Abstract:
System and method for detecting ransomware. A current user behavior pattern is monitored based on user input via a user input device. The user behavior is compared against a reference set of behavior patterns associated with user frustration with non-responsiveness of the user interface module. A current status pattern of the operating system is also monitored. The current status pattern is compared against a reference set of operating system status patterns associated with predefined ransomware behavior. In response to indicia of current user frustration with non-responsiveness of the user interface, and further in response to indicia of the current status pattern having a correlation to the predefined ransomware behavior, an indication of a positive detection of ransomware executing on the computer system is provided.

Description:
CLAIM TO PRIORITY 
     This application claims the benefit of Russian Federation Patent Application No. 2012156439 filed Dec. 25, 2012 the disclosure of which is incorporated by reference herein. 
     FIELD OF THE INVENTION 
     The invention relates generally to information processing and security and, more particularly, to detecting malicious software that prevents normal user interaction with the operating system, such as ransomware. 
     BACKGROUND OF THE INVENTION 
     Malicious computer software, such as viruses, computer worms and trojans, present one of the most important computer security issues. It was calculated that annual financial losses of businesses caused by malicious software amount to tens of billions of dollars. 
     In recent years, a new type of malicious software, ransomware, has become widespread. The term ransomware as used herein refers to a family of malicious programs that block or impede user interaction with the operating system. Oftentimes, these programs demand the transfer of funds to the offenders in exchange for restoration of the computer&#39;s operability. Technically, ransomware programs are a variant of trojan program, which infect their target computer systems using techniques such as attaching executable files to emails, or attacking a vulnerability in the network service to gain access to the target computer system. 
     In order to combat ransomware, antivirus companies have traditionally used basic signature detection methodology, which is based on the comparison of program codes with the signatures of known malicious programs stored in antivirus databases. Other approaches have used a combination of multiple proactive antivirus security technologies, the main goal of which, in contrast to the reactive (signature-based) technologies, is to prevent infection of the user&#39;s system by unknown malicious software. For example, heuristic analysis during code emulation has been combined with behavior analysis in order to prevent infection of the user&#39;s operating system with a ransomware program. 
     In the code emulation mode, the behavior of the operating system and of the central processor is simulated; therefore, the application in question cannot harm the user&#39;s operating system. Heuristic analysis based on the application code analysis determines the parts of the code which are responsible for malicious activity. A behavior analysis of the execution mode determines whether a program is malicious, based on its behavior. 
     The main advantage of these technologies consists in their ability to differentiate safe programs from malicious ones without using a professional virus analyst. One shortcoming, however, is that there is a certain intermediate zone between clearly malicious actions and acceptable actions. Moreover, the same actions or parts of a code can be malicious in a malicious program intended to extort money but useful in legitimate software. For example, running a full-screen application with a window in front of the other windows is used by ransomware programs to complicate the user&#39;s interaction with the operating system&#39;s graphical interface, but this is quite legitimate in computer games—for example, when a computer game application is run full-screen and does not respond to special key combinations like ALT-F4 or ALT-TAB. 
     This creates the possibility that antivirus technologies may be unable to detect an unknown ransomware program, which may result in a blockage of the user&#39;s interaction with the operating system, and, consequently, a blockage of the user&#39;s access to the antivirus product tools. Therefore, there is a need for quickly detecting the presence of an active ransomware program in the operating system, so that the antivirus software can alleviate the problem in a timely manner. 
     SUMMARY OF THE INVENTION 
     Aspects of the invention is directed to detecting active ransomware in a computer system. Ransomware is defined herein as malicious software that interferes with, or obstructs, the user interface with the operating system to such an extent that the computer system is made to appear to the user as being unresponsive to user input. Such malware may also attempt to extort funds from the user (i.e., “seek a ransom”), though this latter feature is not a requirement for a piece of malware to be considered ransomware within the present context. 
