Abstract:
A method and apparatus for preventing compromise of data stored in a memory by assuring the deletion of data and minimizing data remanence affects is disclosed. The method comprises the steps of monitoring the memory to detect tampering, and if tampering is detected, generating second signals having second data differing from the first data autonomously from the first processor; providing the generated second signals to the input of the memory; and storing the second data in the memory. Several embodiments are disclosed, including self-powered embodiments and those which use separate, dedicated processors to generate, apply, and verify the zeroization data.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims benefit of U.S. Provisional Patent Application No. 60/593,015, entitled “HIGH-ASSURANCE PROCESSOR ACTIVE MEMORY CONTENT PROTECTION,” by Michael Masaji Furusawa and Chieu T. Nguyen, filed Jul. 30, 2004, and U.S. Provisional Patent Application No. 60/593,016, entitled “HIGH-ASSURANCE SECURE BOOT CONTENT PROTECTION,” by Michael Masaji Furusawa, and Chieu T. Nguyen, filed Jul. 6, 2004, which applications are hereby incorporated by reference herein.  
         [0002]     This application is related to the following co-pending and commonly assigned patent application, which application is also incorporated by reference herein: 
        application Ser. No. ______, entitled “HIGH-ASSURANCE SECURE BOOT CONTENT PROTECTION,” filed on same date herewith, by Michael Masaji Furusawa and Chieu T. Nguyen.       
 
     
    
     BACKGROUND OF THE INVENTION  
       [0004]     1. Field of the Invention  
         [0005]     The present invention relates to data protection systems and methods, and in particular to a system and method for preventing compromise of data stored in a memory due to data remanence.  
         [0006]     2. Description of the Related Art  
         [0007]     One possible avenue to obtain access to otherwise secure data is to carefully scan the storage media or memory for data that was incompletely deleted from the data storage device. Incomplete erasure of data is a problem with both magnetic and optical storage media and electronic semiconductor memory. Such incomplete erasure arises from magnetic persistence in magnetic media and deformations in optical media. In semiconductors, remanence can have serious affects on volatile random access memory (RAM) and non-volatile memory (Flash) technologies. Remanence is known to be influenced by hot-carrier effects (which charges the semiconductor devices), electro-migration (which physically changes the semiconductor devices), and environmental dependencies affecting remanence including voltage and temperature.  
         [0008]     Data remanence issues can be solved using techniques that range from performing repetitive read and write operations of known data patterns to memories and the development of new semiconductor technologies.  
         [0009]     An effective way to avoid short-term data retention is to ensure that no memory cell can hold a quantity of data for more than a certain amount of time. Similarly, an effective way to avoid long-term storage effects is to periodically flip the stored data bits as suggested in the 1996 paper (Titled “Secure Deletion of Data from Magnetic and Solid-State Memory”, Peter Gutmann,  Proceedings of the  6 th Usenix Security Symposium , July 1996, p. 77.) so that each cell never holds a value long enough for it to be permanently or temporarily “remembered”. Although impractical for large amounts of data, this may be feasible for small amounts of sensitive data such as cryptographic key variables.  
         [0010]     Long-term retention effects are most likely to occur when the same data is repeatedly fed through a specialized circuit. For example, in cryptography there may be a repeated use of an identical private key variable in a cryptographic circuit that performs an encryption algorithm. This condition is common in specialized cryptographic circuits, as opposed to general-purpose processor circuits, which constantly processes all sorts of different data types that cannot be distinguished at any given time. In contrast, a private key stored in a tamper resistant hardware circuit that is input repeatedly by a cryptographic processor will lead to some circuits (and signals) always carrying the same information and leading to pronounced long-term hot-carrier degradation and electro-migration effects.  
         [0011]     One method of actively reducing the effects of electro-migration (as opposed to passively allowing the memory to revert back to its un-programmed ‘ground’ state) is to apply a reverse-current, which reverses the electro-migration stress, effectively undoing the electro-migration damage. Similar techniques are already used in some EEPROM/Flash devices to reduce repeated erasure stress by applying a reverse-polarity pulse after an erase pulse.  
         [0012]     A somewhat more complex and difficult-to-implement approach is to have a cryptographic processor write known false ‘dummy’ data to memory when it isn&#39;t processing real sensitive data or keys. A disadvantage of this method is it requires that a crypto operation be interruptible once started. Unfortunately, alternating dummy and real data is complicated by the design of typical crypto devices.  
