Logic locking encryption method and circuit based on tree locking unit of AND tree and OR tree

Abstract

The invention discloses a logic locking encryption method of a tree locking unit based on an AND tree and an OR tree, and relates to the technical field of integrated circuit design security, and the logic locking encryption method comprises the following steps: obtaining and analyzing a gate-level netlist to obtain specific positions and circuit gates of the AND tree and the OR tree and the equivalent AND tree and the OR tree in a circuit; the trees are selected according to the position relation between the trees, so that the number and the position relation of the trees meet expected safety requirements as much as possible; and according to the selected tree, finding a corresponding circuit gate position, inserting a specific gate, modifying a corresponding connection relation and a circuit structure, and constructing a tree locking unit to construct an encrypted circuit. The encryption circuit is constructed by constructing the tree locking unit which is more coupled with the original logic in the circuit, the resistance of the integrated circuit to reverse engineering and unauthorized modification is remarkably improved, the safety is improved, the tree locking unit is mapped to different original functions according to different keys, the confusion of the circuit is increased, and the safety of the integrated circuit is improved. And different design requirements and security levels are met.

Description

Technical Field

[0001] The present invention relates to the technical field of integrated circuit design security, and more specifically, to a logic locking encryption method and circuit based on a tree locking unit. Background Art

[0002] In the field of integrated circuit design, with globalization and the fragmentation of the supply chain, hardware security issues have become increasingly prominent. Fabless companies outsource designs to foundries and test / assembly companies, which increases the risk of security threats such as IP piracy, hardware Trojans, and reverse engineering. In order to deal with these threats, logic locking technology has emerged. Logic locking makes moderate modifications to the original circuit and inserts additional logic, and then locks and restores the original design by introducing a key. When the correct key is not known or the original circuit structure cannot be inferred, the encrypted design is difficult to understand for malicious actors in the supply chain.

[0003] Existing logic locking technology, especially provably secure logic locking (PSLL), makes the number of iterations of SAT attacks exponentially related to the number of key bits, thereby improving security. However, PSLL shows structural deficiencies when facing advanced removal attacks such as GNN Unlock. This is because traditional PSLL usually places the encryption structure outside the original circuit, resulting in a low degree of coupling between the encryption circuit and the original circuit, which is easy to be identified and removed by attackers. Summary of the invention

[0004] The purpose of the present invention is to overcome the problem in the prior art that the encryption circuit is less coupled with the original circuit and is easily identified and removed by attackers, and to provide a logic locking encryption method and circuit based on a tree locking unit of an AND tree and an OR tree.

[0005] To achieve the above object, the technical solution of the present invention is as follows:

[0006] The present invention provides a logic locking encryption method based on a tree locking unit of an AND tree and an OR tree, comprising the following steps:

[0007] Obtain the gate-level netlist of the original circuit;

[0008] Perform circuit analysis according to the gate-level netlist to obtain specific positions and specific circuit gates of AND trees and OR trees in the original circuit and equivalent AND trees and OR trees;

[0009] After all trees are obtained, trees are selected according to the positional relationships between all AND trees and OR trees, so that the number and positional relationship of the trees meet the desired security requirements as much as possible;

[0010] According to the selected tree, find the corresponding circuit gate position in the gate-level netlist, insert the specific gate, and modify the corresponding connection relationship and circuit structure to construct a tree locking unit;

[0011] An encryption circuit is constructed according to the constructed tree locking unit.

[0012] Furthermore, before selecting a tree according to the positional relationship between trees, it is necessary to first obtain the expected encryption strength of each tree and the encryption strength between trees.

[0013] Furthermore, the step of obtaining the expected encryption strength of each tree includes: peeling each tree and its fan-in cone, performing point function encryption on each tree, performing SAT attack on each tree, and obtaining the expected encryption strength of each tree.

[0014] Furthermore, after obtaining the expected encryption strength of each tree, the single tree whose encryption strength does not meet the expectation is excluded, and among the remaining single trees, the encryption strength between trees is calculated using the SAT attack.

