Abstract:
A method for guaranteeing freshness of results for queries against a non-secure data store extends a write operation to include steps of: receiving a write instruction for application data to be written into a hierarchical tree structure; incrementing a timer responsive to receiving the write instruction; setting a timestamp to the value of the timer; computing a message authentication code based on the received application data and the timestamp; appending control information to the application data and its corresponding check item; the control information including the timestamp, a link to a check entry, and the message authentication code; writing the application data with the appended control information to the data store as a primary item; and updating the control information for each check item associated with the primary item along a path from the primary item to a root by following links.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   Not Applicable. 
   STATEMENT REGARDING FEDERALLY-SPONSORED RESEARCH OR DEVELOPMENT 
   Not Applicable. 
   INCORPORATION BY REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC 
   Not Applicable. 
   FIELD OF THE INVENTION 
   The invention disclosed broadly relates to the field of data security, and more particularly relates to the reliable retrieval of data from a non-trusted data store. 
   BACKGROUND OF THE INVENTION 
   Information Technology (IT) systems depend on reliable data stores and these data stores are often situated outside of the secure computational environment of the IT system and consequently are vulnerable to attack. Secure computational environments can be used to protect their internal applications from physical and logical attacks, but these applications may still depend on external data stores, which cannot be deployed inside the secure computational environment. See Trapp, et al., “Method and Apparatus for Secure Processing of Sensitive Data,” application Ser. No. 10/065,802, hereby incorporated herein by reference. 
   Standard cryptographic techniques can be used to encrypt and authenticate the contents of the data store, and can thus protect the data against spying and unauthorized modifications, but they are not sufficient to guarantee that queries to a data store always return the most accurate and up-to-date data. A replay attack to a query against a data store is an attack in which an attacker answers the query with data that was once stored in the data store, but is no longer current. An attacker who gains entry into the data store can respond to queries made by an application program with outdated data, dissimulating that these are the actual contents of the data store. Such an attack is often called a replay attack, because the attacker “replays” data that was formerly valid in the system. This is a critical problem in many applications today which rely on information from data stores. The severity of this problem becomes apparent if one considers an application querying the amount in a bank account or entries in a watch list of criminals. 
   Although there are known countermeasures to replay attacks for secure communication channels over computer networks, these techniques cannot be adapted to guard against replay attacks for database queries against a data store situated outside the secure computational environment (i.e., a non-secure data store). Most large systems today execute outside of a secure computational environment, therefore there is a need for a method which overcomes the shortcomings of the prior art. 
   SUMMARY OF THE INVENTION 
   According to the invention, a method allows an application that executes inside a secure computational environment to detect and prevent replay attacks during queries to a non-secure data store, e.g. a database or a file system. Briefly, a method according to the invention extends a write operation to comprise the steps of: receiving a write instruction for application data to be written into a hierarchical tree structure; incrementing a timer responsive to receiving the write instruction; setting a timestamp to the value of the timer; computing a message authentication code based on the received application data; appending control information to the application data; the control information comprising the timestamp and the message authentication code; writing the application data with the appended control information to the non-secure data store as a primary item; and updating the control information for each check item associated with the primary item along a path from the primary item to a root by following links. 
   A method according to the invention extends the read operation for reading application data from a non-secure data store to comprise the steps of: receiving a read instruction for application data; determining a location within the non-secure data store for the application data; validating a message authentication code contained at the location; parsing data from the location into application data and control information; authenticating the control information; and transmitting the application data to a calling application. 
   According to an embodiment of the invention, a system for the detection and prevention of replay attacks comprises logic configured to perform the above methods. 
   According to another embodiment, a computer program product comprises instructions for performing the above methods. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates the high-level structure of the secure computational environment and the data store. 
       FIG. 2  illustrates an abstract view of the protection against replay attacks. 
       FIG. 3  illustrates the detailed structure of items and the check tree. 
