1. A system for verifying user identity comprising: a conversational system that receives multi-modal inputs from a user interacting with the conversational system during a user session and transforms the received multi-modal inputs into formal commands executable by a program of instructions executable by a processor; and a behavior verifier coupled to the conversational system to extract 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 wherein the behavior verifier compares 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.
1. A system for verifying user identity comprising: a conversational system that receives multi-modal inputs from a user interacting with the conversational system during a user session and transforms the received multi-modal inputs into formal commands executable by a program of instructions executable by a processor; and a behavior verifier coupled to the conversational system to extract 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 wherein the behavior verifier compares 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. 9. The system as recited in claim 1 , wherein the behavior model includes a plurality of models representative of different interaction behaviors comprising combinations of input modalities of the user.