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 characteristic features of at least one first voice signal obtained from the user uttering at least one enrollment utterance including at least one enrollment word, the method comprising acts of: obtaining a second 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; obtaining at least one characteristic feature from the second voice signal; comparing the at least one characteristic feature with at least a portion of the voice print to determine a similarity between the at least one characteristic feature and the at least a portion of the voice print; and determining whether the speaker is the user based, at least in part, on the similarity between the at least one characteristic feature and the at least a portion of the voice print.