[0015] Furthermore, the trees are selected based on the positional relationships between all the trees and or between the trees, so that the number and positional relationship of the trees can meet the desired security requirements as much as possible. Specifically, according to the positional relationship between the trees, a heuristic algorithm is used to select the trees one by one, and the encryption strength between the trees is used to estimate in real time the encryption strength that can be achieved by the current combination of multiple trees, so as to ensure the reasonable selection of the trees, so that the number and positional relationship of the trees can meet the desired security requirements as much as possible.

[0016] On the other hand, the present invention also provides a logic-locked encryption circuit based on a tree locking unit of an AND tree and an OR tree, wherein the encryption circuit is located inside the original circuit, and the encryption circuit includes at least one tree locking unit, wherein the tree locking unit includes a bypass circuit and an OR tree or an AND tree selected inside the original circuit, and the input of the tree locking unit includes the original input of the OR tree or the AND tree and a key input, and the output of the encryption circuit is XORed with the protected circuit output through an XOR gate to obtain a final circuit output.

[0017] Furthermore, the AND tree and the OR tree selected inside the original circuit are at least three-input trees.

[0018] Furthermore, each original input of the tree corresponds to one of the key inputs and jointly controls the input of the bypass circuit.

[0019] Furthermore, in each of the tree locking units, all outputs obtained after the original input of each tree and the corresponding key input are XORed through an XOR gate are then ANDed through an AND gate to obtain the bypass circuit output of the tree locking unit.

[0020] Further, the encryption circuit includes an AND-tree-tree locking unit and an OR-tree-tree locking unit, the bypass circuit of the AND-tree locking unit also includes a NOT gate, all outputs obtained after each original input of the AND-tree and the corresponding key input are XORed through an XOR gate are further ANDed through an AND gate, and then the final bypass circuit output of the AND-tree-tree locking unit is obtained through the NOT gate, and the final bypass circuit output of the AND-tree-tree locking unit and the original output of the AND-tree are obtained through an AND gate to obtain the AND-tree current output;

[0021] The bypass circuit output of the OR tree locking unit and the original output of the OR tree are coupled through an OR gate to obtain the current output of the OR tree;

[0022] The bypass circuit output of the AND tree locking unit before the NOT gate is also ANDed with the bypass circuit output of the OR tree locking unit through an AND gate to obtain the encryption circuit output, and the encryption circuit output is XORed with the protected circuit output through an XOR gate to obtain the final circuit output.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] The present invention proposes a logic locking encryption method based on a tree locking unit of an AND tree and an OR tree in a circuit. The method has high flexibility and can be used as an independent mechanism or as a security enhancement mechanism of a traditional provably secure logic locking structure and combined with some advanced locking mechanisms to adapt to different design requirements and security levels. By constructing a tree locking unit that is more coupled with the original logic inside the circuit, the present invention significantly improves the resistance of the integrated circuit to reverse engineering and unauthorized modification. Compared with the traditional peripheral encryption structure, the tree locking unit is more difficult to be maliciously identified and removed. The tree locking unit can be mapped to different original functions according to different keys. This design increases the obfuscation of the circuit, making it difficult for malicious behavior objects to infer their true functions and effects even if they discover the tree locking unit. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is a flow chart of a logic locking encryption method of a tree locking unit based on an AND tree and an OR tree of the present application;

[0026] Figure 2 It is a schematic diagram of the traditional provably secure logic locking structure;

[0027] Figure 3 It is a schematic diagram of the internal structure of an original circuit in Example 1 of the present application;

[0028] Figure 4 is a schematic diagram of a tree construction and tree locking unit using three inputs in Example 1 of the present application;

[0029] Figure 5 This is a schematic diagram of the overall structure of a circuit after the circuit is encrypted by a simple logic locking encryption method based on a tree locking unit in the present application;

[0030] Figure 6 This is a schematic diagram of a construction method of a tree locking unit with obfuscation in Example 2 of the present application;

[0031] Figure 7 It is a specific structural diagram of a logic locking encryption circuit of a tree locking unit based on an AND tree and an OR tree according to Embodiment 3 of the present application. DETAILED DESCRIPTION

[0032] In order to more clearly understand the above-mentioned purpose, features and advantages of the present invention, the present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments. It should be noted that the embodiments of the present application and the features in the embodiments can be combined with each other without conflict.

