Patent Publication Number: US-7725786-B2

Title: Protecting an integrated circuit test mode

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a division of prior application Ser. No. 11/041,514, filed on Jan. 24, 2005, entitled “Protecting an Integrated Circuit Test Mode” which application claims the priority benefit of French patent application number 04/00836, filed on Jan. 29, 2004, entitled “Protecting an Integrated Circuit Test Mode” which applications are hereby incorporated by reference to the maximum extent allowable by law. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates, in general terms, to synchronous integrated electronic circuits equipped with combinatorial logic means, flip-flops, and test means. 
   More specifically, the invention relates to an electronic circuit that includes: a plurality of logic cells; a plurality of configurable cells each including at least one multiplexer and one flip-flop; and a plurality of control conductors connected to the configurable cells and in which flow selectively control signals emitted in operation by a control circuit such as an access controller, the configurable cells adopting selectively, as a function of the control signals, a standard operating mode in which they are operationally connected to some at least of the logic cells with which they engage to form a logic circuit, and a test mode in which these configurable cells are operationally connected in a chain to form a shift register equipped with a data input and output. 
   2. Discussion of the Related Art 
   It is a well known practice today to test the correct operation of the operational elements of an integrated circuit by imposing and/or setting, at pre-set moments, data values present at certain internal points of the integrated circuit. 
   A technique of this kind for testing internal paths of an integrated circuit (known as “scanpath” or “internal scan method”;) is for example described in the publication by M. Williams and J. Angel entitled “Enhancing Testability of LSI Circuits Via Test Points and Additional Logic, IEEE Transactions on Computers, vol. C-22, no.1; January 1973”, which is incorporated herein by reference. 
   According to this technique, each of the flip-flops in the logic circuit, for which it is useful to know the state and/or to impose the content during the standard operation of the integrated circuit, is equipped at its input with a multiplexer. 
   The different flip-flops and the multiplexers which are associated with them thus constitute as many configurable cells whose accesses are individually controlled by these multiplexers. 
   The multiplexers of these different configurable cells are collectively controlled by an access controller or “TAP controller” (“TAP” standing for “Test Access Port”) which, depending on a selected operating mode, uses this set of configurable cells either as a standard operating circuit, integrated into the logic circuit which it forms with the logic cells, or as a test circuit. 
   To do this, the TAP controller addresses on different control conductors, by which it is connected to the different configurable cells, control signals, such as a mode control signal, a chaining control signal or a data propagation control signal, which modify the data flow paths within the integrated circuit and which thus allow the controller to capture this data, for the purpose of analyzing it. 
   In standard operating mode, the TAP controller therefore controls the multiplexers of the configurable cells in such a way that the flip-flops of these cells are connected to surrounding logic cells to define one or more operating sub-assemblies of the integrated circuit. 
   In test mode, which is normally triggered on receipt by the TAP controller of a test execution command, this controller produces a chaining control signal to connect the flip-flops of the configurable cells in series in such a way as to form a shift register. 
   This register notably includes a series input and a series output connected to an output and an input of the TAP controller respectively, and a clock input receiving a clock signal to time the data flow. 
   Initially, the TAP controller loads data in series into the flip-flops of the configurable cells through the input of the shift register formed by these cells. 
   Then, the TAP controller changes the switching of the multiplexers to form the operating circuit, and controls the execution of one or more clock cycles by this operating circuit. The data loaded into the flip-flops of the configurable cells is then processed by the operating circuit. 
   The controller then once again changes the switching of the multiplexers to again form the shift register and retrieves, in series at the output of the shift register, the data stored in the flip-flops of the configurable cells during the last clock cycle. 
   Despite proven interest in this testing technique, its practical application may under some circumstances prove problematic, particularly in respect of integrated circuits processing confidential data. 
   Indeed, insofar as activating the test mode may allow a fraudster to read the content of the flip-flops of the configurable cells, this test technique has at first sight the drawback of making such circuits highly vulnerable to fraudulent use. 
   For example, by stopping at various intervals of time a process for the internal loading of confidential data into the integrated circuit and by unloading the content of the shift register, a fraudster may be able to obtain information about confidential data, and even reconstitute it. 
   By activating the test mode, a fraudster may also access the flip-flops of the configurable cells in write mode so as to insert fraudulent data, or to put the integrated circuit in an unauthorized configuration. He may thus for example access a register controlling a security component such as a sensor in order to deactivate it. He may also inject an erroneous data item for the purpose of obtaining information about a confidential data item. 
   The fraud may in fact adopt two different strategies, the first of which consists in taking control of the TAP controller and in observing the shift register cell content on the external contact blocks, and the second of which consists in taking control of the configurable cells by exciting them by micro-probing so as to simulate the control of these cells by the control signals emitted by the TAP controller. 
   An attempt at fraud in accordance with the first strategy may be blocked by a technique which is the subject of a patent application filed simultaneously by the Proprietor. 
   SUMMARY OF THE INVENTION 
   One purpose of the present invention is to propose an electronic circuit designed to foil a fraud attempt in accordance with the second strategy mentioned above. 
