US 7,376,974 B2Grant
Apparatus and method for creating a trusted environment
Issue Date:2008-05-20
•25 Claims
•7 Drawing Sheets
Abstract
A computer apparatus for creating a trusted environment comprising a trusted device arranged to acquire a first integrity metric to allow determination as to whether the computer apparatus is operating in a trusted manner; a processor arranged to allow execution of a first trust routine and associated first operating environment, and means for restricting the first operating environment access to resources available to the trust routine, wherein the trust routine being arranged to acquire the first integrity metric and a second integrity metric to allow determination as to whether the first operating environment is operating in a trusted manner.
Metadata
Assignee
- Hewlett-Packard Development Company, L.P.
Inventors
- Graeme John Proudler
- Boris Balacheff
- John S. Worley
- Chris D. Hyser
- William S Worley, Jr.
Application Information
Application Number:US 10/303,690
Filing Date:2002-11-21
Priority Date:2001-11-22
Art Unit:2137
Classifications
IPC:
G06F7/04H04L9/00
Patent Drawings (7 sheets)
Description
Cross Reference to Related Applications
[0001] The subject matter of the present application may also be related to the following U.S. patent applications: “Operation of Trusted State in Computing Platform,” Ser. No. 09/728,827, filed Nov. 28, 2000; “Performance of a Service on a Computing Platform,” Ser. No. 09/920,554, filed Aug. 1, 2001; “Secure E-Mail Handling Using a Compartmented Operating System,” Ser. No. 10/075,444, filed Feb. 15, 2002; “Electronic Communication,” Ser. No. 10/080,466, filed Feb. 22, 2002; “Demonstrating Integrity of a Compartment of a Compartmented Operating System,” Ser. No. 10/165,840, filed Jun. 7, 2002; “Multiple Trusted Computing Environments with Verifiable Environment Entities,” Ser. No. 10/175,183, filed Jun. 18, 2002; “Renting a Computing Environment on a Trusted Computing Platform,” Ser. No. 10/175,185, filed Jun. 18, 2002; “Interaction with Electronic Services and Markets,” Ser. No. 10/175,395, filed Jun. 18, 2002; “Multiple Trusted Computing Environments,” Ser. No. 10/175,542, filed Jun. 18, 2002; “Performing Secure and Insecure Computing Operations in a Compartmented Operating System,” Ser. No. 10/175,553, filed Jun. 18, 2002; “Privacy of Data on a Computer Platform,” Ser. No. 10/206,8 12, filed Jul. 26, 2002; “Trusted Operating System,” Ser. No. 10/240,137, filed Sep. 26, 2002; and “Trusted Gateway System,” Ser. No. 10/240,139, filed Sep. 26, 2002.
Field of the Invention
[0002] The present invention relates to an apparatus and method for creating a trusted environment.
Background
[0003] Computer platforms used for commercial applications typically operate in an environment where their behaviour is vulnerable to modification by local or remote entities.
[0004] Additionally, with the continuing increase in computer power it has become increasingly common for computer platforms to support multiple users, where each user can have their own operating environment installed on the computer platform.
[0005] The operating system software typically runs at the system privilege level of a processor, where the system privilege level permits the individual operating systems to utilise both privileged and non-privileged instructions provided by the processor hardware. As such, where a number of separate operating systems are running simultaneously on a computer platform the operating systems are sharing full system privilege and are not necessarily isolated or protected from one another. The volume of source code for software components sharing full system privilege is typically so large in modern operating systems that it is virtually impossible to ensure the correctness of the source code and whether the behaviour of the source code will behave as expected.
[0006] Accordingly, this potential insecurity of the platform is a limitation on its use by parties who might otherwise be willing to use the platform.
[0007] Increasing the level of trust in platforms therefore enables greater user confidence that the platform and operating system environment behave in a known manner.