[0033] In the following description, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention may also be implemented in other ways different from those described herein. Therefore, the protection scope of the present invention is not limited to the specific embodiments disclosed below.

[0034] Example 1

[0035] See also Figure 2 , Figure 2 Figure 1 is a schematic diagram of the traditional provably secure logic lock structure. Original circuit / modified circuit: The original circuit is the design circuit that the designer owns the intellectual property rights of. The circuit needs to be produced and tested by a third party. The original circuit may be modified to deal with some security threats, or it may not be modified at all and only rely on the peripheral encryption circuit for protection. Among them, the input and output of the original circuit are circuit input and circuit output in the figure. Encryption circuit: The circuit used to implement the encryption function in the traditional provably secure logic lock. The input of the encryption circuit is the protected input and the key input. The protected input is selected by the designer from the circuit input according to security requirements. The output of the encryption circuit is XORed with the original circuit output through an XOR gate. The correctness of the key determines the correctness of the circuit output. The external encryption circuit is easy to resist SAT attacks, but it is used to remove the attack. Removal attack refers to directly deleting the encryption part of the circuit through analysis.

[0036] There are often many small circuits in the original circuit that are only built by AND gates and OR gates, which can be called AND trees and OR trees. At the same time, there are also many logically equivalent AND trees and OR trees. When the original circuit structure allows, the desired security performance can be achieved. By reasonably selecting a specified number of trees inside the original circuit to construct a specified number of tree locked units (Tree Locked Unit, TLU), and using the tree locked units to build an encryption circuit, the defense performance of provably secure logic locking against SAT attacks can be achieved, while improving their defects of not resisting removal attacks.

[0037] See also Figure 1 The embodiment of the present invention provides a logical locking encryption method based on a tree locking unit of an AND tree and an OR tree, comprising the following steps:

[0038] S1: Get the gate-level netlist of the original circuit.

[0039] A gate-level netlist is a description file in circuit design, which contains all the information of the circuit design. The gate-level netlist describes the connection relationship between each logic gate (such as AND gate, OR gate, NOT gate, etc.) in the circuit and is the basic unit of digital circuit.

[0040] S2: Perform circuit analysis based on the gate-level netlist to obtain the specific positions and specific circuit gates of the AND tree and OR tree in the original circuit and the equivalent AND tree and OR tree.

[0041] S3: After all trees are obtained, trees are selected according to the positional relationship between the trees, so that the number and positional relationship of the trees can meet the desired security requirements as much as possible.

[0042] After obtaining all the trees, it is necessary to screen the trees and select specific trees. First, peel off the single tree and its fan-in cone, then perform a simple point function encryption on the single tree in the circuit, and perform a SAT attack, which can tell the expected encryption strength of the single tree. After judging the expected encryption strength of all single trees in the same way, exclude the single tree whose encryption strength does not meet the expectations. Among the remaining single trees, use the SAT attack to calculate the encryption strength between trees one by one. Use a heuristic algorithm to select trees one by one, and use the encryption strength between trees to estimate in real time the encryption strength that can be achieved by the current combination of multiple trees, to ensure the reasonable selection of trees, so that the number and position relationship of trees can meet the expected security requirements as much as possible.

[0043] S4: According to the selected tree, find the corresponding circuit gate position in the gate-level netlist, insert the specific gate, and modify the corresponding connection relationship and circuit structure to build a tree locking unit.

[0044] S5: Construct an encryption circuit according to the constructed tree locking unit.

