In one aspect, a method for determining a validity of an identity asserted by a speaker using a voice print that models speech of a user whose identity the speaker is asserting is provided. The method comprises acts of performing a first verification stage comprising acts of obtaining a first voice signal from the speaker uttering at least one first challenge utterance; and comparing at least one characteristic feature of the first voice signal with at least a portion of the voice print to assess whether the at least one characteristic feature of the first voice signal is similar enough to the at least a portion of the voice print to conclude that the first voice signal was obtained from an utterance by the user. The method further comprises performing a second verification stage if it is concluded in the first verification stage that the first voice signal was obtained from an utterance by the user, the second verification stage comprising acts of adapting at least one parameter of the voice print based, at least in part, on the first voice signal to obtain an adapted voice print, obtaining a second voice signal from the speaker uttering at least one second challenge utterance, and comparing at least one characteristic feature of the second voice signal with at least a portion of the adapted voice print to assess whether the at least one characteristic feature of the second voice signal is similar enough to the at least a portion of the adapted voice print to conclude that the second voice signal was obtained from an utterance by the user.