       FIG. 4  illustrates the data structures for items and links as class diagrams in the Unified Modeling Language (UML). 
       FIG. 5  illustrates the detailed flow for writing a primary item to the data store and for updating the control information in the check tree. 
       FIG. 6  illustrates the detailed flow for reading a primary item from the data store and for checking the control information in the check tree. 
       FIG. 7  illustrates an alternative to  FIG. 1  for keeping some items within the secure computation environment. 
       FIG. 8  illustrates an alternative to  FIG. 7  with omitted check entries inside the secure computation environment. 
       FIG. 9  shows the equations for computing links to check entries on demand. 
       FIG. 10  illustrates the shared use of a single data store or multiple data stores from multiple secure computation environments. 
       FIG. 11  illustrates an example of a system whereby multiple secure environments access at least one data store through one secure environment, designated as a master. 
       FIG. 12  shows a simplified block diagram of a computer program product on which an embodiment of the invention can be advantageously used. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   We discuss a method to recognize and prevent replay attacks during queries against an external (non-secure) data store made by an application running in a secure computational environment. 
   A secure computational environment (“secure environment”) is one wherein transactions cannot be viewed or accessed from outside of the secure environment. Further, it is impossible to maliciously change the processing or the processed data from outside of the secure environment. Attempts to tamper with a secure environment, the programs running inside, or the data being processed inside are detected by the environment, which then destroys sensitive data stored inside the environment or makes it permanently inaccessible. An example of a secure environment is a general-purpose computing device such as the IBM 4758 cryptographic compressor, which has a FIPS (Federal Information Processing Standards) 140-1 Level 4 validation. See Schneck, et al., “System for Controlling Access and Distribution of Digital Property”, U.S. Pat. No. 6,314,409, issued Nov. 6, 2001, hereby incorporated herein by reference. 
   In a preferred embodiment of the invention, the data store, which can be a database or a file system, is viewed as a set of items. An item is a part of the data store that is always read and written in one piece. An item can, for example, denote a single field or an entire row in a database table, a record in a file, or a whole file. There are two kinds of items: primary items and check items. Application data is stored in primary items. Check items contain the control, or security, information. Primary and check items contain a message authentication code, a link to a check entry and a virtual time (VTime) field. Each check item holds at least two check entries. The check entries represent single VTime values stored as an array within the check item. 
   An important feature of the invention is the use of a global counter, timestamps and a hierarchy of check entries to determine, inside a secure environment, whether a query to an external data store returns the most up-to-date data (the freshest data). Virtual time is the value of a global counter which is logically located within the secure environment. The global counter is incremented each time a primary item is written into the data store. For details see  FIG. 5 . The counter can be implemented in hardware or software, but its value must not be lost when the secure environment is reset or rebooted. Preferably the value of the global counter is stored in battery-backed RAM inside the secure environment. 
   Another aspect of the invention is that items (primary and check items) are augmented (i.e., appended) with a VTime value (timestamp), a unique link to a check entry, and a message authentication code (MAC). All sensitive operations are confined within the secure environment. For example, the MAC is generated and checked inside the secure environment using a unique MAC key, which is kept inside the secure environment where it cannot be altered from the outside. 
   Items and their links form a tree-like hierarchical structure, where the leaves of this tree are primary items, and all other nodes, including the root, are check items. The method embodies processing which extends both the read and write operations to/from a non-secure data store. Replay attacks are detected during read operations. Countermeasures to detect and prevent future replay attacks are implemented during write operations and are described below. 
   Referring to  FIG. 1  there is shown a high level diagram of a system  100  for protection against replay attacks. Inside a secure environment  150 , an application  101  executes a command to store application data  102  into a data store  107 , which is outside the secure environment  150 . The arrow  132  represents a communicative association between the secure environment  150  and the data store  107 . Since the data store  107  is outside of the secure environment  150 , it is considered to be a non-secure system. We define a “non-secure” system as one which can be illegally accessed (i.e., unauthorized access). It therefore follows that a “trusted” system is one which cannot be illegally accessed. 
