U.S. patent application number 11/207917 was filed with the patent office on 2005-12-29 for system and method for secure storage of data using a key.
This patent application is currently assigned to Microsoft Corporation. Invention is credited to De Treville, John D., England, Paul, Lampson, Butler W..
Application Number | 20050289067 11/207917 |
Document ID | / |
Family ID | 28793850 |
Filed Date | 2005-12-29 |
United States Patent
Application |
20050289067 |
Kind Code |
A1 |
Lampson, Butler W. ; et
al. |
December 29, 2005 |
System and method for secure storage of data using a key
Abstract
In one aspect, a data structure to be encrypted is received in a
device, the data structure including content along with a statement
of conditions under which the content may be decrypted. The data
structure is encrypted using a symmetric key of a processor of the
device. In another aspect, a data structure is decrypted using a
processor symmetric key. A statement of conditions under which
content in the data structure can be decrypted is obtained, and
testing is performed as to whether the conditions are satisfied.
The decrypted content is returned only if the conditions are
satisfied.
Inventors: |
Lampson, Butler W.;
(Cambridge, MA) ; De Treville, John D.; (Seattle,
WA) ; England, Paul; (Bellevue, WA) |
Correspondence
Address: |
LEE & HAYES PLLC
421 W RIVERSIDE AVENUE SUITE 500
SPOKANE
WA
99201
|
Assignee: |
Microsoft Corporation
Redmond
WA
|
Family ID: |
28793850 |
Appl. No.: |
11/207917 |
Filed: |
August 19, 2005 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
11207917 |
Aug 19, 2005 |
|
|
|
10430994 |
May 7, 2003 |
|
|
|
10430994 |
May 7, 2003 |
|
|
|
09227568 |
Jan 8, 1999 |
|
|
|
60105891 |
Oct 26, 1998 |
|
|
|
Current U.S.
Class: |
705/51 |
Current CPC
Class: |
G06F 21/575 20130101;
G06F 21/10 20130101; G06F 9/4406 20130101; G06F 9/468 20130101;
H04L 63/0435 20130101; H04L 63/166 20130101; G06F 2221/2113
20130101; H04L 63/0442 20130101 |
Class at
Publication: |
705/051 |
International
Class: |
H04L 009/00 |
Claims
We claim:
1. A method comprising: receiving a data structure to be encrypted,
wherein the data structure includes content along with a statement
of conditions under which the content may be decrypted, wherein the
statement of conditions comprises an operating system identity that
an operating system executing on the device must have in order for
the content to be decrypted; and encrypting the content using a
key.
2. A method as recited in claim 1, wherein the key comprises a
symmetric key.
3. A method as recited in claim 1, wherein the key comprises a
symmetric key of a processor of the device.
4. A method as recited in claim 1, wherein the operating system
identity is maintained in a software identity register (SIR).
5. A method as recited in claim 1, wherein the operating system
identity is identified in a signed certificate from an operating
system vendor.
6. A method as recited in claim 1, wherein the operating system
identity is for an operating system that is different than an
operating system executing when the content to be encrypted is
received.
7. A method as recited in claim 1, wherein encrypting the content
comprises encrypting the data structure.
8. A system comprising: means for obtaining a block of data to be
encrypted, a current operating system identity, and a target
operating system identity; and means for invoking a seal operation
to have the block of data encrypted by a processor of the system
using a symmetric key of the processor.
9. A system as recited in claim 8, further comprising means for
invoking the seal operation so that only an operating system having
the target operating system identity can decrypt the encrypted
block of data.
10. A system comprising: means for invoking an unseal operation in
order to have a data block decrypted using a key; and means for
receiving, in response to invoking the unseal operation, the
decrypted data block only if conditions under which content in the
data block can be decrypted are satisfied.
11. A system as recited in claim 10, wherein the key comprises a
symmetric key.
12. A system as recited in claim 10, wherein the key comprises a
symmetric key of a processor of the device.
13. A system as recited in claim 10, wherein the conditions
comprise an operating system identity that an operating system
invoking the unseal operation must have in order for the content to
be decrypted.
14. A system as recited in claim 13, wherein the operating system
identity of the operating system invoking the unseal operation is
different than an operating system identity of an operating system
that previously had the data block encrypted.
15. A system as recited in claim 10, further comprising means for
receiving, in response to invoking the unseal operation, an error
indication if the conditions are not satisfied.
Description
RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 10/430,994, filed May 7, 2003, entitled
"System and method for secure storage data using a key", which is
hereby incorporated by reference herein, and which is a
continuation of U.S. patent application Ser. No. 09/227,568, filed
Jan. 8, 1999, entitled "Key-Based Secure Storage". U.S. patent
application Ser. No. 09/227,568 is a continuation-in-part of U.S.
provisional patent application Ser. No. 60/105,891 filed on Oct.
26, 1998, which is herein incorporated by reference, and is related
to co-pending and co-filed U.S. patent application Ser. No.
09/266,207 titled "System and Method for Authenticating an
Operating System to a Central Processing Unit, Providing the CPU/OS
with Secure Storage, and Authenticating the CPU/OS to a Third
Party", Ser. No. 09/227,611 titled "Loading and Identifying a
Digital Rights Management Operating System", Ser. No. 09/227,559
titled "Digital Rights Management", and Ser. No. 09/227,561 titled
"Digital Rights Management Operating System".
FIELD OF THE INVENTION
[0002] This invention relates generally to computer operating
systems, and more particularly to systems and methods for secure
storage of data using a key.
COPYRIGHT NOTICE/PERMISSION
[0003] A portion of the disclosure of this patent document contains
material which is subject to copyright protection. The copyright
owner has no objection to the facsimile reproduction by anyone of
the patent document or the patent disclosure as it appears in the
Patent and Trademark Office patent file or records, but otherwise
reserves all copyright rights whatsoever. The following notice
applies to the software and data as described below and in the
drawings hereto: Copyright.COPYRGT. 1998, Microsoft Corporation,
All Rights Reserved.
BACKGROUND OF THE INVENTION
[0004] More and more content is being delivered in digital form,
and more and more digital content is being delivered online over
private and public networks, such as Intranets, the Internet and
cable TV networks. For a client, digital form allows more
sophisticated content, while online delivery improves timeliness
and convenience. For a publisher, digital content also reduces
delivery costs. Unfortunately, these worthwhile attributes are
often outweighed in the minds of publishers by the corresponding
disadvantage that online information delivery makes it relatively
easy to obtain pristine digital content and to pirate the content
at the expense and harm of the publisher.
[0005] Piracy of digital content, especially online digital
content, is not yet a great problem. Most premium content that is
available on the Web is of low value, and therefore casual and
organized pirates do not yet see an attractive business stealing
and reselling content. Increasingly, though, higher-value content
is becoming available. Books and audio recordings are available
now, and as bandwidths increase, video content will start to
appear. With the increase in value of online digital content, the
attractiveness of organized and casual theft increases.
[0006] The unusual property of digital content is that the
publisher (or reseller) gives or sells the content to a client, but
continues to restrict rights to use the content even after the
content is under the sole physical control of the client. For
instance, a publisher will typically retain copyright to a work so
that the client cannot reproduce or publish the work without
permission. A publisher could also adjust pricing according to
whether the client is allowed to make a persistent copy, or is just
allowed to view the content online as it is delivered. These
scenarios reveal a peculiar arrangement. The user that possesses
the digital bits often does not have full rights to their use;
instead, the provider retains at least some of the rights.
[0007] "Digital rights management" is therefore fast becoming a
central requirement if online commerce is to continue its rapid
growth. Content providers and the computer industry must quickly
provide technologies and protocols for ensuring that digital
content is properly handled in accordance with the rights granted
by the publisher. If measures are not taken, traditional content
providers may be put out of business by widespread theft, or, more
likely, will refuse altogether to deliver content online.
[0008] Traditional security systems ill serve this problem. There
are highly secure schemes for encrypting data on networks,
authenticating users, revoking certificates, and storing data
securely. Unfortunately, none of these systems address the
assurance of content security after it has been delivered to a
client's machine. Traditional uses of smart cards offer little
help. Smart cards merely provide authentication, storage, and
encryption capabilities. Ultimately, useful content must be
assembled within the host machine for display, and again, at this
point the bits are subject to theft. Cryptographic coprocessors
provide higher-performance cryptographic operations, and are
usually programmable but again, fundamentally, any operating system
or sufficiently privileged application, trusted or not, can use the
services of the cryptographic processor.
[0009] There appear to be three solutions to this problem. One
solution is to do away with general-purpose computing devices and
use special-purpose tamper-resistant boxes for delivery, storage,
and display of secure content. This is the approach adopted by the
cable industry and their set-top boxes, and looks set to be the
model for DVD-video presentation. The second solution is to use
secret, proprietary data formats and applications software, or to
use tamper-resistant software containers, in the hope that the
resulting complexity will substantially impede piracy. The third
solution is to modify the general-purpose computer to support a
general model of client-side content security and digital rights
management.
[0010] This invention is directed to a system and methodology that
falls generally into the third category of solutions.
[0011] A fundamental building block for client-side content
security is a secure operating system. If a computer can be booted
only into an operating system that itself honors content rights,
and allows only compliant applications to access rights-restricted
data, then data integrity within the machine can be assured. This
stepping-stone to a secure operating system is sometimes called
"Secure Boot." If secure boot cannot be assured, then whatever
rights management system the secure OS provides, the computer can
always be booted into an insecure operating system as a step to
compromise it.
