JP5277825B2 - Program obfuscation device and obfuscation program - Google Patents

Description

  The present invention relates to a program obfuscation apparatus and an obfuscation program.

  Various program obfuscation techniques have been proposed to protect programs from malicious reverse engineering. Program obfuscation is a technique for converting a program so that it is difficult for humans to understand unless the runtime behavior of the program is changed. By obfuscating the program, the cost and time for reverse engineering can be increased. Here, reverse engineering means analyzing a program, elucidating the relationship between modules, and analyzing the basic specifications of the system.

  In Patent Document 1, a plurality of pointer variables to a function are newly introduced into the source code of a program, and a plurality of instruction statements for assigning a function address to one of the function pointer variables are inserted. The call is converted to call the function according to the value of the function pointer variable.

  Patent Document 2 detects a repetition control statement included in a program, and converts the contents of the program so as to change the order of values input to the index variable of the repetition control statement.

  Patent Document 3 discloses a routine X that disguises a plurality of instruction codes constituting an assembly program with a fake instruction code Y, returns the fake instruction code Y to the original correct instruction code y, and a correct instruction returned by the routine X. A routine Z for impersonating the code y again with a false instruction code Y is generated, and the generated routines X and Z are written at predetermined positions on the assembly program to generate a self-rewriting program.

  In Patent Document 4, data in a program is encoded using an encoding rule C (a, b) = ax + b, and an operator conversion rule R (a (a) is added to each operator of an arithmetic expression including the encoded data. , B) to R10 (a, b) are used to convert the arithmetic expression into an encoding arithmetic expression, and the encoding arithmetic expression is set as a decoding arithmetic expression using the decoding rule D (a, b). Non-Patent Documents 1 to 6 disclose documents that obfuscate similar programs.

  In Patent Document 5, a function including a plurality of instruction codes is extracted from a program, and an instruction included in the extracted function is converted into an atypical function call / return instruction. Therefore, it is possible to generate an executable format that can cause abnormal termination of the decompiler and an erroneous output result.

JP 2003-337629 A JP 2004-185064 A Japanese Patent Laid-Open No. 2004-192068 JP 2005-49925 A JP 2007-304726 A C. Collberg, C. Thomborson, and D. Low, “Manufacturing Cheap, Resilient, and Stealthy Opaque Constructs”, Proceedings of the 25th ACM Symposium on Principles of Programming Languages, 1998 C. Collberg, C. Thomborson, and D. Low, “Breaking Abstractions and Unstructuring Data Structures”, Proceedings of the 1998 IEEE International Conference on Computer Languages, 1998 C. Wang and J. Hill and J. Knight and J. Davidson, “Software Tamper Resistance: Obstructing Static Analysis of Programs”, Technical Report CS-2000-12, Department of Computer Science, University of Virginia, 2000 Masato Kadota, Yoshihiro Takada, Koji Torii, “Proposal for Obfuscating Programs Containing Loops”, IEICE Transactions, Vol. J80-D-I, No. 7, 1997 J. Chan and W. Yang, “Advanced Obfuscations on Java (registered trademark) Bytecode”, Journal of Systems and Software, vol. 71 (1? 2), 2004 Non-Patent Document 6): C. Linn and S. Debry, “Obfuscation of Executable Code to Improve Resistance to Static Disassembly”, Proceedings of the 10th ACM Conference on Computer and Communications Security, 2003

  An object of the present invention is to provide a program obfuscation apparatus and an obfuscation program that generate a program that increases the amount of calculation and storage area required for program analysis using inline expansion.

In order to achieve this object, the program obfuscation device according to claim 1 of the present invention comprises an input means for inputting an obfuscation index n (n is an integer of 2 or more) indicating the degree of obfuscation, and at least one command. Selecting means for selecting an obfuscation target program including a sentence; adding means for adding a plurality of n subroutines to the selected obfuscation target program; and at least one imperative sentence in the selected obfuscation target program Is converted into a call instruction for calling a first subroutine that is one of the n subroutines added, and each of the first to n- 1th subroutines has a branch structure, At least one of the branch section of the branched structure but contains a call instruction of another one of the subroutine, the n subroutine is executed one by one in order from the first sub-routine Are associated as, the subroutine of the n that is executed last is characterized by executing the converted instruction statement calling instruction for calling the first subroutine by said converting means.

  According to a second aspect of the present invention, in the first aspect of the present invention, any one of the branching clauses of the branch structure includes a call instruction for another subroutine different from the other.