   According to an embodiment of the invention, an application  101  executes within (is logically located in) a secure environment  150 . The Global Counter (the timer)  104 , the Encoding/Decoding Service  103 , the Message Digest Service  105  and the Encryption Engine  115  are also logically located (i.e., not necessarily physically located) within the secure environment  150 . According to the embodiment described herein, a write operation would proceed as follows: the application  101  transmits the application data  102  to the Encoding/Decoding Service  103 . The Encoding/Decoding Service  103  increments a Global Counter  104  and augments the application data  102  with the actual value of the Global Counter  104  and creates a link  111  from a primary item  110  to an entry in a check item  109 . 
   Several schemes are possible for the creation of the links from items to their check entries: the Encoding/Decoding Service  103  can a) allocate and assign a new check entry whenever it writes an item; or b) allocate and assign the check entry for an item during the first write operation to that item. Consecutive writes to the same item will then reuse the link stored in that item. A variation of b is a scheme c) where the system generates items, check entries, and the links between them during an initialization phase or periodically on demand. In this case, whenever the Encoding/Decoding Service  103  wants to update an item, it reuses these predefined links, which are stored in the initialized items. In the following discussion we will assume, for the sake of simplicity, that scheme c is used. Further details for the allocation of check entries and an alternative representation of links are given in  FIG. 9 . 
   Details of the structure of items and the check tree  140  will be given in  FIG. 3 . The Encoding/Decoding Service  103  uses the Message Digest Service  105  to compute a MAC (Message Authentication Code) for the augmented data (which includes the link and timestamp) using the MAC key  106 . Optionally, the data being written into the primary and check items can be encrypted and decrypted using the Encryption Engine  115 . The augmented and protected data is then written into a primary item  110  inside the data store  107 . Then the Encoding/Decoding Service  103  updates the associated check items  109  by updating the timestamps in the check entries and the MACs for the check items  109 . Inside the check item  109 , two timestamps are updated: one is the check entry, which is referenced by another check item, and the other is the timestamp of the check item itself. The timestamps are updated with the time entry created when the Encoding/Decoding Service  103  incremented the Global Counter  104 . 
     FIG. 1  shows a subset of a check tree  140  as an example: dashed arrows and dots represent arbitrary layers of check items  109 . The VTime that is stored as the timestamp of a primary item  110  or check item  109  is the point in time (as indicated by the value of the Global Counter  104 ) when the item was last written to the data store  107 . This VTime is also recorded in the item&#39;s check entry. Therefore, whenever a primary item  110  is written to the data store  107 , all check items  109  on the check tree path between the primary item  110  and the root  108  of the check tree  140  are also updated with the new timestamp. 
   The Encoding/Decoding Service  103  updates the check items  109 , starting with the check entry that was referenced by the link in the just-written primary item  110 , and then traverses up the tree structure  140  until it has updated the root  108  of the tree  140 , thus updating all of the control information within items along that path. 
   In one embodiment of the invention, it is possible to avoid the explicit storage of links to check entries. The addresses of the check entries are computed on demand from the address of the item in which the link would otherwise be explicitly stored, thus eliminating the need for explicitly storing the links. The required equations for this embodiment are shown in  FIG. 9 . 
   A read operation begins when the application  101 , inside the secure environment  150 , delegates a request for application data  102  to the Encoding/Decoding Service  103 . The Encoding/Decoding Service  103  reads the requested primary item  110  and all check items  109  that are on the path from the requested primary item  110 , up to and including the root  108  of the check tree  140 . For all items that are read, the Encoding/Decoding Service  103  uses the Message Digest Service  105 , with the MAC key  106 , to verify that all MACs in the read items are correct. For these tests, the Encoding/Decoding Service  103  recomputes a MAC for the data (including link and timestamp) stored in the item, and compares it to the MAC that is currently stored in the item. If the data is authentic, i.e. it was generated by the Encoding/Decoding Service  103 , both values (the MAC that is currently stored in the item and the recomputed MAC value) are the same. If the MAC comparison is successful, the Encoding/Decoding Service  103  then determines if the timestamp value of the root  108  of the check tree  140  is equal to the actual value of the Global Counter  104 . If the MAC comparison is not successful, the read operation terminates and an error is reported to the caller. 
