A method for verifying an identity of a user comprising: receiving, by data processing hardware, a request to verify an identity of an unverified user; obtaining, by the data processing hardware, stored audio data representing a particular user speaking a subset of subwords of a verification phrase, the verification phrase generated by the data processing hardware based on: (i) at least one subword for which no stored audio data representing the user speaking the subword has been obtained; and (ii) at least one subword for which stored audio data representing the user speaking the subword has been obtained; wherein the verification phrase comprises the at least one subword for which no stored audio data representing the user speaking the subword has been obtained and the at least one subword for which stored audio data representing the user speaking the subword has been obtained; in response to receiving the request to verify the identity of the unverified user, prompting, by the data processing hardware, the unverified user to speak the verification phrase; receiving, at the data processing hardware, verification audio data representing the unverified user speaking the verification phrase; determining, by the data processing hardware, whether the unverified user speaking the verification phrase is the particular user who spoke the verification phrase based on the stored audio data and the verification audio data; and in response to determining that the unverified user speaking the verification phrase is the particular user who spoke the verification phrase: storing the verification audio data as training data corresponding to the particular user. receiving, by data processing hardware, a request to verify an identity of an unverified user; obtaining, by the data processing hardware, stored audio data representing a particular user speaking a subset of subwords of a verification phrase, the verification phrase generated by the data processing hardware based on: (i) at least one subword for which no stored audio data representing the user speaking the subword has been obtained; and (ii) at least one subword for which stored audio data representing the user speaking the subword has been obtained; (i) at least one subword for which no stored audio data representing the user speaking the subword has been obtained; and (ii) at least one subword for which stored audio data representing the user speaking the subword has been obtained; wherein the verification phrase comprises the at least one subword for which no stored audio data representing the user speaking the subword has been obtained and the at least one subword for which stored audio data representing the user speaking the subword has been obtained; in response to receiving the request to verify the identity of the unverified user, prompting, by the data processing hardware, the unverified user to speak the verification phrase; receiving, at the data processing hardware, verification audio data representing the unverified user speaking the verification phrase; determining, by the data processing hardware, whether the unverified user speaking the verification phrase is the particular user who spoke the verification phrase based on the stored audio data and the verification audio data; and in response to determining that the unverified user speaking the verification phrase is the particular user who spoke the verification phrase: storing the verification audio data as training data corresponding to the particular user.