US 20080215847 A1
A device with mass storage capability that uses a readily available non secure memory for the mass storage but has firmware (and hardware) that provides security against unauthorized copying of data. This is true even though the firmware itself is stored in the non secure mass storage memory, and therefore potentially vulnerable to hacking. An indication of the authenticity of the firmware must be present before it will be executed by the device. This protects the device contents from unauthorized duplication or tampering. Additional functionality can be added to the device with additional firmware applications, and the authenticity of those additional applications will also be verified before they will be executed. This further prevents unauthorized copying or tampering of secure content through any mechanisms that may be unscrupulously introduced. Any data within the mass storage memory may also be encrypted.
1. A flash memory based device comprising:
a mass storage repository comprising flash memory;
a first level of the firmware stored in a physical access partition of the mass storage repository;
a second level of the firmware stored in a logical access partition of the mass storage repository;
a logical to physical interface routine that maps logical addresses to physical addresses in the logical access partition; and
a controller that executes the first level of firmware in the physical access partition without use of the logical to physical interface routine, but executes the second level of firmware through the logical to physical interface routine.
2. The flash memory based device of
3. The flash memory based device of
This application claims priority to provisional Application No. 60/717,164 entitled “Secure Yet Flexible System Architecture for Secure Devices With Flash Mass Storage Memory” filed Sep. 14, 2005 to Micky Holtzman et al.
This application is related to the following applications, each of which is hereby incorporated by this reference in its entirety: “Methods Used in a Secure Yet Flexible System Architecture for Secure Devices With Flash Mass Storage Memory” to Micky Holtzman et al., Attorney Docket No.: SNDK.470US2; “Method of Hardware Driver Integrity Check Of Memory Card Controller Firmware” to Micky Holtzman et al., application Ser. No. 11/284,623, Attorney Docket No. SNDK.408US 1; “Hardware Driver Integrity Check Of Memory Card Controller Firmware” to Micky Holtzman et al., application Ser. No. 11/285,600, Attorney Docket No. SNDK.408US2; “Methods Used in a Secure Memory Card With Life Cycle Phases” to Micky Holtzman, et al. Attorney Docket No. SNDK.383US2; and “Secure Memory Card With Life Cycle Phases” to Micky Holtzman et al., Attorney Docket No. SNDK.383US3.
The present application is generally related to the operation of flash based mass storage devices, and in particular those with copy protection of secure content.
Flash based mass storage devices are used to store very large amounts of content, such as pictures and music or software programs. Examples of these mass storage devices include memory cards, universal serial bus (“USB”) flash drives, flash based music and/or video players, and other portable computing devices that rely on flash for the mass storage of content or files.
User files are frequently updated and modified. This is particularly the case when dealing with photos, music, and documents. Flash memory has a limited number of read/write cycles, and a great deal of research and development has gone into distributing the cycles among the flash memory cells in order to maximize the lifespan and reliability of the devices. For instance, wear leveling techniques such as those taught in U.S. Pat. No. 6,230,233 entitled “Wear Leveling Techniques For Flash EEPROM Systems” to Lofgren et al., U.S. Pat. No. 5,268,870 entitled “Flash EEPROM System and Intelligent Programming and Erasing Methods Therefore” to Harari, PCT Publication No. WO2004040578A2 entitled “Wear Leveling In Non-Volatile Storage Systems” to Chang et al., and U.S. Patent Publication No. 20040083335A1, entitled “Automated Wear Leveling In Non-Volatile Storage Systems” to Gonzalez et al., which are hereby incorporated by this reference in their entireties, are commonly implemented in these devices. These techniques generally involve changing the logical/physical mapping so that physical locations of the memory are used roughly the same amount.
In addition, as the usage of flash based mass storage devices is proliferating, the number of different things that can be done with them is also increasing.
Therefore, there exists a need for a new device operating system architecture that provides flexibility to store and run a wide range of firmware that can be updated and changed to accommodate the range of increasing functionality. In addition to being flexible, this architecture must provide a highly secure and reliable environment for both firmware and content. As is always the case, all of this should be done for the lowest possible cost, using standard components whenever possible.
