Abstract:
Among other disclosed subject matter, a computer-implemented method includes executing a plurality of virtual machines on a physical machine, wherein a first virtual machine of the plurality of virtual machines executes an encryption process. Execution of a hostile process that is configured to compromise the encryption process is detected, wherein the hostile process executes in a second virtual machine of the plurality of virtual machines. Migrating at least the second virtual machine to a different second physical machine based on the detection of the execution of the hostile process.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application of, and claims priority to, U.S. patent application Ser. No. 13/483,880, entitled DEFENSIVE TECHNIQUES TO INCREASE COMPUTER SECURITY, filed on May 30, 2012. The disclosure of the foregoing application is incorporated herein by reference in its entirety for all purposes. This specification relates to computer security. 
    
    
     BACKGROUND 
     Cloud computing is network-based computing in which typically large collections of servers housed in data centers or “server farms” provide computational resources and data storage as needed to remote end users. Some cloud computing services allow end users to execute software applications in virtual machines. For example, an operator of the cloud computing service can allow users to rent, lease or otherwise use virtual machines hosted on the operator&#39;s physical machines (e.g., the collection of servers). Users can execute various applications on the virtual machines. For example, a user can execute encryption algorithms/applications (e.g., password encryption schemes, data encryption schemes, user authentication algorithms, etc.) or other security applications. 
     Some users may execute malicious software applications or processes (e.g., malware, viruses, etc.) that attempt to compromise processes executing on other virtual machines hosted on the same physical machine. For example, a malicious process can perform a side-channel attack of a first virtual machine to compromise an encryption process executing on a second virtual machine that is hosted on the same physical machine. As another example, a malicious process can perform a statistical keystroke analysis attack and attempt to gather information entered from the keyboard based on elapsed times between keystrokes that can be used to predict character combinations. As a third example, a malicious process can perform a covert channel attack that involves two or more virtual machines that are isolated from each other (e.g., cannot communicate with one another) and use subtle variations in events that perturb the physical machine (e.g., disk accesses or accessing a shared resource that can only be accessed by a one virtual machine at a time) as a medium to communicate information. 
     SUMMARY 
     This specification describes technologies relating to defensive techniques for improving computer security. The system can monitor a virtual machine&#39;s clock accesses and determine whether the number of clock accesses exceeds a threshold. If the number of clock accesses exceeds the threshold, the system can determine that the virtual machine is executing a malicious process (e.g., a side channel attack). The system can then limit the virtual machine&#39;s ability to access the clock. For example, the system can limit the frequency at which the virtual machine accesses the clock. In addition, the system can migrate one or more of the virtual machines hosted on the physical machine to a different physical machine. The system can migrate the virtual machines one at a time. 
     In general, one aspect of the subject matter described in this specification can be embodied in methods that include the actions of executing a plurality of virtual machines on a first physical machine, wherein a first virtual machine of the plurality of virtual machines executes an encryption process; detecting, on a second virtual machine of the plurality of virtual machines, execution of a hostile process that is configured to compromise the encryption process; and migrating at least the second virtual machine from the first physical machine to a second physical machine based on the detection of the execution of the hostile process, wherein the second physical machine is separate and distinct from the first physical machine. Other embodiments of this aspect include corresponding systems, apparatus, and computer programs. 
     These and other aspects can optionally include one or more of the following features. Detecting the execution of the hostile process can comprise: determining a count representing accesses to a clock by the hostile process, wherein the clock is within a central processing unit of the physical machine; determining whether the count is beyond a threshold; and detecting the execution of the hostile process based on the determination that the count is beyond the threshold. Migrating at least the second virtual machine can comprise migrating the first virtual machine to a third physical machine. Migrating at least the second virtual machine can comprise migrating the first virtual machine to the second physical machine at a time instance that is different than a time instance of migrating the second virtual machine. The second physical machine may not be hosting a virtual machine executing an encryption process. The hostile process can be configured to monitor events associated with the first virtual machine to determine a cryptographic key associated with the encryption process. The events associated with the first virtual machine can include cache accesses. The cache accesses can be associated with a cache associated with the physical machine. The process configured to compromise the encryption process can comprise a side-channel attack. A processing load associated with the second physical machine can be less than a load threshold before migrating the second virtual machine. 
