Patent Publication Number: US-2022229887-A1

Title: Recognition of unauthorized access of a portable electronic apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to Italian Application No. 102021000000752, filed on Jan. 18, 2021, which application is hereby incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to a method of recognition of an unauthorized access of a portable electronic apparatus. 
     BACKGROUND 
     As is known, electronic apparatuses, in particular portable electronic apparatuses such as mobile phones and wearable devices, are typically provided with mechanisms of protection capable of preventing any undesired or unauthorized access, for example by an unauthorized user. For instance, generally, access to the electronic apparatus may be protected by a secret identifier, which is known only to an authorized user, such as a password, a secret code, a pattern, or by a method of biometric identification, for example based upon fingerprints, voice recognition, and/or face recognition of the authorized user. 
     However, the secret identifier may get stolen or forcefully extorted by the unauthorized user to the detriment of the authorized user. 
     Moreover, the fingerprints, voice, and images of the authorized user may be forged by the unauthorized user in order to be able to gain access to the electronic apparatus. 
     Electronic apparatuses are moreover known, configured to store and analyse data regarding the way in which the authorized user uses the Internet traffic and/or specific applications connected to the Internet, for example social networks, so as to detect possible different or discordant modes of use that could indicate that an unauthorized user has taken over control of the electronic apparatus; for example, the apparatus has been stolen. However, this method has a low accuracy in preventing any undesired access to the electronic apparatus and requires the electronic apparatus to be connected to the Internet. Consequently, this method has a limited applicability, and the level of safety provided is not sufficiently high in specific applications. 
     The aim of the present application is to overcome the drawbacks of the known art. 
     SUMMARY 
     An embodiment includes a method of recognition of an authorized user from an unauthorized user of a portable electronic apparatus, where the portable electronic apparatus includes a plurality of sensors and a user-recognition circuit. The method includes receiving, at the user-recognition circuit, a plurality of electrical signals from the plurality of sensors in the portable electronic apparatus, where each of the plurality of sensors is configured to generate a signal representative of a respective physical quantity associated with the use, by a user, of the portable electronic apparatus in an operating state. The method includes determining, at the user-recognition circuit, a plurality of recognition parameters from the plurality of electrical signals, where each recognition parameter is associated with a specific mode of use of the portable electronic apparatus by the user. The method includes determining a plurality of indicators of use at the user-recognition circuit, one of the indicators being determined for each of the recognition parameters, where each of the indicators of use indicate the probability that, at a time instant, the respective recognition parameter is associable with an unauthorized user; determining, at the user-recognition circuit, a probability of intrusion starting from the plurality of indicators of use; and comparing, at the user-recognition circuit, the probability of intrusion with an intrusion threshold and based thereon recognizing the authorized user from an unauthorized user. 
     In an embodiment, a system includes a portable electronic apparatus comprising a plurality of sensors and a user-recognition circuit, the sensors being each configured to generate a signal representative of a respective physical quantity associated with the use, by a user, of the portable electronic apparatus in an operating state. The user-recognition circuit is configured to receive a plurality of electrical signals from the plurality of sensors in the portable electronic apparatus, where each of the plurality of sensors is configured to generate a signal representative of a respective physical quantity associated with the use, by a user, of the portable electronic apparatus in an operating state. The user-recognition circuit is configured to determine a plurality of recognition parameters from the plurality of electrical signals, each recognition parameter being associated with a specific mode of use of the portable electronic apparatus by the user; determine a plurality of indicators of use at the user-recognition circuit, one of the indicators being determined for each of the recognition parameters, where each of the indicators of use indicate the probability that, at a time instant, the respective recognition parameter is associable with an unauthorized user. The user-recognition circuit is configured to determine a probability of intrusion starting from the plurality of indicators of use; and compare the probability of intrusion with an intrusion threshold and based thereon recognize an authorized user from an unauthorized user of the portable electronic apparatus. 
     According to the present invention, a method of recognition of use of a portable electronic apparatus by an unauthorized user and a portable electronic apparatus are provided, as defined in the annexed claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the present invention, embodiments thereof are now described, purely by way of non-limiting example, with reference to the attached drawings, wherein: 
         FIG. 1  shows a simplified block diagram of the present portable electronic apparatus; 
         FIG. 2  shows a block diagram of the external user recognition circuit of the portable electronic apparatus of  FIG. 1 ; 
         FIG. 3  shows a flowchart of the present method of recognition of use of the portable electronic apparatus of  FIG. 1  by an unauthorized user; 
         FIG. 4  shows a pattern generated by the specific discordance indicators that are calculated by the portable electronic apparatus of  FIG. 1 , applying the method of  FIG. 3 ; 
         FIG. 5  shows a flowchart of a part of the present method of recognition of use of the portable electronic apparatus of  FIG. 1 , according to another embodiment; 
         FIG. 6  shows a block diagram of a specific embodiment of the portable electronic apparatus of  FIG. 1 ; and 
         FIG. 7  shows a block diagram of an electrostatic-charge-variation sensor of a specific embodiment of the portable electronic apparatus of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
       FIG. 1  is a schematic illustration of a portable electronic apparatus  1 , for example a mobile phone, a wearable device, such as a smartwatch or a pair of augmented-reality glasses, etc. 