The present invention allows a device to be both secure in operation and flexible in terms of functionality. This means functionality can be tailored to users' desires and added over time all the while maintaining a high level of security. Therefore the device can be used to store confidential and limited access information such as transactional data and copyrighted artistic works.
The present invention also allows for the device to boot quickly and reliably while still providing for reliable long term data storage through the use of wear leveling techniques where appropriate.
Firmware that is not authentic, and that may potentially compromise the security of the device will not be executed. An indication of the authenticity is verified before execution. In a preferred embodiment, multiple different levels of such indications are provided and are associated with the particular controller of the device that created the indications. In this preferred embodiment, one or more of the different levels of indications can be verified. Without the properly associated indication the firmware will not be executed.
Another aspect of the present invention is that this security is achieved despite the fact that the device utilizes readily available memory without built in security for the mass storage of the data, including the firmware.
Devices incorporating flash memory for mass storage purposes must store large amounts of content that is written and read relatively often. For instance, digital photo and music libraries are regularly updated by users of such devices. With the increase of protected content and the desire to protect content generally, such devices must also provide robust security to prevent unauthorized copying of such “secure” or protected content. At the same time, security should not come at the cost of flexibility. The present invention provides for a device that allows functionality to be added over time, while maintaining a high level of security. This flexibility is essential in a world where devices are expected to provide ever increasing functionality.
A secure device is one that protects the contents of the device from unauthorized copying or alteration. Secure content includes any content or data that it is desirable to safeguard from unauthorized copying or tampering. In addition to billing, transaction and other traditionally confidential personal information, artistic content must also be secured from access and copying by those other than the owner or other authorized persons.
Depending on the architecture of the device, a hacker may try to gain access to the content via data buses, or by directly accessing the mass storage memory. In some prior devices, directly accessing the memory storage unit was difficult as the memory storage unit was often protected by placing it an environment that was logistically hard to access. For instance, Smart Cards utilized programmable read only memories (PROMS) that incorporated a small amount of non volatile memory that was made secure in part by physically isolating it from access.
However, it is desirable to utilize unsecure mass storage memory, that is, among other things, more standardized, readily available, and/or economical. An unsecure memory or storage unit is one where authorization is not required in order to gain (read/write) access to the (encrypted or unencrypted) data stored therein, or one where there are no built in protection mechanisms that prevent copying of the stored data. While this memory may be packaged in a multi functional package with other non-memory components such as a processor, it is commonly in the form of a dedicated memory package with one or more memory chips.
Typically, a device or system incorporating mass storage flash memory utilizes a processor to control the data storage and retrieval operations of the memory. Such a processor is part of a controller and is often referred to as a controller. A controller executes software instructions to control the device. The software that runs and controls the hardware of a device is often referred to as firmware. The firmware is typically executed from random access memory (RAM) after having been copied from some other memory where it is normally stored. Shadowing or copying to RAM is advantageous because although flash is easily updated it is slower and not inherently executable because it does not have random access capability, and because read only memory is not easily updated.
In the case where some amount of security is to be provided in the firmware, there must be some mechanism to prevent execution of the other than the proper firmware that has the requisite security mechanisms. This is especially true when the firmware is stored in an unsecure memory. As mentioned above, it is the firmware that controls the operation of the device, and therefore it is not a simple matter to have the firmware essentially protect itself. Nor is it a simple matter to protect execution of compromised or unauthentic firmware when such firmware is stored in an otherwise unsecure memory package.
The present invention provides for a secure system with mass storage capability even though it uses unsecure memory for the mass storage unit. Furthermore, it creates a secure system where the firmware for running the secure system is stored in the unsecure memory. In order to be able to store the firmware in the unsecure mass storage memory, the present invention employs a system that prevents execution of inauthentic firmware.
Reference will now be made to preferred embodiments depicted in the figures.
