U.S. patent application number 11/615195 was filed with the patent office on 2007-05-10 for 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.
This patent application is currently assigned to Microsoft Corporation. Invention is credited to John D. Detreville, Paul England, Butler W. Lampson.
Application Number | 20070104329 11/615195 |
Document ID | / |
Family ID | 28791747 |
Filed Date | 2007-05-10 |
United States Patent
Application |
20070104329 |
Kind Code |
A1 |
England; Paul ; et
al. |
May 10, 2007 |
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
Abstract
In accordance with certain aspects, an operating system is
booted for execution on a central processing unit (CPU). An atomic
operation is executed, and if the atomic operation completes
correctly then a software identity register of the CPU is set to an
identity of the operating system.
Inventors: |
England; Paul; (Bellevue,
WA) ; Detreville; John D.; (Seattle, WA) ;
Lampson; Butler W.; (Cambridge, MA) |
Correspondence
Address: |
LEE & HAYES PLLC
421 W RIVERSIDE AVENUE SUITE 500
SPOKANE
WA
99201
US
|
Assignee: |
Microsoft Corporation
Redmond
WA
98052
|
Family ID: |
28791747 |
Appl. No.: |
11/615195 |
Filed: |
December 22, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09266207 |
Mar 10, 1999 |
7174457 |
|
|
11615195 |
Dec 22, 2006 |
|
|
|
60105891 |
Oct 26, 1998 |
|
|
|
Current U.S.
Class: |
380/255 |
Current CPC
Class: |
G06F 21/445 20130101;
G06F 2221/0704 20130101; G06F 9/468 20130101; G06F 21/575 20130101;
G06F 2221/2129 20130101; G06F 2221/2103 20130101; G06F 2221/2113
20130101; G06F 21/57 20130101; G06F 9/4406 20130101; G06F 21/10
20130101 |
Class at
Publication: |
380/255 |
International
Class: |
H04K 1/00 20060101
H04K001/00 |
Claims
1. In a computer system having a central processing unit (CPU) and
an operating system (OS), the CPU having a software identity
register, a method for booting the operating system comprising:
computing a cryptographic function of at least a portion of the
operating system; and setting the software identity register to a
result of the computed cryptographic function if atomic execution
of a boot block of the operating system does not fail, and
otherwise setting the software identity register to a value
indicating that the atomic execution of the boot block failed.
2. The method as recited in claim 1, further comprising defining a
secure storage space, access to which is based in part on the
result set in the software identity register.
3. A computer comprising: a memory; a central processing unit (CPU)
coupled to the memory, the CPU having a software identity register;
an operating system stored in the memory, the operating system
having a block of code; and the operating system being booted for
execution on the CPU according to a sequence that begins with an
atomic operation, wherein in an event that the atomic operation
completes correctly, the software identity register is set to the
identity of the operating system.
4. The computer as recited in claim 3, wherein the identity
comprises a digital signature on a block of code from the operating
system.
5. The computer as recited in claim 3, wherein the identity
comprises a hash digest of a block of code from the operating
system.
6. The computer as recited in claim 3, wherein the CPU holds a
manufacturer certificate signed by a manufacturer of the CPU.
7. The computer as recited in claim 3, further comprising a boot
log, wherein the CPU appends the identity of the operating system
to the boot log in the event that the atomic operation completes
correctly.
8. The computer as recited in claim 3, wherein the CPU is assigned
a pair of public and private keys, and CPU is configured to create
an OS certificate containing the identity in the software identity
register, information describing the operating system, and the CPU
public key, the CPU signing the OS certificate using the CPU
private key.
9. The computer as recited in claim 3, wherein the CPU is
configured to form a generator seed from a CPU-specific secret and
OS-specific data and to generate a private storage key based on a
function of the generator seed.
10. A central processing unit comprising: software identity
register; a boot log; and processing means to process an atomic
operation such that in an event that the atomic operation completes
correctly, the software identity register is set to an identity of
software code and the identity is appended to the boot log.
