Protocol Security Analysis - MLS Implementation

Overview

Analysis of MLS protocol-level security including TreeKEM implementation, epoch management, commit handling, forward secrecy, and post-compromise security based on RFC 9420 requirements and 2024 security research findings.

Status: 🟡 MODERATE - Core protocol is correct but missing safeguards

TreeKEM Implementation

Ratchet Tree Management

Status: ✅ RFC 9420 Compliant with operational concerns

Key Implementation:

// Line 32-38: Proper trailing null stripping
function stripTrailingNulls(tree: any[]): any[] {
  let lastNonNull = tree.length - 1;
  while (lastNonNull >= 0 && tree[lastNonNull] === null) {
    lastNonNull--;
  }
  return tree.slice(0, lastNonNull + 1);
}

Analysis:

✅ Implements RFC 9420 requirement to strip trailing nulls
✅ Preserves interior nulls (unmerged positions)
✅ Proper tree structure maintained
⚠️ No size limits (could accept massive trees)
⚠️ No node validation before processing

Tree Security Properties

Property	Status	Assessment
Binary tree structure	✅ Correct	ts-mls handles
Path secrets	✅ Secure	Proper HKDF derivation
Leaf node isolation	✅ Correct	Per-member encryption
Parent node updates	✅ Correct	Path update mechanism
Tree consistency	✅ Maintained	Tree hash verification
Size limits	❌ Missing	Vulnerability

Forward Secrecy Analysis

Epoch-Based Ratcheting

Implementation: MLSManager.tsx:450-480

async updateKey(groupId: string): Promise<any> {
  const commitResult = await createCommit(
    { state: groupState, cipherSuite: this.cipherSuite! },
    { forcePathUpdate: true }  // ✅ Forces key rotation
  );
  this.groups.set(groupId, commitResult.newState);
}

Forward Secrecy Properties:

✅ Epoch progression: New epoch = new keys
✅ Key ratcheting: Per-message key derivation
✅ Path updates: Entire key schedule rotates
✅ Old keys discarded: Previous epochs cannot decrypt new messages
✅ Automatic on member changes: Add/remove triggers epoch++

Assessment: ✅ SECURE - Full forward secrecy implemented

Test Evidence:

// From mls-manager.test.js:286-327
test('should perform key rotation', async () => {
  // After rotation, epoch increases
  expect(afterRotation.epoch > beforeRotation.epoch).toBe(true);
  // Messages after rotation use new keys
});

Post-Compromise Security

Key Rotation Recovery

Mechanism: Path update propagates new secrets through tree

Security Properties:

✅ Future messages recover after compromise
✅ Attacker must compromise EVERY epoch to maintain access
✅ Path secret updates heal compromised nodes
⚠️ Inactive user vulnerability: Offline members don't update keys

2024 Research: Inactive User Problem

Finding: Post-compromise security requires ALL users to be active

Inactive users remain at old epoch
Don't update their encryption keys
Represent vulnerability for entire group

This Implementation:

⚠️ No mechanism to detect inactive users
⚠️ No mechanism to force key rotation for inactive members
⚠️ Group can't heal if members stay offline

Mitigation: RFC 9420 Section 11.2 allows removing inactive members

Commit Distribution & Synchronization

RFC 9420 Section 11.2 Compliance

Documentation: MLSManager.tsx:238-249

// RFC 9420 Section 11.2: Commit Distribution
// ⚠️ IMPORTANT: The returned commit MUST be sent to all existing group members
// so they can process it with processCommit() to stay synchronized.
//
// Distribution flow:
// 1. Alice adds Bob: addMembers() returns { welcome, commit }
// 2. Alice sends welcome to Bob (new member)
// 3. Alice sends commit to existing members (Charlie, David, etc.)
// 4. All existing members call processCommit(commit) to update their state

Analysis:

✅ Documentation: Clear instructions for integrators
✅ API Design: Returns both welcome and commit
⚠️ No enforcement: Application must handle distribution
⚠️ No failure handling: What if commit doesn't reach everyone?

