1. A method for generating a spoken password for use by an individual speaker of interest, the method comprising: generating a speaker-specific model from a plurality of speech samples, the speaker-specific model identifying one or more speaker-distinguishing speech features that distinguish the individual speaker of interest as different from a plurality of impostor speakers, the speaker-specific model generated by: extracting a plurality of features from utterances of the speaker of interest and from utterances of other speakers; defining one or more regions within the utterances based on the extracted features; modeling one or more speech features within each of the one or more regions; and based on the modeling, assigning a performance measure to one or more of the extracted features, wherein the performance measure represents a strength with which the one or more extracted features distinguishes the speaker of interest from the other speakers; and using the speaker-specific model to generate a spoken password that would elicit the speaker-distinguishing feature when the spoken passphrase is spoken.