11. For execution on a computer system having a central processing
unit (CPU) and an operating system (OS), the CPU having a software
identity register, a computer program stored on one or more
computer-readable storage media of the computer system, the program
comprising: executing an atomic operation to set an identity of the
operating system into the software identity register of the CPU,
wherein in an event that the atomic operation completes correctly,
the software identity register contains the identity of the
operating system and in an event that the atomic operation fails to
complete correctly, the software identity register contains a value
other than the identity of the operating system; and examining a
content of the software identity register to verify the identity of
the operating system.
Description
RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 09/266,207, filed Mar. 10, 1999, which is
hereby incorporated by reference herein. U.S. patent application
Ser. No. 09/266,207 is a non-provisional application claiming
priority to 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 applications titled "Loading And
Identifying A Digital Rights Management Operating System," U.S.
patent application Ser. No. 09/227,611, now U.S. Pat. No.
6,327,652, "Key-based Secure Storage," U.S. patent application Ser.
No. 09/227,568, "Digital Rights Management," U.S. patent
application Ser. No. 09/227,559, now U.S. Pat. No. 6,820,063, and
"Digital Rights Management Operating System," U.S. patent
application Ser. No. 09/227,561, now U.S. Pat. No. 6,330,670, all
filed on Jan. 8, 1999 and assigned to the same assignee as the
present application.
FIELD OF THE INVENTION
[0002] This invention relates to computer-implemented
authentication systems and methods for authenticating an operating
system (OS) to a processor during its boot sequence in order to
establish a chain of trust rooted in the combination of the OS and
the processor on which it is running. The invention can be used in
conjunction with digital rights management systems to establish
trust with a content provider. This invention further relates to
techniques for securely maintaining the digital content in
persistent local memory (such as on disk) while preventing rogue
operating systems and applications from illicitly accessing the
content.
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
[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 and the Internet.
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
address technologies and protocol 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] 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.
[0011] 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).
[0012] 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.
[0013] 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).
[0014] 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.
[0015] Another secure boot model, known as AEGIS.RTM., 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.RTM. 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.
[0016] 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.RTM.'s Cryptolope.RTM.. 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.RTM." audio rights management system based
on the PolicyMaker rights management system.
SUMMARY
[0017] This invention concerns a system and method for distributing
digital data to a client and handling the digital data at the
client in accordance with the rights granted by the publisher.
Generally, the system involves a general-purpose microprocessor
that enables a new mechanism that facilitates an authenticated boot
sequence in which the operating system can prove its identity to
the microprocessor. The boot sequence provides the building blocks
for client-side rights management when the system is online, and
provides for continued protection of persistent data even when the
user goes offline.
[0018] In one implementation, the client or subscriber-side
computer system has a central processing unit (CPU) and an
operating system (OS). The CPU is manufactured with a public-key
pair, a manufacturer certificate testifying that the manufacturer
built the CPU according to a known specification, and a software
identity register. The operating system includes a block of code,
referred to as the "boot block". The boot block uniquely describes
the operating system in that it will boot that operating system and
no other. An OS identity can be established from the boot block by
examining a digital signature stored with the boot block or by
computing a hash digest of the boot block.
[0019] During booting, the CPU executes the boot block as an atomic
operation to store the identity of the operating system into the
software identity register. Execution of the boot block is such
that the software identity register, which can be read but not
modified, is set to either the OS identity (i.e., boot block digest
or OS public key) if the operation is successful, or zero if some
event or circumstance subverts operation.
[0020] Rooted in this self-authentication, the OS can then continue
to load and validate other blocks of code (including device drivers
to be executed). An identity of each block of code that is
successfully validated is appended to a boot log.
[0021] Following this authenticated boot sequence, the subscriber
unit is prepared to establish a chain of trust to prove its
hardware and software to a content provider. The CPU in the
subscriber unit begins by submitting a request for content
maintained at the content provider.
[0022] In response to the request, the content provider generates a
challenge nonce and returns the challenge nonce to the subscriber
unit. The subscriber unit forms an OS certificate that contains the
OS identity (held in its software identity register), the boot log,
the challenge nonce, and the CPU public key. The CPU signs this
newly minted OS certificate using the CPU private key. The
subscriber unit returns this OS certificate, along with the CPU
manufacturer certificate, to the content provider. The content
provider then has sufficient information to identify the OS and
other software components identified in the log, and the processor,
and to determine whether to trust this combination of software and
hardware. If trust is established, the content provider can choose
to download the content to the subscriber unit, along with a list
of terms under which the content may be used. This list may be in
the form of a license or an Access Control List (ACL), specifying
by which processor, by which OS, by which application(s), and under
which additional terms the content may be used.
