JP6632962B2 - Obfuscation device, obfuscation method and obfuscation program - Google Patents

Description

  The present invention relates to an apparatus, a method and a program for obfuscating software source code and an execution program.

2. Description of the Related Art Conventionally, a method of obfuscating a source code and an execution program to prevent illegal analysis and tampering of software has been proposed.
For example, Non-Patent Document 1 proposes a technique for obfuscating a software control flow using a conditional branch that is always true or a conditional branch that is always false.

See Collberg, C.I. Thomborson, C .; , And Low, D.C. , “A Taxonomi of Obfuscating Transformations,” Technical Report 148, Department of Computer Science The University of Auckland, 1997. Preda, M .; D. , Madou, M .; , Bosschere, K .; D. , And Giacobazzi, R.A. , “Opaque Predicates Detection by Abstract Interpretation,” Proc. of the 11th International Conference Algebraic Methodology and Software Technology (AMAST 2006), Lectur Notes in Computer Science 4019. 81-95, 2006. Ming, J.M. Xu, D .; Wang, L .; , And Wu, D.A. , "LOOP: Logic-Oriented Opaque Predictate Detection in Obfuscated Binary Code," Proc. of the 22nd ACM SIGSAC Conference on Computer and Communications Security (CCS 2015), pp. 146-315. 757-768, 2015.

However, a conditional branch that is always true (for example, if ((n * n + n)% 2) == 0) and a conditional branch that is always false (for example, if (n * n * n-n% 6) == 1) ) Has a characteristic format and is easily found by third parties.
Non-Patent Literature 2 and Non-Patent Literature 3 show that it is possible to determine whether these conditional branches are always true or always false by an algebraic method.

  An object of the present invention is to provide an obfuscation device, an obfuscation method, and an obfuscation program for software that are difficult to analyze.

  The obfuscation device according to the present invention provides an encryption method and a decryption process in which a first parameter whose probability is equal to or more than a first threshold is set for an encryption method in which the probability of successful decryption is variable according to the parameter. A first code, and a generating unit that generates a second code of an encryption process and a decryption process in which a second parameter is set such that the probability is less than a second threshold value. An insertion unit that performs a decryption process after the encryption process using one code or the second code, inserts a conditional branch that determines whether the plaintext has been restored, and assigns a dummy process to one of the conditional branches. , Is provided.

  The generation unit generates a third code of an encryption process and a decryption process in which the third parameter is set such that the probability is equal to or more than a second threshold and less than a first threshold, and the insertion unit generates the third code for the plaintext. A decryption process is performed after the encryption process using one code, the second code, or the third code, a conditional branch that determines whether the plaintext has been restored is inserted, and a conditional branch that uses the third code is inserted. In this case, an equivalent process may be assigned to both of the conditional branches.

  The generator may determine the parameter at random.

  The insertion unit may randomly select a conditional branch code.

  The encryption method may be an LWE type lattice encryption method.

  An obfuscation method according to the present invention provides an encryption method and a decryption process in which a first parameter whose probability is equal to or more than a first threshold is set for an encryption method in which the probability of successful decryption is variable according to the parameter. Generating a second code of an encryption process and a decryption process in which a first code is set and a second parameter in which the probability is smaller than a second threshold value; and a control flow of software includes: Performing a decryption process after the encryption process using one code or the second code, inserting a conditional branch for determining whether the plaintext has been restored, and assigning a dummy process to one of the conditional branches; The computer runs.

  An obfuscation program according to the present invention provides an encryption method and a decryption processing in which, for an encryption method in which the probability of successful decryption according to a parameter is variable, a first parameter in which the probability is equal to or greater than a first threshold is set. Generating a second code of an encryption process and a decryption process in which a first code is set, and a second parameter in which the probability is smaller than a second threshold value; and Performing a decryption process after the encryption process using one code or the second code, inserting a conditional branch for determining whether the plaintext has been restored, and assigning a dummy process to one of the conditional branches; Is executed by the computer.

  According to the present invention, it becomes more difficult to analyze software source codes and execution programs.

It is a block diagram showing the functional composition of the obfuscation device concerning an embodiment. It is a figure showing the 1st pattern of obfuscation concerning an embodiment. It is a figure showing the 2nd pattern of obfuscation concerning an embodiment. It is a figure showing the 3rd pattern of obfuscation concerning an embodiment.

Hereinafter, an example of an embodiment of the present invention will be described.
FIG. 1 is a block diagram illustrating a functional configuration of the obfuscation apparatus 1 according to the present embodiment.
The obfuscation device 1 is an information processing device (computer) such as a server device or a personal computer, and includes, in addition to the control unit 10 and the storage unit 20, an input / output device or a communication device for various data.

