Patent Publication Number: US-6658328-B1

Title: Passive function control system for a motor vehicle

Description:
TECHNICAL FIELD 
     The present invention relates to a passive function control system for a motor vehicle. More particularly, the present invention relates to a passive entry system for a keyless vehicle. 
     BACKGROUND OF THE INVENTION 
     Passive entry systems for gaining access to the interior of a vehicle are known. Known passive entry systems include a vehicle based transceiver and a portable transceiver that is carried by an authorized user. When the authorized user approaches the vehicle, the vehicle based transceiver transmits a low frequency challenge signal. In one known system, the challenge signal is transmitted in response to the authorized user triggering a sensor in a door handle of the vehicle. The challenge signal is a random number. 
     In response to receiving the challenge signal, the portable transceiver generates a challenge response signal. In generating the challenge response signal, the portable transceiver encrypts the random number using an encryption key. The encrypted random number is transmitted as the challenge response signal. 
     While the vehicle based transceiver is waiting for the challenge response signal, the vehicle based transceiver encrypts the random number using an encryption key that is identical to the encryption key of the portable transceiver. The result of the encrypted random number is an expected response. Upon receiving the challenge response signal from the portable transceiver, the vehicle based transceiver compares the challenge response signal received to the expected response. The vehicle based transceiver controls a locking mechanism of the vehicle to allow access into the interior of the vehicle when the challenge response signal matches the expected response. 
     Known passive entry systems are susceptible to “dictionary” attacks. In a dictionary attack, an unauthorized user uses a device to transmit a plurality of random challenge messages in the vicinity of the portable transceiver. The portable transceiver responds to each random challenge message with a challenge response signal. The unauthorized user uses another device to record the challenge response signals transmitted from the portable transceiver. After building a database or dictionary of challenge response signals, the unauthorized user goes to the vehicle and begins triggering the vehicle based transceiver to transmit challenge signals. The unauthorized user transmits responses from the dictionary. If the unauthorized user&#39;s dictionary has the valid challenge response signal to the challenge signal transmitted from the vehicle based transceiver, the unauthorized user is allowed to access the interior of the vehicle. 
     The dictionary attack is a statistical approach to gaining access to the vehicle. The probability of gaining access through the use of the dictionary attack is dependent upon the number of challenge response signals stored in the dictionary and the word size or number of bits dedicated to the random number of the challenge signal. A need exists for a passive entry system that is not susceptible to a dictionary attack. 
     SUMMARY OF THE INVENTION 
     In accordance with an exemplary embodiment of the present invention, a passive function control system for a vehicle is provided. The system comprises a vehicle based transceiver for transmitting a challenge signal. The vehicle based transceiver includes a first controller, a random number generator, a first encryption key, and a memory for storing an identification code. The first controller provides the challenge signal having a random number from the random number generator and the identification code from the memory. The first controller encrypts at least a portion of the random number and at least a portion of the identification code of the challenge signal using the first encryption key. The system also comprises a portable transceiver for receiving the challenge signal and for transmitting a challenge response signal. The portable transceiver comprises a second controller, a second memory for storing a reference identification code, and a decryption key corresponding to the first encryption key of the vehicle based transceiver. The second controller decrypts the encrypted portions of the challenge signal using the decryption key, compares the identification code to the reference identification code, and outputs the challenge response signal having the random number only in response to identification code comparison indicating a match. The first controller responds to the challenge response signal when the random number of the challenge response signal is related to the random number from the random number generator. 
     In accordance with the present invention, an exemplary method of operation of a passive function control system of a vehicle is provided. During the method, a challenge signal is provided which includes a random number and an identification code. At least a portion of the random number and at least a portion of the identification code of the challenge signal are encrypted. The challenge signal is transmitted from a vehicle based transceiver. The challenge signal is received at a portable transceiver. The encrypted portions of the challenge signal are decrypted. The identification code is compared to a reference identification code. A challenge response signal having the random number is transmitted only in response to identification code comparison indicating a match. The challenge response signal is received at the vehicle based transceiver. The vehicle based transceiver responds to the challenge response signal when the random number of the challenge response signal is related to the random number. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other features of the present invention will become apparent to those skilled in the art to which the present invention relates upon reading the following description with reference to the accompanying drawings, in which: 
     FIG. 1 is a schematic functional block diagram of a passive function control system constructed in accordance with the present invention; 
     FIG. 2 is a flow diagram illustrating a process of operation of a portable transceiver of FIG. 1; 
     FIG. 3 is a flow diagram illustrating a process of operation of a vehicle based transceiver of FIG. 1; and 
     FIG. 4 schematically illustrates the encryption of a random number and a portion of an identification code in the vehicle based transceiver of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a schematic functional block diagram of a passive function control system  10  constructed in accordance with an exemplary embodiment of the present invention. The passive function control system  10  will be discussed below in the context of permitting an authorized user access into the interior of a vehicle  12  through an entryway (i.e., a door of the vehicle). The passive function control system  10  may also be used for other functions, such as starting the vehicle&#39;s ignition once access into the vehicle&#39;s interior is gained. 
