Patent Publication Number: US-8533414-B2

Title: Authentication and securing of write-once, read-many (WORM) memory devices

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional of U.S. patent application Ser. No. 12/572,991, filed Oct. 2, 2009, now U.S. Pat. No. 8,255,655 which is incorporated herein by reference. 
    
    
     BACKGROUND 
     Portable memory devices, such as memory cards, are often used with consumer electronic devices, such as digital cameras, mobile phones, and personal computers (PCs), to store and/or transport data. In addition to a storage medium, many portable memory devices contain circuitry, such as a microprocessor or controller, that can transform logical addresses received from the host device to physical addresses used by the memory device, thereby allowing the circuitry on the memory device to control where data is stored in the storage medium. 
     Many memory devices use a rewritable memory, which allows a memory address to be erased and rewritten for system or user purposes. However, other memory devices use a one-time programmable (OTP) memory array. In an OTP memory array, once a memory cell at a memory address is changed to a programmed state, it cannot be changed back to its original, unprogrammed state. Because of this limitation on the number of times a memory address can be written, memory devices with OTP memory arrays may not be compatible with host devices that use the popular DOS FAT file system or other file systems that expect to be able to rewrite to a memory address. A similar problem occurs, to a lesser extent, with memory devices that use a few-time programmable (FTP) memory array, whose memory cells can be written more than once but not as many times as memory cells in a rewritable memory array. 
     Another type of memory device is a write-once, read-many (WORM) memory device. This memory device is not rewritable, so that data, once written, cannot be later changed, erased, or overwritten. This is useful for applications where data reliability and security are paramount, such as archival document storage or permanent record-keeping. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an illustration of a host device in communication with a memory device of an embodiment. 
         FIG. 2  is an illustration of a dual-mode behavior of a memory device of an embodiment. 
         FIGS. 3 and 4  are illustrations of how a controller of a memory device of an embodiment makes logical memory appear rewritable even though the underlying memory is one-time programmable. 
         FIG. 5  is an illustration of a relationship between logical memory and physical memory of an embodiment. 
         FIG. 6  is an illustration of a dual-mode behavior of a write-once read-many (WORM) memory device of an embodiment. 
         FIG. 7  is an illustration of a logical memory of a write-once read-many (WORM) memory device of an embodiment. 
         FIG. 8  is an illustration of a memory device of an embodiment being authenticated by a remote key server. 
         FIG. 9  is an illustration of a memory device of an embodiment being authenticated by a local key server. 
         FIG. 10  is an illustration of a host device communicating with a memory device of an embodiment having encryption/decryption functionality. 
         FIG. 11  is an illustration of a host device writing data to a memory device of an embodiment. 
         FIGS. 12A ,  12 B, and  12 C are illustrations of a host device reading data from a memory device of an embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS 
     By way of introduction, the following embodiments relate generally to providing compatibility between a memory device that uses a one-time programmable (OTP) (or few-time programmable (FTP)) memory array and host devices that use a file system, such as the DOS FAT file system, that expects to be able to rewrite to a memory address in the memory device. An OTP memory is a memory in which memory cells are fabricated in an initial, un-programmed digital state and can be switched to an alternative, programmed digital state at a time after fabrication. For example, the original, un-programmed digital state can be identified as the Logic 1 (or Logic 0) state, and the programmed digital state can be identified as the Logic 0 (or Logic 1) state. Because the memory cells are OTP, an original, un-programmed digital state of a storage location (e.g., the Logic 1 state) cannot be restored once switched to a programmed digital state (e.g., the Logic 0 state). In contrast, an FTP memory is a memory in which memory cells can be written to more than once but not as many times as a typical rewritable (or write-many) memory array. 
     Many techniques can be used to make a OTP memory device compatible with a rewritable file system of a host device. In one technique, the memory device is configured to behave exactly the same as a standard flash rewritable memory device until the memory is fully consumed, at which point the memory device would stop performing write operations. Until the memory is consumed, the memory device is essentially indistinguishable from a normal rewritable memory device. In this way, such a memory device would be backwards-compatible with existing host applications and devices. U.S. Patent Application Publication No. US 2006/0047920 and U.S. Pat. No. 6,895,490, which are both hereby incorporated by reference, provide further details on backwards-compatible memory devices. However, for certain hosts and certain host behaviors, there may be a danger of corner cases where the host might attempt to write data to the memory device, but the memory device would run out of memory and not be able to store the data (a typical example would be a digital camera attempting to store a picture on a memory card). In the worst case, the host device would not even realize the write had failed and would give no indication to the user that there was a problem. 
     To avoid this risk (and the accompanying negative end-user perception), the following embodiments provide a memory device that leverages the existing definition of a read-only memory (ROM) card type, provide read compatibility with existing host devices (such as existing SecureDigital (SD) host devices), and minimize the effort to modify host devices to write to an OTP memory device. In one presently preferred embodiment, the memory device takes the form of an SD memory card, based on OTP memory, that operates in accordance with a formal OTP card type specification set forth by the SecureDigital (SD) Association. Various features of these embodiments include, but are not limited to, the following:
         A memory device that powers up in a read-only memory (ROM) mode to be compatible with existing specifications and readable in existing SD-compliant host devices.   A memory device that implements a new function (using a protocol defined in the SD specifications) to switch the memory device into a read/write (R/W) mode. When in R/W mode, the memory device generally behaves like a standard rewritable (e.g., flash) card, so minimal changes are needed for host devices to implement support for the OTP card.   Because, unlike a rewritable memory, an OTP memory is finite and can be fully consumed, in one embodiment, the memory card defines a new command (preferably compliant with the definition in the SD specifications) for the host device to track physical memory consumption.   Additional modifications for operations that program registers due to the limitations of OTP memory (as compared to rewritable memory) are also provided.       

