1. A method for determining validity of an identity asserted by a speaker using a voice print associated with a user whose identity the speaker is asserting, the voice print obtained from the user's utterance of at least one enrollment utterance including at least one enrollment word, the method comprising: obtaining a voice signal of the speaker uttering at least one challenge utterance, wherein the at least one challenge utterance includes at least one word that was not in the at least one enrollment utterance; and determining whether the speaker is the user based, at least in part, on the voice signal and the voice print.