def get_session_keys(pairing_data):
"""
HomeKit Controller state machine to perform a pair verify operation as described in chapter 4.8 page 47 ff.
:param pairing_data: the paring data as returned by perform_pair_setup
:return: tuple of the session keys (controller_to_accessory_key and accessory_to_controller_key)
:raises InvalidAuthTagError: if the auth tag could not be verified,
:raises IncorrectPairingIdError: if the accessory's LTPK could not be found
:raises InvalidSignatureError: if the accessory's signature could not be verified
:raises AuthenticationError: if the secured session could not be established
"""
#
# Step #1 ios --> accessory (send verify start Request) (page 47)
#
ios_key = x25519.X25519PrivateKey.generate()
ios_key_pub = ios_key.public_key().public_bytes(
encoding=serialization.Encoding.Raw,
format=serialization.PublicFormat.Raw)
request_tlv = [(TLV.kTLVType_State, TLV.M1),
(TLV.kTLVType_PublicKey, ios_key_pub)]
step2_expectations = [
TLV.kTLVType_State, TLV.kTLVType_PublicKey, TLV.kTLVType_EncryptedData
]
response_tlv = yield (request_tlv, step2_expectations)
#
# Step #3 ios --> accessory (send SRP verify request) (page 49)
#
response_tlv = TLV.reorder(response_tlv, step2_expectations)
assert response_tlv[0][0] == TLV.kTLVType_State and response_tlv[0][
1] == TLV.M2, 'get_session_keys: not M2'
assert response_tlv[1][
0] == TLV.kTLVType_PublicKey, 'get_session_keys: no public key'
assert response_tlv[2][
0] == TLV.kTLVType_EncryptedData, 'get_session_keys: no encrypted data'
# 1) generate shared secret
accessorys_session_pub_key_bytes = bytes(response_tlv[1][1])
accessorys_session_pub_key = x25519.X25519PublicKey.from_public_bytes(
accessorys_session_pub_key_bytes)
shared_secret = ios_key.exchange(accessorys_session_pub_key)
# 2) derive session key
hkdf_inst = hkdf.Hkdf('Pair-Verify-Encrypt-Salt'.encode(),
shared_secret,
hash=hashlib.sha512)
session_key = hkdf_inst.expand('Pair-Verify-Encrypt-Info'.encode(), 32)
# 3) verify auth tag on encrypted data and 4) decrypt
encrypted = response_tlv[2][1]
decrypted = chacha20_aead_decrypt(bytes(),
session_key, 'PV-Msg02'.encode(),
bytes([0, 0, 0, 0]), encrypted)
if type(decrypted) == bool and not decrypted:
raise InvalidAuthTagError('step 3')
d1 = TLV.decode_bytes(decrypted)
d1 = TLV.reorder(d1, [TLV.kTLVType_Identifier, TLV.kTLVType_Signature])
assert d1[0][
0] == TLV.kTLVType_Identifier, 'get_session_keys: no identifier'
assert d1[1][0] == TLV.kTLVType_Signature, 'get_session_keys: no signature'
# 5) look up pairing by accessory name
accessory_name = d1[0][1].decode()
if pairing_data['AccessoryPairingID'] != accessory_name:
raise IncorrectPairingIdError('step 3')
accessory_ltpk = ed25519.VerifyingKey(
bytes.fromhex(pairing_data['AccessoryLTPK']))
# 6) verify accessory's signature
accessory_sig = d1[1][1]
accessory_session_pub_key_bytes = response_tlv[1][1]
accessory_info = accessory_session_pub_key_bytes + accessory_name.encode(
) + ios_key_pub
try:
accessory_ltpk.verify(bytes(accessory_sig), bytes(accessory_info))
except ed25519.BadSignatureError:
raise InvalidSignatureError('step 3')
# 7) create iOSDeviceInfo
ios_device_info = ios_key_pub + pairing_data['iOSPairingId'].encode(
) + accessorys_session_pub_key_bytes
# 8) sign iOSDeviceInfo with long term secret key
ios_device_ltsk_h = pairing_data['iOSDeviceLTSK']
