26. A program storage device readable by machine, tangibly embodying a program of instructions executable by the machine to perform method steps for verifying a user identity, the method steps comprising: receiving multi-modal inputs from a user interacting with a conversational system during a user session and transforming the received multi-modal inputs into formal commands executable by the machine; extracting features from the multi-modal inputs and formal commands, wherein the extracted features include a combination of input modalities representative of the user's current interaction behavior for performing a task during the user session; and comparing the combination of input modalities representative of the user's current interaction behavior for performing the task to a behavior model representative of the user's past interaction behavior comprising a known combination of input modalities for performing the task used by the user during one or more previous user sessions to determine the identity of the user.
26. A program storage device readable by machine, tangibly embodying a program of instructions executable by the machine to perform method steps for verifying a user identity, the method steps comprising: receiving multi-modal inputs from a user interacting with a conversational system during a user session and transforming the received multi-modal inputs into formal commands executable by the machine; extracting features from the multi-modal inputs and formal commands, wherein the extracted features include a combination of input modalities representative of the user's current interaction behavior for performing a task during the user session; and comparing the combination of input modalities representative of the user's current interaction behavior for performing the task to a behavior model representative of the user's past interaction behavior comprising a known combination of input modalities for performing the task used by the user during one or more previous user sessions to determine the identity of the user. 32. The program storage device as recited in claim 26 , wherein the extracted features include features representative of a duration between executed user commands.