     The portable electronic apparatus  1  is configured, when it is on, so as to be able to be in one of two different operating conditions, namely, an operating state, in which an external user can access the functions of the portable electronic apparatus  1  and the data stored therein, and a permanent block state, in which the external user is prevented from accessing the functions of the portable electronic apparatus  1  and the data stored therein. 
     As described in detail hereinafter, the portable electronic apparatus  1  is configured to be in the operating state if the external user is recognized as authorized user, for example an owner of the portable electronic apparatus  1 , and in the permanent block state if the external user is recognized as unauthorized user. In particular, the portable electronic apparatus  1  is configured to verify, at pre-set instants or in pre-set situations, whether the current external user is authorized or not and, if the current external user is not authorized, to switch from the operating state to the permanent block state. The portable electronic apparatus  1  may moreover be configured so that, after being set into the permanent block state, following upon detection of an unauthorized user, it can no longer switch into the operating state, unless a reset operation is carried out by a special authorized user. 
     The portable electronic apparatus  1  comprises a main processing circuit  2 , a main memory  3 , an external user recognition circuit  4 , a plurality of sensors  5 , and a plurality of input and output interfaces  7 , operatively coupled together. 
     For instance, the plurality of input and output interfaces  7  comprises one or more keys, a screen, for example a touchscreen, and/or one or more wireless connection cards. 
     In detail, as illustrated in  FIG. 2 , the external user recognition circuit  4  comprises one or more processing circuits, here a first, a second, and a third processing circuit  10 ,  11 ,  12 , which are operatively coupled together. 
     The sensors  5  are each configured to detect a respective physical quantity associated with use of the portable electronic apparatus  1 , generate a set of electrical signals I and supply the set of electrical signals I to the external user recognition circuit  4 , in particular to the first processing circuit  10 . 
     The set of electrical signals I comprises, for each sensor of the plurality of sensors  5 , one or more electrical signals as a function of the respective physical quantities detected. 
     In detail, and as described in detail hereinafter with reference to  FIG. 6 , the plurality of sensors  5  comprises movement sensors, for example accelerometers and gyroscopes, configured to detect quantities associated, for example, with acceleration, velocity, displacement, and angles of rotation of the portable electronic apparatus  1 ; acoustic sensors, configured to detect quantities associated with the sounds surrounding the portable electronic apparatus  1 ; pressure sensors, configured to detect quantities associated, for example, with intensity, duration, and frequency of touch of a screen or of a key of the electronic apparatus  1 . 
     According to an embodiment, the plurality of sensors  5  also comprises at least one electrostatic-charge-variation sensor, as discussed in detail hereinafter with reference to  FIG. 7 . In particular, the electrostatic-charge-variation sensor is configured to detect a variation of electrostatic charge associated, for example, with the presence, movement, gestures, or other activities of an external user, who is in contact with, or in the proximity of, the portable electronic apparatus  1 , or associated with touching of the portable electronic apparatus  1 . 
     In addition, the plurality of sensors  5  may comprise one or more temperature sensors, humidity sensors, locating devices, for example of a GPS type, proximity sensors, one or more photographic cameras and video cameras, biometric sensors, for example for fingerprint detection, measurement of body temperature, body fat, breathing rate, blood pressure, heartbeat, etc. 
     The first, the second and the third processing circuits  10 ,  11 ,  12  each comprise a respective memory, respectively indicated by  17 ,  18  and  19 , and are configured to determine a probability of intrusion of the portable electronic apparatus  1  by an unauthorized user, starting from the values of the set of electrical signals I, in particular using known machine learning algorithms. 
     The first processing circuit  10  further comprises a plurality of analysis circuits  13 , functionally coupled to the memory  17  and configured to receive the set of electrical signals I and determine a plurality of indicators of use of the portable electronic apparatus  1 , starting from the set of electrical signals I, as discussed in detail hereinafter. 
     Here, the analysis circuits  13  each comprise a respective parameter-extraction circuit  15  and a respective classification circuit  16 . 