[0023] The subscriber unit stores the content in encrypted form
using a storage key that is generated as a function of CPU-specific
and OS-specific data. More particularly, the CPU forms a generator
seed from a CPU-specific secret, a user-supplied seed, and
OS-specific data from the OS identity register. The generator seed,
the CPU-specific key, and the OS digest are input to a hash
function to generate a unique storage key. The storage key is then
used to encrypt the content. In this manner, only the same CPU and
OS that have proven themselves to the content provider are able
subsequently to access the data previously encrypted.
[0024] Alternatively, 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".
[0025] In both of these scenarios for providing secure storage, a
key pair K.sub.CPU and K.sub.CPU.sup.-1 unique to the CPU need be
capable only of signing, not for encryption. Alternatively, if the
key pair K.sub.CPU and K.sub.CPU.sup.-1 is capable of encryption
and decryption as well, ordinary user-level software can implement
the "Seal" operation, even on another processor, and
K.sub.CPU.sup.-1 can be used to implement the "Unseal"
operation.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] FIG. 1 is a diagrammatic illustration of a system having a
subscriber unit and a content provider.
[0027] FIG. 2 is a block diagram of the subscriber unit.
[0028] FIG. 3 illustrates a signed boot block.
[0029] FIG. 4 is a flow diagram showing a method for performing an
authenticated boot operation on the operating system.
[0030] FIG. 5 illustrates a boot log created during booting of an
operating system on the subscriber unit.
[0031] FIGS. 6a and 6b are a flow diagram showing a method for
proving a CPU and operating system resident at the subscriber unit
to the content provider.
[0032] FIG. 7 is a flow diagram showing a method for securely
storing digital content.
[0033] The same numbers are used throughout the drawings to
reference like components or features.
DETAILED DESCRIPTION
[0034] The following discussion assumes that the reader is familiar
with cryptography. For a basic introduction of cryptography, the
reader is directed to a text written by Bruce Schneier and entitled
"Applied Cryptography: Protocols, Algorithms, and Source Code in
C," published by John Wiley & Sons with copyright 1994 (or
second edition with copyright 1996), which is hereby incorporated
by reference.
[0035] This invention concerns a system and method for distributing
digital data to a client and handling the digital data at the
client in accordance with the rights granted by the publisher. In
the most basic scenario, a content provider agrees to deliver
digital content to a subscriber unit, provided that the subscriber
unit promises not to violate the associated terms for using the
digital data and that the content provider can trust the subscriber
unit's promise. For instance, the content provider delivers content
to the subscriber unit with the understanding that the subscriber
unit will not redistribute the content, reproduce the content, or
utilize the content in violation of a predefined use agreement
between the content provider and the subscriber unit.
[0036] 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.
[0037] To place this in a particular context for discussion
purposes, suppose the content provider 22 is a producer of feature
films. Assume that the content provider f22 offers its animated
movies at two different prices. a low price for the right to view
the movie one time as it is delivered online (i.e., akin to
pay-per-view) and a higher price for the right to view the movie as
often as the subscriber likes (i.e., akin to video purchase). In
each case, the agreement specifically prohibits reproduction of the
movie or redistribution of the movie to another subscriber unit. In
the first case, it also prohibits the client device from making a
persistent copy to disk. For the content provider 22 to download a
requested movie, it must trust that the subscriber unit will abide
by the agreement and not permit illicit use of the digital movie
data. This trust involves trust of the hardware components, trust
of the operating system, and trust of the applications, as well as
trust of the manufacturer of the hardware and software.
[0038] FIG. 1 shows a system 20 having a content provider 22 that
is capable of delivering digital content to a subscriber unit 24
over a network 26. The content provider 22 has a content database
30 that stores the content and a media server 32 that serves the
content to the subscriber unit 24. The media server 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 the network 26 to the
subscriber unit 24.