   When a primary item  110  is read from a data store  107 , the item&#39;s timestamp must still match the value in its check entry. The timestamp of the check item, which holds this check entry, must match its own check entry and so on, up to the root  108  of the check tree  140 . The timestamp of the check tree&#39;s root  108  must always be equal to the actual virtual time inside the secure environment  150  (as indicated by the Global Counter  104 ). If one of these conditions fails, or if any MAC of these items is wrong, the data is assumed to have been tampered with or replayed, and the method reports an error. 
   Additionally, the Encoding/Decoding Service  103  determines, for all other items read, whether their timestamp values are identical to the values stored in the check entries referenced by the item&#39;s link  111 . The Encoding/Decoding Service  103  performs a check to determine whether each item on the path from the primary item  110  to the root  108  is fresh. Immediately after a primary item  110  and its check items  109  are written, the timestamps of all these items are set to the actual virtual time (the value of the Global Counter  104 ). When a primary item  110  and its check items  109  are read, they do not necessarily all have the same timestamps, but for each item, the timestamp must be the same as the timestamp recorded in the item&#39;s check entry. The items on the same path from primary item  110  to the root  108  of the check tree  140  only have the same timestamps if no writes to different primary items (and their check items) occurred between the write and the read of a primary item (and its check items). If one write operation to a different primary item b comes after a write to primary item a and before a read of primary item a, then: all of the items on the path from b to the root  108  have the same timestamps; and it follows that now not all of the items on the path from a to the root  108  have the same timestamps anymore (at least the root  108  has changed in the meanwhile, but maybe also the timestamps of additional items, if the path from a to the root, and the path from b to the root  108  have more items in common). What holds in both cases is that the timestamp of each item is identical to the timestamp in its check entry, which is located inside its check item  109 . 
   If all MACs are found to be correct, and all timestamp comparisons match, the Encoding/Decoding Service  103 , using, for example, a parsing algorithm, strips off the timestamp, link, and MAC from the contents of the primary item  110 , and returns the remaining data as application data  102  to the application  101 . If a MAC is incorrect, or a timestamp comparison fails, the Encoding/Decoding Service  103  returns an error code to the calling application  101 , indicating that the data in the data store  107  was tampered with and consequently may contain out-of-date contents, perhaps as the result of a replay attack. 
     FIG. 2  shows the abstract steps for protection against replay attacks. The method extends the read/write operations to and from the data store  107 . Replay attacks are detected during reads from the data store  107   
   The extended write operation comprises steps  201 ,  202 , and  203 . In step  201  the application data  102  is augmented with control information, such as the VTime when the item was stored in, or written to, the data store  107 . This VTime is the value of the Global Counter  104 . In step  202  a tree-like, hierarchical data structure, called the check tree, is maintained, which consists of check entries which are stored in check items. A check entry records the last modification time of the item that links to it. The tree-like structure is maintained with the use of links from primary item  110  to check items  109  along a path to the root  108 . In step  203 , the value of the Global Counter  104 , which shows the last modification time of the root  108  of the check tree  140 , is preserved inside the secure environment  150 . In other words, the value of the Global Counter  104  remains the same until the next time that data is written to the data store. 
   The extended read operation comprises the steps  204 ,  205 , and  206 . In step  204 , a primary item  110  is read from the data store  107 . The modification time of that item (recorded in the item itself) is then compared to the modification time recorded in the item&#39;s check entry in step  205 . This check is repeated for all parent nodes in the check tree  140  up to the root  108 . In step  206  the last modification time of the root  108  is compared to the value of the Global Counter  104  inside the secure environment  150 , which reflects the last point in time when the root was modified. A replay attack will be detected if any of these checks fail, or if the MACs of the items are not valid. 