ios_device_ltpk_h = pairing_data['iOSDeviceLTPK']
ios_device_ltsk = ed25519.SigningKey(
bytes.fromhex(ios_device_ltsk_h) + bytes.fromhex(ios_device_ltpk_h))
ios_device_signature = ios_device_ltsk.sign(ios_device_info)
# 9) construct sub tlv
sub_tlv = TLV.encode_list([(TLV.kTLVType_Identifier,
pairing_data['iOSPairingId'].encode()),
(TLV.kTLVType_Signature, ios_device_signature)])
# 10) encrypt and sign
encrypted_data_with_auth_tag = chacha20_aead_encrypt(
bytes(), session_key, 'PV-Msg03'.encode(), bytes([0, 0, 0, 0]),
sub_tlv)
tmp = bytearray(encrypted_data_with_auth_tag[0])
tmp += encrypted_data_with_auth_tag[1]
# 11) create tlv
request_tlv = [(TLV.kTLVType_State, TLV.M3),
(TLV.kTLVType_EncryptedData, tmp)]
step3_expectations = [TLV.kTLVType_State, TLV.kTLVType_Error]
response_tlv = yield (request_tlv, step3_expectations)
#
# Post Step #4 verification (page 51)
#
response_tlv = TLV.reorder(response_tlv, step3_expectations)
assert response_tlv[0][0] == TLV.kTLVType_State and response_tlv[0][
1] == TLV.M4, 'get_session_keys: not M4'
if len(response_tlv) == 2 and response_tlv[1][0] == TLV.kTLVType_Error:
error_handler(response_tlv[1][1], 'verification')
# calculate session keys
hkdf_inst = hkdf.Hkdf('Control-Salt'.encode(),
shared_secret,
hash=hashlib.sha512)
controller_to_accessory_key = hkdf_inst.expand(
'Control-Write-Encryption-Key'.encode(), 32)
hkdf_inst = hkdf.Hkdf('Control-Salt'.encode(),
shared_secret,
hash=hashlib.sha512)
accessory_to_controller_key = hkdf_inst.expand(
'Control-Read-Encryption-Key'.encode(), 32)
return controller_to_accessory_key, accessory_to_controller_key
def get_session_keys(conn, pairing_data):
"""
HomeKit Controller side call to perform a pair verify operation as described in chapter 4.8 page 47 ff.
:param conn: the http_impl connection to the target accessory
:param pairing_data: the paring data as returned by perform_pair_setup
:return: tuple of the session keys (controller_to_accessory_key and accessory_to_controller_key)
:raises InvalidAuthTagError: if the auth tag could not be verified,
:raises IncorrectPairingIdError: if the accessory's LTPK could not be found
:raises InvalidSignatureError: if the accessory's signature could not be verified
:raises AuthenticationError: if the secured session could not be established
"""
headers = {'Content-Type': 'application/pairing+tlv8'}
#
# Step #1 ios --> accessory (send verify start Request) (page 47)
#
ios_key = py25519.Key25519()
request_tlv = TLV.encode_list([(TLV.kTLVType_State, TLV.M1),
(TLV.kTLVType_PublicKey, ios_key.pubkey)])
conn.request('POST', '/pair-verify', request_tlv, headers)
resp = conn.getresponse()
response_tlv = TLV.decode_bytes(resp.read())
#
# Step #3 ios --> accessory (send SRP verify request) (page 49)
#
response_tlv = TLV.reorder(response_tlv, [
TLV.kTLVType_State, TLV.kTLVType_PublicKey, TLV.kTLVType_EncryptedData
])
assert response_tlv[0][0] == TLV.kTLVType_State and response_tlv[0][
1] == TLV.M2, 'get_session_keys: not M2'
assert response_tlv[1][
0] == TLV.kTLVType_PublicKey, 'get_session_keys: no public key'
assert response_tlv[2][
0] == TLV.kTLVType_EncryptedData, 'get_session_keys: no encrypted data'
# 1) generate shared secret
accessorys_session_pub_key_bytes = response_tlv[1][1]