     Particular embodiments of the subject matter described in this specification can be implemented so as to realize one or more of the following advantages. For example, network and computer security can be increased by preventing a hostile software application or malware from compromising encryption/cryptographic processes executed on a virtual machine or executed by the physical machine (e.g., the host machine). Another possible advantage is computer security can be increased by protecting other processes or applications from hostile software applications that rely on access to precise timing information. For example, covert channel attacks and statistical keystroke analysis can rely on precise timing information and can be thwarted. In addition, the security of a user account or data associated with a user account can be increased. 
     The details of one or more embodiments of the subject matter described in this specification are set forth in the accompanying drawings and the description below. Other features, aspects, and advantages of the subject matter will become apparent from the description, the drawings, and the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an example system to improve computer security. 
         FIG. 2  is a flowchart of an example process for improving computer security. 
         FIG. 3  is a flowchart of an example process for improving computer security. 
         FIG. 4  is a flowchart of an example process for improving computer security. 
     
    
    
     Like reference numbers and designations in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
       FIG. 1  illustrates an example system for improving computer security. The system includes a physical machine  100 . The physical machine  100  can be a computer, server or a collection of data processing apparatuses and can be part of a cloud computing system. The physical machine  100  can include a Central Processing Unit (CPU)  102  that includes a cache  104 , a clock  106 , a counter  108 , a hardware virtualization module  110 , a virtual machine monitor  112 , virtual machines  114  and  116  and a malicious or hostile software application  118  executed on virtual machine  116 . Although not illustrated, the system can include multiple physical machines  100 . 
     The CPU  102  can be various types of computer processors. For example, the CPU  102  can be similar to an x86 processor, an x86 compatible processor or a 64 bit descendant of the x86 processor (e.g., Intel Core 2 processor or AMD Opteron processor). Other CPUs can be used. 
     The CPU  102  can include a cache  104  (e.g., a data cache). For example, the cache  104  can be a memory module that stores data and provides for fast memory accesses. In some implementations, the cache  104  includes several levels of cache (e.g., L1 and L2 cache). The CPU  102  can use the cache to store data that was recently used by a process executed on the virtual machine. For example, the cache  104  can store values recently used in an encryption algorithm (e.g., intermediate calculations or cryptographic key information). 
     The CPU  102  can also include one or more clocks  106 . For example, the CPU  102  can include one or more high precision clocks (e.g., nanosecond or faster clocks) that provide information associated with the CPU timing. For example, the CPU  102  can include a time stamp counter that counts the number of ticks since the CPU  102  was reset. In addition, the CPU  102  can include other clocks. For example, the CPU  102  can include a real time clock, a high precision event timer or a power management timer. The clocks  106  can include clock sources (e.g., a real time clock) or timer sources (e.g., the time stamp counter). Each of the clocks  106  can be accessed by the physical machine  100 , the virtual machine monitor  112 , a virtual machine  114  and  116  and/or a process executed on the virtual machines  114  and  116 . For example, in some implementations, the virtual machine  114  can access a clock  106  using an Application Programming Interface (“API”) associated with the CPU  102  and/or an operating system executing on the physical machine  100 . In some implementations, the one or more clocks  106  are also included in a peripheral device (e.g., a graphics processing unit). 
     Each of the clocks  106  can be associated with a counter  108  that stores the number of times the respective clock  106  has been accessed. Alternatively, the counter  108  stores the rate of access of the clock  106  (e.g., the number of times the clock  106  has been accessed in a time period divided by the length of the time period). For example, a time stamp counter can store the number of times the time stamp clock is accessed. After each time the time stamp clock is read, the time stamp counter can be incremented. The counter  108  can be accessed by the virtual machine monitor  112  using an API (e.g., READ_TSC) associated with the CPU  102  or an operating system executed on the physical machine  100 . For ease of discussion, it will be assumed that the CPU  102  includes a single clock  106  and a counter  108  associated with the clock  106 . 
     The hardware virtualization module  110  can be used to implement virtual devices. For example, in some implementations, the hardware virtualization module  110  can include a virtualization component, e.g., a Kernel-based Virtualization Machine, and a processor emulator, e.g., QEMU, to virtualize and emulate hardware devices, such as virtual machines  114  and  116 . 