  The control unit 10 is a part that controls the entire obfuscation device 1 and realizes various functions in the present embodiment by appropriately reading and executing various programs stored in the storage unit 20. The control unit 10 may be a CPU.

  The storage unit 20 is a storage area for various programs and various data for causing the hardware group to function as the obfuscator 1, and may be a ROM, a RAM, a flash memory, a hard disk (HDD), or the like. Specifically, the storage unit 20 stores an obfuscation program that causes the control unit 10 to execute each function of the present embodiment.

  The control unit 10 includes a generation unit 11 and an insertion unit 12. The obfuscation apparatus 1 obfuscates the input source code or execution program of the software by these functional units and outputs it.

The generation unit 11 sets a first parameter whose probability is equal to or more than a first threshold (for example, 1-1 / 2 ¹²⁸ ) for an encryption method in which the probability of successful decryption is variable according to the parameter. A first code for encryption processing and decryption processing, and a second code for encryption processing and decryption processing in which a second parameter whose probability is less than a second threshold value (for example, ^{１２８ 128} ) are generated.

Here, as the encryption method, for example, LWE (learning with error) type lattice encryption is adopted.
The LWE-type lattice encryption is a lattice encryption technique based on the LWE problem for finding a solution of a system of linear equations including noise in a constant part. The LWE-type lattice encryption has three parameters of dimension n, modulus q, and noise distribution χ. In general, a normal distribution is used for the noise distribution χ, but an LWE type encryption method having noise of another probability distribution may be used.

The generation unit 11 generates codes for key generation processing, encryption processing, and decryption processing in LWE-type lattice encryption. At this time, the generation unit 11 sets the standard deviation σ of the noise distribution χ to a value larger than the normal value in addition to the general parameter setting, for example, the LWE-type lattice encryption in which the probability of successful decryption is 1-1 / 2 ¹²⁸ or more. set, fails to decode with high probability, for example, the success probability generate code for encryption processing and decryption processing in LWE grating cryptographic less than 1/2 ^128.
Further, the generation unit 11 generates a third code of the encryption processing and the decryption processing in which the third parameter is set such that the probability of successful decryption is equal to or more than the second threshold and less than the first threshold. The decoding process using the third code succeeds with a probability of, for example, about 1/2.
Note that the encryption parameters may be determined randomly within a range that satisfies the probability condition.

The insertion unit 12 performs a decryption process on the input source code of the software or the control flow of the execution program after encrypting the plaintext with the first code, the second code, or the third code, and restores the original plaintext. A conditional branch for determining whether or not the operation has been performed is inserted.
The conditional branch can be described as “if (D (E (m)) == m)” using plaintext m, encryption function E, and decryption function D, for example.
Here, a conditional branch that can always be regarded as true (hereinafter, a true and true branch) is created from the first code, and a conditional branch that can always be regarded as false (hereinafter, a true / false branch) is created from the second code. You. From the third code, a conditional branch (undefined branch) whose truth is uncertain is created.

  Further, the insertion unit 12 assigns a dummy process to a process on the false side of the true / false branch and a process on the true side of the true / false branch. Further, the insertion unit 12 assigns an equivalent process having a different description to both of the undefined branches.

Here, as the conditional branch to be inserted, a true branch, a false branch, or an undefined branch may be randomly selected.
Further, the plaintext m to be encrypted is an arbitrary character string, and may be selected at random, or a value such as a predetermined work area may be selected.
The location where the conditional branch is inserted by the insertion unit 12 may be selected by an existing method.

FIG. 2 is a diagram showing a first pattern of obfuscation according to the present embodiment.
Here, a true branch is used, the original process P is assigned to the YES side of the conditional branch, and the dummy process Q is assigned to the NO side of the conditional branch.
Since the determination in the conditional branch is always YES, the dummy process Q on the NO side is not executed, and the control flow after obfuscation is equivalent to the original process P.

FIG. 3 is a diagram showing a second pattern of obfuscation according to the present embodiment.
Here, a false branch is used, a dummy process Q is assigned to the YES side of the conditional branch, and the original process P is assigned to the NO side of the conditional branch.
Since the determination in the conditional branch is always NO, the dummy process Q on the YES side is not executed, and the control flow after obfuscation is equivalent to the original process P.