shared_secret = ios_key.get_ecdh_key(
py25519.Key25519(pubkey=bytes(accessorys_session_pub_key_bytes),
verifyingkey=bytes()))
# 2) derive session key
hkdf_inst = hkdf.Hkdf('Pair-Verify-Encrypt-Salt'.encode(),
shared_secret,
hash=hashlib.sha512)
session_key = hkdf_inst.expand('Pair-Verify-Encrypt-Info'.encode(), 32)
# 3) verify auth tag on encrypted data and 4) decrypt
encrypted = response_tlv[2][1]
decrypted = chacha20_aead_decrypt(bytes(),
session_key, 'PV-Msg02'.encode(),
bytes([0, 0, 0, 0]), encrypted)
if type(decrypted) == bool and not decrypted:
raise InvalidAuthTagError('step 3')
d1 = TLV.decode_bytes(decrypted)
d1 = TLV.reorder(d1, [TLV.kTLVType_Identifier, TLV.kTLVType_Signature])
assert d1[0][
0] == TLV.kTLVType_Identifier, 'get_session_keys: no identifier'
assert d1[1][0] == TLV.kTLVType_Signature, 'get_session_keys: no signature'
# 5) look up pairing by accessory name
accessory_name = d1[0][1].decode()
if pairing_data['AccessoryPairingID'] != accessory_name:
raise IncorrectPairingIdError('step 3')
accessory_ltpk = py25519.Key25519(pubkey=bytes(),
verifyingkey=bytes.fromhex(
pairing_data['AccessoryLTPK']))
# 6) verify accessory's signature
accessory_sig = d1[1][1]
accessory_session_pub_key_bytes = response_tlv[1][1]
accessory_info = accessory_session_pub_key_bytes + accessory_name.encode(
) + ios_key.pubkey
if not accessory_ltpk.verify(bytes(accessory_sig), bytes(accessory_info)):
raise InvalidSignatureError('step 3')
# 7) create iOSDeviceInfo
ios_device_info = ios_key.pubkey + pairing_data['iOSPairingId'].encode(
) + accessorys_session_pub_key_bytes
# 8) sign iOSDeviceInfo with long term secret key
ios_device_ltsk_h = pairing_data['iOSDeviceLTSK']
ios_device_ltsk = py25519.Key25519(
secretkey=bytes.fromhex(ios_device_ltsk_h))
ios_device_signature = ios_device_ltsk.sign(ios_device_info)
# 9) construct sub tlv
sub_tlv = TLV.encode_list([(TLV.kTLVType_Identifier,
pairing_data['iOSPairingId'].encode()),
(TLV.kTLVType_Signature, ios_device_signature)])
# 10) encrypt and sign
encrypted_data_with_auth_tag = chacha20_aead_encrypt(
bytes(), session_key, 'PV-Msg03'.encode(), bytes([0, 0, 0, 0]),
sub_tlv)
tmp = bytearray(encrypted_data_with_auth_tag[0])
tmp += encrypted_data_with_auth_tag[1]
# 11) create tlv
request_tlv = TLV.encode_list([(TLV.kTLVType_State, TLV.M3),
(TLV.kTLVType_EncryptedData, tmp)])
# 12) send to accessory
conn.request('POST', '/pair-verify', request_tlv, headers)
resp = conn.getresponse()
response_tlv = TLV.decode_bytes(resp.read())
#
# Post Step #4 verification (page 51)
#
response_tlv = TLV.reorder(response_tlv,
[TLV.kTLVType_State, TLV.kTLVType_Error])
assert response_tlv[0][0] == TLV.kTLVType_State and response_tlv[0][
1] == TLV.M4, 'get_session_keys: not M4'
if len(response_tlv) == 2 and response_tlv[1][0] == TLV.kTLVType_Error:
error_handler(response_tlv[1][1], 'verification')
# calculate session keys
hkdf_inst = hkdf.Hkdf('Control-Salt'.encode(),
shared_secret,
hash=hashlib.sha512)
controller_to_accessory_key = hkdf_inst.expand(
'Control-Write-Encryption-Key'.encode(), 32)
hkdf_inst = hkdf.Hkdf('Control-Salt'.encode(),
shared_secret,
hash=hashlib.sha512)
accessory_to_controller_key = hkdf_inst.expand(
'Control-Read-Encryption-Key'.encode(), 32)
return controller_to_accessory_key, accessory_to_controller_key