3 Security and Cryptography

3. Security and Cryptography Model

Tincan is engineered to guarantee privacy, zero-knowledge authentication, and transport confidentiality.

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3.1 Two-Tier Security Architecture

Security is enforced at two distinct layers:

1. Transport Layer Encryption: End-to-End Encryption (E2EE) over QUIC TLS 1.3.
2. Admission Control: Zero-knowledge challenge-response password authentication.

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3.2 Argon2id Challenge-Response Authentication

When joining a password-protected room:

  Joining Peer                                  Coordinator (Host)
       │                                                 │
       ├───────────── QUIC TLS 1.3 Connect ──────────────►│
       │                                                 │
       │◄───────────── 32-Byte Nonce N ──────────────────┤
       │                                                 │
  Compute Proof:                                         │
  K = Argon2id(password, N)                              │
       │                                                 │
       ├───────────── Proof Key K ──────────────────────►│
                                                    Verify Proof:
                                                    Expected = Argon2id(password, N)
                                                    Match -> Grant Entry

Key Security Benefits

• Zero Password Transmission: The plain-text password is never sent over the wire.
• Replay Attack Defense: Because the host generates a cryptographically random 32-byte nonce $N$ for every join attempt, a captured proof cannot be replayed by eavesdroppers.
• Argon2id Memory Hardness: Protects against GPU/ASIC brute-force dictionary attacks.

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3.3 Transport Security & Network Privacy

• QUIC TLS 1.3: Every connection is encrypted using QUIC TLS 1.3 backed by Ed25519 public key pairs via Iroh (src/net/endpoint.rs).
• Invite Code = Public Key: The 63-character invite code is the Base32 representation of the coordinator's public key.
• DERP Relay Privacy: When direct P2P hole punching fails, traffic flows through encrypted DERP relays. Relays cannot read audio or text payloads because they lack decryption keys.

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3.4 Threat Model Summary

Threat	Risk Level	Defense Mechanism
Password Interception	Low	Zero-knowledge Argon2id nonce challenge
Replay Attacks	Low	Fresh host nonce on every connection attempt
Wire Eavesdropping	Low	QUIC TLS 1.3 encryption for streams & datagrams
Man-in-the-Middle	Low	Iroh Ed25519 public key verification
Relay Tampering	Low	E2E encrypted QUIC payload