     FIG. 2  gives a very abstract view. Details about the structure of data items, the structure of the check tree  140 , protection of items with MACs, data definitions, algorithms and variations of the method are given below. 
     FIG. 3  shows the detailed structure of primary items  110  and check items  109  in the check tree  140 . It shows a subset  300  of check tree  140  as an example: dashed arrows stand for arbitrary layers of check items  109 . Primary items  110  consist of two parts: the content,  302 , which holds application data, and the control information which contains: a timestamp  303 , a link  304  which holds the address of a check entry, and a MAC  305 . Check items  109  contain an array  307  of at least two check entries, and, as in primary items, a timestamp  308 , a link  309  to a check entry, and a MAC  310 . Timestamps  308  denote the point in virtual time (which value is provided by the Global Counter  104 ) when the item was last written. The MAC  310  protects the content of the items against unrecognized modification. 
   The root  108  of the check tree  140  is special. It is a check item  109  which does not have a link to a check entry. All other items  109  and  110  have links that point to check entries. Links are pairs &lt;c, s&gt;, where c denotes a check item ( 109 ), and s denotes an index into c&#39;s array of check entries ( 307 ). A link &lt;c x , s x &gt; points from an item x to x&#39;s check entry, which is located in check item c x . Each item has a unique link, i.e. no two items are linked to the same check entry. The leaves of the check tree  140  are always primary items  110 . The root  108  and the inner nodes of the check tree  140  are all check items  109 . 
   Check items  109  have to be created and linked whenever a new primary item  110  is allocated in (i.e., written to) the Data Store  107 . If the number of primary items  110  is fixed, all items and links  111  can be created beforehand. It is also possible to proportionally increase the number of primary items  110  and thereby create the necessary check items  109  and links  111  accordingly. 
   The size of the check entry array  307  in check items  109  is arbitrary, the only requirement is that it contains at least two entries. This guarantees that the size of a check tree  140  for a finite set of primary items  110  will also be finite. Different check items  109  may have check entry arrays of different sizes. The larger the check entry arrays are, the shorter will be the height of the resulting check tree, producing a flatter hierarchical structure. This results in less check entries on a path in the check tree, but also in larger check items  109  that have to be updated and checked during read/write operations. 
   In another implementation of the system to prevent replay attacks some or all of the data that is stored in the primary items  110  and/or the check items  109  are encrypted, using the Encryption Engine  115 . While this is a desirable feature, it is not necessary for replay prevention. 
     FIG. 4  shows the definition of items and links as a class diagram in the Unified Modeling Language (UML). The Item class  401  is the common super class of the PrimaryItem class  403 , and the CheckItem class  404 . Item class  401  defines a timestamp, a MAC, and aggregates a value of the Link class  402 . All of these fields are inherited by primary and check item objects. The type “MacType” is a type suitable to hold a MAC. The type “VTime” of the timestamp is an integral type large enough to express a sufficiently large number of points in virtual time, e.g. &gt;32 Bit. VTime values are incremented during write operations and must not overflow during the lifetime of the system. While objects of the PrimaryItem class  403  have a content field of some arbitrary type, objects of the CheckItem class  404  hold check entries. A check entry array is an array of VTime values. A single check entry is one value in this array. A single check entry records the last modification time of the item that references this entry. Note that a CheckItem  404  holds several timestamps: at least two timestamps are stored in the check entry array, and an additional timestamp is inherited from the Item super class. A timestamp in a check entry records the last modification time of the item which links to it, whereas the inherited timestamp records the last modification time of the check item itself. Link  402  contains two values: a reference to a CheckItem  404 , expressed by the location association, and an integer value called idx, which is used to select a check entry in the check item. 