[0039] The subscriber unit 24 is coupled to receive the digital
content from the network 26. The subscriber unit 24 is illustrated
as a general-purpose computer that is linked to the network 26 via
a network connection, a digital cable interface, a modem, or other
interface. The subscriber computer has memory to buffer or to store
the digital content received from the content provider, a monitor
to display any visual content (video, pictures, images, text,
etc.), and a sound system (not shown) to play audio content. The
subscriber unit may be implemented, however, as other devices that
are capable of receiving and presenting digital content. For
instance, the subscriber unit 24 might be a television, or a
television/set-top box system, or a portable-computing device
(e.g., laptop, palmtop, portable information device, Web-enabled
phone, etc.).
[0040] The network 26 is representative of many diverse types of
networks, including wire-based networks, such as an enterprise
network (e.g., a local area network, wide area network) or a public
network (e.g., the Internet), and wireless networks (e.g.,
satellite network, RF network, microwave). The network 26 can also
be implemented as a telephone network, or an interactive television
network, or any other form for linking the subscriber unit 24 to
the content provider 22.
[0041] Exemplary Subscriber Unit
[0042] FIG. 2 shows general components in the subscriber unit 26.
They include a central processing unit (CPU) 40, nonvolatile memory
42 (e.g., ROM, disk drive, CD ROM, etc.), volatile memory 44 (e.g.,
RAM), and a network interface 46 (e.g., modem, network port,
wireless transceiver, etc.). The subscriber unit 26 may also
include a sound system 48 and/or a display 50. These components are
interconnected via conventional busing architectures, including
parallel and serial schemes (not shown).
[0043] The CPU 40 has a processor 60 and may have a cryptographic
accelerator 62. The CPU 40 is capable of performing cryptographic
functions, such as signing, encrypting, decrypting, and
authenticating, with or without the accelerator 62 assisting in
intensive mathematical computations commonly involved in
cryptographic functions.
[0044] The CPU manufacturer equips the CPU 40 with a pair of public
and private keys 64 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 the content provider, as
is discussed below in more detail. The CPU manufacturer may further
embed a second secret key K.sub.2 in the CPU 40 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 blow
under the heading "Secure Storage". 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.
[0045] The manufacturer also issues a signed certificate 66
testifying that it produced the CPU according to a known
specification. Generally, the certificate testifies that the
manufacturer created the key pair 64, placed the key pair onto the
CPU 40, and then destroyed its own knowledge of the private key
"K.sub.CPU.sup.-1". In this way, nobody but the CPU knows the CPU
private key K.sub.CPU.sup.-1; the same key is not issued to other
CPUs. The certificate can in principle be stored on a separate
physical device but still logically belongs to the processor with
the corresponding key.
[0046] 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 66 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: [0047] Mfr.
Certificate=(K.sub.MFR, Certifies-for-Boot, K.sub.CPU), signed by
K.sub.MFR.sup.-
[0048] 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.
[0049] Another implementation in which a `chain of certificates`
leading back to a root certificate held by the processor
manufacturer is also acceptable.
[0050] The CPU 40 has an internal software identity register (SIR)
68, 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 80, 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
68 is set to a predetermined false value (e.g., zero). This process
is described below in more detail under the heading "Authenticated
Boot".
[0051] The CPU 40 also utilizes a second internal register (LOGR)
69, 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.
[0052] The CPU 40 also maintains a "boot log" 70 to track software
modules and programs that are loaded. In one implementation, the
boot log 70 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 70 can be comfortably
included in the main CPU. Alternatively, the CPU 40 can store the
boot log 70 in volatile memory 44 in a cryptographic
tamper-resistant container.
[0053] 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.
[0054] 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.
[0055] The operating system (OS) 80 is stored in the memory 42 and
executed on the CPU 40. The operating system 80 has a block of code
82 used to authenticate the operating system on the CPU during the
boot operation. The boot block 82 uniquely determines the operating
system, or class of operating systems (e.g. those signed by the
same manufacturer). The boot block 82 can also be signed by the OS
manufacturer.
[0056] FIG. 3 shows an example of a signed boot block 90 created by
signing the block of code 82. It contains the
BeginAuthenticatedBoot opcode 92, a length 94 specifying the number
of byte in the block of code, the code 82, a signature 96, and a
public key 98 used to verify the signature 96. The boot block will
also contain as a constant or set of constants, keys, or other
information 99 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.
[0057] 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 96 and public key 98 are then not
needed.
[0058] 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.