     Each analysis circuit  13 , in particular each parameter-extraction circuit  15 , is configured to receive in input a respective portion of the set of electrical signals I, starting from which to determine a respective set of signal-description parameters, indicative of a respective recognition parameter, which is associated with a specific way of use of the portable electronic apparatus  1  by the external user. 
     For instance, a recognition parameter may be the acoustic background noise, the way of movement, the way of touching the portable electronic apparatus  1 , the surrounding environment, for example according to one or more quantities chosen among temperature, humidity, and light intensity, the number of stairs climbed per day, the number of steps made per day, the position of the cellphone during a call, the typical mode and timetable of battery charging, etc. 
     In detail, each set of signal-description parameters comprises the value of one or more statistical parameters, which are calculated starting from the respective portion of signals of the set of electrical signals I. 
     For instance, each parameter-extraction circuit  15  can determine, as set of signal-description parameters, the mean and/or variance of the respective portion of the set of electrical signals I, in a specific time window. 
     For instance, each parameter-extraction circuit  15  can determine, in a specific time window, as set of signal-description parameters, one or more among auto-correlation, number of zero-crossings, peak-to-peak value, number of peaks, number and position of local maxima and minima, inflection points, or a combination of the foregoing, starting from the respective portion of the set of electrical signals I, in particular starting, for example, from amplitude, mean, variance, frequency spectrum, derivative, or integral of the respective portion of the set of electrical signals I. 
     The classification circuits  16  are each configured to receive the respective set of signal-description parameters and determine the plurality of indicators of use. 
     In detail, the classification circuits  16  are each configured to classify the respective recognition parameter in a respective concordance class or in a respective discordance class by applying one or more known classification algorithms to the values of the respective set of signal-description parameters. 
     The concordance class of a recognition parameter is defined by the values of the respective set of signal-description parameters that are associated with a specific mode of use of the portable electronic apparatus  1  by the authorized user. 
     The discordance class of a recognition parameter is defined by the values of the respective set of signal-description parameters that are associated with a specific mode of use of the portable electronic apparatus  1  by the unauthorized user. 
     In order to carry out the classification, the classification circuits  16  each compare current values of the respective set of signal-description parameters with previous values of the respective set of signal-description parameters, using specific weights stored in the memory  17  of the first processing circuit  10 . 
     The classification circuits  16  generate, at output, following upon classification, the plurality of indicators of use, referred to here and in what follows as plurality of specific discordance indicators DISCI 1 (t), . . . , DISCI N (t). Each specific discordance indicator DISCI i (t) indicates the current discordance probability P d (t) of the respective recognition parameter, i.e., the probability of the respective recognition parameter being classified, at a time instant t, in the respective discordance class. 
     The current discordance probability P d (t) of a recognition parameter can be calculated starting from the current concordance probability P c (t) of the same recognition parameter, i.e., from the probability of the recognition parameter being classified, at a time instant t, in the respective concordance class. In fact, the current discordance probability P d (t) of a recognition parameter is a function of the current concordance probability P c (t) of the same recognition parameter; in particular, P c (t)+P d (t)=1. 
     The one or more classification algorithms used by the classification circuits  16  may be of a supervised type, a non-supervised type, or a combination thereof. For instance, the one or more classification algorithms may include a clustering algorithm, a decision tree, a neural network, for example of a recurrent, recursive, deep, or convolutional type, a support-vector machine (SVM), a K-means algorithm, etc. 
     According to an embodiment, as discussed in detail hereinafter with reference to  FIG. 5 , the memory  17  of the first processing circuit  10  comprises training data, which are provided by the authorized user in a step of initialization of the portable electronic apparatus  1  and are used by the classification circuits  16  as examples of values (training values) of the signal-description parameters, and therefore of the respective recognition parameters, belonging to the respective concordance class. 
     The second processing circuit  11  is configured to receive at input the plurality of specific discordance indicators DISCI i (t), . . . , DISCI N (t) supplied at output by the classification circuits  16  and calculate an overall discordance indicator O-DISCI at the time instant t, which indicates the current probability of the portable electronic apparatus  1  being used, at the time instant t, by an unauthorized user. 
     The overall discordance indicator O-DISCI at the time instant t is supplied to the third processing circuit  12  and is stored in the respective memory  19 . The third processing circuit  12  is configured to process the overall discordance indicators O-DISCI stored in the respective memory  19  in a decision interval Δt, and consequently determine whether the external user is recognized or not as authorized user, as described in detail hereinafter. 
     The external user recognition circuit  4  further comprises a block unit  20 , configured to receive at input an intrusion signal INT, from the third processing circuit  12 , in the case where the external user is recognized as unauthorized user, and supply at output a blocking signal B, which is configured to set the portable electronic apparatus  1  in the permanent block state. 
