Patent Publication Number: US-10771062-B1

Title: Systems and methods for enhancing confidentiality via logic gate encryption

Description:
CROSS REFERENCE TO RELATED PATENT APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 15/645,562, entitled “SYSTEMS AND METHODS FOR ENHANCING CONFIDENTIALITY VIA LOGIC GATE ENCRYPTION,” listing as inventors, Robert Michael Muchsel, Donald Wood Loomis, III., Edward Tangkwai Ma, Hung Thanh Nguyen, Nancy Kow Iida, and Mark Alan Lovell, and filed Jul. 10, 2017, which is a continuation of U.S. patent application Ser. No. 14/659,348, entitled “SYSTEMS AND METHODS FOR ENHANCING CONFIDENTIALITY VIA LOGIC GATE ENCRYPTION,” listing as inventors, Robert Michael Muchsel, Donald Wood Loomis, III., Edward Tangkwai Ma, Hung Thanh Nguyen, Nancy Kow Iida, and Mark Alan Lovell, and filed Mar. 16, 2015, which is related to and claims the priority benefit of U.S. Provisional Patent Application No. 62/058,564, entitled “SYSTEMS AND METHODS FOR ENHANCING CONFIDENTIALITY VIA LOGIC GATE ENCRYPTION,” listing as inventors, Robert Michael Muchsel, Donald Wood Loomis, III., Edward Tangkwai Ma, Hung Thanh Nguyen, Nancy Kow Iida, and Mark Alan Lovell, and filed Oct. 1, 2014, which applications, which applications are hereby incorporated herein by reference in their entireties and for all purposes. 
    
    
     BACKGROUND 
     A. Technical Field 
     The present invention relates to security applications in digital electronics and, more particularly, to systems, devices, and methods of encrypting digital logic gates. 
     B. Background of the Invention 
     Methods to reverse engineer physical IP are becoming increasingly powerful, automatable, and affordable. Today, sophisticated attackers can gain access to and reverse engineer secret encryption and decryption keys embedded in hardware without much effort. A complete, annotated, hierarchical netlist of a digital circuit can be obtained for less than $15,000. This includes circuits that cannot be patented or otherwise protected-exposing proprietary information. This creates a number of severe problems to chip manufacturers and their customers. A related problem is the exposure of keys due to theft and unauthorized distribution of devices. For example, a subcontractor might sell excess quantities of a manufactured device to others, or resell substandard devices that failed to conform to the contractor&#39;s manufacturing specifications under an alternate trade name. 
     As a consequence, manufacturers are forced to expend considerable time and money to develop countermeasures to deter adversaries. Numerous methods to encrypt, obfuscate, and hide information have been employed in the software domain for a long time. Until now, however, no equivalent methods have been feasible in the hardware domain. Nor does there exist any generation of hardware that would be capable of implementing such techniques. 
     What is needed are effective systems and methods that allow for the protection of valuable IP and information in the hardware domain, ideally, using automated procedures that are compatible with existing manufacturing tools and processes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Reference will be made to embodiments of the invention, examples of which may be illustrated in the accompanying figures. These figures are intended to be illustrative, not limiting. Although the invention is generally described in the context of these embodiments, it should be understood that this is not intended to limit the scope of the invention to these particular embodiments. 
       FIG. (“FIG.”)  1 A shows a prior art truth table for a two-input NAND gate. 
         FIG. 1B  shows prior art examples of logic functions expressed in NAND logic. 
         FIG. 2  is a general illustration for decrypting a logic key according to various embodiments of the invention. 
         FIG. 3A  shows an exemplary logic function with four logic gates. 
         FIG. 3B  illustrates the logic key bits for the example logic function in  FIG. 3A , according to various embodiments of the invention. 
         FIG. 4  illustrates an example logic function using generic logic blocks according to various embodiments of the invention. 
         FIG. 5  illustrates a general-purpose configurable logic cell according to various embodiments of the invention. 