[0012] Secure boot of an operating system is usually a multi-stage
process. A securely booted computer runs a trusted program at
startup. The trusted program loads an initial layer of the
operating system and checks its integrity (by using a code
signature or by other means) before allowing it to run. This layer
will in turn load and check the succeeding layers. This proceeds
all the way to loading trusted (signed) device drivers, and finally
the trusted application(s).
[0013] An article by B. Lampson, M. Abadi, and M. Burrows, entitled
"Authentication in Distributed Systems: Theory and Practice," ACM
Transactions on Computer Systems v10, 265, 1992, describes in
general terms the requirements for securely booting an operating
system. The only hardware assist is a register that holds a machine
secret. When boot begins this register becomes readable, and
there's a hardware operation to make this secret unreadable. Once
it's unreadable, it stays unreadable until the next boot. The boot
code mints a public-key pair and a certificate that the operating
system can use to authenticate itself to other parties in order to
establish trust. We note that in this scheme, a malicious user can
easily subvert security by replacing the boot code.
[0014] Clark and Hoffman's BITS system is designed to support
secure boot from a smart card. P. C. Clark and L. J. Hoffman,
"BITS: A Smartcard Operating System," Comm. ACM. 37, 66, 1994. In
their design, the smart card holds the boot sector, and PCs are
designed to boot from the smart card. The smart card continues to
be involved in the boot process (for example, the smart card holds
the signatures or keys of other parts of the OS).
[0015] Bennet Yee describes a scheme in which a secure processor
first gets control of the booting machine. B. Yee, "Using Secure
Coprocessors", Ph.D. Thesis, Carnegie Mellon University, 1994. The
secure processor can check code integrity before loading other
systems. One of the nice features of this scheme is that there is a
tamper-resistant device that can later be queried for the details
of the running operating system.
[0016] Another secure boot model, known as AEGIS, is disclosed by
W. Arbaugh, D. G. Farber, and J. M Smith in a paper entitled "A
Secure and Reliable Bootstrap Architecture", Univ. of Penn. Dept.
of CIS Technical Report, IEEE Symposium on Security and Privacy,
page 65, 1997. This AEGIS model requires a tamper-resistant BIOS
that has hard-wired into it the signature of the following stage.
This scheme has the very considerable advantage that it works well
with current microprocessors and the current PC architecture, but
has three drawbacks. First, the set of trusted operating systems or
trusted publishers must be wired into the BIOS. Second, if the
content is valuable enough (for instance, e-cash or Hollywood
videos), users will find a way of replacing the BIOS with one that
permits an insecure boot. Third, when obtaining data from a network
server, the client has no way of proving to the remote server that
it is indeed running a trusted system.
[0017] On the more general subject of client-side rights
management, several systems exist or have been proposed to
encapsulate data and rights in a tamper-resistant software package.
An early example is IBM's Cryptolope. Another existent commercial
implementation of a rights management system has been developed by
Intertrust. In the audio domain, AT&T Research have proposed
their "A2b" audio rights management system based on the PolicyMaker
rights management system.
[0018] Therefore, there is a need in the art for a digital rights
management operating system that protects content downloaded from a
provider while operating on a general purpose personal computer
without the need of specialized or additional hardware.
SUMMARY OF THE INVENTION
[0019] Systems and methods for secure storage of data using a key
are described herein.
[0020] In accordance with one aspect, a data structure to be
encrypted is received in a device, the data structure including
content along with a statement of conditions under which the
content may be decrypted. The content is encrypted using a key.
[0021] In accordance with another aspect, a data structure is
decrypted using a key. A statement of conditions under which
content in the data structure can be decrypted is obtained, and
testing is performed as to whether the conditions are satisfied.
The decrypted content is returned only if the conditions are
satisfied.
[0022] In accordance with another aspect, a data structure to be
encrypted in a device is obtained. The data structure includes
content along with a statement of conditions under which the
content may be decrypted. A seal operation is invoked to have the
data structure encrypted using a symmetric key of a processor of
the device.
[0023] In accordance with another aspect, an unseal operation is
invoked in order to have a data block decrypted using a key. In
response to invoking the unseal operation, the decrypted data block
is received only if conditions under which content in the data
block can be decrypted are satisfied.
[0024] In accordance with another aspect, the processor may contain
a fixed per-processor symmetric key K.sub.S which can be used to
encrypt a data structure containing content along with a statement
of the conditions under which it may be decrypted; key K.sub.S is
also used to decrypt the data structure, test the conditions, and
either return the content or fail. Key K.sub.S is to be used only
for this pair of operations, which are referred to as "Seal" and
"Unseal".
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] FIG. 1A is a diagram of the hardware and operating
environment in conjunction with which exemplary embodiments of the
invention may be practiced;
[0026] FIG. 1B is a diagram of a client computer for use with
exemplary embodiments of the invention;
[0027] FIG. 2 is a diagram illustrating a system-level overview of
an exemplary embodiment of the invention;
[0028] FIG. 3 is a flowchart of a method to be performed by a
client when booting or loading system components according to an
exemplary embodiment of the invention;
[0029] FIG. 4 is a diagram of a certificate revocation list data
structure for use in an exemplary implementation of the
invention;
[0030] FIG. 5 is a flowchart of a method to be performed by a
client to create a boot log according to an exemplary embodiment of
the invention;
[0031] FIG. 6 is a block diagram of an exemplary boot log created
using the method of FIG. 5;
[0032] FIGS. 7A, 7B and 7C are block diagrams of boot blocks for
use in an exemplary embodiment of the invention;
[0033] FIG. 8 is a block diagram of key generation functions
according to an exemplary embodiment of the invention;
[0034] FIG. 9 is a diagram of a rights manager certificate data
structure for use in an exemplary implementation of the
invention;
[0035] FIG. 10 is a diagram of a required properties access control
list data structure for use in an exemplary implementation of the
invention; and
[0036] FIG. 11 is a diagram of a license data structure for use in
an exemplary implementation of the invention.
[0037] FIG. 12 shows an example of a signed boot block created by
signing a block of code.
[0038] FIG. 13 shows steps in a method for performing an
authenticated boot operation on an operating system.
[0039] FIG. 14 shows an exemplary structure of a boot log.
DETAILED DESCRIPTION OF THE INVENTION
[0040] In the following detailed description of exemplary
embodiments of the invention, reference is made to the accompanying
drawings, which form a part hereof, and in which is shown by way of
illustration specific exemplary embodiments in which the invention
may be practiced. These embodiments are described in sufficient
detail to enable those skilled in the art to practice the
invention, and it is to be understood that other embodiments may be
utilized and that logical, mechanical, electrical and other changes
may be made without departing from the spirit or scope of the
present invention. The following detailed description is,
therefore, not to be taken in a limiting sense, and the scope of
the present invention is defined only by the appended claims.
Hardware and Operating Environment
[0041] FIG. 1A is a diagram of the hardware and operating
environment in conjunction with which embodiments of the invention
may be practiced. The description of FIG. 1A is intended to provide
a brief, general description of suitable computer hardware and a
suitable computing environment in conjunction with which the
invention may be implemented. Although not required, the invention
is described in the general context of computer-executable
instructions, such as program modules, being executed by a
computer, such as a personal computer. Generally, program modules
include routines, programs, objects, components, data structures,
etc. that perform particular tasks or implement particular abstract
data types.
[0042] Moreover, those skilled in the art will appreciate that the
invention may be practiced with other computer system
configurations, including hand-held devices, multiprocessor
systems, microprocessor-based or programmable consumer electronics,
network PCs, minicomputers, mainframe computers, and the like. The
invention may also be practiced in distributed computing
environments where tasks are performed by remote processing devices
that are linked through a communications network. In a distributed
computing environment, program modules may be located in both local
and remote memory storage devices.
[0043] The exemplary hardware and operating environment of FIG. 1A
for implementing the invention includes a general purpose computing
device in the form of a computer 20, including a processing unit
21, a system memory 22, and a system bus 23 that operatively
couples various system components, including the system memory 22,
to the processing unit 21. There may be only one or there may be
more than one processing unit 21, such that the processor of
computer 20 comprises a single central-processing unit (CPU), or a
plurality of processing units, commonly referred to as a parallel
processing environment. The computer 20 may be a conventional
computer, a distributed computer, or any other type of computer;
the invention is not so limited.
[0044] The system bus 23 may be any of several types of bus
structures including a memory bus or memory controller, a
peripheral bus, and a local bus using any of a variety of bus
architectures. The system memory may also be referred to as simply
the memory, and includes read only memory (ROM) 24 and random
access memory (RAM) 25. A basic input/output system (BIOS) 26,
containing the basic routines that help to transfer information
between elements within the computer 20, such as during start-up,
is stored in ROM 24. The computer 20 further includes a hard disk
drive 27 for reading from and writing to a hard disk, not shown, a
magnetic disk drive 28 for reading from or writing to a removable
magnetic disk 29, and an optical disk drive 30 for reading from or
writing to a removable optical disk 31 such as a CD ROM or other
optical media.