     The passive function control system  10  of FIG. 1 includes a vehicle based transceiver  14  and a portable transceiver  16 . The vehicle based transceiver  14  is attached to the vehicle  12 , such as in the vehicle&#39;s instrument panel (not shown). The portable transceiver  16  may take to the form of a key fob or a credit card and is easily carried by an authorized person or user. 
     The vehicle based transceiver  14  includes a controller  18 . Preferably, the controller  18  is a microcomputer. Alternatively, the controller  18  may be formed from analog or discrete circuit components or an application specific integrated circuit. 
     The controller  18  is operatively connected to a power source  20 . Preferably, the power source  20  is the vehicle battery through appropriate regulating circuitry (not shown). The controller  18  illustrated in FIG. 1 receives electrical power from the power source  20  and controls application of electrical power to the other components of the vehicle based transceiver  14  that require electrical energy. 
     A user proximity sensor  22  is operatively connected to the controller  18 . The user proximity sensor  22  illustrated in FIG. 1 is a door handle switch. The door handle switch  22  is operative to send a proximity signal to the controller  18  when a user touches a door handle of the vehicle  12 . The door handle switch  22  is generally known in the art. As an alternative to a door handle switch  22 , any known device that senses a user&#39;s proximity to the vehicle and provides a proximity signal to the controller  18  in response to a user&#39;s proximity to the vehicle may be used as the user proximity sensor. 
     The controller  18  has two modes of operation; a sleep mode and a function mode. The sleep mode reduces the power consumption of the vehicle based transceiver  14 . The controller  18  enters the sleep mode after a predetermined period of inactivity. When the controller  18  receives the proximity signal from the door handle switch  22 , the controller enters the function mode. 
     A random number generator  24  is also operatively connected to the controller  18 . Alternatively, the random number generator  24  may form a portion of controller  18  or may include software operating in the controller. The random number generator  24  is a known device that executes a program or algorithm to generate a random number. The random number is placed in the form of a digital word having a given number of bits. Thus, the random number is only random in the fact that the random number generated is generally unpredictable and no number is any more likely to occur at a given time or place in the sequence of the random number than any other number. The random number generator provides the random number to the controller  18 . 
     A memory  26  is also operatively connected to the controller  18 . Alternatively, the memory  26  may form a portion of controller  18 . The memory  26  is a nonvolatile memory in which is stored an identification code. When prompted by the controller  18 , the memory  26  provides the identification code to the controller. 
     The identification code is a digital word having a given number of bits. The identification code includes a most significant bit portion and a least significant bit portion. The most significant bits are the highest order or leftmost bits in the digital word. The least significant bits are the lowest order or rightmost bits of the digital word. 
     Transmit circuitry  28  and a transmitting antenna  30  are also operatively connected to the controller  18 . As will be described in detail below, the controller  18  outputs a challenge signal to the transmit circuitry  28 . The transmit circuitry  28  transmits the challenge signal via the transmitting antenna  30 . Preferably, the transmitted challenge signal is a low frequency signal. In one embodiment, the challenge signal has a frequency of about  125  kHz. Preferably, the low frequency challenge signal has a range of approximately one meter from the transmitting antenna  30 . 
     In the embodiment illustrated in FIG. 1, the transmitting antenna  30  is a loop antenna that extends from the controller  18  to a position near the door handle of the vehicle  12 . Preferably, a transmitting antenna  30  is associated with each door handle of the vehicle  12 . 
     Receive circuitry  32  and a receiving antenna  34  are also operatively connected to the controller  18 . The receiving antenna  34  receives a challenge response signal. The receive circuitry  32  demodulates and filters the challenge response signal and provides the challenge response signal to the controller  18 . The filtering of the challenge response message removes noise that is located outside of a frequency range in which the challenge response message is transmitted. In one embodiment, the challenge response signal received is a radio frequency signal. 
