A mobile communications device including: a plurality of first input devices capable of passively collecting input data; at least one second input device capable of collecting response data based upon a challenge; and a processor coupled to the plurality of first input devices and the at least one second input device and capable of determining a level of assurance, LOA, that possession of the mobile communications device has not changed based upon a statistical behavioral model and the passively collected input data from the plurality of first input devices, comparing the LOA with a security threshold, when the LOA is above the security threshold, performing a given mobile device operation without requiring response data from the at least one second input device, and when the LOA falls below the security threshold, generating the challenge, performing the given mobile device operation responsive to valid response data from the at least one second input device, characterized by adding recent input data to the statistical behavioral model responsive to receipt of the valid response data, or excluding the recent input data from the statistical behavioral model responsive to invalid response data, wherein the determining the LOA is performed by an LOA engine implemented as an artificial neural network providing an artificial intelligence, AI, for learning new passive factors that are indicative of the behavior of an authorized user(s). a plurality of first input devices capable of passively collecting input data; at least one second input device capable of collecting response data based upon a challenge; and a processor coupled to the plurality of first input devices and the at least one second input device and capable of determining a level of assurance, LOA, that possession of the mobile communications device has not changed based upon a statistical behavioral model and the passively collected input data from the plurality of first input devices, comparing the LOA with a security threshold, when the LOA is above the security threshold, performing a given mobile device operation without requiring response data from the at least one second input device, and when the LOA falls below the security threshold, generating the challenge, performing the given mobile device operation responsive to valid response data from the at least one second input device, characterized by adding recent input data to the statistical behavioral model responsive to receipt of the valid response data, or excluding the recent input data from the statistical behavioral model responsive to invalid response data, wherein the determining the LOA is performed by an LOA engine implemented as an artificial neural network providing an artificial intelligence, AI, for learning new passive factors that are indicative of the behavior of an authorized user(s). determining a level of assurance, LOA, that possession of the mobile communications device has not changed based upon a statistical behavioral model and the passively collected input data from the plurality of first input devices, comparing the LOA with a security threshold, when the LOA is above the security threshold, performing a given mobile device operation without requiring response data from the at least one second input device, and when the LOA falls below the security threshold, generating the challenge, performing the given mobile device operation responsive to valid response data from the at least one second input device, characterized by adding recent input data to the statistical behavioral model responsive to receipt of the valid response data, or excluding the recent input data from the statistical behavioral model responsive to invalid response data, wherein the determining the LOA is performed by an LOA engine implemented as an artificial neural network providing an artificial intelligence, AI, for learning new passive factors that are indicative of the behavior of an authorized user(s). generating the challenge, performing the given mobile device operation responsive to valid response data from the at least one second input device, characterized by adding recent input data to the statistical behavioral model responsive to receipt of the valid response data, or excluding the recent input data from the statistical behavioral model responsive to invalid response data, wherein the determining the LOA is performed by an LOA engine implemented as an artificial neural network providing an artificial intelligence, AI, for learning new passive factors that are indicative of the behavior of an authorized user(s).