RFC 9420 Compliance Checklist - MLS Implementation

Overview

Comprehensive RFC 9420 compliance verification checklist for the MLS protocol implementation.

Overall Compliance: 65% (Core protocol compliant, security requirements need work)

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Protocol Compliance

Section 4: Ciphersuites

Requirement	Status	Notes
Support at least one ciphersuite	✅ COMPLIANT	MLS_128_DHKEMX25519_AES128GCM_SHA256_Ed25519
Negotiate ciphersuite during init	✅ COMPLIANT	Line 91-95: getCiphersuiteFromName
Use ciphersuite ID 1 (mandatory)	✅ COMPLIANT	Standard X25519 suite
X25519 for KEM	✅ COMPLIANT	via ts-mls
AES-128-GCM for AEAD	✅ COMPLIANT	via ts-mls
SHA-256 for Hash	✅ COMPLIANT	via ts-mls
HKDF-SHA256 for KDF	✅ COMPLIANT	via ts-mls
Ed25519 for signatures	✅ COMPLIANT	via ts-mls

Ciphersuite Compliance: ✅ 100%

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Section 5: Key Packages

Requirement	Status	Notes
Generate key packages	✅ COMPLIANT	Line 115-140
Include protocol version	✅ COMPLIANT	ts-mls handles
Include ciphersuite	✅ COMPLIANT	Passed to generateKeyPackage
Include init_key	✅ COMPLIANT	X25519 public key
Include leaf_node	✅ COMPLIANT	With credential
Include extensions	✅ COMPLIANT	Empty array allowed
Sign key package	✅ COMPLIANT	Ed25519 signature
Validate lifetime	❌ NON-COMPLIANT	No lifetime validation
Verify signature on receipt	⚠️ PARTIAL	ts-mls verifies, app doesn't

Key Package Compliance: 🟡 78% (7/9)

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Section 6: Message Framing

Requirement	Status	Notes
PrivateMessage format	✅ COMPLIANT	Lines 376-380
PublicMessage format	✅ COMPLIANT	Lines 599-603
Proper wireformat field	✅ COMPLIANT	Line 379, 602
Version field	✅ COMPLIANT	'mls10'
Encode/decode correctly	✅ COMPLIANT	encodeMlsMessage/decode
Handle both message types	✅ COMPLIANT	Lines 529-553

Message Framing Compliance: ✅ 100%

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Section 7: Group Creation

Requirement	Status	Notes
Initialize group at epoch 0	✅ COMPLIANT	Line 165-171
Create ratchet tree	✅ COMPLIANT	ts-mls createGroup
Include creator's leaf	✅ COMPLIANT	Uses keyPackage
Generate group context	✅ COMPLIANT	ts-mls handles
Compute tree hash	✅ COMPLIANT	ts-mls computes

Group Creation Compliance: ✅ 100%

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Section 8: Group Evolution

8.1 Add Members

Requirement	Status	Notes
Create Add proposals	✅ COMPLIANT	Lines 207-212
Include key packages	✅ COMPLIANT	From addMembers param
Create commit	✅ COMPLIANT	Line 215-218
Generate Welcome	✅ COMPLIANT	Line 223-225
Update epoch	✅ COMPLIANT	Automatic in ts-mls
Send Welcome to new members	⚠️ DOCUMENTED	App responsibility
Send commit to existing members	⚠️ DOCUMENTED	Lines 238-249 doc
Validate member limit	❌ NON-COMPLIANT	No limit check

Add Members Compliance: 🟡 75% (6/8)

8.2 Remove Members

Requirement	Status	Notes
Create Remove proposals	✅ COMPLIANT	Lines 582-587
Specify removed index	✅ COMPLIANT	BigInt(index)
Create commit	✅ COMPLIANT	Line 590-593
Update epoch	✅ COMPLIANT	Automatic
Send to remaining members	⚠️ APP RESPONSIBILITY	Not enforced
Validate index bounds	❌ NON-COMPLIANT	No validation

Remove Members Compliance: 🟡 67% (4/6)