[0059] Once booted the operating system 80 and the applications
named in the license or ACL by the content provider can set aside
space 84 in memory or disk 42 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.
[0060] Software programs 86 (the applications) are also shown
stored in memory 42. These programs may be used to render or
otherwise play the content. Each program 86 has an associated key
or digest 88 for unique identification.
[0061] Authenticated Boot
[0062] 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.
[0063] FIG. 4 shows a method for performing an authenticated boot
operation on the operating system 80. The method is performed by
the CPU 40 and OS 80 resident in the subscriber unit 24. At block
100, the CPU executes the BeginAuthenticatedBoot opcode 92 in the
signed boot block 90 to set an identity for the operating system
80. The identity can be a digest of the boot block's opcodes and
data, or the public key 98 corresponding to a signature on the boot
block of the operating system.
[0064] The BeginAuthenticatedBoot opcode 92 and the boot block 90
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 92 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.
[0065] Execution of the BeginAuthenticatedBoot opcode 92 sets the
internal software identity register 70 to either (1) the OS's
identity (i.e., boot block digest or OS public key 98) 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 block 102), the SIR 70 is
now a unique number or other value that represents the identity of
the operating system 80 (block 104). 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 block 102),
the SIR is set to zero (block 106).
[0066] It is noted that different operating systems may be serially
booted on the subscriber unit 24. Executing the BAB opcode 92 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.
[0067] At block 110, the CPU 40 fills the first entry on the boot
log 70 with the public key (or digest) of the boot block 82. From
now on, any running code can append data to the boot log 70, 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.
[0068] The boot block 82 is free to load the next set of blocks in
the boot-chain (block 112). At block 114, the boot block 82 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 70. 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.
[0069] 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. 5 shows an exemplary
structure of a boot log 70. It contains a seed field 130 and a
block ID field 132. The block ID field 132 holds identities of the
blocks of code that are loaded and verified on the subscriber unit.
The block ID field 132 can hold text or binary data.
[0070] The SIR or the seed field 130 holds an authenticated boot
key generator seed. The CPU uses the seed in field 130 to generate
keys unique to the OS and processor. Since the first entry of the
boot log 70 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 under the
heading "Secure Storage".
[0071] 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".
[0072] Chain of Trust to Content Provider
[0073] Once the CPU has derived an appropriate SIR for the
operating system, the combination of the CPU and the OS has a
unique identification that may be presented to third parties. The
subscriber unit is thus prepared to order content from the content
provider, to specify the CPU and the OS, and prove the identity of
the CPU and operating system to the content provider.
[0074] FIGS. 6a and 6b show a method for proving the CPU 40 and OS
80 to the content provider 22 in order for the content provider 22
to trust that these components will abide by the digital rights
agreement. The method is described with additional reference to
FIGS. 1, 2, 3, and 5. The method is performed by software
components resident at both the subscriber unit 24 and the content
provider 22 and are listed in FIGS. 6a and 6b under corresponding
headings to illustrate generally where the method is performed.
[0075] At block 150, the operating system 80 establishes an SSL
(secure socket layer) connection, or a similar secure connection,
with the content provider 22. This connection is conventional and
establishes a cryptographically secured communication path over an
otherwise insecure network 26. The path prevents others from
intercepting, modifying, replaying or deciphering messages being
exchanged between the subscriber unit 24 and the content provider
22.
[0076] At block 152, the subscriber unit 24 submits a request for
particular content provided by the content provider 22. The request
contains an identification of the content, the CPU, the OS, the
application, the desired rights to play the content (e.g., rent,
purchase, multi-site use, etc.), and payment instructions (or
authorization to pay) for the specified rights. Suppose that the
user wants to rent a particular movie. In the Internet context, the
user may invoke a browser to browse a catalog of movies offered at
a Web site owned by the film company that produced the movie.
Through the browser interface, the user selects the movie, selects
a rental option, and authorizes payment of the rental fee. The
browser software causes the request to be sent to the film
company.
[0077] At block 154, the content provider 22 receives the request
and analyzes it. The content provider generates a challenge nonce
"Challenge-N" to question the subscriber unit for proof of its
processor and of the operating system it is running (block 156). A
different challenge nonce is generated for each request so that the
server can identify the challenge nonce when it is returned by the
subscriber unit. The content provider 22 sends the challenge nonce
to the subscriber unit 24 (block 158).