         FIG. 6  illustrates details of the general-purpose configurable logic cell shown in  FIG. 5 , according to various embodiments of the invention. 
         FIG. 7  illustrates an exemplary logic cell configuration for the general-purpose configurable logic cell of  FIG. 5  and  FIG. 6 , according to various embodiments of the invention. 
         FIG. 8  is an exemplary logic cell output for the general-purpose configurable logic cell of  FIG. 5  and  FIG. 6 , according to various embodiments of the invention. 
         FIG. 9  illustrates the effect decrypting a logic function by using an invalid or wrong key. 
         FIG. 10  illustrates logic key protection using a secure physical element according to various embodiments of the invention. 
         FIG. 11A and 11B  illustrate the computation of equivalent information from an original logic function, according to various embodiments of the invention. 
         FIG. 12  illustrates software processing as applied to a modified version of logic function of  FIG. 3A , according to various embodiments of the invention. 
         FIG. 13  is a flowchart of an illustrative process to determine a logic key in accordance with various embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following description, for the purpose of explanation, specific details are set forth in order to provide an understanding of the invention. It will be apparent, however, to one skilled in the art that the invention can be practiced without these details. One skilled in the art will recognize that embodiments of the present invention, described below, may be performed in a variety of ways and using a variety of means. Those skilled in the art will also recognize that additional modifications, applications, and embodiments are within the scope thereof, as are additional fields in which the invention may provide utility. Accordingly, the embodiments described below are illustrative of specific embodiments of the invention and are meant to avoid obscuring the invention. 
     Reference in the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, characteristic, or function described in connection with the embodiment is included in at least one embodiment of the invention. The appearance of the phrase “in one embodiment,” “in an embodiment,” or the like in various places in the specification are not necessarily referring to the same embodiment. 
     Furthermore, connections between components or between method steps in the figures are not restricted to connections that are affected directly. Instead, connections illustrated in the figures between components or method steps may be modified or otherwise changed through the addition thereto of intermediary components or method steps, without departing from the teachings of the present invention. 
       FIG. 1A  shows a prior art truth table for a two-input NAND gate. It is well-known that digital logic can be expressed using basic blocks of logic elements, such as gates. At a very basic level, any arbitrary digital logic function can be implemented exclusively with NAND gates or NOR gates. In practice, a standard cell library contains more complex devices than just NAND gates, but the same principles apply. For a better understanding of the present invention, only simple gates are shown herein. 
     Truth table  100  in  FIG. 1  shows output values, F, of a NAND gate for two inputs X and Y, indicating that the output of the logic function is determined by its inputs. Examples of logic functions expressed in NAND logic are shown in  FIG. 1B . Logic values  160  can be chosen arbitrarily or by convention. A block of five NAND gates that are appropriately connected to each other can be used to output any of the logic functions  170  shown in  FIG. 1B . As an example, the value #010 in table  150  represents AND function  180 , which can be represented by expression NOT(NOT(X AND Y))  190  if the appropriate combination of NAND gates is used. Based on this principle, any logic function can be expressed by combining multiple, basic NAND gates into larger logic building blocks. 
     Now, if the result of a logic function were determined not only by its inputs, but additionally by a key (subsequently called “logic key”), as suggested by the present invention, then the functionality of that logic function could not be determined by simply reverse engineering the logic function itself, because the logic key would remain unknown and the output of the logic function could take on any possible result. 
     A logic key is typically an encrypted key that can be stored, e.g., in a tamperproof storage element. In one embodiment of the invention, the logic key is used to determine the physical wiring for a given logic building block to, in effect, encrypt the logic building block. As a result, reverse engineering of the building block without knowledge of the logic key would show only that the building block can be used to express any possible logic function. However, a reverse engineered building block would not expose the actual function of the logic. The logic key may be used directly, i.e., without intermediate storage, such that each bit controls one logic element. In one embodiment, the logic key is created by decrypting data using a secret or private decryption key. 