     The hardware virtualization module  110  can be associated with a virtual machine monitor  112  (e.g., a hypervisor). The virtual machine monitor  112  can manage the execution of the virtual machines  114  and  116  executed on the physical machine  100 . The virtual machine monitor  112  can migrate or relocate the virtual machines  114  and  116  from the physical machine  100  to a different physical machine. For example, the virtual machine monitor  112  can migrate each of the virtual machines  114  and  116  from the physical machine  100  one at the same time or at different times. In some implementations, migrating a virtual machine is accomplished by saving the virtual machine&#39;s state on the original physical machine, and then restoring it on the new physical machine. By relocating virtual machines  114  one at a time, the virtual machine monitor  112  can reduce the likelihood that a hostile application will be able to compromise an encryption process executed on a virtual machine hosted on the physical machine  100 . 
     The virtual machines  114  and  116  can be a hardware virtualization of a physical machine (e.g., a computer or server). For example, the virtual machines  114  and  116  can be a virtualized computer that includes an operating system (e.g., a guest operating system) and a software application. The virtual machines  114  and  116  can be associated with an end user. For example, the virtual machine  114  can be assigned to or be associated with an end user of the system (e.g., a user that rents or purchases computing resources from the operator of a cloud computing system). The end user can host various data and/or software applications on the virtual machines  114  and  116 . In addition, the end user can execute various applications on the virtual machine  114  and  116 . For example, a user can execute an encryption algorithm or process associated with a website hosted on the virtual machine  114 . In some implementations, the encryption algorithm (e.g., Advanced Encryption Standard or Data Encryption Standard) can be used to encrypt or otherwise protect data (e.g., a password, user account information, credit card information, etc.). In some implementations, the encryption algorithm encodes communications and data exchanges (e.g., a Secure Sockets Layer (“SSL”) or a Transport Layer Security (“TSL”)). In addition, a second user can execute a malicious process or application (e.g., the side-channel attack  118 ) on the virtual machine  116  that attempts to compromise the cryptographic keys associated with the encryption algorithm. The side-channel attack  118  can attempt to use information associated with the cryptographic calculations to compromise the encryption algorithm. For example, the side-channel attack  118  can monitor cache accesses and attempt to deduce the cryptographic key using timing information associated with the encryption algorithm (e.g., how much time the encryption algorithm takes to perform computations and access the CPU cache  104 ). 
     The virtual machines  114  and  116  can access hardware resources associated with the physical machine  100 . For example, the virtual machines  114  and  116  can read the clock  106  using an API associated with the CPU  102  and/or an operating system executed on the physical machine  100 . In some implementations, the virtual machine monitor  112  can act as an intermediary between the virtual machines  114  and  116  and the physical machine  100 . For example, in some implementations, when an application executed on the virtual machine  114  uses an API to interact with a hardware resource associated with the physical machine  100 , the virtual machine monitor  112  can receive the instruction and interact with the hardware resource on behalf of the virtual machine  114 . As an example, if the side-channel attack  118  uses the READ_TSC instruction to read the clock  106 , the virtual machine monitor  112  can receive the READ_TSC instruction (e.g., trap the READ_TSC instruction), perform its own READ_TSC instruction, receive the value from the CPU  102  and then provide the value to the side-channel attack  118 . 
       FIG. 2  is a flowchart of an example process  200  for improving computer security. The process  200  begins by executing a set of virtual machines (at  202 ). For example, the physical machine  100  can host the virtual machines  114  and  116 . Each of the virtual machines  114  and  116  can execute software applications. In some implementations, each virtual machine  114  and  116  can access the CPU  102  and other hardware resources for a limited amount of time (e.g., time sliced execution). By way of example, a slice can be 1 to 10 milliseconds in duration. 
     As each virtual machine is executed, the virtual machine monitor  112  monitors clock reads/accesses (at  204 ). For example, when a virtual machine  114  or  116  begins its execution in its respective time slice, the virtual machine monitor  112  can reset the counter  108  and monitor the number of times (or the rate at which) a process executed within the virtual machine  114  and  116  accesses/reads the clock (e.g., performs a READ_TSC operation). For example, the virtual machine monitor  112  can use an API associated with the CPU  102  and/or the operating system executed on the physical machine  100  to read the counter  108 . In some implementations, the virtual machine monitor  112  continuously polls the counter  108  using the API. In some implementations, the virtual machine monitor  112  can periodically poll the counter  108  using the API. Each time the clock is read, the CPU  102  can increment the counter  108 . 
