1. An apparatus comprising: at least one sensor; a receiver configured to receive information from the at least one sensor; at least one processing core configured to determine at least one characteristic of a user based at least in part on the information received from the at least one sensor, and to modify execution of a process based at least in part on the at least one characteristic, wherein the at least one characteristic of the user is expressed by a characteristic vector comprising at least one element of the information received from the at least one sensor; and a memory storing at least two reference data items comprising reference data item vectors, wherein the at least one processing core is further configured to monitor whether the user is a legitimate user at the same time that the apparatus is being used by the user based at least on the determined at least one characteristic of the user, wherein the information received from the at least one sensor and the at least one characteristic of the user are updated at the same time that the apparatus is being used by the user, and wherein the at least one processing core is configured to determine the at least one characteristic of the user by determining a best match between the characteristic vector and the at least two reference data item vectors using a sum of squares method comprising: obtaining a difference vector by subtracting a first one of the characteristic vector and a reference data item vector, squaring each element of the difference vector, summing the squared elements of the difference vector, repeating the obtaining and the squaring and the summing for the at least two reference data items, and considering the at least two reference data items with a lowest sum of squared difference as the closest match with the characteristic vector.