  The invention according to claim 3 is the invention according to claim 1 or 2, characterized in that each of the branch clauses of the branch structure includes a call instruction for another subroutine.

  The invention according to claim 4 is the invention according to any one of claims 1 to 3, wherein the adding means adds a plurality of dummy subroutines to the obfuscated program, and At least one of the branch structures includes a call instruction for the plurality of dummy subroutines.

  The invention according to claim 5 is the invention according to any one of claims 1 to 4, wherein the adding means generates a reversible function that reversibly manipulates a variable included in the obfuscated program, At least one of the branch clauses of the branch structure includes a statement that reversibly manipulates the variable based on the reversible function.

  The invention according to claim 6 is the invention according to any one of claims 1 to 5, wherein the conditional expression of the branch structure is a conditional expression that is always false.

The invention according to claim 7 selects an obfuscation target program including an input procedure for inputting an obfuscation index n (n is an integer of 2 or more) indicating the degree of obfuscation and at least one imperative sentence. adding and selection procedure, and additional steps to add multiple subroutine program for adding the n multiple subroutine selected obfuscated target program, at least one statement in the selected obfuscated target program to execute the conversion procedure for converting the call instruction for calling the first subroutine, which is one of the n subroutines, each of the first n-1 of the subroutine from the first has a branched structure, the branched at least one branch node structure includes a call instruction of another one subroutine, the order said n number of subroutines one by one from said first subroutine Are associated as being performed, the subroutine of the n that is finally executed as characterized by realizing that executes the converted instruction statement calling instruction for calling the first subroutine by said converting means Yes.

  The invention of claim 1 generates a program that increases the amount of calculation and the amount of storage area required for program analysis by inline expansion.

  According to the second aspect of the present invention, any branch clause of the branch structure included in the subroutine includes another subroutine call, and generates a program that increases the amount of calculation and storage area necessary for program analysis by inline expansion.

  According to another aspect of the present invention, any branch clause of the branch structure included in the subroutine includes another same subroutine call, and generates a program that increases the amount of calculation and storage area necessary for program analysis by inline expansion.

  The invention of claim 4 includes a dummy subroutine for executing an arbitrary command statement, and generates a program that increases the amount of calculation and the amount of storage area required for program analysis by inline expansion.

  The invention of claim 5 includes a program piece that reversibly manipulates a variable based on a reversible function, and generates a program that increases a calculation amount and a storage area amount necessary for program analysis by inline expansion.

  The invention of claim 6 generates a program that includes a conditional expression that is always false and increases the amount of calculation and the amount of storage area required for program analysis by inline expansion.

  The invention of claim 7 generates a program that increases the amount of calculation and the amount of storage area required for program analysis by inline expansion.

  DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, exemplary embodiments of the invention will be described with reference to the drawings.

  FIG. 1 is a block diagram showing a configuration of a program obfuscation apparatus used in an embodiment of the present invention. The program obfuscation device 10 includes an input / output device 14, an arithmetic device 11, a RAM 12, and an external storage device 13. The input / output device 14 receives an instruction from a user who uses the obfuscation device or displays the result of the obfuscation processing. The external storage device 13 stores the obfuscation target program and the obfuscated program. The RAM 12 is a device that temporarily stores a program that is being obfuscated. The arithmetic device 11 is a device that applies obfuscation to the obfuscation target program on the RAM.

  Next, the program obfuscation method of the first embodiment of the present invention will be described with reference to FIGS. FIG. 2 is a block diagram showing the function of the arithmetic unit 11 for obfuscating a program in an arbitrary execution format. The arithmetic unit 11 includes an input unit 21 that inputs one or more programs before obfuscation, an obfuscation processing unit 22 that performs obfuscation processing of the input program, and an output unit 23 that outputs the obfuscated program. Composed.

  The obfuscation processing unit 22 determines an initial condition necessary for the obfuscation process by using the input unit 221 and the selection unit 222.

First, the input unit 221 will be described.
The obfuscation processing unit 22 includes an input unit 221 for inputting an obfuscation index n indicating the degree of obfuscation. The obfuscation index n may be determined in advance or may be determined based on the designation of the obfuscation index n. Also, the designation of the obfuscation index n may be designated by the arithmetic device 11 at random or by the user via the input / output device 14.

