Abstract:
A method for protection of a chip card from unauthorized use includes: inputting a first identification into a chip card terminal, producing a cipher of at least one first communication parameter using a first symmetric key derived from the first identification, a protected first communication channel being definable between the chip card terminal and the chip card, using the communication parameter, transmitting the cipher via a predefined communication channel from the chip card terminal to the chip card, attempting to decrypt the cipher using a second symmetric key by means of the chip card, the result of decryption only being the first communication parameter if the first symmetric key is identical to the second symmetric key so that the protected first communication channel can only be defined between the chip card terminal and the chip card if the first identification is correct.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to International Patent Application No.: PCT/EP2008/064116, filed Oct. 20, 2008, the entire disclosure of which is hereby incorporated by reference. 
     BACKGROUND 
     The invention relates to a method for protecting a chip card from unauthorized use, to a chip card and to a chip card terminal. 
     To enable a chip card function, prior user identification to the chip card may be necessary, as is known per se from the prior art. The most frequent user identification is the input of a secret identifier, which is generally referred to as a PIN (Personal Identification Number) or as CHV (Card Holder Verification). Such identifiers generally comprise a numeric or alphanumeric character string. For the purpose of user identification, the identifier is input by the user on the keypad of a chip card terminal or a computer to which a chip card reader is connected, and is then sent to the chip card. The latter compares the input identifier with the stored identifier and notifies the terminal or the computer of the result by outputting an appropriate signal. 
     For the PINs, a distinction can be drawn between static PINs and changeable PINs. A static PIN cannot be altered by the user and needs to be memorized by him. If it has been revealed, the card user needs to destroy his chip card in order to prevent misuse by unauthorized parties, and needs to obtain a new chip card with a different static PIN. Similarly, the user requires a new chip card if he or she has forgotten the static PIN. 
     A changeable PIN can be changed by the user as desired. To change the PIN, for security reasons it is always necessary to provide the currently valid PIN at the same time, since otherwise any current PIN could be replaced by a hacker&#39;s own PIN. 
     The situation is different with what are known as Super PINs or PUKs (Personal Unlocking Key). These usually have more places than the actual PIN and are used to reset a PIN&#39;s incorrect input counter (also called “incorrect operation counter”) that is at its maximum value. With the PUK, a new PIN is also transferred to the chip card at the same time because a reset incorrect operation counter is of little use if the PIN has been forgotten. Moreover, this is usually the case, of course, when the incorrect operation counter has reached its maximum value. 
     There are also applications which use transport PINs. The chip card is personalized using a random PIN which the card user received in a PIN letter. When it is first input, however, the chip card asks the card user to replace the personalized PIN with his own. In a similar method, called the “zero PIN method”, the chip card is preassigned a trivial PIN, such as “0000”, and the chip card likewise forces a change when it is first used (in this regard, cf. also DE 35 23 237 A1, DE 195 07 043 A1, DE 195 07 044 C2, DE 198 50 307 C2, EP 0 730 253 B1). Such methods provide what is known as a first user function, which provides the authorized user with the certainty that no unauthorized use of the chip card by a third party has taken place before he uses it for the first time. 
     DE 198 50 307 C2 discloses a method for protecting against misuse in chip cards. The chip card has a first user function which, when the data and/or functions of the chip card are used for the first time, demands that a personal secret number (PIN) which can be selected arbitrarily by the user be prescribed, wherein the input of the personal secret number sets data and/or functions of the chip card to a used status. A later change to the personal secret number is made possible by means of a superordinate unlock code. 
     The prior art has already disclosed methods for checking an identifier in which it is not necessary to transmit the identifier itself, such as Strong Password Only Authentication Key Exchange (SPEKE), Diffie-Hellman Encrypted Key Exchange (DH-EKE), Bellovin-Merritt protocol or Password Authenticated Connection Establishment (PACE). The SPEKE protocol is known from www.jablon.org/speke97.html, U.S. Pat. No. 6,792,533 B2 and U.S. Pat. No. 7,139,917 B2, for example. The DH-EKE protocol is likewise known from www.jablon.org/speke97.html, inter alia. The Bellovin-Merritt protocol is known from U.S. Pat. No. 5,241,599, inter alia. The PACE protocol, which is particularly suitable for elliptic curve cryptography, is known from www.heise.de/security/news/meldung/85024. 