     First and second encryption keys  36  and  38 , respectively, are also operatively connected to the controller  18 . Alternatively, the first and second encryption keys  36  and  38  may form a portion of controller  18  or may be included as part of the software operating in the controller. The first and second encryption keys  36  and  38  each include an encryption code, i.e., a sequence of data, that is used to encrypt other data. The controller  18  uses the encryption codes of the first and second encryption keys  36  and  38  to encrypt portions of the challenge signal. Preferably, the first and second encryption keys  36  and  38  have different encryption codes. 
     A function mechanism  40  is also operatively connected to the controller  18 . The controller  18  controls operation of the function mechanism  40 . For example, in the embodiment of FIG. 1, the function mechanism  40  is a door locking mechanism. The function mechanism  40 , i.e., door locking mechanism, receives function signals from the controller  18  and, in response to the function signals, controls the locking and the unlocking of the vehicle doors. 
     The portable transceiver  16  includes a controller  42 . Preferably, the controller  42  is a microcomputer. Alternatively, the controller  42  may be formed from analog or discrete circuit components or an application specific integrated circuit. 
     The controller  42  is operatively connected to a power source  44 . Preferably, the power source  44  is a long life battery. The controller  42  illustrated in FIG. 1 receives electrical power from the power source  44  and controls distribution of electrical power to the other components of the portable transceiver  16  that require electrical energy. 
     As an alternative to having the power source  44 , the portable transceiver  16  may be powered by induction. When powered by induction, the low frequency challenge signal transmitted by the vehicle based transceiver  14  induces a current in the portable transceiver  16 . The induced power is sufficient for operating the portable transceiver. 
     The controller  42  of the portable transceiver  16  has two operating modes; a sleep mode and a function mode. In the sleep mode, the controller  42  uses very little or no electrical energy. The controller  42  defaults to the sleep mode. Upon receiving a challenge signal, the controller  42  “wakes up” and enters the function mode. Operation of the controller  42  in the function mode is described below. 
     A memory  46  is also operatively connected to the controller  42  of the portable transceiver  16 . Alternatively, the memory  46  may form a portion of controller  42 . The memory  46  is a nonvolatile memory in which is stored a reference identification code. When prompted by the controller  42 , the memory  46  provides the reference identification code to the controller. 
     The reference identification code is a digital word and is identical to the identification code of the associated vehicle based transceiver  14 . Thus, the most significant bits and the least significant bits of the reference identification code are identical to the most significant bits and the least significant bits of the identification code. 
     Receive circuitry  48  and a receiving antenna  50  are operatively connected to the controller  42 . The receiving antenna  50  receives the challenge signal that the vehicle based transceiver  14  transmits. The receive circuitry  48  demodulates and filters the challenge signal and provides the challenge signal to the controller  42 . The filtering of the challenge signal removes noise that is located outside of a frequency range in which the challenge signal is transmitted. 
     Transmit circuitry  52  and a transmitting antenna  54  are also operatively connected to the controller  42 . The controller  42  outputs a challenge response signal to the transmit circuitry  52 . The transmit circuitry  52  transmits the challenge response signal via the transmitting antenna  54 . Preferably, the transmitted challenge response signal is a radio frequency signal. 
     A decryption key  56  and an encryption key  58  are also operatively connected to the controller  42 . Alternatively, the decryption key  56  and the encryption key  58  may form a portion of controller  42  or may be included as part of the software operating in the controller. The decryption key  56  includes a decryption code or a sequence of data that is used to decrypt received messages. The controller  42  uses the decryption code of the decryption key  56  to decrypted data received in the challenge signal. The encryption key  58  includes an encryption code or a sequence of data that is used to encrypt messages. The controller  42  uses of the encryption code of encryption key  58  to encrypted data to be output in the challenge response signal. Preferably, the decryption key  56  and the encryption key  58  have different codes. 
     Operation of the passive function control system  10  is discussed below. During the discussion, the term “user” is used to mean any person or thing that initiates a challenge signal from the vehicle based transceiver  14 . The term “authorized user” is used to mean any user having possession of the associated portable transceiver  16 . The term “unauthorized user” is used to mean any user not having. possession of associated the portable transceiver  16 . A user having a similar portable transceiver with a different reference identification code stored in its memory is an unauthorized user. 
