed), T. and M. Marlinspike, "The Double Ratchet
Algorithm", 20 November 2016,
<https://www.signal.org/docs/specifications/
doubleratchet/>.
Appendix A. Protocol Origins of Example Trees
Protocol operations in MLS give rise to specific forms of ratchet
tree, typically affecting a whole direct path at once. In this
section, we describe the protocol operations that could have given
rise to the various example trees in this document.
To construct the tree in Figure 9:
* A creates a group with B, ..., G
* Each member sends an empty Commit setting its direct path
To construct the tree in Figure 10:
* A creates a group with B, C, D, as well as some members outside
this subtree
* D removes C, setting Z and the top node (as well as any further
nodes in the direct path)
* A member outside this subtree removes B, blanking B's direct path
* A adds a new member at C with a partial Commit, adding it as
unmerged at Z
To construct the tree in Figure 11:
* A creates a group with B, C, D
* B sends a full Commit, setting X and Y
* D removes C, setting Z and Y
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* B adds a new member at C with a full Commit
- The Add proposal adds C as unmerged at Z and Y
- The path in the Commit resets X and Y, clearing Y's unmerged
leaves
To construct the tree in Figure 12:
* A creates a group with B, ..., G
* A removes F in a full Commit, setting T, U, and W
* E sends an empty Commit, setting Y and W
* A adds a new member at F in a partial Commit, adding F as unmerged
at Y and W
Appendix B. Array-Based Trees
One benefit of using left-balanced trees is that they admit a simple
flat array representation. In this representation, leaf nodes are
even-numbered nodes, with the n-th leaf at 2*n. Intermediate nodes
are held in odd-numbered nodes. For example, tree with 11 leaves has
the following structure:
X
|
.---------------------+---------.
/ \
X |
| |
.---------+---------. |
/ \ |
X X X
| | |
.---+---. .---+---. .---+.
/ \ / \ / \
X X X X X |
/ \ / \ / \ / \ / \ |
/ \ / \ / \ / \ / \ |
X X X X X X X X X X X
Node: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Leaf: 0 1 2 3 4 5 6 7 8 9 10
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This allows us to compute relationships between tree nodes simply by
manipulating indices, rather than having to maintain complicated
structures in memory. The basic rule is that the high-order bits of
parent and child nodes indices have the following relation (where x
is an arbitrary bit string):
parent=01x => left=00x, right=10x
Since node relationships are implicit, the algorithms for adding and
removing nodes at the right edge of the tree are quite simple:
* Add: Append a blank parent node to the array of nodes, then append
the new leaf node
* Remove: Remove the rightmost two nodes from the array of nodes
The following python code demonstrates the tree computations
necessary to use an array-based tree for MLS.
# The exponent of the largest power of 2 less than x. Equivalent to:
# int(math.floor(math.log(x, 2)))
def log2(x):
if x == 0:
return 0
k = 0
while (x >> k) > 0:
k += 1
return k-1
# The level of a node in the tree. Leaves are level 0, their parents
# are level 1, etc. If a node's children are at different levels,
# then its level is the max level of its children plus one.
def level(x):
if x & 0x01 == 0:
return 0
k = 0
while ((x >> k) & 0x01) == 1:
k += 1
return k
# The number of nodes needed to represent a tree with n leaves.
def node_width(n):
if n == 0:
return 0
else:
return 2*(n - 1) + 1
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# The index of the root node of a tree with n leaves.
def root(n):
w = node_width(n)
return (1 << log2(w)) - 1
# The left child of an intermediate node. Note that because the tree
# is left-balanced, there is no dependency on the size of the tree.
def left(x):
k = level(x)
if k == 0:
raise Exception('leaf node has no children')
return x ^ (0x01 << (k - 1))
# The right child of an intermediate node. Depends on the number of
# leaves because the straightforward calculation can take you beyond
# the edge of the tree.
def right(x, n):
k = level(x)
if k == 0:
raise Exception('leaf node has no children')
r = x ^ (0x03 << (k - 1))
while r >= node_width(n):
r = left(r)
