DE102005046696A1 - Secured program code producing method for license verification, involves storing modified program code including exchanged references, coded program blocks and program block with decoded function as protected computer program - Google Patents

Abstract

The method involves coding predetermined program code blocks into coded program code blocks (13,14). The predetermined program code blocks are exchanged with associated coded program code blocks. The modified program code including exchanged references, coded program blocks and program block with decoded function is stored as a protected computer program (9). The pre-determined program code block implements license verification. Independent claims are also included for the following: (1) a method for executing a program code of protected computer program (2) a computer program product with a program code for implementing the method for producing secured program code.

Description

The The invention relates to a method for protecting a license check of a Computer program.

computer programs with license verification mechanisms are widely known. For becomes a license review a computer program often provided with an individual license key, the for example, when installing the program using one available from the manufacturer provided license number and user or computer specific data is created.

In installed computer programs are usually program code, of the created license key for availability and correctness checked out and the execution otherwise prevents the program. There are differences the used license mechanisms in their complexity, for example whether only a single license key stored in a locally stored file is checked or whether complex usage profiles of one network installation for multiple Users are recorded and checked centrally.

Practically however, all of the described licensing mechanisms have one common Weak spot. If it is possible is to stop the invocation of the program code, which performs the actual check, or change that, that review always successful, it is possible that Program also without valid License to use. The thus enabled use of such so-called Pirated copies yearly a significant economic damage for the manufacturers and sellers of Computer programs arise.

One such circumvention of the license protection is possible, for example, in that calling a license verification function effectively removed from the program. In doing so, the call, in usually a branch or jump command, simply by one other, ineffective command replaced, for example, a so-called NOP (no operation) statement.

alternative can also use the verification function itself changed will always return a positive result. For example, the return value a license key verification function fixed to the boolean value "true", in case this means a successful check.

Such Manipulations are relatively low Effort is feasible, for example, by using a disassembler with the executable Program code analyzed and changed can be.

Though are of modern operating systems and virus scanner forth procedures known, which is a modification of executable Program code, for example, through the use of certificates for signing the executable Recognize program codes and thus prevent the start. however have to Such features are usually activated by the user himself become. Only for a few systems, for example a special smartphone Operating systems, such procedures are enforced, e.g. if this is enforced by an operator of a mobile network.

One intentional Bypassing the license protection allows usually not prevent themselves with such procedures. Of Furthermore, not all operating systems have such protection on, especially older ones or functionally limited Operating systems such as those on mobile electronic devices how PDAs or mobile phones are used.

task It is therefore an object of the invention to provide improved license protection for computer programs describe. In this case, the protection mechanism should not from one to execution depend on the computer program used operating system.

The The object is achieved by a Method for generating protected Program code and a method for executing program code protected Solved computer program as well as a computer program product, such as they in the claims 1, 10 and 15 are defined.

According to the invention at least encoded a program code block in the executable program code stored. While the execution of the Program becomes the encrypted Program code then no longer executed directly, but first over a decryption function decrypts and then executed.

According to one Embodiment of the invention is in the encrypted Program code block for a license verification function. By the encryption This feature prevents a simple change to the license check so that no longer causes the return of an always valid verification result in a simple manner can be.

Around To increase the protective effect of the method according to the invention, both individual program code blocks as well as the program code as a whole by the use of checksums or Signatures protected become. These checksums or signatures can then before the execution the program code block or program for correctness. If the entire program code is protected by a signature or checksum for example, a separate program used for loading or the protected program yourself verify the signature or checksum. Correspondent Program code may be, for example, in a program called at startup Program code block.

By The use of an asymmetric encryption method is in one advantageous embodiment of the invention prevents a unauthorized person changed Program code itself encrypted and in the executable Inject code. Thus it can be prevented that a program code block of a computer program changed becomes.

In In another advantageous embodiment, at least two Program code blocks encrypted being a first encrypted Program code block a reference to a second encrypted Program code block. This is the reference to the second Program code block itself only in encrypted form, so he in a disassembled program not without considerable effort can be discovered.

Further Details and embodiments of the invention are specified in the subclaims.

The Invention will be described below with reference to an embodiment with reference to the drawings explained in more detail. In show the drawings:

1 an exemplary structure of a computer program according to the prior art,

2 an exemplary structure of a computer program protected according to the invention,

3 an exemplary structure of an encrypted program code block,

4 a flow chart for a method according to the invention for generating protected program code,

5 a flow diagram of a method according to the invention for executing program code of a protected computer program.