         [0013]     High-assurance security methods may also include encryption of the active data in working memory. This method might just be a deterrence, since a similar (as a matter of fact, perhaps even more critical and more elaborate) protection must be provided for the vital secret parameters (crypto variables, credentials, etc.) in conjunction with encrypting the data. If encryption of the memory is performed without protecting the vital secret parameters, the encrypted data could still be vulnerable to attacks, because if the critical secrets were recovered, the encrypted data can thus be decrypted.  
         [0014]     Another solution to this problem is to use zeroization techniques to erase the cryptographic variables under appropriate circumstances. This provides limited security protection if not performed effectively or quickly. Federal Information Processing Standard 140-2 (FIPS 140-2) specifies the requirement for zeroizing plain text data and keys but does not specify the method of performing such action, when such action should take place or how this requirement is to be implemented.  
         [0015]     However, the foregoing solutions are limited in their application and/or effectiveness. For example, the continuous flipping of data is impractical for larger data sets. Zeroizing data is effective, but is vulnerable to malicious software and hardware intervention. Both zeroization and reverse current techniques are typically performed at slower speeds by the same processors that are used in normal operational modes. This limits their effectiveness, and current random access memory (RAM) and FLASH memory technologies are moving to still higher speeds.  
         [0016]     Using alternative data processing techniques such as key switching incurs the overhead of a key schedule. Further, pipelined implementations of block ciphers are generally not interruptible, and require completion of processing of the current block (and in some cases several more blocks to force the data pipeline to be flushed) before a key change can take effect.  
         [0017]     Further, the foregoing techniques are difficult to implement in systems having high-performance computing platforms and associated memories that are decoupled from the computer motherboard. Such designs are also expected to become more commonplace.  
         [0018]     References discussing data remanence and methods to ameliorate it include “Data Remanence in Semiconductor Devices”, Peter Gutmann,  IBM T.J. Watson Research Center , Proceedings of the 10th USENIX Security Symposium, Washington, D.C., USA—Aug. 13-17, 2001; “Relation between the hot carrier lifetime of transistors and CMOS SRAM products”, Jacob van der Pol and Jan Koomen,  Proceedings of the International Reliabily Physics Symposium  (IRPS 1990), April 1990, p. 178; “Hot-carrier-induced Circuit Degradation in Actual DRAM”, Yoonjong Huh, Dooyoung Yang, Hyungsoon Shin, and Yungkwon Sung,  Proceedings of the International Reliabiliy Physics Symposium  ( IRPS  1995), April 1995, p. 72; “Metal Electromigration Damage Healing Under Bidirectional Current Stress”, Jiang Tao, Nathan Cheung, and Chenming Ho,  IEEE Electron Device Letters , Vol. 14, No. 12 (December 1993), p. 554; “An Electromigration Failure Model for Interconnects Under Pulsed and Bidirectional Current Stressing”, Jiang Tao, Nathan Cheung, and Chenming Ho,  IEEE Transactions on Electron Devices , Vol. 41, No. 4 (April 1994), p. 539; “New Write/Erase Operation Technology for Flash EEPROM Cells to Improve the Read Disturb Characteristics”, Tetsuo Endoh, Hirohisa Iizuka, Riichirou Shirota, and Fujio Masuoka,  IEICE Transactions on Electron Devices , Vol. E80-C, No. 10 (October 1997), p. 1317; and “Security Requirements for Cryptographic Modules”, Federal Information Processing Standards Publication, FIPS PUB 140-2 (May 25, 2001), all of which are hereby incorporated by reference herein.  
         [0019]     Accordingly, there is a need for a system and method for protecting stored data that avoids the need to constantly flip data within a large memory space, can be performed reliably high speeds, does not require constant processing of alternative data, allows flexibility in the use of memory modules and in modifying external interfaces between the CPU and the memories, and provides adequate security from malicious software while not requiring that the crypto or general purpose processor used with the memory be a trusted processor. The present invention satisfies that need by providing hardware-based protection that provides higher assurance data zeroization techniques deterring data recovery from semiconductor RAM devices (due to remanence) that can be implemented into conventional computing platforms, without having the expense of inventing new semiconductor technologies.  
       SUMMARY OF THE INVENTION  
       [0020]     To address the requirements described above, the present invention discloses a method and apparatus for preventing compromise of data stored in a memory, by assuring the deletion of data and minimizing data remanence affects. In one embodiment, the method comprises the steps of monitoring the memory to detect tampering, and if tampering is detected, generating second signals having second data differing from the first data autonomously from the first processor; providing the generated second signals to the input of the memory; and storing the second data in the memory. Several embodiments are disclosed, including self-powered embodiments and those which use separate, dedicated processors to generate, apply, and verify the zeroization data. The invention can also be practiced as a circuit for protecting data stored in a memory by a processor. The circuit comprises a tamper detector, for generating a tamper signal indicative of an attempt to tamper with the memory; a zeroization generator, for generating zeroization data in response to the tamper signal autonomously from the processor; and a selector, for selectably coupling a processor and the zeroization generator to the memory according to the tamper signal.  
