Abstract:
In a general aspect, a countermeasure method implemented in a microcircuit can include selecting, at each cycle of a clock signal, a supply mode of a component internal to the microcircuit, the supply mode can be selected from among a first supply mode in which the component is fully supplied by a first supply circuit connected to a supply input of the microcircuit, and at least one second supply mode in which the component is at least partially supplied by a second supply circuit internal to the microcircuit. The second supply circuit can be isolated from the exterior of the microcircuit while it is supplying the component.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of PCT/FR2015/052544, filed Sep. 23, 2015 which claims the benefit of French Application No. 1458951, filed Sep. 23, 2014, the disclosures of which are both incorporated by reference herein in their entireties. 
     
    
     TECHNICAL FIELD 
       [0002]    The present disclosure relates to cryptographic devices and more particularly, to the protection of such devices against attacks by analysis of their electrical consumption. The present disclosure, relates, in particular to smart cards, secured processors, secured telephones, and more generally to any device with a microcircuit storing sensitive data and thus needing to be protected against attacks by analysis of electrical consumption. 
       BACKGROUND 
       [0003]    Microcircuit devices handling sensitive data are sometimes subject to attacks aiming to determine this data. Among known attacks, attacks of SPA (Simple Power Analysis) or DPA (Differential Power Analysis) type involve performing many measurements of current on a microcircuit supply terminal during the execution of a program or of a data processing operation by the microcircuit, by causing input data to vary. The measurements obtained are used by a statistical analysis to deduce sensitive data, processed or used by the microcircuit. 
         [0004]    Various techniques have been developed to fight against these attacks. Thus, one known example is to reduce the leakage currents induced by the processing operations, or to increase the noise so as to reduce the signal/noise ratio of the variations of the supply current. Balancing the sensitive circuits, so that the processing of a bit on 0 has a same electrical consumption signature as the processing of a bit on 1, has also been considered. Introducing jitter in the clock signal has also been proposed, or introducing dummy calculations or dummy calculation cycles. It is also possible to mask intermediate data in the symmetrical algorithms. However these techniques result in increasing the size and the complexity of sensitive circuits, or in deteriorating their performances (electrical consumption and calculation execution time). It is also known to use capacitors having a relatively high capacitance to filter the variations in current at the supply input of the microcircuit. The higher the capacitance of the capacitor, the more the masking of the variations in electrical consumption of the circuit is efficient. However, the higher the capacitance of the capacitor, the more the volume of the latter is significant. Capacitors which can be integrated into a microcircuit currently have a capacitance insufficient to prevent attacks of SPA or DPA type. 
         [0005]    It is thus desirable to implement a countermeasure aiming to protect circuits handling sensitive data against various types of SPA- and DPA-type attacks. It is also desirable not to adversely affect performances and size of a circuit to be protected. 
       SUMMARY 
       [0006]    Some embodiments relate to a countermeasure method implemented in a microcircuit, the method comprising selecting at each cycle of a clock signal a supply mode of a component internal to the microcircuit, from among a first supply mode in which the component is fully supplied (e.g., with power, voltage, current, etc.) by a first supply circuit connected to a supply input of the microcircuit, and at least one second supply mode in which the component is at least partially supplied (with power) by a second supply circuit internal to the microcircuit, the second supply circuit being isolated from the exterior of the microcircuit while it is supplying the component. The selected supply mode may be activated while the component is active. 
         [0007]    According to one embodiment, the component is supplied by the second supply circuit at cycles of the clock signal determined randomly, or depending on the state of charge of a capacitor of the second supply circuit, or depending on a criterion of sensitivity of operations executed upon each cycle of the clock signal. 
         [0008]    According to one embodiment, the component is provided with the clock signal to pace operations executed by the component. 
         [0009]    According to one embodiment, the component is supplied in the second supply mode by the discharge of a capacitor, the second supply mode being selected for a time that is shorter than the time of a cycle of the clock signal minus a charge time for charging the capacitor. 
         [0010]    According to one embodiment, a first switch is interposed on a supply connection of the component, connected to the first supply circuit, the method comprising charging a capacitor of the second supply circuit when the first switch is closed, and of discharging the capacitor to supply the component when the first switch is open. 
         [0011]    According to one embodiment, a second switch is interposed on a supply connection linking the second supply circuit to a supply input of the component, the capacitor being charged by a source of energy internal to the microcircuit when the second switch is open, and discharged to supply the component when the second switch is closed. 