     The controller  18  of the vehicle based transceiver  14  is generally in the sleep mode. In the sleep mode, the controller  18  monitors for a proximity signal from the door handle switch  22 . When a user initiates the door handle switch  22  to provide a proximity signal to the controller  18 , the controller “wakes up” and enters a function mode. 
     In the function mode, the controller  18  of the vehicle based transceiver  14 , prompts the memory  26  to provide the identification code, prompts the random number generator  24  to provide a random number, and prompts the first encryption key  36  to provide its encryption code. The controller  18  uses the encryption code from the first encryption key  36  to encrypt at least a portion of the random number and at least a portion of the identification code. As shown schematically in FIG. 4, in one embodiment, the controller  18  uses the encryption code, indicated at  402  in FIG. 4, from the first encryption key  36  to encrypt the entire random number  404  and the most significant bits of the identification code  406  to get the encrypted portion  412 . 
     The controller  18  then assembles a challenge signal to be transmitted. The challenge response signal includes a message packet. In one exemplary embodiment, the message packet includes a 64-bits. The message packet includes the encrypted portion of the identification code, the encrypted portion of the random number, any non-encrypted portions of the identification code and the random number, and a wake-up code or preamble. The wake-up code is a digital word that is to indicate to the controller  42  of the portable transceiver  16  to enter the function mode. The message packet may also include other bits, such as checksum bits. With reference to FIG. 4, the message packet would include the encrypted portion  412 , including the encrypted random number and the encrypted most significant bits of the identification code, and the non-encrypted least significant bits of the identification code  414 , a wake-up code (not shown), and any other bits, such as checksum bits (not shown). 
     After assembling the challenge signal, the controller  18  of the vehicle based transceiver  14  outputs the challenge signal to the transmit circuitry  28 . The transmit circuitry  28  transmits the challenge signal, which includes the message packet, via the transmitting antenna  30 . 
     After the challenge signal is transmitted, the controller  18  of the vehicle based transceiver  14  performs two functions. First, the controller  18  monitors receive circuitry  32  for a challenge response signal. Second, the controller  18  calculates an expected response from the portable transceiver  16 . 
     To calculate the expected response from the portable transceiver  16 , the controller  18  prompts the second encryption key  38  for its encryption code. After receiving the encryption code from the second encryption key  38 , the controller  18  encrypts the random number that was received from the random number generator  24  using the encryption code from the second encryption key  38 . The controller  18  saves the expected response for comparison to any received challenge response signals. Alternatively, the expected response may be the random number, non-encrypted. 
     The antenna  50  of the portable transceiver  16  receives the transmitted challenge signal. The antenna  50  transfers the received challenge signal to receive circuitry  48 . Receive circuitry  48  demodulates and filters the received challenge signal and transfers the received challenge signal to controller  42 . 
     When the controller  42  of the portable transceiver  16  receives the challenge signal, the wake-up code of the message packet causes the controller  42  of the portable transceiver to enter its function mode. The controller  42  then prompts its memory  46  to provide the reference identification code. Upon receiving the reference identification code, the controller  42  compares the non-encrypted portion of the identification code of the received message packet of the challenge signal, if a portion of the identification code is non-encrypted, with a corresponding portion of the reference identification code. For example, if receiving a message packet having the encrypted portion  412  shown in FIG. 4, the controller  42  compares the least significant bits of the identification code  414  with the least significant bits of the reference identification code. 
     If the non-encrypted portion of the identification code fails to match the corresponding portion of the reference identification code, the controller  42  of the portable transceiver  16  ignores the challenge signal and returns to the sleep mode. If the non-encrypted portion of the identification code matches the corresponding portion of the reference identification code, the controller  42  prompts the decryption key  56  to provide its decryption code. The controller  42  then decrypts the encrypted portions of the message packet of the challenge signal. For example, with reference to FIG. 4, the controller  42  will decrypt the encrypted portion  412  to get the random number  404  and the most significant bits of the identification code  406 . 
     Since the decryption code of the decryption key  56  corresponds to the encryption code of the first encryption key  36 , decryption of the encrypted portions of the message packet results in the random number and a remainder of the identification code. The controller  42  then compares the remainder of the identification code, the most significant bits  406  in FIG. 4, to a corresponding portion of the reference identification code. If the remainder of the identification code fails to match the corresponding portion of the reference identification code, the controller  42  ignores the challenge signal and returns to the sleep mode. If the remainder of the identification code matches the corresponding portion of the reference identification code, the controller  42  assembles a challenge response signal to be transmitted. 
