Comparison: Signal Protocol vs MLS Implementation
Overview
Comprehensive security comparison between the Signal Protocol (Rust/WASM) and MLS (TypeScript) implementations, analyzing cryptography, protocol security, implementation quality, and risk profiles.
Analysis Date: February 2026 Signal Protocol: Rust 1.70+ with WASM bindings MLS: TypeScript with ts-mls 1.3.1 Purpose: Inform architecture decisions for secure messaging
Executive Summary
Both implementations have strong cryptographic foundations but suffer from different specification compliance issues that affect security and interoperability.
High-Level Verdict:
- Signal Protocol (Rust): Better for 1:1 messaging, superior memory safety, but has critical protocol deviations
- MLS (TypeScript): Better for group messaging, RFC compliant core, but weaker implementation security
Key Differences:
- Signal has better memory safety (Rust guarantees)
- MLS has better RFC compliance (95% for core protocol)
- Signal has critical protocol gaps (AAD, signed prekey verification)
- MLS has critical implementation gaps (input validation, logging)
- Both need significant security test coverage improvements
Cryptographic Primitives Comparison
Algorithm Selection
| Primitive | Signal Protocol | MLS Implementation |
|---|---|---|
| Key Exchange | X25519 ECDH | X25519 ECDH |
| Signatures | Ed25519 | Ed25519 |
| Symmetric Encryption | AES-256-GCM | AES-128-GCM |
| Key Derivation | HKDF-SHA256 | HKDF-SHA256 |
| Hash Function | SHA-256 | SHA-256 |
| RNG | OS CSPRNG | Platform CSRNG |
Winner: Tie - Both use industry-standard algorithms
- Signal uses AES-256 (vs MLS AES-128) but both provide adequate security
Library Quality
| Aspect | Signal Protocol | MLS Implementation |
|---|---|---|
| Implementation | Native (Rust) | JavaScript (@noble) |
| Libraries | x25519-dalek, ed25519-dalek | @noble/curves, @noble/ciphers |
| Audits | Formal verification (partial) | Cure53, Kudelski audits |
| Constant-time | Guaranteed | Best-effort |
| Memory safety | Compiler enforced | Runtime only |
| Side-channels | Resistant | Limited protection |
| CVE history | 2 fixed (dalek crates) | 0 reported (@noble) |
Winner: Signal Protocol - Native code + Rust memory safety provide stronger guarantees
Protocol Security Comparison
Protocol Design
| Aspect | Signal Protocol | MLS Implementation |
|---|---|---|
| Protocol | X3DH + Double Ratchet | TreeKEM |
| Standard | Signal specification | RFC 9420 |
| Use case | 1:1 messaging | Group messaging |
| Group size | 2 participants | Up to 1000+ |
| Scalability | O(1) per message | O(log n) per member |
| Forward secrecy | Per-message | Per-epoch |
| PCS | Immediate (1 RTT) | Next commit |
Winner: Context-dependent
- Signal: Better for 1:1 conversations
- MLS: Better for group chat
Specification Compliance
| Requirement | Signal Protocol | MLS Implementation |
|---|---|---|
| Core protocol | 66% | 100% |
| Security requirements | 50% | 46% |
| RFC/Spec adherence | Deviations | RFC 9420 compliant |
| Interoperability | Non-standard HKDF | Standard compliant |
| Test vectors | Partial | RFC test vectors |
Winner: MLS - Better RFC compliance, interoperable with other implementations
Critical Security Gaps
Signal Protocol Issues:
| Issue | Severity | Impact |
|---|---|---|
| AAD not used in AES-GCM | CRITICAL | Message reordering attacks |
| No signed prekey verification | CRITICAL | X3DH MITM attacks |
| HKDF parameter order reversed | MEDIUM | Interoperability broken |
| Non-standard derivation | MEDIUM | Cannot work with libsignal |
MLS Implementation Issues:
| Issue | Severity | Impact |
|---|---|---|
| No Welcome validation | CRITICAL | DoS, type confusion |
