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"""Encryption and Decryption functions for KNX Secure."""
from __future__ import annotations
from cryptography.hazmat.primitives import hashes, serialization
from cryptography.hazmat.primitives.asymmetric.x25519 import X25519PrivateKey
from cryptography.hazmat.primitives.ciphers import Cipher, algorithms, modes
from cryptography.hazmat.primitives.kdf.pbkdf2 import PBKDF2HMAC
from .util import Buffer, byte_pad
def calculate_message_authentication_code_cbc(
key: bytes,
additional_data: bytes,
payload: Buffer = b"",
block_0: bytes = bytes(16),
) -> bytes:
"""Calculate the message authentication code (MAC) for a message with AES-CBC."""
blocks = (
block_0 + len(additional_data).to_bytes(2, "big") + additional_data + payload
)
y_cipher = Cipher(algorithms.AES(key), modes.CBC(bytes(16)))
y_encryptor = y_cipher.encryptor()
y_blocks = (
y_encryptor.update(byte_pad(blocks, block_size=16)) + y_encryptor.finalize()
)
# only calculate, no ctr encryption
return y_blocks[-16:]
def decrypt_ctr(
key: bytes,
counter_0: bytes,
mac: bytes,
payload: Buffer = b"",
) -> tuple[bytes, bytes]:
"""
Decrypt data from SecureWrapper.
MAC will be decoded first with counter 0.
Returns a tuple of (KNX/IP frame bytes, MAC TR for verification).
"""
cipher = Cipher(algorithms.AES(key), modes.CTR(counter_0))
decryptor = cipher.decryptor()
mac_tr = decryptor.update(mac) # MAC is encrypted with counter 0
decrypted_data = decryptor.update(payload) + decryptor.finalize()
return (decrypted_data, mac_tr)
def encrypt_data_ctr(
key: bytes,
counter_0: bytes,
mac_cbc: bytes,
payload: Buffer = b"",
) -> tuple[bytes, bytes]:
"""
Encrypt data with AES-CTR.
Payload is expected a full Plain KNX/IP frame with header.
MAC shall be encrypted with counter 0, KNXnet/IP frame with incremented counters.
Returns a tuple of encrypted data (if there is any) and encrypted MAC.
"""
s_cipher = Cipher(algorithms.AES(key), modes.CTR(counter_0))
s_encryptor = s_cipher.encryptor()
mac = s_encryptor.update(mac_cbc)
encrypted_data = s_encryptor.update(payload) + s_encryptor.finalize()
return (encrypted_data, mac)
def derive_device_authentication_password(device_authentication_password: str) -> bytes:
"""Derive device authentication password."""
kdf = PBKDF2HMAC(
algorithm=hashes.SHA256(),
length=16,
salt=b"device-authentication-code.1.secure.ip.knx.org",
iterations=65536,
)
return kdf.derive(device_authentication_password.encode("latin-1"))
def derive_user_password(password_string: str) -> bytes:
"""Derive user password."""
kdf = PBKDF2HMAC(
algorithm=hashes.SHA256(),
length=16,
salt=b"user-password.1.secure.ip.knx.org",
iterations=65536,
)
return kdf.derive(password_string.encode("latin-1"))
def generate_ecdh_key_pair() -> tuple[X25519PrivateKey, bytes]:
"""
Generate an ECDH key pair.
Return the private key and the raw bytes of the public key.
"""
private_key = X25519PrivateKey.generate()
public_key_raw = private_key.public_key().public_bytes(
serialization.Encoding.Raw, serialization.PublicFormat.Raw
)
return private_key, public_key_raw
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