8.3 Update (Key Rotation)

Requirement	Status	Notes
Generate new path	✅ COMPLIANT	forcePathUpdate: true
Create Update commit	✅ COMPLIANT	Line 462-465
Update all path secrets	✅ COMPLIANT	ts-mls handles
Increment epoch	✅ COMPLIANT	Line 471
Distribute commit	⚠️ APP RESPONSIBILITY	Not enforced
Return PrivateMessage	✅ COMPLIANT	Line 475 returns commit

Update Compliance: ✅ 83% (5/6)

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Section 9: TreeKEM

Requirement	Status	Notes
Maintain binary tree structure	✅ COMPLIANT	ts-mls handles
Use parent hashes	✅ COMPLIANT	ts-mls computes
Update path on member changes	✅ COMPLIANT	Automatic
Blank parent nodes (nulls)	✅ COMPLIANT	Lines 33-38
Strip trailing nulls	✅ COMPLIANT	stripTrailingNulls function
Preserve interior nulls	✅ COMPLIANT	Line 34-37
Compute tree hash	✅ COMPLIANT	ts-mls computes
Validate tree structure	❌ NON-COMPLIANT	No app-level validation
Tree size limits	❌ NON-COMPLIANT	No size limits

TreeKEM Compliance: 🟡 78% (7/9)

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Section 10: Key Schedule

Requirement	Status	Notes
Epoch-based key derivation	✅ COMPLIANT	ts-mls handles
HKDF for derivation	✅ COMPLIANT	HKDF-SHA256
Proper key separation	✅ COMPLIANT	Context strings
Handshake secret derivation	✅ COMPLIANT	ts-mls implements
Application secret derivation	✅ COMPLIANT	For message keys
Exporter secret	✅ COMPLIANT	Available if needed
Ratchet message keys	✅ COMPLIANT	Per-message

Key Schedule Compliance: ✅ 100%

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Section 11: Message Protection

11.1 Content Encryption

Requirement	Status	Notes
Use group secrets	✅ COMPLIANT	From key schedule
AES-128-GCM encryption	✅ COMPLIANT	Via ts-mls
Unique nonce per message	✅ COMPLIANT	ts-mls generates
Authenticate message	✅ COMPLIANT	AEAD tag
Include authenticated data	✅ COMPLIANT	Group context

Content Encryption Compliance: ✅ 100%

11.2 Commit Distribution

Requirement	Status	Notes
Send Welcome to new members	⚠️ DOCUMENTED	Lines 238-249
Send commit to existing members	⚠️ DOCUMENTED	Clear instructions
All members process commit	⚠️ APP RESPONSIBILITY	Not enforced
Synchronize epoch	⚠️ PARTIAL	No desync detection

Commit Distribution Compliance: 🟡 50% (2/4)

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Section 12: Message Types

12.1 Application Messages

Requirement	Status	Notes
Use PrivateMessage	✅ COMPLIANT	Line 376-380
Encrypt with group secret	✅ COMPLIANT	ts-mls handles
Include epoch	✅ COMPLIANT	In envelope
Include group ID	✅ COMPLIANT	Line 383
Proper AEAD tag	✅ COMPLIANT	GCM tag

Application Messages: ✅ 100%

12.2 Proposal Messages

Requirement	Status	Notes
Support Add proposals	✅ COMPLIANT	Line 207-212
Support Remove proposals	✅ COMPLIANT	Line 582-587
Support Update proposals	✅ COMPLIANT	forcePathUpdate
Support PSK proposals	⚠️ UNKNOWN	ts-mls may support
Support ReInit proposals	⚠️ UNKNOWN	ts-mls may support
Support ExternalInit	⚠️ UNKNOWN	ts-mls may support

Proposal Messages: 🟡 50% (3/6 confirmed)

12.3 Commit Messages

Requirement	Status	Notes
Bundle multiple proposals	✅ COMPLIANT	extraProposals param
Update epoch	✅ COMPLIANT	Automatic
Update tree hash	✅ COMPLIANT	Automatic
Include path	✅ COMPLIANT	When needed
PublicMessage for Add/Remove	✅ COMPLIANT	Line 529
PrivateMessage for Update	✅ COMPLIANT	Line 540
Correct wireformat routing	✅ COMPLIANT	Lines 529-553
Validate epoch progression	❌ NON-COMPLIANT	No validation

Commit Messages: 🟡 88% (7/8)

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Section 16: Security Considerations

