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Encryption Layer 2: Per-Queue E2E

Draft

This section is under construction.

Purpose​

Layer 2 provides end-to-end encryption at the queue level, protecting message content from the SMP server.

Key Sources Per Queue Type​

Contact Queue​

The Contact Queue has NO separate Layer 2 encryption. Layer 3 (server-to-recipient) is sufficient because the sender uses the recipient's DH key from the invitation link.

Reply Queue​

For Reply Queues, Layer 2 uses a fresh DH computation:

  • Sender generates an ephemeral X25519 keypair
  • Sender's ephemeral public key is included in the cmHeader (SPKI-encoded)
  • Recipient uses their rcvDhPrivate from queue creation
  • Shared secret computed via X25519 DH + HSalsa20

ClientMsgEnvelope Format​

┌────────────────┬─────────────┬──────────────────┐
│ cmHeader       │ cmNonce(24B)│ cmEncBody        │
│ (phVer, dhKey) │             │ (encrypted)      │
└────────────────┴─────────────┴──────────────────┘
Critical Bug

Bug #17: The nonce for Layer 2 decryption is cmNonce from the envelope header, NOT msgId from the MSG delivery. This was one of the most difficult bugs to find.