Next, the selection unit 222 will be described.
When the obfuscation index n is determined, the obfuscation processing unit 22 selects a program for obfuscation (hereinafter referred to as an obfuscation target program) from one or more pre-obfuscation programs. The obfuscation target program may be determined in advance or may be determined based on the designation of the obfuscation target program. The obfuscation target program may be designated by the arithmetic device 11 at random or by the user via the input / output device 14.

  Next, the obfuscation processing unit 22 selects a routine included in the selected obfuscation target program. As the routines, all the routines included in the selected obfuscation target program may be selected, or only specific (one or more) routines may be selected. The routine to be selected may be determined in advance or may be determined based on the designation of the routine. The routine may be specified by the arithmetic device 11 at random, or by the user via the input / output device 14.

  Next, the obfuscation processing unit 22 selects one or more consecutive command statements included in the selected routine as a program piece. The program piece to be selected may be determined in advance or may be selected by specifying the program piece. The program piece may be designated by the arithmetic device 11 at random or by the user via the input / output device 14.

The obfuscation processing unit 22 adds a subroutine to the program to be obfuscated by the adding unit 223 in accordance with the initial condition and the obfuscation processing rule determined by the input unit 221 and the selection unit 222.
Then, the obfuscation processing unit 22 converts the program piece selected by the selection unit 222 into a subroutine call added by the addition unit 224 by the conversion unit 224.
The obfuscation processing unit 22 ends obfuscation of the obfuscation target program by the adding unit 223 and the converting unit 224.

  The output unit 23 receives a program for which the obfuscation processing unit 22 has finished the obfuscation process. The program becomes the output of the obfuscation device as a whole. The obfuscated program is stored in the external storage device 13, for example.

Next, the obfuscation processing procedure will be described.
First, an operation screen and an operation example of the program obfuscation apparatus according to the first embodiment of the present invention will be described with reference to FIG.

FIG. 3 shows an example of the operation screen of the obfuscation processing apparatus.
The operation screen 30 displays one or more unobfuscated programs input to the input unit 21 in the source file list 31.
The user selects a program to be obfuscated from the source file list 31. When a program to be obfuscated is selected, routines included in the selected program are displayed in the routine list 32.

When the user selects a routine to be obfuscated from the routine list 32, the source code of the selected routine is displayed in the pre-obfuscation routine column 33.
In FIG. 3, the source code of a routine called main is displayed. Through the above operation, the user selects a program to be obfuscated and a routine to be obfuscated.

  Next, the user inputs an integer of 2 or more in the parameter input 34 in order to designate the obfuscation index n that determines the degree of obfuscation. In FIG. 3, 15 is input as an example. Based on the obfuscation index n, the number of subroutines to be added is determined by the adding means of the obfuscation processing unit 22.

  The user uses the mouse cursor 35 to select a program piece included in the subroutine displayed in the pre-obfuscation routine column 33 in order to specify a program piece to be converted into a subroutine by the conversion means. In FIG. 3, the arithmetic instruction of a + = b is selected as the program piece.

Since the initial conditions necessary for the obfuscation process have been set in the above process, the user presses the obfuscation execution button 36. Then, the obfuscation process is executed, and the program after the obfuscation application is displayed in the post-obfuscation application routine column 37. After confirming the obfuscation result, the user can press the obfuscated program save button 38 to save the obfuscated program.
To end the obfuscation process, the user may press the end button 39.

Next, the obfuscation processing procedure performed by the obfuscation processing unit 22 in order to realize the obfuscation processing by the above operation will be described with reference to FIG.
FIG. 4 is a flowchart of the obfuscation processing performed by the obfuscation processing unit 22 in accordance with one obfuscation processing rule.

  First, the user or the computing device 11 designates an obfuscation index n (n is 2 or more) that determines the degree of obfuscation (step S61). This step corresponds to inputting an integer of 2 or more to the parameter input 34 in the example of the operation screen shown in FIG.

  Next, the user or the arithmetic unit 11 selects the obfuscation target program P, the routine P included in the program P, and the program piece B in the routine R (step S62). . This step corresponds to selecting the program to be obfuscated from the source file list 31 in the example of the operation screen shown in FIG. The selection of the routine R corresponds to the selection of the routine R from the routine list 32, and the selection of the program piece B is performed by selecting “a + = b” with the mouse cursor 35 from the routine block 33 before obfuscation. Corresponding to

  The program obfuscation apparatus 10 performs the obfuscation process so that the obfuscated program P is provided to the user without changing the execution operation of the program piece B included in the routine R.

