Abstract:
In a limited-copy protection scheme, a check-out/check-in system is configured to a) verify that the receiving device is a certified conforming device, and b) verify that the device that checks-in content material is the same device that checked-out the content material. The verification of the receiving device is effected via a conventional certification process. The verification that the same device is used for check-in and check-out is effected via a secure challenge-response protocol. As contrast to a conventional contemporaneous challenge-response protocol, the system provides a challenge that is specific to the receiving device when the content material is checked-out, and verifies the appropriate response when the content material is checked-in. Because the challenge-response is specific to the receiving device, only the device that receives the challenge when the content material is checked-out can provide the appropriate response when the content material is checked-in.

Description:
This application claims the benefit of U.S. Provisional Application No. 60/131,993 filed 30 Apr. 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to the field of consumer devices, and in particular to techniques for preventing or discouraging the illicit duplication of copy protected material. 
     2. Description of Related Art 
     Techniques are continually being proposed and developed to prevent or discourage the illicit duplication of copy-protected material, such as commercial music recordings. These techniques generally attempt to limit the number of copies that can be made from a legitimate copy of the copy-protected material. At the same time, the purchaser of this legitimate copy expects to have unlimited rights for copying this material for his or her private purposes. For example, the typical purchaser has access to multiple means for playing and recording the material, and expects to be able to play the purchased material on each of these means, without constraints. 
     Increasingly common in the art is the use of flash memory cards to record content material for playback on small portable devices. These flash memory cards, or similar electronic memory devices, have an advantage over conventional recording media such as discs or tapes, in that they contain no moving parts and are thus more reliable and robust. Similarly, the playback devices for these memory cards need not contain movement mechanisms and are therefore also more reliable, robust, and, in general, less expensive than conventional players. The electronic memory devices and corresponding players are also generally much smaller than conventional discs or tapes and corresponding players, and generally consume less power, further increasing their suitability for use as portable playback systems. 
     One method for limiting the ability to copy the content material is a “check-out/check-in” system. In this, as in other protection schemes presented herein, it is assumed that the copying and playback devices are “conforming” devices, in that they conform to the standards used to protect copy-protected material. When a copy of the material is made from a providing device to a portable medium, the conforming providing device prevents additional copies from being made until the portable medium containing the copy is returned to the providing device. That is, the check-out/check-in system provides a “one-at-a-time”, or an “at-most-N-at-a-time”, copy scheme to limit the number of simultaneously available copies of protected content material. 
     A check-out/check-in system is susceptible to a variety of attacks intended to overcome the security provided by a check-out/check-in system. The most straightforward attack is one in which a non-conforming device is used to receive the material. After receiving the material, the non-conforming device provides unlimited copies. Another attack is one in which a non-conforming device “checks-in” material that another device received. A conforming device can receive/check-out the material, the non-conforming can “check-in” the material, and another conforming device can then receive the material, because the check-out/check-in system believes that the other copy has been returned. In this manner, an unlimited number of copies can be made to conforming devices, such as the aforementioned flash memory cards. 
     BRIEF SUMMARY OF THE INVENTION 
     It is an object of this invention to provide a reliable check-out/check-in system and method for limiting the number of copies of protected content material that are simultaneously available. It is a further object of this invention to provide a reliable system and method for assuring that protected content material is provided only to devices that conform to copy protection standards established for protecting the content material. It is a further object of this invention to provide a reliable system and method for assuring that the device that checks-out content material is the same device that checks-in the content material. 
     These objects and others are achieved by a check-out/check-in system that is configured to a) verify that the receiving device is a certified conforming device, and b) verify that the device that checks-in content material is the same device that checked-out the content material. The verification of the receiving device is effected via a conventional certification process. The verification that the same device is used for check-in and check-out is effected via a secure challenge-response protocol. As contrast to a conventional contemporaneous challenge-response protocol, the system provides a unique challenge that is specific to the receiving device when the content material is checked-out, and verifies the appropriate response when the content material is checked-in. Because the challenge-response is specific to the receiving device, only the device that receives the challenge when the content material is checked-out can provide the appropriate response when the content material is checked-in. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is explained in further detail, and by way of example, with reference to the accompanying drawings wherein: 
         FIG. 1  illustrates an example block diagram of a check-out/check-in system in accordance with this invention. 