Memory 108 is preferably flash type memory and is used for mass storage purposes. This means that the memory is used for general purpose storage of user files, such as audio, video, and picture files, among other things. It is a principal memory storage unit of device 108 and can be used to store any type of file a user wishes to store in it. It is designed to allow a user to frequently update and access his library of files. A mass storage memory is generally larger than other random access memory (“RAM”) and read only memory (“ROM”) that SD 100A may also comprise (not shown) in this and other embodiments. Also, as a general file storage device, a mass storage memory is distinct from code storage devices that are designed to store comparatively small amounts of operating code that are infrequently updated. A ROM or flash memory may be used as a code storage device, but it should be understood that a code storage device is different in purpose and generally in size than a mass storage device.
SD 100A also comprises a data or memory bus 106 and a host bus 102. SD 100A may be a complete electronic device such as a digital camera or music player, cellular telephone etc. It may also have the form factor of a memory card or universal serial bus (“USB”) drive designed to be used in conjunction with any type of processor controlled electronic device. For simplicity in describing SD100A and the other embodiments depicted in the figures, the embodiments may often be referred to as a memory card, but it should be understood that such reference is to a preferred embodiment and should not limit the scope of the present invention which is defined by the appended claims. Currently, the preferred form factor for a memory card in which the present invention is especially useful is the well known Secure Digital (“SD”) Card.
Data and commands are communicated to and from SD100A via host bus 102. The host, which is not shown, may be a personal computer or other electronic device. Secure controller 104 controls the read and write operations to and from unsecure memory 108 via memory bus 106. In doing so, it also limits access to the contents of the unsecure memory 108. As mentioned above, the firmware that runs the device is stored in unsecure memory 108. This firmware, which will be described in more detail later with regard to
Ensuring the authenticity of the firmware is much more difficult when it is in an unsecure memory. However, given that the unsecure memory 108 is used for mass storage purposes, it is quite large and is easily updated. Therefore, it makes sense to use the capacity of the unsecure memory to store the firmware. This may eliminate or a least reduce the size of a code storage device dedicated to storing the firmware. Alternatively it reduces the need for such storage within the controller. This cost saving is important in a competitive market. There are 3 main paths to the contents stored in memory 108: reading the contents of the memory 108 directly; monitoring the signals on bus 102; and monitoring the signals on bus 106. Even though any or all of the information in the unsecure memory 108 or on buses 102 and 106 may be in an encrypted format, there is always the danger that the encryption key(s) could be compromised. If the firmware were to be compromised and replaced with another firmware that lacked the security features of the authentic firmware, and then executed by the system, restricted or limited access files and private data on the mass storage memory could be copied or tampered with. For example, a user's banking or social security information could be copied or altered without authorization, with obvious negative ramifications. In another example, copyrighted or otherwise protected content could also be copied without authorization. Digital rights management schemes could be thwarted. As another example, cryptographic codes or user passwords could also be compromised.
The cryptographic engine 110 is hardware based and can encrypt and/or decrypt data as it passes through secure controller 104. For example, data encrypted with a first encryption algorithm as it arrives at the controller from host bus 102 can be decrypted and then encrypted with a second algorithm before it is sent to flash memory 108 via data bus 106. Of course, data encrypted in memory 108 can be decrypted by engine 110 and passed in a decrypted state over host bus 102 although it is preferably in an encrypted format as it passes over host bust 102 so as to avoid potential unauthorized copying of the data.
The cryptographic engine 110, also referred to as encryption engine 110, may comprise numerous sub engines and is capable of utilizing numerous encryption standards and algorithms. Examples of the various encryption techniques and algorithms include: Message Authentication Codes (“MACs”); Data Encryption Standard (“DES”), Triple DES, Advanced Encryption Standard (“AES”), RSA and Diffie-Helman that are often used in a Public Key Infrastructure (“PKI”), and other hash based encryption such as SHA-1 and MD5. The encryption engine may use other currently available algorithms and techniques and others yet to be developed or well accepted, and the aforementioned list is only meant to provide some examples.
A Message Authentication Code is a hash computed from a message and some secret data. It is difficult to forge without knowing the secret data. The MAC is computed using an algorithm based on the DES or AES ciphers, which use a secret key. The secret key 112, or one or more keys derived from the secret key are stored in controller 104, and therefore the hash or message authentication code created by the controller is associated with that controller, and cannot be duplicated by another controller. Therefore hash values from a particular controller are associated with the controller and can act as a type of signature of the controller and device, because the signature is unique and cannot be duplicated.
