15. A computer-readable storage device storing software comprising instructions executable by one or more computers which, upon such execution, cause the one or more computers to perform operations comprising: receiving, by a hotword detector of a mobile computing device that includes (a) the hotword detector, (b) an ultrasonic audio subsystem, and (c) a replay attack engine and that is (i) operating in a mode in which access to one or more resources is disabled, and (ii) configured to exit the mode in which access to the one or more resources is disabled upon detecting an utterance of a hotword, an initial audio input corresponding to an utterance of an initial portion of a hotword; while a remaining portion of the hotword is being uttered and before the hotword is fully uttered, providing, by the ultrasonic audio subsystem of the mobile computing device, verification audio for output through a speaker of the mobile computing device; and selectively exiting, by the replay attack engine of the mobile computing device, the mode in which access to the one or more resources is disabled after the hotword is fully uttered.
15. A computer-readable storage device storing software comprising instructions executable by one or more computers which, upon such execution, cause the one or more computers to perform operations comprising: receiving, by a hotword detector of a mobile computing device that includes (a) the hotword detector, (b) an ultrasonic audio subsystem, and (c) a replay attack engine and that is (i) operating in a mode in which access to one or more resources is disabled, and (ii) configured to exit the mode in which access to the one or more resources is disabled upon detecting an utterance of a hotword, an initial audio input corresponding to an utterance of an initial portion of a hotword; while a remaining portion of the hotword is being uttered and before the hotword is fully uttered, providing, by the ultrasonic audio subsystem of the mobile computing device, verification audio for output through a speaker of the mobile computing device; and selectively exiting, by the replay attack engine of the mobile computing device, the mode in which access to the one or more resources is disabled after the hotword is fully uttered. 20. The computer-readable storage device of claim 15 , the operations further comprising: receiving, by the mobile computing device, an additional audio input corresponding to an utterance of the remaining portion of a hotword; determining that the initial audio input corresponding to an utterance of an initial portion of a hotword and the additional audio input corresponding to an utterance of the remaining portion of a hotword, collectively, correspond to the entire hotword, wherein selectively exiting the mode is based at least in part on determining that the initial audio input corresponding to an utterance of an initial portion of a hotword and the additional audio input corresponding to an utterance of the remaining portion of a hotword, collectively, correspond to the entire hotword.