Class HOTP

src/cryptography/hazmat/primitives/twofactor/hotp.py:45–101

class HOTP:
    def __init__(
        self,
        key: Buffer,
        length: int,
        algorithm: HOTPHashTypes,
        backend: typing.Any = None,
        enforce_key_length: bool = True,
    ) -> None:
        if len(key) < 16 and enforce_key_length is True:
            raise ValueError("Key length has to be at least 128 bits.")

        if not isinstance(length, int):
            raise TypeError("Length parameter must be an integer type.")

        if length < 6 or length > 8:
            raise ValueError("Length of HOTP has to be between 6 and 8.")

        if not isinstance(algorithm, (SHA1, SHA256, SHA512)):
            raise TypeError("Algorithm must be SHA1, SHA256 or SHA512.")

        self._key = key
        self._length = length
        self._algorithm = algorithm

    def generate(self, counter: int) -> bytes:
        if not isinstance(counter, int):
            raise TypeError("Counter parameter must be an integer type.")

        truncated_value = self._dynamic_truncate(counter)
        hotp = truncated_value % (10**self._length)
        return "{0:0{1}}".format(hotp, self._length).encode()

    def verify(self, hotp: bytes, counter: int) -> None:
        if not constant_time.bytes_eq(self.generate(counter), hotp):
            raise InvalidToken("Supplied HOTP value does not match.")

    def _dynamic_truncate(self, counter: int) -> int:
        ctx = hmac.HMAC(self._key, self._algorithm)

        try:
            ctx.update(counter.to_bytes(length=8, byteorder="big"))
        except OverflowError:
            raise ValueError(f"Counter must be between 0 and {2**64 - 1}.")

        hmac_value = ctx.finalize()

        offset = hmac_value[len(hmac_value) - 1] & 0b1111
        p = hmac_value[offset : offset + 4]
        return int.from_bytes(p, byteorder="big") & 0x7FFFFFFF

    def get_provisioning_uri(
        self, account_name: str, counter: int, issuer: str | None
    ) -> str:
        return _generate_uri(
            self, "hotp", account_name, issuer, [("counter", int(counter))]
        )