  In this embodiment, the obfuscation process when the main routine main of FIG. 5 included in an arbitrary program P is selected as the routine R and the operation instruction “a + = b” included in main is selected as the program piece B. explain.

  The obfuscation processing unit 22 includes, in the program P, a subroutine subB (FIG. 6) including the source code of the program piece B selected in the preprocessing (step S62) and a dummy subroutine subDmmy (FIG. 7) including an arbitrary source code. ) Is added (step S63). Therefore, when subB is called, the selected program fragment B is executed.

Next, the obfuscation processing unit 22 adds the first subroutine sub_1 to the program P to the program P (step S64). However, sub_1 uses a conditional expression that is always false (hereinafter abbreviated as a false conditional expression) as a condition determination, calls subDmmy from a true conditional clause, and calls subB from a false conditional clause. Assume that it is composed of branches (FIG. 8). Here, “k * (k−1)% 2! = 0” in FIG. 8 is a conditional expression that is always false.
When sub_1 added by this process is called, subB is always called, and as a result, the selected program piece B is executed.

The obfuscation processing unit 22 adds a subroutine sub_i (i = 2,..., N) corresponding to i to the program P for i = 2,..., N (step S65).
However, sub_i uses an arbitrary false conditional expression as a condition determination, and a true conditional clause consists of a call of sub_i-1 and one or more consecutive command statements Dmmy_i created arbitrarily, and a false conditional clause Is composed of conditional branches consisting of calls to sub_i-1 (FIG. 9).

  As a result of this process, a subroutine for calling a subroutine added before one process is added. That is, sub_2 calls sub_1 added in the pre-processing, and sub_3 calls sub_2.

The obfuscation processing unit 22 converts the program piece B into a call to the subroutine sub_n added at the end of the preprocessing (step S65) (step S66).
Thus, in this embodiment, when the user executes the main routine, sub_n-1 is called by executing sub_n, and sub_n-2 is called by sub_n-1.
After n times of subroutine calls, the subroutine subB is called, and the selected program piece B is executed.

  The obfuscation of the program P is completed by the above processing. Therefore, the obfuscation processing unit 22 outputs the obfuscation-applied program P to the output unit 23 (step S67).

  According to the present embodiment, the program P including the routine main before obfuscation in FIG. 5 is obfuscated as shown in FIG.

The obfuscated program obtained by the present embodiment increases the storage area and processing amount necessary for inline expansion.
Therefore, the obfuscation effect will be described with reference to FIG. 11 schematically showing changes in the program when inline expansion is performed on the obfuscated program FIG.
In FIG. 11, a circle represents a subroutine, and a circle-to-circle arrow represents a subroutine call. If there are k arrows from circle 1 to circle 2, it indicates that there are k subroutine calls corresponding to circle 2 in the function corresponding to circle 1.

  The obfuscated program 41 is a schematic representation of the obfuscated program FIG. When sub_n of the obfuscated program 41 is expanded inline, a first inline expansion 42 is obtained. The first inline expansion 42 schematically represents that one content of sub_n is expanded in main.

  When sub_n−1 of the first inline expansion 42 is further expanded inline, a second inline expansion 43 is obtained. Since there are two calls of sub_n-1 in main, in the second inline expansion 43, two contents of sub_n-1 are expanded in main. When sub_n-2 of the second inline expansion 43 is further expanded inline, a third inline expansion 44 is obtained.

Since there are four calls of sub_n-2 in main, in the third inline expansion 44, the contents of sub_n-2 are expanded in main.
As described above, when inline expansion is sequentially performed on the obfuscated program, the number of expansions of the subroutine becomes exponential with respect to n. Therefore, when inline expansion is applied to the obfuscated program, the size of the routine main increases exponentially with respect to n. Therefore, when n becomes large to some extent (for example, n = 50), inline expansion becomes practically impossible.

  Next, an obfuscation process performed by the obfuscation processing unit 22 according to an obfuscation process rule different from the above-described obfuscation process rule will be described with reference to FIG.

  First, the user or the arithmetic unit 11 designates a program P that performs obfuscation and an integer n (n is 2 or more) that determines the degree of obfuscation (step S131). This step corresponds to inputting an integer of 2 or more to the parameter input 34 in the example of the operation screen shown in FIG.