   There is therefore a need for an electronic circuit which resolves one or more of these drawbacks. To this end, the electronic circuit of the invention, comprises: 
   a logic circuit having a plurality of logic cells; 
   storage cells able to form a shift register, able to be connected to the logic cells, and having terminals for the reception of respective control signals conducive to entering data into the cells and reading data entered in the cells; 
   a connection control module having an input for the reception of an identification key, the module connecting the storage cells in accordance with a set order so as to form a test shift register when the receive input receives a valid identification key, and when the storage cells receive a test control signal in read or write mode, and the module connecting the storage cells so as to form randomly a diversion circuit when the input does not receive a valid identification key and when the storage cells receive a test control signal in read or write mode. 
   According to one variant, the logic cells and the storage cells are connected so as to form an operating circuit when the storage cells do not receive a test read or write control signal. 
   According to another variant, each storage cell includes: 
   a multiplexer with its first input having an operating circuit formation connection and with its second input connected to the control module so as to form selectively the test shift register or the diversion circuit; 
   a D flip-flop with its input connected to the output of the multiplexer and with its output having an operating circuit formation connection and a connection to the control module. 
   According to another variant, the control module includes: 
   a random address generator; 
   a selection circuit having: 
   a first test address receive input; 
   a second input connected to the random generator; 
   an output reproducing the state of the first input when the identification key is valid and otherwise reproducing the state of the second input; 
   multiplexers, each associated with a storage cell and having: 
   an input for the formation of the test shift register; 
   at least one input for the formation of a diversion circuit; 
   an output connected to the second input of the multiplexer of the associated storage cell; 
   an addressing input connected to the selection circuit output. 
   According to yet another variant, the random address generator randomly generates a distinct address for each multiplexer associated with a storage cell. 
   According to one variant, the control module includes XOR gates the output of which is connected to a diversion circuit formation input and the inputs of which are connected to memory cells or logic cells. 
   According to another variant, diversion circuit formation inputs are connected at a pre-set logic level. 
   According to another variant, the diversion circuit is a shift register formed by the storage cells connected in random order. 
   According to yet another variant, the module includes an input and an output for entering and reading data respectively in the cells forming the test shift register. 
   The circuit may also be expected to include a control circuit, such as an access controller, connected by respective control conductors to the storage cells and delivering in operation said control signals on the control conductors, and applying the data for entry to the module input and reading the data at the module output. 
   The invention also relates to a smart card that includes an electronic circuit of this kind. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other characteristics and advantages of the invention will emerge clearly from the description which is given hereinafter, by way of example and in no way restrictively, with reference to the appended drawings in which: 
       FIG. 1  is a general view of a circuit according to the invention; 
       FIG. 2  is in diagrammatic illustration of the connections when an operating circuit is formed; 
       FIG. 3  is in diagrammatic illustration of the connections when a test shift register is formed; 
       FIG. 4  is a diagrammatic illustration of the connections when a diversion circuit is formed; 
       FIG. 5  shows a variant intended to form a diversion circuit. 
   

   DETAILED DESCRIPTION 
   A module controls the connection of storage cells associated with a logic circuit and able to form a test shift register. The invention provides the random formation of one or more diversion circuits with the storage cells when a read or write command is applied to these storage cells without an identification key being provided to the control module. In this way, if an attempt is made to enter or to read data fraudulently by forming the test shift register, the data will in fact be entered or read in a diversion circuit. This data will not have the expected effect on the logic circuit, nor will it have any meaning when read. In this way, the fraudulent retrieval of a secret key stored in the logic circuit is much more arduous. 
     FIG. 1  shows an example of an electronic circuit  1  according to the invention. The circuit  1  is an integrated circuit, including a logic circuit  2  equipped with a plurality of logic cells not shown. The circuit  1  also includes storage cells  3  connected to these logic cells and able to receive control signals for entering or reading data in the cells. The circuit  1  additionally has a module for controlling the connections  4 , to which an input and the output of each storage cell are connected. The control module  4  has an input  45  for entering test data and an output for reading test data  46 . 
   Storage cells  3  are known per se. A storage cell typically includes a multiplexer  31 , of which a first input  311  is connected to the logic circuit  2  and of which a second input  312  is connected to the control module  4  in a way presented in detail below. The multiplexer selection contact block  33  allows either the state of the first input, or the state of the second input to be reproduced selectively at the output of the multiplexer. The output of the multiplexer  31  is connected to the input of a D flip-flop  32 . An output of the D flip-flop forms the output of the storage cell and is connected on the one hand to the logic circuit  2 , and on the other hand to the control module  4 . The D flip-flop has a clock input  34  which may possibly be controlled in test mode at a test frequency different from the normal operating frequency of the circuit  2 . 
   According to the prior art, the contact block  33  typically receives a so-called scan-enabled signal supplied in a way known per se by a TAP controller. As described in the introduction, a test is conducted of the internal path of the logic circuit  2  by initially applying a scan-enabled signal to the contact block  33  of the multiplexers. The storage cells then form a shift register according to the connection shown diagrammatically in  FIG. 3 . In this way a test circuit  22  is obtained. This shift register is then loaded with the data applied to its input. Next an operating circuit  21  is formed as shown in  FIG. 2 , by no longer applying the scan-enabled signal to the contact block  33  of each multiplexer. The operating circuit  21  is led to execute one or more clock cycles with the loaded data. The test circuit  22  is then formed again and then the data entered in the shift register is read in series at its output. 