   A location is a value that uniquely identifies an item in the data store  107 . The structure of this value depends on the granularity of an item, such as whether the item is a database row, database field, or a record in a file. For example, to refer to a database row, the location value could be the name of the database table together with a key value that uniquely selects a row in this table. As another example, if items are records in an indexed file, then a location value would consist of the filename and the index of the record in the file. 
   The PrimaryItem class  403  provides two operations, read and write, that implement the method for detection of and protection against replay attacks. The write operation saves application data  102  to a primary item  110  in the data store  107  and also maintains the additional control information in the check tree  140 . The read operation reads application data  102  from a primary item  110  in the data store  107  and also validates the control information in the check tree  140  to detect a replay attack. Details about these operations are given in  FIGS. 5 and 6 . 
     FIG. 5  shows the logic flow for writing application data  102  to a primary item  110  in the data store  107 , and for maintaining the control information in the check tree  140 . The operation has two parameters: the application data  102  to be stored, and the location where it should be stored. The location has to refer to a primary item  110 . It must not refer to a check item  109 . 
   In step  500  the Global Timer  104  is incremented to provide a new VTime value for this write operation. Step  501  reads the item that is currently stored in the data store  107  at the given location. The actual content of the item is not considered at this point, but rather its link information, which is required later in steps  506  and  507 . The current content is overridden with the new application data  102  in step  502 . In step  503  the item&#39;s timestamp is set to the actual virtual time. The item is written back to the data store  107  in step  505  after the MAC for the item is computed and set in step  504 . Step  506  determines whether the link field of the item points to a check entry. If it does not point to a check entry, the item is the root  108  of the check tree  140  and the write operation is completed in step  510 . 
   If the link field points to a check entry, processing continues in step  507 . In step  507  the location of the item and the index idx of the check entry within this item are fetched from the link field of the actual item. In step  508  the check item is read from its location in the data store. Step  509  updates the timestamp at index idx in the check entry array of the just read check item with the actual virtual time. Processing then loops back to step  503  to protect and store the check item  109  and to continue with additional check entries up to the root  108  of the check tree  140  (which will eventually be detected in step  506 ). 
     FIG. 6  shows the flow for reading application data  102  from a location in the data store  107 , and for checking the control information in the check tree  140 . The parameter for the read operation is the location of a primary item  110 . The operation will return the application data  102  stored in the item in the data store  107 . 
   In step  601  the primary item  110  is read from the given location in the data store  107 . The primary item&#39;s MAC is checked in step  602 . If this check fails, processing ends in step  613  and an error is reported to the calling application  101 . If the MAC is correct, step  603  assigns the content field of the just read item to a variable data, so that it can later be returned to the calling application  101  in step  612 . Step  604  saves the item&#39;s timestamp in a variable stamp for later comparison. Step  605  then determines whether the link field of the actual item points to a check entry. If it does not point to a check entry, the actual item is the root  108  of the check tree  140  and processing continues with step  611 . If the link of the actual item points to a check entry, processing continues to step  606  where the location of the check item  109  and the index idx of the check entry within this check item  109  are fetched from the link field of the actual item. This check item  109  is read from its location in the data store  107  in step  607 . Step  608  checks the MAC of the just read check item. If the MAC is not correct, processing ends in step  613 . If the MAC is correct, processing continues to step  609 . 
   Steps  609  and  610  compare the timestamp at index idx in the check entry array of the just read check item with the value of stamp (which was set to the virtual time obtained from the former item in step  604 ). If these values are different, processing ends in step  613 . If the values are the same, processing loops back to step  604  to check additional check entries up to the root  108  of the check tree  140  (which will eventually be detected in step  605 ). When processing reaches step  611  the check tree  140  has been successfully checked up to the root. Step  611  now compares the stamp variable, which now holds the timestamp of the root item, with the actual virtual time, i.e. the value of the Global Counter  104 . If both values are the same, no replays have been detected and the read operation terminates successfully in step  612  by returning the value of data, which was set in step  603 . Note that only the value of data (the content only) is transmitted to the calling application. If the two values differ, processing ends in step  613 , with an error reported to the caller. 