[0078] At block 160, upon receipt of the challenge nonce, the CPU
40 mints an OS certificate that contains the challenge nonce from
the content provider and an identity of the OS. The OS certificate
takes the following form: [0079] OS Certificate=(SIR, Reply,
Challenge-N, K.sub.CPU) signed by K.sub.CPU.sup.-1
[0080] In addition to the challenge nonce, the OS certificate
contains the SIR value, a reply, and the CPU's public key
K.sub.CPU. The "reply" can optionally contain all of the data
written to the boot log 70 so that the content provider can
evaluate what software components are currently loaded and
executing. In other cases, the content providers could just trust
the OS publisher (and hence simply the value of the SIR). The OS
certificate is signed using the CPU's private key K.sub.CPU.sup.-1.
Effectively, the OS certificate says "The processor named K.sub.CPU
was running the Operating System SIR with the specified boot log
when it received the challenge Challenge-N". (In the case where the
boot log is included, the CPU is including more information,
effectively saying "Further, it was running this OS revision, with
these version components, and these device drivers.")
[0081] The CPU 40 uses the key pair K.sub.CPU, K.sub.CPU.sup.-1
only for this operation, or for other similarly restricted classes
of operations; the CPU is unable to sign an arbitrary block of
data. As a result, there is no way that the CPU or another party
could falsify an OS certificate. One might imagine certain attacks,
such as saving a certificate from a previous reboot, or
impersonating a real content provider to get one of the
certificates. However, as noted above, the content provider
generates and sends a different challenge nonce each time, thereby
preventing such "replay attacks". Another possible attack is for
the subscriber unit to return a proper certificate challenge but
then be quickly rebooted into a different OS so that it may
illicitly use the digital content. This attack is also futile
because the OS maintains the SSL connection session key in secrecy
and the new OS would not know the current session key being used by
the SSL connection.
[0082] At block 162, the subscriber unit 24 returns the
newly-minted OS certificate to the content provider 22. The
subscriber unit 24 also returns the CPU manufacturer's certificate
66. At block 164, the content provider 22 receives and validates
the OS certificate and manufacturer's certificate using a series of
tests, enumerated as blocks 166 and 170-180. Failure of any one of
the tests results in the request for content being rejected by the
content provider.
[0083] The first test is whether the content provider recognizes
the SIR value contained in the OS certificate and trusts the
associated operating system (block 166). The content provider can
also evaluate the boot log in the reply portion of the OS
certificate to decide whether to trust other software components
running on the subscriber unit. If the content provider chooses not
to trust the OS or other components, the request is rejected (block
168).
[0084] Otherwise, assuming the OS and other modules are trusted
(i.e., the "yes" branch from block 166), the content provider 22
next determines whether the challenge nonce is the same as it
generated and supplied to the subscriber unit (block 170). If the
nonce returned in the reply fails to match the nonce generated by
the content provider, the request is rejected (block 168). However,
if the two match, the content provider evaluates whether the OS
certificate is properly signed with the CPU's private key
K.sub.CPU.sup.-1 (block 172). The content provider makes this
evaluation using the enclosed public key K.sub.CPU.
[0085] With respect to the CPU manufacturer's certificate, the
content provider determines whether the certificate names the same
public key K.sub.CPU used in the OS certificate (block 174). If so,
the content provider continues to the next test; otherwise, the
request is rejected (block 168).
[0086] The content provider next examines at block 176 whether the
manufacturer certificate is signed by the manufacturer's private
key K.sub.MFR.sup.-1 by using the manufacturer's public key
K.sub.MFR. If the signature is proper, the content provider decides
whether it trusts this manufacturer (block 178).
[0087] If all tests prove true and the content provider trusts the
processor, operating system, and the manufacturers of both the
processor and the operating system, the content provider can choose
to download the content to the subscriber unit, along with a list
of terms under which the content may be used (block 180). This list
may be in the form of a license or an Access Control List (ACL),
specifying by which processor, by which OS, by which
application(s), and under which additional terms the content may be
used. The subscriber unit 24 stores the content in the secure space
84 of memory 42 (block 182).