   The invention aims particularly to protect the electronic circuit against an attack by microprobing. Such an attack may include applying a microprobe to a control of the contact blocks  33  so as to form the test shift register fraudulently and in applying another microprobe so as to enter or read data in this shift register. The connection module  4  only forms the test shift register when it receives a correct identification key. 
   The connection module  4  has an input  41  for the reception of an identification key. This identification key may be presented in the form of a word with an appropriate number of bits. If a correct key is applied to the input  41 , the module  4  connects the output  35  of a storage cell to the second input  312  of the multiplexer  31  of another storage cell, in a set order. The module  4  connects the input  45  to the input  312  of the multiplexer  32  of the first storage cell of the test shift register. The module  4  connects the output of the last storage cell of the test shift register to the output  46 . In this way, when the contact blocks  33  receive a scan-enable signal, the test shift register is formed as shown in  FIG. 3  and data can be entered or read by the input  45  and the output  46 . 
   If no identification key is applied to the input  41  or if an erroneous identification key is applied, the module  4  connects the inputs  312  in such a way that the application of the scan-enable signal to the contact block  33  generates a diversion circuit  23  as shown in  FIG. 4 . The connections of this diversion circuit are expected to be fixed randomly, by connection of flip-flops, logic gates or pre-set logic states to the inputs  312 . The connections of the diversion circuits  23  may in particular be expected to be modified at each clock cycle of the electronic circuit. 
     FIG. 1  shows in detail a possible embodiment of a connection module  4  according to the invention. The module  4  has multiplexers  5 . The output  53  of each multiplexer  5  is connected to the input  312  of an associated storage cell. The multiplexer has an input  52  connected to the output of another storage cell. This connection is intended to form the test shift register  22 . 
   The module  4  has a multiplexer  43  for each multiplexer  5 . The output of a multiplexer  43  is connected to the selection contact block  54  of the associated multiplexer  5 . The multiplexer  43  is provided so as to apply either a test address or a diversion address to the selection contact block  54 . 
   To this end, the module  4  has a test address generator  44 . The generator  44  is connected to the input  41  and therefore receives the keys applied to this input. The generator  44  compares the key received with an identification key which it stores. When the received key is identical to the stored identification key, the generator  44  provides the addresses corresponding to the test circuit to the input  431  of the multiplexers  43 . The generator  44  additionally has an output  441  connected to the selection contact block  433  of the multiplexer  43 . The generator  44  applies to the output  441  a validation signal when the identification key received is correct. 
   The module  4  has a random address generator  42 . The generator  42  randomly applies an address to an input  432  of a multiplexer  43 . The addresses applied to the different contact blocks  432  are preferably generated randomly in a disassociated way. 
   When a validation signal is applied by the output  441  to the contact block  433 , the multiplexer  43  reproduces the test address at output. The associated multiplexer  5  thus reproduces at its output  53  the state of its input  52 . In the absence of a validation signal, the multiplexer  43  reproduces at output the diversion address applied by the random generator. The multiplexer  5  then reproduces at output the state of the input  51  or  52  designated by the diversion address. 
   The multiplexer  5  may have only an input  51  and an input  52 . Indeed, if random multiplexing is carried out between these two inputs for each multiplexer  5 , diversion circuits  23  may also be formed randomly. Of course, the greater the number of inputs  51 , the larger the number of randomly generated diversion circuits may be. The number of diversion circuits generated also grows with the number of storage cells connected to the module  4 . 
     FIG. 5  shows an example of an embodiment of a diversion circuit. The circuit shown in broken lines connects the input  51  of the multiplexers  5  to the XOR gate outputs. Using XOR gates allows the number of possible diversion circuits to be increased. It is possible to apply to the input of a XOR gate the output of a storage cell, a pre-set logic state or any other signal able to make the diversion circuit random and unrepresentative of the test circuit. 
   As shown in  FIG. 4 , the diversion circuits formed may possibly be expected to be shift registers with their configuration changing randomly at regular intervals. 
   Although an example has previously been described in which the connection of each storage cell in the test shift register can be modified, it is also conceivable to simplify the electronic circuit  1  by applying a random connection only to a limited number of storage cells  3 . 
   Depending on the configuration of the electronic circuit  1 , the man skilled in the art will choose whether or not to include therein a control circuit such as a TAP controller. The connection module  4  might be expected to be integrated into a TAP controller. The controller will deliver in a way known per se any other signal necessary for reading or writing in the test shift register. The electronic circuit may also be without a TAP controller and be controlled by an external TAP controller connected to an input/output interface of the electronic circuit. 
   The electronic circuit  1  may be integrated into a smart card. 
   Having thus described at least one illustrative embodiment of the invention, various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description is by way of example only and is not intended as limiting. The invention is limited only as defined in the following claims and the equivalents thereto.