       FIG. 2  is a general illustration for decrypting a logic key, according to various embodiments of the invention. Encrypted logic key, d,  204  and decryption key, x,  206  are used to generate decrypted logic key, k,  210  using mathematical function f(d, x)  208 . Logic key k  210  ultimately determines the appropriate connections for a given logic building block (not shown) as previously mentioned. Both secret decryption key, x,  206  (e.g., a secret or private key) and encrypted logic key, d,  204  are needed to compute k  210 . In one embodiment, decryption key x  206  has different length than decrypted logic key  210 . For example, x may be a 128-bit AES key that is used to decrypt a relatively larger logic key k  204 . One of ordinary skill in the art will appreciate that both symmetric as well as asymmetric cryptography may be used. 
     Encrypted logic key  204  may be stored in a secure memory. Another example of an indirectly storing the logic key will be discussed with respect to  FIG. 10 . A person of ordinary skill in the art will appreciate that each approach has its own advantages depending on the implementation and the particulars of a given system. One of ordinary skill in the art will also appreciate that there are numerous possibilities how logic building blocks can be arranged and implemented, only some of which are discussed in detail herein. 
     In one embodiment, encrypted logic key d  204  is automatically determined for a given to-be-encrypted logic circuit, for example, a two-dimensional x-y circuit that performs a sensitive algorithm. Ideally, the algorithm has been tested and its proper working condition had been verified. 
       FIG. 3A  shows an exemplary logic function having four logic gates. The four logic gates  302 - 308  in digital logic  300  are connected with each other as shown in  FIG. 3A , and are labeled by their serial numbers S 0   302  through S 3   308 , wherein S 0   302  is an OR gate, S 1   304  is an AND gate, S 2   306  is a NOT gate, and S 3   308  is a NAND gate. For any given input  320  w, x, y, and z, logic  300  outputs the function NOT(AND(z, OR(NOT(y), AND(w,x)))). Logic key bits  360  corresponding to each logic gate type  370  are displayed in  FIG. 3B , according to various embodiments of the invention. The concatenation of logic key bits  360  yields the complete logic key for the example function, here, 011010001000. 
     The logic gates identified in the original design in  FIG. 3A  may be replaced with generic logic blocks, such that logic  300  comprising the four logic gates  302 - 308  will appear as shown in  FIG. 4 , according to various embodiments of the invention. In the example in  FIG. 4 , each logic gate has been replaced with a corresponding generic logic block  402 - 408 . Generic logic block  402 - 408  represents a universal logic gate. For this purpose, first, the logic gates used in the logic function in  FIG. 3A  may be serialized into a sequence in order to create a known sequence of the circuit. Serialization and synthesis of the logic function may be accomplished automatically, for example, by using commercially available tools that can generate an ordered string of gates. This serialization is similar to the process used by known scan mechanisms that are used to create scan chains. Each type of logic gate or a subset thereof (e.g., OR) is analyzed and a sequence of key bits k[i:j]  410  is assigned to it. The identified logic gates or blocks in the original design are then removed and replaced with generic building blocks  402 - 408  that are capable of performing the equivalent function (here, k[2:0]=#011=OR). As a result, a replaced AND gate, for example, cannot be distinguished from an OR gate. 
     While generic building blocks  402 - 408  can be configured to perform certain desired functions, configuration  400  in  FIG. 4  is of no use to the attacker, unless building blocks  402 - 408  are also properly configured with the information that was previously contained in the now replaced logic gates, such that the logic can perform its intended function. In one embodiment, the key bits that have been recorded are loaded back into the silicon using a scan chain to shifting the bits accordingly. This has the advantage that the order of bits remains intact, thereby, eliminating the need to transpose between different orders. 
     In one embodiment, configuration information represented by key bits  410  is stored, in a tamperproof memory, for later use as a decryption key. Upon a power-up condition, or as needed, a state machine or software may extract key bits  410  from the tamperproof memory and shift them into their corresponding logic gates. As a result, logic function  400  regains the properties of the logic function shown in  FIG. 3A  and, thus, operates in the desired manner. 