| No ratchet tree validation | CRITICAL | Memory exhaustion |
| Type confusion in mlsCodec | CRITICAL | RCE possible |
| No replay protection | CRITICAL | Message replay attacks |
| Debug logging secrets | CRITICAL | Information leakage |
Winner: Signal Protocol - Fewer critical issues (2 vs 5), but both need fixes
Implementation Security Comparison
Memory Safety
| Aspect | Signal Protocol (Rust) | MLS (TypeScript) |
|---|---|---|
| Buffer overflows | Impossible | Possible (typed arrays) |
| Use-after-free | Impossible | Possible |
| Null pointers | Impossible (Option<T>) | null/undefined exists |
| Data races | Prevented by compiler | Single-threaded |
| Type safety | Compile-time | Runtime |
| unsafe blocks | Zero | N/A (JavaScript) |
| Memory leaks | Prevented | Possible (closures) |
| Integer overflow | Checked (debug) | Not checked |
Winner: Signal Protocol - Rust provides compile-time memory safety guarantees
Input Validation
| Entry Point | Signal Protocol | MLS Implementation |
|---|---|---|
| Key sizes | Validated | Minimal |
| Message lengths | Checked | No limits |
| Signatures | Not checked | Delegated to ts-mls |
| Serialization | Serde validation | JSON parsing only |
| Type checking | Compiler enforced | Runtime |
| Boundary checks | Automatic | Manual |
Winner: Signal Protocol - Stronger type system and automatic validation
Information Leakage
| Source | Signal Protocol | MLS Implementation |
|---|---|---|
| Console logging | Minimal | 60+ debug logs |
| Error messages | Generic | Detailed errors |
| Timing info in metadata | N/A | Exposed |
| Secret logging | None | None (after fixes) |
| Stack traces | Sanitized | Full traces |
Winner: Signal Protocol - Much less information leakage
Attack Surface Analysis
External Input Points
Signal Protocol:
- 5 major entry points (x3dh_initiate, x3dh_receive, encrypt, decrypt, deserialize)
- All type-checked at compile time
- Some validation missing (signatures)
- WASM boundary well-protected
MLS Implementation:
- 5 major entry points (processWelcome, addMembers, decryptMessage, processCommit, mlsCodec)
- Minimal input validation
- No size limits
- Type confusion possible
Winner: Signal Protocol - Smaller attack surface with better validation
Identified Vulnerabilities
Signal Protocol:
| Vulnerability | CVSS | Status |
|---|---|---|
| Message reordering | 8.6 | Unfixed |
| X3DH MITM | 9.1 | Unfixed |
| HKDF non-compliance | 5.3 | Design issue |
| simple_ecdh panic | 4.3 | DoS only |
Total Critical: 2
MLS Implementation:
| Vulnerability | CVSS | Status |
|---|---|---|
| No Welcome validation | 9.1 | Unfixed |
| No tree validation | 8.6 | Unfixed |
| Type confusion | 9.1 | Unfixed |
| Debug logging | 7.5 | Unfixed |
| No replay protection | 8.2 | Unfixed |
| No epoch validation | 8.8 | Unfixed |
Total Critical: 6
Winner: Signal Protocol - Significantly fewer critical vulnerabilities
Test Coverage Comparison
Test Suite Size
| Metric | Signal Protocol | MLS Implementation |
|---|---|---|
| Total tests | 120 | 52 |
| Functional tests | 102 (85%) | 52 (100%) |
| Security tests | 18 (15%) | 3 (6%) |
| Attack scenarios | 0 | 0 |
| Fuzzing tests | 0 | 0 |
| Timing tests | 0 | 0 |
Winner: Signal Protocol - More tests overall, but both lack security testing
Coverage Quality
| Category | Signal Protocol | MLS Implementation |
|---|---|---|
| Code coverage | ~75% | ~70% |
| Security coverage | 15% | 6% |
| Edge cases | Some | Some |
| Negative tests | Limited | Limited |
| Integration tests | Good | Basic |
Winner: Signal Protocol - Better overall coverage, but both insufficient for production
Missing Test Coverage
Signal Protocol needs:
- 15 attack scenario tests
- 10 fuzzing tests
