GB2376386A

GB2376386A - Locking a mobile telephone from a radiotelephone network

Info

Publication number: GB2376386A
Application number: GB0210454A
Authority: GB
Inventors: Denis Dumont; Joel Chevillard
Original assignee: Sagem SA
Current assignee: Sagem SA
Priority date: 2001-05-10
Filing date: 2002-05-08
Publication date: 2002-12-11
Anticipated expiration: 2022-05-08
Also published as: DE10220637A1; FR2824692A1; GB0210454D0; GB2376386B; FR2824692B1

Abstract

A method or apparatus for controlling access to a mobile telecommunications network from a mobile unit 100 including: storing a cipher key K in a unique electronic circuit 102 of the unit 100; encrypting by means of the cipher key K an access control value Vp that is unique to the unit 100 and allows access to a specific cellular network, in order to obtain an encrypted access control value Vpc unique to the device 100 which is then stored in the unit 100; using the unique circuit 102 to perform a read operation of an access identification value 111 enabling access to a cellular network; using the cipher key K to decrypt the unique encrypted access control value Vpc to give a unique access control value Vp; using the electronic circuit to compare the unique access control value Vp with the access identification value 111 and authorising access, or not, on the basis of this comparison. If the values match, then access to the network is allowed. The unique electronic circuit 102 is preferably a application-specific integrated circuit that controls critical functionalities of the unit 100, such as power supply or radio emissions.

Description

Mobile Telephone With Protection Of A System Locking Access To A

Telecommunications Network And Associated Protection Method The object of the present invention is a mobile telephone with 5 protection of a system locking access to a telecommunications network of a cellular radiotelephony network type. The further object of the invention is a method for protection by locking access to such a telecommunications network from a mobile telephone.

The mobile telephone, or mobile handset, and the method according 10 to the invention provide an additional level of security in the locking system of a mobile telephone on a given network.

The field of the invention is that of mobile telephony. The invention will

be described more specifically with reference to its use in a network compliant with the Global System for Mobile Communication (GSM) 15 standard, the General Packet Radio Service (GPRS) standard, or the Universal Mobile Telecommunications Systems (UMTS) standard, but it may be implemented for cellular radiotelephony networks operating according to other standards. In this field, a variety of mobile telephony operators coexist

within the same territory. A mobile telephone user may thus choose to 20 telephone using a particular cellular telephony network set up by one of the available operators. In practice, this choice is most often made at the time of purchase of a mobile handset. Indeed today, a user most frequently buys a mobile telephone as part of package that including not only the mobile handset itself, but also a commitment to an operator, such commitment 25 frequently taking the form of a subscription. Thus, a user who has just bought a mobile telephone can only use it on a single telecommunications network established by the operator that also sold the set. In return, this operator heavily subsidises the purchase of the mobile handset.

Technically, the access restriction of the mobile telephone to a 30 predetermined cellular radiotelephony network is assured by the use of a specific value, the access control value specific to a network, also known as the simlock value, which is stored in encrypted form in a memory on the mobile telephone. This encrypted value is associated with the cellular radiotelephony network in question. Every time the mobile telephone is used, 35 this encrypted value is read, decrypted and interpreted by an operating

system incorporated in the mobile telephone. The interpretation of this decrypted value is what allows the mobile telephone to determine the specific cellular telecommunications network with which it can exchange data. 5 More precisely, according to the state of the art, a system, for locking a mobile telephone onto a given network, also known as a simlock system, with which it is possible to determine whether a mobile telephone is authorised for access to a cellular radiotelephony network desired by the telephone user, functions as follows: 10 In general, a mobile telephone is equipped with a microchip called a SIM (Subscriber Identification Module) chip. This SIM chip includes certain information and/or applications dedicated to the mobile phone, particularly including an identification value for access to a radiotelephony cellular network. Thus in general, a SIM chip is dedicated to a specific operator entity 15 which uses a particular network. When a mobile telephone is activated, an access control value stored in encrypted form in a particular memory unit of the mobile telephone is decrypted by means of a cipher key contained in a different memory unit of the mobile telephone in order to obtain the unencrypted access control value. In principle, it is the same cipher key that 20 was used to derive the encrypted access control value from the unencrypted access control value. A comparison operation then takes place between the decrypted access control value and the access identification value contained in the SIM chip. If the two values are identical, the user is authorised to communicate on the cellular radiotelephony network associated with the SIM 25 chip inserted in the mobile telephone.