     The external user recognition circuit  4  is configured to execute the method illustrated in the flowchart of  FIG. 3  and designated by the number  40 . 
     The method  40  comprises an initial step S o , wherein the portable electronic apparatus  1  passes, for example, from an OFF state to an ON state, entering the operating state. 
     During operation of the portable electronic apparatus  1 , and therefore during its use by an external user, an acquisition step  50  follows, wherein the sensors  5  each acquire the respective physical quantities associated with use of the portable electronic apparatus  1  by the external user, generate the set of electrical signals I, and supply the set of electrical signals I to the first processing circuit  10 , in particular to the parameter-extraction circuits  15  of the analysis circuits  13 . 
     The sensors  5  may each be configured to acquire and supply, to the first processing circuit  10 , the respective electrical signals in a continuous way, for example on the basis of the period of a respective clock. 
     Consequently, the analysis circuits  13  may be configured (step  55 ) to use a moving window each for processing a signal of the respective portion of electrical signals I. In this way, the input data used by the parameter-extraction circuits  15  are obtained at a time instant t. For instance, within each moving window, the analysis circuits  13  may be configured to execute operations such as averaging, integration, differentiation, etc. 
     Next, the parameter-extraction circuits  15  each process a respective portion of the input data, which is a function of the respective portion of the set of electrical signals I, each to extract the respective set of signal-description parameters, as discussed above. 
     The classification circuits  16  then each classify (step  60 ) the respective set of signal-description parameters, which indicates the respective recognition parameter, and supply at output the specific discordance indicators DISCI i (t), . . . , DISCI N (t), one for each recognition parameter, wherein, as discussed above, each specific discordance indicator DISCI i (t) indicates the probability of the respective recognition parameter, at the time instant t, to belong to the discordance class, i.e., that the respective recognition parameter indicates a specific use of the electronic apparatus  1 , by the external user, that is associable with an unauthorized user. 
     The specific discordance indicators DISCI i (t), . . . , DISCI N (t) are then supplied to the second processing circuit  11 . 
     The second processing circuit  11  (step  65 ) is configured to process the specific discordance indicators DISCI i (t), . . . , DISCI N (t) and supply at output a corresponding overall discordance indicator O-DISCI(t), which indicates the probability that, at the time instant t, the external user is associable with an unauthorized user. 
     According to an embodiment, in step  65 , the second processing circuit  11  is configured to execute a weighted average of the specific discordance indicators DISCI i (t), . . . , DISCI N (t). 
     The weighted average is calculated by attributing a weight w i  to each discordance indicator DISCI i (t). The weights w are stored in the portable electronic apparatus  1 , for example in the memory  18  of the second processing circuit  11 , and can be established during initialization or calibration of the portable electronic apparatus  1  and can be modified in time, following upon subsequent steps of calibration and/or training of the second processing circuit  11 . 
     According to a different embodiment, in step  65 , the second processing circuit  11  is configured to process the specific discordance indicators DISCI i (t), . . . , DISCI N (t), using a known machine-learning algorithm, in particular a pattern-recognition algorithm, for example an artificial neural network, to calculate the overall discordance indicator O-DISCI(t), whose weights w i  are determined during a training step and stored in the memory  18  of the second processing circuit  11 . The specific discordance indicators DISCI i (t), . . . , DISCI N (t) can be represented graphically as illustrated in  FIG. 4  in the case of twelve specific discordance indicators DISCI i (t), . . . , DISCI 12 (t). In particular, the representation of the specific discordance indicators DISCI i (t), . . . , DISCI N (t) uses a plurality of axes, one for each recognition parameter, wherein the value of the respective specific discordance indicator DISCI i  at the time instant t is indicated on each axis. 
     By connecting the value of a specific discordance indicator DISCI i (t) of an axis with the value of the specific discordance indicator DISCI j (t) of an adjacent axis, a pattern is formed (represented in light grey in  FIG. 4 ). 
     The pattern-recognition algorithm is trained to calculate at output the overall discordance indicator O-DISCI(t), receiving at input the specific discordance indicators DISCI i (t), . . . , DISCI N (t) and using the weights w i . 
     The overall discordance indicator O-DISCI(t) is then (step  70 ) supplied to and stored in the memory  19  of the third processing circuit  12  for subsequent processing. 
     The external user recognition circuit  4  verifies whether a decision interval Δt has elapsed (step  75 ). 
     If the decision interval Δt has not elapsed (output NO from step  75 ), the steps  55 - 65 , wherein the specific discordance indicators DISCI 1 , . . . , DISCI N  and the overall discordance indicator O-DISCI are calculated, are repeated at a time instant t+t′ subsequent to the time instant t. Moreover, also step  70  is repeated, wherein the value of the overall discordance indicator O-DISCI at the time instant t+t′ is stored in the memory  19  of the third processing circuit  12 . 