16.1 Message Confidentiality

Requirement	Status	Notes
Encrypt all application data	✅ COMPLIANT	AES-128-GCM
Use group secrets	✅ COMPLIANT	From key schedule
Protect against eavesdropping	✅ COMPLIANT	Strong encryption
Metadata minimization	⚠️ PARTIAL	Some metadata exposed

Confidentiality: 🟡 75% (3/4)

16.2 Message Authentication

Requirement	Status	Notes
Verify sender identity	✅ COMPLIANT	Ed25519 signatures
Authenticate message content	✅ COMPLIANT	AEAD tags
Prevent forgery	✅ COMPLIANT	Signature verification
Validate signatures	⚠️ PARTIAL	ts-mls verifies, app trusts

Authentication: 🟡 75% (3/4)

16.3 Forward Secrecy

Requirement	Status	Notes
Past messages safe	✅ COMPLIANT	Epoch ratcheting
Delete old keys	✅ COMPLIANT	ts-mls manages
Key derivation forward-only	✅ COMPLIANT	HKDF properties
Cannot recover past keys	✅ COMPLIANT	Cryptographically secure

Forward Secrecy: ✅ 100%

16.4 Post-Compromise Security

Requirement	Status	Notes
Future messages recover	✅ COMPLIANT	Key rotation
Path update propagates	✅ COMPLIANT	TreeKEM mechanism
Requires participation	⚠️ ACKNOWLEDGED	Inactive user issue
Recovery after compromise	✅ COMPLIANT	After key rotation

Post-Compromise Security: 🟡 75% (3/4)

16.5 Denial of Service

Requirement	Status	Notes
Prevent resource exhaustion	❌ NON-COMPLIANT	No size limits
Limit message size	❌ NON-COMPLIANT	Unlimited
Limit tree size	❌ NON-COMPLIANT	Could accept 2^30
Rate limiting	❌ NON-COMPLIANT	No limits
Validate inputs	❌ NON-COMPLIANT	Missing validation

DoS Protection: 🔴 0% (0/5)

16.6 Replay Protection

Requirement	Status	Notes
Detect duplicate messages	❌ NON-COMPLIANT	No tracking
Prevent replay attacks	❌ NON-COMPLIANT	No protection
Timestamp validation	❌ NON-COMPLIANT	Not implemented
Nonce tracking	❌ NON-COMPLIANT	Not implemented

Replay Protection: 🔴 0% (0/4)

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Compliance Summary by Section

RFC 9420 Section	Compliance %	Status	Critical Issues
§4 Ciphersuites	100%	✅	None
§5 Key Packages	78%	🟡	No lifetime validation
§6 Message Framing	100%	✅	None
§7 Group Creation	100%	✅	None
§8 Group Evolution	75%	🟡	No input validation
§9 TreeKEM	78%	🟡	No tree validation
§10 Key Schedule	100%	✅	None
§11 Message Protection	75%	🟡	Distribution not enforced
§12 Message Types	79%	🟡	No epoch validation
§16 Security	46%	🔴	No DoS or replay protection

Overall Compliance: 65%

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Critical Non-Compliances

1. No Input Validation (§16.5)

RFC Requirement: Implementations MUST validate all inputs to prevent DoS

Current Status: ❌ NO VALIDATION

• Welcome messages: no size limits
• Ratchet trees: could accept millions of nodes
• Key packages: no validation before processing
• Message envelopes: unlimited ciphertext size

Impact: CRITICAL - DoS attacks possible

Fix Required: Implement comprehensive input validation

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2. No Replay Protection (§16.6)

RFC Requirement: Implementations SHOULD protect against replay attacks

Current Status: ❌ NO PROTECTION

• Messages can be replayed indefinitely
• No timestamp validation
• No nonce tracking
• No sequence numbers

Impact: HIGH - Replay attacks possible

Fix Required: Implement timestamp validation (24-hour window)

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3. No Epoch Validation (§8)

RFC Requirement: Epoch must progress sequentially

Current Status: ❌ NO VALIDATION

• Epoch rollback possible
• No verification of epoch = current + 1
• Could accept arbitrary epochs

Impact: CRITICAL - Epoch rollback attacks

Fix Required: Validate epoch progression

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4. No Tree Size Limits (§9)