     Turning now to the drawings,  FIG. 1  is an illustration of a host device  10  in communication with a memory card  20  of an embodiment. As used herein, the phrase “in communication with” means in direct communication with or in indirect communication with through one or more components, which may or may not be shown or described herein. In this particular illustration, the host device  10  is in communication with the memory card  20  via mating ports. It should be noted that although a memory card  20  is being used for illustration in  FIG. 1 , a memory card  20  is just one example of a memory device that can be used with these embodiment. In general, a “memory device” can take any suitable form, such as, but not limited to, a memory card, a Universal Serial Bus (USB) device, and a hard drive. In one presently preferred embodiment, the memory device takes the form of a solid-state memory card, such as a SecureDigital (SD) memory card. 
     As shown in  FIG. 1 , in this embodiment, the memory card  20  comprises a controller  30  in communication with one or more memory devices  40 . The memory device(s)  40  can comprise any suitable type of memory, such as, but not limited to, solid state, optical, and magnetic memory. In one embodiment, at least some of the memory device(s)  40  comprise OTP and/or FTP memory. In the event that multiple memory devices  40  are used, it should be understood that the various memory devices can each use the same or different memory technologies (e.g., (i) all OTP, (ii) OTP and FTP, or (iii) OTP, FTP, and rewritable). Preferably, the memory device  40  comprises a field-programmable solid-state memory array. The memory cells in the memory array can be organized in a two-dimensional or three-dimensional fashion. In one preferred embodiment, the memory array is a three-dimensional array, such as an array described in U.S. Pat. No. 6,034,882 to Johnson et al., which is hereby incorporated by reference. It should be noted that with reference to  FIG. 1  only, the term “memory device” refers to the memory die itself. In other instances of this document, “memory device” refers more generally to the overall device (such as a memory card) that includes the memory die and other components (such as a controller, input/output ports, etc.). 
     The controller  30  is operative to perform various functions, some of which are described below. While a controller  30  is shown in  FIG. 1 , it should be understood that the memory card  20  can comprise any suitable circuitry  130 . As used herein, “circuitry” can include one or more components and be a pure hardware implementation and/or a combined hardware/software (or firmware) implementation. Accordingly, “circuitry” can take the form of one or more of a controller, microprocessor, or processor that executes computer-readable program code (e.g., software or firmware stored in the memory device(s)  40 , logic gates, switches, an application specific integrated circuit (ASIC), a programmable logic controller, and an embedded microcontroller, for example. 
     Also, a “host device” refers to any device that can be put in communication with a memory device and be used to store data in the memory device and/or read data from the memory device. Examples of host devices include, but are not limited to, consumer electronic devices such as mobile phones, digital media players, digital cameras, game devices, personal computers (e.g., desktop or portable (such as laptop or notebook) computers), email and/or text messaging devices, and personal digital assistants (PDAs). A host device can be a relatively mobile device (such as a mobile phone) or a relatively stationary device (such as a desktop personal computer). 
     In this embodiment, the OTP memory card  20  implements two modes of operation. In the first mode, the memory card  120  powers up in a configuration compatible with an existing ROM card definition already defined in the SD specifications (and, therefore, supported by compliant SD host devices). In the second mode, the memory card  20  is switched into a writable mode before accepting and performing writes. (A suitable ROM card configuration and behavior are described in the SD Part 1 Physical Layer Specifications v2.00, as well as in the Part 1 Physical Layer Specification version 2.00 Supplementary Notes version 1.00. The switch command protocol was defined in the SD specifications for the general situation of enabling expanded or additional features for an SD card (see section 4.3.10 of the SD Part 1 Physical Layer Specification, version 2.00.).) By powering up in a read-only mode that is compatible with an existing card definition, existing host devices can still read from the memory card  20  but cannot write to the memory card  20 , providing read compatibility with existing host devices but avoiding the dangers that those non-enabled host devices will write to the memory card  20  and run into the problematic corner case discussed above. 