     SUMMARY OF THE INVENTION 
     By contrast, the invention is based on the object of providing an improved method for protecting a chip card from unauthorized use. The invention is also based on the object of providing an improved chip card and an improved chip card terminal. 
     The invention provides a method for protecting a chip card from unauthorized use. The method involves not only the chip card itself but also a chip card terminal. 
     In this context, “chip card terminal” is understood to mean any appliance which is designed for communication with a chip card in order to send chip card commands to the chip card, for example, and to receive corresponding responses from the chip card. The communication between the chip card and the chip card terminal can take place in contact-based fashion, wirelessly, for example using an RFID method, or either in contact-based fashion or wirelessly, particularly using what is known as a dual-mode interface. The chip card terminal may be what is known as a class 1, 2 or 3 chip card reader with or without its own keypad or a computer to which a chip card reader is connected. The chip card terminal may also be a terminal provided for a particular purpose, such as a bank terminal for handling bank transactions, a payment terminal, for example for purchasing electronic tickets, or an access terminal for enabling access to a protected area. 
     In this context, the term “protection of a chip card” is understood to mean the protection of the chip card as a whole or the protection of one or more chip card functions of the chip card. By way of example, the invention involves the protection of a chip card function which is particularly worthy of protection on the chip card, such as a signature function for generating an electronic signature, a payment function, an authentication function or the like. 
     In accordance with one embodiment of the method according to the invention, the authorized user receives a secret identifier, which is generally referred to as a PIN, from the office issuing the chip card. To use the chip card, it is first of all necessary to input an identifier into the chip card terminal, said identifier subsequently being referred to as the PIN′. Only if the PIN′ is identical to the PIN will use of the chip card or of the protected chip card function be possible. 
     To this end, the chip card terminal produces a cipher from at least one first communication parameter using a first symmetric key. The first symmetric key may be the PIN′ itself or may be a symmetric key derived from the PIN′. By way of example, the PIN′ is used as what is known as a seed value for the generation of the first symmetric key by the chip card terminal. 
     The at least one communication parameter is of a nature such that it can define a protected first communication channel between the chip card terminal and the chip card. To be able to set up said protected first communication channel between the chip card and the chip card terminal, the cipher of the first communication parameter, which cipher is obtained using the first symmetric key, is first of all transmitted via a predefined communication channel from the chip card terminal to the chip card. Said predefined communication channel is thus defined as standard for setting up an initial communication between the chip card terminal and the chip card. 
     Following the transmission of the cipher via said predefined communication channel from the chip card terminal to the chip card, the chip card attempts to decrypt said cipher using a second symmetric key. This decryption is successful only if the second symmetric key is the same as the first key, i.e. if the prerequisite PIN′=PIN is met. 
     A communication link can thus be set up via the protected first communication channel only if the condition PIN′=PIN is met, since only in this case is the chip card provided with knowledge of the first communication parameter which allows the protected first communication channel to be stipulated. 
     By way of example, the first communication parameter may be the specification of a transmission frequency, of a frequency hopping scheme, of a coding method and/or of a modulation method. 
     If the condition PIN′=PIN is not met, on the other hand, the first key derived from the PIN′ does not match the second key of the chip card. The result of this is that the decryption of the cipher received from the chip card terminal by the chip card using the second key does not yield the first communication parameter, but rather a second communication parameter, for example, which is different than the first communication parameter. 
     The second communication parameter may have defined a second communication channel which is different than the first communication channel. If the chip card receives a signal on the first communication channel, said signal is ignored, however, since the chip card expects a signal on the second communication channel. As a result, there is thus no communication between the chip card terminal and the chip card if the condition PIN′=PIN is not met. 
     According to one embodiment of the invention, the communication parameter may be a public key from an asymmetric key pair of the chip card terminal. To stipulate a symmetric key for the communication between the chip card terminal and the chip card, for example using the Diffie-Hellman method, the public key of the chip card terminal is encrypted using the first symmetric key obtained from the first identifier and is sent to the chip card via the predefined communication channel. 
     Only if the condition PIN′=PIN is met does the chip card obtain the correct public key for the chip card terminal. The chip card terminal takes the public key of the chip card, which is requested from a key server, for example, and uses the Diffie-Hellman method to generate the third key, whereas the chip card generates a fourth key, likewise using the Diffie-Hellman method, from its private key and the cipher decrypted using the second symmetric key, wherein the fourth key is the same as the third key only if the condition PIN′=PIN is met. 
