8. A system comprising: a processor; and a computer-readable storage medium having instructions stored which, when executed by the processor, cause the processor to perform operations comprising: generating a feature coefficient based on a speech signal from a user; comparing the feature coefficient to a user-specific codebook associated with the user, to yield a similarity value, wherein the user-specific codebook utilizes utterances from both the user and a group of non-users; and when the similarity value meets a threshold: adding the speech signal to a database of reference speech signals associated with the user-specific codebook; and adding the feature coefficient to the user-specific codebook.
14. The system of claim 8 , the computer-readable storage medium having additional instructions stored which result in operations comprising verifying an identity of the user based on the similarity value.