     Because, in this embodiment, host devices must issue a command to switch the memory card  20  into a writable mode, only host devices that have been enabled to work with the OTP memory card  20  and understand its unique features will be able to write to it.  FIG. 2  illustrates the memory card&#39;s  20  dual-mode behavior. The left-hand illustration in  FIG. 2  shows that (1) the memory card  20  powers up in a read-only mode so legacy host devices can only read from the card  20 , (2) only enabled host devices know how to switch the card  20  into a read/write mode, and (3) in read/write mode, enabled host devices can both read and write to the card  20 . 
     In its writable mode, the memory card  20  behaves similarly to a “normal” flash rewritable memory card, at least until the memory card&#39;s OTP memory  40  is fully consumed. So, for example, if the host device  10  overwrites a sector of data with different data (which is often done for rewritable memory cards for a variety of reasons), the memory card  20  accepts and performs the requested write operation. (Because the underlying memory device  40  is OTP in this embodiment, memory on the device  40  itself cannot be changed after being written, but the card  20  firmware can automatically write updated data to a new location in memory and “remap” the old location to the new location. This remapping functionality is similar to the remapping that occurs in firmware in existing flash memory devices (see, for example, U.S. Pat. No. 7,139,864, which is hereby incorporated by reference).) This “overwrite” behavior ensures that there are minimal changes that the host device  10  must make to support the OTP memory card  20 . The host device  10  can use any file system (most use the industry-standard FAT file system) and can still perform all the operations that it does for rewritable memory cards, including file rename, change, and delete operations, for example. 
     With reference again to  FIG. 1 , the host device  10  interfaces to the memory card  20  using logical addresses, the controller  30  acts as the interface between the host device  10  and the physical memory device(s)  40  and performs logical-to-physical addressing. The interface between the controller  30  and the memory device(s)  40  uses physical addresses. This interface implementation is standard to existing flash memory devices (see, for example, U.S. Pat. No. 7,139,864, which is hereby incorporated by reference).  FIGS. 3 and 4  demonstrate how the controller  30  makes the logical memory appear rewritable even though the underlying memory  40  is OTP.  FIG. 3  shows that original data stored at a logical address is stored in a physical location within the memory device  30 .  FIG. 4  shows that when the host device  10  overwrites the data at the logical address, the new updated data is stored at a new physical address, and the controller  30  updates its logical-to-physical addressing to reference the updated data instead of the now super-ceded original data. Again, the process is similar to the existing flash memory device implementation noted above; however, a difference between this embodiment and the flash implementation is that the memory  40  in this implementation (here, OTP) is not erased and re-used. 
     In its writable mode, the memory card  20  implements a new command (Read_Mem_Remaining) for the host device  10  to track the amount of physical (OTP) memory remaining. (The command code was allocated and defined in the SD Part 1 Physical Layer Specification, version 2.00, section 4.3.12 Command System as part of the switch command protocol, but the data values and format was defined specifically for the OTP card application.) The following table lists the values returned by the Read_Mem_Remaining, in a presently preferred embodiment (of course, other implementations can be used). Values are preferably returned in most-significant-byte, most-significant-bit order. 
     