         [0021]     The foregoing provides hardware-based protection that yields higher assurance data zeroization techniques, thus deterring remanence data recovery from semiconductor RAM, EEPROM, or FLASH devices. This technique can also be implemented with conventional computing platforms, without incurring the expense of new semiconductor technologies.  
         [0022]     One embodiment of the invention provides for a self-powered passive zeroization mode, which provides protection against discovery of remanence-related data even when the primary power of the computing platform under protection has been removed or defeated. The invention can be implemented by a module that can be added to commercial CPU circuit boards, or an embeddable circuit that can be designed into CPU circuit boards. Although aiming at RAM, this invention applies to other Semiconductor technologies (such as EEPROM, FLASH) as well.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]     Referring now to the drawings in which like reference numbers represent corresponding parts throughout:  
         [0024]      FIG. 1  illustrates an exemplary computer system that could be used to implement the present invention;  
         [0025]      FIG. 2  is a diagram of a circuit depicting one embodiment of the present invention;  
         [0026]      FIG. 3  is a diagram illustrating another embodiment of the invention in which the circuit  300  active memory  206  is external to the circuit  200 ;  
         [0027]      FIG. 4A  is a flow chart illustrating exemplary process steps that can be used to perform the active data zeroization techniques described above; and  
         [0028]      FIG. 4B  is a flow chart illustrating exemplary process steps that can be used to perform passive data zeroization. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0029]     In the following description, reference is made to the accompanying drawings which form a part hereof, and which is shown, by way of illustration, several embodiments of the present invention. It is understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention.  
       Hardware Environment  
       [0030]      FIG. 1  illustrates an exemplary computer system  100  that could be used to implement the present invention. The computer  102  comprises a processor  104  and a memory, such as random access memory (RAM)  106 . The computer  102  is operatively coupled to a display  122 , which presents images such as windows to the user on a graphical user interface  118 B. The computer  102  may be coupled to other devices, such as a keyboard  114 , a mouse device  116 , a printer, etc. Of course, those skilled in the art will recognize that any combination of the above components, or any number of different components, peripherals, and other devices, may be used with the computer  102 .  
         [0031]     Generally, the computer  102  operates under control of an operating system  108  stored in the memory  106 , and interfaces with the user to accept inputs and commands and to present results through a graphical user interface (GUI) module  118 A. Although the GUI module  118 A is depicted as a separate module, the instructions performing the GUI functions can be resident or distributed in the operating system  108 , the computer program  110 , or implemented with special purpose memory and processors. The computer  102  also implements a compiler  112  which allows an application program  110  written in a programming language such as COBOL, C++, FORTRAN, or other language to be translated into processor  104  readable code. After completion, the application  110  accesses and manipulates data stored in the memory  106  of the computer  102  using the relationships and logic that was generated using the compiler  112 . The computer  102  also optionally comprises an external communication device such as a modem, satellite link, Ethernet card, or other device for communicating with other computers.  
         [0032]     In one embodiment, instructions implementing the operating system  108 , the computer program  110 , and the compiler  112  are tangibly embodied in a computer-readable medium, e.g., data storage device  120 , which could include one or more fixed or removable data storage devices, such as a zip drive, floppy disc drive  124 , hard drive, CD-ROM drive, tape drive, etc. Further, the operating system  108  and the computer program  110  are comprised of instructions which, when read and executed by the computer  102 , causes the computer  102  to perform the steps necessary to implement and/or use the present invention. Computer application program  110  and/or operating instructions may also be tangibly embodied in memory  106  and/or data communications devices  130 , thereby making a computer program product or article of manufacture according to the invention. As such, the terms “article of manufacture,” “program storage device” and “computer program product” as used herein are intended to encompass a computer program accessible from any computer readable device or media.  
         [0033]     Those skilled in the art will recognize many modifications may be made to this configuration without departing from the scope of the present invention. For example, those skilled in the art will recognize that any combination of the above components, or any number of different components, peripherals, and other devices, may be used with the present invention.  
         [0034]      FIG. 2  is a diagram of a circuit  200  depicting one embodiment of the present invention. The circuit  200  comprises a tamper detector  218 , which provides a tamper signal  216  indicative of an attempt to tamper with a memory  206  to a communicatively coupled zeroization data generator (ZDG)  202 .  