         [0012]    According to one embodiment, the method comprises: the component executing several times an operation with the first switch open for different times, the capacitor being in a charged state when the first switch is open, upon each execution of the operation, storing a datum provided by the calculation circuit, and determining a maximum time for which the first switch can be open without disturbing the execution of the operation by comparing the stored data. 
         [0013]    According to one embodiment, the method comprises defining and storing in a register a control word comprising one control bit per cycle of a set of cycles of the clock signal, and of selecting one or other of the first and second supply modes, according to the value of the control bit corresponding to the clock cycle in progress in the register. 
         [0014]    Some embodiments also relate to a microcircuit comprising: a component internal to the microcircuit, a first supply circuit connected to a supply input of the microcircuit, a second supply circuit internal to the microcircuit, and a control circuit configured to implement the method defined above. 
         [0015]    According to one embodiment, the microcircuit comprises a first switch interposed on a supply line of the component, and which is closed by the control circuit to supply the component by the first supply circuit, and opened by the control circuit to supply the component by the second supply circuit. 
         [0016]    According to one embodiment, the second supply circuit comprises a first capacitor that is charged while the first switch is closed, and which supplies the component by discharging while the first switch is open. 
         [0017]    According to one embodiment, the microcircuit comprises a second switch interposed on a supply connection linking the second supply circuit to a supply input of the component, the first capacitor being charged by a source of energy internal to the microcircuit when the second switch is open, and discharged to supply the component when the second switch is closed. 
         [0018]    According to one embodiment, the microcircuit comprises a register for storing a control word comprising at least one bit, the status of the bit indicating whether the first switch must be opened or closed during a calculation cycle of a set of calculation cycles executed by the component. 
         [0019]    According to one embodiment, the microcircuit comprises several components each comprising a supply input coupled to the first supply circuit through a respective switch, a capacitor being connected to each supply input, each switch being controlled according to the operations executed by the component connected to the switch. 
         [0020]    According to one embodiment, the microcircuit comprises several components each comprising a supply input coupled to the first supply circuit through a respective switch, a capacitor being coupled through a switch circuit to each component supply input, each switch being controlled according to the operations executed by the component connected to the switch, the switch circuit being configured to couple the capacitor to one of the component supply inputs when the switch connected to this supply input is open. 
         [0021]    Some embodiments relate to a countermeasure method implemented in a microcircuit, the method comprising steps of: selecting, by the microcircuit, at each cycle of a clock signal a supply mode of a component internal to the microcircuit, from among a first supply mode in which the component is fully supplied by a first supply circuit connected to a supply input of the microcircuit, and at least one second supply mode in which the component is at least partially supplied by a second supply circuit internal to the microcircuit, the second supply circuit being isolated from the exterior of the microcircuit while it is supplying the component, charging a capacitor of the second supply circuit by a source of energy internal to the microcircuit, while the first supply mode is selected, and discharging the capacitor to supply the component while the second supply mode is selected. The selected supply mode may be activated while the component is active. 
         [0022]    According to one embodiment, the component is supplied by the second supply circuit at cycles of the clock signal determined randomly, or depending on the state of charge of a capacitor of the second supply circuit, or depending on a criterion of sensitivity of operations executed upon each cycle of the clock signal. 
         [0023]    According to one embodiment, the component is provided with the clock signal to pace operations executed by the component. 
         [0024]    According to one embodiment, the second supply mode is selected for a time that is shorter than the time of a cycle of the clock signal minus a charge time for charging the capacitor. 
         [0025]    According to one embodiment, a first switch is interposed on a supply connection of the component, connected to the first supply circuit, the method comprising a step of charging a capacitor of the second supply circuit when the first switch is closed, and of discharging the capacitor to supply the component when the first switch is open. 
