1. A method of identifying a source of data input to a computing system: comprising: a) receiving speech utterance from an entity related to randomly selected challenge text; wherein said challenge text represents a selected set of one more contiguous words which when articulated have a measurable difference in acoustical characteristics between a reference human voice and a reference computer synthesized voice that exceeds a target threshold; b) processing said speech utterance with said computing system to compute first acoustical characteristics of said entity; c) generating a determination of whether said speech utterance originated from a machine or a human.