Summary of the Invention
[0008] In accordance with a first aspect of the present invention there is provided a computer apparatus for creating a trusted environment comprising a trusted device arranged to acquire a first integrity metric to allow determination as to whether the computer apparatus is operating in a trusted manner; a processor arranged to allow execution of a first trust routine and associated first operating environment, and means for restricting access of the first operating environment to resources available to the trust routine, wherein the trust routine is arranged to acquire the first integrity metric and a second integrity metric to allow determination as to whether the first operating environment is operating in a trusted manner.
[0009] In accordance with a second aspect of the present invention there is provided a computer apparatus for creating a trusted environment comprising a trusted device arranged to acquire a first integrity metric to allow determination as to whether the computer apparatus is operating in a trusted manner and a processor arranged to allow execution of a first trust routine and an associated first operating environment, the processor is arranged to restrict access of the first operating environment to resources available to the trust routine, wherein the trust routine is arranged to acquire the first integrity metric and a second integrity metric to allow determination as to whether the first operating environment is operating in a trusted manner.
[0010] In accordance with a third aspect of the present invention there is provided a computer apparatus for creating a trusted environment comprising a trusted device arranged to acquire a first integrity metric to allow determination as to whether the computer apparatus is operating in a trusted manner and a processor arranged to allow execution of a first trust routine in a first privilege level of the processor and to allow execution of an associated first operating environment in a second privilege level of the processor such that access to resources available to code executed in the first privilege level is restricted to code executed in the second privilege level, wherein the trust routine is arranged to acquire the first integrity metric and a second integrity metric to allow determination as to whether the first operating environment is operating in a trusted manner;
[0011] Preferably the trusted device is a tamper resistance device.
[0012] Most preferably the processor is arranged to allow execution of a plurality of trust routines in the first privilege level and to allow execution of respective associated operating environments in the second privilege level; each trust routine being arranged to acquire the first integrity metric and an integrity metric associated with the respective associated operating environment to allow determination as to whether the respective operating environment is operating in a trusted manner.
[0013] Preferably the trust routine is arranged to incorporate cryptographical functionality for restricting access to data associated with the trust routine.
[0014] Preferably the trusted device is arranged to store one or more secrets.
[0015] Suitably one or more secrets are associated with the trust routine.
[0016] Most suitably at least one secret is a private asymmetric encryption key.
[0017] Preferably the trusted device is arranged to store, on powering down of the computer apparatus, secrets utilised by a trust routine.
[0018] Preferably the code executed in the second privilege level has restricted access to data associated to a trust routine executed in the first privilege level.
[0019] Preferably all data associated with the trust module is protected from alteration from code executed in privilege level 2.
[0020] Preferably secrets associated with a trust module are not accessible to code executed in privilege level 2.
[0021] Suitably the trusted device vouches for a public key associated with the trust module using a private key of the trusted device.
[0022] Suitably a trusted third party vouches for a public key associated with the trust module using a private key of the trusted parties.
[0023] Preferably the trusted device is arranged to transfer boot-up sequence instructions to allow initiation of boot-up of the computer apparatus.
[0024] Most preferably the processor is arranged to cause the boot-up sequence instructions to be the first instructions executed by the processor after release from reset.
[0025] In accordance with a fourth aspect of the present invention there is provided a method for creating a trusted environment comprising acquiring a first integrity metric to allow determination as to whether a computer apparatus is operating in a trusted manner and executing a first trust routine in a first privilege level of a processor and executing an associated first operating environment in a second privilege level of the processor, restricting access to resources available to code executed in the first privilege level from code executed in the second privilege level, acquiring the first integrity metric and a second integrity metric to allow determination as to whether the first operating environment is operating in a trusted manner.
[0026] In accordance with a fifth aspect of the present invention there is provided a method for creating a trusted environment comprising acquiring a first integrity metric to allow determination as to whether a computer apparatus is operating in a trusted manner; executing a first trust routine and an associated first operating environment, restricting the first operating environments access to resources available to the trust routine, and arranging the trust routine to acquire the first integrity metric and a second integrity metric to allow determination as to whether the first operating environment is operating in a trusted manner.