     In assembling the challenge response signal, the controller  42  prompts the encryption key  58  for its encryption code. The controller  42  uses the encryption code from the encryption key  58  to encrypt at least a portion of the random number. The encryption code of the encryption key  58  corresponds to the encryption code of the second encryption key  38  of the vehicle based transceiver  14 . The challenge response signal may also include other portions such as a preamble. As an alternative to including the encrypted random number, the challenge response signal may include the random number, non-encrypted. 
     The controller  42  then outputs the challenge response signal to transmit circuitry  52  of the portable transceiver  16 . The transmit circuitry  52  transmits the challenge response signal via its antenna  54 . 
     The receiving antenna  34  of the vehicle based transceiver  14  receives the challenge response signal. The challenge response signal is transferred to the receive circuitry  32  of the vehicle based transceiver  14 . In the receive circuitry  32 , the challenge response signal is demodulated and filtered. 
     The challenge response signal is sent to the controller  18 . 
     In response to receiving the challenge response signal, the controller  18  compares the encrypted random number of the challenge response signal, or the non-encrypted random number if the random number is not encrypted in the portable transceiver  16 , to the expected response that the controller calculated. If the encrypted random number (or non-encrypted) and the expected response fail to match, the message packet is ignored and access into the vehicle  12  is denied. If the encrypted (or non-encrypted) random number and the expected response match, the controller  18  outputs a function signal to the function mechanism  40  to control the function mechanism to permit access into the interior of the vehicle  12 . 
     FIG. 2 is a flow diagram illustrating a process  200  of operation of a portable transceiver  16  of FIG.  1 . The process  200  starts at step  202  in which the controller  42  is initialized, memories are cleared and set to initial values, and flags are set to initial conditions, etc. The process  200  then proceeds to step  204 . At step  204 , the portable transceiver  16  is in a sleep mode or a low power consumption mode. The process  200  proceeds to step  206 . At step  206 , a determination is made as to whether a challenge signal is received. If the determination at step  206  is negative, the process  200  returns to step  204 . If the determination at step  206  is affirmative, the process  200  proceeds to step  208 . 
     At step  208 , the controller  42  of the portable transceiver  16  wakes up and enters the function mode. As part of step  208 , the controller  42  prompts memory  46  to provide the reference identification code. The process  200  then proceeds to step  210 . 
     At step  210 , a determination is made as to whether the non-encrypted portion of the identification code sent in the challenge signal matches a corresponding portion of the reference identification code. In one embodiment, at step  210 , a determination is made as to whether the least significant bits of the identification code  414  match the least significant bits of the reference identification code. If the determination at step  210  is negative, the process  200  returns to step  204 . If the determination at step  210  is affirmative, the process  200  proceeds to step  212 . 
     At step  212 , the process  200  decrypts the encrypted portions of the received challenge signal. During step  212 , the controller  42  of the portable transceiver  16  prompts the decryption key  56  to provide its decryption code. The controller  42  uses the decryption code to decrypt the encrypted portions. After the encrypted portions are decrypted, the process  200  proceeds to step  214 . In one embodiment, the decryption at step  212  results in the random number  404  and the most significant bits of the identification code  406 . 
     At step  214 , the process  200  determines whether the encrypted portion of the identification code, that was decrypted at step  212 , matches the corresponding portion of the reference identification code. For example, step  214  determines if the most significant bits of the identification code  406  match the most significant bits of the reference identification code. If the determination at step  214  is negative, the process  200  returns to step  204 . If the determination at step  214  is affirmative, the process  200  proceeds to step  218 . At step  218 , the portable transceiver transmits the challenge response signal having the random number. 
     Alternatively, in response to an affirmative determination at step  214 , the process  200  may proceed to step  216 . At step  216 , the controller  42  of the portable transceiver  16  prompts the encryption key  58  for its encryption code. The controller  42  encrypts the random number using the encryption code. The controller  42  outputs a challenge response signal that includes the encrypted random number. The process  200  then proceeds to step  218  in which the portable transceiver  16  transmits the challenge response signal. 