       
         
           
               
               
               
             
               
                   
               
               
                 Byte 
                   
                   
               
               
                 Position 
                 Parameter 
                 Description 
               
               
                   
               
             
            
               
                 511:508 
                 Main Area 
                 Amount of memory remaining in the “main” area 
               
               
                   
                 Remaining 
                 of the card. Reported in units of sectors (200h 
               
               
                   
                   
                 bytes). 
               
               
                 507:504 
                 Reserve 
                 Amount of memory remaining in the “main” area 
               
               
                   
                 Area 
                 of the card. Reported in units of sectors (200h 
               
               
                   
                 Remaining 
                 bytes). 
               
               
                   
               
            
           
         
       
     
     By making the host device  10  responsible for tracking memory consumption, the memory card  20  definition avoids the problem with the original OTP card implementation, where the card tries to discern what the host device is doing and why. With this new definition, the memory card  20  is free to accept and perform all write operations, since only enabled host devices will be able to write to it, and they are responsible for tracking memory consumption and avoiding illegal operations. 
     In this embodiment, the first two values returned by the new command (“main” and “reserve”) report to the host device  10  the amount of physical space remaining on the card  20  (1) for general/unspecified uses (file data and file system operations) (the “main area”) and (2) that is reserved for file system operations only (no file data) (the “reserved area”). The main value was defined so that the host device  10  can use it the same way it currently uses the FAT tables: to both determine how many more files (or, more generally, how much more data) can be written to the card  20  and to decide when the card  20  is full and the host device  10  should stop writing data to the card  20 . The reserve value was defined so that the host device  10  would always be able to perform the file-system related operations necessary to complete a file that had been in progress when the card  20  was completely consumed (as reported by the main area), in the same way as it does for the FAT file system structures. This minimizes the changes a host device  10  must make in order to support OTP cards in addition to rewritable cards. 
       FIG. 5  illustrates the relationship between logical memory (visible to the host device  10 ) and the physical memory  40  (not directly visible to the host device  10 ). The amount of unused logical memory is tracked by file system structures, typically a “File Allocation Tables” for the standard FAT file system. The controller  30  in the memory card  20  manages logical-to-physical addressing. Overwritten data, which is represented is this diagram by cross-hatches, is no longer referenced but still consumes physical memory. The controller  30  reports the amount of physical memory remaining using a new command, so the host device can determine the amount of space available. The amount of physical memory remaining may be smaller or larger than the amount of logical memory remaining. 
     In this embodiment, the behavior of “Program_CSD” and “Lock_Unlock” commands are also modified to reflect the fact that the card memory  40  is OTP and not rewritable. That is, these commands are preferably used only once to update/program the related registers. The Card Status Defaults (CSD) register reports the card&#39;s  20  operating conditions and supported commands and features. It also includes a small number of writable bits that are used to indicate the card&#39;s  20  write protect status and if the card  20  is original data or is a copy. These bits can only be set once, collectively; if any bits are changed by issuing the Program_CSD command, then the command can not be used again to further update the CSD register. The Lock_Unlock command is used to both set the card  20  password and to supply the password to later unlock the card  20 . For the OTP card  20 , the password can only be set once and, once set, can never be changed or cleared. If the password has been set, then this has implications on the future behavior of the card  20  as required by the SD specifications. Also, the OTP card  20  preferably does not support the “force erase” operation described in the SD specifications, where if the password is forgotten, the card  20  can be completely erased (clearing the password), because the OTP card  20  does not support the erase operation. Specifying that these two registers can be changed only once each achieves two purposes. First, it fixes the amount of space the card  20  must reserve in order to store the update register value, and, second, it allows the host device  10  to determine if the register has already been changed or not (if the register does not match its default value, it must have been modified, and since it has been modified, it may not be modified again). This would obviously not be true if the host device  10  could change the register values more than one time. 
     As noted above, the examples described herein are only some of the many implementations these embodiments can take. Also, while the use of OTP memory was used in these examples, it should be noted that the teachings described herein can also be applied to FTP memories. Further, although the memory card  20  in these embodiments took the form of an SD card, as mentioned above, any suitable type of memory device can be used, including, but not limited to, those in compliance with MultiMedia, Memory Stick, Compact Flash, Smart Media, xD, USB, or HS-MMC specifications. 
     As another alternative, the above embodiments can be modified to add features to prevent accidental or deliberate overwrites, changes, or deletions to existing data in a write-once, read-many (WORM) memory device. Preferably, these features are implemented in firmware logic and algorithms and do not rely on any particular memory type. Accordingly, a WORM memory card can be built using either one-time programmable (OTP), few-time programmable (FTP), or read/write memory devices, and can be built using standard two-dimensional flash memory or three-dimensional memory. (More generally, any of the embodiments described herein, including the OTP embodiments described above, can be built using any type of memory (e.g., OTP or rewritable) because, in these embodiments, it is the controller that makes a card “OTP” or “WORM” and not the underlying memory.) 
     In general, a WORM card is designed for OEM/industrial applications where data reliability and security are paramount, such as when storing digital still images, video images, and audio recordings used for police evidence; point-of-sale receipts for tax collection purposes; and long-term storage of financial records. For these and other applications, the WORM card&#39;s ability to ensure the integrity and safety of data written to the card is its primary attribute. Accordingly, features that prevent accidental or deliberate overwrites, changes, or deletions to existing data are desired. This embodiment uses some of the features of the OTP embodiment described above while adding additional features. Specifically, like the OTP card, the WORM card of this embodiment:
         Powers-up in a read-only mode, compatible with the defined “ROM” SD memory card type, and readable in existing SD-compliant host devices.   Requires the host device to switch the card into a different mode (here, a read/write WORM mode, which is different from the OTP mode used in the OTP card); the WORM mode implements additional features noted below to ensure data integrity.   Implements a similar command as the OTP card for the host device to track physical memory consumption.   Behaves in a similar manner as the OTP card for the Program_CSD and Lock/Unlock commands.       

     However, the WORM card of this embodiment implements new features to ensure that data can be added to the card but not changed or deleted after being written. Specifically, the WORM card of this embodiment:
         Implements new write “filters” that analyze write commands issued by the host device to determine if the write should be performed or not.   Implements a new command in WORM mode for the host device to designate writes as either “open” or “closed.” This feature is used along with the write filters to ensure data is not changed after being written.       