[0027] In accordance with a sixth aspect of the present invention there is provided a method for creating a trusted environment comprising acquiring a first integrity metric to allow determination as to whether a computer apparatus is operating in a trusted manner and executing a first trust routine and an associated first operating environment, restricting the first operating environment's access to resources available to the trust routine, acquiring the first integrity metric and a second integrity metric to allow determination as to whether the first operating environment is operating in a trusted manner.
[0028] In accordance with a seventh aspect of the present invention there is provided a computer system for creating a trusted environment comprising a trusted device arranged to acquire a first integrity metric to allow determination as to whether the computer apparatus is operating in a trusted manner; a processor arranged to allow execution of a first trust routine and associated first operating environment, and means for restricting access of the first operating environment to resources available to the trust routine, wherein the trust routine is arranged to acquire the first integrity metric and a second integrity metric to allow determination as to whether the first operating environment is operating in a trusted manner.
Brief Description of the Drawings
[0029] For a better understanding of the present invention and to understand how the same may be brought into effect reference will now be made, by way of example only, to the accompanying drawings, in which:
[0030] FIG. 1 illustrates a system capable of implementing embodiments of the present invention;
[0031] FIG. 2 illustrates a motherboard including a trusted device;
[0032] FIG. 3 illustrates privilege levels of a processor;
[0033] FIG. 4 illustrates the trusted device in more detail;
[0034] FIG. 5 illustrates the steps involved in acquiring an integrity metric of the computing apparatus;
[0035] FIG. 6 illustrates a system capable of implementing embodiments of the present invention;
[0036] FIG. 7 illustrates a virtual trusted device;
[0037] FIG. 8 illustrates an embodiment of the present invention.
Description of an Embodiment of the Invention
[0038] The present embodiment provides the incorporation into a computing platform of a physical trusted device and a software trust routine (i.e. a virtual trusted device). The function of the physical trusted device is to bind the identity of the platform to reliably measured data that provides an integrity metric of the platform, while the virtual trusted device binds the identity of an associated software operating environment (e.g. an operating system) to reliably measured data that provides an integrity metric of the operating environment. The identities and the integrity metrics are compared with expected values provided by a trusted party (TP) that is prepared to vouch for the trustworthiness of the platform. Optionally, the expected values provided by the trusted third party are securely stored in the respective physical trusted device and the virtual trusted device. If there is a match, the implication is that at least part of the platform and operating system is operating correctly, depending on the scope of the integrity metric.
[0039] A user verifies the correct operation of the platform and operating environment before exchanging other data with the platform. A user does this by requesting the identities and integrity metrics of the physical trusted device and the virtual trusted device. (Optionally the trusted devices will refuse to provide evidence of identity if it itself was unable to verify correct operation of the platform.) The user receives the proof of identity and the identity metric, and compares them against the values provided by the trusted third party. If the measured data reported by the trusted devices are the same as that provided by the trusted third party, the user can trust the platform.
[0040] Additionally, where the computer platform is arranged to support a plurality of separate operating environments, each operating environment having their own respective virtual trusted device, the users of the respective operating environments can trust that their operating environment is isolated from any other operating environment running on the computer platform.
[0041] Once a user has established trusted operation of the platform and operating environment, he exchanges other data with the platform. For a local user, the exchange might be by interacting with some software application running within the operating environment on the platform. For a remote user, the exchange might involve a secure transaction. In either case, the data exchanged is typically ‘signed’ by one of the trusted devices. The user can then have greater confidence that data is being exchanged with a platform whose behaviour can be trusted.
[0042] The trusted devices use cryptographic processes but do not necessarily provide an external interface to those cryptographic processes.