     By first comparing a clear or non-encrypted portion of the identification code and then, if a match is found, comparing the encrypted portion of the identification code, the verification speed of the portable transceiver  16  is increased and power consumption within the portable transceiver is decreased if a match is not determined. For example, if the least significant bits of the identification code  414  and the reference identification code do not match, the controller  42  immediately resumes the sleep mode without further comparison of the identification code. Since fewer than all of the bits of the identification code are compared when the non-encrypted portion of the identification code does not match the corresponding portion of the reference identification code, the controller  42  returns to the sleep mode sooner than if all of the identification code bits were compared and thus, power consumption within the portable transceiver  16  is decreased. 
     FIG. 3 is a flow diagram illustrating a process  300  of operation of a vehicle based transceiver  14  of FIG.  1 . The process  300  begins at step  302  in which the controller  18  is initialized, memories are cleared and set to initial values, and flags are set to initial conditions. The process  300  then proceeds to step  304 . At step  304 , a determination is made as to whether a user has touched the door handle of the vehicle  12 , i.e., initiated a proximity signal to the controller  18 . If the determination in step  304  is negative, the process  300  cycles back to step  304  until an affirmative response is determined. If the determination at step  304  is affirmative, the process  300  proceeds to step  306 . 
     At step  306 , the controller  18  prompts the random number generator for a random number. The process  300  proceeds to step  308 . At step  308 , the controller  18  prompts the first encryption key  36  for its encryption code. The controller  18  uses the encryption code from the first encryption key  36  to encrypt at least portions of the random number and at least a portion of the identification code. For example, in FIG. 4, the controller  18  encrypts the entire random number  404  and the most significant bits of the identification code  406 . The controller  18  then assembles a challenge signal having the encrypted portions of the random number and the identification code and any non-encrypted portions of the random number and the identification code. The process  300  then proceeds to step  310 . 
     At step  310 , the vehicle based transceiver  14  transmits the challenge signal. The challenge signal includes an encrypted portion and a non-encrypted portion. The process  300  then proceeds to step  312 . 
     At step  312 , the controller  18  of the vehicle based transceiver  14  calculates an expected response from the portable transceiver  16 . If the process  200  described above for the portable transponder  16  proceeds from directly to step  218  in response to an affirmative determination at step  214 , then the expected response is the random number that was generated by the random number generator  24 . However, if the process  200  proceeds to step  216  in response to an affirmative determination at step  214 , then to calculate the expected response, the controller  18  encrypts the random number using the encryption code of the second encryption key  38 . The process  300  then proceeds to step  314 . 
     At step  314 , a determination is made as to whether a challenge response signal has been received. If the determination in step  314  is negative, the process  300  proceeds to step  316 . At step  316 , a count is set equal to the previous count plus one. The count is initially set equal to zero at step  302 . The process  300  proceeds from step  316  to step  318 . At step  318 , a determination is made as to whether the count equals a predetermined value, shown as X. If the determination at step  318  is negative, the process  300  returns to step  314 . If the determination at step  318  is affirmative, the process  300  proceeds to step  332 . At step  332 , the count is reset equal to zero. The process  300  then returns to step  304 . 
     If the determination at step  314  is affirmative, the process  300  proceeds to step  320 . At step  320 , the controller  18  compares the random number received in the challenge response signal to the expected response that the controller calculated. If the process  200  of the portable transceiver  16  included step  216 , then the encrypted random number is compared to the expected response. The process  300  then proceeds to step  322 . At step  322 , a determination is made as to whether the received random number matches the expected response. If the determination is negative, the process  300  proceeds to step  324  in which access to the interior of the vehicle  12  is denied. From step  324 , the process  300  proceeds to step  326 . In step  326 , the expected response is cleared or reset. The process  300  then returns to step  304 . Alternatively, the process  300  may proceed from step  324  back to step  314  and wait for another response. 
     If the determination in step  322  is affirmative, the process  300  proceeds to step  328 . At step  328 , the controller  18  outputs a function signal to the function mechanism  40  and access into the interior of the vehicle  12  is permitted. The process  300  then proceeds to step  330  and the process ends. 
     The processes illustrated in the flow diagrams of FIGS. 2 and 3 prevent dictionary attacks. The portable transceiver  16  will not generate a challenge response signal unless the identification code is received. A portion of the identification code is encrypted along with a random number. Thus, an attacker is hindered from obtaining the identification code. As a result, the attacker is unable to build a dictionary for use in attacking the system. 
     From the above description of the invention, those skilled in the art will perceive improvements, changes and modifications. Such improvements, changes and modifications within the skill of the art are intended to be covered by the appended claims.