Such a method presents a number of weakness: firstly, the cipher key is written in memory so that the representative data of this cipher key may be accessed directly by a software application designed to perform encryption and decryption operations on the simlock value. As a consequence, the data 30 pertaining to the cipher key may be recovered fraudulently simply by reading the memory unit containing this cipher key. The mere knowledge of the cipher key may enable a range of fraudulent operations to be carried out within the telephone.

Moreover, the simple fact that the comparison operation is performed 35 by a software application weakens the protection of the locking system used.

Indeed, a software application of such kind does nothing more than execute several lines of computer code, some of which are specifically dedicated to the comparison operation indicated previously, which is critical for access authorization to a cellular radiotelephony network. The result of the 5 comparison operation is a value, the validation value, this validation value being a function of the result of the comparison. The validation value is then used by other software applications within the mobile telephone to carry out those operations that may be necessary to connect the mobile telephone to the cellular radiotelephony network associated with the access identification 10 value which has just been compared to the access control value.

A simple way of defeating the simlock system installed in mobile telephones is to design a software application that makes it possible to always obtain the validation value corresponding to the value that is obtained when the comparison operation is deemed satisfactory, regardless of the 15 access identification value used. In other words, one of the essential weakness of such a simlock system consists in the fact that the reading, encryption and/or decryption, and comparison operations of the access control values and/or of the access identification values are all executed at a software level which may be easily circumvented by fraudulent users. This 20 fraud operation is becoming more common because it may be advantageous to purchase a mobile phone operating on one cellular radiotelephony network and to attempt to circumvent the simlock system in order to be able to use the mobile telephone on other cellular radiotelephony networks.

The telephone and the method according to the invention make it 25 possible to counteract the problems outlined in the foregoing.

The invention proposes a method and a mobile telephone which permit the protection of the simlock system. To this end, the method and the telephone according to the invention implement a simlock system incorporating a unique electronic circuit, preferably an ASIC (Application 30 Specific Integrated Circuit). A set of operations will be performed within this unique electronic circuit, and particularly a reading operation of an access identification value, for example by a phase interrogating the SIM chip instead of the mobile phone, or the comparison operation previously described. Moreover, the unique electronic circuit may provide for the 35 management of certain critical functions, for example power supply

management, radio emission management, within the mobile phone....

In the invention, a unique electronic circuit of such kind used in a mobile telephone possesses a unique key, which is known only to itself and which permits It to authenticate; enc.rvot and decrypt data located outside 5 this circuit, preferably in non-volatile, rewritable memories. The unique electronic circuit is thus tasked with testing the consistency of the different elements permitting locking with respect to the corresponding data contained in the SIM chip and, according to requirements, satisfying or otherwise the critical functions for which it is responsible within the mobile telephone.

10 In addition, the access control value may be changed within the mobile telephone, thus allowing access to one or more cellular radiotelephony networks other than the one associated with the access control value initially stored in the telephone memory. Generally, the different operators are willing to make this value change free of charge after a certain 15 subscription period has elapsed, or for payment if this minimum subscription duration has not elapsed. Under certain conditions, locking may also be entirely removed by the operator.

For these reasons, according to the invention, the access control value, protected beforehand by an encryption operation with the cipher key, 20 is stored outside the unique electronic circuit. A cryptographic operation is implemented for this purpose. In particular, it incorporates the International Mobile Equipment Identity (IMEI) and certain data selected by the operator.

In the invention the unique electronic circuit is therefore tasked with modifying the access control value so as to change the network onto which 25 the mobile telephone is locked, or with unlocking the mobile telephone completely. For this purpose, a correct password, previously defined within the mobile telephone, is typed on the keypad by the user. The implementation variations of the invention thus allow this controlled change of the cellular radiotelephony network.

30 According to the invention, therefore, the integrity of the locking is assured by a unique electronic circuit which is much more difficult to modify and circumvent than a software application. One of the advantages of the invention is that the unique electronic circuit providing the locking cannot be removed from the mobile telephone because it manages certain critical 35 functions of the device.

The invention therefore relates particularly to a method for protecting a system for locking access of a mobile telephone to a set of telecommunications networks, characterized in that it includes the following 5 different steps: - storing a cipher key within a unique electronic circuit of the mobile telephone; - encrypting by means of a cipher key an access control value that is unique to the telephone and that allows access to a particular cellular 10 radiotelephony network, to obtain an encrypted access control value unique to the device; - storing the encrypted access control value unique to the device in a memory module of the mobile telephone; - using the unique electronic circuit, performing a read operation of an 15 identification value enabling access to a cellular radiotelephony network; - using the cipher key, decrypting the unique encrypted access control value to obtain the unique access control value; - using the unique electronic circuit, comparing the unique access control value with the access identification value for a cellular radiotelephony 20 network; - if the access identification value for a cellular radiotelephony network and the unique access control value are identical, authorising access to the specific cellular radiotelephony network associated with the access identification value.