[0088] Secure Storage
[0089] The CPU provides for secure (Authenticated OS-specific)
storage with the addition of one further opcode. The opcode is
"GenerateKey(Seed)". The opcode takes a seed and generates a unique
storage key SK. The seed comprises a second CPU secret key K.sub.2
(distinct from the CPU public key K.sub.CPU), the SIR or the first
two entries in the boot log 70 of FIG. 5 (i.e., SIR and the
following four bytes expressing a version number "2.01"), and a
user-supplied seed. The seed is input to a cryptographic
"pseudo-random" number generator, which is implemented as part of
the cryptography accelerator 62 (or in the processor if no
accelerator is present). The opcode GenerateKey(Seed) is a
protected-mode (kernel accessible) instruction.
[0090] One possible implementation of the opcode instruction is as
follows: SK=SHA(K.sub.2, SIR, seed) (1) where SHA is a specific
secure digest function called the "secure hash algorithm". The
resulting storage key SK can be used to encrypt the content
received from the content provider. The encrypted content is then
stored in the store 84.
[0091] A second implementation is to include all or part of the
boot log, as follows: SK=SHA(K.sub.2, SIR, seed, Boot Log Entries)
(2) In this implementation, access to storage can be made dependent
on all or part of the details of the remainder of the operating
system running (service packs, device drivers, etc.).
[0092] Note that the same storage key can be generated from the
same user seed whenever the same authenticated OS is running on the
same CPU. If a different authenticated OS is booted, a different
storage key is returned and the original storage key cannot be
obtained. Similarly, since key generation is based on a unique key
in each processor, the encrypted data cannot be moved to another
machine and decrypted. Since the CPU-internal key K.sub.2 is kept
secret, there is no way that a non-Authenticated OS or a different
authenticated OS can ever recover this number if the original
authenticated OS does not reveal it.
[0093] FIG. 7 shows a method for securely storing digital content
supplied by the content provider within the secure store 84 on the
subscriber unit 24. The method is performed by software/hardware
components at the subscriber unit.
[0094] At block 200, the CPU receives an OS-supplied or
application-supplied number, character string, or alphanumeric
value for use as a seed. The CPU concatenates the user seed, the
CPU secret key K.sub.2, and the SIR or boot log entries to form a
composite identifier (block 202). This identifier is input to a
secure hash algorithm, which produces a storage key SK (block 204).
As content arrives from the content provider, the application or
operating system encrypts the content using the storage key SK
(block 206). The encrypted content is then stored in memory 42 to
form the secure store 84 (block 208).
[0095] The storage key SK can be regenerated as illustrated in
blocks 200-204 each time the encrypted content is read. However,
this mechanism does not allow the operating system to be upgraded
without the SIR changing and hence the storage keys being lost. To
accomplish upgrades that change the SIR, a different scheme for
storing secrets (escrowed encryption keys, for instance) is
used.
[0096] 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.
[0097] 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.
[0098] 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.
[0099] In this way, a processor can store encrypted information
that can be decrypted only by the same processor running a
specified operating system.
[0100] 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.
[0101] 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 Ks 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.
[0102] ATTEST Operation
[0103] The action of signing a statement of the current value of
the SIR can be generalized into a technique for the operating
system to make a signed attestation of the current value of any
arbitrary region of memory and/or a register. In one
implementation, the ATTEST operation is performed as follows:
[0104] ATTEST(Register Name, Region of Memory)
[0105] This operation produces a signed result: [0106] [K.sub.CPU,
"ATTEST", Register Name, Register Value, Memory Contents] (signed
with K.sub.CPU.sup.-1)
[0107] The ATTEST operation can be used to sign the current value
of the SIR or any other register (including a log register LOGR,
described below). The processor also signs the data contained in an
arbitrary region of memory. This can be used to include the
challenge value or some other signed statement desired by the
operating system or application. The ATTEST operation can be used
to provide an implementation of a more general form of OS
certificates, as discussed earlier.
[0108] Boot Log Implementation
[0109] The boot log is an append-only record of all or selected
components loaded by the operating system. This can be managed
entirely in hardware, but can be simplified by means of a LOG
operation.
[0110] The LOG operation constructs a secure one way digest of a
supplied parameter, an internal LOGR register 69, and stores the
result back in the LOGR register. At power up, or at processor
reset, the LOGR register is set to zero. The LOGR register can be
read, but not written apart from execution of the LOG operation.