   Step  613  is reached only if a replay attack or a data modification is detected, i.e., if a comparison of timestamps failed in step  610  or  611 ; or if validation of a MAC failed in step  602  or  608 . If step  613  is reached, the read operation terminates and signals the error to the caller in order to alert the caller to a possible contamination of the data. Any subsequent operation which depends on the read value is now unsafe with a high probability of producing invalid results. The caller has several courses of action in dealing with this error. Three possible choices are:
         a) retry the read operation in the expectation that the modification was only temporary. This would be possible if an attacker didn&#39;t permanently modify the data store  107 , but intercepted and altered the communication between the data store and the secure environment  150 ;   b) if the system has a backup of the item, and a complete log of the transactions that were executed in the meantime, and if this information can be read successfully without encountering further replay attacks, the system can restore the account and the caller can try the read again; or   c) the caller can precipitate a processing failure, thus forcing a rollback of the actual transaction, and signal the event to an operator.       

     FIG. 7  and  FIG. 8  show alternative embodiments to the data layout in  FIG. 2 . 
     FIG. 7  shows the secure environment  701  but it omits the components that had been shown in  FIG. 1  inside the secure environment  150 . It also shows a non-secure data store  702  with primary items  706 . The important distinction between this data store  702  and the data store  107  of  FIG. 1  is that only some of the check items  705  are placed in the data store  702 . The root  703  of the check tree, and maybe several layers of check items  704 , which are close to the root  703 , are placed inside the secure environment.  FIG. 7  shows a subset of a check tree as an example: dashed arrows stand for arbitrary layers of check items. 
   This embodiment is functionally equivalent to the one shown in  FIG. 1 , but it reduces the height of the part of the check tree that is kept in the data store  702 . A disadvantage is that the check items  703 - 704  inside the secure computational environment  701  may consume precious memory resources of the secure environment  701 . The advantage is that for the flows shown in  FIG. 5  and  FIG. 6  the number of items that have to be read from and written to the data store  702  can be reduced, which in turn can significantly speed up the process. A decision on which embodiment to employ has to consider this trade-off between speed and memory allocation. 
     FIG. 8  illustrates another embodiment of the invention which is essentially a modification of  FIG. 7 . In comparison to  FIG. 7 , the check items  803  inside the secure environment  801  do not have links to check entries. Links are required for check items  804  and primary items  805  in the data store  802 , but they are not necessary for items inside the secure environment  801 . The root of the check tree and other check items can be omitted, as long as there are no direct links that reach them from items  804 - 805  in the data store  802 . If these check items are omitted, the flows in  FIG. 5  and  FIG. 6  have to be changed accordingly: check items  803  are not read from or written to disk, and step  611  is omitted and replaced by step  612 . For check items  803  inside the secure environment  801  it is also possible to omit the timestamp and MAC fields.  FIG. 8  shows a subset of a check tree as an example: dashed arrows stand for arbitrary layers of check items. 
     FIG. 8  can also be viewed as a forest of check trees with several roots, and therefore also shows how the method for protection against replay attacks can be used to protect different parts of a data store  801  or even different data stores with their own check trees, as indicated by the curved line in  FIG. 8 . 
     FIG. 9  shows recursive equations that can be used to compute links to check entries on demand from the address of a primary item x. If links to check entries are computed on demand, it is not necessary to store the links explicitly in the items. Computation of links on demand is simple if the number N of primary entries, and the number n of check entries per check item, are fixed. Here &lt;c 1 , s 1 &gt;=&lt;c x , s x &gt; is the check entry for the primary item x, &lt;c 2 ,s 2 &gt; is the check entry for the check item c 1 , &lt;c 3 , s 3 &gt; is the check entry for the check item c 2 , and so on up to the root of the check tree, c r , where r is the first index for which l r =1 holds, where l is the length function as defined in  FIG. 9 . 