     The third and the identical fourth symmetric key are used for encrypting signals, particularly chip card commands and responses to such chip card commands, which are interchanged between the chip card terminal and the chip card via the first communication channel. Said first communication channel is at least additionally defined by means of the third key, which is used to encrypt the communication via the first communication channel using a symmetric encryption method. 
     In accordance with one embodiment of the invention, a method of discrete logarithmic cryptography (DLC) is used for the generation of a third key by the chip card terminal and of a fourth key by the chip card, the fourth key being the same as the third key only if the condition PIN′=PIN is met. 
     The third key is stipulated using any method of discrete logarithmic cryptography, in principle, as are described by way of example in the National Institute of Standards and Technology (NIST) standard, NIST Special Publication 800-56A, March 2007, and in Standards for Efficient Cryptography, SEC1: Elliptic Curve Cryptography, Certicom Research, Sep. 20, 2000, Version 1.0. Such methods require the production of what are known as domain parameters for the purpose of the generation of the identical third and fourth keys by the chip card terminal or the chip card. 
     In accordance with one embodiment of the invention, the DLC used is a method for elliptic curve cryptography (ECC), particularly Elliptic Curve Diffie-Hellman (ECDH). 
     In accordance with one embodiment of the invention, the first identifier, i.e. the PIN′, which is input into the chip card terminal is used as what is known as a seed value for the derivation of the first symmetric key. This produces a key of greater length than would be the case if the first identifier were used directly as a key. 
     In accordance with one embodiment of the invention, the chip card stores a second identifier, i.e. the PIN, from which it is possible to derive the second key for the decryption of the cipher initially received from the chip card terminal. The second key can be derived from the second identifier using the second identifier as a seed value. 
     In accordance with one embodiment of the invention, it is not the PIN itself which is stored in the chip card but rather only the second key. The second key is preferably stored in a nonvolatile protected memory area of the chip card. In contrast to the prior art, it is thus not necessary to store the PIN as a reference value in the chip card. 
     In accordance with one embodiment of the invention, the chip card has an incorrect operation counter. If incorrect input of the PIN′ means that the first and second communication channels do not match, the chip card increments or decrements the incorrect operation counter with every message which the chip card receives on a communication channel other than the second or the predefined communication channel. Such messages which the chip card receives on a communication channel other than the second or the predefined communication channel are otherwise ignored by the chip card. If the number of incorrect operations exceeds a prescribed threshold value, the chip card as a whole or a particular chip card function is reversibly or irreversibly locked. 
     In accordance with one embodiment of the invention, the chip card has a first-user function. The unused chip card is in its first-use state, in which a particular communication parameter is stipulated for a first selection of the first communication channel. The chip card changes from its first-use state to a used state when it receives a chip card command on said first communication channel for the first time. For further use of the chip card, the chip card terminal then needs to select another communication parameter. 
     In a further aspect, the invention relates to a chip card with a chip card having an interface for communication with a chip card terminal via a predefined communication channel and a plurality of further communication channels, means for the decryption of a cipher received on the predefined channel, which cipher has been encrypted using a first symmetric key, using a second symmetric key, wherein the decryption yields at least one communication parameter if a first identifier which has been input into the chip card terminal previously is correct, wherein the communication parameter explicitly stipulates one of the further communication channels for the protected communication between the chip card and the chip card terminal. 
     In a further aspect, the invention relates to a chip card terminal having means for the input of a first identifier, means for the production of a cipher from at least one first communication parameter using a first symmetric key derived from the first identifier, wherein the communication parameter can be used to define a protected first communication channel between the chip card terminal and the chip card, and means for sending the cipher to the chip card via a predefined communication channel. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention are explained in more detail below with reference to the drawings, in which: 
         FIG. 1  shows a block diagram of a first embodiment of a chip card based on the invention and of a chip card terminal, 
         FIG. 2  shows a flowchart of an embodiment of a method based on the invention, 
         FIG. 3  shows a block diagram of a further embodiment of a chip card based on the invention and of a chip card terminal, 
         FIG. 4  shows a flowchart of a further embodiment of a method based on the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the figures below, elements which correspond to one another in the various embodiments are denoted by the same reference symbols. 