     If the value of the counter  108  is less than a threshold (e.g., an access threshold) (at  206 ), the virtual machine monitor  112  continues to monitor the number of clock accesses or the rate of clock accesses (at  204 ). For example, the threshold can be a rate of 100 to 1000 times per second. Other threshold values are possible. In some implementations, profiling of a “typical” workload could be used to establish the threshold. If the value is greater than or equal to the threshold, the virtual machine monitor  112  can determine that the virtual machine  116  includes a malicious process (e.g., the side-channel attack  118 ) and can limit the virtual machine&#39;s ability to access the clock. For example, the virtual machine monitor  112  can decrease or limit the rate at which an application executing on the virtual machine can read the clock  106  by trapping the READ_TSC instruction (e.g., rate limiting or frequency limiting access to the clock). The virtual machine monitor  112  can rate limit the application&#39;s ability to access the clock  106  for the remainder of the time period the virtual machine is executed (e.g., the remainder of the time-slice). In some implementations, the virtual machine monitor  112  can prevent the virtual machine  116  from reading the clock  106 . In some implementations, the virtual machine monitor  112  can continue to rate limit the application&#39;s ability to access the clock  106  in future time-slices. In some implementations, instead of rate limiting or frequency limiting the virtual machine&#39;s ability to access the clock, the virtual machine monitor  112  can trap the READ_TSC instruction and return a value corresponding to a less precise clock. For example, instead of returning a value corresponding to a counter that has a microsecond degree of accuracy, the virtual machine monitor  112  can return a value of a counter that has a ten millisecond degree of accuracy. Because some side-channel attacks rely on frequent clock accesses, the side-channel attack  118  can be thwarted because the side-channel attack  118  cannot access the clock  106  at a sufficient rate. 
       FIG. 3  is a flowchart of an example process  300  for improving computer security. The process  300  begins by executing n virtual machines (at  302 ). For example, n virtual machines  114  and  116  can be hosted on a physical machine  100 . In some implementations, each virtual machine  114  and  116  can access the CPU  102  and other hardware resources for a limited amount of time (e.g., time sliced execution). Each of the virtual machines  114  and  116  can execute one or more applications. 
     The length of time (T) to compromise an encryption algorithm executed in a virtual machine can be determined (at  304 ). For example, if the virtual machine  114  is executing a particular encryption algorithm (e.g., SSL), the virtual machine monitor  112  or an administrator/operator of the physical machine  100  can determine that the time T to compromise the encryption algorithm is equal to 5 minutes (i.e., T=300 seconds). In some implementations, the virtual machine monitor  112  or the administrator/operator of the physical machine  100  or system can access a database that provides estimates of the time necessary to compromise a particular encryption algorithm. The time necessary for the attack can also depend on the rate at which the encryption operation is being performed, the accuracy of the available clock (and how quickly it can be accessed), and the amount of background activity. 
     After determining the time to compromise the encryption algorithm, the virtual machine monitor can migrate each of the n virtual machines from the physical machine during the time T (at  306 ). For example, the virtual machine monitor  112  can migrate each of the n virtual machines from physical machine  100  to a different physical machine  100  (e.g., a destination physical machine) such that none of the n virtual machines  114  and  116  are executed on physical machine  100  for longer than T seconds (e.g., T−1 seconds, T/2 seconds, etc.) In some implementations, the virtual machine monitor  112  migrates a virtual machine  114  or  116  every T/n seconds. The virtual machine monitor  112  can migrate each of the virtual machines  114  or  116  to different destination physical machines. In some implementations, the virtual machine monitor  112  can check the processing load of a destination physical machine before migrating a virtual machine  114  or  116 . In some implementations, the virtual machine monitor  112  will relocate a virtual machine to a destination physical machine if the processing load is less than a threshold (e.g., load threshold). Migrating each of the n virtual machines can reduce the likelihood that a side-channel attack  118  will be able to compromise the encryption algorithm because each time the virtual machine  116  executing the side-channel attack  118  is migrated, the data it has collected will be obsolete and will have to start collecting a new set of data. Although not illustrated in  FIG. 3 , virtual machines can be migrated from a different physical machine to the physical machine  100 . 
