Patent Publication Number: US-5841786-A

Title: Testing of memory content

Description:
The invention relates to the testing of the content of memory associated with a microprocessor. The memory may be memory whose contents cannot be altered (Read Only Memory or ROM) or memory whose contents can be altered such as Electrically Programmable Read Only Memory (EPROM) or RAM (Random Access Memory). 
     There are many instances in which memory which holds predetermined information is required to be tested. An example is in the use of integrated credit cards, otherwise know as smart cards. Conventionally, smart card memory is tested by connecting the card to a reader, reading out the memory contents and performing a check on the readout, perhaps by way of check-sum verification or by comparison of the read-out with data which should be present. Often, the memory contents are confidential and it is possible that unauthorised access to the information could be obtained by simulating a memory test. An object of the invention is to overcome this problem. 
     According to a first aspect of the invention there is provided a method of testing the proper functioning of an electrically alterable memory associated with an integrated circuit, the method consisting in writing test data to the memory, applying predetermined data as input to the integrated circuit, comparing the data internally within the integrated circuit with the test data held in the memory, and issuing a verification output from the integrated circuit whereby the write function and the retention properties of the memory may be verified as correct or not according to the verification output. 
     By this method, the write function and retention properties of an electrically alterable memory can be tested. 
     According to a second aspect of the invention there is provided a method of testing the content of memory associated with a microprocessor in a module, the method consisting in applying predetermined data as input to the module, encrypting the predetermined data, comparing the encrypted predetermined data internally within the module with data held in the memory and issuing a verification output from the module according to whether the memory content is verified as correct or not by the comparison. 
     According to the second aspect of the present invention, the data held in the memory may be data permanently or semi-permanently resident in the memory, such as program data and/or initialisation data, or it may be data written to the memory as a first step in the test method. By this latter method, the write function and retention properties of an electrically alterable memory can be tested. 
     The module may be an integrated circuit including the memory and the microprocessor or may be a set of integrated circuits physically bonded by encapsulation or the like. Typically the module is an integrated circuit in a smart card. 
     By use of an internal memory comparison technique as described above the memory contents are externally inaccessible directly. All that is output is a verification signal. 
     It is possible that fraudulent attempts to determine the content of the memory may be made by automatically inputting successive sets of test data which are systematically changed in content until a verification output indicates that the memory content corresponds with the input data. There are ways of dealing with this problem. 
     According to a feature of the invention the microprocessor is programmed to delay the issue of each verification or non-verification output. The delay between successive outputs may be a matter of microseconds or seconds. The object is to render the time taken for systematic checking to be prohibitively long. 
     Another way of preventing fraudulent memory checking is to arrange that the input data is encrypted, for example by being operated on by a secret algorithm, before comparison. Then the input data would have no clear correlation with the memory content. The algorithm or a key to it could be different from batch to batch of the smart cards. 
    
    
     The invention will further be described with reference to the accompanying drawing, of which the sole FIGURE is a schematic diagram illustrating a method embodying the invention. 
    
    
     Referring to the drawing there is shown an integrated circuit 1 having a memory 2 in the form of an EEPROM which contains certain data and a comparison device 3, for example in the form of a microprocessor which is programmed to effect data comparisons. An interface to the external world is represented by the dotted line 4. On the interface 4 are three ports 5, 6 and 7 whereby signals are passed to and fro between the integrated circuit 1 and the outside world. The internal route from port 7 splits into two paths: a write path 8 passing to the memory 2 and a read path 9 passing to comparison device 3. 
     Testing is carried out by inputting program data to the integrated circuit 1 and performing a comparison internally by means of the comparison device 3. The program data is generated by external means (not shown) and is passed to the comparison device 3 via the data input port 7 and the read path 9. Corresponding data stored in the memory 2 is passed to the comparison device 3 via a read path 10. In the comparison device, the data entering from the two paths 9 and 10 is compared and the outcome of the comparison is communicated to the outside world via port 5 by means of a verification signal identifying the comparison as Pass or Fail. This method prevents the memory contents from ever leaving the integrated circuit 1 and thus ensures high security. The read address and control signals can be generated by external means (not shown) and input to the integrated circuit through an address/control port 6 at the interface 4. The address/control signals are passed to the memory and control the passing of data from the memory 2 to the comparison device 3 in the conventional manner. 
     In an alternative embodiment (not shown) the ports 5 and 7 are combined into a single multiplexed read/write port and the port 6 eliminated. Address and control signals which would otherwise have used the port 6 are instead input on a multiplex basis via the sub-port 7 of the combined port 5/7 and the path 8. 
     The scheme also allows for the testing of the write function of electrically alterable memory by allowing test data to be written to the memory 2 via the data input port 7 at the interface 4 and the write path 8. A correct write can then be verified in the manner outlined above, again under the control of the address/control signals. In this way properties such as endurance, retention time and cell margin can all be tested. 
     It might still be possible for fraudulent access to the contents of memory 2 by exhaustively testing all possible combinations of memory. It can be shown that this becomes more difficult the fewer times a pass/fail signal is generated. For instance, if the pass/fail is generated after reading a single 8 bit byte, then a maximum of 256 reads are required to exhaustively test a byte. If a pass/fail signal is only generated after reading a 16 bit word however then a maximum of 65,536 reads would be required to exhaustively test a word. Similarly 4,294,967,296 reads are required to exhaustively test a 32 bit word. Exhaustive testing to extract the memory contents can easily be made unfeasible using this scheme. 
     An additional aid to security is to ensure that words to be tested extend over memory page boundaries, perhaps by variable amounts.