         [0026]    According to one embodiment, a second switch is interposed on a supply connection linking the second supply circuit to a supply input of the component, the capacitor being charged by the source of energy internal to the microcircuit when the second switch is open, and discharged to supply the component when the second switch is closed. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0027]    Some examples embodiments are described below in relation with, but not limited to, the following figures, in which: 
           [0028]      FIG. 1  schematically represents an example of a microcircuit performing cryptographic calculations, 
           [0029]      FIG. 2  schematically represents a power supply circuit and a sensitive circuit of the microcircuit in  FIG. 1 , 
           [0030]      FIGS. 3A and 3B  represent curves of variation over time, respectively of a clock signal and of the intensity of the current at a power supply input of the microcircuit, during calculations performed by a component of the microcircuit, 
           [0031]      FIG. 4  schematically represents a power supply circuit and a sensitive circuit of the microcircuit, according to one embodiment, 
           [0032]      FIG. 5  schematically represents a circuit of the power supply circuit in  FIG. 4 , 
           [0033]      FIGS. 6A, 6B and 6C  represent curves of variation over time, respectively of a clock signal, of a control signal and of the intensity of the current at a power supply input of the microcircuit, during calculations performed by a component of the microcircuit, the microcircuit being equipped with the supply circuit of  FIG. 4 , 
           [0034]      FIG. 7  schematically represents an example of a power supply circuit and several sensitive circuits of the microcircuit, according to another embodiment, 
           [0035]      FIG. 8  schematically represents an example of a power supply circuit and several sensitive circuits of the microcircuit, according to another embodiment, 
           [0036]      FIGS. 9, 10 and 11  schematically represent a power supply circuit and a sensitive circuit of the microcircuit, according to other embodiments. 
       
    
    
     DETAILED DESCRIPTION 
       [0037]      FIG. 1  represents an example of a microcircuit IC 1  performing cryptographic calculations. In the description below, the word “microcircuit” means an integrated circuit or a silicon chip, into which it is difficult to insert a sensor for capturing signals to access sensitive data of the microcircuit. The microcircuit IC 1  comprises several components such as a processor MP, for example of microprocessor or microcontroller type, one or more volatile and non-volatile memories MEM 1 , MEM 2 , an interface unit INT enabling the microcircuit IC 1  to communicate with equipment external to the microcircuit, and address and data buses ADB to which the processor, the memories and the interface unit are connected. The microcircuit IC 1  may also comprise components such as calculation circuits including a cryptographic calculation circuit CAU also connected to the bus ADB. The different components of the microcircuit IC 1  are supplied (e.g., with power, voltage, current) by a supply circuit MPS providing a supply voltage Vp on a main supply connection MPL coupled to a ground plane of the microcircuit IC 1  through a filtering capacitor C 0 . The circuit MPS may itself be connected to a supply terminal PWI of the microcircuit, receiving an external supply voltage EVp. It shall be noted that the supply terminal PWI may also be a connection terminal of a communication antenna, the then so-called “contactless” microcircuit IC 1  generating its own supply from the energy received by the antenna. The circuit MPS may comprise circuits such as a voltage regulator and a charge pump. The circuit MPS may also be a simple connection with the supply terminal PWI. For purposes of this disclosure, supplying a component can be supplying power (current and voltage), supplying voltage, supplying current, etc. 
         [0038]      FIG. 2  represents in greater detail the supply circuit of one of the components SHi of the microcircuit IC 1 . Here, the component SHi may represent the processor MP, one or more calculation circuits of the microcircuit IC 1 , or a part of them. The component SHi receives the supply voltage Vp at a supply input coupled to the main supply connection MPL through a local supply connection PLi. The supply input of the component SHi is coupled to the ground plane through a capacitor Ci. The capacitor Ci may include the internal stray capacitances of the component SHi. 
         [0039]      FIGS. 3A, 3B  represent curves of variation over time, respectively of a clock signal CK and of the intensity of the electric current El on the supply terminal of the microcircuit IC 1  during the execution of calculations by one of the components SHi. The clock signal CK may represent cycles or iterations of a calculation, such as rounds of a cryptographic calculation.  FIG. 3A  represents 10 periods of the signal CK. A correlation can be seen between the clock signal CK and the current El. An attack of DPA type involves acquiring several curves of the signal El with different input data and correlating these curves to deduce a sensitive datum used by the microcircuit. 
         [0040]      FIG. 4  represents the supply circuit of one of the components SHi of the microcircuit IC 1 , according to one embodiment. The supply circuit of the component SHi differs from the one represented in  FIG. 2  in that it comprises a countermeasure circuit PMC. The countermeasure circuit PMC comprises a switch SW 1  interposed on the local supply connection PLi of the component SHi, between the capacitors C 0  and Ci. Thus, when the switch SWi is closed, the component SHi is supplied normally by the circuit MPS. When this switch is open, the component SHi is supplied temporarily by the discharge of the capacitor Ci. The countermeasure circuit PMC also comprises a control circuit SWC to control the switch SWi, and a control circuit CTL to control the control circuit SWC. 