       FIG. 1  shows a block diagram of a chip card terminal  100 . The chip card terminal  100  has an interface  102  for communicating with a chip card  104  which has a corresponding interface  106 . Preferably, the interfaces  102  and  106  are designed for wireless communication, for example by radio, particularly on the basis of an RFID method. 
     By way of example, the interfaces  102  and  106  are of a nature such that various communication channels can be set up between the interfaces  102 ,  106 , said communication channels differing from one another on a physical and/or logical level. By way of example, communication channels at different transmission frequencies can be set up. It is also possible to set up communication channels on the basis of various frequency hopping schemes. In this context, “frequency hopping” is understood to mean frequency hopping methods which involve the frequencies used for the data transmission being continually changed on the basis of a defined scheme. 
     The interfaces  102 ,  106  may also be in a form such that different communication channels are set up using different coding methods and/or modulation methods, such as frequency modulation, amplitude modulation, phase modulation, pulse width modulation or other modulation methods. 
     The various communication channels which can be set up between the interfaces  102  and  106  are subsequently referred to as the “set of communication channels”. 
     One of the communication channels  108  from the set of communication channels is predefined for the initial communication between the chip card terminal  100  and the chip card  104 . By way of example, the communication channel is predefined in terms of its transmission frequency and the modulation and coding methods to be used. 
     The predefined communication channel is used for the transmission of a cipher  110  for the at least one communication parameter K 1  from the chip card terminal  100  to the chip card  104  in order to notify the chip card  104  of which of the communication channels  112  in the set of communication channels is intended to be used for the subsequent communication with the chip card terminal  100 . 
     The communication parameter K 1  thus contains a specification which explicitly specifies said communication channel  112 . This specification may be provided in the form of a codeword. The chip card  104  may store what is known as a lookup table in a nonvolatile memory, said lookup table containing a respective specification for one of the communication channels in the set of communication channels in association with the possible codewords. 
     For the selection of a communication channel from the set of communication channels, all possible communication channels which can be set up between the interfaces  102 ,  106  may be available or a selection thereof, in which case each of the communication channels in the set of communication channels that is actually able to be used for the communication between the interfaces  102 ,  106  is associated with an explicit codeword which can be transmitted as a communication parameter  110  from the chip card terminal  100  to the chip card  104 . 
     The chip card terminal  100  has a user interface  114 , such as a keypad or a graphical user interface, which can be used to input a first identifier  116 . Said first identifier is subsequently referred to as PIN′ without restricting the general nature. 
     The chip card terminal  100  has at least one processor  118  for executing an application program  120 . The application program  120  can prompt the generation of a chip card command  122  in order to call a particular chip card function  124  of the chip card  104 . In one example, the application program  120  needs the chip card function  124  for an authorization check, for the generation of a digital signature, for checking an authorization, particularly an access authorization, performing a financial transaction or the like. 
     In addition, the processor  118  is further used to execute the program instructions of a communication module  126  which is used for selecting the communication channel  112  from the set of communication channels and hence for selecting the communication parameter  110 . The communication parameter  110  can be selected on the basis of a prescribed scheme or randomly, particularly pseudo-randomly. By way of example, the communication module  126  stores a list of different communication parameters  110  which is processed cyclically. 
     The processor  118  is also used to execute program instructions  128  for symmetric encryption of the communication parameters  110 . The encryption is performed using the PIN′. To this end, the program instructions  128  may contain a key generator  130 . 
     The key generator  130  may be in a form such that it takes the PIN′ as a seed value and generates a first symmetric key, which is subsequently referred to as S 1 . The key S 1  is used for the symmetric encryption of the communication parameter K 1  selected by the communication module  126 . The cipher obtained for the communication parameter K 1  from the symmetric encryption with the key S 1  is transmitted from the interface  102  to the interface  106  via the predefined communication channel  108 . 
     The chip card  104  has a processor  132  which is used to execute the program instructions of a communication module  134 . The communication module  134  is designed to process the communication parameter K 1  possibly received from the chip card terminal  100 . By way of example, the communication module  134  can use the communication parameter K 1  as a key to access an association table, particularly a lookup table, in order to request the parameters for the communication channel  112  selected by the chip card terminal  100 , such as the transmission frequency of said communication channel and/or the coding and modulation methods to be used. 
