Patent Publication Number: US-7900001-B2

Title: System and method for obscuring hand-held device data traffic information

Description:
BACKGROUND 
     Hand-held data processing devices are frequently wireless and allow users to communicate with a variety of other devices and systems. As part of such communication functionality, hand-held data processing devices generate and store data relating to the communications which have occurred using the hand-held devices. To improve operating functionality of such devices, the storage of certain data relating to communication information may be carried out using a cache. In such an arrangement, a hand-held data processing device includes a processor which operates in conjunction with a memory cache component in the device. The storage of data relating to device communication as entries in a fast access memory cache allows for operating efficiencies in the hand-held device. 
     For example, a device may allow for security certificate information to be stored as an entry in a cache to allow for faster access to such certificate information as part of secure communication operations carried out by the hand-held device. It is known to operate such caches in an ordered manner, such as a “move to front” cache in which the most recently accessed entry in the cache is placed at the front of the ordered cache. 
     Hand-held data processing device design includes the determination of what information is to be retained in caches, for how long, and whether to retain cache data when the device enters a locked state. With respect to the last design consideration, hand-held devices may be placed in a locked state in which much of the communication and other functionality of the device is disabled. Typically, a user must enter a password on the hand-held device to continue use of the device after it has been placed in a locked state (although certain functions, such as incoming telephone call reception may be permitted). The intention of the device entering the locked state is to heighten the security of the hand-held device. Unauthorized access is intended to be prevented by the device being placed in a locked state as it is intended that only the authorized user will be able to take the device out of the locked state by entering an appropriate password. 
     However, it is possible for attacks to take place on the security of the hand-held device even when the device is in a locked state. An unauthorized access to the device may allow a probe to be used to access the contents of memory on the device. Another type of attack involves a malicious application (“Trojan horse” software) being loaded on the device which will covertly execute to inspect the contents of device memory and make information about the memory contents available to an unauthorized attacker. To prevent information being extracted from a locked device, it is desirable to limit the information that is retained by the data in the device&#39;s memory cache when the device is in the locked state. To completely delete the data in the memory cache on entering a locked state, however, will mean inefficiencies are introduced in the operation of the hand-held device. In such a case, it would be necessary for the various memory caches on the device to be re-built whenever the hand-held device returns from a locked state. It is therefore the case in certain hand-held devices that the cache data is retained in the memory caches during the time that the device is in a locked state. 
     As set out above, such retained cache data may be subject to an unauthorized attack in which the memory cache is accessed. The information stored in the data in the cache is potentially placed at risk. In addition, traffic information relating to communications carried out by the hand-held device is potentially made available to the attacker if such communication information is stored in a cache. For such a cache in which the entries are ordered based on their most recent access, such as a move to front ordering, information about the recent communications of the hand-held device user will be available from an analysis of the location of entries in the relevant cache. Based on such information, an unauthorized access to the device may allow an attacker to determine traffic patterns for the device and hence provide knowledge of common communications carried out by the user. 
     It would accordingly be advantageous to provide a system and method to allow communication-related data to be retained in a hand-held device cache during a locked state but which makes difficult the acquisition of traffic pattern information by an unauthorized access of the hand-held device cached memory. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In drawings which illustrate an embodiment of the invention by way of example only, 
         FIG. 1  is a block diagram of a system overview of a hand-held data processing device of the preferred embodiment. 
         FIG. 2  is a block diagram showing a high-level abstraction of the relationship between a processor and cache memory of the preferred embodiment. 
         FIG. 3  is a block diagram showing a high-level abstraction of index and memory block for a cache of the preferred embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     According to an aspect of the invention there is provided an improved method for secure operation of a hand-held data processing device with communication functionality. 
     In accordance with an aspect of the invention there is provided a method for use in increasing the security of the communication device, the device being a hand-held data processing communication device comprising an cache memory for storing cache entries corresponding to device communication information, the cache memory being access-ordered during user operation of the communication device, the device being configured to enter a locked state, the method comprising the steps of the communication device defining a reordering set of entries in the cache memory for reordering within the cache memory, the communication device accepting or generating a trigger to enter a locked state, responsive to the trigger to enter a locked state, the communication device redefining the ordering of the cache entries in the reordering set of cache entries, relative to the contents of the cache memory, so as to no longer be access-ordered. 
     In accordance with an aspect of the invention there is provided the above method in which the access-ordered cache memory is a move to front cache memory with a front portion and in which the step of defining a reordering set of entries in the cache memory further comprises the step of selecting entries for the reordering set from entries located at the front portion of the cache memory. 