     For instance, the time t′ between two successive repetitions of the steps  55 - 70  may be fixed or variable, and may, for example, be of a few seconds, minutes, or hours, in particular one minute, according to the specific application. 
     When the decision interval Δt has elapsed (output YES from step  75 ), the third processing circuit  12  processes (step  80 ) the values of the overall discordance indicators O-DISCI stored during the decision interval Δt and supplies at output an overall discordance indicator over time O-DISCIT(Δt). 
     The decision interval Δt may have a fixed or variable duration, for example comprised between one day and thirty days. In other words, the overall discordance indicator over time O-DISCIT(Δt) is calculated once a week or once every multiple weeks. 
     For instance, the overall discordance indicator over time O-DISCIT(Δt) can be determined by making a weighted average of the overall discordance indicators O-DISCI(t) in the decision interval Δt. 
     Alternatively, the overall discordance indicator over time O-DISCIT(Δt) can indicate the number of times in which, in the decision interval Δt, the overall discordance indicators O-DISCI(t) are higher than a threshold chosen during calibration or training of the third processing circuit  12 . 
     Then (step  85 ), the third processing circuit  12  compares the overall discordance indicator over time O-DISCIT(Δt) with an intrusion threshold, which is also chosen during calibration or training of the third processing circuit  12 . Step  85  can be executed via a known machine-learning algorithm, for example using an artificial neural network. 
     If the third processing circuit  12  verifies that the overall discordance indicator over time O-DISCIT(Δt) is lower than the intrusion threshold (branch YES at output from step  85 ), then steps  55 - 75  are repeated, and the third processing circuit  12  calculates a new value of the overall discordance indicator over time O-DISCIT(Δt), using the values of the overall discordance indicators O-DISCI stored in the memory  19  of the third processing circuit  12 . 
     If the third processing circuit  12  verifies that the overall discordance indicator over time O-DISCIT(Δt) is higher than the intrusion threshold (branch NO at output from step  85 ), then the third processing circuit  12  supplies the intrusion signal INT to the block unit  20 . Next (step  88 ), the block unit  20  generates and supplies at output the block signal B, for example to the main processing circuit  2 , which is configured to set the portable electronic apparatus  1  in the permanent block state (end S f ). 
     In other words, step  88  indicates that, during the decision interval Δt, the external user who has used the portable electronic apparatus  1  corresponds to an unauthorized user; for example, the portable electronic apparatus  1  has been stolen. The permanent block state guarantees that the unauthorized user has no longer access to the functionalities of the portable electronic apparatus  1  and to the data stored therein. 
     According to an embodiment, the portable electronic apparatus  1  may be further configured so that a special authorized user, for example the manufacturer of the portable electronic apparatus  1 , can restore the portable electronic apparatus  1  from the permanent block state to the operating state. 
     According to this embodiment, as illustrated in  FIG. 5 , the method, designated by  87 , also comprises an initialization step  92 , executed before the steps of the method  40  of  FIG. 3 . In detail, the method  87  starts with a step  90 , corresponding to a first start-up of the portable electronic apparatus  1 . For instance, the first start-up corresponds to the first time when the authorized user starts the portable electronic apparatus  1 . 
     A step of initialization  92  of the external user recognition circuit  4  of the portable electronic apparatus  1  follows, wherein the machine-learning algorithms used by the first processing circuit  10  are trained on the basis of information of use supplied directly by the authorized user. In detail, the information of use supplied by the authorized user is converted, by using known conversion factors stored in the portable electronic apparatus  1 , for example in the memory  17  of the first processing circuit  10 , into expected values of the set of electrical signals I, i.e., values of the recognition parameters belonging to the concordance class, thus corresponding to the use of the portable electronic apparatus  1  by the authorized user. Then, the method  87  proceeds with step  50  of the method  40  of  FIG. 3 . 
     The information of use can also be used for training the machine-learning algorithms used by the second processing circuit  11  and by the third processing circuit  12 ; thus, for example, to train the algorithm of recognition of the pattern formed by the specific discordance indicators DISCI i (t), . . . , DISCI N (t), or to determine the weights w i  used in step  65 , and to train the algorithm of determination of the overall discordance indicator over time O-DISCI(Δt), or to determine the weights used in step  80  and the intrusion threshold used in step  85 . 
     For instance, the information of use can be determined on the basis of questions, which are submitted to the authorized user for example via a graphic interface of the portable electronic apparatus  1 , and which are indicative of the way in which the authorized user uses the portable electronic apparatus  1 . 