     In one presently preferred embodiment, the implementation of the WORM card follows the same card architecture as other SD card products. Namely, the controller  30  implements the features and algorithms of the card  20  and acts as the interface between the host device  10  and the internal memory device(s)  40 , as illustrated in  FIG. 1 . As noted above, the WORM card does not require any particular memory technology and can be built with OTP, FTP, or read/write memory, for example. 
     As shown in  FIG. 6 , like the OTP card  20  described above, the WORM card  60  in this embodiment implements two modes of operation, where it powers up in a ROM mode, readable by compliant SD host devices, and must be switched into a writable mode before writes can be performed. However, unlike the OTP card  20  described above, the WORM card  60  is switched into a “WORM” mode (instead of an “OTP” mode), which can be performed using the same command but a different argument. In WORM mode, the card  60  implements features to ensure that data can be added to the card  60  but not changed afterward. 
     Internally, after write commands are accepted by the card  60  (and not rejected by the write filters the card  60  implements to ensure that data is not changed after being written), the card  60  operates similar to the OTP card  20  described above and updates internal data structures for writes that update existing sectors of data. If the underlying memory device(s) are read/write, the now-superceded “original data” sectors may be erased and re-used. If the underlying memory device(s) are OTP, the updated data sectors will never be re-used and will simply never be referenced or accessed again. 
     The WORM card  60  preferably implements the same Read_Mem_Remaining command as the OTP card  20  for the host to track physical memory consumption. Also, the WORM card&#39;s  60  behavior with the Program_CSD and Lock_Unlock commands is preferably the same as the OTP card  20  implementation described above. However, the WORM card  60  preferably implements a new command for the host to mark write operations as either “open” or “closed.” This information is used in conjunction with the write filters described below to ensure that data cannot be changed or deleted after being written. (The command code was allocated and defined in the SD Part 1 Physical Layer Specification, version 2.00, section 4.3.12 Command System as part of the switch command protocol, but the data values and format was defined specifically for the WORM card application.) In one embodiment, the host uses this command by specifying either an argument of 01h for “open” or 02h for “closed.” Once set, the mode is “sticky” and is not changed until the command is issued again. When the card  60  is first switched into WORM mode, the write mode defaults to “open.” 
     In this embodiment, the WORM card  60  uses the FAT file system to organize files and directories written to the card  60 . (This is the file system recommended by the SD specifications, in Part 2 File System Specification.) As was noted in the OTP card description above, a host device that uses the FAT file system will often issue write operations overwriting existing data in the course of performing normal file-system operations. In this embodiment, the WORM card  60  requires the host device to make changes to its FAT file system implementation to satisfy the WORM card data overwrite requirements, but there are some FAT file system operations that overwrite existing data that the WORM card  60  must still support in order to be generally compatible with the FAT file system. Therefore, in this embodiment, the WORM card  60  preferably implements write filters to analyze write commands issued by the host and determine which are legal and should be performed and which are illegal and should be rejected. At a high level, the WORM card  60  allows write operations that are related to adding or creating new files or directories but rejects write operations that change already-written data. The rejected write operations include, for example, those that change file data, such as overwriting, deleting, or appending data to a completed file, as well as those that change a file&#39;s (or directory&#39;s) file entry, such as renaming the file entry, deleting the file entry (or marking it “deleted,” a different operation), changing its attributes (read-only, archive, etc.), or changing the creation or last-modified date and time stamps, and other similar operations. 
     To support the write filters, the WORM card  60  preferably divides its logical memory into three regions, as shown in  FIG. 7 . The System Area begins at the “Master Boot Record” structure, located at the beginning of memory (logical address 00h), up to and including the “Partition Boot Record” structure. The FAT Area consists of the two copies of the File Allocation Tables and is located immediately after the System Area. The User Area consists of the remainder of the card&#39;s memory, beginning at the Root Directory, immediately after the second FAT, up to the end of the card&#39;s logical memory. The addresses of the FAT file system structures which mark boundaries between the various regions can be determined by decoding the structures, beginning with the MBR, which is located at logical address 00h. 
     In this embodiment, the WORM card  60  implements write rules to ensure that data cannot be modified after it has been written. These rules analyze every write command, on a sector-by-sector basis, to determine if each sector should be written as requested by the host. The write rules preferably use three pieces of information when determining if a write should be allowed or rejected: the location of the write operation (System Area, FAT Area, or User Area), whether that sector had been previously written and if it had been written “open” or “closed,” and the data pattern the host is attempting to write. 
     In one presently preferred embodiment, the write rules implemented by the WORM card  60  are as follows:
         No writes are allowed to the System Area, whether open or closed.   Writes to the FAT area can only change unprogrammed cluster entries. Programmed cluster entries cannot be changed to any value, including 0000h (the unprogrammed value). Cluster entries are two-byte values aligned on even byte addresses; if any bit of a cluster entry is non-zero, then that cluster entry is considered programmed and cannot be changed.       