Although the aforementioned standards and various other algorithms and/or standards are well known to those skilled in cryptography, the following publications are informative and are hereby incorporated by reference in their entireties: RFC 3566—The AES-XCBC-MAC-96 Algorithm and Its Use With IPsec by Sheila Frankel, NIST—National Institute of Standards and Technology, 820 West Diamond Ave, Room 677, Gaithersburg, Md. 20899, available at http://www.faqs.org/rfcs/rfc3566.html; Performance Comparison of Message Authentication Code (MAC) Algorithms for the Internet Protocol Security (IPSEC) by Janaka Deepakumara, Howard M. Heys and R. Venkatesan, Electrical and Computer Engineering, Memorial University of Newfoundland, St. John's, NL, Canada, A1B3S7 available at http://www.engr.mun.ca/˜howard/PAPERS/necec—2003b.pdf; and Comments to NIST concerning AES Modes of Operations: A Suggestion for Handling Arbitrary-Length Messages with the CBC MAC by John Black, University of Nevada, Reno, Phillip Rogaway, University of California at Davis, available at http://csrc.nist.gov/CryptoToolkit/modes/proposedmodes/xcbc-mac/xcbc-mac-spec.pdf.
As a further security precaution, the hash values themselves can be encrypted one or more times. In the example of MAC values, a MAC entry that protects the MAC table 200 c 2 is created so even if a hacker finds a way to switch or alter the firmware and recalculate the appropriate MACs, he is still facing a problem because he must calculate the MAC of MACs (or MAC of SHA-1s). Furthermore, in one embodiment the MAC of MACs is again encrypted and stored in another (different) memory field, for example the non volatile memory of encryption engine 110 or the controller 104. This multi-level distributed hierarchy ensures that the signatures cannot be forged, or rather, that a forged signature will not be accepted as authentic. As an illustration, if one were to access the flash memory 108 and replace the firmware and table 200 c, the system would then check one level up the hierarchy and see if the MAC of table 200 c indicates that table 200 c has not been tampered with. If the stored MAC of the table does not match the calculated MAC, this indicates a problem with the authenticity. However, if the MAC of table 200 c has also been altered to match the replaced table 200 c, then the system would verify the signature in error. This error is avoided by storing a copy of the MAC of table 200C in another (inaccessible) memory, and comparing the copy in the other (inaccessible) memory with the value in the flash memory 108. If the values do not match, this indicates an authenticity problem. Although only a few levels were illustrated, this multi-level distributed structure may have numerous levels and incorporate numerous different memories depending on the size and complexity of the firmware to be protected.
This multi-level distributed hierarchy employed in conjunction with the overlay structure of the firmware also results in a very efficient and rapid authentication process. Dividing the firmware into overlays and signing each overlay greatly speeds up the overall authentication process. This is because it is much faster to verify the signature of a smaller amount of code. In order to calculate a hash value, all of the data for which the hash is to be calculated must be read. The larger the portion of firmware to be read, the longer it will take to calculate the signature, and then verify that the signature is authentic. Calculating the signature for a large amount of data is potentially very time consuming and inefficient.
Also stored within the flash memory are various firmware applications 202A . . . X, shown as APP FW 1, 2 . . . X, and, of course, user files (not shown). The firmware applications may be configured differently for various product configurations. The number and type of these applications will vary from one product to another. The firmware applications are also preferably divided into overlays if the applications are larger than the RAM module. A map of the application firmware overlays 201A indicates the location in memory of the various overlays. A table of hash values (SHA-1 digests or MAC values etc.) 201B for the various firmware applications, encrypted with a secret key, which may be secret key 104 or a key derived from secret key 104, is also stored in the flash memory. A firmware application is akin to other applications that run on a base system, e.g. a word processing application in the Windows® environment running on the Windows® operating system.
As discussed in the background, flash memory cells have a limited lifetime and the cells degrade with each read and write operation. Therefore data in the flash memory is generally moved from time to time in order to distribute the read and write operations evenly among the cells and distribute the “wear” evenly amongst the cells. This wear leveling, along with all read/write operations, is controlled by the firmware 200, and in particular by the system firmware 200B. In order to be able to easily move data, the data is logically stored. This means that a logical address is mapped to a physical address, and that while the logical address remains the same, it can be mapped to a different physical address. Again, this logical to physical mapping is carried out by the system firmware.