     The processor  132  is also used to execute program instructions  136  for the symmetric decryption of the cipher  110  which the chip card  104  has received from the chip card terminal  100 . By way of example, the chip card  104  has a protected memory area  138  which stores a second identifier  140 . The second identifier is subsequently referred to as the PIN without restricting the general nature. The PIN is communicated to the authorized user of the chip card separately when the chip card  104  is issued, for example in the form of what is known as the PIN letter. 
     The program instructions  136  may contain a key generator  142  which uses the PIN as what is known as a seed value in order to derive a second key therefrom. This symmetric second key is subsequently referred to as S 2 . 
     Alternatively, the key S 2  may be stored in the protected memory area  138  of the chip card  104  instead of the PIN  140 . The key generator  142  and storage of the PIN  140  in the chip card  104  are then superfluous. In contrast to the prior art, the chip card  104  therefore does not necessarily need to store the PIN  140  as a reference value for checking the correctness of the PIN′  116 . 
     The chip card  104  may also have an incorrect operation counter  144 . The incorrect operation counter  144  is designed such that every incorrect operation of the chip card  104  is counted. The number of incorrect operations is compared with a prescribed threshold value. If this threshold value is reached, at least the chip card function  124  with which the incorrect operation counter  144  is associated is reversibly or irreversibly locked. 
     The chip card  104  may also have a first-use function. By way of example, the first-use status of the chip card  104  is defined by a particular communication parameter which specifies one of the communication channels in the set which needs to be used for the first use of the chip card. 
     To use the chip card  104 , the procedure is as follows: a user inputs the PIN′  116  into the chip card terminal  100  via the user interface  114 . This can be done upon an appropriate request via the application program  120 . The communication module  126  then selects a first of the possible communication parameters from the prescribed list of communication parameters, for example, that is to say the communication parameter K 1 . 
     The key generator  130  takes the PIN′ and generates the key S 1 . The communication parameter K 1  is then encrypted using the symmetric key S 1  by executing the program instructions  128 . The resultant cipher  110  for the communication parameter K 1  is then sent from the interface  102  to the interface  106  of the chip card  104  via the predefined communication channel  108 . 
     If required, the chip card  104  derives the key S 2  from the PIN or accesses the key S 2  directly in the protected memory area  138 . The key S 2  is used to make the attempt at decrypting the cipher  110  received for the communication parameter K 1  from the chip card terminal  100  by virtue of the execution of the program instructions  136  by the chip card  104 . 
     The result of this decryption attempt is a second communication parameter, which is subsequently referred to as K 2  and which is transferred to the communication module  134 . Said communication parameter K 2  is identical to the communication parameter K 1  only if the condition PIN′=PIN is met, since only then can the key S 1  which has been used for the symmetric encryption be the same as the key S 2 , which has been used for the symmetric decryption of the cipher for the communication parameter K 1 . 
     The communication parameter K 2  may define a second communication channel  146 , namely by virtue of the communication module  134  using the communication parameter K 2  to access its association table. Said second communication channel  146  is in turn identical to the first communication channel  112  only if the condition PIN′=PIN is met. 
     Following the transmission of the cipher for the communication parameter K 1  via the predefined communication channel  108 , the chip card terminal  100  generates the chip card command  122 , which is sent via the first communication channel  112  from the interface  102  to the interface  106 . The chip card  104  and the communication module  134  thereof are set to the second communication channel  146  for reception on the basis of the communication parameter K 2 . 
     If the second communication channel  146  matches the first communication channel  112 , the chip card command  122  is processed by the chip card  104  and the chip card function  124  is called. As a result, the chip card  104  generates a response to the chip card command  122  and transmits said response back to the chip card  100  via the first communication channel  112 . 
     If, by contrast, the second communication channel  146  is not identical to the first communication channel  112 , the chip card  104  ignores the chip card command received on the first communication channel  112  and increments the incorrect operation counter  144 . 
     By way of example, the communication channel  108  is defined by a transmission frequency of 9 GHz, the communication channel  112  is defined by a transmission frequency of 10 GHz and the communication channel  146  is defined by a transmission frequency of 11 GHz, the transmission frequencies of the communication channels  112  and  146  differing from one another, since the PIN′ which has been input into the chip card terminal  100  is not the same as the PIN. If the chip card  104  receives a signal on the frequency 10 GHz from the chip card terminal  100  in this case, even though it expected reception on the frequency 11 GHz, this signal is ignored and the incorrect operation counter is incremented. This provides an implicit check on the PIN′ without the need for the PIN′ to be compared directly with the PIN and without the PIN needing to be stored in the chip card. 