25 In a preferred embodiment of the invention, the unique electronic circuit is an application-specific integrated circuit, which may also control certain critical functionalities of the mobile telephone, such as control of the power supply to the mobile telephone and/or some of its components, or handling of radio emission operations by the mobile telephone. In certain 30 configurations, the cipher key is stored in memory in the unique electronic circuit when the mobile telephone is manufactured, for example by a fuse cut-out operation within the unique electronic circuit.

In a particular embodiment, the method according to the invention may include the additional step according to which if the unique access 35 identification value to a cellular radiotelephony network and the access

control value are not identical, access to the specific cellular radiotelephony network associated with the access identification value is denied and the critical functionalities controlled by the specific electronic circuit are blocked.

The method according to the invention further allows the unique 5 access control value to be replaced with a general access control value, and the various operations previously described to be performed, incorporating the unique access control value by replacing this with the unique access control value.

A further object of the invention is a mobile telephone having 10 protection for the system locking access to a telecommunications network, characterized in that it includes means for implementing the method according to one of the previous claims. Such means particularly include the unique electronic circuit.

The invention and its various applications will be better understood 15 with reference to the description that follows and to the single accompanying

drawing. These are presented for exemplary purposes only are not intended to limit the invention in any way. Figure 1 shows a schematic diagram of the principal elements operating in the method and in the mobile telephone according to the invention.

20 In the figure, a mobile telephone 100 is represented schematically.

Mobile telephone 100 is particularly equipped with microprocessor 101 which manages all operations and particularly exchanges of data taking place within the mobile telephone under consideration. A number of essential functions 103 may include for example the entire radio emission mechanism 25 104 of the mobile telephone, a power amplifier 105 activated when the telephone is functioning, or even a series of circuits 106 requiring an electrical supply, is connected symbolically to a unique electronic circuit 102 of type ASIC. A re-writable type memory 107 including a number of different memory modules is also connected to ASIC 102 so as to be able to 30 exchange data therewith. ASIC 102 is also capable of receiving various types of data for example from an external circuit 108 of the SIM micromodule type. Data may also be exchanged between microprocessor 101 and ASIC 102, which are connected to this end.

The operation of the method according to the invention will be 35 explained in the following. As soon as it is installed in mobile telephone 100,

ASIC 102 stores a key (K) for encryption 109. Such storage may be effected by for example by known method, by a cut-out operation in a number of fuses in ASIC 102. Each telephone 100 is thus equipped with one ASIC 102 possessing its own unique cipher key. When created in this manner, the 5 cipher key K is no longer contained in an independent memory unit, and is therefore much less easily accessible to a possible fraudulent user. By means of cipher key K, a unique access control value, Vp, is encrypted to be stored in encrypted form, Vpc, in a first memory unit 1 10 of memory 107. The unique access control value stored when the mobile telephone was 10 manufactured is already associated with a specific cellular radiotelephony network, the purchaser of mobile telephone 100 is thus initially limited to use of this specific network.

When a user inserts SIM chip 108 into mobile telephone 100, different data reading operations contained in SIM chip 108 are performed by mobile 15 telephone 100. These various reading operations are controlled by microprocessor 101. In the invention, all the data read in SIM chip 108 passes through ASIC 102 before being processed by microprocessor 101 and various program memories connected thereto. In addition, SIM micromodule 108 includes an identification value 111 for access to a specific 20 network. ASIC 102 is dedicated to a certain number of specific applications 112, particularly including an application 146 for reading the SIM chip, an application 147 for reading memory 107, and an application 148 for writing to memory 107.

One of these applications is an application 113 for detecting the 25 access identification value to a specific network. Thus, when ASIC 102 detects this access identification value 111, it invokes another application of the set 112, this being application 141 which uses key K to encrypVdecrypt encrypted unique access control value Vpc that is contained in memory unit 110. Once this decryption application is invoked, another application 142 of 30 the set 112 is also implemented. This application is a comparison application that enables the decrypted unique access control value, Vp, to be compared with the access identification value for a specific network 111.