1 shows an exemplary structure of a computer program 1 according to the prior art. The computer program 1 includes four program code blocks 2 . 3 . 4 and 5 and four references 6a . 6b . 7 and 8th on the program code blocks 5 . 3 respectively. 4 ,

The first program code block 2 contains a function called "main", with which the execution of the program 1 starts. The program code block 3 performs a first program function A and the program code block 4 a second program function B off. The program code block labeled C 5 performs a verification of a license key for the computer program 1 out. From the program code block 2 find references 6a and 7 on the program code blocks 5 respectively. 3 instead of. The program function A of the program code block 3 refers by the reference 8th to the program code block 4 , The program function B of the program code block 4 refers by the reference 6b also on the program code block 5 ,

2 shows an exemplary structure of a protected computer program according to the invention 9 with five program code blocks 10 . 11 . 12 . 13 and 14 as well as four references 15a . 15b . 16 and 17 , The program code blocks 10 . 11 and 12 are unencrypted. By contrast, the program code blocks are located 13 and 14 in an encrypted state.

The program code block 10 corresponds to the function "main" of the program code block 2 , Opposite the original program code block 2 just became the reference 15a changed. The program code block 12 corresponds to the program code block 3 the unprotected computer program 1 and was merely by adjusting the reference 17 changed. The program code blocks 13 and 14 Functions B and C are now in encrypted form in the computer program 9 before, allowing a direct call of functions B and C from the program code blocks 10 . 12 and 13 is no longer possible. In particular, the reference 15b from the now encrypted program function B to the program code block 14 is now itself part of the encrypted program code block 13 , The program code block 11 corresponds to an additional decryption function D, which is used to decrypt the program code blocks 13 and 14 serves.

The references 15a . 15b and 17 from the program code blocks 10 . 12 respectively 13 are now on the decryption tion D in the program code block 11 directed. The start address of the program code blocks is used as the call parameter 14 respectively 13 pass what's in of the 2 indicated by the dashed arrows.

Upon reaching the reference 15a of the program code block 10 be in the protected computer program 9 following steps are executed: First, the decryption function D of the program code block 11 and the parameter start address of the program code block 14 to hand over. The decryption function D then loads the encrypted program code block 14 starting at the transmitted start address and decrypts that in the program code block 14 contained program code using an asymmetric decryption method. For example, the required public key in the program code block 11 be included or on the computer on which the protected program 9 expires, be saved.

After decryption of the program code block 14 this is for example in the working memory of the executing computer with the decrypted program code block 5 the function C overwrites and the execution of the program 9 at the start address of the decrypted program code block 5 continued. After processing the program code block 5 becomes the execution of the program code block 10 after the reference 15a continued.

The second encrypted program code block 13 as well as the references 17 respectively. 15b are processed in an equivalent way. In order to limit the amount of computation required for the decryption, once decrypted program code blocks 4 and 5 the encrypted program code blocks 13 and 14 for the duration of the execution of the protected program 9 be kept in the working memory. Alternatively, it is also possible to decrypt the program code blocks 4 and 5 again through the encrypted program code blocks 13 and 14 after execution of the decrypted program code blocks 4 and 5 replace, so the decrypted program code 4 respectively 5 is stored in main memory only for the duration of the execution.

In both cases, the program code block is located 14 in the embodiment, the function C for verifying a license key, only in encrypted form on a disk of a computer before. Thus, changing the program code block 5 practically impossible. For a successful change of the first time to decrypt the program code block 14 this would have to be encrypted again after the change. In order to encrypt a program code block, however, a private key of an asymmetric key pair is required in the illustrated embodiment, which is contained in the protected program code 9 not included.

But also the replacement of the references 15a and 15b by an ineffective operation is in the protected program 9 very difficult. Because the reference 15a refers as well as the reference 17 to the program code block 11 and not the encrypted program code block 14 , Thus, a distinction of references 15a and 17 in a disassembled, protected program 9 only possible with difficulty. However, both are references 15a and 17 from the program code of the program 9 This removes the program code block 13 disabled. Provided the program code block 13 contains one for the use of the program 9 important function, thus becomes the program 9 rendered useless. The same also applies to a manipulation of those in the program code block 11 contained decryption function D, since without this function or with a functional impairment of this function and the program code block 13 can no longer be executed successfully.