     One aspect is directed to a method for detecting malware in a computer system comprising computing hardware that includes a processor and data store, a user input device and a display device. The computer system also includes an operating system executable on the computing hardware, the operating system including a user interface module interfaced with the user input device and the display device. In the method, a current user behavior pattern is monitored based on user input via the user input device. The user behavior is compared against a reference set of behavior patterns associated with user frustration with non-responsiveness of the user interface module. This reference set of behavior patterns may be stored in a synamically-updatable database according to some embodiments. A current status pattern of the operating system is monitored. Monitoring of the status pattern may be as simple as reading certain indicia of the operating system&#39;s state, such as a configuration of open windows and handling of input device data, for example. The current status pattern is compared against a reference set of operating system status patterns associated with predefined ransomware behavior. In response to a result of the comparing of the current user behavior pattern against the reference set of behavior patterns being indicative of current user frustration with non-responsiveness of the user interface module, and further in response to a result of the comparing of the current status pattern against the reference set of operating system status patterns being indicative of the current status pattern having a correlation to the predefined ransomware behavior, an indication of a positive detection of ransomware executing on the computer system is provided. 
     Embodiments of the method may further take remedial actions such as identifying which process is associated with the ransomware, and disabling, or removing, the ransomware to restore normal operability of the computer system for the user. 
     In another aspect of the invention, a system for detecting ransomware is a computer-based system that includes computing hardware (i.e., including a processor and data store, a user input device and a display device), and is implemented in a computer system that has an operating system executable on the computing hardware and comprising instructions stored in a non-transitory storage medium that, when executed, implement a user interface module that operationally communicates with the user input device and the display device. 
     The system includes a user input monitoring module, an input data analysis module, an operating system status analysis module, and a decision module, each of which is executable on the computing hardware and comprising instructions stored in a non-transitory storage medium. When executed, the user input monitoring module causes the computing hardware to collect a current user behavior pattern based on user input via the user input device. The input data analysis module causes the computing hardware to compare the user behavior against a reference set of behavior patterns associated with user frustration with non-responsiveness of the user interface module. The operating system status analysis module causes the computing hardware to compare a current status pattern of the operating system against a reference set of operating system status patterns associated with predefined ransomware behavior. The decision module causes the computing hardware to provide an indication of a positive detection of ransomware executing on the computer system in response to an output of the input data analysis module being indicative of current user frustration with non-responsiveness of the user interface module, and further in response to an output of the operating system status analysis module being indicative of the current status pattern having a correlation to the predefined ransomware behavior. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which: 
         FIG. 1  illustrates an example of ransomware displaying a message on a computer screen with instructions for unlocking the operating system by completing a transfer of funds. 
         FIG. 2A  is a block diagram illustrating a system architecture according to one embodiment of the invention. 
         FIG. 2B  is a block diagram illustrating interactions during operation of a user behavior analysis module according to an embodiment of the invention. 
         FIG. 2C  is a diagram illustrating a system architecture according to another embodiment of the invention. 
         FIG. 3  is a flow diagram illustrating a process for detecting a ransomware program according to one embodiment of the invention. 
         FIG. 4  is a block diagram depicting a general-purpose computer system with which aspects of the invention may be realized according to certain embodiments. 
     
    
    
     While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Ransomware programs can be spread through sites offering free software, file sharing sites and hacked legitimate web resources. Often, users download and run malicious programs voluntarily, believing that they are installing legitimate software. 
     An example of a sequence of typical actions taken by a malicious ransomware program is as follows: 
     1. Once admitted into the system and run, the malicious program, in order to establish itself in the system, creates a file and writes it into the operating system&#39;s autorun. 
     2. Usually, the malicious program disables the operating system&#39;s task manager and the possibility of booting the system in the safe mode. This is done in order to complicate the user&#39;s resistance against the ransomware program, which blocks the user&#39;s interaction with the operating system. 
     3. This is followed by the actual mission of the ransomware program. The malicious program blocks the user&#39;s interaction with the operating system, displaying, typically over all windows, a window with a message that the computer is locked, and a purported instruction for unlocking it. In the instruction, the offenders, under various pretexts, demand that the user pays for the unlocking codes, and tell the user how he or she will receive the codes to be able to unlock the computer after the payment. A typical example of a window displaying a pretext for locking the computer system, along with instructions is illustrated in  FIG. 1 . 
     Lately, authors of ransomware programs sometimes don&#39;t even include the unlocking function in the logic of the malicious program&#39;s operation. Moreover, this function may no longer be provided in updated versions of the malicious programs which used to “faithfully” allow the user to unlock the computer. 
     The reasons for this trend are simple. First, it is quite cumbersome for the authors of malicious programs to work with unlocking codes: this requires entering new codes in new versions of malicious programs and constantly monitoring the telephone number match database and the unlocking code database. It is much easier to compile new versions of malicious programs without taking care of the codes, but simply changing mobile phone/account numbers to receive money. 
