feat: Add Protocol 3.4 Session Key Negotiation

[damacus]

feat: Add Protocol 3.4 Session Key Negotiation

Summary

Protocol 3.4 devices require a 3-way session key negotiation before encrypted
communication can begin. This is required for some newer Eufy vacuum models.

Technical Details

Based on tinytuya protocol notes:

Session Key Negotiation Flow

1. Client → Device: Send 16-byte nonce
2. Device → Client: Respond with 16-byte device nonce + 32-byte HMAC-SHA256
   of client nonce
3. Client → Device: Send 32-byte HMAC-SHA256 of device nonce

Session Key Calculation

tmp_key = bytes([a^b for (a,b) in zip(device_nonce, client_nonce)])
cipher = AESCipher(real_key)
session_key = cipher.encrypt(tmp_key, use_base64=False, pad=False, iv=client_nonce[:12])[12:28]

Known quirk: If the first byte of the resulting session key is 0x00, the
device considers it invalid and negotiation must restart.

Current State

• ✅ Protocol 3.4 HMAC-SHA256 checksum support added
• ❌ Session key negotiation not implemented

Affected Models (Suspected)

Models that may require session key negotiation:

• T2266 (X8 Pro) - Issue [T2266] Add support for RoboVac X8 Pro #101
• T2265 (L50) - Issue [MODEL] Add support for RoboVac model L50 #245
• T2070 (E20 3-in-1) - Issues [T2070] Add support for RoboVac model 3-in-1 E20 #156, [MODEL] Add support for RoboVac model T2070G10 (E20 3-in-1) #274
• Other newer models released after 2023

Implementation Notes

• Session keys are per-connection and must be renegotiated on reconnect
• The existing TuyaCipher class will need modification to support session keys
• Connection flow in TuyaDevice needs to include negotiation step

References

• tinytuya Protocol Notes
• tinytuya source code