     In accordance with an aspect of the invention there is provided the above method in which the step of redefining the ordering of the cache entries in the reordering set of cache entries comprises the steps of stepping through the entries in the reordering set to successively select each of the entries in the set, for each selected entry randomly selecting an exchange target entry in the cache memory and exchanging the selected entry in the reordering set with the exchange target entry in the cache memory. 
     In accordance with an aspect of the invention there is provided a program product for improving the security of a communications device, the program product comprising a computer or communications device usable medium having computer or communications device readable program product code embodied in said medium, said program code being executable by a processor of a communications device for implementing the above methods. 
     In accordance with an aspect of the invention there is provided in a hand-held data processing communication device comprising an cache memory and cache manager for storing cache entries corresponding to device communication information, the cache memory entries being access-ordered during user operation of the communication device, and the device being configured to enter a locked state, an improved cache manager responsive to a trigger for the device to enter a locked state, and operative to redefine the ordering of the cache entries so as to no longer be access-ordered during the locked state. 
     Advantages of the preferred embodiment of the invention include the storage of cache information relating to communications of a hand-held data processing device such that when the device is placed in a locked state the cached data is retained in the data cache but traffic information for the device is obscured. 
     The preferred embodiment will be described in detail below, by way of example only, in the context of hand-held data processing devices  10  having wireless communications capabilities and a battery, as generally illustrated in  FIG. 1 . However, although the system and method is particularly advantageous in the hand-held data processing devices  10 , it will be appreciated that the principles of the system apply to other hand-held data processing devices having communication functionality and the system and method described are not intended to be limited thereby. The preferred embodiment may be implemented as a data processing device program product that includes program code for operation on a microprocessor to carry out the steps in the process described. The computing-device program product may be embodied in, and delivered to an intended recipient device by, signals carried by networks, including the Internet, or may be embodied in media such as magnetic electronic or optical storage media. The process described may be carried out by a combination of executable code and hardware embodied in a hand-held data processing device (the process being in this case a data processing device-implemented method). It is contemplated that the preferred embodiment will be implemented on wireless hand-held mobile devices in which resources such as power, memory and processing capacity are constrained by desired limits on the size and weight of the devices. 
     Hand-held data processing devices  10  each include a housing, a keyboard  14  and an output device  16 . The output device shown in  FIG. 1  is a display  16 , which is preferably a full graphic LCD. Other types of output devices may alternatively be utilized. A processor  18 , which is shown schematically in  FIG. 1 , is contained within the housing and is coupled between the keyboard  14  and the display  16 . The processor  18  controls the operation of the display  16 , as well as the overall operation of the data processing device  10 , in response to actuation of keys on the keyboard  14  by the user. 
     The housing may be elongated vertically, or may take on other sizes and shapes (including clamshell housing structures). The keyboard may include a mode selection key, or other hardware or software for switching between text entry and telephony entry. The housing contains the primary power supply, in the embodiment shown a battery  170 , a secondary power supply, which may be a small rechargeable battery or capacitor capable of sustaining power to the processor  18 , NAND flash  116  and SDRAM  118  for the duration of an AutoSave interval and a Power Controller  190 . 
     In addition to the processor  18 , other parts of the data processing device  10  are shown schematically in  FIG. 1 . These include a communications subsystem  100 ; a short-range communications subsystem  102 ; the keyboard  14  and the display  16 , along with other input/output devices  106 ,  108 ,  110  and  112 ; as well as memory devices  116 ,  118  and various other device subsystems  120 . The data processing device  10  is preferably a two-way RF communication device having voice and data communication capabilities. In addition, the data processing device  10  preferably has the capability to communicate with other computer systems via the Internet. 
     Operating system software executed by the processor  18  is preferably stored in a persistent store, such as a flash memory  116  , but may be stored in other types of memory devices, such as a read only memory (ROM) or similar storage element. In addition, system software, specific device applications, or parts thereof, may be temporarily loaded into a volatile store, such as a random access memory (RAM including also DRAM and SDRAM)  118 . Communication signals received by the data processing device  10  may also be stored to the RAM  118 . 