     Second, the use of unlocking codes forces the offenders to utilize only such payment methods that allow to send replies to users (generally, this scheme uses premium-type mobile numbers). Without being tied to an unlocking code, offenders can exploit any convenient methods for receiving money. 
     Finally, the absence of an unlocking code significantly complicates life for the antivirus companies. Previously, companies used to add the unlocking codes found in malicious programs to special unlocking services of known ransomware programs, such as Kaspersky Deblocker. Such services provide users with unlocking codes matching the telephone/account numbers used by offenders and with instructions for curing the operating system. Today, if the ransomware programs do not contain unlocking codes, antivirus companies fight them with special utilities, such as Kaspersky Windows Unlocker. This utility works separately from the infected operating system and is therefore able to eliminate the consequences of the infection, removing files and system registry keys created by the malicious program. 
     It is clear that if antivirus security modules do not detect a ransomware program, infection of the operating system will result in blockage of the user&#39;s interaction with the operating system, and, consequently, the impossibility to access antivirus product modules from the user&#39;s side. Therefore, at present, in order to remove the blockage, the user inevitably must utilize special services through an outside computer system. 
     Aspects of the invention allow the computer system to handle situations where the user&#39;s interaction with the operating system is locked by ransomware programs, without using outside utilities or resources, but by identifying situations involving violations in the user&#39;s interaction with the operating system&#39;s interface. 
       FIG. 2A  illustrates an exemplary system architecture of one of the embodiments. The system for detecting active ransomware programs in the operating system includes four modules: a pattern storage module  201 , a user behavior analysis module  205 , an operating system status analysis module  210  and a decision-making module  215 . 
     The term module as used herein means a real-world device, component, or arrangement of components implemented using hardware, such as by an application specific integrated circuit (ASIC) or field-programmable gate array (FPGA), for example, or as a combination of hardware and software, such as by a microprocessor system and a set of program instructions that adapt the module to implement the particular functionality, which (while being executed) transform the microprocessor system into a special-purpose device. A module can also be implemented as a combination of the two, with certain functions facilitated by hardware alone, and other functions facilitated by a combination of hardware and software. In certain implementations, at least a portion, and in some cases, all, of a module can be executed on the processor(s) of one or more computers that execute an operating system, system programs, and application programs, while also implementing the module using multitasking, multithreading, distributed (e.g., cloud) processing where appropriate, or other such techniques. Accordingly, each module can be realized in a variety of suitable configurations, and should generally not be limited to any particular implementation exemplified herein, unless such limitations are expressly called out. 
     The pattern storage module  201  contains at least two updatable databases—a behavior pattern database  202 , which determines possible user behavior scenarios in case of interrupted communication between the user and the operating system interface; and a status pattern database  203 , which determines how the operating system handles an active ransomware program. The user behavior analysis module  205  is configured to discover such signs in the user&#39;s behavior which are typical for certain situations—for example, related to violations or abnormalities in the user&#39;s interaction with the operating system interface. During operation, the user behavior analysis module  205  interacts with the pattern storage module  201  to look up known behavior patterns stored in the updatable behavior pattern database  202 . The operating system status analysis module  210  compares the current operating system status with the status patterns from the status pattern database  203 , characterizing how the operating system handles an active ransomware program. Based on the results of the operation of these two modules, the user behavior analysis module  205  and the operating system status analysis module  210 , the decision-making module  215  generates a determination as to whether an active ransomware program is present in the operating system. 
     The operating system status can only indicate that a ransomware program is active in the operating system, obstructing interaction with the operating system user interface, without stating specifically what application represents that ransomware program. The following are examples of characteristics included in the status patterns from the status pattern database  203 , indicating that the operating system has an active ransomware program:
         restriction of the cursor operation area; or   disabling one or more control elements, such as the Start button, the task manager, etc.       

     In one type of embodiment, the operating system status analysis module  210  performs a non-primary role (i.e., a support role) for determining the presence of a ransomware program in the operating system. In one such embodiment, the operating system status analysis module  210  operates to determine whether or not the operating system is free from a ransomware program. In this embodiment, the operating system status analysis module  210  does not independently discover a presence of a ransomware program in the first instance. Instead, the operating system status analysis module  210  can help to check if the user behavior analysis module may have provided a false positive detection of a ransomware program. Therefore, the decision-making module  215  concludes that an active ransomware program is present in the operating system based on data from two analytical modules—the user behavior analysis module  205 , which is checked against the output from the operating system status analysis module  210 . 