  Next, the user or the computing device 11 selects the obfuscation program P, the routine P included in the program P, and the program piece B in the routine R (step S132). This step corresponds to selecting the program to be obfuscated from the source file list 31 in the example of the operation screen shown in FIG. The selection of the routine R corresponds to the selection of the routine R from the routine list 32, and the selection of the program piece B is performed by selecting “a + = b” with the mouse cursor 35 from the routine block 33 before obfuscation. Corresponding to

  The program obfuscation apparatus 10 performs the obfuscation process so that the obfuscated program P is provided to the user without changing the execution operation of the program piece B included in the routine R.

  In this embodiment, the obfuscation process when the main routine main of FIG. 5 included in an arbitrary program P is selected as the routine R and the operation instruction “a + = b” included in main is selected as the program piece B. explain.

  The obfuscation processing unit 22 includes, in the program P, a subroutine subB (FIG. 6) including the source code of the program piece B selected in the preprocessing (step S132) and a dummy subroutine (FIG. 7) including an arbitrary source code. Is added (step S133). Therefore, when subB is called, the selected program fragment B is executed.

Next, the obfuscation processing unit 22 adds the first subroutine sub_1 to the program P as the obfuscation target program (step S134). However, it is assumed that sub_1 is composed of a conditional branch that uses an arbitrary false conditional expression as a condition determination, calls subDmmy from a true conditional clause, and calls subB from a false conditional clause (FIG. 8). Here, “k * k−1% 2! = 0” in FIG. 8 is a conditional expression that is always false.
When sub_1 added by this process is called, subB is always called, and as a result, the selected program piece B is executed.

Next, the user or the arithmetic unit 11 selects an arbitrary integer variable x from the program P, and the obfuscation processing unit 22 generates a reversible function g having the selected integer x as an input / output (step S135). In this embodiment, it is assumed that g (x) = 2 * x + 3 is generated as the reversible function g.
The reversible function g is used in a subroutine added in the process described later.

The obfuscation processing unit 22 adds a subroutine sub_i (i = 2,..., 2n + 1) corresponding to i to the program P for i = 2,..., 2n + 1 (step S136). However, for i = 3, 5,..., 2n + 1, sub_i is a program fragment that uses an arbitrary conditional expression as a condition determination and returns the same result as x = g (x) for both true / false conditional clauses. And sub_i-1 (FIG. 13).
Also, for i = 2, 4,..., 2n, sub_i is a program fragment that uses an arbitrary conditional expression as a condition determination and returns the same result as x = g to (x) together with a true / false conditional clause. And a call of sub_i-1 (FIG. 14). Here, g˜ is an inverse function of g. That is, g to (x) = (x−3) / 2.

The obfuscation processing unit 22 converts the program piece B into a call to the subroutine sub_2n + 1 added at the end of the preprocessing (step S136) (step S137).
Thereby, in this embodiment, when the user executes the main routine, sub_2n is called by executing sub_2n + 1, and sub_2 (n-1) +1 is called by sub_2n.
Then, after 2n + 1 subroutine calls, the subroutine subB is called, and the selected program piece B is executed.

  Since the obfuscation processing of the program P is completed by the above-described processing, the obfuscation processing unit 22 outputs the obfuscated application program P to the output unit 23 (step S138).

  According to the present embodiment, the program P including the routine main before obfuscation in FIG. 5 is obfuscated as shown in FIG.

The obfuscated program obtained by the present embodiment increases the storage area and processing amount necessary for inline expansion.
Therefore, the obfuscation effect will be described with reference to FIG. 16 schematically showing the change of the program when inline expansion is performed on the obfuscated program FIG.
In FIG. 16, a circle represents a subroutine, and a circle-to-circle arrow represents a subroutine call. If there are k arrows from circle 1 to circle 2, it indicates that there are k subroutine calls corresponding to circle 2 in the function corresponding to circle 1.

  The obfuscated program 51 is a schematic representation of the obfuscated program FIG. When sub_2n + 1 of the obfuscated program 51 is expanded inline, the first inline expansion 52 is obtained. The first inline expansion 42 schematically represents that one content of sub_2n + 1 is expanded in the main.

  When sub_2n + 1 of the first inline expansion 52 is further expanded inline, a second inline expansion 53 is obtained. Since there are two calls of sub_2n in main, in the second inline expansion 53, two contents of sub_2n are expanded in main. When sub_2n of the second inline expansion 53 is further expanded inline, a third inline expansion 44 is obtained.

