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import hashlib
from .constants import (
BASE_ITERATION_COUNT,
CUSTOMIZATION_STRING_ORIG,
ID_LENGTH_BITS,
ROUND_COUNT,
)
from .utils import bits_to_bytes
def _xor(a: bytes, b: bytes) -> bytes:
return bytes(x ^ y for x, y in zip(a, b))
def _round_function(i: int, passphrase: bytes, e: int, salt: bytes, r: bytes) -> bytes:
"""The round function used internally by the Feistel cipher."""
return hashlib.pbkdf2_hmac(
"sha256",
bytes([i]) + passphrase,
salt + r,
(BASE_ITERATION_COUNT << e) // ROUND_COUNT,
dklen=len(r),
)
def _get_salt(identifier: int, extendable: bool) -> bytes:
if extendable:
return bytes()
identifier_len = bits_to_bytes(ID_LENGTH_BITS)
return CUSTOMIZATION_STRING_ORIG + identifier.to_bytes(identifier_len, "big")
def encrypt(
master_secret: bytes,
passphrase: bytes,
iteration_exponent: int,
identifier: int,
extendable: bool,
) -> bytes:
if len(master_secret) % 2 != 0:
raise ValueError(
"The length of the master secret in bytes must be an even number."
)
l = master_secret[: len(master_secret) // 2]
r = master_secret[len(master_secret) // 2 :]
salt = _get_salt(identifier, extendable)
for i in range(ROUND_COUNT):
f = _round_function(i, passphrase, iteration_exponent, salt, r)
l, r = r, _xor(l, f)
return r + l
def decrypt(
encrypted_master_secret: bytes,
passphrase: bytes,
iteration_exponent: int,
identifier: int,
extendable: bool,
) -> bytes:
if len(encrypted_master_secret) % 2 != 0:
raise ValueError(
"The length of the encrypted master secret in bytes must be an even number."
)
l = encrypted_master_secret[: len(encrypted_master_secret) // 2]
r = encrypted_master_secret[len(encrypted_master_secret) // 2 :]
salt = _get_salt(identifier, extendable)
for i in reversed(range(ROUND_COUNT)):
f = _round_function(i, passphrase, iteration_exponent, salt, r)
l, r = r, _xor(l, f)
return r + l
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