     Therefore, the information of use allows to have positive-labelled examples linked to use of the portable electronic apparatus  1  by the authorized user and therefore allows to speed-up the step of training of the machine-learning algorithms used by the first, the second and the third processing circuits  10 ,  11 ,  12 . This also allows to obtain a high level of accuracy in determining whether the external user corresponds to the authorized user, even though a limited time has elapsed since the first start-up of the portable electronic apparatus  1 . 
       FIG. 6  shows another embodiment of the portable electronic apparatus  1  wherein the method  40  of  FIG. 3  and the method  87  of  FIG. 5  are executed using three specific discordance indicators, referred to as first specific discordance indicator DISCI 1 , second specific discordance indicator DISCI 2 , and third specific discordance indicator DISCI 3 . 
     In detail, in  FIG. 6 , the plurality of sensors  5  comprises one or more movement sensors  105 , for example one or more accelerometers, of a uni-axial, bi-axial, or tri-axial type and/or a gyroscope; one or more acoustic sensors  110 , for example a microphone; and one or more pressure sensors  115 , for example a force sensor for detecting touching of a screen or of a key of the portable electronic apparatus  1 . 
     The set of electrical signals I therefore comprises a movement signal I 1 , which is generated by the movement sensors  105  and comprises, for example, data regarding the acceleration and orientation of the portable electronic apparatus  1 ; an acoustic signal I 2 , which is generated by the acoustic sensors  110  and comprises, for example, data regarding sounds external to the portable electronic apparatus  1 ; and a pressure signal I 3 , which is generated by the pressure sensors  115  and comprises, for example, data regarding the force with which the screen or the key of the portable electronic apparatus  1  is touched. 
     Herein, the analysis circuits  13  comprise a movement-analysis circuit  120 , a background-noise-analysis circuit  125  and a touch-analysis circuit  130 , which are configured to receive at input, respectively, the movement signal I 1 , the acoustic signal I 2 , and the pressure signal I 3 . 
     In detail, the movement-analysis circuit  120  comprises a respective parameter-extraction circuit ME 1  and a respective classification circuit MC 1 , which are configured, as discussed above, to process, at the time instant t, the values of the movement signal I in a first previous acquisition interval; to extract the corresponding set of movement-signal-description parameters, indicating a movement-recognition parameter; and to supply at output the first specific discordance indicator DISCI 1 , which represents the discordance of movement of the portable electronic apparatus  1 , and therefore of the external user, with respect to the movement of the portable electronic apparatus  1  when used by the authorized user, as stored in the memory  17  of the first processing circuit  10 . The values of the movement signal I comprise, for example, amplitude and phase of the acceleration and/or of the velocity along three axes of a cartesian reference system and/or of the angles of rotation of the portable electronic apparatus  1 . 
     The background-noise-analysis circuit  125  comprises a respective parameter-extraction circuit ME 2  and a respective classification circuit MC 2 , which are configured to process, at the time instant t, the values of the acoustic signal I 2  in a second previous acquisition interval; to extract the corresponding set of acoustic-signal-description parameters, indicative of a background-noise-recognition parameter; and to supply at output the second specific discordance indicator DISCI 2 , which represents the discordance of the external background noise of the portable electronic apparatus  1 , used by the external user, with respect to the external background noise when the portable electronic apparatus  1  is used by the authorized user, as stored in the memory  17  of the first processing circuit  10 . The values of the acoustic signal I 2  comprise, for example, amplitude, phase, and frequency of the sounds detected by the acoustic sensors  110 . 
     The touch-analysis circuit  130  comprises a respective parameter-extraction circuit ME 3  and a respective classification circuit MC 3 , which are configured to process, at the time instant t, the values of the pressure signal I 3  in a third previous acquisition interval; to extract the corresponding set of pressure-signal-description parameters, indicating a touch-recognition parameter; and to supply at output the third specific discordance indicator DISCI 3 , which represents the discordance of the way of touching the screen and/or the keys of the portable electronic apparatus  1  by the external user, with respect to the way of touching the screen and/or the keys of the portable electronic apparatus  1  when the latter is used by the authorized user, as stored in the memory  17  of the first processing circuit  10 . The values of the pressure signal I 3  comprise, for example, amplitude and phase of the intensity of touch and duration of the touch detected by the pressure sensors  115 . 
     In the case of the portable electronic apparatus  1  of  FIG. 6 , the information of use can be obtained via questions regarding, for example, the habits of displacement and the means of transport used by the authorized user, in order to extract the training values for the movement-analysis unit  120 . 
     In addition, the authorized user can be asked to carry out movement tests, for example walking or running tests, in order to extract further training values of the movement-analysis circuit  120 . 
     Moreover, the information of use can be obtained via questions regarding the places of habitual frequentation of the authorized user in order to extract the training values of the background-noise-analysis circuit  125 . 
