def __init__(self, hash_function, info_string):
super(RootKeyKDF, self).__init__()<EOL>if not hash_function in RootKeyKDF.HASH_FUNCTIONS:<EOL><INDENT>raise ValueError("<STR_LIT>")<EOL><DEDENT>if not isinstance(info_string, bytes):<EOL><INDENT>raise TypeError("<STR_LIT>")<EOL><DEDENT>self.__hash_function = RootKeyKDF.HASH_FUNCTIONS[hash_function]<EOL>self.__info_string   = info_string<EOL>
Prepare a RootKeyKDF, following a recommendation by WhisperSystems.

:param hash_function: One of (the strings) "SHA-256" and "SHA-512".
:param info_string: A bytes-like object encoding a string unique to this usage
    within the application.