[0045] The hard disk drive 27, magnetic disk drive 28, and optical
disk drive 30 are connected to the system bus 23 by a hard disk
drive interface 32, a magnetic disk drive interface 33, and an
optical disk drive interface 34, respectively. The drives and their
associated computer-readable media provide nonvolatile storage of
computer-readable instructions, data structures, program modules
and other data for the computer 20. It should be appreciated by
those skilled in the art that any type of computer-readable media
that can store data that is accessible by a computer, such as
magnetic cassettes, flash memory cards, digital video disks,
Bernoulli cartridges, random access memories (RAMs), read only
memories (ROMs), and the like, may be used in the exemplary
operating environment.
[0046] A number of program modules may be stored on the hard disk,
magnetic disk 29, optical disk 31, ROM 24, or RAM 25, including an
operating system 35, one or more application programs 36, other
program modules 37, and program data 38. A user may enter commands
and information into the personal computer 20 through input devices
such as a keyboard 40 and pointing device 42. Other input devices
(not shown) may include a microphone, joystick, game pad, satellite
dish, scanner, or the like. These and other input devices are often
connected to the processing unit 21 through a serial port interface
46 that is coupled to the system bus, but may be connected by other
interfaces, such as a parallel port, game port, or a universal
serial bus (USB). A monitor 47 or other type of display device is
also connected to the system bus 23 via an interface, such as a
video adapter 48. In addition to the monitor, computers typically
include other peripheral output devices (not shown), such as
speakers and printers.
[0047] The computer 20 may operate in a networked environment using
logical connections to one or more remote computers, such as remote
computer 49. These logical connections are achieved by a
communication device coupled to or a part of the computer 20; the
invention is not limited to a particular type of communications
device. The remote computer 49 may be another computer, a server, a
router, a network PC, a client, a peer device or other common
network node, and typically includes many or all of the elements
described above relative to the computer 20, although only a memory
storage device 50 has been illustrated in FIG. 1. The logical
connections depicted in FIG. 1 include a local-area network (LAN)
51 and a wide-area network (WAN) 52. Such networking environments
are commonplace in offices, enterprise-wide computer networks,
intranets and the Internet.
[0048] When used in a LAN-networking environment, the computer 20
is connected to the local network 51 through a network interface or
adapter 53, which is one type of communications device. When used
in a WAN-networking environment, the computer 20 typically includes
a modem 54, a type of communications device, or any other type of
communications device for establishing communications over the wide
area network 52, such as the Internet. The modem 54, which may be
internal or external, is connected to the system bus 23 via the
serial port interface 46. In a networked environment, program
modules depicted relative to the personal computer 20, or portions
thereof, may be stored in the remote memory storage device. It is
appreciated that the network connections shown are exemplary and
other means of and communications devices for establishing a
communications link between the computers may be used.
[0049] The hardware and operating environment in conjunction with
which embodiments of the invention may be practiced has been
described. The computer in conjunction with which embodiments of
the invention may be practiced may be a conventional computer, a
distributed computer, or any other type of computer; the invention
is not so limited. Such a computer typically includes one or more
processing units as its processor, and a computer-readable medium
such as a memory. The computer may also include a communications
device such as a network adapter or a modem, so that it is able to
communicatively couple to other computers.
[0050] One exemplary embodiment of a suitable client computer is
described in the related application titled "System and Method for
Authenticating an Operating System to a Central Processing Unit,
Providing the CPU/OS with Secure Storage, and Authenticating the
CPU/OS to a Third Party," and illustrated in FIG. 1B as subscriber
unit 124. The CPU 140 in the subscriber unit 124 is able to
authenticate the identity of the boot block and OS components that
have been loaded into the computer, and to provide quoting and
secure storage operations based on this identity as briefly
described next. Full descriptions of various embodiments for the
subscriber unit 124 are provided in the related application.
[0051] FIG. 1B shows general components in the subscriber unit 124.
They include a central processing unit (CPU) 140, nonvolatile
memory 142 (e.g., ROM, disk drive, CD ROM, etc.), volatile memory
144 (e.g., RAM), and a network interface 146 (e.g., modem, network
port, wireless transceiver, etc.). The subscriber unit 124 may also
include a sound system 148 and/or a display 150. These components
are interconnected via conventional busing architectures, including
parallel and serial schemes (not shown).
[0052] The CPU 140 has a processor 160 and also can have a
cryptographic accelerator 162. The CPU 140 is capable of performing
cryptographic functions, such as signing, encrypting, decrypting,
and authenticating, with or without the accelerator 162 assisting
in intensive mathematical computations commonly involved in
cryptographic functions.
[0053] The CPU manufacturer equips the CPU 140 with a pair of
public and private keys 164 that is unique to the CPU. For
discussion purpose, the CPU's public key is referred to as
"K.sub.CPU" and the corresponding private key is referred to as
"K.sub.CPU.sup.-1". Other physical implementations may include
storing the key on an external device to which the main CPU has
privileged access (where the stored secrets are inaccessible to
arbitrary application or operating systems code). The private key
is never revealed and is used only for the specific purpose of
signing stylized statements, such as when responding to challenges
from a content provider, as is discussed below.
[0054] The CPU manufacturer may further embed a second secret key
K.sub.2 in the CPU 140 or other secure hardware. The second key is
distinct from the first key pair, and is used to generate a secure
storage key, as is described below. Alternatively, as described
below, a symmetric key K.sub.S may be used with "Seal" and "Unseal"
operations to encrypt a data structure along with a statement of
the conditions under which the data structure may be decrypted.
[0055] The manufacturer also issues a signed certificate 166
testifying that it produced the CPU according to a known
specification. Generally, the certificate testifies that the
manufacturer created the key pair 164, placed the key pair onto the
CPU 140, and then destroyed its own knowledge of the private key
"K.sub.CPU.sup.-1". In this way, only the CPU knows the CPU private
key K.sub.CPU.sup.-1; the same key is not issued to other CPUs and
the manufacturer keeps no record of it. The certificate can in
principle be stored on a separate physical device associated with
the processor but still logically belongs to the processor with the
corresponding key.
[0056] The manufacturer has a pair of public and private signing
keys, K.sub.MFR and K.sub.MFR.sup.-1. The private key
K.sub.MFR.sup.-1 is known only to the manufacturer, while the
public key K.sub.MFR is made available to the public. The
manufacturer certificate 166 contains the manufacturer's public key
K.sub.MFR, the CPU's public key K.sub.CPU, and the above testimony.
The manufacture signs the certificate using its private signing
key, K.sub.MFR.sup.-1, as follows:
Mfr. Certificate=(K.sub.MFR, Certifies-for-Boot, K.sub.CPU), signed
by K.sub.MFR.sup.-1
[0057] The predicate "certifies-for-boot" is a pledge by the
manufacturer that it created the CPU and the CPU key pair according
to a known specification. The pledge further states that the CPU
can correctly perform authenticated boot procedures, as are
described below in more detail. The manufacturer certificate 66 is
publicly accessible, yet it cannot be forged without knowledge of
the manufacturer's private key K.sub.MFR.sup.-1.
[0058] The CPU 140 has an internal software identity register (SIR)
168, which contains the identity of an authenticated operating
system 180 or a predetermined false value (e.g., zero) if the CPU
determines that the operating system 180 cannot be authenticated.
The operating system (OS) 180 is stored in the memory 142 and
executed on the CPU 140. The operating system 180 has a block of
code 182 that is used to authenticate the operating system to the
CPU during the boot operation. The boot block 182 uniquely
determines the operating system, or class of operating systems
(e.g. those signed by the same manufacturer). The boot block 182
can also be signed by the OS manufacturer.
[0059] Another implementation in which a `chain of certificates`
leading back to a root certificate held by the processor
manufacturer is also acceptable.
[0060] The CPU 140 has an internal software identity register (SIR)
168, which is cleared at the beginning of every boot. The CPU
executes an opcode "BeginAuthenticatedBoot" or "BAB" to set an
identity of a corresponding piece of software, such as operating
system 180, and stores this identity in the SIR; the boot block of
the operating system (described below) is atomically executed as
part of the BAB instruction. If execution of the BAB opcode and the
boot block fails (e.g., if the execution was not atomic), the SIR
168 is set to a predetermined false value (e.g., zero). This
process is described below in more detail under the heading
"Authenticated Boot".
[0061] The CPU 140 also utilizes a second internal register (LOGR)
169, which holds contents produced as a result of running a LOG
operation. This operation, as well as the register, is described
below in more detail.
[0062] The CPU 140 also maintains a "boot log" 171 to track
software modules and programs that are loaded. In one
implementation, the boot log 171 is a log in an append-only memory
of the CPU that is cleared at the beginning of every boot. Since it
consumes only about a few hundred bytes, the boot log 171 can be
comfortably included in the main CPU. Alternatively, the CPU 140
can store the boot log 171 in volatile memory 144 in a
cryptographic tamper-resistant container.
[0063] A further implementation is by means of a software module
that allows each section of the booting operating system to write
entries into the boot log that cannot be removed by later
components without leaving evidence of tampering. Yet
alternatively, the SIR can hold a cryptographic digest of a data
structure comprising the initial boot block and the subsequent
contents of the boot log. The operation of appending to the boot
log (call this operation "Extend") replaces the SIR with the hash
of the concatenation of the SIR and the entry being appended to the
boot log. A straightforward implementation of this operation may be
seen to modify the SIR, potentially disallowing future "Unseal"
operations that depend on the value of the SIR. Note, however, that
the operating system, when booting, can choose to add elements to
the boot log without loading the corresponding components, and so a
more privileged combination of software components can impersonate
a less privileged one. This allows the controlled transfer of
secrets across privilege levels. In this approach, software will
keep its own plaintext copy of the boot log entries, along with the
initial value of the SIR following boot, and this plaintext copy is
validated by knowledge of the current composite SIR.