It presents some difficulty if the firmware is in charge of moving itself. This is especially true when the firmware is responsible for copy protection of the data in the flash memory, and should therefore preferably be verified as authentic before execution. Also, while it is true that the system firmware may be updated from time to time, it will be written very infrequently when compared with other data stored in the flash memory 108. Therefore, the firmware 200, including the boot loader 200 a is physically (without logical mapping) written to and read from flash memory 108.
The application firmware provides additional functionality not present in the system firmware, and may be loaded into the device at any time. It is unknown how much application firmware may be loaded into the device, and when each application may be loaded. Therefore space within the physical partition is not allocated and the application firmware is stored in the logical partition 214 and logically addressed like any other user files and data in the flash memory 108.
Firmware 200, including system firmware 200B, is stored in physical area 212. Application firmware 202A . . . X is stored in logical area 214 where the user files are also stored. The application firmware and all other data in logical area 214 is moved around from time to time by the wear leveling routines of the system firmware.
The authenticity of all of the firmware is preferably checked before it is executed. This is done because, as discussed earlier, the mass storage memory 108 does not have its own built in protection mechanisms. The flowchart of
Hash values for BLR 200A are stored and calculated in a unique process that is described in copending application entitled “Hardware Driver Integrity Check Of Memory Card Controller Firmware” to Micky Holtzman et al. having attorney docket number SNDK.408US2. Please refer to that application for further information on that process.
In one preferred embodiment involving the aforementioned life cycle phases or states, the firmware can only be signed in certain states, and unsigned firmware cannot be executed. In particular, in state 150 (not shown), which is the secure operating state that the device will generally be in while in the hands of the consumer, firmware update and signing will not be allowed. This prevents installation of substitute firmware that may not be authentic. The system in that embodiment only enables the encryption engine to sign the firmware in states other than the secure state. In other embodiments, updating of the firmware is allowed in the field (i.e. while in state 150) as long as the firmware is signed before it is loaded and that signature can be verified by the card. The source of the firmware can also be identified and verified, as well as verifying the signature of the firmware itself. The firmware should be supplied by a trusted entity before it is loaded, and in a preferred embodiment the trust is established using a public key infrastructure (“PKI”) certificate. This certificate could be in addition to or alternatively in lieu of the hash based signature. For example, if trust is established (by the certificate in this illustrative embodiment) then the encryption engine would sign the firmware. As another added precaution, a secure connection can be established with the supplier of the firmware. The secure connection would be encrypted to protect the data passing between the device and the supplier. The secure connection would preferably be encrypted according to the aforementioned AES standard, but could employ any known encryption standard.
As mentioned previously, the system firmware is broken up into overlays of smaller size so that each overly can be loaded into RAM for execution. A map 200 c 1 of the various overlays is stored in the flash memory. Each overlay is individually signed. A table 200 c 2 of the signatures, which are preferably MAC values, is also stored in the flash memory as part of firmware 200. The system or device 200 allocates sufficient room in the RAM for complete table 200 c 2 to be loaded, and the entire table is loaded and resident in RAM during operation of the device.
Each firmware application 202A . . . X is also broken up into overlays, and each overlay is likewise signed. Currently, as with the system firmware, it is preferable to calculate key dependent hash (e.g. MAC) values to sign the firmware applications, although as mentioned previously, other hash values may be used. Table 201B contains the signatures for each application firmware overlay in map 201A. A one sector buffer is pre-allocated in the RAM as a workspace for the application firmware signatures.
Although it is preferable to sign each overlay of any of the firmware because this prevents replacement of a piece of firmware that may have critical decision making functionality, any amount of firmware can rely on one signature. For example, although not preferred, one signature could be used for all the firmware. Furthermore, the size of the overlays or portions to be signed may also vary. Referring again to
Although the various aspects of the present invention have been described with respect to exemplary embodiments thereof, it will be understood that the present invention is entitled to protection within the full scope of the appended claims.