Abstract:
A method for providing an anti-theft mechanism for devices, in particular for car radios, in that a code which identifies the device must be entered directly into the device in order to activate the device. The code is derived from an item of check information assigned to the device, using an irreversible encryption method.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a method for providing an anti-theft mechanism for devices, in particular for car radios, in which a code that identifies the device must be entered directly into the device in order to activate the device. 
   BACKGROUND INFORMATION 
   Conventional means of providing car radios with an anti-theft code are known. For this purpose, a secret code is stored in a memory assigned to the car radio. This code can be stored, for example, in an internal or an external EEPROM (Electrically Erasable Programmable Read-Only Memory). The first time the car radio is activated, or when it is reactivated following an interruption of the power supply for the car radio, the code must be entered directly into the car radio. This can be done either manually using input keys or using an external code card. A processor unit integrated into the car radio compares the stored secret code with the entered code and, if the two codes match, enables unrestricted use of the car radio. 
   The disadvantage of this method is that the secret code stored in the car radio can be read relatively easily. Unauthorized reading of the code enables unauthorized users to gain access to the code and thus to activate the car radio as well. 
   SUMMARY OF THE INVENTION 
   The method according to the present invention features offers the advantage over the conventional means in that a device provided with an anti-theft mechanism cannot be used without authorization. Because the code is derived from an item of check information assigned to the device using an irreversible encryption method, the anti-theft code itself is not stored in the device. This prevents unauthorized persons from reading this code and from improperly using it to activate the device without authorization. For example, asymmetrical methods, in particular the RSA (Revest Shamir Edelmann) method, are used as the irreversible encryption method. The advantage of these methods is that, they offer a very high cryptographic quality, which means that information that has been encrypted by an asymmetrical method can be decrypted only by systematic trial and error, which involves a great deal of time and computing power and is not economically justified by the benefits gained. 
   According to an embodiment of the present invention, a public key of the encryption method and the encryption output information (check information), in particular a serial number of the device to be protected, are stored in a processor or in a memory element assigned to the processor in the device. Knowledge of this information does not enable an unauthorized user to determine the code for unrestricted use of the to be protected device. Only after the authorized user has entered the code externally, for example manually or using a code card, is the check information calculated and then compared with the stored check information, with the device enabled if the two items of check information match. 
   According to a another embodiment of the present invention, both the check information and a check algorithm are stored in the protected device. This makes it advantageously possible to minimize the amount of effort needed for encryption, at the same time maintaining an irreversible encryption method. An irreversible encryption method of this type can be provided even without the relatively complicated use of a complete asymmetrical method by storing a check algorithm for a code, but not the algorithm for generating the code. The check algorithm and the check information, which can be read out, also make it possible to obtain the algorithm for generating the code, and thus the code itself, only with a great deal of effort. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a first schematic representation of an encryption system for a device according to the present invention. 
       FIG. 2  shows a first schematic representation of a method for activating an encrypted device according to the present invention. 
       FIG. 3  shows a second schematic representation of an encryption system for a device according to the present invention. 
       FIG. 4  shows a second schematic representation of a method for activating a device encrypted according to the present invention. 
       FIG. 5  shows a third schematic representation of a method for activating a device encrypted according to the present invention. 
   

   DETAILED DESCRIPTION 
   Based on the embodiments explained below, the description concerns the generation and use of an anti-theft code for car radios. However, this method can, in principle, be used for all devices in need of protection. 
     FIG. 1  shows the generation of the code according to the present invention. In this case, an asymmetrical encryption algorithm  10  is provided with an item of check information  12  to which a secret code  14  is applied, thus forming a code  16 . For example, the serial number of the protected device i.e., the serial number of the car radio, for example, can be used as check information  12 . Code  16  is generated from this serial number, or a portion of the serial number, for example a selection of any individual digits in the serial number, using secret key  14  in a protected environment during production of the car radio. Code  16  is supplied with the car radio when it is first shipped from the factory or when it is intentionally transferred to another party. This can be done, for example, by communicating the code in writing or by supplying a code card on which code  16  is stored. 
   The car radio itself contains a processor unit in which check information  12  in the form of invariable information and a public key  18  of asymmetrical encryption method  10  are stored. 
   Car radio  20  indicated in  FIG. 2  contains public key  18  and check information  12 , but not code  16 , which is needed to activate or reactivate car radio  20 . To activate or reactivate car radio  20  in an authorized manner, the authorized user enters code  16  into the processor of car radio  20 . This can be done, for example, by entering code  16 , which was communicated in writing, using a key or by inserting a code card. Asymmetrical encryption method  10  determines a calculated item of check information  12 ′ on the basis of entered code  16  and public key  18 . Check information  12 ′ is compared with invariable stored check information  12  in a comparator  22 . If comparator  22  finds a match between check information  12  and calculated check information  12 ′, car radio  20  is enabled for unlimited user  24 . 
   It is apparent that, upon the theft of car radio  20 , public key  18  of asymmetrical encryption method  10 , to which anyone can gain access in any case, and the check information, for example the serial number, which is also generally accessible, are available, while code  16  for determining calculated check information  12 ′ remains inaccessible. In addition, code  16  cannot be determined on the basis of public key  18  of asymmetrical encryption method  10 . 
   It is also perfectly apparent that car radio  20  cannot be activated or reactivated without knowledge of code  16 . 
   Because the irreversible asymmetrical encryption method described on the basis of  FIGS. 1 and 2  involves an amount of effort that is not justified by the economic benefit gained for certain groups of protected devices a simplified irreversible encryption method is described on the basis of  FIGS. 3 and 4 . 
   As illustrated in  FIG. 1 , check information  12 , for example the serial number of the car radio, is supplied to an encryption method  10 ′. A generation algorithm  26  is used to derive code  16  from the check information. Code  16  is then supplied with the car radio either in writing or in the form of a code card. 
   As shown in  FIG. 4 , car radio  20  is activated or reactivated by entering code  16  into car radio  20 . Encryption method  10 ′ applies a check algorithm  28  and check information  12  to code  16 . Check algorithm  28  checks code  16  to see whether the latter can be associated with check information  12 . If check algorithm  28  determines that code  16  belongs to car radio  20  containing check information  12 , it enables car radio  20  for unrestricted use  24 . 
   The irreversible encryption method explained on the basis of  FIGS. 3 and 4  is also characterized in that car radio  20  contains neither code  16  nor algorithm  26  for generating code  16 . It includes only check information  12  and check algorithm  28 , which does not provide direct access to code  16  even if it is output by an unauthorized person. Only after completing a complicated analysis of check algorithm  28  is it possible to gain access to algorithm  26  for generating code  16 . As a result, even this simplified irreversible encryption method provides an anti-theft mechanism for car radios  20  or other devices in need of protection. 
   Depending on the level of protection required by the protected devices, it is possible to select either the more complicated asymmetrical encryption method described according to  FIGS. 1 and 2  or the simpler pseudo-asymmetrical encryption method described according to  FIGS. 3 and 4 . 
     FIG. 5  shows a method for activating a device encrypted according to the present invention in which code  16  and check algorithm  28  are supplied to encryption method  10 ′, which generates check information  12 ′ from them. Check algorithm  28  and check information  12  are both stored in car radio  20 . Comparator  22  compares check information  12 ′ to check information  12  in the car radio and; if the two items of information match, enables the car radio for unrestricted use  24 .