5. The method of claim 4 , wherein each of the plurality of words in the challenge vocabulary has at least one syllable that rhymes with at least one syllable of at least one word used in the at least one enrollment utterance.
4. 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 selected from a challenge vocabulary comprising a plurality of challenge words, wherein the challenge vocabulary includes words having substantial phonetic overlap with the at least one word used in the at least one enrollment utterance.