Client-Side Boot Domains and Boot Rules
TECHNICAL FIELD
This invention relates generally to computer operating systems, and more particularly to verifying components loaded by an operating system.
BACKGROUND OF THE INVENTION
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.
To prevent their content from being stolen or misused, content providers will download content only to trusted software, and therefore only to subscriber computers that can prove that the software executing on the subscriber computer is trusted. This trust concept is predicated on having a trusted operating system executing on the computer, which, by its nature, only loads trusted components and provides some kind of secure storage. The problem then becomes one of identifying an operating system with such peculiarity that the content provider can make an intelligent decision whether to trust its content to the operating system.
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" discloses one
embodiment of a unique operating system identifier that is a cryptographic digest of
all the software components loaded by the operating system. 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. While the
content provider can maintain a list of those identities it trusts, or delegate the
maintenance of such a list to a third-party, what is needed in the art is a way to
reduce the number of trusted operating system identities without limiting the
choices of software components available to a user.
SUMMARY OF THE INVENTION
The above-mentioned shortcomings, disadvantages and problems are
addressed by the present invention, which will be understood by reading and
studying the following specification.
Each software component loaded for a verified operating system on a client
computer must satisfy a set of boot rules for a boot certificate. The appropriate boot
certificate is selected by the user or by default upon each boot of the computer. A
verified operating system identifier is created from the boot certificate. The boot
certificate is published and signed by a boot authority that attests to the validity of
the operating system booted under the boot certificate. Each software component
for the operating system is associated with a component certificate published and
signed by the same boot authority that signed the boot certificate. The boot rules
determine the validity of the software component based on the contents of the
component and boot certificates.
The client computer transmits the verified operating system identity and the
boot certificate to a server computer, such as a content provider, and the content
provider determines whether to trust the verified operating system with its content.
Downloaded data is secured on permanent storage through a key derived from the
verified operating system identifier. The boot certificate, component certificates,
and secured content define the boot domain.
A combination of two or more boot components can be used to boot a
verified operating system. Updating of the boot and component certificates, the
underlying components, and the changing of the verified operating system identity
and its affect on the boot domain are also described.
Because a content provider must only decide which boot authorities, and
which boot certificates from those authorities, to trust, the content provider must
keep track of only a small number of identities. The client computer is restricted
only in that components loaded into a verified operating system must be attested to
by one of the boot authorities trusted by the content provider. The client computer
can operate under an unverified operating system but data stored under a verified
boot domain will not be accessible.
The present invention describes systems, clients, servers, methods, and
computer-readable media of varying scope. In addition to the aspects and
advantages of the present invention described in this summary, further aspects and
advantages of the invention will become apparent by reference to the drawings and
by reading the detailed description that follows.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1A is a diagram of the hardware and operating environment in
conjunction with which embodiments of the invention may be practiced;
FIG. IB is a diagram of a client computer for use with exemplary
embodiments of the invention;
FIG. 2 is a diagram illustrating a system-level overview of an exemplary
embodiment of the invention;
FIG. 3 is a time line diagram illustrating interactions between boot and
component certificates used the exemplary embodiment of the invention shown in
FIG. 2;
FIG. 4 is a flowchart of a method to be performed by a client when booting
according to an exemplary embodiment of the invention;
FIG. 5 is a flowchart of a method to be performed by a client in obtaining a
new component certificate according to an exemplary embodiment of the invention;
FIG. 6 is a flowchart of a method to be performed by a client in obtaining an
updated component certificate to an exemplary embodiment of the invention;
FIGs. 7A and 7B are flowcharts of a method to be performed by a client in
obtaining downloaded content according to an exemplary embodiment of the
invention;
FIG. 8A is a diagram of a boot certificate data structure for use in an
exemplary implementation of the invention;
FIG. 8B is a diagram of an entry in a revocation list data structure for use in
an exemplary implementation of the invention;
FIG. 9 is a diagram of a component certificate data structure for use in an
exemplary implementation of the invention; and
FIG. 10 is a diagram of an time certificate data structure for use in an
exemplary implementation of the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 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.