     In one presently preferred embodiment, writes to the User Area are subject to the following rules:
         Data can only be modified in multiples of 32-byte quantities (the size of a directory entry), aligned on 32-byte addresses. If any bit of a 32-byte group is non-zero, then that 32-byte group is considered programmed and cannot be changed.   If a sector (512 bytes) has been previously written with any non-zero data, additional data to that sector can only be appended (written after) in programmed 32-byte quantities. If a sector has one or more 32-byte quantities previously programmed such that there is a “gap” of one or more 32-byte quantities that are unprogrammed (all 00h values), then those intervening blank 32-byte quantities cannot be programmed.   If a sector has previously been written “closed” then that sector can never be written to again, regardless of the location or data pattern requested by the host.       

     If the host violates any of these rules, the card will return a Write Protection Violation error. 
     Of course, the above rules are merely examples, and other rules can be used. For example, the rule that data can only be modified in multiples of 32-byte quantities may be unnecessarily restrictive in some embodiments, and it may be better to allow updates to previously-programmed 32-byte quantities if those 32-byte quantities had left the “starting cluster” and/or “file length” fields blank and only to update those fields and not any others. This modification would make it easier for host devices to support a WORM card because they would have to make fewer changes to their existing FAT file system implementation. As another example, the rule that data can only be appended in 32-byte quantities may be unnecessary if the WORM card relies on the host device to properly follow the rules. For a host device that follows the recommended implementation, this situation should not arise. 
     In addition, note that the above rules describe a preferred embodiment to support a “FAT16” FAT file system; similar rules can be developed for FAT12, FAT32, and exFAT file systems, which are also commonly used for memory devices. 
     The WORM card  60  preferably requires the host device to modify its FAT file system implementation to satisfy the write rules. The WORM card  60  preferably requires the host device to write the file data (the data a file contains, as opposed to file-system related data such as the file name) as a “closed” write; by doing this, the file data can never be changed, regardless of the contents of the file. If the host does not write the file data as “closed,” and the file data contained one or more blank sectors (all 00h values), then a malicious device could corrupt the file by changing that sector such that some of the 00h values were changed to other values. The file-system related information should be written “open” because those sectors might later be updated, since a sector might contain information on more than one file. 
     As mentioned above and in U.S. patent application Ser. Nos. 12/421,229 and 12/421,238, which are hereby incorporated by reference, WORM memory devices can be used to store data in archival situations where data integrity is critical, such as when storing digital evidence or other types of court-admissible content, national security information, insurance records, confidential information, and voting machine data, for example. In such situations, it is important for the user to be sure that a WORM memory device will function as expected and maintain data integrity. To provide such assurance, before a host is allowed to store data in a memory device, the memory device is first authenticated. This ensures that the memory device is coming from a trusted source and is not a counterfeit device, which may potentially delete or alter critical data, either by intent or by accident. This authentication can take place only once per memory device (e.g., a one-time registration), every time a host is attempting to store data in the memory device (e.g., authentication required every session), or at any other desired interval. While any suitable technique for authenticating a memory device can be used, the following paragraphs provide some exemplary techniques. 
     In one embodiment, the WORM memory device is preloaded with a security key (or, more generally, “security information,” which can be any information used to authenticate the memory device), and the memory device is authenticated if the security key appears on a list of authorized security keys. A security key can take any suitable form and can be unique to the memory device (e.g., randomly generated by the controller) or to a batch of memory devices. The security key can be stored in any suitable location in the memory device and, in one embodiment, is stored in a hidden area to prevent the key from being accessed by unauthorized entities. In this embodiment, before the controller of the memory device initializes and enters an operational mode, a host must first register the memory device by validating the security key against a list of security keys. This list may be stored on the host or on a connected server. In case of a connected server, the host doing the validating is preferably connected to the server, e.g., via the Internet, during validation in order to access the most up-to-date list of security keys. If the memory device&#39;s security key is on the list, the memory device is put into operational mode. This can occur in any suitable manner. For example, in one embodiment, upon validating the security key, a host can automatically register the memory device by setting a flag in the memory device, which would unlock the memory device, allowing it to enter into its operational mode. In another embodiment, a human inspector (e.g., a person with credentialed password access to oversee the authentication process) manually sets this flag. In yet another embodiment, instead of setting a flag in the memory device to put it into its operational mode, the memory device is manufactured in an operational mode, and a human inspector simply separates those memory devices that are authenticated from those that are not, such that only authenticated memory devices are distributed. This “chain of custody” provides that desired assurance of an authenticated memory device. Validation can also be based on visual inspection, confirming authenticity of the supply channel, or electronically comparing the memory device to a master “gold copy.” Irrespective of the particular method used, the above techniques ensure that a WORM memory device will function as advertised. 