         [0035]     The ZDG  202  provides signals a first signal group  210  comprising address data, control data, and zeroization data to a communicatively coupled selector  204 . Also coupled to the selector  204  is second signal group  208  comprising address data, control data, and operating data. The selector  204  selectably provides either the first signal group  210  or the second signal group  208  as a selected signal group  214  to the memory  206 , based upon a switch signal  212  obtained from the ZDG  202 .  
       Tamper Detector  
       [0036]     A variety of different tampering techniques can be detected, including (1) physical intrusion of the memory package, (2) excessively low or high temperatures, (3) excessively low or high primary power voltage  224  and (4) excessively low or high battery voltages. If tampering is detected, the tamper detector  218  generates a tamper signal  216  and optionally generates a reset signal  226  as well.  
         [0037]     In one embodiment, the tamper signal is provided to the ZDG  202  to generate zeroization data if tampering is detected and primary power remains available via the primary power signal  224  (hereinafter referred to as “active zeroization”), while a reset signal  226  is provided to the memory  206  if tampering is detected and primary power is not available (hereinafter referred to as “passive zeroization”). In another embodiment, the tamper signal  216  and the reset signal  226  are provided when tampering is detected, regardless of the status of the primary power provided by signal  224 .  
       Zeroization Generator  
       [0038]     The ZDG  202  is powered by the primary power signal  224 . Power may also be provided by the local power supply  222  if tampering is detected. The ZDG  202  is also nominally reset by the processor  104  upon power-on or reset conditions via power-on-reset (POR) signal  204 .  
         [0039]     The ZDG  202  comprises an internal oscillator  228  and a clock  230  that is independent from the that of the processor  104 . This oscillator  228  and clock  230  remains in a standby or inactive state until the tamper signal  216  is received from the tamper detector  218 . Upon receiving the tamper signal  216 , the ZDG  202  generates zeroization data  232  that is used to effectively erase the data stored in the memory  206  without remanence effects, and generates and sends a switch signal  212  to the selector  204  to command the selector  204  to provide the zeroization data  232  along with the appropriate address  234  and control  236  data that forms the first signal group  208  to the memory  206  in place of the ordinary (non-tamper) operational data that is provided in second signal group  210 . To generate the zeroization data  232 , the ZDG tamper signal activates circuitry that is in a standby or inactive state before tampering is detected. This circuitry may include special purpose discrete circuitry, special purpose processor(s), or general purpose processor(s) or any combination thereof. In one embodiment, activation of the ZDG  202  upon receipt of the tamper signal  216  from the tamper detector  218  includes enabling a ZDG oscillator  228  and a ZDG clock  230 . Using this circuitry and/or processors, the ZDG  202  generates one or more sets of data signals that are used to zeroize the data in memory  206 , and also, to optionally verify that the zeroization process was successfully completed, as described in further detail below.  
       Local Power Supply  
       [0040]     The circuit  200  also comprises a local power supply  222 , communicatively coupled to the memory  206 , the tamper detector  218 , and optionally, the zeroization data generator  202 . The local power supply  222  provides local power to these components so that they can complete their function when and after tampering is detected, even if the primary power  224  is removed. This includes providing local power to the tamper detector  218  upon removal of the primary power  224  and providing burst power to the memory  206  to allow the memory  206  to be reset.  
         [0041]     In one embodiment, the local power supply  222  is a battery that is charged by the primary power signal  224 .  
         [0042]     The local power supply  222  may also provide power to the appropriate components even when no tampering is detected. For example, if the local power signal  220  is continuously provided to the tamper detector  218  (even during periods when the memory or packaging is not tampered with), this signal can be used to determine if there has been any tampering with the local power generator  222 , perhaps as the first step to tampering with the memory  206  or other components of the circuit  200 .  
         [0043]     The diagram shown in  FIG. 2  illustrates an embodiment of the invention in which the circuit  200  is a custom memory module. In this embodiment, the active memory  206 , ZDG  202 , tamper detector  218 , and local power supply  222  are all in a single package, and interface with the processor  104  via connector  302 .  
         [0044]      FIG. 3  is a diagram illustrating another embodiment of the invention in which the circuit  300  active memory  206  is external to the circuit  200 . In this embodiment, an external memory module  206 ′ is coupled to the processor  104  via an alternative circuit  200 ′ via processor/circuit connector  302  and circuit/memory connector  304 . This embodiment operates in substantially the same way as the embodiment shown in  FIG. 2 , however this embodiment provides local power to the memory  206  via connector  304  using the same conductor as the primary power line from the selector  204  to the connector  304 . This embodiment can be added to commercial CPU boards to prevent remanence problems from compromising the security of the data stored in the memory  206 .  