     The processor  18 , in addition to its operating system functions, enables execution of software applications  130 A- 130 N on the device  10 . A predetermined set of applications that control basic device operations, such as data and voice communications  130 A and  130 B, may be installed on the device  10  during manufacture. In addition, a personal information manager (PIM) application may be installed during manufacture. The PIM is preferably capable of organizing and managing data items, such as e-mail, calendar events, voice mails, appointments, and task items. The PIM application is also preferably capable of sending and receiving data items via a wireless network  140 . Preferably, the PIM data items are seamlessly integrated, synchronized and updated via the wireless network  140  with the device user&#39;s corresponding data items stored or associated with a host computer system. 
     Communication functions, including data and voice communications, are performed through the communication subsystem  100 , and possibly through the short-range communications subsystem. The communication subsystem  100  includes a receiver  150 , a transmitter  152 , and one or more antennas  154  and  156 . In addition, the communication subsystem  100  also includes a processing module, such as a digital signal processor (DSP)  158 , and local oscillators (LOs)  160 . The specific design and implementation of the communication subsystem  100  is dependent upon the communication network in which the data processing device  10  is intended to operate. For example, a data processing device  10  may include a communication subsystem  100  designed to operate with the Mobitex™, Data TAC™ or General Packet Radio Service (GPRS) mobile data communication networks and also designed to operate with any of a variety of voice communication networks, such as AMPS, TDMA, CDMA, PCS, GSM, etc. Other types of data and voice networks, both separate and integrated, may also be utilized with the data processing device  10 . 
     Network access requirements vary depending upon the type of communication system. For example, in the Mobitex and DataTAC networks, mobile devices are registered on the network using a unique personal identification number or PIN associated with each device. In GPRS networks, however, network access is associated with a subscriber or user of a device. A GPRS device therefore requires a subscriber identity module, commonly referred to as a SIM card, in order to operate on a GPRS network. 
     When required network registration or activation procedures have been completed, the data processing device  10  may send and receive communication signals over the communication network  140 . Signals received from the communication network  140  by the antenna  154  are routed to the receiver  150 , which provides for signal amplification, frequency down conversion, filtering, channel selection, etc., and may also provide analog to digital conversion. Analog-to-digital conversion of the received signal allows the DSP  158  to perform more complex communication functions, such as demodulation and decoding. In a similar manner, signals to be transmitted to the network  140  are processed (e.g. modulated and encoded) by the DSP  158  and are then provided to the transmitter  152  for digital to analog conversion, frequency up conversion, filtering, amplification and transmission to the communication network  140  (or networks) via the antenna  156 . 
     In addition to processing communication signals, the DSP  158  provides for control of the receiver  150  and the transmitter  152 . For example, gains applied to communication signals in the receiver  150  and transmitter  152  may be adaptively controlled through automatic gain control algorithms implemented in the DSP  158 . 
     In a data communication mode, a received signal, such as a text message or web page download, is processed by the communication subsystem  100  and is input to the processor  18 . The received signal is then further processed by the processor  18  for an output to the display  16 , or alternatively to some other auxiliary I/O device  106 . A device user may also compose data items, such as e-mail messages, using the keyboard  14  and/or some other auxiliary I/O device  106 , such as a touchpad, a rocker switch, a thumb-wheel, or some other type of input device. The composed data items may then be transmitted over the communication network  140  via the communication subsystem  100 . 
     In a voice communication mode, overall operation of the device is substantially similar to the data communication mode, except that received signals are output to a speaker  110 , and signals for transmission are generated by a microphone  112 . Alternative voice or audio I/O subsystems, such as a voice message recording subsystem, may also be implemented on the device  10 . In addition, the display  16  may also be utilized in voice communication mode, for example to display the identity of a calling party, the duration of a voice call, or other voice call related information. 
     The short-range communications subsystem enables communication between the data processing device  10  and other proximate systems or devices, which need not necessarily be similar devices. For example, the short-range communications subsystem may include an infrared device and associated circuits and components, or a Bluetooth™ communication module to provide for communication with similarly-enabled systems and devices. 
     In  FIG. 1  microprocessor  18  of hand-held data processing device  10  is shown with a connection to RAM  118  and a connection to flash memory  116 . As is typical in data processing devices, and is the case in the preferred embodiment, microprocessor  18  has available to it memory storage units that operate at different speeds. In the example of  FIG. 1 , RAM  118  is faster access memory and flash memory  116  is a slower memory unit. As will also be appreciated, microprocessors may also include memory storage units within the microprocessors themselves. Although this description of the preferred embodiment refers to memory storage units, memory components or memory storage devices, it will be understood that the physical arrangement of such memory can vary. The description of the preferred embodiment relates to a hand-held data processing device that is used for communication and which maintains a memory unit, memory device or memory component for the storage of data relating to information concerning the communication that has been carried out by the hand-held device. As is described below, for certain device designs, such information concerning the communications of the device permits communication traffic information for the device to be obtained directly or deduced. 