         FIG. 2  illustrates an example flow diagram of a check-out/check-in system in accordance with this invention. 
     
    
    
     Throughout the drawings, same reference numerals indicate similar or corresponding features or functions. 
     DETAILED DESCRIPTION OF THE INVENTION 
     This invention is based on the premise that a reliable check-in/check-out system has two fundamental requirements. Such a system requires a reliable means of verifying that the receiving device is a certified conforming-device. Otherwise, the copy provided to a potentially non-conforming device may be illicitly reproduced, thereby obviating the copy protection provided by a check-out/check-in system. Additionally, the system requires a reliable means of verifying that the checked-in material is being returned from the same certified conforming-device that initially checked-out the material. Otherwise, a non-conforming device can be used to ‘check-in’ material that another device, including a conforming device, received. Other security techniques, common in the art, may also be applied, for increased copy and distribution protection. 
       FIG. 1  illustrates an example block diagram of a check-out/check-in system in accordance with this invention. The check-out/check-in system includes a check-out/check-in device  100  and a receiving device  200 . The check-out/check-in device includes a catalog controller  110  that controls access to a catalog of content material  150  to certified receiving devices. The controller  110  limits the number of copies of each content material  150  that are simultaneously available. In a preferred embodiment, the controller  110  maintains a count of the number of copies of the content material  150  that have been provided to, or “checked-out” to, certified receiving devices, and refuses to provide additional copies when a pre-defined limit is reached. The check-out/check-in system allows a receiving device  200  to “return”, or “check-in” a copy of the content material  150 , so that it can be provided to another receiving device. The receiving device  200  is presumed to be a conforming device, and a “return” of the material  150  corresponds to a termination of access to the copy of the content material  150  by the receiving device  200 , and the “return” is a notification to the check-out/check-in device  100  that the receiving device  200  has terminated the access. Upon receipt of the “return” notification, the check-out/check-in device  100  decrements the count of the number of copies of the content material  150  that have been provided to receiving devices, thereby allowing another copy of the content material  150  to be provided to a certified receiving device. 
     A conforming receiving device  200  includes a “certificate”  211  that is issued by a trusted authority to certify that the receiving device is a device that is configured to conform to standards that have been established to protect copy protected content material. This certificate  211  typically includes a public key of a public-private key pair that is associated with the receiving device  200 , the identity of the receiving device, and a digital signature based on a private key that is associated with the trusted authority. The digital signature binds the identity of the receiving device to the public key to the public key of the receiving device. The receiving device  200  communicates this certificate  211  to a certification verifier  120  in the check-out/check-in device  100 . The certification verifier  120  applies a public key associated with the trusted authority to verify a correspondence between the communicated public key and the identity and authenticity of the receiver by means of the digital signature in the certificate  211 . Because only the trusted authority can be expected to provide a digital signature that can be verified by the trusted authority&#39;s public key, the proper verification of the digital signature is a certification that the receiving device  200  is an authorized recipient for protected content material. 
     When the certificate is verified, the catalog controller  110  provides/checks-out the selected content material  150  to the receiving device  200 , provided that the selected content material  150  has not already been checked-out to the maximum number of simultaneous receivers, based on the count parameter discussed above. That is, for example, if the content material  150  is limited to a single-copy-at-a-time distribution, then the catalog controller  110  will check-out the content material  150  to the receiving device  200  if and only if no other receiving device has checked-out this content material  150  and has not yet returned it. 
     In accordance with this invention, when the catalog controller  110  provides the copy of the content material  150  to the receiving device  200 , the catalog controller  110  also issues a secure “challenge” to the receiving device, using any of a number of challenge-response protocols. In the example embodiment, the challenge is an encryption of a random number  135 , based on the aforementioned public key of the public-private key pair that is associated with the receiving device  200 . The random number  135  is provided by any number of techniques common in the art, including a pseudo-random number generator, a selection from a list, and so on. As noted above, preferably the certificate  211  contains this public key, and the certification verifier  120  provides this certified public key to an encrypter  130  to effect the encryption of the random number  135 . The receiving device  200  stores the challenge in its memory  210 , along with the associated content material  150 . 