     For additional security, in addition to authenticating the security key, one or more values can also be authenticated along with the security key before the memory device is considered authentic. These additional values can take any suitable form, such as, but not limited to, a time stamp or a memory device identifier (e.g., a serial ID of the memory device). These additional values can also be validated by the same list that validates the security key, or another list can be used. For added security, the security key and/or one or more of the additional values can be encrypted using a predetermined key, with the host decrypting these encrypted elements before validating them. 
     Without intending to be a limitation on the claims, in one presently-preferred embodiment, the authentication method uses an RSA (or equivalent, such as ECC) asymmetrical protocol and a certificate plus key pair on a WORM memory device to authenticate the WORM memory device through a key server. This example is shown in  FIGS. 8 and 9 . In both  FIGS. 8 and 9 , a WORM Card Certification Authority (CA)  800  issues a security certificate  810  for a WORM memory device  820  (or for a batch of WORM memory devices). The root certificate for the CA, along with a certificate revocation list (CRL), which is used to identify revoked certificates, is stored on a key server  830 . This same certificate  810 , along with a private key pair (not shown), is stored on the WORM memory device  820  during production. Before initializing the WORM memory device  820  for usage, the certificate on the memory device  820  is validated against the certificate and CRL on the key server  830 . The key server  830  can keep a list of valid memory devices that have been enabled through this process. 
     In  FIG. 8 , the key server  830  is remote from the host  840  and is accessed by the host  840  via a network, such as the Internet  850 , in order to access the most up-to-date list of certificates and CRLs. In this situation, the host  840  can identify and authenticate the key server  830  through a secured connection. In contrast, in  FIG. 9 , the key server  830  is part of the host  840 . In this situation, the key server  830  is a trusted local server, and server authentication is not required, as memory authentication may be done locally. The host/local server can be a PC or digital camera, for example. It should be noted that other key server arrangements can be used. For example, the key server can be located within an intranet of an organization, separate from the host. 
     Turning now to another embodiment, data that is stored on a WORM memory device can be highly sensitive and confidential. As such, it may be desired to implement one or more security features on the WORM memory device to maintain the integrity and confidentiality of stored data. With reference to  FIG. 10 , a controller  1000  of a WORM memory device  1010  can operate in a storage mode in which the controller  1000  encrypts data as the data is written to the memory  1020  and will only provide decrypted data to a host  1030  that has provided the appropriate credentials. Any encryption method can be used. In one embodiment, the WORM memory device  1010  uses a hardware encryption method. For example, the controller  1000  in the WORM memory device  1010  can use the hardware encryption methods described in U.S. Patent Application Publication Nos. 2006/0242064 and 2008/0010685, which are hereby incorporated by reference herein. Alternatively, the WORM memory device  1010  can use software encryption methods or a combination of hardware and software encryption methods. Examples of suitable encryption techniques that can be used include, but are not limited to, AES, DES, 3DES, PKI, SHA-1 Hash, RSA, and random number generation. In one presently preferred embodiment, the memory device  1010  uses a TrustedFlash™ security platform from SanDisk Corporation. Various forms of encryption can be available for use, and the host can set the encryption type during the authentication of the card. This may be useful in situations where an organization may want all their cards to be operating in a certain mode, and they can ensure this by setting it during the authentication process. In this case, they would have appropriate hosts that are set to write data in that mode as well. 
     The controller  1000  of the memory device  1010  can also be operative to implement several storage modes, where the host  1030  can provide the memory device  1010  with a command (e.g., an op code) to select one of these modes. This allows the host  1030  to decide what mode best fits a given application usage model. For example,  FIG. 11  shows the host device  1030  sending three different op codes (“Write A,” “Write B,” “Write C”) for three different files (“file  1 ,” “file  2 ,” “file  3 ”). In response to these op codes, the WORM memory device  1010  stores the files using their respective storage modes. So, as shown in  FIG. 11 , the file  1  would be stored using storage mode A, file  2  would be stored using storage mode B, and file  3  would be stored in storage mode C. Of course, fewer (even one) or more storage modes can be used. Although the various storage modes can have any desired characteristics,  FIGS. 12A-12C  illustrate three particular storage modes of an embodiment. Storage mode A ( FIG. 12A ) is an unsecured mode. Accordingly, when the host device  1030  sends a command to the WORM memory device  1010  to read (or, more generally, “access” (e.g., read or copy)) a file (here, file  1 ) that was stored using storage mode A, the WORM memory device  1010  simply provides file  1  to the host device  1030 . That is, in this mode, no security is required, and the WORM memory device  1010  will allow open access to the file. Storage mode B ( FIG. 12B ) is a standard security mode, with a password required to read a file (here, file  2 ) that was stored using storage mode B. As illustrated in FIG.  12 B, to read file  2 , the host device  1030  sends both a command to read file  2 , as well as valid credentials (here, a user ID and password). The WORM memory device  1010  checks those credentials against a stored list of valid credentials and, if there is a match, provides file  2  to the host device  1030 . The WORM memory device  1010  can keep an access log of attempts (successful or not) to read file  2  and can provide such log to an authenticated host device  1030 . The access log can include data, such as, but not limited to, user ID, user password, and time and date stamps, for the purposes of keeping accurate records. In another implementation of storage mode B, the host already has a valid key to access the card; however, the system (e.g., a program running on a PC) requires the user to input a valid user ID and password before allowing the host to access the card. This would be a sort of “local authentication,” where it is up to the organization to control access within their walls. 