[0043] To ensure there is a minimum risk that the virtual trusted device is susceptible to software attack by rogue software running on the computer platform the virtual trusted device is arranged to be executed in a processor privilege level that restricts access to other software applications being executed on the computer platform (as described below). Additionally, secrets associated with the virtual trusted device are stored such that the secrets are inaccessible to software applications being executed in a processor privilege level that is lower than that in which the virtual trusted device is executed. Also, a most desirable implementation would be to make the physical trusted device tamperproof, to protect secrets by making them inaccessible to other platform functions and provide an environment that is substantially immune to unauthorised modification. Since tamper-proofing is impossible, the best approximation is a trusted device that is tamper-resistant, or tamper-detecting. The trusted device, therefore, preferably consists of one physical component that is tamper-resistant.
[0044] Techniques relevant to tamper-resistance are well known to those skilled in the art of security. These techniques include methods for resisting tampering (such as appropriate encapsulation of the trusted device), methods for detecting tampering (such as detection of out of specification voltages, X-rays, or loss of physical integrity in the trusted device casing), and methods for eliminating data when tampering is detected.
[0045] A trusted platform 10 is illustrated in the diagram in FIG. 1 . The platform 10 includes the standard features of a keyboard 14, mouse 16 and visual display unit (VDU) 18, which provide the physical ‘user interface’ of the platform. In the platform 10, there are a plurality of modules 15: these are other functional elements of the trusted platform of essentially any kind appropriate to that platform (the functional significance of such elements is not relevant to the present invention and will not be discussed further herein).
[0046] As illustrated in FIG. 2 , the motherboard 20 of the trusted computing platform 10 includes (among other standard components) a main processor 21 with internal memory 25, main memory 22, a trusted device 24, a data bus 26 and respective control lines 27 and lines 28, BIOS memory 29 containing the BIOS program 28 for the platform 10 and an Input/Output (IO) device 23, which controls interaction between the components of the motherboard, the keyboard 14, the mouse 16 and the VDU 18. The main memory 22 is typically random access memory (RAM).
[0047] In this embodiment the processor 21 has four execution privilege levels PL0, PL1, PL2, PL3. Examples of such processors are the Hewlett-Packard's PA-RISC processor or Intel's IA-64 processor, however other processor configurations having a plurality of privilege levels can also be used.
[0048] Running in the processor 21 is a secure platform architecture (SPA) 31, as shown in FIG. 3 .
[0049] The SPA 31 includes BIOS program or firmware 28 that runs on the processor 21 at execution privilege level 0 (PL0), the most privileged level of processor 21. SPA 31 includes a four-layer software ring that runs on top of BIOS firmware 28 in processor 21.
[0050] The innermost software ring, running on top of BIOS firmware 28, is referred to as the secure platform kernel (SPK) 32 and is the only software ring that runs as a privileged task. SPK 32 runs at PL0 and forms the foundation layer of SPA 31 and is the only ring layer that accesses privileged system registers and executes privileged instructions.
[0051] A secure platform global services module (SPGS) 33 runs on top of the SPK 32 as an unprivileged task. SPGS 33 runs at execution privilege level 1 (PL1), the second most privileged level of processor 21. SPK 32 and SPKGS 33 are collectively referred to as secure platform (SP) 34.
[0052] At least one operating system image 35 runs on top of SPGS 33 as an unprivileged task. Operating system image 35 runs at execution privilege level 2 (PL2), the third most privileged level of processor 31. End user applications 36 run on top of operating system image(s) 35 as unprivileged tasks. End user applications 36 run at execution privilege level 3 (PL3), the fourth privileged level (i.e., the least privileged level) of processor 21.
[0053] SPK 32 is preferably a small kernel of trusted, provably correct code that performs security critical services where the small size contributes to the SPK's security and correctness. Examples of security critical services include memory and process management, trap and interrupt handling, and cryptographic services, where some of these security services may be performed via a virtual trust device, as described below. SPGS 33 is constructed with trusted code, but utilizes hardware security capabilities of the processors 21, such as IA-64 processors, to minimize the impact of a failure. SPGS 33 runs as an unprivileged task and employs SPK 32 to perform privileged operations.
