    def generate_otp(self, input: int) -> str:
        """
        :param input: the HMAC counter value to use as the OTP input.
            Usually either the counter, or the computed integer based on the Unix timestamp
        """
        if input < 0:
            raise ValueError("input must be positive integer")
        hasher = hmac.new(self.byte_secret(), self.int_to_bytestring(input), self.digest)
        if hasher.digest_size < 18:
            raise ValueError("digest size is lower than 18 bytes, which will trigger error on otp generation")
        hmac_hash = bytearray(hasher.digest())
        offset = hmac_hash[-1] & 0xF
        code = (
            (hmac_hash[offset] & 0x7F) << 24
            | (hmac_hash[offset + 1] & 0xFF) << 16
            | (hmac_hash[offset + 2] & 0xFF) << 8
            | (hmac_hash[offset + 3] & 0xFF)
        )
        str_code = str(10_000_000_000 + (code % 10**self.digits))
        return str_code[-self.digits :]

    def byte_secret(self) -> bytes:
        secret = self.secret
        missing_padding = len(secret) % 8
        if missing_padding != 0:
            secret += "=" * (8 - missing_padding)
        return base64.b32decode(secret, casefold=True)

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