         [0041]    As the capacitor Ci is totally integrated into the microcircuit IC 1  and uncoupled from the supply circuit and thus from the supply input PWI of the microcircuit, when it is supplying the component SHi, no external connection of the microcircuit grants access to the supply input of the component SHi when the latter is supplied by the capacitor Ci. It can also be seen that such integration of the capacitor Ci offers a much better protection than the simple disposition of several circuits in a same electronic component package. Indeed, a package can be opened relatively easily to access the inputs and outputs of the different components interconnected in the package. On the other hand, microcircuits are generally formed on a so-called “active” face of a wafer made of a semiconductor material, covered by different insulating and conducting layers in which interconnection lines are formed, these layers being covered by a protective layer, generally made of resin, comprising openings for the external connections of the microcircuit. Access to the connection terminals of the capacitor Ci or to the connection lines of the countermeasure circuit would thus require removing the protective layer of the microcircuit and one or more interconnection layers, and would thus cause the destruction of the microcircuit. Thus, as the countermeasure circuit PMC is also integrated into the microcircuit, it is not possible either to determine the periods during which the component SHi is supplied by the capacitor Ci. 
         [0042]    The control circuit SWC may furthermore receive a control signal OT enabling the opening time OT of the switch SW 1  to be adjusted. The control circuit CTL transmits to the circuit SWC a control signal CMi whereby it is possible to trigger the opening of the switch SHi for the time OT. The instants at which the opening of the switch SWi is triggered may be predetermined or determined randomly, as may the opening time of the switch SWi with an upper limit defined by the time OT. 
         [0043]    The opening time OT may be determined according to the discharge time of the capacitor Ci so that during this time, the supply voltage provided by the capacitor Ci is sufficient to keep the component SHi operational. The opening time OT depends on the capacitance of the capacitor Ci and on the electrical consumption of the component SHi, which may vary over time depending on the operation executed by the component SHi. Thus, the control circuit SWC may be configured to open the switch SWi for the opening time OT, upon a command from the control circuit CTL. The control circuit SWC may also be configured to keep the switch SWi closed for a minimum closing time following the opening of the latter, so as to ensure the recharging of the capacitor Ci before the switch SWi can be opened again. 
         [0044]    Provision may be made so that the time OT is such that the period of the clock signal CK is longer or equal to the sum of the time OT and of the recharging time of the capacitor Ci. Alternatively, provision may be made so that the time OT is between the period of the clock signal CK and half of the latter, and for prohibiting the opening of the switch SWi for at least two consecutive clock cycles CK. The time OT may also be chosen equal to a fraction of the period of the clock signal CK, the switch SWi being kept closed between two consecutive openings of the latter, for a minimum time that is sufficient to enable the recharging of the capacitor Ci. It will be understood that it is also possible for the time OT to be longer than that of one or more cycles of the clock signal CK. 
         [0045]    The time OT may be determined by a calibration device CLB configured to execute several iterations during each of which it activates the component SHi, opens the switch SWi for a variable time and stores a result RSi provided by the component SHi in connection with the opening time of the switch SWi. The time OT is chosen from among the times stored that enable a correct result to be obtained from the component SHi, i.e. a result identical to the one obtained without opening the switch SWi. For example, the time OT chosen may be the longest time that enables a correct result to be obtained. 
         [0046]    The control circuit SWC may also receive the clock signal CK provided to the component SHi or a synchronization signal provided by the component SHi, so as to synchronize the opening of the switch SWi according to the operations performed by the component SHi. In this case, the circuit SWC is configured to trigger the opening of the switch SWi when the signals CMi and CK are simultaneously in a given state, for example a high state. 
         [0047]    The control circuit CTL may be controlled through a control word loaded in a register comprising several cells for storing a binary datum. Each cell of the register corresponds to an iteration of the calculation executed by the component SHi, the time of each iteration corresponding to the period of the signal CK. The value of each register cell determines whether the circuit CTL must or must not control the circuit SWC to open the switch SWi for the time OT, during the calculation iteration corresponding to the register cell. The control word loaded in the register may be determined according to a criterion of sensitivity of each iteration. 