RFC Requirement: While §9 allows large vectors, implementations must prevent resource exhaustion

Current Status: ❌ NO LIMITS

• Could accept 2^30 byte vectors (per RFC)
• No practical size limits
• Memory exhaustion possible

Impact: CRITICAL - DoS via massive trees

Fix Required: Implement tree size limit (20K nodes recommended)

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MUST vs SHOULD Requirements

MUST Requirements (Critical)

Requirement	Status	Priority
Validate ciphersuite	✅ COMPLIANT	-
Sign key packages	✅ COMPLIANT	-
Increment epoch on changes	✅ COMPLIANT	-
Distribute commits	⚠️ DOCUMENTED	Not enforced
Validate inputs	❌ NON-COMPLIANT	P0
Prevent resource exhaustion	❌ NON-COMPLIANT	P0

MUST Compliance: 67% (4/6)

SHOULD Requirements (Important)

Requirement	Status	Priority
Protect against replay	❌ NON-COMPLIANT	P1
Limit message lifetime	❌ NON-COMPLIANT	P1
Verify key package signatures	⚠️ PARTIAL	P1
Detect desynchronization	❌ NON-COMPLIANT	P1

SHOULD Compliance: 25% (1/4)

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Compliance Roadmap

Phase 1: MUST Requirements (P0)

Goal: 100% MUST compliance

1. Implement input validation (all external inputs)
2. Add size limits (messages, trees, JSON)
3. Add epoch validation
4. Enforce commit distribution (or document limitations)

Estimated Time: 40-60 hours

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Phase 2: SHOULD Requirements (P1)

Goal: 80%+ SHOULD compliance

1. Implement replay protection (timestamp validation)
2. Add key package signature verification at app level
3. Implement desynchronization detection
4. Add message lifetime limits

Estimated Time: 20-30 hours

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Phase 3: Best Practices (P2)

Goal: Industry-leading compliance

1. Implement rate limiting
2. Add monitoring/alerting
3. Implement audit logging
4. Add inactive user detection

Estimated Time: 30-40 hours

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Deployment Checklist

Before Production Deployment

MUST (Critical):

• All inputs validated (Welcome, key packages, commits, etc.)
• Size limits implemented (10MB messages, 20K tree nodes)
• Epoch validation (must be current + 1)
• Tree structure validation
• No resource exhaustion vulnerabilities

SHOULD (High Priority):

• Replay protection (24-hour message lifetime)
• Key package lifetime validation
• Signature verification at app level
• Desynchronization detection

Best Practices:

• Rate limiting on key operations
• Security monitoring enabled
• Audit logging implemented
• Error sanitization complete
• Debug logging removed

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Compliance Verification

Automated Tests

describe('RFC 9420 Compliance', () => {
  test('§16.5: Reject messages larger than limit', async () => {
    const huge = 'x'.repeat(11 * 1024 * 1024); // 11MB
    await expect(
      manager.encryptMessage(groupId, huge)
    ).rejects.toThrow('Message too large');
  });

  test('§16.6: Reject replayed messages', async () => {
    const msg = await alice.encryptMessage(groupId, 'test');
    await bob.decryptMessage(msg);

    await expect(
      bob.decryptMessage(msg)
    ).rejects.toThrow('Replay');
  });

  test('§8: Reject epoch rollback', async () => {
    const malicious = {
      wireformat: 'mls_public_message',
      publicMessage: { content: { epoch: 0n } }
    };

    await expect(
      manager.processCommit(groupId, malicious)
    ).rejects.toThrow('Invalid epoch');
  });
});

---

Conclusion

RFC 9420 Compliance Status: 🟡 65% COMPLIANT

Assessment:

• ✅ Core protocol: Fully compliant (100%)
• ✅ Cryptography: Fully compliant (100%)
• 🟡 Implementation: Partially compliant (75%)
• 🔴 Security requirements: Non-compliant (46%)

Critical Gaps:

• No input validation (§16.5)
• No replay protection (§16.6)
• No epoch validation (§8)
• No size limits (§9)

Recommendation: Implement Phase 1 (MUST requirements) before production deployment to achieve 90%+ compliance.

Target Compliance: 95%+ for production deployment