Commit Processing

Implementation: MLSManager.tsx:491-565

async processCommit(groupId: string, commit: any): Promise<void> {
  if (commit.wireformat === 'mls_public_message') {
    result = await processPublicMessage(...);  // Add/remove
  } else if (commit.wireformat === 'mls_private_message') {
    result = await processPrivateMessage(...);  // Update/rotation
  }
}

Security Properties:

✅ Correct routing based on wireformat
✅ Handles both public and private commits
✅ Updates group state atomically
❌ No epoch validation (epoch rollback possible)
❌ No sender validation (who created commit?)

Epoch Management

Current Implementation

State:

export interface MLSGroupInfo {
  groupId: Uint8Array;
  members: string[];
  epoch: bigint;  // ✅ Uses BigInt for large epochs
}

Epoch Progression:

✅ Increments on every group change
✅ Tracked per-group correctly
✅ Exposed via getGroupKeyInfo()
❌ No validation of epoch sequence
❌ No protection against epoch rollback

Epoch Rollback Attack

Vulnerability: processCommit doesn't validate epoch++

Attack:

const rollback = {
  wireformat: 'mls_public_message',
  publicMessage: {
    content: {
      epoch: 0n  // ⚠️ Rollback to epoch 0!
    }
  }
};
await victim.processCommit('group1', rollback);
// If accepted: group security compromised

Impact: Attacker forces group to old epoch with known/compromised keys

Fix Required:

// Validate epoch must be currentEpoch + 1
if (msg.content.epoch !== currentState.groupContext.epoch + 1n) {
  throw new Error('Invalid epoch progression');
}

External Operations Security

2024 Research Finding

ETK Research (2025): "External-Operations TreeKEM and the Security of MLS in RFC 9420"

External operations (external commits/proposals) were major RFC 9420 addition
Previous security analyses didn't cover external operations
This implementation delegates to ts-mls library

This Implementation:

✅ Uses ts-mls which implements external operations
⚠️ No additional validation at application layer
⚠️ Trusts ts-mls security properties

Assessment: ACCEPTABLE - Proper delegation to audited library

Authentication & Integrity

Signature Scheme

Ed25519 (NOT ECDSA):

✅ Critical: Avoids 2024 ECDSA vulnerability
✅ Provides SUF-CMA security (stronger than EUF-CMA)
✅ TreeKEM consistency guaranteed
✅ No signature malleability

Message Authentication

Mechanism	Implementation	Status
Member signatures	Ed25519 via ts-mls	✅ Secure
AEAD tags	AES-128-GCM	✅ Secure
Membership tags	RFC 9420 §6.1	✅ Implemented
Sender authentication	ts-mls	✅ Delegated

Group State Management

State Storage

private groups: Map<string, ClientState> = new Map();

Analysis:

✅ In-memory storage (no persistence risks)
✅ Per-group isolation
✅ ClientState from ts-mls (proper structure)
⚠️ No state backup/recovery
⚠️ Application restart = state loss

State Synchronization

Mechanisms:

Commit distribution (documented, not enforced)
Welcome message processing (automated)
Epoch tracking (per-group)

Issues:

❌ No detection of desynchronized members
❌ No automatic resynchronization
❌ No state reconciliation protocol

Impact: If members miss commits → permanent desync

Security Properties Verification

RFC 9420 Required Properties

Property	Requirement	Status	Evidence
Confidentiality	Only members decrypt	✅	AES-GCM + group keys
Authentication	Verify sender identity	✅	Ed25519 signatures
Forward Secrecy	Past messages safe	✅	Epoch ratcheting
Post-Compromise	Future messages recover	🟡	Requires active users
Integrity	Detect tampering	✅	AEAD tags
Group Agreement	Same view	🟡	No desync detection

Attack Scenarios

1. Epoch Desynchronization Attack

Scenario:

Alice, Bob, Charlie in group at epoch 5
Alice sends commit (epoch 5→6) to Charlie only
Bob never receives commit, stays at epoch 5
Alice/Charlie at epoch 6, Bob at epoch 5
Bob cannot decrypt messages from epoch 6

Current Protection: ❌ None

Mitigation: Implement epoch validation and sync detection

2. Commit Injection Attack

Scenario:

