def root_signature_provider(signature_dict, key_id, _key, _data):
    """Root signature provider used to return signatures created remotely.

    Args:
        - signature_dict(dict): Dict where key is key_id and value is signature
        - key_id(str): Key id from targets metadata file
        - _key(securesystemslib.formats.RSAKEY_SCHEMA): Key info
        - _data(dict): Data to sign (already signed remotely)

    Returns:
        Dictionary that comforms to `securesystemslib.formats.SIGNATURE_SCHEMA`

    Raises:
        - KeyError: If signature for key_id is not present in signature_dict
    """
    from binascii import hexlify
    return {'keyid': key_id, 'sig': hexlify(signature_dict.get(key_id)).
        decode()}
