bthome_v1_encryption.py

"""Example showing encoding and decoding of BTHome v1 advertisement"""

from __future__ import annotations

import binascii

from cryptography.hazmat.primitives.ciphers.aead import AESCCM


def parse_value(data: bytes) -> dict[str, float]:
    """Parse decrypted payload to readable BTHome data"""
    vlength = len(data)
    if vlength >= 3:
        temp = round(int.from_bytes(data[2:4], "little", signed=False) * 0.01, 2)
        humi = round(int.from_bytes(data[6:8], "little", signed=False) * 0.01, 2)
        print("Temperature:", temp, "Humidity:", humi)
        return {"temperature": temp, "humidity": humi}
    print("MsgLength:", vlength, "HexValue:", data.hex())
    return {}


def decrypt_payload(
    payload: bytes, mic: bytes, key: bytes, nonce: bytes
) -> dict[str, float] | None:
    """Decrypt payload."""
    print("Nonce:", nonce.hex())
    print("CryptData:", payload.hex())
    print("Mic:", mic.hex())
    cipher = AESCCM(key, tag_length=4)
    print()
    print("Starting Decryption data")
    try:
        data = cipher.decrypt(nonce, payload + mic, b"\x11")
    except ValueError as error:
        print()
        print("Decryption failed:", error)
        return None
    print("Decryption succeeded, decrypted data:", data.hex())
    print()
    return parse_value(data=data)


def decrypt_aes_ccm(key: bytes, mac: bytes, data: bytes) -> dict[str, float] | None:
    """Decrypt AES CCM."""
    print("MAC:", mac.hex())
    print("Bindkey:", key.hex())
    adslength = len(data)
    if adslength > 15 and data[0] == 0x1E and data[1] == 0x18:
        pkt = data[: data[0] + 1]
        uuid = pkt[0:2]
        encrypted_data = pkt[2:-8]
        count_id = pkt[-8:-4]
        mic = pkt[-4:]
        # nonce: mac [6], uuid16 [2], count_id [4] # 6+2+4 = 12 bytes
        nonce = b"".join([mac, uuid, count_id])
        return decrypt_payload(encrypted_data, mic, key, nonce)
    else:
        print("Error: format packet!")
    return None


def encrypt_payload(
    data: bytes, mac: bytes, uuid16: bytes, count_id: bytes, key: bytes
) -> bytes:
    """Encrypt payload."""
    nonce = b"".join([mac, uuid16, count_id])  # 6+2+4 = 12 bytes
    cipher = AESCCM(key, tag_length=4)
    associated_data = b"\x11"
    result = cipher.encrypt(nonce, data, associated_data)
    ciphertext = result[:-4]
    mic = result[-4:]
    print("MAC:", mac.hex())
    print("Binkey:", key.hex())
    print("Data:", data.hex())
    print("Nonce:", nonce.hex())
    print("CryptData:", ciphertext.hex(), "Mic:", mic.hex())
    return b"".join([uuid16, ciphertext, count_id, mic])


# =============================
# main()
# =============================
def main() -> None:
    """Example to encrypt and decrypt BTHome payload."""
    print()
    print("====== Test encode -----------------------------------------")
    data = bytes(bytearray.fromhex("2302CA090303BF13"))  # BTHome data (not encrypted)
    parse_value(data)  # Print temperature and humidity

    print()
    print("Preparing data for encryption")
    count_id = bytes(bytearray.fromhex("00112233"))  # count id (change every message)
    mac = binascii.unhexlify("5448E68F80A5")  # MAC
    uuid16 = b"\x1e\x18"
    bindkey = binascii.unhexlify("231d39c1d7cc1ab1aee224cd096db932")

    payload = encrypt_payload(
        data=data, mac=mac, uuid16=uuid16, count_id=count_id, key=bindkey
    )
    print()
    print("Encrypted data:", payload.hex())
    print()
    print("====== Test decode -----------------------------------------")
    decrypt_aes_ccm(key=bindkey, mac=mac, data=payload)


if __name__ == "__main__":
    main()