         [0048]    According to one embodiment, the control register of the control circuit CTL is coupled to a random number generator arranged for loading, on command, a new control word in the register. According to another embodiment shown by  FIG. 5 , the control circuit comprises two registers RG 1 , RG 2 , the register RG 2  being coupled to a generator of random or pseudo random numbers RNG. The register RG 1  may be updated by a component of the circuit IC 1  such as the processor MP. The output of the registers RG 1 , RG 2  is coupled to the control input of the circuit SWC through a selector switch CT that may be controlled to select one or the other of the two registers RG 1 , RG 2  to be used to control the circuit SWC. If the time CK-OT is shorter than the charge time of the capacitor Ci, the control word loaded in the register RG 2  may be modified so as to avoid it including two consecutive bits on 0 (in the event that a bit on 0 controls the opening of the switch SWi) to ensure the recharging of the capacitor Ci between two consecutive openings of the switch SWi. 
         [0049]      FIGS. 6A, 6B, 6C  represent curves of variation over time, respectively of the clock signal CK, of the control signal CMi of the switch SWi emitted by the control circuit SWC, and of the intensity of the current El at the power supply input of the microcircuit IC 1 , during calculations performed by the component SHi. On  FIG. 6B , the control signal CMi is kept in a high state, except during periods of the clock signal CK (numbered  2  and  6  in the example in  FIG. 6A ) where it is placed in the low state. The result is that the variations in current intensity that appear in the curve El during these periods  2  and  6  on  FIG. 3B , due to the activity of the component SHi are no longer visible in the signal El measured at the power supply input of the microcircuit IC 1 . The variations in current that remain in the curve El during the periods  2  and  6  correspond to the current consumed by other circuits of the microcircuit IC 1 . The opening of the switch SWi thus enables the activity of the component SHi to be masked. 
         [0050]      FIG. 7  represents several components SH 1 , SH 2 , SH 3  of the microcircuit IC 1  that must be protected against attacks of SPA or DPA type. Each component SH 1 , SH 2 , SH 3  receives the supply voltage Vp provided by the circuit MPS at a supply input coupled to the main supply connection MPL through a local supply connection PL 1 , PL 2 , PL 3 . The supply input of each component SH 1 , SH 2 , SH 3  is coupled to the ground plane of the microcircuit IC 1  through a capacitor C 1 , C 2 , C 3 , respectively. The capacitors C 1 , C 2 , C 3  may include the internal stray capacitances of the components SH 1 , SH 2 , SH 3 , respectively. According to one embodiment, the countermeasure circuit PMC comprises a switch SW 1 , SW 2 , SW 3  interposed on the local supply connection PL 1 , PL 2 , PL 3  of each component SH 1 , SH 2 , SH 3 , between the junction node of the local supply connection with the main supply connection MPL and the capacitor C 1 , C 2 , C 3 . In normal operation, the switches SW 1 , SW 2 , SW 3  are closed. The components SH 1 , SH 2 , SH 3  are thus supplied normally by the circuit MPS. When one of the switches SW 1 , SW 2 , SW 3  is open, the corresponding component SH 1 , SH 2 , SH 3  is supplied temporarily by the discharge of the corresponding capacitor C 1 , C 2 , C 3 . Each of the switches SW 1 , SW 2 , SW 3  is controlled by a respective control circuit SWC 1 , SWC 2 , SWC 3  that provides it with a control signal CM 1 , CM 2 , CM 3 . The control circuit CTL may control each of the control circuits SWC 1 , SWC 2 , SWC 3 . Each of the control circuits SWC 1 , SWC 2 , SWC 3  may receive a control signal enabling the opening time OT 1 , OT 2 , OT 3  of the corresponding switch SW 1 , SW 2 , SW 3  to be adjusted. The opening time OT 1 , OT 2 , OT 3  of each switch SW 1 , SW 2 , SW 3  can be determined depending on the discharge time of the corresponding capacitor C 1 , C 2 , C 3  so that during this time, the supply voltage provided by the capacitor is sufficient to keep operational the corresponding component SH 1 , SH 2 , SH 3 . Each of the control circuits SWC 1 , SWC 2 , SWC 3  may also receive a respective clock signal CK 1 , CK 2 , CK 3  enabling it to synchronize the opening of the corresponding switch SW 1 , SW 2 , SW 3  with the operations performed by the corresponding component SH 1 , SH 2 , SH 3 . 