[0054] Additionally, the SPK 32 includes code to allow execution of one or more virtual trusted devices 37 within the SPK 32. The virtual trusted device(s) 37 are associated with an operating environment executed in PL2 and PL3 and allow a user to establish whether the associated operating environment can be trusted, as described below. It is not essential, however, for the virtual trust device code to be incorporated within the SPK code, the code can be housed elsewhere, for example in the trusted device 24.
[0055] To ensure that the virtual trusted device 37 can be trusted it is desirable for the manufacture of the SPK code to be validated by a trusted third party. On validation a validation credential signed with the trusted third parties private key is associated with the SPK code.
[0056] SPGS 33 typically includes all the services that do not have to be included in SPK 32. One reason that secure platform 34 is split into SPK 32 and SPGS 33 is to permit SPK 32 to be small, stable and verifiable.
[0057] Interfaces between BIOS firmware 28 and processor hardware 21 include a privileged application binary interface (ABI) and a non-privileged ABI. The interfaces between SPK 32 and BIOS firmware 28 include a privileged ABI, a non-privileged ABI, and processor abstraction layer (PAL)/system abstraction layer (SAL)/extensible firmware interface (EFI) interfaces. The interfaces between SPGS 33 and SPK 32 include a secure platform interface (SPI) and a non-privileged ABI. The interfaces between operating system image(s) 35 and SPGS 33 include a SPI, a global services interface (GSI), and a non-privileged ABI. The interfaces between end user applications 36 and operating system image(s) 35 include an application program interface (API) and a non-privileged ABI.
[0058] SPGS 33 can partition operating system image layer 35 into multiple independent protection domains which operate at PL2. A protection domain is herein referred to as a software partition and associated collection of system resources, such as memory, I/O, processors, and the like, created by SPGS 33 for the purpose of loading and executing a single operating system image 35. Each of the multiple independent protection domains are capable of booting and executing an operating system image 35 or any other program capable of operation using only SPK 32 and SPGS 33 services, such as a specialized application control program.
[0059] The multiple independent protection domains running at PL2 are protected from each other through the memory protection capabilities of the four privilege level processor hardware 21, such as the memory protection capabilities of the IA-64 processor. Therefore, a failure in one of the independent protection domains typically has no effect on the other independent protection domains, even if the failure is an operating system crash. The independent protection domains provide the capability to manage system utilization on a fine-grain basis while maintaining security. Operating system images 35 are ported to secure platform 34 of SPA 31 similar to how operating systems are ported to a new hardware platform industrial standard archicture ISA in the classical architecture for operating systems.
[0060] End user applications 36 run at the least privileged level, PL3, as unprivileged tasks under the control of an operating system image 35 in a secure platform 34 protection domain. Typically, from the end user application perspective, the end user application 36 operates under the control of an operating system image 35 as the end user application would run under the control of an operating system in the classical architecture for operating systems.
[0061] In order for the computer platform 10 and operating environment(s) to be trusted, a chain of trust from the system hardware, through the boot process, to final running code is established. In addition, all software code is preferably authenticated before being executed, and a properly authenticated piece of code is preferably unchangeable except by a similarly trusted component to maintain the chain of trust. The software authentication should be more than a simple check sum or other forgeable scheme. Thus, SPA 31 preferably employs strong authentication using cryptographic methods, such as public key encryption, such that software can be undetectably corrupt only if a private key is known.
[0062] The chain of trust extends back to the trusted device 24. As described below, after system reset the processor 21 is initially controlled by the trusted device 24, which then after performing a secure boot process hands control over to the BIOS firmware 28. During the secure boot process, the trusted device 24 acquires an integrity metric of the computer platform 10, as described below.
