1. A method comprising: inputting, by a computing device, speech data that corresponds to a particular utterance of a particular speaker to a neural network having parameters trained based on propagation between an input layer and an output layer through one or more hidden layers located between the input layer and the output layer, wherein the one or more hidden layers were trained using utterances of multiple speakers, and wherein the multiple speakers do not include the particular speaker; generating, by the computing device and in response to inputting the speech data that corresponds to the particular utterance to the neural network, a representation of activations occurring at a particular layer of the neural network that was trained as one of the hidden layers located between the input layer and the output layer; comparing, by the computing device, the generated representation of activations occurring at the particular layer of the neural network in response to the speech data that corresponds to the particular utterance with a reference representation of activations occurring at the particular layer of the neural network in response to speech data that corresponds to one or more past utterances of the particular speaker; based on comparing the generated representation and the reference representation, determining, by the computing device, that the particular utterance was likely spoken by the particular speaker; and providing, by the computing device, access to the computing device based on determining that the particular utterance was likely spoken by the particular speaker.