Patent Publication Number: US-7900045-B2

Title: Method to authenticate an accessory

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to electronic devices, and, more specifically to a system for authenticating an accessory used with an electronic device. 
     2. Background of the Invention 
     Manufacturers of electronic devices (e.g. cellular telephones) have an interest in ensuring that accessories (e.g., batteries) operating with the devices meet the standards of the manufacturer. One way of doing this is to employ a system that authenticates accessories used with the manufactured devices. 
     To reduce the number of unauthorized accessories, certain relatively low-cost semiconductors enable an accessory to be authenticated by a device to which it is connected. These semiconductors typically contain a hashing function and a secret hashing key. To use such a system, both the authenticating device and the accessory have “pre-shared” knowledge of the secret hashing key. The authenticating device generates a seemingly random challenge message and sends it to the accessory. The accessory computes a hash over a combination of the secret hashing key, the received challenge message, and possibly additional padding data. Since the authenticating device knows the secret hashing key, it performs the same computation to determine the expected hash digest. The accessory returns the resulting digest value to the authenticating device, which compares an internally-calculated response to the response received from the accessory. If the expected and received hash digests match, then the device has been authenticated and normal operation of the device will be allowed. Such devices may be vulnerable because the authenticating device must store the secret hashing key value. 
     Another method of authentication uses public key encryption, that uses both a public key that is made publicly available and a private key that is stored only by the device. The public and private keys are inverses and are asymmetrical If the accessory were to store a private asymmetric key, the phone would send a random challenge to the accessory. The accessory would either encrypt or digitally sign the challenge using the private key and return it. The device would use a trusted public key either to decrypt the response and see if it matched what was sent, or to verify the signature on the response. However, implementing this in an accessory would add substantially to the cost of the accessory. 
     Certain devices lack the ability to store a key securely. Even in devices with a hardware-based capability of securing the value of the key, there is still a problem of how to provision the value of the key into the accessories in a secure manner. 
     There are several different techniques used to disable authentication algorithms in devices, including collision attacks and preimage attacks. A collision attack on a cryptographic hash tries to find two arbitrary inputs that will produce the same hash value, i.e., a hash collision. A hash collision is a situation that occurs when two distinct inputs into a hash function produce identical outputs. One desirable property of cryptographic hash functions is that it may be impractical to find a collision. Most hash functions have potential collisions, but with good hash functions they occur less often than with bad ones. 
     There are two types of preimage attacks: (1) a first preimage attack: given a hash h, find a message m such that hash(m)=h; and (2) a second preimage attack: given a fixed message m1, find a message m2 such that hash(m2)=hash(m1). A preimage attack differs from a collision attack in that there is a fixed hash or message that is being attacked. These attacks can be used by a sophisticated attacker to determine the value of the hashing key, or at least determine the expected response to a challenge, from such devices. Doing so could allow an attacker to make cloned or counterfeit devices. 
     Therefore, there is a need for a system that authenticates an accessory in which a key to authenticating an accessory cannot be derived from either the accessory or the device with which the accessory is used. 
     SUMMARY OF THE INVENTION 
     The disadvantages of the prior art are overcome by the present invention which, in one aspect, is an apparatus used by a plurality of devices to authenticate an accessory configured to operate with a device of the plurality of devices. The accessory stores a first key and employs an authentication circuit that applies an authentication algorithm to the first key and a challenge received from the device, thereby generating a response corresponding to the challenge. The apparatus includes a challenge and response memory and an enabling circuit. A challenge and response memory is in each device of the plurality of devices and stores a subset of a set of stored challenges and stored responses. Each stored response corresponds to a different one of the stored challenges and is pre-computed by applying the corresponding stored challenge and a second key to the authentication algorithm. The subset stored in the challenge and response memory of a first device of the plurality of devices is different from the subset stored in the challenge and response memory of a second device of the plurality of devices. The enabling circuit in each device of the plurality of devices transmits a stored challenge to the accessory and receives a received response therefrom. The enabling circuit also compares the received response to the stored response corresponding to the stored challenge sent to the accessory. The enabling circuit is configured to generate a normal operation enabling signal that enables normal operation of the device when the received response matches the stored response and is configured not to generate the normal operation enabling signal when the received response does not match the stored response. 