[0045] like Figure 3 The figure shows the internal structure of an original circuit including an AND tree and an OR tree. A circuit composed of multiple gates is called a tree. A circuit composed of only AND gates is called an AND tree. A circuit composed of only OR gates is called an OR tree. The output of an AND tree is 1 only under one input condition, while the output of an OR tree is 0 only under one input condition. At the same time, small circuits that are functionally equivalent to AND trees and OR trees are also classified as AND trees and OR trees, and the number of inputs of AND trees and OR trees is not necessarily 3. Small logic trees that are often ubiquitous inside the original circuit include AND trees, OR trees, and equivalent AND trees and OR trees (equivalent means that they still only output 1 or 0 under one input condition) of various input sizes. The dotted lines indicate that these trees are part of the original circuit rather than independent existences, and their inputs and outputs are often connected to other gates inside the original circuit. The present invention utilizes the ubiquitous small logic trees inside the original circuit to construct a tree locking unit and an encryption circuit. Among them, one way to construct a tree locking unit is as follows Figure 4 See Figure 5 , Figure 5 This is a schematic diagram of the overall structure of the circuit after the circuit is encrypted by the logic encryption method based on the tree locking unit (TLU) proposed in this embodiment 1. The dotted line indicates that the encryption circuit is a part of the circuit, and together with other parts, the function of the original circuit is realized. The output of the encryption circuit is XORed with the output of the protected circuit through an XOR gate to obtain the final circuit output. The encryption structure constructed by the logic locking encryption method proposed in this embodiment 1 is the same as Figure 2 Different from the traditional provably secure logic locking, it no longer directly attaches a large encryption structure to the periphery of the original circuit, but builds a tree locking unit based on the tree inside the original circuit, and builds an encryption circuit based on the tree locking unit. This increases the degree of coupling between the encryption circuit and the original circuit, making it more difficult to distinguish than the traditional structure, and increases the difficulty of implementing the removal attack.

[0046] Through a reasonable tree selection strategy, when the original circuit structure itself allows, the structure of the encryption circuit constructed using the encryption method of this embodiment 1 can achieve the same level of SAT attack resistance as traditional provably secure logic locks.

[0047] In the design of the tree locking unit, this embodiment 1 adopts the concept of an obfuscated tree locking unit. Through the careful selection and modification of the tree and the specific design of the locking unit, the obfuscated tree locking unit can have the possibility of multiple different original logical functions under the same local structure, which increases the difficulty of maliciously inferring the original circuit function, making it difficult for the malicious behavior object to restore the original design by removing or modifying the locking unit when the correct key or the original circuit structure is not known, further increasing the difficulty of implementing the removal attack.

[0048] The logic lock encryption method of embodiment 1 of the present invention has a good fit with the traditional provably secure logic lock and can be combined with it to enable it to obtain the ability to resist advanced removal attacks such as GNN Unlock, while still retaining the security characteristics of the original encryption mechanism.

[0049] Example 2

[0050] Based on Example 1, Example 2 further describes a confusing tree locking unit in Example 1, as follows:

[0051] See also Figure 6 , Figure 6 This is a schematic diagram of a simple obfuscating tree locking unit construction method of this embodiment 2. The tree locking unit itself is located inside the circuit and is more coupled to the original circuit than the traditional provably secure logic locking. By carefully modifying the original tree structure and making specific adaptations to the encryption circuit, an obfuscating locking unit can be implemented. Figure 6 As shown, for a small AND tree consisting of two AND gates, its structure is modified, and one of the AND gates is changed to an OR gate. Subsequently, a small tree consisting of an AND gate and an OR gate can be found in the original circuit, but its structure is no longer modified. For these two trees with the same current structure but inconsistent original circuit structure, the same encryption circuit can be used for encryption to form a confusing tree locking unit. The only difference between the two locking units is the correct key value. The confusing locking unit can also be constructed in the same way for a small tree consisting of an OR gate and an AND gate. This allows the same locally identical circuit structure to map different original circuit functions, and this unit structure can exist in large numbers in the circuit. This increases the difficulty of maliciously inferring the original circuit function, so that when the malicious behavior object does not know the correct key or the original circuit structure, there will be a huge risk every time the encryption structure in the tree locking unit is removed.

