1. An information processing system comprising: a memory; and processing circuitry configured to when an analysis log of a monitoring target program is generated, capture a library function invoked by the monitoring target program which processes source information, the library function being included as part of an operating system of the information processing system; store, in the memory, the analysis log including at least input data to the library function together with a tag for uniquely identifying the invocation of the library function every time the library function is invoked during the processing of the source information; specify a library function invoked during the processing of the source information by using as a key the tag set to the stored analysis log; and extrapolate the source information based on characteristics of the specified library function, even when obfuscation data remains obfuscated, wherein the processing circuitry when the tag is set to output data from the library function and when the analysis log of the monitoring target program is generated, propagates the tag set to the output data from the library function to any subsequent processing performed by a next library function that is invoked by the monitoring target program, and when the tag is not set to the output data from the library function and when the analysis log of the monitoring target program is generated, sets the tag for uniquely identifying the invocation of the library function to be propagated to any subsequent processing performed by a next library function that is invoked by the monitoring target program.