     It is noted that the process of replacing generic building blocks  402 - 408  may be repeated for any and all remaining logic gates in the sequence. One advantage of an automated, computer-controlled replacement process is that it eliminates the need to re-implement or re-build an existing circuit that is to be protected. One of ordinary skill in the art will appreciate scan chains and serialized logic may be combined in the creation step and in the hardware implementation. An example of the details of each generic block, e.g., S 1   404  is provided in  FIG. 5 . 
       FIG. 5  illustrates a general-purpose configurable logic cell according to various embodiments of the invention. In the implementation shown in  FIG. 5 , logic cell  500  comprises external input signals  502 - 508  and output signal  512 , including clock signal  504 . Decoder  530  decodes input signals Q 0 -Q 2   522 - 524  into function  F   540 . Function  F   540  is, for example, an AND function that receives gate signals S 0 -S 4   532 - 536  from the output of decoder  530  and input signals A and B  506  and  508  and generates therefrom output signal Y  512 . One of ordinary skill in the art will appreciate that logic cell  500  may be designed to process any number of bits in serial and/or parallel configuration, and that many variations of loading and configuring functions are possible. 
       FIG. 6  illustrates details of the general-purpose configurable logic cell shown in  FIG. 5 , according to various embodiments of the invention. In particular, details of function  F   540  are shown. A and B  602 - 604  are the actual inputs to function  F   540 , and Y  608  is the output. Tables  700  and  800  shown in  FIG. 7  and  FIG. 8  describe the configurable logic cell according to various embodiments of the invention. Input  702  and output  704  of the decoder are displayed in  FIG. 7 . Input  802  and output  804  of the function are displayed in  FIG. 8  accordingly. 
     Returning to  FIG. 4 , to decrypt logic  400 , at system startup, during runtime, or on demand, logic key bits  410  are loaded into the logic building blocks  402 - 408  in the same order as previously used to encrypt the logic function. This configures logic building blocks  402 - 408  and causes them to behave like the original logic function shown in  FIG. 3A . 
     In scenarios where there are any errors in logic key bits  410 , logic building blocks  402 - 408  will perform unknown or invalid operations. In one embodiment, a built-in self-test (BIST) is performed upon power-up and combined with the loading of logic key bits  410  so as to take advantage of the fact that both the BIST and the key loading make use of serialized logic by, e.g., a logic scan. The effect decrypting a logic function by using an invalid or wrong key is illustrated in  FIG. 9 , according to various embodiments of the invention. 
       FIG. 9  comprises logic function  900  that uses generic logic blocks that represent an erroneous logic key 010010001000 instead of the correct logic key 011010001000. As shown in example in  FIG. 9 , the attempt to decrypt logic function  900  by applying an invalid logic key that has a single bit error results in the wrong logic expression NOT(AND(z, AND(NOT(y), AND(w,x)))). While output  930  may still be some operational logic function, i.e., using other keys may or may not result in some operational logic, it is not the desired logic function that can perform the operation the system was designed for. In other words, if there is any error in the logic key bits, the entire logic function is rendered invalid, such that logic building blocks  902 - 908  will output unknown or invalid operations. 
     It is noted that unlike field programmable gate arrays, this system does not have to be designed to be capable of expressing more than one arbitrary logic function during runtime. Typically, only the originally designed, valid logic function is activated, while all other incorrect combinations are inactive. Additionally, errors in logic key bits may cause system latch-up and other violations. Therefore, in one embodiment, generic logic blocks are specifically designed to avoid these unwanted effects. 
     Regarding testability in manufacturing, the desired function is tested and verified as correct, while any undesired invalid (i.e., wrongly configured) function does not have to be tested except to the extent required to ensure reliable operation of the correct function. For example, the logic key bits could be loaded to unlock the logic function, and scanning could be performed just as it would in a regular test flow to verify correct timing. For incorrect logic functions, the circuit may be tested to ensure that the incorrect logic does not permanently negatively impact the system (e.g., by causing a destructive latch-up). It is noted that, unlike for the correct function, timing is of no concern and may or may not be met for incorrect functions. 
