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"""Calculate Thread PSKc.
Based on https://github.com/openthread/ot-br-posix/blob/main/src/utils/pskc.cpp
"""
import struct
from cryptography.hazmat.primitives import cmac
from cryptography.hazmat.primitives.ciphers import algorithms
AES_128_KEY_LEN = 16
ITERATION_COUNTS = 16384
BLKSIZE = 16
SALT_PREFIX = "Thread".encode()
def _derive_key(passphrase: str) -> bytes:
"""Derive key from passphrase according to RFC 4615."""
passphrase_bytes = passphrase.encode()
if len(passphrase_bytes) == AES_128_KEY_LEN:
return passphrase_bytes
c = cmac.CMAC(algorithms.AES128(b"\0" * AES_128_KEY_LEN))
c.update(passphrase_bytes)
return c.finalize()
def compute_pskc(ext_pan_id: bytes, network_name: str, passphrase: str) -> bytes:
"""Compute Thread PSKc."""
salt = SALT_PREFIX + ext_pan_id + network_name.encode()
key = _derive_key(passphrase)
block_counter = 1
prf_input = salt + struct.pack("!L", block_counter)
# Calculate U_1
c = cmac.CMAC(algorithms.AES128(key))
c.update(prf_input)
prf_output = c.finalize()
pskc = bytearray(prf_output)
for _ in range(ITERATION_COUNTS - 1):
prf_input = prf_output
# Calculate U_i
c = cmac.CMAC(algorithms.AES128(key))
c.update(prf_input)
prf_output = c.finalize()
# xor
for i in range(BLKSIZE):
pskc[i] ^= prf_output[i]
return pskc
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