     To efficiently make use of the memory in hand-held data processing device  10  of  FIG. 1 , the preferred embodiment provides for memory caching. The use of a memory cache involves a fast-access portion of memory being set aside for storage of data that is identified as likely to be accessed by the processor of the hand-held device. In the example configuration shown in  FIG. 1 , data persistently stored in flash memory  116  may be copied into a memory cache location defined in RAM  118  for use during execution of particular software applications selected from  130 A . . .  130 N. The presence of such data in RAM  118  permits more efficient software operation as access to such cached data will be faster than access to data in flash memory  116 , for example. 
     As is shown in the example block diagram of  FIG. 2 , in the preferred embodiment, caches are separately defined and generated for different types of data. The simple example of  FIG. 2  shows processor  18  able to access RAM  118 . RAM  118  includes, by way of example, security certificate cache  200  and web page cache  202 . For secure communication functionality in particular, cache  200  for security certificates is desirable. Security certificates are used in secure communication between the hand-held device and other data processing devices and systems. A security certificate is typically stored (in flash memory  116 , for example) in a byte-array format. However, for use in software executing on microprocessor  18 , the security certificate is made available as a Java object. The conversion from byte array form to Java object form is typically a relatively complex operation and therefore it is preferable to retain the Java object form in a memory cache for future use. For this reason, the preferred embodiment includes a defined cache for security certificates. 
     In an alternative design for hand-held data processing device  10  a single communications-related cache may be defined to include heterogeneous cache entries. For example, such a cache may include security certificate information entries and web browser page entries. 
     Security certificate cache  200  is defined to include an entry in the cache corresponding to a certificate. In the preferred embodiment, the entry includes the byte array form of the certificate, a cyclic redundancy code for the byte array form, as well as the object form of the certificate. The example web page cache  202  shown in  FIG. 2  has a cache entry structure that differs from the cache entry structure defined for cache  200 . As will be appreciated, however, the definition of the cache entry structure will typically not be significant for the cache reordering described. For this reason, the description set out below relates to cache  200  as a representative cache defined in the examples of the figures. 
     In the example of  FIG. 2 , operating system  204  is shown executing on microprocessor  18 . Further, cache manager  206  is shown also executing on processor  18  and interoperating with operating system  204 . As will be appreciated, the specifics of how caches  200 ,  202  are managed will depend on implementation design decisions for hand-held data processing device  18 . In the preferred embodiment, cache manager  206  is executable program code that is executable by microprocessor  18  and is operative to manage access to and the maintenance of caches defined in RAM  118 . 
       FIG. 3  is a block diagram showing, at a high level of abstraction, the format of cache  200 . In the preferred embodiment, entries in the memory caches are ordered relative to a specified first entry, or front entry, of the cache. As will be appreciated by those skilled in the art, the ordering of the cache entries, and the definition of the front of the cache, may be implemented in different ways. These implementations provide for identifiable entries in the cache that are ordered, based on the most recent access to the cache entries by the microprocessor. In the preferred embodiment the ordering is relative to a front entry and to each other. 
     In the preferred embodiment as shown in  FIG. 3 , cache  200  is provided with an index  210  that defines the ordering of the entries in cache  200 . Entry order is maintained by operation of cache manager  206  in a move to front arrangement in which the most recently accessed entry in cache  200  is moved to the front of cache  200 . Index  210  is shown with entry  0  being the front of the cache. Cache  200  entries are shown in memory block  212  and are denoted by letters A-F. Thus in the example of  FIG. 3 , the front of cache  200  is defined by index entry  0  in index  210 , which points to cache entry C in memory block  212 . The preferred embodiment is described with respect to the move to front ordering of caches. However, the approach of the preferred embodiment is also applicable to other defined cache ordering designs where the ordering is based on the relative timing of access to the various cache entries (referred to in this description as “access-ordered”). For example, a move to back cache arrangement will potentially provide the same type of traffic information as a move to front arrangement and the reordering of the cache entries described below may be carried out on such an ordered cache, with the appropriate modifications. 
