Abstract:
An improvement to a graphics controller to prevent the contents of selected portions of the frame buffer from being read by devices external to the graphics controller. The invention defines one or more viewable rectangles in the frame buffer as a protected write-only area. Any attempt to read data from the protected area of the frame buffer triggers a security violation which can delete or destroy the contents of that area to prevent it from being read. The controller can also operate in a bypass mode in which the security functions are bypassed so the graphics controller operates in a conventional manner. A security violation may return the controller to the bypass mode. The invention can prevent protected data, such as copyrighted data downloaded over the Internet, from being copied from the frame buffer and used in an unauthorized manner.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention pertains generally to computer security. In particular, it pertains to protecting electronic documents on a computer from unauthorized copying or other harmful intervention. 
   2. Description of the Related Art 
   Widespread use of the Internet and email has left millions of personal computers (PCs) vulnerable to downloaded viruses and other types of malicious software that can destroy programs, copy and upload private documents, and perform other harmful acts, frequently without the PC operator&#39;s knowledge. The increasing popularity of downloaded programs has multiplied the problem significantly, since it creates so many more opportunities to unknowingly download the malicious software. Due to their open architecture, most PCs provide very little protection against such destructive software. It is this very openness that has made the PC platform the general-purpose solution provider that it is. Other types of computers are also vulnerable to such attacks in varying degrees, but the pervasive use of PCs has drawn much attention to the problem as it applies to PCs. 
   In the past, owners of copyrighted information or other intellectual property have been reluctant to allow their property to be viewed on the PC platform (books, movies, sensitive corporate documents, etc.) as the nature of the open PC platform makes the property vulnerable to mischievous software that may be running in parallel. Although self-replicating destructive software (viruses) attracts the most attention, copyright owners are more concerned with the illegal copying and distribution of any document that they permit to be downloaded to a computer. This is particularly true of e-books, or books that are available electronically by downloading the text of those books over a network such as the Internet. The ease of copying documents downloaded into a PC makes it easy to illicitly reproduce and forward copyrighted materials without detection of this activity by the copyright owner. 
     FIG. 1  shows a conventional system  10 . Protected content in the form of encrypted data is provided over channel  11  to storage subsystem  12 , where it is stored for subsequent use. Channel  11  could be an Internet connection and the portion of a PC that receives and processes network data. Storage subsystem  12  could be main memory, the hard disk on the PC, or some other form of storage. When the data is ready for presentation, it can be retrieved from storage  12  and presented to player  14  for processing. Player  14  is generally software running in the PC. Decryption of the encrypted data can take place in player  14 , which can also reformat the data. The processed data can then be passed over channel  15  to graphics sub-system  16 , where it is formatted for presentation over channel  17  to the actual display device, such as a video monitor. Note: although the terms “document” and “display” are used here, this scenario applies equally well to graphics video data and to audio data, such as music, that is played through speakers. 
     FIG. 2  shows a conventional graphics controller  16 . Previous attempts to protect downloaded data have focused primarily on encrypting the data for delivery and storage. However, once the data is decrypted, formatted, and sent to the graphics controller  16  through primary interface  21 , the bit-image of that data is generally placed in a video memory  22  where the data is repeatedly read out and transmitted through output port  23  to a display device. For reasons of flexibility and usability, the contents of video memory  16  can generally be read through primary interface  21  by the PC that implements player  14 , and may be read by other devices as well through that same interface. Many graphics controllers also have a secondary interface  24  that also permits both read and write capability of video memory  22  by other devices, as well as permitting direct transmission of video data to output port  23  when that capability is needed. Output port  23  generally does not provide memory read capability. However, interfaces  21  and  24  provide two ports through which the data in video memory  22  can be captured, and subsequently saved and/or transmitted, for later display in an unauthorized manner. For efficiency of transmission, the offending software that captures the bit-image from video memory can also use the text font maps stored in the PC to interpret the bit image and convert any displayable text back to a standard word processing format. Thus, by using resources freely available in the PC, the supposedly protected data in graphics controller  16  can not only be retrieved and stored and/or transmitted to another device, but the retrieved data can be reverse-engineered into a much more compact and usable form before such storage/transmission. 