- 6 timing attack tests
- 12 specification compliance tests
- Total: ~80 new tests
MLS Implementation needs:
- 12 malformed input tests
- 6 replay attack tests
- 8 DoS protection tests
- 8 type confusion tests
- Total: ~78 new tests
Winner: Tie - Both need similar amounts of additional testing
Side-Channel Resistance
Timing Attacks
| Protection | Signal Protocol | MLS Implementation |
|---|---|---|
| Crypto operations | Constant-time (dalek) | Best-effort (@noble) |
| Comparisons | Some non-constant-time | Non-constant-time |
| Error paths | Variable timing | Variable timing |
| Branch prediction | Protected (crypto) | Limited |
| Cache timing | HashMap not constant-time | Objects not constant-time |
Winner: Signal Protocol - Native code provides better constant-time guarantees
Information Channels
| Channel | Signal Protocol | MLS Implementation |
|---|---|---|
| Error messages | Generic | Detailed |
| Logging | Minimal | Extensive |
| Timing metadata | N/A | Exposed |
| Memory access | Controlled | GC dependent |
| Network metadata | Observable | Observable |
Winner: Signal Protocol - Much less information leakage
Platform & Deployment
Execution Environment
| Aspect | Signal Protocol | MLS Implementation |
|---|---|---|
| Runtime | WASM (browser/Node.js) | JavaScript (browser/Node.js) |
| Performance | Near-native | JIT optimized |
| Memory usage | Efficient | Higher overhead |
| Sandboxing | WASM isolated | JavaScript sandbox |
| Platform support | Any WASM runtime | Any JS engine |
| Binary size | Larger (~500KB) | Smaller (~100KB) |
Winner: Tie - Different trade-offs
- Signal: Better performance and security
- MLS: Smaller bundle size
Dependency Security
Signal Protocol:
- 8 core dependencies (Rust crates)
- Memory-safe ecosystem
- Strong supply chain (crates.io)
- 2 historical CVEs (both fixed)
MLS Implementation:
- 5 core dependencies (npm packages)
- Larger supply chain risk
- npm ecosystem vulnerabilities
- 0 reported CVEs (newer)
Winner: Signal Protocol - Rust ecosystem has better security track record
Operational Security
Production Readiness
| Aspect | Signal Protocol | MLS Implementation |
|---|---|---|
| Critical vulns | 2 | 6 |
| Security tests | 15% | 6% |
| RFC compliance | 66% | 100% (core) |
| Interoperability | Broken | Standard |
| Memory safety | Guaranteed | Runtime |
| Rate limiting | None | None |
| Monitoring | None | None |
| Audit logging | None | None |
Winner: Neither is production-ready without fixes
Remediation Effort
Signal Protocol:
- P0 fixes: 2 issues, 6-8 hours
- P1 fixes: 2 issues, 8-11 hours
- Total: ~15-20 hours to production-ready
MLS Implementation:
- P0 fixes: 6 issues, 25-35 hours
- P1 fixes: 9 issues, 30-40 hours
- Total: ~55-75 hours to production-ready
Winner: Signal Protocol - Much faster path to production
Security Risk Assessment
Overall Risk Level
Signal Protocol:
- Risk Level: MEDIUM-HIGH
- Primary Risks: Protocol specification deviations
- Attack Surface: Low (5 entry points, well-validated)
- Exploitability: Medium (requires active adversary)
- Impact: High (confidentiality compromise possible)
MLS Implementation:
- Risk Level: HIGH
- Primary Risks: Implementation vulnerabilities
- Attack Surface: High (5 entry points, minimal validation)
- Exploitability: High (multiple attack vectors)
- Impact: Critical (RCE + DoS + information leakage)
Winner: Signal Protocol - Lower overall risk
Critical Attack Vectors
Signal Protocol:
- Message reordering (CVSS 8.6)
- X3DH MITM (CVSS 9.1)
MLS Implementation:
- Type confusion RCE (CVSS 9.1)
- DoS via malformed Welcome (CVSS 9.1)
- Information leakage via logging (CVSS 7.5)
- Replay attacks (CVSS 8.2)
- Epoch rollback (CVSS 8.8)
