This document defines the explicit threat model for the Ultra-Secure Messenger system.

1. Nation-State Attackers

   • Sophisticated, well-funded
   • Access to advanced cryptanalysis
   • Potential for traffic analysis
   • Long-term persistence

2. Malicious Servers

   • Compromised server infrastructure
   • Insider threats
   • Government coercion
   • Server-side attacks

3. Compromised Clients

   • Malware on client devices
   • Physical device access
   • Side-channel attacks
   • Memory scraping

4. Network Attackers

   • Man-in-the-middle (MITM)
   • Traffic analysis
   • Replay attacks
   • Denial of service

1. Confidentiality: Messages remain secret from unauthorized parties
2. Integrity: Messages cannot be modified without detection
3. Authentication: Participants can verify each other's identity
4. Forward Secrecy: Past messages remain secure after key compromise
5. Post-Compromise Security: Future messages become secure after key update
6. Deniability: Sender can plausibly deny sending (optional)

Threat: Adversary monitors network traffic between clients and server.

Mitigation:

• All messages encrypted end-to-end
• Server cannot decrypt messages
• Traffic analysis resistance (message padding, timing)
• TLS/WebSocket encryption for transport

Residual Risk: Low

• Adversary can observe metadata (message sizes, timing)
• Cannot read message contents

Threat: Adversary intercepts and modifies traffic.

Mitigation:

• Cryptographic authentication (Ed25519 signatures)
• Handshake authentication prevents MITM
• Message authentication codes (MACs)
• Certificate pinning (recommended)

Residual Risk: Low

• Requires compromise of identity keys
• Detected by authentication failures

Threat: Server infrastructure is compromised.

Mitigation:

• Zero-knowledge architecture (server cannot decrypt)
• No plaintext message storage
• Stateless design (minimal data exposure)
• Rate limiting prevents abuse

Residual Risk: Medium

• Adversary can observe metadata
• Can perform denial of service
• Cannot read message contents

Threat: Client device is compromised (malware, physical access).

Mitigation:

• Forward secrecy (past messages protected)
• Secure key storage (OS secure enclave when available)
• Automatic key rotation
• Minimal key exposure

Residual Risk: High (unavoidable)

• Adversary can read current messages
• Can impersonate user
• Past messages remain protected (forward secrecy)

Threat: Adversary replays old messages.

Mitigation:

• Timestamps in handshake
• Message sequence numbers
• Nonces in handshake
• Clock skew tolerance

Residual Risk: Low

• Replayed messages detected and rejected

Threat: Adversary compromises long-term key and impersonates user.

Mitigation:

• Ephemeral keys in handshake
• Double Ratchet provides post-compromise security
• Key rotation
• Trust-on-first-use with upgrade paths

Residual Risk: Medium

• Initial compromise allows impersonation
• Future messages become secure after key update

Threat: Adversary analyzes message patterns, sizes, timing.

Mitigation:

• Message padding (future enhancement)
• Timing randomization (future enhancement)
• Minimal metadata exposure

Residual Risk: Medium

• Some metadata leakage unavoidable
• Message contents remain protected

Threat: Adversary disrupts service availability.

Mitigation:

• Rate limiting per client
• Connection limits
• Graceful degradation
• Distributed architecture

Residual Risk: Medium

• DoS attacks possible but mitigated
• Service can degrade gracefully

Threat: Adversary exploits timing, power, or other side channels.

Mitigation:

• Constant-time operations where possible
• Secure memory handling
• Minimal key exposure

Residual Risk: Medium

• JavaScript/TypeScript limitations
• Best-effort mitigations

Threat: Adversary learns who is talking to whom, when.

