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ETH-MLS-OFFCHAIN

Abstract​

The following document specifies Ethereum authenticated scalable and decentralized secure group messaging application by integrating Message Layer Security (MLS) backend. Decentralization refers each user is a node in P2P network and each user has voice for any changes in group. This is achieved by integrating a consensus mechanism. Lastly, this RFC can also be referred to as de-MLS, decentralized MLS, to emphasize its deviation from the centralized trust assumptions of traditional MLS deployments.

Motivation​

Group messaging is a fundamental part of digital communication, yet most existing systems depend on centralized servers, which introduce risks around privacy, censorship, and unilateral control. In restrictive settings, servers can be blocked or surveilled; in more open environments, users still face opaque moderation policies, data collection, and exclusion from decision-making processes. To address this, we propose a decentralized, scalable peer-to-peer group messaging system where each participant runs a node, contributes to message propagation, and takes part in governance autonomously. Group membership changes are decided collectively through a lightweight partially synchronous, fault-tolerant consensus protocol without a centralized identity. This design enables truly democratic group communication and is well-suited for use cases like activist collectives, research collaborations, DAOs, support groups, and decentralized social platforms.

Format Specification​

The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in 2119.

Assumptions​

  • The nodes in the P2P network can discover other nodes or will connect to other nodes when subscribing to same topic in a gossipsub.
  • We MAY have non-reliable (silent) nodes.
  • We MUST have a consensus that is lightweight, scalable and finalized in a specific time.

Roles​

The three roles used in de-MLS is as follows:

  • node: Nodes are members of network without being in any secure group messaging.
  • member: Members are special nodes in the secure group messaging who obtains current group key of secure group messaging.
  • steward: Stewards are special and transparent members in secure group messaging who organizes the changes upon the voted-proposals.

MLS Background​

The de-MLS consists of MLS backend, so the MLS services and other MLS components are taken from the original MLS specification, with or without modifications.

MLS Services​

MLS is operated in two services authentication service (AS) and delivery service (DS). Authentication service enables group members to authenticate the credentials presented by other group members. The delivery service routes MLS messages among the nodes or members in the protocol in the correct order and manage the keyPackage of the users where the keyPackage is the objects that provide some public information about a user.

MLS Objects​

Following section presents the MLS objects and components that used in this RFC:

Epoch: Fixed time intervals that changes the state that is defined by members, section 3.4 in MLS RFC 9420.

MLS proposal message: Members MUST receive the proposal message prior to the corresponding commit message that initiates a new epoch with key changes, in order to ensure the intended security properties, section 12.1 in MLS RFC 9420. Here, the add and remove proposals are used.

Application message: This message type used in arbitrary encrypted communication between group members. This is restricted by MLS RFC 9420 as if there is pending proposal, the application message should be cut. Note that: Since the MLS is based on servers, this delay between proposal and commit messages are very small. Commit message: After members receive the proposals regarding group changes, the committer, who may be any member of the group, as specified in MLS RFC 9420, generates the necessary key material for the next epoch, including the appropriate welcome messages for new joiners and new entropy for removed members. In this RFC, the committers only MUST be stewards.

de-MLS Objects​

Voting proposal Similar to MLS proposals, but processed only if approved through a voting process. They function as application messages in the MLS group, allowing the steward to collect them without halting the protocol.

Flow​

General flow is as follows:

  • A steward initializes a group just once, and then sends out Group Announcements (GA) periodically.

  • Meanwhile, eachnodecreates and sends theircredential includes keyPackage.

  • Each membercreates voting proposals sends them to from MLS group during epoch E.

  • Meanwhile, the steward collects finalized voting proposals from MLS group and converts them into MLS proposals then sends them with correspondng commit messages

  • Evantually, with the commit messages, all members starts the next epoch E+1.