Attacker intercepts commit from Alice
Modifies commit (changes epoch, proposals)
Sends to Bob

Current Protection:

✅ Ed25519 signature verification (via ts-mls)
✅ Membership tag validation
❌ No application-level sender validation

Assessment: PROTECTED by ts-mls signatures

3. Replay Attack

Scenario:

Attacker captures legitimate commit/message
Replays to victim later

Current Protection: ❌ None

Impact:

Old messages re-accepted
Could cause state confusion
May bypass access controls

Mitigation: Implement timestamp validation (24-hour window)

Comparison with RFC 9420 Requirements

Section 16: Security Considerations

RFC 9420 §16 Requirement	Status	Notes
Message confidentiality	✅	AES-128-GCM
Message authentication	✅	Ed25519 + AEAD
Member authentication	✅	Credentials in leaf nodes
Forward secrecy	✅	Epoch-based ratcheting
Post-compromise security	🟡	Requires active users
DoS resistance	❌	No size limits
Replay protection	❌	Missing
Privacy protection	🟡	Metadata exposed

Recommendations

Critical (P0)

Implement epoch validation
- Verify epoch = current + 1
- Reject rollback attempts
- Detect desynchronization
Add replay protection
- Timestamp validation (24-hour window)
- Nonce tracking for commits
- Sequence number validation
Implement size limits
- Max tree size (20,000 nodes)
- Max members per group (1,000)
- Max commit size

High (P1)

Add state synchronization detection
- Track expected epoch per member
- Detect desynchronized members
- Implement resync protocol
Add commit sender validation
- Verify sender is current member
- Verify sender has permission (admin check)
Implement inactive user detection
- Track last activity per member
- Warning for inactive members
- Consider Quarantined-TreeKEM (2024 research)

Conclusion

Protocol Implementation: 🟡 MODERATE

Strengths:

✅ Core RFC 9420 protocol correctly implemented
✅ Forward secrecy via epoch ratcheting
✅ Correct signature scheme (Ed25519 not ECDSA)
✅ Proper TreeKEM structure maintenance
✅ Authentication and integrity via signatures + AEAD

Critical Gaps:

❌ No epoch validation (rollback possible)
❌ No replay protection
❌ No synchronization detection
❌ No DoS protection (size limits)
⚠️ Inactive user vulnerability

Recommendation: Implement P0 fixes before production deployment.

Overview​

TreeKEM Implementation​

Ratchet Tree Management​

Tree Security Properties​

Forward Secrecy Analysis​

Epoch-Based Ratcheting​

Post-Compromise Security​

Key Rotation Recovery​

2024 Research: Inactive User Problem​

Commit Distribution & Synchronization​

RFC 9420 Section 11.2 Compliance​

Commit Processing​

Epoch Management​

Current Implementation​

Epoch Rollback Attack​

External Operations Security​

2024 Research Finding​

Authentication & Integrity​

Signature Scheme​

Message Authentication​

Group State Management​

State Storage​

State Synchronization​

Security Properties Verification​

RFC 9420 Required Properties​

Attack Scenarios​

1. Epoch Desynchronization Attack​

2. Commit Injection Attack​

3. Replay Attack​

Comparison with RFC 9420 Requirements​

Section 16: Security Considerations​

Recommendations​

Critical (P0)​

High (P1)​

Conclusion​

Overview

TreeKEM Implementation

Ratchet Tree Management

Tree Security Properties

Forward Secrecy Analysis

Epoch-Based Ratcheting

Post-Compromise Security

Key Rotation Recovery

2024 Research: Inactive User Problem

Commit Distribution & Synchronization

RFC 9420 Section 11.2 Compliance

Commit Processing

Epoch Management

Current Implementation

Epoch Rollback Attack

External Operations Security

2024 Research Finding

Authentication & Integrity

Signature Scheme

Message Authentication

Group State Management

State Storage

State Synchronization

Security Properties Verification

RFC 9420 Required Properties

Attack Scenarios

1. Epoch Desynchronization Attack

2. Commit Injection Attack

3. Replay Attack

Comparison with RFC 9420 Requirements

Section 16: Security Considerations

Recommendations

Critical (P0)

High (P1)

Conclusion