[0064] As an optimization, regardless of the implementation of the
boot log, the OS may choose not to extend the boot log with the
identities of certain software components, if these components are
judged to be as trustworthy as the OS itself, or if they will
execute only in a protected environment from which they will be
unable to subvert operation.
[0065] FIG. 12 shows an example of a signed boot block 190 created
by signing the block of code 182. It contains the
BeginAuthenticatedBoot opcode 192, a length 194 specifying the
number of byte in the block of code, the code 182, a signature 196,
and a public key 198 used to verify the signature 196. The boot
block will also contain as a constant or set of constants, keys, or
other information 199 that is used to validate the subsequent
operating system components (for instance a public key or keys). In
this implementation, the CPU will set the SIR to the public key of
the boot block, but only if the boot block code signature is
correct for the stated boot block public key.
[0066] In an alternative implementation, the SIR is set to the
cryptographic hash or digest of the code and constants that make up
the boot block. The signature 196 and public key 198 are then not
needed.
[0067] A key observation of both of these implementations is that
no one can boot an untrusted operating system in which the SIR is
set to the value of a trusted operating system.
[0068] Once booted the operating system 180 and the applications
named in the license or ACL by the content provider can set aside
space 184 in memory or disk 142 to hold the digital content from
the content provider in a secure manner, without fear of other
operating systems or rogue applications reading the data in the
space. The persistent content is protected by encryption using a
key that is generated based in part upon a seed supplied by an
authenticated and trusted OS, in part by a secret key stored in the
CPU, and in part by the software identity register (SIR).
(Alternatively, the persistent content is stored using the "Seal"
and "Unseal" operations, described below in more detail, or using
the processor's public key pair for encryption.) The persistent
content is stored with a license or ACL naming the applications
that can use the content and the terms under which they can use
it.
[0069] Software programs 186 (the applications) are also shown
stored in memory 142. These programs may be used to render or
otherwise play the content. Each program 186 has an associated key
or digest 188 for unique identification.
Authenticated Boot
[0070] Traditional approaches to secure boot attempt to secure the
BIOS or other loader, and have the BIOS check later components
before allowing them to execute. In contrast to this traditional
approach, the authenticated boot process allows any software at any
point in the boot sequence to initiate an authenticated boot.
[0071] FIG. 13 shows steps in a method for performing an
authenticated boot operation on the operating system 180. These
steps are performed by the CPU 140 and OS 180 resident in the
subscriber unit 124. At step 1300, the CPU executes the
BeginAuthenticatedBoot opcode 192 in the signed boot block 190 to
set an identity for the operating system 180. The identity can be a
digest of the boot block's opcodes and data, or the public key 198
corresponding to a signature on the boot block of the operating
system.
[0072] The BeginAuthenticatedBoot opcode 192 and the boot block 190
execute as one atomic operation, with the implication that if they
execute completely and correctly, the resulting operating system
can be trusted. Measures are taken to ensure that the CPU is not
interrupted and that the boot code that has just been validated
cannot be modified. This can involve locking the memory bus and
switching off interrupts. It could also involve having the CPU
watch for interrupts or for writes by other bus agents and
invalidate the authenticated boot sequence if they occur. The BAB
opcode 192 can be executed at any time, with one exemplary time
being at the start of the OS loader, right after the OS-selector
executes. An alternative implementation is to provide both a
BeginAuthenticatedBoot (BAB) and an EndAuthenticatedBoot (EAB)
instruction. The BAB instruction computes the secure hash of the
boot block and the EAB instruction sets the SIR if the execution of
the boot block was not interrupted or potentially modified by
memory writes from another processor or another bus master.
[0073] Execution of the BeginAuthenticatedBoot opcode 192 sets the
internal software identity register 168 to either (1) the OS's
identity (i.e., boot block digest or OS public key 198) if the
operation is successful, or (2) zero if some event or circumstance
has potentially subverted operation. Assuming the operation is
successful (i.e., the "yes" branch from step 1302), the SIR 168 is
now a unique number or other value that represents the identity of
the operating system 180 (step 1304). Any two processors running
the same operating system will produce the same SIR. If the BAB
opcode operation is unsuccessful (i.e., the "no" branch from step
1302), the SIR is set to zero (step 1306).
[0074] It is noted that different operating systems may be serially
booted on the subscriber unit 124. Executing the BAB opcode 192 for
different signed OS boot blocks results in different SIR values.
However, it is possible for multiple boot blocks to result in the
same SIR, when desired.
[0075] At step 1310, the CPU 140 fills the first entry on the boot
log 171 with the public key (or digest) of the boot block 182. From
now on, any running code can append data to the boot log 171, and
it is generally used by code in the boot chain to identify code
versions as they are loaded and executed. As noted earlier,
appending data to the boot log can be simulated by modifying the
SIR via the "Extend" operation.
[0076] The boot block 182 is free to load the next set of blocks in
the boot-chain (step 1312). At step 1314, the boot block 182 checks
the validity of the modules (by signature or other means) and loads
them so that they can be executed. An identity for each module is
appended to the boot log 171. The OS will also retain additional
information on components that it loads (e.g., version numbers,
device driver IDs, etc.). Loading and executing the code may result
in loading more code, validating it, and executing it, etc. This
process continues through to the loading of device drivers. When
the boot sequence is complete, the OS is operational and the
software identity register and the boot log store non-modifiable
data captured during the boot sequence. We can recommence loading
new device drivers at any point, possibly causing the operating
system to become less privileged, with the possible termination of
access to protected content.
[0077] The CPU can generate a signed certificate containing the
boot log data to attest to the particular operating system
(including drivers) that is running. It could also generate a
signed statement containing just the SIR. FIG. 14 shows an
exemplary structure of a boot log 171. It contains a seed field
1400 and a block ID field 1402. The block ID field 1402 holds
identities of the blocks of code that are loaded and verified on
the subscriber unit. The block ID field 1402 can hold text or
binary data.
[0078] The SIR or the seed field 1400 holds an authenticated boot
key generator seed. The CPU uses the seed in field 1400 to generate
keys unique to the OS and processor. Since the first entry of the
boot log 171 can only be generated by the execution of a particular
boot block or the holder of the boot block private key, the keys
can only be re-generated by the same OS, or another OS from the
same publisher under control of the publisher. OS-specific key
generation provides a building block for secure persistent storage
of data and the continued enforcement of digital usage rights even
if the computer is physically compromised, or the computer is
booted into another operating system. Use of OS-specific storage
keys for secure storage is described below in more detail.
[0079] Alternatively, the processor may use the "Seal" and "Unseal"
instructions to store persistent protected content, or when
possible may encrypt it with the processor's public key and decrypt
it with the "Unseal" instruction, which is called "Reveal" when
used with public keys. These operations are described below in more
detail under the heading "Secure Storage".
System Level Overview
[0080] A system level overview of the operation of an exemplary
embodiment of the invention is described by reference to FIG. 2. A
subscriber computer 200, such as client computer 20 in FIG. 1, is
connected to a content provider server computer 220, such as remote
computer 49, through a wide-area network, such as WAN 52. Processes
performed by the components of the subscriber computer 200 and the
content provider 200 are illustrated by arrows in FIG. 2. Many of
these processes incorporate either public/private key pairs,
digital signatures, digital certificates, and/or encryption
algorithms, or a combination of these standard cryptographic
functions. Such functions are assumed to be provided by the CPU of
the subscriber computer in the descriptions that follow, but can be
provided by other well-known cryptographic mechanisms as will be
immediately understood by one skilled in the art.
[0081] The content may be essentially any type of content that can
be expressed as digital data, including video, still pictures,
audio, graphical images, and textual data or executable content
(computer programs). Examples of possible content include
feature-length movies, TV shows, games, software programs, news,
stock information, weather reports, art, photographs, and so
on.
[0082] To prevent their content from being stolen or misused,
content providers will download content only to known software, and
therefore only to subscriber computers that can prove that their
operating systems will enforce the limitations the provider places
on the content. Such a digital rights management operating system
(DRMOS) must load and execute only OS components that are
authenticated as respecting digital rights ("trusted"), and must
allow access to the downloaded content by only similarly trusted
applications.
[0083] The first requirement is met in the exemplary embodiment of
the invention by having all trusted operating system-level
components digitally signed by their developers or a trusted
third-party, with the signature acting as a guarantee that the
components respect digital rights. The signature is validated
before the component is loaded. The resulting DRMOS is assigned a
unique trusted identity, as explained in detail below, which is
recorded in an internal register in the CPU, such as SIR 168 in
FIG. 1B. FIG. 2 illustrates a DRMOS 205, with its identity 206,
after it has been loaded into the CPU 201 of a subscriber computer
200 through such a loading process 1.
[0084] The second requirement has two aspects. First, trusted
applications must be identified in some fashion, and, second, the
DRMOS must prevent non-trusted applications from gaining access to
the content when it is stored, either permanently or temporarily,
on the subscriber computer.
