Abstract: Herein disclosed is a method for automatically automatically infer a recursive state machine (RSM) describing the space of acceptable input of an arbitrary binary program. This method automatically identifies atomic fields of fixed and variable lengths and syntactic elements, such as separators and terminators, and generalizes them into regular expression tokens. It constructs an RSM of tokens to represent structures such as arrays and records. Further, it constructs nested states in RSM to represent complex, nested structures. The RSM may serve as an independent parser for the program's acceptable input.