15. The system of claim 14 , wherein identifying subwords to be included in the verification phrase comprises: identifying candidate subwords, for which no stored acoustic data is associated with the user, as one or more of the subwords to be included in the verification phrase, wherein obtaining a candidate phrase that includes at least some of the identified subwords as the verification phrase comprises: obtaining a candidate phrase that includes at least one candidate subword for which stored acoustic data is associated with the user and at least one candidate subword for which no stored acoustic data is associated with the user.
16. The system of claim 15 , the operations comprising: storing acoustic data from the obtained acoustic data that corresponds to the identified candidate subwords, for which no stored acoustic data is associated with the user, in association with the user.