     FIG. 10  illustrates another embodiment of the invention in which two or more secure environments  1001 - 1002  are afforded protection from replay attacks while accessing one or more data stores. In this example two data stores,  1004  and  1005 , are shown. In this embodiment  1000  the secure environments  1001  and  1002  establish a secure channel  1003  between them. This channel  1003 , or communication link, can be secured by any one of a number of standard cryptographic protocols, such as SSL/TLS (Secure Sockets Layer/Transport Layer Security), which protects against modifications and replay attacks in the communication between two or more secure environments. The secure channel  1003  can then be used to administer and synchronize the sharing of the one or more global timers among all secure environments. This embodiment requires one timer for each data store. 
     FIG. 11  shows an alternative embodiment to the one shown in  FIG. 10  wherein one secure environment  1123  is designated as the “Master” and a plurality of other secure environments (outside of the Master&#39;s secure environment) can access the application data  102  in a data store  1104  through the Master  1123 . Only the Master  1123  has access to the items in the data store  1104 . The other secure environments  1101  through  1109  act as Clients of the Master  1123 , tracking the server/client paradigm. The clients  1101  through  1109  delegate read/write requests to the Master  1123 . The Master  1123  implements a service for the Clients. This service provides the read and write operations for the Clients. The service can be accessed through a number of different protocols, such as the TCP/IP (Transmission Control Protocol/Internet Protocol) and RPC (Remote Procedure Calls). RPC is a programming interface that allows one program to use the services of another program in a remote machine. The communication between the Master  1123  and Clients  1101 - 1109  is represented as the arrows  1130  and  1139 . The communication layer must be protected against modifications and replays. This can be done with standard cryptographic techniques such as the SSL/TLS protocol of  FIG. 10 . 
   In the example shown in  FIG. 11  the Master  1123  provides a cache  1127  which caches read accesses made by all of the Clients  1101 - 1109 . The cache  1127  is logically located inside the Master&#39;s secure environment  1123  and is therefore protected against possible attacks. When a Client  1101 - 1109  requests application data from a data store the Master  1123  first queries the cache  1127  since the requested data might have been cached there from a previous successful request. If the data is available in the cache  1127  the Master  1123  can return that data to the requesting Client without having to access the Data Store  1124 . Retrieving cached data is a known method for increasing the speed of data retrievals. In addition, since all modifications to the Data Store  1124  must be channeled through the Master  1123 , and no other system can access the Data Store  1124  directly, the cached data is always known to be fresh and not contaminated. It should be understood that the embodiment just described could also be advantageously used without the Cache  1127 . Additionally, multiple data stores could be accessed in this manner, in keeping with the spirit and scope of the invention. Data Store  1125  is shown with dashed lines to represent an alternative configuration with multiple data stores. 
   Referring to  FIG. 12  there is shown is a simplified block diagram of a programmable computer that can be configured to operate according to an embodiment of the invention. According to an embodiment of the invention, a computer readable medium, such as a CDROM  1201  can include program instructions for operating the programmable computer  1200  according to the invention. The processing apparatus of the programmable computer  1200  comprises: random access memory  1202 , read-only memory  1204 , a processor  1206  and input/output controller  1208 . These are linked by a CPU bus  1207 . Additionally, there is an input/output bus  1209 , and input/output interface  1210 , a disk drive controller  1212 , a mass storage device  1220 , a mass storage interface  1214 , and a removable CDROM drive  1216 . What has been shown and discussed is a highly-simplified depiction of a programmable computer apparatus. Those skilled in the art will appreciate that other low-level components and connections are required in any practical application of a computer apparatus. 
   It is to be understood that the provided illustrative examples are by no means exhaustive of the many possible uses for the invention. Therefore, while there have been described what are presently considered to be preferred embodiments, it will be understood by those skilled in the art that other modifications can be made within the spirit and scope of the invention.