1. A method comprising: selecting a set of printable characters as test inputs for an unknown binary module having no known valid input and for which a specification, documentation, and a source code associated with the unknown binary module are unavailable; and building a string of printable characters that includes one or more printable characters that are valid inputs for the unknown binary module at one or more index positions in the string of printable characters, the building including: executing the unknown binary module with each printable character of the set of printable characters at an index position in the string of printable characters as one of the test inputs for the unknown binary module; determining, for each of the printable characters included in the set of printable characters at the index position, a number of instructions executed by the unknown binary module responsive to being executed with each printable character of the set of printable characters at the index position; generating set data including each of the printable characters associated with the number of instructions executed for each of the printable characters at the index position; and analyzing the set data to identify one or more printable characters of the set of printable characters at the index position as one or more valid inputs for the unknown binary module based on a comparison of the number of instructions associated with each of the printable characters at the index position and a threshold range.