   Since many players are in the form of a PC that is vulnerable to modification by maliciously loaded software, this exposure of the graphics subsystem creates a security problem that discourages the use of PCs for any displayable data that needs to be protected, such as copyrighted video material. Once the data is placed in graphics subsystem  16 , that data is vulnerable to unauthorized monitoring and capture by software that has been illicitly placed in the computer. 
   It is not only downloaded malicious software that can compromise the security of the data. The PC operator might wish to illicitly copy the data, which he or she legitimately downloaded, for subsequent illicit use or distribution. 
   SUMMARY OF THE INVENTION 
   An embodiment of the invention includes a method that includes receiving data in a presentation buffer of a presentation controller, and receiving a request from a requestor to read the data in the presentation buffer. It further includes deleting the data from the presentation buffer in response to the request and not delivering the data to the requestor in response to the request. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a prior art system. 
       FIG. 2  shows a prior art graphics controller. 
       FIG. 3  shows a graphics controller of the invention. 
       FIG. 4  shows a control circuit of the invention. 
       FIG. 5  shows a system of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   When electronic documents are downloaded from a publisher/owner to a computer for display, unauthorized copying, diversion, modification, destruction, or other harmful effects to that data can take place at several points. The data needs to be protected from those harmful effects during delivery to the player, inside the player, outside the player, and at the display. This invention focuses on protecting the data in the graphics subsystem, and can include elements in the player to implement that protection. 
     FIG. 3  shows a graphics controller  30  of the invention. Rendered data from the player can be received through primary interface  31  for subsequent storage in video memory  32 . Memory  32  includes a frame buffer for storing the portion of the stored image that is actually displayed. Data from the frame buffer can then be sent to an output interface  33 , which sends a properly formatted signal to the display device. Output interface  33  can include a random access memory digital-to-analog converter (RAMDAC), which converts the digitized data into one or more properly formatted analog signals with the specified color rendition. Other types of display devices might require a different output interface to format the data in a different manner, but the overall process within graphics controller  30  is basically the same. A secondary interface  34  can also provide a secondary port to a data channel  18  for data destined for video memory  32 , or provide video data directly to output interface  33 . Unlike a conventional graphics controller, however, graphics controller  30  can contain a control circuit  35  to monitor and/or control data flowing between video memory  32  and either of interfaces  31  or  34 , and to control video memory  32  in a manner that protects secure data stored therein from being illicitly read by devices external to graphics controller  30 . Thus control circuit  35  can act as a gatekeeper between video memory  32  and devices external to graphics controller  30 . 
   The operation of control circuit  35  can be broken down into several functional areas: mode control, security setup, security violation detection, response to violation, and termination. These are described in more detail below: 
   Mode Control 
   Control circuit  35  (and therefore graphics controller  30 ) can have two modes: a security mode and a by-pass mode. In the by-pass mode, the security features of control circuit  35  are by-passed, and graphics controller  30  can effectively perform as a conventional graphics controller. In the security mode, control circuit  35  can perform security functions to prevent all or a portion of the contents of video memory  32  from being read through interface  31 . If interface  34  has a read capability, control circuit  35  can also be coupled to interface  34  to prevent video memory  32  from being read through interface  34 . The mode can be established by one or more commands from the computer controlling circuit  35 . In one embodiment, mode commands, other commands, associated addresses and video data can all be input through interface  31  along with other commands and data to be written to video memory. In another embodiment, commands and addresses can be input through one or more separate interfaces (not shown). In one embodiment, the security mode can be entered simply with an external command, but the security mode can be exited only if the secure data is deleted first. This prevents illicit software from simply turning off the security mode so that the protected data can be read with impunity. 