[0085] In the exemplary embodiment shown in FIG. 2, a trusted
application 209 has agreed to operate in accordance with the
limitations placed on content by a provider. The trusted
application 209 is identified through a "rights manager"
certificate 210. In one embodiment, the rights manager certificate
210 extends a standard digital certificate, which includes such
items as date of publication and name of the application, by adding
a list of services, or properties, provided by the application,
i.e., content type handled, version of the application, whether it
saves content to disk, etc. For purposes of the exemplary
embodiment shown in FIG. 2, the certificate 210 also identifies the
trusted application; alternate mechanisms for identifying a trusted
application are described later in the methods section.
[0086] The DRMOS 205 provides key-secured storage for permanently
stored content to prevent unauthorized access to the content. For
temporarily stored content, the DRMOS 205 prevents an untrusted
process from reading the memory holding the content. These and
other safeguards are also described in detail below. The permanent
and temporary storage within subscriber computer 200 are
collectively represented by device 203, which is illustrated in
FIG. 2 as a disk drive. Such illustration is not intended to limit
the range of devices that can serve as secured storage for a
DRMOS.
[0087] Turning now to the reminder of the processes depicted in
FIG. 2, application 209 requests 2 the download of content 221 from
provider 220. The DRMOS 205 sends a message 3 to the provider 220
requesting the content 221. The content provider 220 transmits a
challenge message 4 to the DRMOS 205 asking for the identity of the
CPU 201, the DRMOS 205, and the application 209. The DRMOS 205
transmits a response message 5 containing a certificate 202 for the
CPU 201, its own identity 206, and the rights manager certificate
210 for the application 209.
[0088] The challenge-response process follows the common protocol
for such interchanges, the difference being only in the data
exchanged between the subscriber computer and the content provider.
In one exemplary embodiment of a suitable challenge-response
process described in the related application titled "System and
Method for Authenticating an Operating System to a Central
Processing Unit, Providing the CPU/OS with Secure Storage, and
Authenticating the CPU/OS to a Third Party," the certificate 202
contains the challenge message 3, the identity of the DRMOS 206,
the public key of the CPU 201, and data representing all software
components that are currently loaded and executing on the
subscriber computer 200. The certificate 202 is signed using the
private key of the CPU 201. The content provider 220 examines the
CPU certificate 202, the DRMOS identity 206, and the properties
specified in the rights manager certificate 210 to determine
whether it should establish a trust relationship with the DRMOS 205
on the subscriber computer 200.
[0089] In an alternate exemplary embodiment, the challenge-response
protocol runs over a secure connection such as SSL (Secure Socket
Layer) or TLS (Transport Level Security), which relies on a session
key to encrypt the data transferred between the subscriber computer
200 and the content provider 220. This stops an attacker (such as
the legitimate owner of the machine) from rebooting the PC into a
different operating system after the DRMOS has authenticated
itself, or using a different computer on the network for snooping
on the data destined for the DRMOS.
[0090] If the trust relationship is established, the provider
downloads 6 the content 221, an access predicate 222, and a
"license" 223 to the DRMOS 205 on the subscriber computer 200. The
access predicate 222 specifies the properties that an application
must have in order to process the content 221, such as read-only or
minimum/maximum video resolution. The access predicate 222 may also
specify specific applications or families of applications allowed
to process the content 221. The license 223 places restrictions on
the use of the content 221 by an approved application, such as the
number of times the content can be accessed or what derivative use
can be made of the content. A media server of the content provider
may be configured to download the entire content as a file, or to
stream the content continuously over the network. As an example,
the content provider may implement a server computer system
comprising one or clustered server computers that handle requests
from subscribers, manage the digital files locally, and facilitate
delivery of requested digital files over a network to the
subscriber 200.
[0091] When the DRMOS 205 receives the content 221, the access
predicate 222 and the license 223, it determines whether the
content should be permanently stored in a key-secured storage. If
so, it requests an application storage key from the CPU 201. In the
present example, the application storage key is specific to the
application 209 that requested the content 221. The content 221 and
the license 223 are encrypted using the application storage key and
the access predicate 222 is attached to the encrypted information.
If the content 221 is to be stored only temporarily, the DRMOS 205
places various safeguards around the memory area holding the
content so that the content cannot be accessed by an untrusted
application. The generation of application storage keys and the
memory safeguards are described in detail below.
[0092] Each time application 209 wants to access the stored content
221, it passes its rights manager certificate 210 and the
appropriate application storage key (action 8) to the DRMOS 205.
The DRMOS 205 validates the key and compares the rights manager
certificate 210 against the access predicate 222. Assuming the
storage key is authenticated and the rights manager certificate 210
satisfies the access predicate 222, the content 221 and the license
223 are decrypted. The DRMOS determines if the application's use of
the content is permitted under the license 223 and allows access 9
if it is.
[0093] The system level overview of the operation of an exemplary
embodiment of the invention has been described in this section of
the detailed description. A series of processes and data structures
on a subscriber computer control the loading of a digital rights
management operating system, identify the DRMOS and trusted
applications to a content provider, and secure content downloaded
by the provider to the subscriber computer. While the invention is
not limited to any particular hardware and software, for sake of
clarity only a minimal hardware and software configuration
necessary to process multimedia has been assumed for the subscriber
computer.
Methods of Exemplary Embodiments of the Invention
[0094] In the previous section, a system level overview of the
operation of exemplary embodiments of the invention was described.
In this section, the particular methods performed by a subscriber
computer, or client, of such exemplary embodiments are described by
reference to a series of flowcharts and operational diagrams. The
methods to be performed by the client constitute computer programs
made up of computer-executable instructions. Describing the methods
by reference to flowcharts and operational diagrams enables one
skilled in the art to develop such programs including such
instructions to carry out the methods on suitable computerized
clients (e.g., on the processor of a client executing the
instructions from computer-readable media). Data structures
necessary to perform the methods are also described in this
section. The methods of the content provider server computer are
described to complete the understanding of the methods performed by
the client.
[0095] Although many of the methods are interrelated, they have
been divided into four groups to facilitate understanding. The
boot/load process and various mechanisms for creating identities
for different versions of a digital right management operating
system (DRMOS) are first described. The functions that must be
provided by the DRMOS to ensure the enforcement of the content
providers' rights are described next. The third group consists of
methods directed toward providing permanent storage of the content
on the subscriber computer once downloaded, and protecting that
content from unauthorized access. Finally, the identification of
trusted applications and the rights management functions are
described.
[0096] Booting/Loading and Identifying the DRMOS
[0097] Referring first to FIG. 3, a flowchart of a method to be
performed by a subscriber computer according to an exemplary
embodiment of the invention is shown. This method is inclusive of
the acts required to be taken by the computer to boot a DRMOS or to
load additional components after the boot process is complete.
Exemplary embodiments of boot block data structures are described
below in conjunction with FIGS. 7A-C.
[0098] Shortly after a computer is turned on or is reset, a small
program called a boot loader is executed by the CPU (block 301).
The boot loader loads a boot block for a particular operating
system. Code in the boot block then loads various drivers and other
software components necessary for the operating system to function
on the computer. The totality of the boot block and the loaded
components make up the identity of the operating system.
[0099] For a DRMOS, that identity can be trusted only if the boot
block and the loaded components are trusted. In the embodiments
described herein, all components are signed by a trusted source and
provided with a rights manager certificate. An exemplary embodiment
of the rights manager certificate is described below in conjunction
with FIG. 9.
[0100] The operating system checks the signature of a component
before loading it (block 303). If the signature is valid (block
305), the component has not been compromised by someone attempting
to circumvent the boot process and the process proceeds to check
the level of trust assigned to the component (block 307). If the
signature is not valid (or is there is no signature) but the
component must be loaded (block 309), the operating system will not
assume the identity of a DRMOS upon completion of the boot process
as explained further below.
[0101] A plug-and-play operating system provides an environment in
which devices and their supporting software components can added to
the computer during normal operation rather than requiring all
components be loaded during the boot process. If the device
requires the loading of an untrusted component after the boot
process completes, a plug-and-play DRMOS must then "renounce" its
trusted identity and terminate any executing trusted applications
(block 323) before loading the component. The determination that an
untrusted component must be loaded can be based on a system
configuration parameter or on instructions from the user of the
computer.
[0102] Assuming the signature is valid (block 305) and the
component is trusted (block 309), it is loaded (block 311). The
trustworthiness of a component can be decided using various
criteria. In one embodiment, only components provided by the
operating system developer are trusted. At the other end of the
scale, in another embodiment, all components are assumed
trustworthy by the DRMOS, leaving the final decision to the content
provider as described in more detail below. Still a third alternate
embodiment provides that components signed by any of a select
number of entities can be considered as equivalent to components
provided by the DRMOS developer. In this embodiment, the identity
of the resulting operating system is considered equivalent to the
"pure" DRMOS provided by the DRMOS developer. The content provider
decides whether it trusts the equivalent operating system.
[0103] Furthermore, not all versions of a component may be trusted.
Because the rights manager certificate contains the version number
of the component, it can be used to verify the trust level of a
particular version. One embodiment of the loading process checks a
component certification revocation list (CRL) to determine whether
a component signature has been revoked. The CRL can be provided by
the content provider or the DRMOS developer. An exemplary
embodiment of a CRL is illustrated in FIG. 4. Each entry 401
contains the name of the component 403, the version 405, and the
signer 407 whose signature is revoked. The particular CRL used
becomes part of the operating system identity using a standard
hashing function described further below.
