Pairing & Cryptographic Security

CDUS treats security as a foundational primitive rather than an afterthought. All data transferred between paired devices is end-to-end encrypted with authenticated forward secrecy.

The Noise Protocol Handshakes

CDUS implements two distinct handshake patterns from the formal Noise Protocol Framework:

1. Initial Device Pairing (Noise XX)

When introducing two previously untrusted devices:

• Neither device knows the other’s static public key in advance.
• An out-of-band QR code transmits ephemeral connection metadata and a visual confirmation digest.
• The devices execute a 3-message Noise_XX_25519_ChaChaPoly_BLAKE2s handshake:
  • Ephemeral key exchange
  • Mutual static public key transmission
  • Authenticated identity confirmation
• Both devices display matching 6-digit confirmation pins derived from the cryptographic handshake hash to prevent Man-in-the-Middle (MitM) attacks.

2. Fast Reconnection (Noise IK)

When reconnecting with a previously paired device:

• The initiator already knows the responder’s static public key.
• The connection completes in a single round-trip Noise_IK_25519_ChaChaPoly_BLAKE2s handshake.
• Ephemeral keys generate fresh session keys with forward secrecy for every connection cycle.

Data Integrity via BLAKE3

Files are partitioned into 1MB chunks prior to transmission. Each chunk includes:

• A progressive chunk index
• A 32-byte BLAKE3 cryptographic hash
• The complete file root BLAKE3 hash

The receiving daemon verifies each chunk against its hash immediately upon arrival on disk. Corrupted or interrupted chunks are re-requested independently without re-transmitting the entire file.

Key Storage

• Desktop (Linux/macOS/Windows): Static private keys reside in the operating system’s native keychain (libsecret on Linux, macOS Keychain, and Windows Credential Manager).
• Android: Keys are stored in the hardware-backed Android KeyStore with StrongBox or TEE isolation when available.