       FIG. 2B  illustrates an exemplary structure and principle of operation of the user behavior analysis module  205 . In this example, the behavior analysis module  205  includes two main modules. The user input monitoring module  206  comprises various data monitoring tools, executed during the user&#39;s interaction with the input interfaces. In certain embodiments, at least one of the data monitoring tools is implemented in hardware, which makes those tool(s) more resilient against attacks. Data analysis module  207  analyzes data sequences and interacts with the pattern storage module  201 , using known behavior patterns stored in the updatable behavior pattern database  202 , which characterize possible user behavior scenarios in case of interruption of the user&#39;s interaction with the operating system interface. 
     In general, operating system user interfaces include user interface modules and devices for user interaction with a computer&#39;s operating system or a user application. These include a graphical user interface that facilitates interaction with the operating system using control elements such as icons, menus, interactive windows, etc.), and input devices that facilitate the user&#39;s interaction with the computer hardware. Input devices can include, for example, data input devices, such as a keyboard, a mouse or sensor control elements, as well as devices for photo, video and audio data capture, and the like. The user interface module of a computer system implements a set of operating system functions (including device drivers and in some cases, applications) that receive and process user input from the input devices. 
     The launch of a ransomware program results in interference with the graphical user interface. The user&#39;s interaction via the input devices of computer system  211  generates at least the following data, interception of which is read by data monitoring modules  206 :
         sequences of symbols entered from a keyboard and temporal relationships between the entered symbols;   commands and coordinates from pointing control devices such as mice, touchpads, and touchscreens;   photo, video and audio data; and   other input data.       

     From the intercepted data within a certain time window, sequences of data are built and analyzed by the analysis module  207  by comparing them with the behavior patterns from the behavior pattern database  202 , characterizing typical user behavior tendencies in response to interruption in the user&#39;s interaction with the operating system interface. 
     The time window for monitoring can be chosen in an interval from a few seconds to several minutes. An alternative to the time window can be, for example, a buffer of a certain size, which builds data sequences for analysis once it is filled. In this case, in one embodiment, the buffer can operate using the FIFO (first in, first out) principle, i.e. when a new pressed key code comes from the keyboard or an event comes from the mouse, the first received data at the beginning of the queue are bumped out by the last data at the end of the queue. In another embodiment, the buffer can be emptied if it does not receive any new data after a period from one second to several minutes. 
     One example of the description of the user&#39;s behavior indicating an interruption in the user&#39;s interaction with, for example, a graphic operating system user interface, can be the pressing of a certain combination of preset keys specified in a behavior pattern from the behavior pattern database  202 . In a more complex embodiment, an interruption in the user&#39;s interaction with the operating system interface can be determined and described in the behavior pattern based on the nature and sequences of the symbols entered by the user—for example, multiple repeated pressing of special key combinations, such as CTRL+ALT+DELETE, ALT+F4, ALT+TAB, CTRL+SHIFT+ESC, etc. Also, the frequency and incoherence of user-pressed keys can indicate the user&#39;s panic caused by the blockage of the user&#39;s interaction with the operating system. In another embodiment, such signs of user panic can be determined, for example, by analyzing photo/video/audio data, by detecting increased vibrations during the user&#39;s interaction with the sensor control module, by detecting deviations from the user&#39;s usual behavior, or by other such behavioral criteria. 
     An exemplary architecture of a behavior analysis module  205 , shown in  FIG. 2B , allows to implement various scenarios for detecting situations related to obstructions in the user&#39;s interaction with the operating system&#39;s interface. The capabilities of the behavior analysis module  205  are defined only by the set of input interfaces available in a specific computer system  211 , which determine what data monitoring modules  206  are used, and, consequently, determine the methods for describing behavior templates from the behavior template database  202 , characterizing the possible user behavior scenarios in case of interruption in the user&#39;s interaction with the operating system interface. 
     The above-identified user behavior scenarios can, in the general case, indicate that the user is experiencing difficulties, and, more specifically, indicate an interruption in the user&#39;s interaction with the operating system interface. In certain embodiments, only in combination with a positive verdict from the operating system status analysis module  210 , will the presence of such behavior patterns lead the decision-making module  215  to conclude that an active ransomware program is present in the operating system. 