       FIG. 2  shows an appropriate flowchart. In step  200 , the PIN′ is input in the chip card terminal. Next, in step  202 , the chip card terminal  100  stipulates the communication parameter K 1  for selecting one of the communication channels from the set of communication channels. In step  204 , the communication parameter K 1  is symmetrically encrypted using the PIN′. This can be done by virtue of a key generator being used to derive from the PIN′ the symmetric key S 1 , which is then used to encrypt the communication parameter K 1 . 
     In step  206 , the cipher produced for the communication parameter K 1  using the key S 1  is transmitted from the chip card terminal to the chip card via a predefined communication channel. 
     In step  208 , the chip card  104  makes the attempt at decrypting the communication parameter K 1  on the basis of the PIN. The correct PIN may be stored in a protected memory area of the chip card and is used to derive a symmetric key S 2 . Alternatively, the key S 2  may also be stored in the protected memory area of the chip card directly. 
     The decryption of the cipher for the communication parameter K 1  with the key S 2  results in a communication parameter K 2 . Said communication parameter K 2  may define a second communication channel in the set. Only if the PIN′ is correct, i.e. if the condition PIN′=PIN is met, are the communication channels specified by the communication parameters K 1  and K 2  identical. 
     In step  210 , the chip card terminal generates a chip card command and sends it to the chip card via the first communication channel, specified by the communication parameter K 1  (step  212 ). In step  214 , the chip card can receive the chip card command only if the second communication channel, for which the chip card is set up for reception, is identical to the first communication channel, i.e. if the condition PIN′=PIN is met. Conversely, the chip card ignores the cipher received on the first communication channel and increments the incorrect operation counter therefor. 
     In one embodiment of the invention, the communication parameter K 1  may be a public key of the chip card terminal. The cipher for said public key, which cipher has been generated using the key S 1  by means of symmetric encryption, is transmitted from the chip card terminal to the chip card. The chip card receives the correct public key of the chip card terminal only if in turn the condition PIN′=PIN is met, since only then is the decryption of the cipher using the key S 2  successful (cf. the embodiment in  FIG. 1 ). The public key of the chip card can be requested by the chip card terminal from an external keyserver, for example, via a network, particularly the Internet. 
     The chip card terminal can use the Diffie-Hellman method to derive a symmetric key S 3  from the private key of the chip card terminal and the public key of the chip card. Accordingly, the chip card can likewise use the Diffie-Hellman method to derive a symmetric key S 4  from the public key of the chip card terminal and its private key. The keys S 3  and S 4  are identical if the condition PIN′=PIN is met. 
     The first communication channel (cf. communication channel  112  in  FIG. 1 ) is at least additionally defined by means of the symmetric keys S 3 =S 4  in this embodiment. This is because the chip card command sent from the chip card terminal to the chip card is encrypted with the symmetric key S 3 , and can be decrypted, i.e. received, by the chip card only if the chip card command can be decrypted using the key S 4 . Otherwise, the chip card command is ignored and the incorrect operation counter is incremented. 
       FIG. 3  shows an embodiment of a chip card based on the invention and of a chip card terminal based on the invention, wherein a method for discrete logarithmic cryptography is used to generate the keys S 3  and S 4 . As an addition to the embodiment shown in  FIG. 1 , the processor  118  is used to execute program instructions  148 , which provide what is known as a Key Establishment Scheme for generating the symmetric key S 3 . 
     The Key Establishment Scheme operates on the basis of a method for discrete logarithmic cryptography (DLC), particularly elliptic curve cryptography (EEC), preferably on the basis of an elliptic curve Diffie-Hellman method (ECDH). To generate the symmetric key S 3 , the program instructions  148  first of all produce first domain parameters, which are subsequently referred to as D 1 . 
     In addition, the communication module  126  can produce a first channel parameter KA 1  or can read it from a prescribed list, such a first channel parameter specifying the physical properties of the first communication channel, for example. The first channel parameter KA 1  corresponds to the channel parameter K 1  in the embodiment in  FIG. 1 . 
     The domain parameters D 1  and the channel parameter(s) KA 1  are encrypted by the program instructions  128  using the key S 1 . The cipher  110  obtained from KA 1 , D 1  using the key S 1  is transmitted from the interface  102  to the interface  106  via the predefined communication channel  108 . 