     While the invention as described above results in a powerful hurdle for reverse engineering, additional steps may be taken to protect the keying material to make it inaccessible to potential intruders. In one embodiment, the logic key that holds the secret is therefore stored in a tamper-resistant, battery-backed non-volatile memory. Indirect storage of the logic may be achieved by employing alternatives that do not require a battery. One embodiment uses Physically Uncloneable Functions (PUFs) as secure physical elements. A PUF is typically a random, device-unique but constant number that may change as soon as the device is being probed. Therefore, such unique identification elements serve as excellent encryption keys. 
     Logic key protection using a secure physical element is illustrated in  FIG. 10 , according to various embodiments of the invention. PUF  1002  provides a device-unique unique secure physical element, d, that is determined by the hardware of a particular device. PUF  1002  may be used to secure the secret key. Unlocking key  1004  (denoted as x) is pre-computed based on the non-secure physical element and the secret or private key. This unlocking key  1004  may be different from device to device, such that even if an attacker manages to extract unlocking key  1004  from one chip, it would be of no use, since unlocking key  1004  is individualized to each device. Logic key k  1006  comprises key bits computed previously. Mathematical function f(d, x)=k  1010  can be designed in a manner that its inverse function delivers a value for unlocking key x  1004 , i.e., f −1 (d, k)=x. Then, for a given PUF d  1002  and logic key k  1006 , unlocking key x  1004  can be computed from x=f −1 (d, k). 
     One simple example used for illustrative purposes is an XOR operation. Assuming that x=(d XOR k), then k=(d XOR x), i.e., both PUF d  1002  and unlocking key x  1004  are needed to calculate logic key k  1006 . In other words, because x  1004  is dependent on PUF d  1002 , PUF d  1002  is needed to compute k from unlocking key x  1004 . But this also means that unlocking key x  1004  is computable, since all the necessary information is known or determinable. In particular, logic key k  1006  is known from designing the function, and the value of PUF d  1002  can be determined from measurements, for example, as part of the manufacturing process. Given PUF d  1002  and logic key k  1006 , unlocking key x  1004  can be computed from x=f −1 (d, k). In practice, strong cryptographic functions f c ( ) rather than XOR are used. 
     The value of unlocking key x  1004  may then be stored, for example, inside the device&#39;s OTP, Flash memory, battery-backed SRAM or other non-volatile memory. For a potential attacker, the value of obtaining unlocking key x  1004  is extremely low since, by itself, unlocking key x  1004  cannot be used to activate other devices. Nor does unlocking key x  1004  unlock the logic function of the device. 
     In one embodiment, at device startup, or upon use of the logic function, the device computes logic key k  1006  as k=f(d, x) and loads (i.e., shifts) logic key k  1006  into the logic block configuration, thereby activating the correct logic function. As an advantage, only unlocking key x  1004  needs to be stored on the chip, and not secret key k  1006  itself, such that logic key k  1006  is successfully obfuscated. Note that if k were stored directly, PUF  1002  would not have any bearing on key k  1006 . One of ordinary skill in the art will appreciate that a multitude of functions of varying speeds, sizes, and more complex cryptographic properties can be used, including public key cryptography. 
     In one embodiment, the system described in  FIG. 10  is extended such that encrypted logic function  1020  is not automatically activated upon power-up or upon first use. Instead, the value of unlocking key x  1004  is transmitted to the logic building blocks, for example, by using a bus connection to a microprocessor, a remote link such as a network connection to an external server, or similar. This allows for protection of logic functions based on achieving an overall secure environment as determined by other system components, as well as implementation of hardware licensing features that have not been available using traditional designs. For example, remote decryption could be made contingent upon the satisfaction of licensing requirements such as the receipt of licensing fees, etc. 
