8. A system comprising: one or more computers and one or more storage devices storing instructions that are operable, when executed by the one or more computers, to cause the one or more computers to perform operations comprising: obtaining training acoustic data corresponding to a user's utterance of one or more words that include one or more particular subwords; dynamically generating a verification phrase based at least on one or more of the particular subwords included in the words uttered by the user in the training acoustic data; prompting, by a mobile device that is in a locked mode, the user to speak the dynamically generated verification phrase; and in response to determining that the user has likely spoken the dynamically generated verification phrase, determining, by the mobile device, whether to exit the locked mode.
8. A system comprising: one or more computers and one or more storage devices storing instructions that are operable, when executed by the one or more computers, to cause the one or more computers to perform operations comprising: obtaining training acoustic data corresponding to a user's utterance of one or more words that include one or more particular subwords; dynamically generating a verification phrase based at least on one or more of the particular subwords included in the words uttered by the user in the training acoustic data; prompting, by a mobile device that is in a locked mode, the user to speak the dynamically generated verification phrase; and in response to determining that the user has likely spoken the dynamically generated verification phrase, determining, by the mobile device, whether to exit the locked mode. 10. The system of claim 8 , wherein dynamically generating a verification phrase based at least on one or more of the particular subwords included in the words uttered by the user in the training acoustic data comprises: generating a verification phrase that includes at least one or more of the particular subwords.