       FIG. 4  is a flowchart of an example process  400  for improving computer security. 
     Process  400  begins by executing n virtual machines (at  402 ). For example, n virtual machines  114  and  116  and applications executed on the virtual machines  114  and  116  can be executed on a physical machine  100 . Each virtual machine can access the CPU  102  and other hardware resources for a limited amount of time (e.g., time sliced execution). 
     As each virtual machine is executed, the virtual machine monitor  112  monitors accesses to the clocks (at  404 ). For example, when a virtual machine  114  or  116  begins its executing in its time slice, the virtual machine monitor  112  can reset the counter  108  and monitor the number of times (or rate at which) a process executed within the virtual machine reads the clock (e.g., performs a READ_TSC operation). Each time the clock is read, the CPU  102  can increment the counter  108 . In some implementations, the virtual machine monitor  112  continuously polls the counter  108  or periodically polls the counter  108 . 
     If the value of the counter  108  is less than a threshold (e.g., an access threshold) (at  406 ), the virtual machine monitor  112  continues to monitor clock accesses or the rate of clock access (at  404 ). For example, the threshold can be a rate of 100 to 1000 times per second. Other threshold values are possible. In some implementations, profiling of a “typical” workload could be used to establish the threshold. If the value is greater than or equal to the threshold (at  406 ), the virtual machine monitor  112  can determine that the virtual machine  116  includes a hostile application and can determine whether any of the virtual machines  114  hosted on the physical machine  100  are executing an encryption algorithm (at  408 ). For example, the virtual machine monitor  112  can access a configuration file associated with the virtual machine  116  that indicates whether any of the virtual machines  114  are configured to execute an encryption algorithm (e.g., the configuration file can include a token or string that indicates that an encryption algorithm can be performed or executed by one of the virtual machine  114 ). Alternatively, or in addition to, the virtual machine monitor  112  can determine whether any of the virtual machines  114  hosted on the physical machine  100  are executing an encryption algorithm by analyzing a list of virtual machines  114  that are executing an encryption algorithm. The virtual machine monitor  112  can generate the list by intercepting CPU instructions that are commonly used in or specifically designed for the execution of encryption algorithms during normal execution of the virtual machines  114  or by monitoring the virtual machines  114  access to a key repository or cryptographic device provided by the virtual machine monitor  112  during normal execution of the virtual machines  114 . In some implementations, the virtual machine monitor  112  can suspend or terminate the virtual machine  116  after it determines that the virtual machine  116  includes the hostile application. 
     If the virtual machine monitor determines that none of the virtual machines  114  are executing an encryption algorithm (at  408 ), the virtual machine monitor  112  can limit the virtual machine&#39;s  116  ability to access the clock (at  410 ). For example, the virtual machine monitor  112  can limit the rate at which an application executing on the virtual machine  116  can read the clock  106  by trapping the READ_TSC instruction (e.g., rate limiting or frequency limiting access to the clock). The virtual machine monitor  112  can rate limit the application&#39;s ability to access the clock  106  for the remainder of the time period the virtual machine is executed (e.g., the remainder of the time-slice). In some implementations, the virtual machine monitor  112  can continue to rate limit the application&#39;s ability to read the clock  106  in future time-slices. In some implementations, if the virtual machine monitor  112  determines that none of the virtual machines  114  and  116  are executing an encryption algorithm, the virtual machine monitor  112  can continue monitoring clock accesses and not rate limit the virtual machine&#39;s ability to access the clock. 
     If the virtual machine monitor determines that at least one of the virtual machines are executing an encryption process (at  408 ), the virtual machine monitor can migrate a virtual machine (at  412 ). For example, the virtual machine monitor  112  can migrate the virtual machine  114  that is executing the encryption algorithm to a different physical machine (e.g., a physical machine that has not detected a hostile application). As another example, the virtual machine monitor  112  can migrate the virtual machine  116  that is hosting the side-channel attack  118  to a physical machine that is not executing any virtual machines performing encryption applications). In some implementations, the virtual machine monitor  112  can migrate each of the virtual machines  114  or  116  hosted on the physical machine  100  to different destination physical machines. In some implementations, the virtual machine monitor  112  can check the processing load of a destination physical machine before migrating a virtual machine  114  or  116 . In some implementations, the virtual machine monitor  112  will relocate a virtual machine to a destination physical machine if the processing load is less than a threshold (e.g., a load threshold). 