   Security Setup 
   The display device in a conventional system frequently shows multiple windows at the same time, some of them overlapping others. Thus the frame buffer may contain multiple windows, or portions of windows, at any given time. Since all the displayed windows are in the frame buffer, and the frame buffer can be read by external devices in a conventional system, the displayed contents of any window are freely accessible to external devices in a conventional system. 
   When a copyrighted document or other secure data is being displayed, the window containing that secure data may be only one of several windows that are being simultaneously shown on various parts of the display device. Several of those windows may contain non-secure data that the operator wishes to handle in a standard manner. For example, while viewing portions of a secure copyrighted e-book in one window, the operator may wish to read e-mail or look up an appointment notebook in another window without having to exit from the e-book application. Therefore the protected portion of the data may be only a subset of the frame buffer, and the secure portion of the data should be defined separately from the remaining displayed data. 
   The invention can define the coordinates of a secure window within the frame buffer. These coordinates can be contained in a set of registers that define opposite corners of a rectangular displayed window, such as the upper left and lower right corners of that window. Any data that is located within this window is considered secure, and can be protected. In one embodiment, one or more sets of registers are dedicated to defining secure windows in this manner. In another embodiment, existing registers that define a window are temporarily designated as secure registers as long as the defined window contains secure data, but those registers can return to non-secure status once the secure data is automatically deleted and/or the window is closed. In both embodiments, multiple sets of registers can be used to define multiple secure windows, so that the security function can be performed simultaneously on different windows. Note: although the term ‘registers’ is used here, the invention can also use other forms of data storage to hold the coordinates of the secure windows, such a block of memory containing multiple memory locations. Such obvious design tradeoffs are within the capability of an average circuit designer. 
   Security registers can be loaded with the coordinates of the secure window by appropriate setup commands passed through interface  31 . This assumes interface  31  supports conveyance of a combination of commands and data. Alternately, setup commands can be passed to control circuit  35  through another interface (not shown) specially designated for this purpose. In one embodiment, once these setup commands are entered, the designated security coordinates cannot be altered without deleting the protected data defined by these coordinates. This protects against malicious software that accesses the secure data simply by changing the coordinates of the protected area to another location. 
   Security Violation Detection 
   Regardless of the register configuration used, the data within the secure window can be treated as write-only data for all devices other than output circuit  33  which, by necessity, must read the contents of video memory so that it can display the image on a display device. All other devices, i.e., devices that can read video memory through interfaces  31 ,  34 , or any other accessory ports, are prevented from reading any data in the window defined by the contents of the secure registers. Data in the frame buffer that is outside this defined security window can be read in the normal manner. This effectively prevents the pre-defined secure data in the frame buffer from being illicitly read, copied, or transmitted by malicious software, while not interfering with normal operations for the rest of the data in the frame buffer. 
   Detection of an attempted security violation can be accomplished by monitoring the addresses of any requests to read data from video memory  32 . If the requested address falls between the two stored addresses that define the opposite corners of a protected window, then a violation has occurred. When multiple security windows are defined at the same time, a separate comparison can be made for each secure window. A violation of any secure window can trigger a response. 
   Response to Violation 
   The no-read function can be enforced in various ways. In one embodiment, when a device attempts to read data from the secure area defined by the security registers, a data protector in graphics controller  30  will return video data, but not the requested video data. The controller might return a solid color for all of the locations in the protected area (such as blue, black, white, etc.). The controller might also return random data, resulting in an image of static. Another option is to return a window with an warning message, alerting the operator to the fact that protected data has been requested. 
   A second embodiment provides greater protection. In this embodiment, any attempt to read data from the protected portion of video memory can result in purging the protected portion of the data by the data protector. This can be done by overwriting the protected data with other data, such as the solid color, random data, or error message described above. This step can be followed by exiting the secure mode, so that the requested window is available for reading, but the secure data is no longer in it. These actions can also trigger other defensive mechanisms, such alerting the operator, deleting the other secure data that is still stored in encrypted form on disk, or severing the connection to the remote source of the secure data. Such drastic actions can prevent alternate, repeated attacks on the secure data by removing the secure data from the system altogether. 