         [0045]      FIG. 4A  is a flow chart illustrating exemplary process steps that can be used to perform the active data zeroization techniques described above. In ordinary (non-tamper condition) operation, data  240  is passed between the processor  104  and the memory  206 , as shown in block  402 . The memory  206  is monitored for a tampering condition as discussed above, as shown in block  404 . This can be accomplished by detecting a tamper condition such as a physical intrusion of the package in which the memory  206  or other elements are contained, an aberrant package temperature (e.g. excessively high, low, or with a temporal history that is abnormal), or an aberrant supply of voltage to the package (e.g. also abnormally high, low or with an abnormal temporal history). A tamper condition may also be determined as a function of the foregoing conditions (e.g. higher than normal temperature and lower than normal voltage) to prevent false alarms. Block  406  continues the monitoring function of block  404  until tampering is detected, in which case, processing is passed to blocks  408  and  418 .  
         [0046]     If tampering is detected, a check is made to determine if primary power is present, or if it is absent or has been defeated, as shown in block  407 . If primary power is absent or defeated, processing passes to block “A” which describes passive zeroization. If primary power is present, active zeroization is initiated. As shown in blocks  408 - 412 , signals having data different than the ordinary data  240  are generated, applied to the memory  206  and stored in the memory  206 . In one embodiment, this is accomplished by the ZDG  202 , and the selector  204  in response to the tamper detector  218  shown in  FIGS. 2 and 3 .  
         [0047]     In one embodiment, the generation, application, and storage of the zeroization data  232  is accomplished by the use of a plurality of zeroization data sets. For example, in a preferred embodiment of the invention, zeroization data  232  comprises a first data set comprising first pseudorandom data, a second data set comprising second pseudorandom data, a third data set comprising only ones and a fourth data set comprising only zeroes. These data sets are applied to and stored in the memory  206  in order, first overwriting the data stored by the processor  104  and later overwriting the previous zeroization data sets. Hence, the data previously stored in the memory  206  is first overwritten by pseudorandom data, that pseudorandom data is written over by pseudorandom data, the second pseudorandom data is written over by all ones and the ones are written over by zeroes. Other data set patterns can also be used. For example, the last step could be to store all ones rather than all zeroes.  
         [0048]     Note that the ZDG can generate all of the zeroization data sets all at once, store them for use in the memory one at a time, or can generate them and pass them along to the memory as they are generated. Also note that in embodiments where pseudorandom data is used, that data can be generated by a pseudorandom number generator in the ZDG  202 , or by a number of techniques known in the art.  
         [0049]     Optionally, the zeroization data can be read to verify that the zeroization process has satisfactorily eliminated data remanence. In one embodiment, this is accomplished by reading the data from the memory and comparing it to the zeroization data that was last generated and stored in the memory  206 . The method by which the zeroization data is read from the memory  206  preferably mimics that which a hacker might use to take advantage of data remanence to gain access to the data (for example, by modifying the power supply to the memory). If the read zeroization data matches the data that was last stored (the data that last overwrote what was stored in the memory  206 ), the process has completed, and the process ends. If the read data does not match, or if the read zeroization data otherwise indicates that complete zeroization has not occurred, processing loops to block  408  to repeat the process as many times as is required. This is shown in blocks  414  and  416 .  
         [0050]     While the operations in blocks  408 - 416  are performed, local power is provided to zeroization elements (e.g. the tamper detector  218 , memory  206 , and optionally, the ZDG  202 ). This is shown in block  418 .  
         [0051]      FIG. 4B  is a flow chart illustrating exemplary process steps that can be used to perform passive data zeroization. These steps are performed if tampering is detected (as shown in block  407  of  FIG. 4A , and primary power is either absent or defeated). In this instance, burst power is provided to the memory  206  as shown in block  420 , while the memory  206  is reset, as shown in block  422 .  
         [0052]     While  FIGS. 4A and 4B  illustrate that the memory  206  is reset only if primary power is unavailable or disabled, the present invention can be implemented by resetting the memory before the zeroization process depicted in blocks  408 - 414  and  418  take place.  
       CONCLUSION  
       [0053]     This concludes the description of the preferred embodiments of the present invention. The foregoing description of the preferred embodiment of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto. The above specification, examples and data provide a complete description of the manufacture and use of the composition of the invention. Since many embodiments of the invention can be made without departing from the spirit and scope of the invention, the invention resides in the claims hereinafter appended.