     As in conventional transfer systems, the content material  150  is provided to the receiving device in a secure form. The content material  150  may be encrypted using the aforementioned public key of the receiving device, and subsequently decrypted by the receiving device using a decrypter  230  and the receiving device&#39;s corresponding private key  212 . Alternatively, the content material  150  may be encrypted using a particular key, and this particular key is encrypted using the public key of the receiving device. The receiving device  200  decrypts the encrypted key using the private key  212 , and uses the decrypted key to decrypt the content material  150 . These and other techniques for communicating protected content material are common in the art. 
     When a user of the receiving device  200  decides to return/check-in the content material to the check-out/check-in device, to allow another device to receive a copy of the content material, the receiving device is placed in communication with the check-out/check-in device, and the check-in process is initiated. In accordance with this invention, when placed in a check-in state, a security device  220  in the receiving device  200  erases the selected content material  150  from its memory  210 , or otherwise terminates access to this content material  150 , and communicates a “response” to the aforementioned “challenge” that was received when the content material  150  was received. In the example embodiment, the decrypter  230  within the security device  220  decrypts the encrypted random number that was received, and communicates the decrypted random number to the return verifier  140  of the check-out/check-in device  100 . The return verifier compares the received decrypted random number to the original random number  135  to verify that the receiving device  200  is the same device that received the content material  150 . Note that because the receiving device is assumed to be the only device having access to the receiving device&#39;s private key  212 , and the encryption of the random number  135 , and the content material  150 , is based on the receiving device&#39;s public key, only the receiving device can return a decrypted random number that matches the original random number  135 . 
     As will be evident to one of ordinary skill in the art, the catalog controller will typically contain a variety of content material that can be checked-out, and will typically check-out selected content material to a variety of receiving devices. Not illustrated, the catalog controller  110  and/or the return verifier  140  will contain a list of each checked-out content material and the random number associated with each checked-out content material, to effectively manage the check-out/check-in process. In like manner, each content material may have a different limit to the number of copies that may be simultaneously provided, with some content material allowing an unlimited number of simultaneous copies, and the catalog controller  110  is configured to enforce each limit as required. 
       FIG. 2  illustrates an example flow diagram of a check-out/check-in system in accordance with this invention. At  310 , the check-out/check-in device receives a transaction request that includes an identification of the content material, and a certificate that verifies that the receiving device is a conforming device. As discussed above, in a preferred embodiment, the certificate includes the public key of a public-private key pair that is associated with the receiving device. If, at  315 , the certificate is determined to be invalid, the process is aborted; otherwise, the type of transaction is determined, at  325 . If the transaction is a request to check-out content material, the current count of the number of copies of the requested content material is compared to the limit of the number of simultaneous copies permitted, at  335 . If the number of currently checked-out copies is not less than the limit, the process is aborted. If the number of currently checked-out copies is less than the limit, a challenge is generated, at  340 , and the challenge and the content material are transmitted to the receiving device, at  350 . The current count of the number of checked-out copies is incremented, at  360 , thereby corresponding to a check-out of the content material. 
     If, at  325 , the transaction is a return/check-in of content material, the check-out/check-in device receives the response to the challenge that was given to the receiving device checked-out the content material, at  370 . If the response is an appropriate response to the challenge, at  375 , the current count of the number of checked-out copies of the content material is decremented, at  380 , thereby corresponding to a “return” of the content material, and the process continues. If the response does not correspond to the challenge, at  375 , the count is not decremented. 
     After incrementing or decrementing the count, or after aborting, the process continues at  390 , typically by looping back to step  310 , to await another transaction request. Note that the flow  310 – 325  can be modified to bypass the ‘re-certification’ of the receiving device when content material is being returned, on the assumption that only a previously certified receiving device will be able to provide an appropriate response to the challenge, at  370 . 
     The foregoing merely illustrates the principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the invention and are thus within the spirit and scope of the following claims.