       FIG. 11A and 11B  illustrate the computation of equivalent information from the original logic function, according to various embodiments of the invention. In the absence of secure physical elements, there is typically only a single logic key for any given type of silicon die. Theft or accidental exposure of the logic key would negate many of the benefits of the systems and methods discussed herein. Therefore, in one embodiment, modified logic function  1154  is used instead of original logic function  1102  and software operations are performed at the inputs and outputs of modified logic function  1154  such that the overall behavior of system  1150  is the equivalent of that of system  1100 . 
     Moving certain logic operations on inputs and outputs of a logic function into software allows the use a different logic key k 2    1160  instead of original logic key k 1    1110  and, thus, provides additional control over logic keys. Multiple pairs of keys and software may be used in computing the equivalent information. In one embodiment, software library enables software operations  1170  and  1180  on the inputs and outputs of logic function  1154 , respectively, to negate a predetermined number of the input bits  1152  to logic function  1154  by inverting corresponding generic logic blocks such that the results computed by logic function  1154  are identical to results of non-inverted inputs computed with a different software library. As illustrated in  FIG. 11B , a similar configuration may be applied at the output  1158  of logic function  1154 . 
       FIG. 12  illustrates software processing as applied to a modified version of logic function of  FIG. 3A , according to various embodiments of the invention. As shown, logic function  1200  comprises the same elements as in  FIG. 3A , except that the original generic logic block S 3  has been replaced with a different output gate  1218 . In example in  FIG. 12 , the logic key has been changed from 011010001000 to 011010001 010 . In other words, only the last element in the chain has been replaced by simply inverting it. In addition, software processing  1220  is applied to the output for the purpose of inverting the output of modified gate  1218 . 
     In operation, the software bit inversion results in equivalent processing as that in example in  FIG. 3A . This allows, for example, a manufacturer to give to two different customers two different logic keys, wherein one logic key creates the inverted output of the other, such that both customers receive two different versions of software and a small code is used to negate the effect of the hardware change. As a result, both customers receive different secret keys, so that the device of one does not operate with the secret key of the other, thus, discouraging the sale of devices. One of ordinary skill in the art will appreciate that more complex logic operations other than inversion can be used. 
     Some embodiments of the present invention may greatly increase the gate count of a logic implementation, e.g., by a factor of five, and result in a decrease of the achievable speed. Therefore, in one embodiment, the systems and methods of the present invention are applied only to critical blocks in a given design. In another embodiment, custom cells are used to reduce the footprint of individual generic logic blocks. In yet another embodiment, a subset of the chip design is run at reduced clock speeds to reduce the required die area and/or mitigate the impact of an increased gate count. 
       FIG. 13  is a flowchart of an illustrative process to determine a logic key in accordance with various embodiments of the invention. The process for determining the logic key  1300  starts at step  1302  when logic gates used in a given logic function are serialized into a sequence to generate a known sequence of, e.g., a two-dimensional x-y circuit. 
     At step  1304 , each type of logic gate used is analyzed and identified. 
     At step  1306 , a sequence of key bits is obtained and assigned to the logic gate, such that a generic logic building block can perform the equivalent function. 
     At step  1308 , the logic gate is replaced with a generic logic gate or building block. 
     At step  1310 , key bits are stored, e.g., in a database. 
     Finally, steps  1304  through  1310  are repeated for some or all of the remaining logic gates in the sequence. 
     It will be appreciated by those skilled in the art that fewer or additional steps may be incorporated with the steps illustrated herein without departing from the scope of the invention. No particular order is implied by the arrangement of blocks within the flowchart or the description herein. 
     It will be further appreciated that the preceding examples and embodiments are exemplary and are for the purposes of clarity and understanding and not limiting to the scope of the present invention. It is intended that all permutations, enhancements, equivalents, combinations, and improvements thereto that are apparent to those skilled in the art, upon a reading of the specification and a study of the drawings, are included within the scope of the present invention. It is therefore intended that the claims include all such modifications, permutations, and equivalents as fall within the true spirit and scope of the present invention.