One embodiment provides a method, including: receiving at least two challenge test mechanisms of different challenge test modalities, wherein a challenge test mechanism comprises a challenge portion of a challenge-response test for distinguishing between a human operator and a computer; receiving challenge test operators for combining the at least two challenge test mechanisms; generating a composite challenge task by combining the at least two challenge test mechanisms using the identified challenge test operators; identifying any errors in the composite challenge task by running the composite challenge task; evaluating the composite challenge task to determine (i) a challenge difficulty for a human operator and (ii) a challenge difficulty for a computer; and implementing the composite challenge task if (i) no errors are identified at the composite challenge task analyzer, (ii) the challenge difficulty for a human operator is below a predetermined threshold, and (iii) the challenge difficulty for a computer is above a predetermined threshold.