     Lastly, storage mode C ( FIG. 12C ) is a high security mode, which requires the host device  1030  to be connected to a key server  1210  (e.g., via the Internet  1220 ) in order to validate a certificate  1200  (or, more generally, a user ID and an access key) stored on the host device  1030 . This level of security can ensure that a host device attempting to access data in the WORM memory device  1010  is a currently-authenticated host. In this embodiment, a WORM Host Certificate Authority provides the certificate  1200  to the host device  1010 . The WORM Host Certificate Authority also stores the certificate and a certificate revocation list (CRL) on the key server  1210 . When the host device  1030  wants to read a file (here, file  3 ) stored using storage mode C, the host device  1030  first presents its certificate to the key server  1210  for validation. If the certificate  1200  matches the one on the key server  1210  and is not on the CRL, the key server  1210  will validate the certificate  1200  and provide the host device  1030  with valid credentials. When the host device  1030  presents these valid credentials to the WORM memory device  1010  along with a request to read file  3 , the WORM memory device  1010  allows the host device  1030  to read file  3 . As with storage mode B, the WORM memory device  1010  can keep an access log of attempts to read a file. The key server  1210  can also keep an access log of attempts to validate certificates, as well as a list of enabled host devices. 
     After a file has been written to the WORM memory device  1010  using a certain storage mode, that file (and/or other files stored in the WORM memory device  1010 ) can be set to a higher security level. For example, a file written using storage mode A can be set by an authenticated host to storage mode B, and a file written using storage mode B can be set to storage mode C. However, it may be preferred to prevent a file from being set to a lower storage mode (e.g., a storage mode C file should not be set to storage mode A). One usage example is when a camera host captures an image. The camera host may write the image using storage mode A to allow the user to review the image on the camera&#39;s display device. However, after reviewing the image, the user may decide to then “secure” the image on the WORM memory device and, therefore, switch that image (and perhaps other images on the WORM memory device) to storage mode C, at which point read access is only granted to an authenticated host. In another implementation of storage mode C, the certificate on the host must be authenticated with a remote key server first, after which the system (e.g., a program running on a PC) requires the user to input a valid user ID and password before allowing the host to access the card. The key server can be in any suitable location, such as, but not limited to, on a local intranet separate from the host. 
     These embodiments provide several advantages over prior storage techniques. For example, while analog media, such as film and cassette tapes, can be used to store critical data, such as evidence, in an unalterable state, analog media is harder to replicate than digital media (e.g., flash memory), as the data must be converted to a digital format to be stored and distributed electronically. While the original analog media can be kept for archival purposes, outside of locking the analog media in an external enclosure to physically limit access to the data, it is difficult to keep the data confidential and difficult to track the access history. Further, analog media is becoming rare and is being phased out by most manufacturers, making analog media hard to find. Also, when traditional flash media devices, such as standard SD cards or CF cards, are used to store sensitive data, often, a strict chain of custody process for the cards is needed to ensure that the data stored therein is not altered in any way during the handling of the cards since traditional flash media devices can be altered. The security features discussed above can be used to address these concerns, as the functionality of ensuring that the stored data is not altered is built-in to the WORM memory device itself. Also, by using the registration features discussed above, a user can be assured that the WORM memory device is legitimate and will support any specialized command from the host needed to implement the WORM functionality. 
     Other alternatives can be used. For example, to keep the WORM memory device tamperproof, test pads that can provide direct access to the memory can be removed from the memory device&#39;s substrate during production of the memory device. 
     Finally, as noted above, the memory used in these embodiments can take any suitable form, as it is the controller (e.g., pure hardware, or hardware running firmware or software) that enforces the write-once, read-many nature of the memory device. Accordingly, the memory can be one-time programmable, few-time programmable, or re-writable (e.g., NAND). In the case of a re-writable memory, using the controller to limit the number of writes (to as low as one write) can provide longer data retention. Also, as noted above, in one embodiment, the memory device works in accordance with the SDA standard and uses standard SD commands from the SDA physical layer specification, along with some additional WORM-specific commands. While the memory device has an SD card form factor in that embodiment, it should be noted that other form factors can be used. 
     Some of the following claims may state that a component is operative to perform a certain function or is configured for a certain task. It should be noted that these are not restrictive limitations. It should also be noted that the acts recited in the claims can be performed in any order—not necessarily in the order in which they are recited. Also, it is intended that the foregoing detailed description be understood as an illustration of selected forms that the invention can take and not as a definition of the invention. It is only the following claims, including all equivalents, that are intended to define the scope of this invention. Finally, it should be noted that any aspect of any of the preferred embodiments described herein can be used alone or in combination with one another.