     In some implementations, if the virtual machine monitor  112  determines that at least one of the virtual machines are executing an encryption process, the virtual machine monitor  112  can migrate each of the n virtual machines  114  hosted on the physical machine  100  as explained in connection with  FIG. 3 . For example, the virtual machine monitor  112  can determine the length of time (T) needed to compromise the encryption algorithm and migrate each of the n virtual machines from physical machine  100  to a different physical machine  100  such that none of the n virtual machines  114  and  116  are executed on the physical machine  100  for longer than T seconds. 
     Alternatively, or in addition to, if the virtual machine monitor  112  determines that at least one of the virtual machines is executing an encryption process, the virtual machine monitor  112  can suspend the virtual machine  116  that is hosting the side-channel attack  118  until the encryption algorithm is completed or can terminate the virtual machine  116  such that the virtual machine  116  and any processes executed by the virtual machine  116  are no longer executed. 
     Embodiments of the subject matter and the operations described in this specification can be implemented in digital electronic circuitry, or in computer software, firmware, or hardware, including the structures disclosed in this specification and their structural equivalents, or in combinations of one or more of them. Embodiments of the subject matter described in this specification can be implemented as one or more computer programs, i.e., one or more modules of computer program instructions, encoded on computer storage medium for execution by, or to control the operation of, data processing apparatus. Alternatively or in addition, the program instructions can be encoded on an artificially-generated propagated signal, e.g., a machine-generated electrical, optical, or electromagnetic signal, that is generated to encode information for transmission to suitable receiver apparatus for execution by a data processing apparatus. A computer storage medium can be, or be included in, a computer-readable storage device, a computer-readable storage substrate, a random or serial access memory array or device, or a combination of one or more of them. Moreover, while a computer storage medium is not a propagated signal, a computer storage medium can be a source or destination of computer program instructions encoded in an artificially-generated propagated signal. The computer storage medium can also be, or be included in, one or more separate physical components or media (e.g., multiple CDs, disks, or other storage devices). 
     The operations described in this specification can be implemented as operations performed by a data processing apparatus on data stored on one or more computer-readable storage devices or received from other sources. 
     The term “data processing apparatus” can encompass apparatuses, devices, and machines for processing data, including by way of example a programmable processor, a computer, a system on a chip, or multiple ones, or combinations, of the foregoing The apparatus can include special purpose logic circuitry, e.g., an FPGA (field programmable gate array) or an ASIC (application-specific integrated circuit). The apparatus can also include, in addition to hardware, code that creates an execution environment for the computer program in question, e.g., code that constitutes processor firmware, a protocol stack, a database management system, an operating system, a cross-platform runtime environment, a virtual machine, or a combination of one or more of them. The apparatus and execution environment can realize various different computing model infrastructures, such as web services, distributed computing and grid computing infrastructures. 
     A computer program (also known as a program, software, software application, script, or code) can be written in any form of programming language, including compiled or interpreted languages, declarative or procedural languages, and it can be deployed in any form, including as a stand-alone program or as a module, component, subroutine, object, or other unit suitable for use in a computing environment. A computer program may, but need not, correspond to a file in a file system. A program can be stored in a portion of a file that holds other programs or data (e.g., one or more scripts stored in a markup language document), in a single file dedicated to the program in question, or in multiple coordinated files (e.g., files that store one or more modules, sub-programs, or portions of code). A computer program can be deployed to be executed on one computer or on multiple computers that are located at one site or distributed across multiple sites and interconnected by a communication network. 
     The processes and logic flows described in this specification can be performed by one or more programmable processors executing one or more computer programs to perform actions by operating on input data and generating output. The processes and logic flows can also be performed by, and apparatus can also be implemented as, special purpose logic circuitry, e.g., an FPGA (field programmable gate array) or an ASIC (application-specific integrated circuit). 