return r
# The immediate parent of a node. May be beyond the right edge of the
# tree.
def parent_step(x):
k = level(x)
b = (x >> (k + 1)) & 0x01
return (x | (1 << k)) ^ (b << (k + 1))
# The parent of a node. As with the right child calculation, we have
# to walk back until the parent is within the range of the tree.
def parent(x, n):
if x == root(n):
raise Exception('root node has no parent')
p = parent_step(x)
while p >= node_width(n):
p = parent_step(p)
return p
# The other child of the node's parent.
def sibling(x, n):
p = parent(x, n)
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if x < p:
return right(p, n)
else:
return left(p)
# The direct path of a node, ordered from leaf to root.
def direct_path(x, n):
r = root(n)
if x == r:
return []
d = []
while x != r:
x = parent(x, n)
d.append(x)
return d
# The copath of a node, ordered from leaf to root.
def copath(x, n):
if x == root(n):
return []
d = direct_path(x, n)
d.insert(0, x)
d.pop()
return [sibling(y, n) for y in d]
# The common ancestor of two nodes is the lowest node that is in the
# direct paths of both leaves.
def common_ancestor_semantic(x, y, n):
dx = set([x]) | set(direct_path(x, n))
dy = set([y]) | set(direct_path(y, n))
dxy = dx & dy
if len(dxy) == 0:
raise Exception('failed to find common ancestor')
return min(dxy, key=level)
# The common ancestor of two nodes is the lowest node that is in the
# direct paths of both leaves.
def common_ancestor_direct(x, y, _):
# Handle cases where one is an ancestor of the other
lx, ly = level(x)+1, level(y)+1
if (lx <= ly) and (x>>ly == y>>ly):
return y
elif (ly <= lx) and (x>>lx == y>>lx):
return x
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# Handle other cases
xn, yn = x, y
k = 0
while xn != yn:
xn, yn = xn >> 1, yn >> 1
k += 1
return (xn << k) + (1 << (k-1)) - 1
Appendix C. Link-Based Trees
An implementation may choose to store ratchet trees in a "link-based"
representation, where each node stores references to its parents and/
or children. (As opposed to the array-based representation suggested
above, where these relationships are computed from relationships
between nodes' indices in the array.) Such an implementation needs
to update these links to maintain the left-balanced structure of the
tree as the tree is extended to add new members, or truncated when
memebers are removed.
The following code snippet shows how these algorithms could be
implemented in Python.
class Node:
def __init__(self, value, parent=None, left=None, right=None):
self.value = value # Value of the node
self.parent = parent # Parent node
self.left = left # Left child node
self.right = right # Right child node
def leaf(self):
return self.left == None and self.right == None
def span(self):
if self.leaf():
return 1
return self.left.span() + self.right.span()
def full(self):
span = self.span()
while span % 2 == 0:
span >>= 1
return span == 1
def rightmost_leaf(self):
X = self
while X.right != None:
X = X.right
return X
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class Tree:
def __init__(self):
self.root = None # Root node of the tree, initially empty
def extend(self, N):
if self.root == None:
self.root = N
return
# Identify the proper point to insert the new parent node
X = self.root.rightmost_leaf()
while X.full() and X != self.root:
X = X.parent
# If X is not full, insert the new parent under X
P = Node("_", right=N)
N.parent = P
if not X.full():
P.parent = X
P.left = X.right
X.right.parent = P
X.right = P
return
# If X is full, then X is the root, so P replaces the root
P.left = self.root
self.root.parent = P
self.root = P
return
def truncate(self):
X = self.root.rightmost_leaf()
if X == self.root:
self.root = None
return
# If X's parent is the root, then shift the root to the left
if X.parent == self.root:
self.root = self.root.left
self.root.parent = None
return
# Otherwise, reassign the right child of the parent's parent
Q = X.parent.parent
Q.right = X.parent.left
Q.right.parent = Q
return
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Authors' Addresses
Richard Barnes
Cisco
Email: rlb@ipv.sx
Benjamin Beurdouche
Inria & Mozilla
Email: ietf@beurdouche.com
Raphael Robert
Email: ietf@raphaelrobert.com
Jon Millican
Facebook
Email: jmillican@fb.com
Emad Omara
Google
Email: emadomara@google.com
Katriel Cohn-Gordon
University of Oxford
Email: me@katriel.co.uk
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