FIG. 4 is a diagram showing a third pattern of obfuscation according to the present embodiment.
Here, an indefinite branch is used, a process P ′ equivalent to the original process P is assigned to the YES side of the conditional branch, and a process P equivalent to the original process P is also assigned to the NO side of the conditional branch. P '' is assigned. Here, P ′ and P ″ are processes equivalent to P.
The determination in the conditional branch may be YES or NO, and either P ′ or P ″ processing is executed.

According to the present embodiment, the obfuscation apparatus 1 generates a condition branch that is always true and a condition branch that is always false by adjusting the parameters of the encryption process and the decryption process, and converts any one of the software Insert into control flow.
Thereby, the obfuscation apparatus 1 can divide the control flow of the software into the dummy processing that is not executed and the original processing, thereby making the analysis difficult. Further, since it is difficult to determine which pattern of the conditional branch is inserted, it becomes more difficult to analyze the software source code and the execution program.

  Further, the obfuscation apparatus 1 can perform obfuscation by inserting conditional branches that can be stochastically true or false and describing two equivalent processes. Thus, the obfuscation apparatus 1 selectively incorporates three patterns of control flows, and makes it more difficult to analyze software source codes and execution programs.

  Furthermore, the obfuscation apparatus 1 randomly analyzes the parameters of the encryption process and the decryption process, and the pattern of the conditional branch (true, false, indefinite) to analyze the software source code and the execution program more. Can be difficult.

  The obfuscation apparatus 1 can appropriately generate three patterns of conditional branches by adjusting the standard deviation σ of the noise distribution, which is a parameter, by using the LWE-type lattice encryption as the encryption method.

  The embodiments of the present invention have been described above, but the present invention is not limited to the above-described embodiments. Further, the effects described in the present embodiment merely enumerate the most preferable effects resulting from the present invention, and the effects according to the present invention are not limited to those described in the present embodiment.

  The obfuscation method by the obfuscation device 1 is realized by software. When realized by software, a program constituting the software is installed in an information processing device (computer). Further, these programs may be recorded on a removable medium such as a CD-ROM and distributed to the user, or may be distributed by being downloaded to the user's computer via a network. Furthermore, these programs may be provided to a user's computer as a Web service via a network without being downloaded.

DESCRIPTION OF SYMBOLS 1 Obfuscator 10 Control part 11 Generating part 12 Insertion part 20 Storage part

Claims (7)

For an encryption method in which the probability of successful decryption according to the parameter is variable, the first code of the encryption and decryption processes that sets the first parameter in which the probability is equal to or greater than a first threshold, and the probability is A generation unit configured to generate a second code of an encryption process and a decryption process in which a second parameter that is smaller than a second threshold is set;
In the software control flow, a decryption process is performed on the plaintext after the encryption process using the first code or the second code, and a conditional branch for determining whether the plaintext has been restored is inserted. An obfuscation device comprising: an insertion unit that assigns a dummy process to one of them. The generation unit generates a third code of an encryption process and a decryption process in which a third parameter in which the probability is equal to or more than a second threshold and less than a first threshold is set,
The insertion unit performs a decryption process on the plaintext after the encryption process using the first code, the second code, or the third code, and inserts a conditional branch that determines whether the plaintext has been restored, The obfuscation apparatus according to claim 1, wherein when a conditional branch using the third code is inserted, an equivalent process is assigned to both of the conditional branches.   The obfuscation device according to claim 1, wherein the generation unit determines the parameter at random.   The obfuscation device according to claim 1, wherein the insertion unit randomly selects a code of a conditional branch.   5. The obfuscation device according to claim 1, wherein the encryption system is an LWE-type lattice encryption system. For an encryption method in which the probability of successful decryption according to the parameter is variable, the first code of the encryption and decryption processes that sets the first parameter in which the probability is equal to or greater than a first threshold, and the probability is A generation step of generating a second code of an encryption process and a decryption process in which a second parameter that is less than a second threshold is set;
In the software control flow, a decryption process is performed on the plaintext after the encryption process using the first code or the second code, and a conditional branch for determining whether the plaintext has been restored is inserted. An obfuscation method in which a computer executes an insertion step of assigning a dummy process to one of them. For an encryption method in which the probability of successful decryption according to the parameter is variable, the first code of the encryption and decryption processes that sets the first parameter in which the probability is equal to or greater than a first threshold, and the probability is A generation step of generating a second code of an encryption process and a decryption process in which a second parameter that is less than a second threshold is set;
In the software control flow, a decryption process is performed on the plaintext after the encryption process using the first code or the second code, and a conditional branch for determining whether the plaintext has been restored is inserted. An obfuscation program for causing a computer to execute an insertion step of assigning a dummy process to one of them.

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