The detailed description is divided into five sections. In the first section, the hardware and the operating environment in conjunction with which embodiments of the invention may be practiced are described. In the second section, a system level overview of the invention is presented. In the third section, methods for an exemplary embodiment of the invention are provided. In the fourth section, a
particular implementation of the invention is described for use with the Microsoft Corporation "WINDOWS" family of operating system. Finally, in the fifth section, a conclusion of the detailed description is provided.
Hardware and Operating Environment
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.
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.
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 include the
system memory 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.
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.
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 which 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.
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.
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. 1A 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.
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.
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 other
computers.
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. IB 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.
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.
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 "KcPU" and the corresponding private key is referred to as "KCPU "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.
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 "Kcpu"1". In
this way, only the CPU knows the CPU private key K u"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.
The manufacturer has a pair of public and private signing keys, KMFR and
K-MFR A ■ The private key KMFR " is known only to the manufacturer, while the public
key KMFR is made available to the public. The manufacturer certificate 166 contains
the manufacturer's public key KMFR, the CPU's public key KCpu, and the above
testimony. The manufacturer signs the certificate using its private signing key,
KMFR \ as follows:
Mfr. Certificate = (KMFR, Certifies-for-Boot, KCpu), signed by K PR "1
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 166
is publicly accessible, yet it cannot be forged without knowledge of the
manufacturer's private key KMFR " .
A chain of certificates can be extended from the manufacturer's
certificate using intermediate certificates and additional key pairs. When
verification is performed using an intermediate certificate in the chain instead of the
manufacturer's certificate, the frequency of use of the important "root" key pairs,
Kcpu /KCPU "1 and K PR/K PR "1, is greatly reduced and hence the root keys are less
likely to be stolen.
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).
Svstem Level Overview
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.1A, 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 220 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.
In the exemplary embodiment illustrated in FIG. 2, component certificates
and boot certificates are used in combination to prove the identity of an operating
system executing on the subscriber computer 200 to the content provider 220. Each
component certificate is published and signed by a boot authority, using a private
key, to indicate that the boot authority attests to the validity and security of the
corresponding component. Each boot authority also publishes a boot certificate that
contains the public key necessary to verify the signature on the component
certificate. When the client computer 200 is booted, a particular combination of
boot authorities is chosen to verify the loaded components. Only two component
certificates and two boot certificates are shown in FIG. 2 for the sake of clarity in
the following discussion. One of skill in the art will readily recognize that, in
practice, more than two component certificates will be typically used, and that a
different number of boot certificates is equally applicable
The boot block (the basic components) for the operating system 211 is
associated with component certificate 205, which is signed by the operating system
vendor and verified using a boot certificate 201 published by the vendor. An
additional software component, such as a hardware device driver, required to
operate the computer 200 is associated with component certificate 207. The
software component's component certificate 207 is signed by an independent boot
authority and verified using the boot certificate 203 published by the independent
boot authority.
When the client computer 200 is booted, a boot loader 209 for the operating
system 211 verifies the component certificate for each component to be loaded
against the boot certificates 201, 203. The identity for the resulting operating
system 211 is a cryptographic digest of the combination of the boot block and the
boot certificates 201, 203, and is recorded in an internal software identity register
(SIR) 213, as explained previously for SIR 168 in FIG. IB. If a component that
cannot be verified is loaded, the identity is set to a default value indicating an
unverified operating system.
When the subscriber computer 200 requests download of content 221 from
provider 220, the content provider 220 transmits a challenge message ("nonce") to
the subscriber computer 200 requesting the boot certificates used to verify the
components of the current operating system 211. The subscriber computer 200
transmits an authentication message containing the boot certificates 201, 203, the
current value of the SIR 213, and the nonce. The subscriber computer attests to the
identity in the SIR 213 by signing the value with the private key of its CPU.
The content provider 220 examines the certificates 201, 203 and the identity
to determine whether it should establish a trust relationship with the subscriber
computer 200. If so, the content provider 220 downloads the content to the
subscriber computer 200. If the content is to be permanently stored on the
subscriber computer 200, the operating system 211 writes the content to a storage
device 215 and keys the access of the stored content, or "secret," 217 to the identity
to prevent unauthorized access to the content. Device 215 is illustrated in FIG. 2 as
a disk drive but such illustration is not intended to limit the range of devices that
can serve as secured storage.
In an exemplary embodiment, each component certificate is valid for a finite
period of time, and each boot certificate has an issue date. The interaction between
the certificates is based on relative dates as illustrated using a time line in FIG. 3.