Even more difficult would be removing or replacing the encrypted link 15b to the license check function C. Because this reference is in fact only after decrypting the program code block 13 in decrypted form. Thus, the reference 15b by merely disassembling the protected program 9 not be recognized.

3 shows a possible structure of the encrypted program code block 13 , At the beginning is initially a length specification 19 the length of subsequent encrypted program code 20 contains. The encrypted program code 20 then follow a checksum 21 and a padding area 22 , the remaining space until the beginning of the next program code block 14 occupies.

Since unencrypted program code usually contains redundant information, this is the encrypted program code 20 often shorter than the original unencrypted program code block 4 , Optionally, this can be accomplished by the use of well-known compression algorithms. Alternatively, the unencrypted program code block 4 even be artificially extended by a translation program, for example, by inserting ineffective commands. This makes it possible to use the unencrypted program code block 14 by an encrypted program code block 13 without replacing the subsequent program code block 14 to damage.

For decryption, however, the decryption function D must be the length of the encrypted program code 20 know. Therefore, the in 3 illustrated structure at the beginning of the encrypted program code block 13 the length specification 19 for example, the length of the encrypted program code 20 in bytes. After the length specification 19 immediately follows the encrypted program code 20 of the program code block 13 ,

To increase the security of the method according to the invention was further a checksum 21 for the encrypted program code 20 created and in the encrypted program code block 13 added. The checksum can be, for example, a so-called CRC checksum or a signature for the encrypted program code 20 which has been generated and can be checked, for example, with the same asymmetric key pair, which is also used to encrypt or decrypt the program code block 4 or the program code 20 is used.

The filling area 22 , ie the remaining space until the beginning of the subsequent program code block 14 , for example, can be filled by zeros or other predetermined values, so that here not safety-related information, for example, parts of the previously unencrypted program code block 4 , remain.

When loading and decrypting the encrypted program code block 13 the decryption function D first loads the length specification 19 the encrypted program code 20 , Following is the encrypted program code 20 read by decryption function D and decrypted using a publicly available key. Optionally, previously, the integrity of the encrypted program code 20 using the checksum 21 be verified. Alternatively, the checksum may be 21 of course also on the unencrypted program code of the program code block 4 Respectively. In this case, the integrity of the program code block becomes 4 checked after its decryption.

4 shows a flow chart for a method according to the invention for generating protected program code.

In a first step 41 become the start adresses of the program code blocks 2 . 3 . 4 and 5 determined. For example, this can be done when translating source text blocks into the program code blocks 2 . 3 . 4 and 5 of the computer program 1 be made. Examples of such source text blocks are function definitions or procedures, or even modules such as objects of a program source text. There are references in the program source text 6a . 6b . 7 and 8th to these source text blocks, usually in the form of function calls. Alternatively, the required information, so in particular the start addresses of the individual program code blocks 2 to 5 , for example, when linking the program code blocks 2 . 3 . 4 and 5 determined or from separately stored information, such as a so-called map file, are determined.

In one step 42 becomes an additional program code block 11 with a decryption function in the changed program code 9 inserted. For example, this may be at the end of the modified program code 9 be attached as in

2 is shown. In this case, the decryption function contains all the instructions that are needed to form an encrypted program code block 13 or 14 to decrypt again. For example, a decryption function can be put into the program code block via one 11 integrated public key. Alternatively, the decryption function may also provide program code that downloads a corresponding public key from a server accessible to the computer system.

In one step 43 become the references 6a . 6b and 8th to predetermined, to be encrypted program code blocks 4 and 5 by references 15a . 15b and 17 to the program code block 11 replaced. At the same time, the decryption function contained therein is also the one in the step 41 determined starting address of the predetermined program code blocks 4 respectively. 5 to hand over. In this way, the decryption function can be told what to decrypt.

In a further step 44 become the predetermined program code blocks 4 and 5 encrypted. For example, by appropriate comments or instructions in the program source text itself or separate instructions to a computer program used for translation or linking, the program code blocks 4 and 5 be selected for encryption. In this case, each encrypted program code block becomes 4 and 5 the length is determined and recorded.