The processor can also sign a statement attesting to the current
value of the LOGR register and a supplied challenge. Symbolically:
LOGR'=SHA.sup.-1(LOGR, DATA) (3) where LOGR is the current value of
the register, DATA is supplied to the LOGR' and is the contents of
the LOGR register after the LOG operation is performed, and
SHA.sup.-1 is an exemplary one way hash function (the Secure Hash
Algorithm).
[0111] The operating system can use the LOG operation to record the
digest of each component as it is loaded. When online to a content
provider, the ATTEST operation can be used to provide an
un-tamperable attestation of all components loaded into the
operating system.
[0112] In order for the content provider to be able to interpret
the LOGR value, the operating system also conveys the digests of
all components that made up the boot log. A content provider can
then: [0113] 1. Check that all of the components revealed are known
and trusted. [0114] 2. Check that the composite value obtained when
the digests are combined according to equation (3) match that
quoted by the microprocessor. [0115] 3. Check that the signature on
the quoted statement is valid for the microprocessor public key.
[0116] 4. Check the processor certificate is valid and matches the
key used in the quoted statement.
[0117] If these conditions are met, the content provider can trust
the client with the premium content. If they are not met, then the
content provider can return a statement of the components that are
not trusted so that the user or operating system can upgrade the
untrusted component.
[0118] Exemplary Chipset Implementation
[0119] 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.
[0120] An agent on the bus can also check for unauthorized
interrupts during the execution of the authenticated operations or
execution of the boot block.
[0121] 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.
[0122] BIOS Implementation
[0123] 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.
[0124] Improved Security
[0125] Hash algorithms and signature schemes can be broken. One
example of a security break is that an attacker finds a second boot
block that has the same identity (same signature, or same digest).
If such a boot block is found, then a different operating system
can be booted, and all content security is lost.
[0126] Greater security can be obtained by combining security
schemes. For instance, the OS-identity can be formed as the
concatenation of two or more digests calculated using different
hash algorithms, or the same algorithm applied to boot block data
in a different order (for instance, backwards).
[0127] In the case of signature based identity, the boot block can
be signed several times using different signature algorithms and
keys. Again the software identity becomes the concatenation of the
relevant public keys. This technique also provides protection
against private key compromise. If one of the signing keys is
compromised, not all security is lost.
[0128] Recertification
[0129] Microprocessor key compromise is inevitable and is
traditionally handled by revocation lists (list of untrusted
hosts). However, if the certificates that vouch for the
microprocessor keys never expire, then revocation lists will grow
uncomfortably large. This problem can be ameliorated by finite
lifetime processor certificates. Content providers will require
valid certificates (not expired), and the chip vendor (or other
trusted party) will be required to re-issue certificates for chips
still considered in good standing (not revoked). Note that the
certificates are not used when offline, so machines do not stop
working when the certificates expire. However, in online
transactions, on occasional (probably automated) extra step would
be to get a new certificate.
CONCLUSION
[0130] The authenticated boot technique has many advantages. It
enables use of open platforms for building trusted systems, where
the open platforms can run arbitrary operating systems and
arbitrary software. Moreover, it can authenticate the combination
of the hardware and the software in the subscriber unit. One
resulting advantage of this invention is that the content provider
and not the owner or manufacturer of the PC or of the OS is
responsible for determining trust. Another advantage is that when
data is stored persistently, the storage key need not be stored
with it, and the client need not go back online to obtain a new
key. The data can be decrypted at any time by the same OS on the
same processor, but cannot be decrypted by an unauthorized OS or on
a different processor. Another advantage is that any number of
independent Authenticated or non-Authenticated operating systems
can be booted serially on the same system. The CPU automatically
provides each with independent services for cryptographic key
generation and data storage. Moreover, a secure OS can be easily
modified for both authentication and security by changing the boot
code and checking signatures before loading.
[0131] Although the invention has been described in language
specific to structural features and/or methodological actions, it
is to be understood that the invention defined in the appended
claims is not necessarily limited to the specific features or
actions described. Rather, the specific features and actions are
disclosed as preferred forms of implementing the claimed
invention.
* * * * *