Since there are four calls of sub_2 (n−1) +1 in the main, in the third inline expansion 44, the contents of sub_2 (n−1) +1 are expanded in the main.
As described above, when inline expansion is sequentially performed on the obfuscated program, the number of expansions of the subroutine becomes exponential with respect to n. Therefore, when inline expansion is applied to the obfuscated program, the size of the routine main increases exponentially with respect to n. Therefore, when n becomes large to some extent (for example, n = 50), inline expansion becomes practically impossible.

  The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited to the specific embodiments according to the present invention, and various modifications can be made within the scope of the gist of the present invention described in the claims. Deformation / change is possible.

It is a block diagram which shows an example of a structure of a program obfuscation apparatus. It is an example of a functional block diagram of a computing device. It is an example of the operation screen of an obfuscation processing apparatus. It is a flowchart which shows an example of an obfuscation process. It is an example routine before obfuscation. It is an example of a subroutine including a program piece. It is an example of a dummy subroutine. It is an example of a first subroutine. It is an example of a subroutine i corresponding to the number i. It is an example program after obfuscation. It is a schematic diagram of inline expansion. It is a flowchart which shows an example of an obfuscation process. It is an example of the subroutine i corresponding to the number i when i is an even number. It is an example of the subroutine i corresponding to the number i when i is an odd number. It is an example program after obfuscation. It is a schematic diagram of inline expansion.

Explanation of symbols

10 ... Program obfuscation device 11 ... Arithmetic device 12 ... RAM
DESCRIPTION OF SYMBOLS 13 ... External storage device 14 ... Input / output device 21 ... Input unit 22 ... Obfuscation processing unit 23 ... Output unit 30 ... Operation screen 31 ... Source file list 32 ... Routine list 33 ... Pre-obfuscation routine column 34 ... Parameter input column 35 ... obfuscation execution button 37 ... obfuscated routine column 38 ... obfuscated program storage 39 ... end button 41 ... obfuscated program 42 ... first inline expansion 43 ... second inline expansion 44 ... third inline expansion 51 ... Obfuscated program 52 ... first inline expansion 53 ... second inline expansion 54 ... third inline expansion 221 ... input means 222 ... selection means 223 ... addition means 224 ... conversion means

Claims (7)

An input means for inputting an obfuscation index n (n is an integer of 2 or more) indicating the degree of obfuscation,
Selecting means for selecting an obfuscation target program including at least one command statement;
An additional means for adding a plurality of n subroutines to the selected obfuscated program;
Conversion means for converting at least one instruction sentence in the selected obfuscation target program into a call instruction for calling a first subroutine which is one of the added n subroutines;
Each of the first to (n-1) -th subroutines has a branch structure, and at least one of the branch nodes of the branch structure includes a call instruction for another subroutine, and the n subroutines are the first subroutine. The n-th subroutine executed last is executed by the conversion means to execute the instruction sentence converted to a call instruction for calling the first subroutine. Feature program obfuscation device.   2. The program obfuscation apparatus according to claim 1, wherein one of the branch clauses of the branch structure includes a call instruction of another subroutine different from the other.   3. The program obfuscation apparatus according to claim 1, wherein each of the branch clauses of the branch structure includes a call instruction for another subroutine. The adding means adds a plurality of dummy subroutines to the obfuscated program,
4. The program obfuscation apparatus according to claim 1, wherein at least one of the branch structures of the plurality of subroutines includes a call instruction for the plurality of dummy subroutines. 5. The additional means generates a reversible function that reversibly manipulates variables included in the obfuscated program,
5. The program obfuscation according to claim 1, wherein at least one of the branch clauses of the branch structure includes a statement that reversibly manipulates the variable based on the reversible function. apparatus.   The program obfuscation apparatus according to claim 1, wherein the conditional expression of the branch structure is a conditional expression that is always false. On the computer,
An input procedure for inputting an obfuscation index n (n is an integer of 2 or more) indicating the degree of obfuscation,
A selection procedure for selecting an obfuscated program including at least one statement;
An additional procedure for adding a plurality of subroutines to a program for adding a plurality of n subroutines to the selected obfuscation program;
A conversion procedure for converting at least one command statement in the selected obfuscation target program into a call command for calling a first subroutine that is one of the added n subroutines;
Each of the first to (n-1) -th subroutines has a branch structure, and at least one of the branch nodes of the branch structure includes a call instruction for another subroutine, and the n subroutines are the first subroutine. The n-th subroutine executed last is executed by the conversion means to execute the instruction sentence converted to a call instruction for calling the first subroutine. Obfuscation program to realize.

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