11. At least one computer readable medium encoded with at least one program for execution on at least one processor, the program having instructions that, when executed by the at least one processor, perform a method of determining a 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.
11. At least one computer readable medium encoded with at least one program for execution on at least one processor, the program having instructions that, when executed by the at least one processor, perform a method of determining a 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. 16. The at least one computer readable medium of claim 11 , wherein the at least one enrollment word is selected from an enrollment vocabulary, and wherein obtaining the voice signal of the speaker uttering the at least one challenge utterance includes obtaining at least one challenge utterance selected from a challenge vocabulary comprising a plurality of challenge words, and wherein the challenge vocabulary includes more words than the enrollment vocabulary.