Boot certificate 301 has an issue date of time mark 1 and boot certificate 302 has an
issue date of time mark 4. Component certificate 303 expires at time mark 5,
component certificate 304 at time mark 3, component certificate 305 at time mark 2,
and component certificate 306 at time mark 6. When the subscriber computer 200
is booted any time between time marks 1 and 4, it checks the expiration dates of
each component certificate for a component to be loaded against the issue date of
the boot certificate. As long as the component certificate is valid as of the issue
date, the component will be loaded. Thus, the components associated with
component certificates 303, 304, 305, 306 are always loaded when using boot
certificate 301 and the computer will always be able to be booted into a verified
operating system.
When booting with boot certificate 302, however, the components associated
with component certificates 304, 305 cannot be verified since their valid periods
have expired before the issue date of boot certificate 302. If these components must
be loaded, the identity of the operating system is set to the value for an unverified
operating system. New component certificates are required to load the components
in a verified operating system under the boot certificate 302. The process of
obtaining a new component certificate is handled automatically by the operating
system and is described in the next section.
Because a particular version of a component can be discovered to
have a vulnerability before the expiration date of its associated component
certificate, an alternate embodiment of the invention associates a revocation list
with each boot certificate. If a component appears in the revocation list, the boot
certificate will not verify the component until an updated version of the component
is obtained. As before, if an unverified component is loaded, the identity is set to
the unverified value. In one embodiment, the revocation list also contains an on¬
line address (URL) for the updated version of the component so the operating
system can automatically obtain the new version as described in the next section.
One exemplary embodiment of a revocation list in which each vulnerable
component is identified by its name and version or by its cryptographic digest is
described in further detail below.
If the content provider refuses to download content because one or
more of the boot certificates are considered out-dated, the current operating system
can obtain later-issued boot certificates to be used on subsequent boots under the
newer boot certificates. Because existing content is secured through the identity of
the operating system, booting with the new boot certificates will cause that content
to be inaccessible. Therefore, the current operating system re-stores the content
using the new identity before the system is rebooted. The process of obtaining
later-issued boot certificates and re-securing content is also described in the next
section.
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 boot domain is created on a client, subscriber computer, which is
defined by boot certificates, component certificates, and stored secrets. Software
components to be loaded in the subscriber computer are verified using the
certificates in accordance with specified boot rules. An operating system identity is
created that reflects the boot certificates used to verify the components. A content
provider determines whether to trust the operating system based on the operating
system identity and the boot certificates. While the invention is not limited to any
particular combination of boot and component certificates, for sake of clarity a
simplified operating system using two boot certificates and two component
certificates has been described.
Methods of an Exemplary Embodiment of the Invention
In the previous section, a system level overview of the operation of an
exemplary embodiment of the invention was described. In this section, the
particular methods performed by the client, or subscriber computer, of such an
exemplary embodiment are described by reference to a series of flowcharts. The
methods to be performed by the clients constitute computer programs made up of
computer-executable instructions. Describing the methods by reference to a
flowchart enables one skilled in the art to develop such programs including such
instructions to carry out the methods on suitable computerized clients (the processor
of the clients executing the instructions from computer-readable media).
Referring first to FIG. 4, 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 as part of the boot
process. When the boot process begins, a boot manager presents the user with a
choice of boot certificates that can be used to verify the operating system
components (block 401). The user chooses the desired boot certificate or
certificates, and the boot loader creates the verified operating system identity based
on the boot block and the chosen boot certificates (block 403). Each component that
is to be loaded is verified by comparing its component certificate against one of the
boot certificates (block 405). The verification process checks the signature on the
component certificate (block 407), the expiration date of the component certificate
(block 409), and the revocation list (block 411) to determine if the component can
be loaded. If the component certificate passes all three tests, i.e., satisfies the boot
rules, then the corresponding component is loaded (block 413).
If the component certificate fails to satisfy one of the boot rules, the boot
loader presents the user with the choice of loading the corresponding component
and booting into an unverified operating system, or not loading the component
(block 415). If the component is required, the boot loader sets the identity of the
operating system to a value indicating an unverified operating system (block 417)
and loads the component (block 413). If the user chooses not to load the
component, the verified identity is preserved.