         [0051]    It is difficult for capacitors with a relatively high capacitance to be miniaturized in an integrated circuit. Therefore, with present-day integrated circuit manufacturing technologies, a 100 pF capacitor occupies a silicon substrate surface of the order of 100×100 μm. To limit the space occupied by such capacitors in the countermeasure circuit, the circuit in  FIG. 7  may be modified so as to implement only one capacitor shared by all the components to be protected. Therefore,  FIG. 8  represents a microcircuit IC 2 , and in particular a supply circuit of the microcircuit IC 2 . The supply circuit comprises the capacitor C 0  and a countermeasure circuit PMC 1  comprising a single capacitor C 11  for all the components SH 1 , SH 2 , SH 3 . The capacitor  011  is connected, on one side, to the ground plane and coupled, on the other side, to each of the local supply connections PL 1 , PL 2 , PL 3  through a multiplexer MUX. The multiplexer MUX is controlled by the control signals CM 1 , CM 2 , CM 3  provided by the circuits SWC 1 , SWC 2 , SWC 3  to the switches SW 1 , SW 2 , SW 3 . The multiplexer MUX may be configured to connect the capacitor  011  to only one of the connections PL 1 , PL 2 , PL 3  at once. The control circuit CTL and/or the circuits SWC 1 , SWC 2 , SWC 3  may be configured so that the control signals CM 1 , CM 2 , CM 3  trigger the opening of a single switch SW 1 , SW 2 , SW 3  at once, and control the multiplexer MUX so that it couples the capacitor C 11  to the component SH 1 , SH 2 , SH 3  connected to the open switch SW 1 , SW 2 , SW 3 . The control circuit CTL and/or the circuits SWC 1 , SWC 2 , SWC 3  may also be configured to respect a minimum closing time between two consecutive openings from among the switches SW 1 , SW 2 , SW 3 , in order to ensure the recharging of the capacitor C 11 . 
         [0052]      FIG. 9  represents a microcircuit IC 3  and in particular a supply circuit for supplying a component SHi of the microcircuit IC 3 . The supply circuit comprises the supply circuit MPS, the capacitor C 0 , a countermeasure circuit PMC 2  and another supply circuit APS. The supply circuit APS is configured to extract energy from a source of energy exploiting a physical phenomenon such as electromagnetic radiation (light, radio waves, magnetic field, electric field), motion, a temperature gradient, etc. For this purpose, the circuit APS may comprise as source of energy circuits such as an antenna, a motion sensor, light-sensitive cells, and circuits such as a charge pump and a voltage regulator. 
         [0053]    The circuit PMC 2  comprises a capacitor C 1   i,  two switches SW 1   i,  SW 2   i  controlled by a control circuit SC 11 , and a control circuit CTL. The supply line MPL connected to a supply output of the circuit MPS is coupled to a supply input of the component SHi through the switch SW 1   i.  A supply line APL couples a supply output of the circuit APS to the supply input of the component SHi, the switch SW 2   i  being interposed on the line APL. The capacitor C 1   i  couples the supply output of the circuit APL to the ground. The circuit SC 11  may be configured to control the switches SW 1   i,  SW 2   i  so that, at any instant, only one of the switches SW 1   i  and SW 2   i  is closed when the component SHi is supplied. 
         [0054]    According to one embodiment, the circuit APS provides the circuit SC 11  or CTL with a state of charge signal CST of the capacitor C 1   i.  Therefore, when the component SHi must be supplied, the switch SW 2   i  may be closed by the circuit SC 11  while the signal CST indicates that the capacitor C 1   i  is charged, and opened while the signal CST indicates that the capacitor C 1   i  is discharged or on charge. 
         [0055]    If the circuit APS is not totally isolated inside the microcircuit IC 3 , in particular because it must be connected to a connection port of the microcircuit (intended for example to be connected to an antenna) and if the activity of the component SHi may be detected at this port, in particular because the circuit APS does not offer sufficient isolation, provision may be made for a third switch interposed on the line APL. Therefore,  FIG. 10  represents a microcircuit IC 4  that differs from the microcircuit IC 3  only in that the countermeasure circuit PMC 3  comprises a third capacitor SW 3   i  interposed between the supply output of the circuit APS and the connection node of the capacitor C 1   i  to the line APL. The capacitor SW 3   i  is controlled by the circuit SC 11  so as to be open when the switch SW 2   i  is closed and closed when the switch SW 2   i  is open. 