     The chip card  104  decrypts the cipher  110  using the symmetric key S 2 . As a result of the decryption, the chip card  104  obtains the second channel parameter KA 2 , which corresponds to the communication parameter K 2  in the embodiment in  FIG. 1 . In addition, the chip card obtains the domain parameters D 2 . The channel parameter KA 2  is processed by the communication module  134  in order to establish the physical specification of the second communication channel  146 , for example. 
     As an addition to the embodiment in  FIG. 1 , the chip card  104  has program instructions  150 , the functionality of which corresponds to that of the program instructions  148  and which implement the Key Establishment Scheme on the chip card. 
     The chip card terminal executes the program instructions  148  in order to derive the symmetric key S 3 , which is stored in a memory  152  of the chip card terminal  100 , from the domain parameters D 1 . Accordingly, execution of the program instructions  150  by the chip card  104  derives a symmetric key S 4 , which is stored in a memory  154  of the chip card  104 , from the domain parameters D 2 . 
     The chip card command  122  is encrypted by the chip card terminal with the symmetric key S 3  before being sent and is then transmitted via the first communication channel  112  specified by the channel parameters KA 1 . The chip card command  122  can be received by the chip card  104  only if both KA 2 =KA 1  and D 2 =D 1 , which in turn is possible only if the condition PIN′=PIN is met. 
     Of particular advantage in this embodiment is the fact that the transmission of the domain parameters D 1  via the predefined communication channel  108  cannot be spied out by third parties, since the domain parameters D 1  are transmitted in an encrypted form. 
       FIG. 4  shows an appropriate flowchart. In step  400 , a PIN′ is input into the chip card terminal by a user. The symmetric key Si is derived from the PIN′. 
     In step  402 , the Key Establishment Scheme is started. Next, in step  404 , a set of domain parameters D 1  is produced. The domain parameters D 1  are used for the generation of the symmetric key S 3  by the chip card terminal. In addition, in step  406 , the chip card terminal generates the channel parameter KA 1  or reads it from a prescribed list. 
     In step  408 , the domain parameters D 1  and/or the channel parameters KA 1  are encrypted with the key S 1 . By way of example, the domain parameters D 1  and the channel parameters KA 1  are appended to one another, which results in a single communication parameter which is then encrypted with the key S 1 . Alternatively, only the domain parameters D 1  or only the channel parameters KA 1  or a respective subset of the domain and/or channel parameters are encrypted with the key S 1 . The cipher resulting from the encryption with the key S 1  and any remaining unencrypted domain and/or channel parameters are transmitted from the chip card terminal to the chip card via the predefined channel (cf. communication channel  108  in  FIGS. 1 and 3 ) in step  410 . 
     In step  412 , the chip card attempts to decrypt the cipher using the key S 2 . From this, the chip card  104  obtains the channel parameters KA 2  and the domain parameters D 2 . The chip card  104  derives the key S 4  from the domain parameters D 2 . 
     In step  414 , the chip card terminal  100  generates a chip card command, which is encrypted with the key S 3  (step  416 ) in order to transmit it via the first communication channel defined by the channel parameters KA 1  (cf. communication channel  112  in the embodiments in  FIGS. 1 and 3 ). The chip card terminal  100  sends the chip card command in step  418 . 
     Correct reception of the cipher by the chip card is possible in step  420  only if the second communication channel  146  matches the first communication channel  112 , i.e. if KA 2 =KA 1 , and if, furthermore, the chip card command can be decrypted with the key S 4 , i.e. if S 4 =S 3 . However, the conditions KA 2 =KA 1  and S 4 =S 3  can be met only if the correct PIN′ has been input into the chip card terminal by the user, i.e. if PIN′=PIN. 
     List of Reference Symbols 
     
         
           100  Chip card terminal 
           102  Interface 
           104  Chip card 
           106  Interface 
           108  Predefined communication channel 
           110  Communication parameter 
           112  First communication channel 
           114  User interface 
           116  PIN′ 
           118  Processor 
           120  Application program 
           122  Chip card command 
           124  Chip card function 
           126  Communication module 
           128  Program instructions 
           130  Key generator 
           132  Processor 
           134  Communication module 
           136  Program instructions 
           138  Protected memory area 
           140  PIN 
           142  Key generator 
           144  Incorrect operation counter 
           146  Second communication channel 
           148  Program instructions 
           150  Program instructions 
           152  Memory 
           154  Memory