     In another aspect, the invention is a wireless telephone that includes a wireless telephone communication circuit, a battery, which powers the wireless telephone communication circuit, a memory and a processor. The battery stores a key and employing an authentication circuit that applies an authentication algorithm to the key and to a challenge received from the telephone so as to generate a response corresponding to the challenge. The memory stores a set of challenges and a set of pre-computed responses. Each response corresponds to a different challenge of the set of challenges. The processor transmits a selected challenge of the set of challenges to the battery and receives a response from the battery. The processor also compares the response received from the battery to a response stored in the memory corresponding to the challenge sent to the battery. The processor also causes a selected one of the battery and the wireless telephone communication circuit to operate in an other than normal operating state when the response received from the battery does not correspond to the response corresponding to the challenge stored in the memory. 
     In yet another aspect, the invention is a method of authenticating accessories configured to work with a device, in which an authentication algorithm is applied to a set of challenges and a key, thereby generating a corresponding set of responses. The set of challenges and the corresponding set of responses is stored in a memory associated with the device, without storing the key. A challenge is transmitted from the set of challenges to an accessory having circuitry that applies the challenge and the key to the authentication algorithm and that generates a response based on the challenge and the key. The response is received from the accessory and the response received from the accessory is compared to the response stored in the memory that corresponds to the challenge sent to the accessory. If the response received from the accessory matches the response corresponding to the challenge sent to the accessory, then normal operation of the device is enabled. Otherwise, normal operation of the device is not enabled. 
     These and other aspects of the invention will become apparent from the following description of the preferred embodiments taken in conjunction with the following drawings. As would be obvious to one skilled in the art, many variations and modifications of the invention may be effected without departing from the spirit and scope of the novel concepts of the disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES OF THE DRAWINGS 
         FIG. 1  is a schematic diagram showing one embodiment of a system for authenticating an accessory. 
         FIG. 2A  is a schematic diagram showing a cellular telephone embodiment. 
         FIG. 2B  is a Venn diagram showing the set of all possible challenge/response pairs and subsets thereof. 
         FIG. 2C  is a schematic diagram showing challenge/response pairs corresponding to the subsets shown in the Venn diagram shown in  FIG. 2B . 
         FIG. 3  is a flow chart showing one method for authenticating an accessory. 
         FIG. 4  is a flow chart showing an accessory acting in cooperation with the method shown in  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A preferred embodiment of the invention is now described in detail. Referring to the drawings, like numbers indicate like parts throughout the views. As used in the description herein and throughout the claims, the following terms take the meanings explicitly associated herein, unless the context clearly dictates otherwise: the meaning of “a,” “an,” and “the” includes plural reference, the meaning of “in” includes “in” and “on.” 
     As shown in  FIG. 1 , one embodiment includes an apparatus  100  used by a device  110  that works in cooperation with an accessory  120 . The device  110  includes an enabling circuit  112  that is in communication with a challenge/response memory  114 . The accessory  120  includes an authentication circuit  122  that manipulates data included in a challenge  130  received from the device  110  and that generates a response  132  based on the challenge  130  and a key stored by the accessory  120 . When the device  110  transmits a challenge to the accessory  120 , the accessory  120  generates a response thereto and transmits the response to the device  110 . The device  110  compares the response to a response stored in the memory  114  and, if they are the same, allows normal operation of the device  110  and the accessory  120 . If the responses are different, then normal operation of the device  110 , the accessory  120 , or both, is not allowed. 
     A wireless telephone-specific embodiment is shown in  FIG. 2 , in which the device is a cellular telephone  210  and the accessory is a battery unit  220  used to power the cellular telephone  210 . The accessory includes an authentication circuit  222  that is coupled to an electrical contact  224  used to make contact with the telephone  210 . The authentication circuit  222  applies an authentication algorithm to a challenge received from the telephone  210 , along with a securely stored key, to generate a response that it transmits to the telephone  210 . The authentication algorithm could include a cryptographic hashing algorithm or an encryption algorithm (such as, for example, a symmetric encryption algorithm or an asymmetric encryption algorithm). The telephone  210  includes a processor  212  or other type of logic circuit that is used to initiate an authentication action with the battery  220  through an electrical contact  216  (that may be electrically coupled to the contact  224  of the battery  220 ) and enable operation of the telephone  210 . The processor  212  is in data communication with a memory  214  that stores a plurality of challenges  232  and pre-computed responses  230  (which are computed using an algorithm and key that is functionally the same as the algorithm and key employed by the accessory) each of which corresponds to a different one of the challenges  232 . 