     Furthermore, for example, the authorized user can be asked to conduct pressure tests of the screen or of the keys of the electronic apparatus  1  in order to extract the training values of the touch-analysis circuit  130 . 
     According to an embodiment, the plurality of sensors  5  of the portable electronic apparatus  1  comprises one or more electrostatic-charge-variation sensors  150 , one of which is illustrated in  FIG. 7 . 
     The electrostatic-charge-variation sensor  150  comprises a sensitive element, or electrode,  155 , with which a part of the body of an external user of the portable electronic apparatus  1 , for example his hand or finger, can be set in electrical contact. For instance, the part of the body of the external user of the portable electronic apparatus  1  can be set in direct or indirect electrical connection with the electrode  155 . 
     The electrode  155  may be integrated within the case of the portable electronic apparatus  1  or may be integrated in an external device, for example a smartwatch, connected to the portable electronic apparatus  1  by a cable or in wireless mode. 
     The electrode  155 , which is configured to collect the external charge, may be a metal surface or an electrode coated with dielectric material or a metal surface set under the case of the device that integrates it. 
     For instance, it is possible to use a touchscreen of the portable electronic apparatus  1  as electrode for acquisition of the environmental electrostatic charge or the electrostatic charge deriving from touching by the external user. 
     The electrical or electrostatic charge is, for example, transferred from the external user to the sensitive element  155  during use of the portable electronic apparatus  1  when the external user rests his finger, or hand, or some other portion of his body, in direct or indirect electrical contact with the sensitive element  155 . 
     The electrostatic-charge-variation sensor  150  is sensitive to the variation of electrostatic charge caused by the movements of the external user. 
     Each movement of the external user generates in fact a variation of electrostatic charge through the body of the external user himself, which is due to an exchange of charges with the ground/floor or the surrounding environment and is detected by the electrostatic-charge-variation sensor  150 , in particular by the electrode  155 . 
     For instance, the variation of electrostatic charge can be generated by a step taken by the external user, for example caused by raising or lowering his foot or leg, or by a gesture, for example, made by his hand. 
     For instance, the variation of electrostatic charge may be generated by the way of touching the portable electronic apparatus  1  by the external user, for example according to the duration and force of touch. 
     A signal associated with the variation of electrostatic charge deriving from specific movements can be isolated and identified, by the first processing circuit  10 , with respect to other movements not of interest and with respect to the background noise present in the case of inactivity of the external user. 
     In this embodiment, as illustrated in  FIG. 7 , the electrostatic-charge-variation sensor  150  also comprises an instrumentation amplifier  160 , which has a first input  165 A, electrically coupled to the electrode  155 , and a second input  165 B, which is electrically coupled to the first input  165 A by a resistor R o  and a capacitor C o  in parallel with one another. The resistor R o  and the capacitor C o  have the function of accumulating the charges collected by the electrode  155  and managing the bandwidth of the input signal (for filtering signals and noise of undesired frequency). 
     An input voltage V d , associated with the variation of electrostatic charge and therefore to the use of the portable electronic apparatus  1  by the external user, is thus generated between the first input  165 A and the second input  165 B. 
     The values of capacitance of the capacitor C o  and of resistance of the resistor R o  can be chosen according to the type of filter that it is desired to provide, for example a lowpass filter, with a cutoff frequency of some tens of hertz, for example 20 Hz. For instance, the capacitance of the capacitor C o  is chosen in the range between 5 pF and 5 nF. For instance, the resistance of the resistor R o  is chosen in the range between 500 MΩ and 50 GΩ. The values of capacitance of the capacitor C o  and resistance of the resistor R o  may likewise be chosen as a function of the impedance of the stage to which they are connected, of the useful frequency of the input voltage V d  and of the frequency of the interferences to be filtered, for example the frequency of the power mains supply, the high-frequency electrical noises of the supply circuits, etc. 
     The instrumentation amplifier  160  includes two operational amplifiers OP 1  and OP 2 . A biasing stage (buffer) OP 3 , comprising a resistor R CM , is used for biasing the instrumentation amplifier  160  to a common-mode voltage V CM . 
     The inverting terminals of the operational amplifiers OP 1  and OP 2  are connected together by a resistor R 2 . 