[0104] Alternatively, if the rights manager certificates on the
components are short-lived and must be renewed periodically, then a
version that is found to be untrustworthy will not have its
certificate renewed. This alternate embodiment requires a secure
time source to be available on the subscriber computer so the user
cannot simply turn back the system clock on the subscriber
computer. A monotonic counter in the CPU can serve as this secure
time source since it only counts up and cannot be reset "back in
time." For example, a monotonic counter that is periodically
incremented while the CPU is active, and that cannot be reset, can
be used in conjunction with a secure time service, such as a secure
Internet time service, to provide a lower bound on the current time
in a trusted manner. Such exemplary use of a monotonic counter is
described in detail below as part of the functions of the
DRMOS.
[0105] Once all components are loaded, the operating system assumes
its identity (block 315). In one embodiment, a one-way hashing
function provided by the CPU is used to create a cryptographic
"digest" of all the loaded components. The digest becomes the
identity for the operating system and is recorded in an internal
register in the CPU. Alternate methodologies of assigning an
identity to the loaded components are equally applicable as long as
a non-trusted configuration cannot have the same identity as a
DRMOS. Signing the operating system identity with a private key
particular to the type of CPU serves to identify both the operating
system and the processor on which it is executing.
[0106] If all computers were identically configured, a single,
signed operating system identity would suffice to authenticate a
particular operating system executing on a particular type of CPU.
However, computers contain a myriad different hardware components,
and the corresponding supporting software components are frequently
updated to add enhancements and fix problems, resulting in a
virtually unlimited number of operating system identities.
Therefore, the content provider would have to maintain a registry
of each subscriber's DRMOS identity or delegate that function to a
trusted third party.
[0107] The problems attendant on having a vast number of DRMOS
identities can be alleviated in at least three ways. First, an
identity is generated or assigned for the basic configuration of
each operating system. Such a basic configuration includes only
components supplied by the operating system vendor. The identity is
generated (or assigned) and stored when the basic components have
been loaded. Different versions of the basic operating system will
generate (or be assigned) different identities.
[0108] Once the basic configuration of a DRMOS is loaded and its
trusted identity is stored, subsequent components required to
support the particular hardware configuration must be verified and
loaded as explained in conjunction with FIG. 3. Such additional
software components can also include updates to the basic
components provided by vendors other than the operating system
developer. Each additional loaded component has an individual
identity (such as a cryptographic digest) generated/assigned and
stored. All the identities are uploaded to the content provider
when the DRMOS identity is requested. Because the basic DRMOS and
additional components always have the same identities when
executing on a specific type of processor, the content provider has
only to maintain a list of the identities for the combinations of
the basic DRMOS and additional components that the provider trusts.
Each identity uploaded is then checked against the list.
[0109] In a second alternate embodiment, the operating system
maintains a "boot log," containing the identity of the basic DRMOS
and the identities of the additional OS components that have been
loaded. The identity is a cryptographic digest of the code for the
component, or a well-known name, or any other string that is
uniquely associated with the component. The CPU also maintains a
composite identity register that holds a one-way cryptographic
function of the boot log. Whenever a component is loaded, its
identity is appended to the boot log and folded into the composite
identity register, such as by setting this register to a secure
hash of its old value appended with the new component's identity.
Whenever the CPU certifies the current value of its composite
identity register, it also verifies that the operating system's
boot log has not been tampered with. Because the log is indelible,
the loaded component cannot erase the record that shows it was
loaded.
[0110] An alternate exemplary embodiment of the boot log holds the
entire boot log in the CPU. The DRMOS uses an instruction provided
by the CPU that appends the identity of each loaded component to
the log. The CPU then signs the boot log to attest to its validity
and delivers the signed boot log to the content provider as the
identity for the DRMOS.
[0111] In another alternate embodiment, DRMOS uses a chain of
public and private key pairs newly generated by the CPU to create
an indelible boot log. The method is shown in FIG. 5 and an
exemplary embodiment of the completed boot log is illustrated in
FIG. 6. The boot loader generates or obtains a first key pair
(K.sub.0, K.sub.0.sup.-1) and records the first key pair in memory
(block 501). The first public key is also saved to secure storage
in the CPU. The boot loader loads the boot block into memory and
records the identity of the boot block in the boot log (block 503).
Before turning control over to the boot block code, the boot loader
obtains a second key pair (K.sub.1, K.sub.1.sup.-1) (block 505),
writes the second public key (K.sub.1) to the boot log (block 507),
and then signs the boot log with the first private key
(K.sub.0.sup.-1) (block 509). The boot loader deletes the first
private key (K.sub.0.sup.-1) from its memory (block 511) and
relinquishes control to the boot block.
[0112] The boot block code loads additional components into memory,
records the identities of those components to the boot log (block
515), obtains a third key pair (K.sub.2, K.sub.2.sup.-1) (block
505), appends the boot log with the third public key (K.sub.2)
(block 507), and signs its portion of the boot log with the second
private key K.sub.1.sup.-1 (block 509). The boot block erases the
second private key (K.sub.1.sup.-1) (block 511) from memory and
turns control of the boot process over to the first loaded
component. Each loaded component that will load additional
components obtains a new key pair (K.sub.n, K.sub.n.sup.-1) and
uses the private key of the previous key pair (K.sub.n-1.sup.-1) to
sign its portion of the boot log. The boot process continues in
this iterative manner through until all components are loaded or,
in the case of a plug-and-and play DRMOS, a content provider
challenge is received (block 513).
[0113] When a non-plug-and-play DRMOS resumes control after the
final component is loaded, it places a "sentinel" on the boot log
(block 519) to indicate that the log is complete and to prevent a
loaded component from deleting entire lines of the log. The
characteristics of the sentinel are that is a known, unique value
that is signed using the last private key (K.sub.n.sup.-1). In the
present embodiment, the sentinel is a signed zero entry. The DRMOS
deletes the last private key and all public keys from memory after
creating the sentinel.
[0114] Because a plug-and-play DRMOS cannot arbitrarily declare
that all components are loaded at the end of the boot process, the
DRMOS cannot add a sentinel to the end of the boot log at that
time. Instead, the DRMOS attests to its most recent public key
K.sub.n as well as its first public key K.sub.0 to certify the
contents of the boot log when challenged.
[0115] Using a chain of key pairs 606, 607, as shown in FIG. 6,
guarantees the boot log reflects the loaded components. Each public
key in a log section is used to authenticate the signature on the
next section. The first public key remains in memory to
authenticate the signature on the boot block section of the log.
While each set of components is free to load more components, a
component cannot change the recording of its identity in a previous
portion of the log because doing so would cause the validity check
on the corresponding signature to fail. Similarly, a section in the
middle of the log cannot be deleted because that would break the
chain of keys. Deleting multiple sections of the log through to the
end also breaks the chain. In this case, attempting to insert a new
sentinel in an effort to make the log appear unaltered will fail
because the private key necessary to add the sentinel is not longer
available. Finally, the entire boot log cannot be replaced since
the signature on the boot block section of the log would not be
validated by the first public key.
[0116] Turning now to the boot block, one exemplary embodiment
suitable for use with a digital rights management operating system
is shown in FIG. 7A. The boot code 701 is signed (signature 703) by
the developer of the DRMOS using its private key. The corresponding
public key 705 of the developer is attached to the boot block 700.
In an alternate embodiment, the public key 705 is not attached to
the boot block 700, but instead is persistently stored in an
internal register in the CPU. The public key 705 is used to
validate the signature 703.
[0117] If the DRMOS developer's private key used to sign the boot
block is compromised, the key pair must be changed and thus all
boot blocks must be reissued to subscriber computers. FIG. 7B
illustrates an alternate embodiment of a boot block that
ameliorates this problem. Boot block 710 comprises a basic boot
section 711 and an intermediate boot section 713. The basic boot
section 711 contains boot code 715 that validates and loads the
intermediate boot section 713 and components not provided by the
DRMOS developer. The intermediate boot section 713 contains boot
code 717 that validates and loads components from the DRMOS
developer. The intermediate boot section 713 is signed with a
special boot block private key. The basic boot code 715 uses a
corresponding boot block public key 719 stored in the basic boot
section 711 to validate the intermediate boot section 713.
Components 727 from the DRMOS developer are signed 729 with the
developer's standard private key and the intermediate boot section
713 uses the DRMOS developer's standard public key 721 to validate
those components.
[0118] If the standard private key used to sign components is
compromised, the developer creates a new standard key pair and
provides a replacement intermediate boot block 713 containing the
new standard public key. Replacement components signed with the new
standard private key are also issued. Because the special boot
block private key is used for few, if any, other purposes than
signing boot blocks, it is less likely to be compromised and
replacement of the basic boot section 711 will rarely be
necessary.
[0119] In FIG. 7C, an alternate embodiment of the single section
boot block 730 also uses a special boot block key pair. The boot
block 730 contains the special boot block, or master, public key
733. The master private key is used to certify ephemeral keys that
are valid for a short period of time. Certificates signed 737 by
the master private key attest to the validity of the ephemeral
keys. A component is signed with one of the ephemeral private keys
and the corresponding certificate 739 is attached. The boot block
determines that the certificate on the component is valid using the
master public key. When the ephemeral key expires, the DRMOS
developer issues replacement components. As with the two-section
boot block shown in FIG. 7B, the master private key is only used to
sign the certificates for the ephemeral keys so it is less likely
to be compromised. Because the ephemeral keys are valid for only a
short duration, public release of a private ephemeral key has
limited impact.