[0052] Example 3

[0053] Based on the embodiment 1, the embodiment 3 applies the logic locking encryption method based on the tree locking unit of the AND tree and the OR tree of the embodiment 1, and provides a logic locking encryption circuit based on the tree locking unit of the AND tree and the OR tree, which is as follows:

[0054] See also Figure 7 , Figure 7 This is a schematic diagram of the specific structure of a logic locking encryption circuit of a tree locking unit based on an AND tree and an OR tree according to Embodiment 3.

[0055] An AND tree with three inputs and one output is found in the original circuit. A tree locking unit constructed based on the AND tree adds a small bypass circuit and an AND gate compared to the original circuit.

[0056] The bypass circuit is controlled by the original input of the AND tree and the key inputs KI1, KI2, and KI3, and includes three XOR gates, a three-input AND gate, and a NOT gate. The output of the bypass circuit and the original output of the AND tree are ANDed through an AND gate to obtain the current output of the AND tree. The inverter (NOT gate) in the bypass circuit of the AND tree lock unit makes the circuit output 0 instead of 1 when the AND tree input meets the comparison relationship with KI.

[0057] For a single tree, the tree locking unit built on it is as follows Figure 7 As shown in the lower part, similar to the tree locking unit structure based on the AND tree, the final encryption effects of the two types of trees are basically the same. Finally, the bypass circuit outputs of the AND tree-tree locking unit and the OR tree-tree locking unit pass through an AND gate to form an encryption circuit marked by a dotted line. The encryption circuit will eventually still perform an XOR operation with the protected circuit output, i.e., the AND tree current output and the OR tree current output, through an XOR gate to achieve the expected SAT attack resistance. It should be noted that the encryption circuit of this embodiment 3 includes an AND tree-tree locking unit and an OR tree-tree locking unit, and several tree locking units can be set as needed in the actual circuit.

[0058] In this embodiment 3, the structure of the encryption circuit is basically consistent with the comparison structure in the traditional provably secure logic lock. In addition, the small bypass circuit only injects errors in very rare cases and hardly affects the operation of the comparison structure. If the original circuit structure itself allows, after reasonable tree selection, the structure can achieve almost the same desired resistance as the traditional provably secure logic lock.

[0059] In summary, the present invention proposes a logic locking encryption method and encryption circuit based on a tree locking unit of an AND tree and an OR tree, which has the following advantages:

[0060] 1) Improved security: By building a locking unit that is more coupled to the original logic inside the circuit, the present invention significantly improves the resistance of the integrated circuit to reverse engineering and unauthorized modification. Compared with traditional peripheral encryption structures, the tree locking unit is more difficult to be maliciously identified and removed.

[0061] 2) Enhanced obfuscation: The tree locking unit can be mapped to different original functions according to different keys. This design increases the obfuscation of the circuit, making it difficult for malicious actors to infer its true function and role even if they discover the locking unit.

[0062] 3) Improved flexibility and adaptability: The technical solution of the present invention is highly flexible and can be combined with traditional logic locking technology and some advanced locking mechanisms to adapt to different design requirements and security levels.

[0063] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the embodiments of the present invention. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the embodiments here. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the claims of the present invention.

Claims

1. A logical locking encryption method based on a tree locking unit of an AND tree and an OR tree, characterized in that: The steps include: Obtain the gate-level netlist of the original circuit; Perform circuit analysis according to the gate-level netlist to obtain specific positions and specific circuit gates of AND trees and OR trees in the original circuit and equivalent AND trees and OR trees; After all AND trees and OR trees are obtained, trees are selected according to the positional relationships between all AND trees and OR trees, so that the number and positional relationship of the trees meet the desired security requirements as much as possible; According to the selected tree, find the corresponding circuit gate position in the gate-level netlist, insert the specific gate, and modify the corresponding connection relationship and circuit structure to construct a tree locking unit; An encryption circuit is constructed according to the constructed tree locking unit.