     The telephone  210  does not store the key used to compute the responses  230 . Typically, during manufacture, the memory  214  is loaded with a subset of all possible challenges and their corresponding responses. Different telephones&#39; memories  214  are loaded with different subsets of challenge/response pairs (which could be chosen using a pseudo-random process of a type well known to the art), thereby making it extremely difficult for a reverse engineer to create an accessory that would work with any randomly-selected telephone. For example, if each challenge included 32 bits (in the example shown), the complete set of possible challenges (and corresponding responses) would exceed four billion in quantity. A 64 bit challenge would give rise to over 1.8×10 19  permutations of challenges. Each telephone&#39;s memory  214  could be loaded with a pseudo-randomly selected subset of, for example, ten challenge/response pairs. In this scenario, the cost of examining enough telephones  210  to create a set of challenge/response pairs that would result in a high probability of a battery unit  220  being authenticated by a telephone  210 , and the cost of storing the set in the battery unit  220 , would make such a battery unit  220  prohibitively expensive. 
     When authenticating the battery  220 , if the stored response does not correspond to the response received from the battery  220 , then the telephone might allow the user to use the telephone  210  but not allow the battery  220  to be recharged, or the telephone  210  could be enabled only when the user is calling an emergency number. In another embodiment, the system could allow the battery to be recharged, but only to a level that is less than the maximum recharge level for the battery. Also, the telephone  210  could be completely disabled when the authentication fails. Many other non-normal operating states for the telephone  210  or the battery  220  are well known to the art and could be employed by this embodiment. 
     A Venn diagram  236  showing the set of all possible challenge/response pairs is shown in  FIG. 2B . Different subsets of the set are possible and may even intersect each other (such as the case of subset A intersecting subset C, as shown). Several possible subsets  214   a ,  214   b ,  214   c  . . .  214   n  are shown in  FIG. 2C . As can be seen subset  214   a  and subset  214   c  both employ challenge/response pair  240  and, thus, they intersect. A large number of challenge/response pairs may be generated and then assigned to different subsets through a pseudo-random assignment algorithm. By doing so, it becomes nearly impossible to predict which device stores which challenge/response pair. 
     One embodiment of a method for authenticating an accessory is shown in  FIG. 3 , in which an authentication operation may be initiated  300  when a device is powered up. A test  302  is performed to determine if a last challenge/response pair has been used and, if so, the system resets a counter  304  so as to cause the system to recycle the set and begin with the first pair. Otherwise, the counter is incremented  306 , thereby causing the system to use the next pair. Alternatively, the device could pseudo-randomly pick a challenge/response pair from the installed subset of pairs, rather that sequentially cycling through the pairs. The challenge is transmitted to the accessory  308  and the system waits to receive a response from the accessory  310 . The response corresponding to the challenge sent to the accessory is retrieved from memory  312  and is compared to the response received from the accessory  314 . If the responses do not match, then normal operation of the device is not enabled  316 , otherwise the responses do match and normal operation of the device is enabled  318 . 
     As shown in  FIG. 4 , the accessory initially waits to receive a challenge from the device  400 . Once received, the system applies the authentication algorithm to the challenge and the key  402 , thereby generating a response. The response is then transmitted  404  to the device. 
     This system provides an economical method of authenticating accessories. The embodiments shown could be used with computer batteries, computer cards in a computer mainframe, computer accessories, secure communications and many other systems that require authentication. 
     The above described embodiments, while including the preferred embodiment and the best mode of the invention known to the inventor at the time of filing, are given as illustrative examples only. It will be readily appreciated that many deviations may be made from the specific embodiments disclosed in this specification without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is to be determined by the claims below rather than being limited to the specifically described embodiments above.