2. The method of claim 1 , wherein obtaining the voice signal of the speaker uttering the at least one challenge utterance includes obtaining at least one challenge utterance consisting preponderantly of words not used in the at least one enrollment utterance.
2. The method of claim 1 , wherein obtaining the voice signal of the speaker uttering the at least one challenge utterance includes obtaining at least one challenge utterance consisting preponderantly of words not used in the at least one enrollment utterance. 3. The method of claim 2 , wherein obtaining the voice signal of the speaker uttering the at least one challenge utterance includes obtaining at least one challenge utterance consisting substantially of words not used in the at least one enrollment utterance.