In one step 45 become the predetermined unencrypted program code blocks 4 and 5 by encrypted program code blocks 13 and 14 replaced. For encryption, for example, the so-called Blowfish algorithm or another asymmetric encryption algorithm known to those skilled in the art can be used.

alternative can also be used a symmetric encryption method become. Although these are less secure, they have the advantage of being faster than asymmetric encryption methods. Thus offers especially in devices With low computational power the use of symmetric encryption methods at. Of course can also combined solutions be used, where one for a symmetric encryption method used, relatively short key protected by means of an asymmetric encryption method.

The structure of the encrypted program code block 13 corresponds to the in 3 shown. First, the length of the encrypted program code block 13 as a length specification 19 to the one in step 41 written start address written. Subsequently, the encrypted program code 20 written, possibly followed by a check digit 21 , If the encrypted program code 20 shorter than the one in the step 42 determined length of the unencrypted program code block 4 respectively 5 , the area becomes 22 optionally filled with zeros, the remaining space of the unencrypted program code block 4 against the encrypted program code 20 plus the fields 19 and 21 accounts. This also avoids that the subsequent program code block 14 is moved in its position, leaving no further adjustments to references in the modified program 9 necessary. The program code block 5 is done in the same way by the encrypted program code block 14 replaced.

In one step 46 may be the modified, protected program code 9 still to be signed. For example, the same asymmetric key pair can be used to sign the program code 9 which is also used to encrypt the program code blocks 4 and 5 was used. This allows a user or an operating system to check if the program code 9 was still changed after its creation. Alternatively, a checksum can also be calculated. However, both steps are optional for the illustrated method of protecting program code.

In one step 47 becomes the created program code 9 saved. Optionally, an additional signature can be added in step 46 was calculated to be stored. The program code 9 can now be delivered to a user, possibly in conjunction with a signature and the public part of the asymmetric key pair.

5 shows a flowchart for a method for carrying out inventively protected program code.

In one step 51 becomes program code with at least one encrypted program code block 13 respectively 14 loaded. The protected computer program 9 further includes a decryption function in the program code block 11 ,

In one step 52 First, a predetermined program code block 10 which is at a predetermined address of the program 9 stands, executed. This can be, for example, the start address of the program 9 or that of a program code block determined by predetermined information, such as the name of a function.

The execution of the program code block 10 will continue until one step 53 a reference 15a to the program code block 11 is recognized with the decryption function.

Became a reference 15a Detected on the decryption function, this is in one step 54 called. The decryption function is replaced by the reference 15a the start address of the encrypted program code block 14 to hand over. The decryption function now reads the length of the encrypted program code in a first step 20 from the length specification 19 one. Below is the encrypted program code 20 itself in previously allocated memory to be loaded.

In one step 55 becomes the encrypted program code 20 using the checksum 21 or a signature checked for its correctness. In this step, a change to the encrypted program code block 20 detected, the execution of the protected program 9 for safety reasons.

In one step 56 becomes the encrypted program code 20 decrypted. For this purpose, the decryption function D usually has dynamic memory for storing the decrypted program code block 5 Request. Alternatively, the encrypted program code block 14 through the decrypted program code block 5 be replaced because they have the same length.

The reference 15a to the program code block 11 will now go to the decrypted program code block 5 forwarded. In this case, all parameters that were initially transferred to the decryption function are sent to the decrypted program code block 5 forwarded. This happens in the step 57 ,

After finishing the execution of the unv keyed in program code blocks 5 becomes the execution of the calling program code block 10 continued. In the 5 this is by going back to the step 52 characterized. It is in the step 53 continue to have references 15a . 15b and 17 to the decryption function in the program code block 11 supervised.

Whether a once decrypted program code block 13 respectively 14 as a decrypted program code block 4 respectively 5 remains in the working memory of the executing computer or not, for example, depends on the available memory and predetermined security criteria.

For example, at the end of execution of a decrypted program code block 4 or 5 this will be removed from the memory again. For example, this function can take over the decryption function before returning control to the calling program code block.

Alternatively, the decryption function D may also use a table in which entries on already decrypted program code blocks 4 respectively 5 are included. Upon a repeated call to an encrypted program code block 13 respectively 14 can directly access the decrypted program code block 4 or 5 without further decryption or verification of the encrypted program code block 13 or 14 becomes necessary.