     Processors suitable for the execution of a computer program include, by way of example, both general and special purpose microprocessors, and any one or more processors of any kind of digital computer. Generally, a processor will receive instructions and data from a read-only memory or a random access memory or both. The essential elements of a computer are a processor for performing actions in accordance with instructions and one or more memory devices for storing instructions and data. Generally, a computer will also include, or be operatively coupled to receive data from or transfer data to, or both, one or more mass storage devices for storing data, e.g., magnetic, magneto-optical disks, or optical disks. However, a computer need not have such devices. Moreover, a computer can be embedded in another device, e.g., a mobile telephone, a personal digital assistant (PDA), a mobile audio or video player, a game console, a Global Positioning System (GPS) receiver, or a portable storage device (e.g., a universal serial bus (USB) flash drive), to name just a few. Devices suitable for storing computer program instructions and data include all forms of non-volatile memory, media and memory devices, including by way of example semiconductor memory devices, e.g., EPROM, EEPROM, and flash memory devices; magnetic disks, e.g., internal hard disks or removable disks; magneto-optical disks; and CD-ROM and DVD-ROM disks. The processor and the memory can be supplemented by, or incorporated in, special purpose logic circuitry. 
     To provide for interaction with a user, embodiments of the subject matter described in this specification can be implemented on a computer having a display device, e.g., a CRT (cathode ray tube) or LCD (liquid crystal display) monitor, for displaying information to the user and a keyboard and a pointing device, e.g., a mouse or a trackball, by which the user can provide input to the computer. Other kinds of devices can be used to provide for interaction with a user as well; for example, feedback provided to the user can be any form of sensory feedback, e.g., visual feedback, auditory feedback, or tactile feedback; and input from the user can be received in any form, including acoustic, speech, or tactile input. In addition, a computer can interact with a user by sending documents to and receiving documents from a device that is used by the user; for example, by sending web pages to a web browser on a user&#39;s client device in response to requests received from the web browser. 
     Embodiments of the subject matter described in this specification can be implemented in a computing system that includes a back-end component, e.g., as a data server, or that includes a middleware component, e.g., an application server, or that includes a front-end component, e.g., a client computer having a graphical user interface or a Web browser through which a user can interact with an implementation of the subject matter described in this specification, or any combination of one or more such back-end, middleware, or front-end components. The components of the system can be interconnected by any form or medium of digital data communication, e.g., a communication network. Examples of communication networks include a local area network (“LAN”) and a wide area network (“WAN”), an inter-network (e.g., the Internet), and peer-to-peer networks (e.g., ad hoc peer-to-peer networks). 
     A system of one or more computers can be configured to perform particular operations or actions by virtue of having software, firmware, hardware, or a combination of them installed on the system that in operation causes or cause the system to perform the actions. One or more computer programs can be configured to perform particular operations or actions by virtue of including instructions that, when executed by data processing apparatus, cause the apparatus to perform the actions. 
     The computing system can include clients and servers. A client and server are generally remote from each other and typically interact through a communication network. The relationship of client and server arises by virtue of computer programs running on the respective computers and having a client-server relationship to each other. In some embodiments, a server transmits data (e.g., an HTML page) to a client device (e.g., for purposes of displaying data to and receiving user input from a user interacting with the client device). Data generated at the client device (e.g., a result of the user interaction) can be received from the client device at the server. 
     While this specification contains many specific implementation details, these should not be construed as limitations on the scope of any inventions or of what may be claimed, but rather as descriptions of features specific to particular embodiments of particular inventions. Certain features that are described in this specification in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination. 
     Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. In certain circumstances, multitasking and parallel processing may be advantageous. Moreover, the separation of various system components in the embodiments described above should not be understood as requiring such separation in all embodiments, and it should be understood that the described program components and systems can generally be integrated together in a single software product or packaged into multiple software products. 
     Thus, particular embodiments of the subject matter have been described. Other embodiments are within the scope of the following claims. In some cases, the actions recited in the claims can be performed in a different order and still achieve desirable results. In addition, the processes depicted in the accompanying figures do not necessarily require the particular order shown, or sequential order, to achieve desirable results. In certain implementations, multitasking and parallel processing may be advantageous.