[0063] Specifically, the trusted device 24 comprises, as shown in FIG. 4 : a controller 40 programmed to control the overall operation of the trusted device 24, and interact with the other functions on the trusted device 24 and with the other devices on the motherboard 20; a measurement function 41 for acquiring the integrity metric from the platform 10; a cryptographic function 42 for signing, encrypting or decrypting specified data; an authentication function 43; and interface circuitry 44 having appropriate ports (46, 47 & 48) for connecting the trusted device 24 respectively to the data bus 26, control lines 27 and address lines 28 of the motherboard 20. Each of the blocks in the trusted device 24 has access (typically via the controller 40) to appropriate volatile memory areas 4 and/or non-volatile memory areas 3 of the trusted device 24. Additionally, the trusted device 24 is designed, in a known manner, to be tamper resistant.
[0064] For reasons of performance, the trusted device 24 may be implemented as an application specific integrated circuit (ASIC). However, for flexibility, the trusted device 24 is preferably an appropriately programmed micro-controller. Both ASICs and micro-controllers are well known in the art of microelectronics and will not be considered herein in any further detail.
[0065] One item of data stored in the non-volatile memory 3 of the trusted device 24 is a certificate 350. The certificate 350 contains at least a public key 351 of the trusted device 24 and optionally an authenticated value 352 of the platform integrity metric measured by a trusted party (TP). The certificate 350 is signed by the TP using the TP's private key prior to it being stored in the trusted device 24. In later communications sessions, a user of the platform 10 can verify the integrity of the platform 10 and operating environment by comparing the acquired integrity metric (i.e. measured integrity metric) with an authentic integrity metric 352, as described below. Knowledge of the TP's generally-available public key enables simple verification of the certificate 350. The non-volatile memory 45 also contains an identity (ID) label 353. The ID label 353 is a conventional ID label, for example a serial number, that is unique within some context. The ID label 353 is generally used for indexing and labelling of data relevant to the trusted device 24, but is insufficient in itself to prove the identity of the platform 10 under trusted conditions.
[0066] The trusted third party that is requested to supply the authentic integrity metric will inspect the type of the platform to decide whether to vouch for it or not. This will be a matter of policy. If all is well the TP measures the value of integrity metric of the platform. Then, the TP generates a certificate for the platform. The certificate is generated by the TP by appending the trusted device's public key, and optionally its ID label, to the measured integrity metric, and signing the string with the TP's private key.
[0067] The trusted device 24 can subsequently prove its identity by using its private key to process some input data received from the user and produce output data, such that the input/output pair is statistically impossible to produce without knowledge of the private key. Hence, knowledge of the private key forms the basis of identity in this case. Clearly, it would be feasible to use symmetric encryption to form the basis of identity. However, the disadvantage of using symmetric encryption is that the user would need to share his secret with the trusted device. Further, as a result of the need to share the secret with the user, while symmetric encryption would in principle be sufficient to prove identity to the user, it would insufficient to prove identity to a third party, who could not be entirely sure the verification originated from the trusted device or the user.
[0068] The trusted device 24 is initialised by writing the certificate 350 into the appropriate non-volatile memory locations 3 of the trusted device 24. This is done, preferably, by secure communication with the trusted device 24 after it is installed in the motherboard 20. The method of writing the certificate to the trusted device 24 is analogous to the method used to initialise smart cards by writing private keys thereto. The secure communication is supported by a ‘master key’, known only to the TP, that is written to the trusted device (or smart card) during manufacture, and used to enable the writing of data to the trusted device 24; writing of data to the trusted device 24 without knowledge of the master key is not possible.
[0069] At some later point during operation of the platform, for example when it is switched on or reset the trusted device 24 measures and stores the integrity metric 361 of the platform.
[0070] The trusted device 24 is equipped with at least one method of reliably measuring or acquiring the integrity metric of the computing platform 10 with which it is associated to enable comparison with the authentic integrity metric supplied by the trusted third party. In the present embodiment, the integrity metric is acquired by the measurement function 41 by generating a digest of the BIOS instructions in the BIOS memory and the SPK code. The measured integrity metric is signed using the trusted device 24 private key to provide confidence that the integrity metric has been acquired by the trusted device 24. Such an acquired integrity metric, if verified as described above, gives a potential user of the platform 10 a high level of confidence that the platform 10 has not been subverted at a hardware, or BIOS program, level.