Creating Voting Proposal​

A member MAY initializes the voting with the proposal payload which is implemented using protocol buffers v3 as follows:


syntax = "proto3";

message Proposal {
string name = 10;                 // Proposal name
string payload = 11;              // Describes the what is voting fore 
int32 proposal_id = 12;           // Unique identifier of the proposal
bytes proposal_owner = 13;        // Public key of the creator
repeated Vote votes = 14;         // Vote list in the proposal
int32 expected_voters_count = 15; // Maximum number of distinct voters
int32 round = 16;                 // Number of Votes
int64 timestamp = 17;             // Creation time of proposal
int64 expiration_time = 18;       // Time interval that the proposal is active
bool liveness_criteria_yes = 19;  // Shows how managing the silent peers vote
}
message Vote {
int32 vote_id = 20;             // Unique identifier of the vote
bytes vote_owner = 21;          // Voter's public key
int64 timestamp = 22;           // Time when the vote was cast
bool vote = 23;                 // Vote bool value (true/false)
bytes parent_hash = 24;         // Hash of previous owner's Vote
bytes received_hash = 25;       // Hash of previous received Vote
bytes vote_hash = 26;           // Hash of all previously defined fields in Vote
bytes signature = 27;           // Signature of vote_hash
}

The voting proposal MAY include adding a node or removing a member. After the member creates the voting proposal, it is emitted to the network via the MLS Application message with a lightweight, epoch based voting such as hashgraphlike consensus. This consensus result MUST be finalized within the epoch as YES or NO.

If the voting result is YES, this points out the voting proposal will be converted into the MLS proposal by the steward and following commit message that starts the new epoch.

Creating welcome message​

When a MLS MLS proposal message is created by the steward, a commit message SHOULD follow, as in section 12.04 MLS RFC 9420 to the members. In order for the new member joining the group to synchronize with the current members who received the commit message, the steward sends a welcome message to the node as the new member, as in section 12.4.3.1. MLS RFC 9420.

Single steward​

To naive way to create a decentralized secure group messaging is having a single transparent steward who only applies the changes regarding the result of the voting.

This is mostly similar with the general flow and specified in voting proposal and welcome message creation sections.

  1. Each time a single steward initializes a group with group parameters with parameters as in section 8.1. Group Context in MLS RFC 9420.
  2. steward creates a group anouncement (GA) according to the previous step and broadcast it to the all network periodically. GA message is visible in network to all nodes.
  3. The each node who wants to be a member needs to obtain this anouncement and create credential includes keyPackage that is specified in MLS RFC 9420 section 10.
  4. The steward aggregates all KeyPackages utilizes them to provision group additions for new members, based on the outcome of the voting process.
  5. Any member start to create voting proposals for adding or removing users, and present them to the voting in the MLS group as an application message.

However, unlimited use of voting proposals within the group may be misused by malicious or overly active members. Therefore, an application-level constraint can be introduced to limit the number or frequency of proposals initiated by each member to prevent spam or abuse. 6. Meanwhile, the steward collects finalized voting proposals with in epoch E, that have received affirmative votes from members via application messages. Otherwise, the steward discards proposals that did not receive a majority of "YES" votes. Since voting proposals are transmitted as application messages, omitting them does not affect the protocol’s correctness or consistency. 7. The steward converts all approved voting proposals into corresponding MLS proposals and commit message, and transmits both in a single operation as in MLS RFC 9420 section 12.4, including welcome messages for the new members. Therefore, the commit message ends the previous epoch and create new ones.

Multi stewards​

Decentralization has already been achieved in the previous section. However, to improve availability and ensure censorship resistance, the single-steward protocol is extended to a multi-steward architecture. In this design, each epoch is coordinated by a designated steward, operating under the same protocol as the single-steward model. Thus, the multi-steward approach primarily defines how steward roles rotate across epochs while preserving the underlying structure and logic of the original protocol. Two variants of the multi-steward design are introduced to address different system requirements.

Multi steward with single consensus​

In this model, all group modifications, such as adding or removing members, must be approved through consensus by all participants, including the steward assigned for epoch E. A configuration with multiple stewards operating under a shared consensus protocol offers increased decentralization and stronger protection against censorship. However, this benefit comes with reduced operational efficiency. The model is therefore best suited for small groups that value decentralization and censorship resistance more than performance.

Multi steward with two consensuses​

The two-consensus model offers improved efficiency with a trade-off in decentralization. In this design, group changes require consensus only among the stewards, rather than all members. Regular members participate by periodically selecting the stewards but do not take part in each decision. This structure enables faster coordination since consensus is achieved within a smaller group of stewards. It is particularly suitable for large user groups, where involving every member in each decision would be impractical.

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References​