Mitigation:

• Minimal metadata collection
• No message content logging
• Encrypted message storage
• Client-side message deletion

Residual Risk: Medium

• Some metadata necessary for operation
• Cannot be completely eliminated

• Algorithm: AES-256-GCM
• Key Size: 256 bits
• IV: 96 bits (12 bytes), randomly generated
• Authentication: Built-in GCM authentication tag

Security Level: 128 bits (quantum-resistant: 256 bits)

• Algorithm: X25519 (Curve25519)
• Key Size: 256 bits
• Security Level: ~128 bits (quantum-resistant: 256 bits)

• Algorithm: Ed25519
• Key Size: 256 bits
• Security Level: ~128 bits (quantum-resistant: 256 bits)

• Algorithm: HKDF-SHA-256
• Output: 256 bits
• Security Level: 256 bits

• Source: crypto.getRandomValues() (Web Crypto API)
• Quality: Cryptographically secure
• Platform: OS-provided CSPRNG

The system implements the Double Ratchet protocol for forward secrecy:

1. Per-Message Keys: Each message uses a unique encryption key
2. Key Evolution: Keys evolve with each message
3. Chain Keys: Separate chains for sending and receiving
4. Key Deletion: Old keys deleted after use

Properties:

• Compromise of current key does not reveal past messages
• Compromise of past key does not reveal future messages
• Automatic key rotation

• Trigger: Every message (via Double Ratchet)
• Method: HKDF-based key derivation
• Storage: Keys stored only in memory
• Deletion: Keys zeroed after use (best-effort)

After key compromise:

1. New Handshake: Establishes new shared secret
2. Key Update: Ratchet state updated
3. Future Messages: Protected by new keys
4. Past Messages: Remain protected (forward secrecy)

Recovery Time: Immediate (next message after key update)

1. End-to-End Encryption: Messages encrypted, server cannot decrypt
2. Forward Secrecy: Past messages secure after key compromise
3. Authentication: Participants authenticated via cryptographic signatures
4. Integrity: Message tampering detected via MACs
5. Replay Protection: Replayed messages rejected

1. Metadata Privacy: Some metadata (timing, sizes) may leak
2. Traffic Analysis Resistance: Patterns may be observable
3. Deniability: Messages are authenticated (not deniable)
4. Perfect Forward Secrecy: Requires perfect key deletion (best-effort)
5. Side-Channel Resistance: Limited by JavaScript/TypeScript

1. Secure Key Storage: Use OS secure enclave when available
2. Regular Updates: Keep software updated
3. Device Security: Use device encryption, strong passwords
4. Trust Verification: Verify identity keys out-of-band
5. Key Backup: Secure backup of identity keys (encrypted)

1. Server Security: Harden server infrastructure
2. Monitoring: Monitor for abuse and attacks
3. Updates: Keep dependencies updated
4. Audits: Regular security audits
5. Incident Response: Plan for security incidents

1. JavaScript Memory: Cannot guarantee secure memory zeroing
2. Timing Attacks: Some operations may have timing variations
3. Metadata: Some metadata leakage unavoidable
4. Client Compromise: Cannot protect against fully compromised client
5. Quantum Computing: Not quantum-resistant (but can be upgraded)

Before production deployment:

1. Cryptographic Review: Audit all cryptographic implementations
2. Protocol Review: Verify protocol correctness
3. Code Review: Security-focused code review
4. Penetration Testing: Adversarial testing
5. Fuzz Testing: Automated fuzz testing
6. Side-Channel Analysis: Timing and power analysis

1. Revoke Identity: Generate new identity key
2. Notify Contacts: Inform contacts of compromise
3. Re-establish Sessions: Perform new handshakes
4. Audit Logs: Review for suspicious activity

1. Isolate: Isolate compromised servers
2. Rotate Keys: Rotate server identity keys
3. Notify Users: Inform users of incident
4. Forensics: Investigate compromise
5. Remediate: Fix vulnerabilities

• NIST Guidelines: Follows NIST cryptographic guidelines
• OWASP: Addresses OWASP security concerns
• Signal Protocol: Inspired by Signal Protocol security model
• Noise Framework: Based on Noise Protocol Framework principles