   Some computer systems, such as laptop PCs, provide an external connector on the graphics controller so that other display devices can be attached. Besides the previously described features, additional protection can be provided by disabling the external graphics connector when secure data is being displayed so that external equipment can&#39;t just record the signal. 
   Termination 
   When the need to display protected data is over, commands can be issued to control circuit  35  to delete the secure window and/or to change the mode of the affected window from security to by-pass. In either case, the protected data within that window can be purged from video memory first so that it cannot be subsequently read by external devices. One or more commands can be implemented that terminate the secure mode in this manner. Alternately, termination can be triggered by simply attempting a read of the protected data, thereby artificially forcing a termination in the manner described above under ‘Response to Violation’. 
     FIG. 4  shows a view of an embodiment of control circuit  35  in more detail.  FIG. 4  is intended to show functional relationships rather than circuit connections, although a circuit can be modeled after the figure. Commands, addresses, and data can be received from interface  31  by circuit  41 . Although  FIG. 4  shows all commands, addresses, and data entering circuit  41  through a common port, different input ports and circuits can also be provided to handle commands and data separately. Logic  41  can also be divided into separate sections (not shown), a command handler for handling commands and/or the associated addresses, and another for handling video data to/from video memory. After receipt from interface  31 , video data can be passed by data handling logic in circuit  41  along data path  40  en route to video memory  32 . Read data requested from video memory  32  can also travel in the opposite direction along bi-directional data path  40  and be provided to the requestor when such read data transactions are permitted. 
   Command handling logic in circuit  41  can decode mode or setup commands input into circuit  35 . A security mode command can cause line  49  to be set, thereby enabling mode enable gate  46 . This in turn enables the security mode, so that any subsequent commands and data will be processed according to the security requirements. Command handling logic in circuit  41  can also accept the upper left and lower right coordinates of a secure window, placing them into upper-left address register  44  and lower-right address register  45 , respectively. These can be security registers dedicated to the security function, or can be existing registers that are temporarily designated as security registers. Subsequently, when a read command is received, command decode logic  41  can place the requested read address in register  42 , where it can be compared with the two coordinate addresses in registers  44 ,  45 . If the address in the read command falls between the upper-left and lower-right addresses, inclusively, address compare logic  43  can set the line to mode enable gate  46 , which can act as a security violation detector. If the other input of mode enable gate  46  is already set (because the circuit is in security mode) then gate  46  can activate the line to data protector  47 , triggering a series of steps that respond to the detected security violation by deleting the secure data in the frame buffer and possibly disabling the security mode. 
     FIG. 5  shows a system  50  of the invention. Protected content from a provider can be input through channel  11  and stored in storage medium  12 , as in the prior art. The data for presentation can then be passed to player  54 , which has the capability to implement the aforementioned security features in graphics controller  30 . Channels  55  and  57  can pass the video data to graphics controller  30  and the display, respectively. 
   The invention can be implemented in circuitry or as a method. The functional steps in the previous paragraphs can be performed with dedicated logic, a state machine, a processor, or any combination of these. The invention can also be implemented as instructions stored on a machine-readable medium, which can be read and executed by at least one processor to perform the functions described herein. A machine-readable medium includes any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computer). For example, a machine-readable medium can include read only memory (ROM); random access memory (RAM); magnetic disk storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other form of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.), and others. 
   The invention has been described in terms of a frame buffer in a video controller. However, it may be applied to other forms of data presentation. The video controller may be generalized as a presentation controller, which can also take the form of an audio controller that presents downloaded audio information such as music or spoken words. The frame buffer may be generalized as a presentation buffer, which can also take the form of an audio buffer that temporarily stores the audio data to be played, including a combination of protected and non-protected audio data. An embodiment of the invention using audio data can be used to play music or to present an audible e-book for the vision-impaired. 
   The foregoing description is intended to be illustrative and not limiting. Variations will occur to those of skill in the art. Those variations are intended to be included in the invention, which is limited only by the spirit and scope of the appended claims.