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.