[0120] Functions of a DRMOS
[0121] As described above, components may be valid only until a
specified date and time, and content may also be licensed only
until a certain date and time. The monotonic counter described
earlier can also used to ensure that the computer's clock cannot be
set backwards to allow the replacement of a trusted component by an
earlier, now untrusted version. The DRMOS connects on a regular
basis to a trusted time server and presents the value of its
monotonic counter, whereupon the trusted time server returns a
certificate binding that value to the current time. If the
monotonic counter is updated periodically, such as every hour that
the DRMOS is running, then the monotonic counter in conjunction
with the most recent time certificate can serve as a useful
approximation to a trusted clock.
[0122] A DRMOS must also protect the content once it is loaded into
the client computer's memory by a trusted application. In
particular, the DRMOS must prohibit the use of certain types of
programs and refrain from performing certain common operating
system procedures when content is in memory.
[0123] An example of one kind of procedure that must be prohibited
is loading a kernel debugger because it would allow the user to
make a copy of the content loaded in memory. If the user of the
subscriber computer attempts to load a kernel debugger into memory,
the DRMOS can either 1) refuse to load the debugger, or 2) renounce
its trusted identity and terminate the trusted application that was
accessing the content before loading the debugger. In the latter
case, the memory must also be purged of the content before the
debugger is loaded. The choice of action can be pre-determined or
chosen by the user when the user attempts to load the kernel
debugger. One of skill in the art will immediately identify other
types of programs that will need to be treated in the same manner
as a kernel debugger.
[0124] Virtual memory operating systems maintain a page file that
holds sections of program code and data that are not currently
active. Under normal circumstances, the contents of the page file
are accessible by the user of the computer, either by running a
privileged program or by booting another operating system that
allows inspection of the disk. Therefore, a DRMOS must either
protect content stored on the page file or must not page content
and similar protected information at all.
[0125] Protecting content on the page file can be accomplished in
at least three ways. First, the DRMOS can prohibit all "raw" access
to page file device when a trusted application is running. Second,
Second, the DRMOS can terminate all trusted applications and erase
the page file before allowing such access. Third, the DRMOS can
encrypt the content and similar protected information before
writing it to the page file.
[0126] Often, a DRMOS must allow the user to perform certain
standard functions but prohibit other, related functions. The DRMOS
can assign the user permissions based on the granularity of the
normally permitted function. For example, the DRMOS can allow the
user to delete an entire content file but not a portion of it.
Another example is that the DRMOS can allow the user to terminate
all the threads of execution for a trusted application but not just
a single thread.
[0127] Finally, a DRMOS must protect the trusted application itself
from tampering. The DRMOS must not allow other processes to attach
to the process executing the trusted application. When the trusted
application is loaded into memory, the DRMOS must prevent any other
process from reading from, or writing to, the sections of memory
allocated to the trusted application without the explicit
permission or cooperation of the trusted application
[0128] Key-based Secure Storage
[0129] In order to protect content permanently stored on the
subscriber computer, the DRMOS must provide a secure storage space.
In essence, the DRMOS must securely store private keys or session
keys for use with encrypted content, or provide some other
mechanism for keeping these keys secret from other OSs or system
level software. These keys can be used for the secure storage and
retrieval of protected information. In the exemplary embodiments
described in this section, the information to be stored in a
protected format is encrypted using one of a set of keys that may
be generated by a function 800 provided by the CPU. The storage key
generation process is tightly coupled to the DRMOS so that the same
key cannot be generated by the CPU for an unrelated operating
system, or by any software on another computer. Three types of
storage keys are envisioned as illustrated in FIG. 8: an OS storage
key 801, an application storage key 811, and a user storage key
821. Each key is specific to the entity that requests it.
[0130] Beginning with the OS storage key 801, the DRMOS passes a
"seed" 803 as an operand of a key-generation instruction
("GenerateKey") 805 to the CPU and receives an OS storage key based
on the seed 803 and the identity of the DRMOS. The CPU will always
return the same OS storage key 801 when the same seed 803 is
provided by the same DRMOS but will return a different OS storage
key if the same seed 803 is provided by an unrelated operating
system. Because an unrelated operating system cannot get the same
key 801, it cannot read any data encrypted by the DRMOS.
[0131] In an alternate embodiment, only a single operating system
storage key is used by the DRMOS as described below. Therefore, in
this embodiment only the identity of the DRMOS is factored into the
key generation function 800 and the seed 803 is not necessary.
[0132] An application storage key 811 is generated when an
application calls an operating system instruction
("GenerateApplKey") 815 using a seed 813. The DRMOS passes the seed
813 through an application-specific one-way hash function 817 to
produce a hashed seed 819. The hashed seed 819 is then passed to
the CPU through the GenerateKey instruction described above. The
resulting application storage key 811 is returned to the
application for use. Because the GenerateKey function uses the
operating system's identity, the same application executing under
an unrelated operating system cannot get the same key, and
therefore cannot access the encrypted data, even if it requests the
key using the same seed 813. Similarly, an unrelated application
using the same seed 813 gets a different key because the DRMOS
passes the seed 813 through a different hash algorithm for each
application.
[0133] In an alternate embodiment, the operating system stores
decryption keys for applications using its own identity; the
applications call the operating system to retrieve application
keys. This also provides a way for an application to allow other
applications access to its key and therefore to the content
encrypted by the key. Instead of creating a secret using a seed
813, the application passes in the access predicate for the
content. The access predicate designates values that must be
present in the rights manager certificate for an application
wishing access to the content. An exemplary embodiment for an
access predicate is shown in FIG. 9 and described in detail in the
following section. The DRMOS supplies the seed 813 that is required
to generate the application specific key and passes the seed 813
through a generic one-way hash. The DRMOS encrypts the seed 813 and
the access predicate using an OS storage key and associates the
encrypted access predicate with the encrypted seed. When any
application requests access to a key protected by an access
predicate, the DRMOS compares the criteria in the access predicate
against the rights manager certificate of the requesting
application. An application that meets the criteria is given access
to the seed 813 and therefore to the application storage key.
Because the seed 813 is encrypted using an OS storage key, an
application that is running under an unrelated operating system
will be unable to gain access to the encrypted data because the
unrelated operating system cannot decrypt the seed 813.
[0134] Finally, a particular user can request a key that is based
on a user identity assigned by the DRMOS or another facility that
guarantees a unique identity for each user. The user supplies a
seed 823 in a "GenerateUserKey" call 825. The operating system
passes the seed 823 through a one-way hash 828, and then passes the
resulting first hashed seed 827 through a keyed hash routine 829 to
generate a second hashed seed 833. The operating system factors the
user identity 831 into the keyed hash routine 829 so that the
second hashed seed 833 is unique to the user. The second hashed
seed 833 is passed to the CPU, which returns the user storage key
821. As described above, only the same user will be able to access
data encrypted with the storage key 821 when the DRMOS that
generated the key is executing. Analogously, the keyed hash routine
829 guarantees that the user storage key will not duplicate either
an OS storage key or an application storage key based on the same
seed. Such a facility is used when downloaded content can be
accessed only by a particular user. Moreover, if downloaded content
is to be accessed only by a particular user and by a particular
application, the secret to be stored may be divided into parts,
with one part protected by an application-specific key and the
other part protected by a user-specific key.
[0135] Once the data is encrypted using the storage keys, there
must be a way to recover the keys when the DRMOS identity changes
(as when the operating system is upgraded to an incompatible
version or an unrelated operating system is installed) or the
computer hardware fails. In the exemplary embodiments described
here, the keys are stored off-site in a "key vault" provided by a
trusted third party. In one embodiment, the DRMOS contains the IP
addresses of the key vault providers and the user decides which to
use. In another embodiment, the content provider designates a
specific key vault and the DRMOS enforces the designation. In
either embodiment, when the user requests the restoration of the
storage keys, the key vault provider must perform a certain amount
of validation before performing the download. The validation
process can include such actions as recording the identity of the
original operating system (or computer) in a revocation list,
checking the frequency of the requests, and requiring a credit card
number before downloading the storage keys.
[0136] Rights Management
[0137] Most operating systems do not directly process media
content, such as video or audio. That function is usually available
through special application programs. Therefore, a content provider
must not only trust the operating system but must also trust the
application that will process the content. Content also can be
accompanied by a predicate stating which applications are to be
trusted to access that content, and this statement can include a
list of generic properties that implicitly define a set of
applications. Further associating a rights manager certificate with
the application provides identification of the application and
certification of its properties. This allows the content provider
to determine if the application fulfills the requirements of the
content provider before downloading content, and also allows the
operating system to restrict future access to only the appropriate
applications.
[0138] One exemplary embodiment of a right manager certification is
shown in FIG. 9. A list of application properties 903 is appended
to the digital certificate fields 1001 standard in some digital
certificate format such as X.509. The certificate names the
application. Each entry 905 in the list 903 defines a property 906
of the application, along with optional arguments 907. For example,
one property might be that the application cannot be used to copy
content. Another example of a property is one that specifies that
the application can be used to copy content, but only in analog
form at 480P resolution. Yet another example of a property is one
that specifies that the application can be used to copy content,
but only if explicitly allowed by an accompanying license.