     When, as part of application execution, microprocessor  18  execution seeks access to another entry in the cache, cache manager  206  code executes such that the identifier for the other entry in memory block  212  (often the memory address) will be placed at index entry  0  in index  210 , and the other entries in cache index  210  will be pushed down the index. In this manner, cache  200  effectively maintains a record of the most recently accessed cache entries. In the simple example of  FIG. 3 , therefore, cache entry D is shown as having been accessed prior to entry C, and F is shown as accessed prior to D, as is reflected by the relative locations of those entry identifiers in index  210  (at index entries  0 ,  1 ,  2 , respectively). In the preferred embodiment, a similar arrangement is defined (using cache manager  206  operation) for other caches such as web page cache  202  that maintains copies of web pages accessed by hand-held data processing device  10 . 
     The access-ordering of cache entries, such as that found in the move to front arrangement described above, provides for efficient use of the memory cache. However, knowledge of the use of such an arrangement and access to the cache contents may potentially be used by an attacker to obtain information about communication traffic on the hand-held device. As will be apparent, by access-ordering the cache entries, such as in a move to front manner, traffic pattern information will be potentially revealed by inspecting the ordering of cache entries (in the example of  FIG. 3 , by inspecting the ordering index  210 ). 
     In hand-held data processing device  10  of the preferred embodiment, security of operation is supported by the device entering a locked state. During such a state it is intended that many of the communication functions of the device will be unavailable. In the preferred embodiment, the device is capable of being used to answer an incoming mobile telephone call, but is otherwise unusable. The implementation of such a locked state may vary for different hand-held devices. 
     In the preferred embodiment, on a user attempting to access device  10  using keyboard  14  or other input devices  106  (such as a thumbwheel) during a locked state, display  16  will merely show the user a screen requiring the user to input a password. The user can unlock the device by entering the correct password on the device using keyboard  14 . If the password is incorrect the device will remain in a locked state. However, after successfully exiting the locked state, the communication and other functionality of device  10  are made available to the user. 
     In the preferred embodiment device  10 , a locked state may be entered by the user using keyboard  14  or input device  106  to choose to lock the device by selecting a lock icon displayed on display  16 . Alternatively, a locked state may be entered after an inactivity timeout (as may be defined by the user or an administrator for the device). Further triggers for entering a locked state include: device  10  being physically returned to a cradle or holster; expiry of a defined amount of time since the previous locked state exit (independent of the activity or inactivity of the device); and receipt of a command by device  10  from an administrator to force the device  10  to lock. As suggested by this list of potential triggers, device  10  of the preferred embodiment is designed to often be placed in a locked state in typical use. As device  10  is a mobile, wireless hand-held unit that potentially stores confidential communication and other information, the use of a locked state is part of the device operation intended to maintain the security of the device. 
     Because the locked state is entered and exited frequently in typical use of the hand-held device of the preferred embodiment, entries in caches such as cache  200  are retained in device  10  during the locked state. Efficiency of operation of device  10  is increased if the cached data remains available for use by processor  18  on exit from a locked state. 
     Although placing hand-held data processing device  10  in a locked state will provide a sufficient level of security in most cases, attacks on the data of the device may be made using probes, Trojan horse applications, or other techniques developed to determine the content of memory in device  10 , despite the fact that the device is in a locked state. As indicated above, without other steps being taken, the use of an ordered arrangement of the cache elements in cache  200  will potentially provide traffic information to an attacker who is able to obtain unauthorized access to the memory contents of device  10  during a locked state. 
     In the preferred embodiment, however, such traffic information is obscured by the software cache manager software  206  executable by microprocessor  18  upon device  10  entering into a locked state. According to the preferred embodiment, when a trigger for the device to enter a locked state occurs, the ordering of entries in cache  200  is altered so that during the locked state, cache  200  no longer has a move to front ordering. In this way, the entries in cache  200  are retained to allow for continued efficient access to such data, but traffic information, that could otherwise be obtained by inspecting the move to front ordering of the cache, is obscured. 
       FIG. 2  shows cache manager  206  interoperating with operating system  204 . In the preferred embodiment, operating system  204  will communicate the request to enter a locked state to cache manager  206  (amongst other executing applications and utilities). Cache manager  206  includes code executable by microprocessor  18  and operative to carry out the reordering of the communication-related memory caches, as described below. An alternative architecture (not shown) provides a different cache manager for each of caches  200 ,  202  so as to provide cache management functions related to the characteristics of the different types of caches used in device  10 . 