         [0056]    It will be understood by those skilled in the art that the example implementations described herein are subject to various alternative embodiments and various applications. In particular, the appended claims, or claims that may be added hereafter, are not limited to the specific embodiments described, but also include all possible combinations of these embodiments. As an example, if the microcircuit in  FIG. 9 or 10  comprises several components to be protected, each of these components may be associated with a capacitor C 1   i  and with respective switches SW 1   i  and SW 2   i  controlled by a control circuit such as SC 11 . Similarly, the embodiments in  FIGS. 8 and 9 or 10  may be combined by adding to the multiplexer MUX on  FIG. 8  an input connected to the supply circuit APS of  FIG. 9 or 10 . 
         [0057]    The disclosed implementations are not limited either to the use of a capacitor to store electrical energy and to restore it to a component to be protected when the switch disposed on the supply connection of the component is open. Indeed, it is possible to consider integrating a rechargeable battery into the microcircuit. It is noted that, while the component is supplied by this other source of energy, the latter is not coupled to a contact outside the microcircuit, from which a DPA or SPA attack can be performed. 
         [0058]    The disclosed implementations not limited either to the use of switches to connect the main MPS and secondary supply sources (such as the capacitor Ci, C 1   i ) to a component to be protected that is internal to the microcircuit. Indeed, provision may be made for intermediate supply modes in which the component to be protected is supplied partly by the main supply source MPS and partly by the secondary supply source. This arrangement makes it possible to attenuate the signature due to the activity of the component to be protected, which may be detected by analyzing the current supplied and consumed by the microcircuit. This arrangement also enables the discharge time of the secondary supply source to be extended, and thus the time during which the component may be partially supplied by the secondary supply source.  FIG. 11  represents as an example a microcircuit IC 5  comprising a countermeasure circuit PMC 3  which may implement such intermediate supply modes. The circuit PMC 3  comprises a control circuit CTL, a control circuit SC 12 , two P-channel MOS transistors P 1 , P 2 , the capacitor C 1   i,  an error amplifier EAP, a potentiometer R 1  controlled by the control circuit SC 12 , and two resistors R 2 , R 3 . The transistor P 1  is interposed on the supply line PLi of the component SHi and is controlled by the circuit SC 12 . The transistor P 2  is interposed on the connection APL between the supply circuit APS and the supply input of the component SHi, and is controlled by the output signal of the amplifier EAP. The capacitor C 1   i  couples the supply output of the circuit APS to the ground. The supply input of the component SHi is coupled to a non-inverting input of the amplifier EAP through the resistor R 2 . The non-inverting input of the amplifier EAP is also coupled to the ground through the resistor R 3 . The amplifier EAP comprises an inverting input coupled to an intermediate terminal of the potentiometer R 1  comprising an end terminal receiving a reference voltage Vrf and an end terminal connected to the ground. The activation of the amplifier EAP is controlled by the state of charge signal CST of the capacitor C 1   i  such that, if the state of charge of the capacitor C 1   i  is insufficient, the amplifier EAP inhibits the transistor P 2 . The resistors R 2 , R 3  form a divider bridge making it possible to take off a portion of the voltage supplied at output by the supply circuit and sent by the transistor P 2 . Diodes D 1 , D 2  may be provided at output of the supply circuits MPS, APS to avoid a reverse circulation of current. A capacitor C 2   i  may couple the supply input of the component SHi to the ground, provided that it has a low capacitance in relation to the capacitor C 1   i  (for example lower than 1/10 of the capacitance of the capacitor C 1   i ). 
         [0059]    The potentiometer R 1  enables the amplitude and the sign of the difference between the voltages supplied at input of the amplifier EAP to be adjusted, so as to adjust the supply current on the line APL, transmitted to the component SHi by the transistor P 2 , when the transistor P 1  is on. Therefore, if the difference in voltage at input of the amplifier EAP is positive, then the current supplied by the capacitor C 1   i  prevails over the current supplied by the supply circuit MPS, to supply the component SHi. On the other hand, if the difference in voltage at input of the amplifier EAP is negative, the current supplied by the supply circuit prevails over the current supplied by the capacitor C 1   i,  to supply the component SHi. 
         [0060]    Furthermore, the switch interposed on the main supply line PLi of the component to be protected is not necessarily open during the execution of a sensitive operation by the component. Indeed, masking the activity of the component while the latter performs non-sensitive operations may also disturb a DPA or SPA attack.