1

computer program

2

Program code block (Main program function main)

3

Program code block (Program function A)

4

Program code block (Program function B)

5

Program code block (License check function C)

6-8

references

9

protected computer program

10

Program code block (Main program function main)

11

Program code block (Decryption function D)

12

Program code block (Program function A)

13

encrypted Program code block (program function B)

14

encrypted Program code block (license verification function C)

15-17

references

19

length specification

20

encrypted program code

21

checksum

22

padding

41-47

steps the process of creating protected program code

51-57

steps the method for performing protected program code

Claims (15)

Method for generating protected program code, comprising - providing a computer program ( 1 ) with several program code blocks ( 2 . 3 . 4 . 5 ), in particular functions, and references ( 6a . 6b . 7 . 8th ) to the program code blocks ( 2 . 3 . 4 . 5 ), in particular function calls, - determining a start address of at least one predetermined program code block ( 4 . 5 ), - providing a program code block ( 11 ) with a decryption function by which an encrypted program code block ( 13 . 14 ) can be decrypted, - replacing the references ( 6a . 6b . 8th ) to the predetermined program code block ( 4 . 5 ) by references ( 15a . 15b . 17 ) to the program code block ( 11 ) with the decryption function with transmission of the start address of the predetermined program code block ( 4 . 5 ), - encrypting the predetermined program code block ( 4 . 5 ) into an encrypted program code block ( 13 . 14 ), - replacing the predetermined program code block ( 4 . 5 ) with the associated encrypted program code block ( 13 . 14 ), and - storing the changed program code, comprising the replacing references ( 15a . 15b . 16 . 17 ), the encrypted program code block ( 13 . 14 ) and the program code block ( 11 ) with the decryption function as a protected computer program ( 9 ). Method according to claim 1, characterized in that the predetermined program code block ( 4 . 5 ) carries out a license check. Method according to claim 1 or 2, characterized in that, in the step of determining the start address, additionally the length of the predetermined program code block ( 4 . 5 ) and, in the step of encrypting, the length of the encrypted program code block ( 13 . 14 ) is determined. A method according to claim 3, characterized in that in the step of replacing shorter encrypted program code ( 20 ) to the length of a longer unencrypted program code block ( 4 . 5 ) with a filling area ( 22 ) with vorbestimm is filled up. Method according to claim 3 or 4, characterized in that the encrypted program code block ( 13 . 14 ) the length ( 19 ) of the encrypted program code ( 20 ) is advanced. Method according to one of Claims 1 to 5, characterized in that the encrypted program code block ( 13 . 14 ) a checksum ( 21 ) or signature. Method according to one of claims 1 to 6, characterized in that for the protected computer program ( 9 ) a checksum or signature is calculated and the checksum or signature is stored with the program code. Method according to one of claims 1 to 7, characterized that the encryption process an asymmetric encryption method is. Method according to one of claims 1 to 8, characterized in that at least two program code blocks ( 4 . 5 ), a first encrypted program code block ( 13 ) a reference ( 15b ) to a second encrypted program code block ( 14 ) having. Method for executing program code of a protected computer program ( 9 ), comprising - loading a computer program ( 9 ) with at least one encrypted program code block ( 13 . 14 ) and a program code block ( 11 ) with a decryption function in a working memory, - starting the execution of the computer program ( 9 ) with a predetermined program code block ( 10 ), - calling the decryption function for decrypting an encrypted program code block ( 13 . 14 ), - decrypting the encrypted program code block ( 13 . 13 ) into a decrypted program code block ( 4 . 5 ) by the decryption function, and - executing the decrypted program code block ( 4 . 5 ). A method according to claim 10, characterized in that in the step of executing the decrypted program code block ( 4 . 5 ) a license check is performed on the computer program. Method according to claim 10 or 11, characterized in that the encrypted program code block ( 13 . 14 ) encrypted program code ( 20 ) as well as a length indication ( 19 ) and in the step of decrypting first the length specification ( 19 ) of the encrypted program code block ( 13 . 14 ) and to determine the encrypted program code ( 20 ) is used. Method according to one of Claims 10 to 12, characterized in that the encrypted program code block ( 13 . 14 ) a checksum ( 21 ) or signature and in the step of decrypting the correctness of the checksum ( 21 ) or signature is checked. Method according to one of claims 10 to 13, characterized in that in the step of loading additionally a checksum or signature for the protected computer program ( 9 ) is loaded and the correctness of the checksum or signature is checked. Computer program product with program code for performing a Method according to one of the claims 1 to 14 when the program code runs on a processor.