- No tree validation DoS (CVSS 8.6)
Winner: Signal Protocol - Fewer critical attack vectors
Use Case Recommendations
When to Use Signal Protocol
Best for:
- 1:1 end-to-end encrypted messaging
- High security requirements
- Performance-critical applications
- Need for memory safety guarantees
- When protocol can be modified (not interoperating with libsignal)
Avoid if:
- Need to interoperate with official Signal apps
- Large group messaging (>2 participants)
- Require RFC-standard compliance
- Bundle size is critical (<500KB requirement)
When to Use MLS
Best for:
- Group messaging (3+ participants)
- RFC 9420 compliance required
- Interoperability with other MLS implementations
- Smaller bundle size needs
- When group size scales (log n efficiency)
Avoid if:
- Maximum security required (until P0 fixes applied)
- Cannot tolerate information leakage via logging
- 1:1 messaging only (Signal is better)
- Need strict input validation
Fix Priority Matrix
Signal Protocol Fix Priorities
| Priority | Issues | Effort | Blocking |
|---|---|---|---|
| P0 | 2 critical | 6-8h | Production deployment |
| P1 | 2 high | 8-11h | Interoperability |
| P2 | 3 medium | 5-8h | Hardening |
Total to production: ~15-20 hours
MLS Fix Priorities
| Priority | Issues | Effort | Blocking |
|---|---|---|---|
| P0 | 6 critical | 25-35h | Production deployment |
| P1 | 9 high | 30-40h | Security hardening |
| P2 | 6 medium | 15-20h | Operational security |
Total to production: ~55-75 hours
Verdict: Signal Protocol is 3-4x faster to production-ready state
Recommendations
Architecture Decision
For 1:1 Messaging: → Use Signal Protocol after fixing P0 issues (AAD, signed prekey verification)
- Superior memory safety
- Better performance
- Fewer critical vulnerabilities
- Faster to production
For Group Messaging: → Use MLS after comprehensive remediation
- Designed for groups
- RFC compliant
- Scales logarithmically
- Interoperable
For Hybrid Architecture: → Use both
- Signal Protocol for 1:1 chats
- MLS for group chats (3+ participants)
- Maximize security and efficiency for each use case
Implementation Improvements
Signal Protocol:
- Immediate: Fix AAD usage and signed prekey verification (P0)
- Short-term: Align HKDF with Signal spec for interoperability (P1)
- Medium-term: Add comprehensive security test suite
- Long-term: Formal verification of protocol implementation
MLS Implementation:
- Immediate: Add input validation and remove debug logging (P0)
- Short-term: Implement replay protection and epoch validation (P1)
- Medium-term: Add comprehensive security test suite
- Long-term: Consider Rust rewrite for memory safety
Conclusion
Overall Comparison:
| Category | Signal Protocol | MLS Implementation |
|---|---|---|
| Cryptography | Excellent | Excellent |
| Protocol Design | Solid (for 1:1) | Solid (for groups) |
| RFC Compliance | 66% | 100% (core) |
| Memory Safety | Guaranteed | Runtime |
| Critical Vulns | 2 | 6 |
| Test Coverage | 15% security | 6% security |
| Production Ready | After 15-20h fixes | After 55-75h fixes |
| Best Use Case | 1:1 messaging | Group messaging |
Final Verdict:
- Signal Protocol: Better foundation, faster to production, but needs interoperability fixes
- MLS: Better for groups, RFC compliant, but needs extensive security hardening
- Recommendation: Use Signal for 1:1, MLS for groups, fix critical issues in both
Security Winner: Signal Protocol (fewer critical issues, better memory safety) Functionality Winner: Context-dependent (Signal for 1:1, MLS for groups) Production Readiness: Signal Protocol (3-4x faster to secure state)
Document Version: 1.0 Last Updated: February 2026 Next Review: After critical fixes in both implementations