1. An information processing device comprising: a processor; and a storage medium coupled to the processor and storing a program that, when executed by the processor, causes the device to implement: an input section configured to input a plurality of symbol strings and attribute information desired to be extracted from each symbol string; a feature quantity function generation section configured to randomly select a plurality of functions from a predetermined function group including a function for converting a symbol string into a numerical value, and generate a plurality of feature quantity functions for outputting a feature quantity from the symbol string by combining the plurality of functions; a feature quantity calculation section configured to input each symbol string input by the input section to each feature quantity function generated by the feature quantity function generation section and to calculate a feature quantity corresponding to each symbol string; an estimation function generation section configured to execute machine learning using the attribute information corresponding to each symbol string input by the input section and the feature quantity corresponding to each symbol string calculated by the feature quantity calculation section, and generate an estimation function for estimating the attribute information from the feature quantity; and an output section configured to output the feature quantity functions generated by the feature quantity function generation section and the estimation function generated by the estimation function generation section.