     Since the two inputs of each operational amplifier OP 1 , OP 2  must be at the same potential, the input voltage V d  is applied also across the resistor R 2  and causes, through the resistor R 2 , a current equal to I 2 =V d /R 2 . The current I 2  does not come from the input terminals of the operational amplifiers OP 1 , OP 2  and therefore passes through the two resistors R 1  connected between the outputs of the operational amplifiers OP 1 , OP 2 , in series to the resistor R 2 ; the current I 2  passing through the series of the three resistors R 1 -R 2 -R 1 , thus produces an output voltage V d ′ given by V d ′=I 2 ·(2R 1 +R 2 )=V d ·(1+2R 1 /R 2 ). Consequently, the overall gain of the circuit of  FIG. 7  is Ad=(1+2R 1 /R 2 ). The differential gain depends upon the value of the resistor R 2  and can therefore be modified by acting on the resistor R 2 . 
     The output voltage V d ′, which is thus proportional to the input voltage V d  between the first and second inputs  165 A,  165 B, is supplied at input to an analog-to-digital converter  170 , which supplies at output a charge-variation signal S Q . 
     The charge-variation signal S Q  is supplied to the external user recognition circuit  4 , in particular to the first processing circuit  10 , for subsequent processing, as described above in detail. 
     The charge-variation signal S Q  can therefore be used by one or more analysis circuits  13  for determining one or more respective recognition parameters. 
     For instance, the charge-variation signal S Q  can be used for identifying a specific activity of the external user, such as running, swimming, diving, etc. 
     For instance, the charge-variation signal S Q  can be used as movement signal I 1  and/or as pressure signal I 3 , in addition to or instead of signals generated by other sensors of the plurality of sensors  5 . In other words, the charge-variation signal S Q  can be used, for example, by the movement-analysis circuit  120  and/or by the touch-analysis circuit  130 . 
     The analog-to-digital converter  170  is optional in so far as the first processing circuit  10  may be configured to work directly on the analog signal, or may itself comprise an analog-to-digital converter designed to convert the output signal V d ′. 
     It emerges from the foregoing that the charge-variation signal S Q  associated with a specific movement of an external user of the portable electronic apparatus  1  can then be used for identifying and recognizing the way of walking, gesticulating, or other patterns of movement that are characteristic of the external user and therefore can be associated with an authorized or unauthorized user of the portable electronic apparatus  1 , as discussed above in detail. 
     Finally, it is clear that modifications and variations may be made to the method of recognition of use of a portable electronic apparatus by an unauthorized user and to the corresponding portable electronic apparatus described and illustrated herein, without thereby departing from the scope of the present invention, as defined in the annexed claims. 
     For instance, the first, the second and the third processing circuits  10 ,  11 ,  12  may be formed by a single processing circuit, for example by the main processing circuit  2  or by a dedicated processing circuit, or by a number of dedicated processing circuits according to the requirements of a specific application. 
     For example, the analysis circuits  13 , the respective parameter-extraction circuits  15  and/or the respective classification circuits  16  may be formed by respective dedicated processing circuits or be formed by just one processing circuit. 
     For instance, the memories  17 ,  18  and  19  of the first, the second and the third processing circuits  10 , n,  12 , respectively, may be each formed by a respective portion of the main memory  3  or by a dedicated memory. 
     For example, the time t′ that elapses between the calculation of two successive overall discordance indicators O-DISCI may vary so as to increase in specific hours, for example at night, when the portable electronic apparatus is less used, and decrease in hours or parts of the day when the portable electronic apparatus is more used or in parts of the day detected as being more significant. For instance, it is possible to configure the external user recognition circuit for executing recurrent analysis of historical data, previously acquired and processed, and determining the aforesaid more significant parts of the day. 
     In addition, the plurality of sensors  5  may be configured to detect also further data regarding the use of the portable electronic apparatus  1 , starting from which the user recognition circuit  4  can determine further recognition parameters, additional to the ones described above. For instance, these further data may be relative to the connection of the portable electronic apparatus  1 , for example relative to the Internet traffic, the name of networks and external devices to which the portable electronic apparatus  1  is connected in different periods of the day, the level of the data-connection signal, and the level of call signal, or the further data may be relative to the type of software installed, the time of use of a software application, the telephone numbers called, the frequency of use of messaging software, the time elapsed between the reception of messages and reading thereof or personalization of settings of the portable electronic apparatus  1 , such as volume at which to listen to music, brightness of screen, etc. 
     For instance, the user-recognition circuit  4  may be configured to carry out steps of periodic training, for example on a weekly basis, of the known machine-learning algorithms used by the first, second, and third processing circuits  10 ,  11 ,  12 . In the above steps of periodic training, the values of the data previously detected by the sensors  5 , and consequently the values of the respective signal-description parameters and of the respective recognition parameters, are used as labelled examples indicating the use of the portable electronic apparatus  1  by the authorized user. In this way, there is an improvement in the capacity of the external user recognition circuit  4  to identify an intrusion of an unauthorized user, which instead entails a sudden change of the mode of use of the portable electronic apparatus  1 .