2. The method for logical locking encryption based on a tree locking unit of an AND tree and an OR tree according to claim 1, characterized in that: Before selecting a tree according to the positional relationships between all the AND trees and / or trees, it is necessary to first obtain the expected encryption strength of each tree and the encryption strength between trees.

3. The method for logical locking encryption based on a tree locking unit of an AND tree and an OR tree according to claim 2, characterized in that: The step of obtaining the expected encryption strength of each tree includes: peeling each tree and its fan-in cone, performing point function encryption on each tree, performing SAT attack on each tree, and obtaining the expected encryption strength of each tree.

4. The method for logical locking encryption based on a tree locking unit of an AND tree and an OR tree according to claim 2, characterized in that: After obtaining the expected encryption strength of each tree, the single tree whose encryption strength does not meet the expectation is excluded, and the encryption strength between trees is calculated using the SAT attack in the remaining single trees.

5. The method for logical locking encryption based on a tree locking unit of an AND tree and an OR tree according to claim 2, characterized in that: The method of selecting trees according to the positional relationships between all the trees and or between the trees so that the number and positional relationships of the trees meet the desired security requirements as much as possible is specifically as follows: according to the positional relationships between the trees, a heuristic algorithm is used to select the trees one by one, and the encryption strength between the trees is used to estimate in real time the encryption strength that can be achieved by the current combination of multiple trees, thereby ensuring the reasonable selection of the trees so that the number and positional relationships of the trees meet the desired security requirements as much as possible.

6. A logic locking encryption circuit based on a tree locking unit of an AND tree and an OR tree, used for applying a logic locking encryption method based on a tree locking unit of an AND tree and an OR tree as claimed in any one of claims 1 to 5, characterized in that: The encryption circuit is located inside the original circuit, and the encryption circuit includes at least one tree locking unit, and the tree locking unit includes a bypass circuit and an OR tree or an AND tree selected inside the original circuit. The input of the tree locking unit includes the original input of the OR tree or the AND tree and a key input. The output of the encryption circuit is XORed with the protected circuit output through an XOR gate to obtain a final circuit output.

7. A logic locking encryption circuit based on a tree locking unit of an AND tree and an OR tree according to claim 6, characterized in that: The AND tree and the OR tree selected inside the original circuit are at least three-input trees.

8. The logic locking encryption circuit based on the tree locking unit of the AND tree and the OR tree according to claim 7, characterized in that: Each original input of the tree corresponds to one of the key inputs and jointly controls the input of the bypass circuit.

9. The logic locking encryption circuit based on the tree locking unit of the AND tree and the OR tree according to claim 8, characterized in that: In each of the tree locking units, the original input of each tree and the corresponding key input are XORed through an XOR gate respectively, and all the outputs obtained are then ANDed through an AND gate to obtain the bypass circuit output of the tree locking unit.

10. The logic locking encryption circuit based on the tree locking unit of the AND tree and the OR tree according to claim 9, characterized in that: The encryption circuit includes an AND-tree-tree locking unit and an OR-tree-tree locking unit, the bypass circuit of the AND-tree locking unit also includes a NOT gate, all outputs obtained after each original input of the AND-tree and the corresponding key input are XORed through an XOR gate are then ANDed through an AND gate to obtain the final bypass circuit output of the AND-tree-tree locking unit through the NOT gate, and the final bypass circuit output of the AND-tree-tree locking unit and the original output of the AND-tree are obtained through an AND gate to obtain the current output of the AND-tree; The bypass circuit output of the OR tree locking unit and the original output of the OR tree are coupled through an OR gate to obtain the current output of the OR tree; The bypass circuit output of the AND tree locking unit before the NOT gate is also ANDed with the bypass circuit output of the OR tree locking unit through an AND gate to obtain the encryption circuit output, and the encryption circuit output is XORed with the protected circuit output through an XOR gate to obtain the final circuit output.

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