The boot process proceeds according to the boot rules until all components
have been loaded (block 419).
When the operating system is completely booted, if one or more of the
components could not be verified the user is given the opportunity to obtain either
valid component certificates or updated components as illustrated in FIGs. 5 and 6,
respectively.
If the component certificate has expired and the user chooses get a new
certificate (block 501 in FIG. 5), the operating system requests the new component
certificate from the boot authority that signed the expired certificate (block 503).
Upon receipt, the new component certificate is stored in place of the expired
certificate (block 505) for use upon subsequent boots.
If the version of the component is listed in the revocation list and the user
chooses to obtain an updated version (block 601 in FIG. 6), the operating system
requests the updated version from a download site associated with the component
(block 603). In one embodiment, the URL for the site is stored in the revocation
list. A new component certificate for the updated version is also downloaded. The
updated component and new component certificate are stored for verification and
loading upon the next boot (block 605).
FIGs. 7A and 7B illustrate the processes performed by the subscriber
computer in requesting content from a provider. The subscriber computer sends a
request message to the provider (block 701) and receives a challenge nonce in
response (block 703). The subscriber computer sends an authentication message
containing the boot certificate(s), the operating system identity, and the nonce back
to the provider (block 705).
If the provider trusts the operating system based on the boot certificate(s)
and operating system identity (block 707), it downloads the content to the
subscriber computer (block 709), which secures the content on permanent storage
based on the operating system identity (block 711).
If the provider determines that the boot certificate(s) used to verify the
components are out-dated, the subscriber computer presents the user with the
opportunity to obtain newer boot certificate(s) (block 713). The subscriber
computer requests the newer boot certificate(s) from the corresponding boot
authorities (block 715) and stores the new boot certificates without overwriting the
older versions (block 717).
The current operating system determines if the operating system defined by
the new boot certificate(s) will boot by verifying each component certificate signed
by the boot authorities that published the new boot certificate(s) (block 719). The
new operating system can fail because one or more of the component certificates
have expired when compared to the new boot certificate(s) (block 735), or because
one or more of the components are versions listed in the revocation list associated
with the new boot certificate(s) (block 737). New component certificates are
obtained using the process described above in conjunction with FIG. 5; updated
components are obtained using the process described above in conjunction with
FIG. 6.
When the current operating system determines the new operating system will
boot, it creates a new operating system identity based on the new boot component(s)
(block 721). Because each operating system identity controls the secrets in the
associated boot domain, a new boot domain must be created for the new operating
system identity. Each secret is evaluated to determine if it will be part of the new
boot domain (block 723). Three rules govern the choice of secrets for the new boot
domain: the new boot certificate(s) must be from the boot authorities as the current
boot certificate(s) (block 725), the new boot certificate(s) must be issued after the
current boot certificate(s) (block 727), and the content must not have not expired
before the issue date of the new boot certificate(s) (block 729). A qualifying secret
is unsealed using the current operating system identity, a copy made of the content,
and the copy is re-sealed using the new operating system identity (block 731). The
process continues until all appropriate secrets have been sealed in the new boot
domain (block 733). The next time the subscriber computer is booted under the
particular boot certificate combination, the new boot certificate(s) will take effect
and the secrets in the new boot domain will be accessible. One of skill in the art
will readily appreciate that this procedure can be automated as part of the normal
operating system update process.
In one embodiment, the secrets in the old boot domain are deleted upon the
first successful boot under the new boot certificate(s). In an alternate embodiment,
only expired content is deleted.
The particular methods performed by a client of an exemplary embodiment
of the invention have been described. The method performed by the client
computer has been shown by reference to flowcharts including all the acts from 400
until 417, from 501 until 505, from 601 until 605, and from 701 until 737.
1. Microsoft "WINDOWS" Implementation
In this section of the detailed description, a particular implementation of the
invention is described for the Microsoft Corporation "WINDOWS" family of
operating systems. Microsoft acts as a boot authority by publishing boot certificates
and component certificates for the boot blocks of the various versions of the
"WINDOWS" operating system. The "WINDOWS" operating system boot
manager and boot loader are extended to operate according to the methods
described in the previous section. Exemplary embodiments of a boot certificate and
a component certificate for use in the "WINDOWS" operating system
implementation are illustrated in FIGs. 8 A and 9, respectively. An exemplary
embodiment of a revocation list entry is illustrated in FIG. 8B. An exemplary
embodiment of an additional certificate, a time certificate, is illustrated in FIG. 10.