Based on the result of the comparison, ASIC 102 triggers one of the following operating applications: 35 - if the comparison has established that the compared values are

identical, a normal operation application 143 is invoked. This normal operation application particularly has the effect of providing a set of voltages appropriate for essential functionalities 103 via ASIC 102.

- if the comparison operation has determined that the two compared 5 values are not identical, and that the unique access control value is not a general access control value, ASIC 102 invokes a null operation application 144, meaning that it does not supply adequate voltages to the set of essential functionalities 103. In certain special cases, the ASIC may trigger an application 145, which is an interrupt operation and with which it supplies 10 voltages higher than normally anticipated to the set of the essential functionalities 103, so that one or more of these functionalities is damaged, thus rendering mobile telephone 100 permanently unusable. This last configuration may be conceivable when the comparison operation detects a non-identity status between the two compared values a certain number of 15 times, a counter being provided within ASIC 102 for this purpose.

A second unit 113 of memory 107 may include a password. This password is used by ASIC 102 when, with the authorization of the operator to whose network the phone is nominally attached, the user of mobile telephone 100 unlocks the access control system for a specific network. To 20 do this, the user types on the keypad a password value that has been communicated him by the operator. This password value is then compared within the ASIC 102 with the password value contained in the memory unit 113, and which has been decrypted previously using cipher key K, or possibly by means of a different cipher key. This procedure enables the user 25 to replace unique access control value 110 with a different unique access control value or a general access control value which will authorise him to access all cellular radiotelephony networks.

When unique access control value 110 has been replaced by a general access control value, a variant of the method according to the 30 invention may be implemented. According to this variant, when ASIC 102 identifies general access control value after decryption, the comparison operation with the access identification value for a specific network, which is contained in a SIM chip, may be suppressed.

The essential benefit of the method according to the invention and the 35 mobile telephone implementing such method lies in the fact that the set of

operations for unlocking access to a given network, and all the control operations for access rights to this given network are performed within an ASIC-type integrated circuit. Such a circuit cannot be circumvented as could a software application such as was hitherto used for such various operations.

5 A second level of security consists in the fact that ASIC 102 manages a number of functions essential for the operation of mobile telephone 100.

Claims

1. A method for protecting a system for locking access of a mobile te!PnhonP- to a set of teler:ommunicatinns nr-tworksi characterized in that it 5 includes the following different steps: - storing a cipher key within a unique electronic circuit of the mobile telephone; - encrypting by means of the cipher key an access control value that is unique to the telephone and that allows access to a specific cellular 10 radiotelephony network, to obtain an encrypted access control value unique to the device; storing the encrypted access control value unique to the device in a memory module of the mobile telephone; - using unique electronic circuit to perform a read operation of an 15 access identification value enabling access to a cellular radiotelephony network; - using the cipher key to decrypt the unique encrypted access control value to obtain an unique access control value; - using the unique electronic circuit to compare the unique access 20 control value with the access identification value for access to a cellular radiotelephony network; - if the access identification value for a cellular radiotelephony network and the unique access control value are identical, authorising access to the specific cellular radiotelephony network associated with the access 25 identification value.

2. A protection method according to claim 1, characterized in that the unique electronic circuit is an application-specific integrated circuit.

3. A method according to any one of claims 1 or 2, characterized in that the unique electronic circuit controls critical functionalities of the 30 mobile telephone.

4. A method according to claim 3, characterized in that a critical functionality controlled by the unique electronic circuit is a functionality controlling supply of power to certain components of mobile telephone.

5. A method according to any one of claims 3 or 4, characterized 35 in that the critical functionality controlled by unique electronic circuit is a

management functionality for radio emission operations of the mobile telephone.

6. A method according to any one of the preceding claims, characterized in that it includes the additional step of replacing the unique 5 access control value with a general access control value.

7. A method according to any one of claims 3 to 6, characterized in that it includes the additional step wherein, if the unique identification value for access to a cellular radiotelephony network and the access control value are not identical, access to the specific cellular radiotelephony network 10 associated with the access identification value is denied and the critical functionalities controlled by the unique electronic circuit are blocked.

8. A method according to any one of the preceding claims, characterized in that the cipher key is stored within the unique electronic circuit at the time of manufacture of the mobile telephone.

15

9. A method according to claim 8, characterized in that the cipher key is stored by a fuse cut-out operation within unique electronic circuit.

10. A mobile telephone characterized in that it includes means for implementing the method according to any one of the preceding claims.

11. A method as substantially herein described.

20

12. A method as substantially herein described with reference to, and as shown in the accompanying drawings.