     In a related embodiment of this invention, the system for detection of active ransomware programs in the operating system shown in  FIG. 2C  additionally includes an active ransomware program detection module  220  and an active ransomware program de-activation module  230 . 
     The active ransomware program detection module  220  compares the sets of characteristics of active processes with the known ransomware program characteristics database  225 . Such characteristics can include characteristics of windows and behavior of applications, for example:
         a full-screen window or a window larger than the screen resolution;   a window with no system window menu (SYSMENU, such buttons as MINIMIZE, MAXIMIZE, etc.) or a CHILD, POPUP window;   a window with a set TOPMOST flag (window over all other windows) or a TOP flag (a milder condition);   a window which constantly changes its Z-ORDER parameter to TOP or TOPMOST;   installed hooks (software handlers of intercepted input) for the keyboard or mouse without further transfer of control (a stringent condition) or simply hooks for the keyboard and mouse (a mild condition);   an application launching itself in case of deletion from memory, i.e.  2  processes are run from a single executable file, each process tracking the presence of the other one in the memory, etc.       

     In case of detection of active processes whose characteristics match characteristics of known ransomware programs from the known ransomware program characteristics database  225 , the active ransomware program de-activation module  230  performs their forced stoppage. 
       FIG. 3  illustrates an exemplary process for detecting an active ransomware program in the operating system. Pursuant to this method, block  301  analyzes the data sequences built during the user&#39;s interaction with input interfaces, after which block  305  generates a conclusion on the detection of an event characterized by an interruption in the user&#39;s interaction with the operating system interface. If no such event is detected, the process loops back to the first block  301 . If such event is detected, the process proceeds to block  310 , which analyzes the operating system status. If the analysis  310  at block  315  does not detect a status typical for an operating system with an active ransomware program, the method loops back to  301 . The presence of an active ransomware program in the operating system at is detected at block  320  only in case of two positive verdicts, when, at block  305 , there is an interruption in the user&#39;s interaction with the operating system interface, and, additionally, block  315  detects a state of the operating system characterized by the presence of an active ransomware program. 
       FIG. 4  is a block diagram illustrating in greater detail a computer system  1  on which aspects of the invention as described herein may be implemented according to various embodiments. The computer system  1  may include a computing device such as a personal computer  2 . The personal computer  2  includes one or more processing units  4 , a system memory  6 , a video interface  8 , an output peripheral interface  10 , a network interface  12 , a user input interface  14 , removable  16  and non-removable  18  memory interfaces and a system bus or high-speed communications channel  20  coupling the various components. In various embodiments, the processing units  4  may have multiple logical cores that are able to process information stored on computer readable media such as the system memory  6  or memory attached to the removable  16  and non-removable  18  memory interfaces  18 . The computer  2  system memory  6  may include non-volatile memory such as Read Only Memory (ROM)  22  or volatile memory such as Random Access Memory (RAM)  24 . The ROM  22  may include a basic input/output system (BIOS)  26  to help communicate with the other portion of the computer  2 . The RAM  24  may store portions of various software applications such as the operating system  28 , application programs  30  and other program modules  32 . Further, the RAM  24  may store other information such as program or application data  34 . In various embodiments, the RAM  24  stores information that requires low-latencies and efficient access, such as programs and data being manipulated or operated on. In various embodiments RAM  24  comprises Double Data Rate (DDR) memory, Error Correcting memory (ECC) or other memory technologies with varying latencies and configurations such as RAMBUS or DDR2 and DDR3. In this way, in various embodiments, the system memory  6  may store the input data store, access credential data store, operating memory data store, instruction set data store, analysis result data store and the operating memory data store. Further, in various embodiments, the processing units  4  may be configured to execute instructions that limit access to the aforementioned data stores by requiring access credential before access to the information is granted. 
     The removable  16  and non-removable  18  memory interfaces may couple the computer  2  to disk drives  36  such as SSD or rotational disk drives. These disk drives  36  may provide further storage for various software applications such as the operating system  38 , application programs  40  and other program modules  42 . Further, the disk drives  36  may store other information such as program or application data  44 . In various embodiments, the disk drives  36  store information that doesn&#39;t require the same low-latencies as in other storage mediums. Further, the operating system  38 , application program  40  data, program modules  42  and program or application data  44  may be the same information as that stored in the RAM  24  in various embodiments mentioned above or it may be different data potentially derivative of the RAM  24  stored data. 
