Member-only Secrets in the Messaging Layer Security Protocol

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Table of Contents

1. Introduction

This is a Work in Progress mechanism to keep some secrets with respect to non-members, including from the MLS Distribution Service (DS).¶

Realted work on a similar topic includes Minimal Metadata Rooms (MMR) [MMR].¶

2. Conventions and Definitions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶

In MLS, the term Directory Server (DS) refers to...¶

3. Mechanisms

One key per group (MMR) in PR #102¶

Use a separate MLS group using PrivateMessage to generate secrets¶

struct {

} EphemeralSecret

EphemeralSecret EphemeralSecretsData<V>;

struct {

} EphemeralSecretsUpdate;

4. Member-only disclosures

This document defines two new MLS application components to facilitate sharing secrets with members, that are hidden from the DS. It defines a new MLS key schedule Exporter Label, member_identity_disclosure_secret.¶

These two application components provide a way to efficiently update encryption secrets inside LeafNode and GroupContext objects, when needed to achieve privacy from the DS, including former members colluding with the DS.¶

During any epoch when a disclosure is added or updated, the new (added or updated) ephemeral keys are encrypted using the member_identity_disclosure_secret for the epoch about to be committed. Also the member_identity_disclosure_secrets for any previous epochs that are still being used to encrypt individual credentials are encrypted using the target epoch member_identity_disclosure_secret in the OldEpochMemberEncryptionKeys struct.¶

struct {
    opaque bytes<V>;
} BlindedClaimHash;

struct {
    uint32 leaf_index;
    BlindedClaimHash blinded_claim_hash;
    uint64 encrypted_during_epoch;
} PerDisclosureEpochEncryptor;

struct {
    PerDisclosureEpochEncryptor per_disclosure_epoch_map<V>;
} PerDisclosureEncryptionMap;

PerDisclosureEncryptionMap per_disclosure_encryption_map;

struct {
    BlindedClaimHash removed_disclosures<V>;
    PerDisclosureEpochEncryptor updated_disclosures<V>;
} PerDisclosureEncryptionMapUpdate;

PerDisclosureEncryptionMapUpdate per_disclosure_encryption_map_update;

struct {
    uint64 epoch;
    opaque encrypted_member_secret<V>;
} OldEpochEncryptionKey;

struct {
    OldEpochEncryptionKey old_epoch_encyption_keys<V>;
} OldEpochMemberEncryptionKeys;

OldEpochMemberEncryptionKeys old_epoch_member_encryption;
OldEpochMemberEncryptionKeys old_epoch_member_encryption_update;

5. Generic form of EncryptWithLabel/DecryptWithLabel

To possibly modify later with the SafeEncryptWithLabel. ???¶

encrypted_disclosure = EncryptWithLabel(public_key, label, context, plaintext)

disclosure = DecryptWithLabel(private_key, label, context, kem_output, ciphertext)

6. Security Considerations

This mechanism could¶

If the same signature keys are used across groups, a former member colluding¶

7. IANA Considerations

This document has no IANA actions.¶

8. References

Acknowledgments

TODO acknowledge.¶

Author's Address