Additional examples include the right to store an encrypted copy of
the content and to restrict such storage to only certain,
acceptable peripheral devices. The property 906 can also be used to
specify acceptable helper applications, such as third-party
multimedia processing stacks or other libraries, to be used in
conjunction with the application named in the certificate. The
certificate is signed by an operating system vendor, content
provider, or third party, certifying the properties of the
application.
[0139] Because the content provider must trust the DRMOS and
application to protect the content from misuse once downloaded, the
content provider attaches an access predicate to the content. This
access predicate can also include a license to the content. The
basic functions of both the access predicate and the license, which
were described in the system overview, are explained in detail
next.
[0140] In one embodiment, the access predicate takes the form of a
required properties access control list (ACL) as shown in FIG. 10.
The required properties ACL 1000 contains a basic trust level field
1001, which specifies the minimum rights management functions that
must be provided by any application wishing to process the content.
These minimum functions can established by a trade association,
such as the MPAA (Motion Picture Association of America), or by the
DRMOS vendor. A unique identifier is used to reference a list of
the minimum functions. The minimum functions list can include CPU,
DRMOS, and application specific requirements.
[0141] The required properties ACL 1000 can also contain one or
more extended trust level fields 1003. The extended trust level
fields 1003 contains identifiers that specify additional rights
management function that must be provided by the subscriber
computer. For example, a required properties ACL can require that
only a certain version of a particular application be allowed
access to the content. The required properties ACL 1000 is compared
against the certificates for the CPU, the DRMOS, and the
application starting at the hardware level, i.e., CPU, DRMOS,
application name, version, and specific properties for the
application. One of skill in the art will readily recognize that
the required properties ACL 1000 can require that all properties
must be present, or at least one of the properties, or some
specified subset.
[0142] The content license (FIG. 11) imposes additional
restrictions on what kind of processing can be performed on the
content once an application has access to the content. As described
briefly above, the license data structure 1100 can limit the number
of times the content can be accessed (usage counter 1101),
determine what use can be made of the content (derivation rights
1103), such as extracting still shots from a video, or building an
endless loop recording from an audio file, or an time-based
expiration counter 1105.
[0143] The license can also specify whether or not a trusted
application is permitted to validate other client computers and
share the content with them (sublicense rights 1107), in effect
having the subscriber computer act as a secondary content provider.
The sublicense rights 1107 can impose more restrictive rights on
re-distributed content than those specified in a license for
content downloaded directly from the original content provider. For
example, the license 1100 on a song purchased directly from the
music publisher can permit a song to be freely re-played while the
sublicense rights 1107 require a limit on the number of times the
same song can be re-played when re-distributed. To enforce the
sublicense rights 1107, in one embodiment, the trusted application
modifies the original license 1100 to specify the additional
restrictions and downloads the modified license with the
re-distributed content. In an alternate embodiment, the original
content provider downloads a sublicense along with the content and
that sublicense is re-distributed by the trusted application when
it re-distributes the content. The sublicense is structurally
identical to the license data structure 1100 although the content
of the fields differs.
[0144] Additional licensing restrictions will be readily apparent
to one skilled in the art and are contemplated as within the scope
of the invention.
[0145] The license 1100 is stored with the content on secured
storage. In one embodiment, the required properties ACL 1000 is
also stored with the license 1100 and the content. In an alternate
embodiment, the ACL 1000 is secured separately and controls access
to the storage key for the content as described above.
[0146] In the embodiments described above, the DRMOS is responsible
for checking the required properties ACL and for enforcing the
licensing restrictions. By providing the validation functions in
the DRMOS, the functions are centralized and can be utilized by any
process. In an alternate embodiment, the validation functions
concerning the application are coded directly into the trusted
applications programs. A similar effect is achieved in yet another
alternate embodiment that places the application validation
functions in a library that is incorporated into the trusted
applications.
[0147] One of skill in the art will immediately perceive that
certain rights are more easily enforced at the DRMOS level, such as
the right for a certain application to access a key or other
content, or the ability to open a file a limited number of times,
while other types of rights are best enforced by the application
itself. Since the DRMOS enforces the restriction that only
explicitly stated applications can access restricted content, the
application can be trusted to enforce the additional restrictions.
Alternate embodiments in which the enforcement of certain rights is
allocated to the DRMOS and the enforcement of others to the
application is therefore contemplated as within the scope of the
invention.
[0148] As described above in conjunction with FIG. 2, the content
provider 220 delivers content to the subscriber computer 200 after
trust is established by transmitting the appropriate
certificates/identities for the CPU, the DRMOS, and the application
to the provider. The content can be explicitly encrypted by the
content provider for this combination of CPU, DRMOS, and
application, as described above, or, if the content is sent over a
secured link (with, for example, Secure Socket Layer services), the
content provider can encrypt the content using the session key for
the secure link. In the latter embodiment, the DRMOS writes the
encrypted content to permanent storage and uses one of the storage
keys generated by the CPU to securely store the session key for
later use. Alternately, the content provider can choose not to
encrypt the content if it is transmitted to the application in a
secure fashion, in which case the application performs the
encryption if it stores a persistent copy of the content.
[0149] The particular methods performed by a subscriber computer of
an exemplary embodiment of the invention have been described. The
methods performed by the subscriber computer have been shown by
reference to flowcharts, operational diagrams, and data structures.
Methods performed by the content provider have also been
described.
Secure Storage
[0150] As an alternative to the GenerateKey operation, two new
operations referred to as "Seal" and "Unseal" may be introduced,
which provide the ability to seal secrets only for subsequent use
on the same machine.
[0151] The "Seal" instruction takes as inputs an arbitrary block of
data, the current OS identity (the SIR), and a target OS identity
(a specified SIR value that must be current at the point of future
decryption). The processor encrypts this data structure using a
symmetric key, K.sub.S.
[0152] The data block can now only be decrypted via an "Unseal"
operation on the same processor, using the same symmetric key. This
symmetric key is only used by the "Seal" and "Unseal" operations,
and will only decrypt the secret if the target OS identity is equal
to the current value of the SIR. If this check succeeds, the
processor decrypts and returns the secret, otherwise it returns an
error.
[0153] In this way, a processor can store encrypted information
that can be decrypted only by the same processor running a
specified operating system.
[0154] As a special case, the operating system can choose to seal
information for a different operating system whose identity it
knows and trusts. An example of this occurs when the operating
system is about to be upgraded and has a signed certificate from
the operating system vendor confirming the identity of the new
operating system. In this case the operating system will seal its
secrets for the new operating system that is about to run.
[0155] Alternatively, another approach is to employ encryption with
the processor's public key and decryption using the "Reveal"
operation, as described earlier. Instead of using K.sub.S for
encryption and decryption, the processor's public key pair is used.
This allows the "Seal" operation to be performed in software, even
on another processor.
Exemplary Chipset Implementation
[0156] The fundamental requirements of atomicity and privileged
access to keys for the microcode that implements authenticated boot
can be met in a variety of alternative implementations. In one
implementation, components in the chipset may examine the bus to
infer operation and permit or deny access to keys depending on the
code executing. Components on the chipset can also examine the bus
for unauthorized agents writing to protected code, or reading
unauthorized secrets.
[0157] An agent on the bus can also check for unauthorized
interrupts during the execution of the authenticated operations or
execution of the boot block.
[0158] Similarly, there is no fundamental requirement for the
microcode that implements the authenticated boot operations to by
physically resident on the microprocessor chip. It could also be
stored in ROM, EPROM, or protected flash memory in a physically
separate device on the bus.
BIOS Implementation
[0159] The authenticated boot technique can be implemented by
existing CPU operating modes using code in the computer's BIOS
code. The System Management Mode (SMM), supported by Intel
microprocessors, provides for a region of memory that is
inaccessible to normal operating system operation, but can provide
subroutines that operating systems or applications can use. Such
SMM protected memory could be used for the storage of keys and the
code that manages those keys.
CONCLUSION
[0160] A digital rights management system has been described
whereby certain cryptographic secrets are reliably associated with
a particular digital rights management operating system or family
of operating systems running on a particular general-purpose
computer. The operating system uses these secrets to authenticate
itself to a third party, to receive encrypted content or
information, to store this content or information securely, and to
retrieve it later. No unrelated operating system or other software
running on the same computer can obtain these secrets and perform
these actions, nor can any operating system or other software
running on any other computer. By using these cryptographic
secrets, the digital rights management operating system can
recursively provide derived cryptographic secrets for the same uses
by applications running on the same operating system on the same
computer.
[0161] Although specific embodiments have been illustrated and
described herein, it will be appreciated by those of ordinary skill
in the art that any arrangement which is calculated to achieve the
same purpose may be substituted for the specific embodiments shown.
This application is intended to cover any adaptations or variations
of the present invention.
[0162] For example, those of ordinary skill in the art will
appreciate that various combination of the exemplary embodiments
are applicable to solve the digital rights management problem
depending on the exact computing environment. Furthermore, those of
ordinary skill in the art will recognize that the invention can be
practiced on a large scale although illustrated herein with only a
single subscriber and content provider.
[0163] The terminology used in this application with respect to is
meant to include all hardware and software configuration and all
networked environments. Therefore, it is manifestly intended that
this invention be limited only by the following claims and
equivalents thereof.
* * * * *