     Further, the removable non-volatile memory interface  16  may couple the computer  2  to magnetic portable disk drives  46  that utilize magnetic media such as the floppy disk  48 , Iomega® Zip or Jazz, or optical disk drives  50  that utilize optical media  52  for storage of computer readable media such as Blu-Ray®, DVD-R/RW, CD-R/RW and other similar formats. Still other embodiments utilize SSD or rotational disks housed in portable enclosures to increase the capacity of removable memory. 
     The computer  2  may utilize the network interface  12  to communicate with one or more remote computers  56  over a local area network (LAN)  58  or a wide area network (WAN)  60 . The network interface  12  may utilize a Network Interface Card (NIC) or other interface such as a modem  62  to enable communication. The modem  62  may enable communication over telephone lines, coaxial, fiber optic, powerline, or wirelessly. The remote computer  56  may contain a similar hardware and software configuration or may have a memory  64  that contains remote application programs  66  that may provide additional computer readable instructions to the computer  2 . In various embodiments, the remote computer memory  64  can be utilized to store information such as identified file information that may be later downloaded to local system memory  6 . Further, in various embodiments the remote computer  56  may be an application server, an administrative server, client computers, or a network appliance. 
     A user may enter information to the computer  2  using input devices connected to the user input interface  14  such as a mouse  68  and keyboard  70 . Additionally, the input device may be a trackpad, fingerprint scanner, joystick, barcode scanner, media scanner or the like. The video interface  8  may provide visual information to a display such as a monitor  72 . The video interface  8  may be an embedded interface or it may be a discrete interface. Further, the computer may utilize a plurality of video interfaces  8 , network interfaces  12  and removable  16  and non-removable  18  interfaces in order to increase the flexibility in operation of the computer  2 . Further, various embodiments utilize several monitors  72  and several video interfaces  8  to vary the performance and capabilities of the computer  2 . Other computer interfaces may be included in computer  2  such as the output peripheral interface  10 . This interface may be coupled to a printer  74  or speakers  76  or other peripherals to provide additional functionality to the computer  2 . 
     Various alternative configurations and implementations of the computer are contemplated. These variations may include, without limitation, additional interfaces coupled to the system bus  20  such as universal serial bus (USB), printer port, game port, PCI bus, PCI Express or integrations of the various components described above into chipset components such as the northbridge or southbridge. For example, in various embodiments, the processing unit  4  may include an embedded memory controller (not shown) to enable more efficient transfer of data from the system memory  6  than the system bus  20  may provide. 
     It should be noted that aspects of the invention may be implemented using a computer system that is a subset of the general-purpose computer system described above. For instance, the computer system may be a blade server having a relatively limited set of input/output facilities. The computer system may also be implemented as an embedded system operating on a micro controller digital signal processor, application-specific integrated circuit, field programmable gate array, or the like, provided that the system includes sufficient input/output facilities to enable it to interface with a subject computer system being managed or with other computing devices. 
     The embodiments above are intended to be illustrative and not limiting. Additional embodiments are within the claims. In addition, although aspects of the present invention have been described with reference to particular embodiments, those skilled in the art will recognize that changes can be made in form and detail without departing from the spirit and scope of the invention, as defined by the claims. 
     Persons of ordinary skill in the relevant arts will recognize that the invention may comprise fewer features than illustrated in any individual embodiment described above. The embodiments described herein are not meant to be an exhaustive presentation of the ways in which the various features of the invention may be combined. Accordingly, the embodiments are not mutually exclusive combinations of features; rather, the invention may comprise a combination of different individual features selected from different individual embodiments, as understood by persons of ordinary skill in the art. 
     Any incorporation by reference of documents above is limited such that no subject matter is incorporated that is contrary to the explicit disclosure herein. Any incorporation by reference of documents above is further limited such that no claims that are included in the documents are incorporated by reference into the claims of the present application. The claims of any of the documents are, however, incorporated as part of the disclosure herein, unless specifically excluded. Any incorporation by reference of documents above is yet further limited such that any definitions provided in the documents are not incorporated by reference herein unless expressly included herein. 
     For purposes of interpreting the claims for the present invention, it is expressly intended that the provisions of Section 112, sixth paragraph of 35 U.S.C. are not to be invoked unless the specific terms “means for” or “step for” are recited in a claim.