     In the preferred embodiment example of  FIG. 3 , a reordering set  214  of entries in cache  200  is defined. The index arrangement of cache  200  results in this definition of the set of cache entries being carried out with reference to index entries. In other cache implementations, such a set of cache entries may be defined in other ways. For example, where a cache is defined without an index and the (move to front) ordering of the cache is accomplished by a rearrangement of memory within the block of cache entries itself, the definition of reordering set  214  will be made with respect to the entries in the block of entries directly. 
     In the preferred embodiment, the set of cache entries include entries located at the front portion of the cache. For appropriate cache sizes, such as that shown in  FIG. 3 , set  214  may be defined to be equal to the cache size itself (in such a case the step of defining reordering set  214  may become trivial or may be found implicitly in the reordering step). In other cases, set  214  may be a subset of the cache. The set of cache entries  214  is defined such that the members of set  214  will be reordered and therefore, in the move to front arrangement of cache  200  in the preferred embodiment, those entries in the reordering set  214  include entries at the front portion of the cache. As will be appreciated, as reordering set  214  becomes larger and comes close to the size of cache  200 , the traffic information obtainable by inspecting the contents of cache  200  becomes increasingly more obscured after reordering. In the example of  FIG. 3 , all six entries in cache  200  are included in reordering set  214 , as reordering set  214  is defined to include index entries  0  . . .  5 . For other move to front caches, the definition of the front portion of the cache will vary, depending on the size of the cache and the communication patterns of the device. Where for a particular communication mode there are limited communication-related cache entries likely to be generated in typical device use, the size of the front portion of the cache for such communications may be relatively small, and conversely where the number of entries is anticipated to be large. 
     Where the cache is access-ordered (based on relative access times for the cache entries) but is not a move to front arrangement, reordering set  214  is defined to include that portion of the ordered cache whose entries includes data reflecting the aspect of the device traffic that is intended to be kept most secure. Typically, this will be the cache entries most recently accessed. However, it may be that in some cases the least recently accessed cache entries will be of interest to a potential attacker and reordering set  214  can therefore be defined to include the cache entries that have remained unaccessed for the longest time. As will be apparent, where constraints of the performance of device  10  permit reordering set  214  to be defined to be the size of the entire cache whereby, all entries may be subject to reordering. 
     As referred to above, it may also be possible for hand-held data processing device  10  to include a cache capable of storing heterogeneous entries. In such a case, reordering set  214  may also be defined such one or more types of entries are included in the set while other types of entries are not (ie. entries are selected based on pre-defined entry attributes). For example, security certificate entries might be defined to be potential members of reordering set  214  while telephone number entries may be considered not to be of security significance and to therefore be excluded from reordering set  214 . 
     In preferred embodiment, the step of reordering the cache entries in reordering set  214  is carried out by stepping through the entries in reordering set  214  to successively select each entry in the set. For each selected entry, the reordering includes the step of randomly selecting an exchange target entry in the cache and then exchanging the selected entry in the reordering set with the target entry in the cache. In this manner, a random reordering of the entries in reordering set  214  in cache  200  is accomplished. As will be appreciated, other reordering techniques may be used (such as sorting the entries according to a selected hashing function). Such techniques will result in the cache entries being reordered such that the previously defined access-order of the cache entries is disrupted so that previously available traffic information is obscured. 
     As will be appreciated, other methods of carrying out such a reordering are possible. For example, where the cache size is large relative to the reordering set, it may be possible to interleave the reordering set entries within the larger set of cache entries without disturbing the ordering of the reordering set entries relative to each other. Such a reordering will, for certain cache structures, effectively destroy the relationship between the position of entries in the ordered cache with the recent communication activity of the device. These techniques are defined such that when device  10  enters a locked state, following such a reordering, the ordering of entries in cache  200  as a whole is no longer an access-ordering and information about device traffic, such as the most recent communication carried out on the device, is thus obscured. 
     As will be apparent, communication-related traffic information stored in different caches may be handled in the same manner. For example, device  10  may include caches for telephone numbers, web pages (as in cache  202 ), or calendar items updated using the device communication functions. Traffic information concerning all such cache entries may be sensitive and it is potentially desirable to maintain such cache entries in their respective caches during a locked state while obscuring the traffic information that could otherwise be gleaned from the ordering of such caches. In such cases, the approach described above may be used so as to obscure such traffic information. 
     An embodiment having been thus described in detail by way of example, it will be apparent to those skilled in the art that variations and modifications may be made without departing from the invention.