Reads the given file as a hex key.

    Args:
        key_filename: The filename where the key is stored. If None,
            defaults to the default key for the current user.

    Returns:
        Signer: the signer

    Raises:
        CliException: If unable to read the file.

def _read_signer(key_filename):
    """Reads the given file as a hex key.

    Args:
        key_filename: The filename where the key is stored. If None,
            defaults to the default key for the current user.

    Returns:
        Signer: the signer

    Raises:
        CliException: If unable to read the file.
    """
    filename = key_filename
    if filename is None:
        filename = os.path.join(os.path.expanduser('~'),
                                '.sawtooth',
                                'keys',
                                getpass.getuser() + '.priv')

    try:
        with open(filename, 'r') as key_file:
            signing_key = key_file.read().strip()
    except IOError as e:
        raise CliException('Unable to read key file: {}'.format(str(e)))

    try:
        private_key = Secp256k1PrivateKey.from_hex(signing_key)
    except ParseError as e:
        raise CliException('Unable to read key in file: {}'.format(str(e)))

    context = create_context('secp256k1')
    crypto_factory = CryptoFactory(context)
    return crypto_factory.new_signer(private_key)