Turning first to FIG. 8A, the exemplary embodiment of the boot certificate
800 comprises five constant fields and three optional fields. The constant fields
contain an identifier 801 for the boot authority, the issue date 802 of the certificate,
the digest 803 of the certificate, the public key 804 of the boot authority, and the
digital signature 808 of the boot authority that issued the certificate 800, i.e., the
boot authority identified by field 801. The public key 804 is used to verify the
digital signature 808 on the boot certificate 800 as well as being used to verify the
component certificates signed by the same boot authority.
In one embodiment, the boot certificate 800 includes an expiration date 805
for the boot certificate. The "WINDOWS" operating system uses the expiration
date 805 to determine when a new boot certificate is required, without having to be
so informed by a content provider.
In an alternate embodiment, the boot certificate 800 includes a revocation list
806. The function of the revocation list has been described in the previous sections.
An exemplary embodiment of an entry in the revocation list 806 is illustrated in
FIG. 8B. Each entry 810 comprises the name and version 811 of the component
that is revoked, a cryptographic digest 812 of the revoked component, and a URL
813 for a download site containing an updated version of the component.
In still another embodiment, the boot certificate 800 includes boot rules 807
in the form of a command script that is executed by the boot process before loading
each component signed by the boot authority identified by field 801. In yet another
alternate embodiment, the boot rules 807 are a series of operating system
commands.
The exemplary embodiment of the component certificate shown in FIG. 9 is
implemented by all boot authorities that attest to components for loading in a
"WINDOWS" operating system. The component certificate 900 contains the
identity 901 of the boot authority, the name and version 902 of the corresponding
component, the digest 903 of the component, the issue date 904 of the component
certificate, the expiration date 904 of the certificate, and the digital signature 906 of
the boot authority. The public key in the boot certificate 800 for the boot authority
is used to verify the digital signature 906 of the component certificate.
When the boot certificate 800 contains a revocation list 806, the
"WINDOWS" operating system compares name and version 902 and digest 903 in
the component certificate against each entry 810 in the revocation list 806 to
determine if the component has been revoked. If the component has been revoked,
a new version is obtained from the URL address in the entry 810 when the user so
chooses.
The "WINDOWS" operating system implementation also uses a time
certificate that enables the setting of time limits on content that is of shorter
duration than is possible using only the boot certificates. The exemplary
embodiment of a time certificate 1000 shown in FIG. 10 contains the same constant
fields as the boot certificate 800, except that the time certificate contains an issue
time as well as an issue date 1002. The time certificate 1000 is folded into the
operating system identity along with the boot certificate(s), and is sent to the
content provider as part of the authentication message. Use of the time certificate
1000 is equivalent to an attestation by the "WINDOWS" operating system that the
computer was booted at a date and time after the issue date and time 1002.
Before downloading quickly expiring content, the content provider
determines if the current time certificate, if any, is too old. If so, or if there is no
current time certificate, the subscriber computer must acquire a new time certificate
and reboot under an operating system identity that includes the new time certificate.
Because the time certificate is part of the operating system identity, secrets sealed
under the old identity must be evaluated for copying into the new boot domain as
described in the previous section for boot certificates. In this case, the expiration of
existing content is determined based on the issue date and time of the time
certificate. Additionally, any expired content is deleted when the new operating
system identity becomes effective.
Conclusion
Client boot domains and boot rules have been described. 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.
For example, those of ordinary skill within the art will appreciate that any organization can act as a boot authority. It is solely the decision of each individual content provider as to which boot authorities it trusts. Furthermore, those of ordinary skill within the art will appreciate that a component certificate will typically have a shorter validity period than a boot certificate since changing versions of a boot certificate requires unsealing and resealing all secrets in the boot domain. In addition, although the invention has been described in terms of loading an operating system as a result of powering on a computer, one of skill in the art will recognize that the invention can be used to load an operating system under the control of an already-executing operating system, or to load an operating system
into a protected address space on a computer running an unverified operating system.
The terminology used in this application with respect is meant to include all operating system environments in which different software components must be loaded in order for the computer to function.