[0071] The measurement function 41 has access to: non-volatile memory 3 for storing a hash program 354 and a private key 355 of the trusted device 24, and volatile memory 4 for storing acquired integrity metric in the form of a digest 361. In appropriate embodiments, the volatile memory 4 may also be used to store the public keys and associated ID labels 360a-360n of one or more authentic smart cards (not shown) that can be used to gain access to the platform 10.
[0072] In one preferred implementation, as well as the digest, the integrity metric includes a Boolean value, which is stored in volatile memory 4 by the measurement function 31, for reasons described below.
[0073] A preferred process for acquiring an integrity metric for the computer platform 10 will now be described with reference to FIG. 5 .
[0074] In step 500, at switch-on, the measurement function 41 monitors the activity of the main processor 21 on the data, control and address lines (26, 27 & 28) to determine whether the trusted device 24 is the first memory accessed. Processor 21 is directed to the trusted device 24, which acts as a memory. In step 505, if the trusted device 24 is the first memory accessed, in step 510, the measurement function 41 writes to volatile memory 3 a Boolean value which indicates that the trusted device 24 was the first memory accessed. Otherwise, in step 515, the measurement function writes a Boolean value which indicates that the trusted device 24 was not the first memory accessed.
[0075] In the event the trusted device 24 is not the first accessed, there is of course a chance that the trusted device 24 will not be accessed at all. This would be the case, for example, if the main processor 21 were manipulated to run the BIOS program first. Under these circumstances, the platform would operate, but would be unable to verify its integrity on demand, since the integrity metric would not be available. Further, if the trusted device 24 were accessed after the BIOS program had been accessed, the Boolean value would clearly indicate lack of integrity of the platform.
[0076] However, if a user is prepared to trust the BIOS the computer platform 10 can be arranged to use the BIOS instructions as the first instructions accessed.
[0077] In step 520, when (or if) accessed as a memory by the main processor 21, the main processor 21 reads the stored native hash instructions 354 from the measurement function 41 in step 525. The hash instructions 354 are passed for processing by the main processor 21 over the data bus 26. In step 530, main processor 21 executes the hash instructions 354 and uses them, in step 535, to compute a digest of the BIOS memory 29, by reading the contents of the BIOS memory 29 and processing those contents according to the hash program. In step 540, the main processor 21 writes the computed digest 361 to the appropriate non-volatile memory location 4 in the trusted device 24. In a similar manner the measurement function 41 initiates the calculation of a digest for the SPK 32 that is correspondingly stored in an appropriate nonvolatile memory location 4 in the trusted device 24. The measurement function 41, in step 545, then calls the BIOS firmware 28 in the BIOS memory 29, and execution continues, as described below.
[0078] Clearly, there are a number of different ways in which the integrity metric of the platform may be calculated, depending upon the scope of the trust required. The measurement of the BIOS program's integrity provides a fundamental check on the integrity of a platform's underlying processing environment. The integrity metric should be of such a form that it will enable reasoning about the validity of the boot process—the value of the integrity metric can be used to verify whether the platform booted using the correct BIOS. Optionally, individual functional blocks within the BIOS could have their own digest values, with an ensemble BIOS digest being a digest of these individual digests. This enables a policy to state which parts of BIOS operation are critical for an intended purpose, and which are irrelevant (in which case the individual digests must be stored in such a manner that validity of operation under the policy can be established).
[0079] Other integrity checks could involve establishing that various other devices, components or apparatus attached to the platform are present and in correct working order. In one example, the BIOS programs associated with a SCSI controller could be verified to ensure communications with peripheral equipment could be trusted. In another example, the integrity of other devices, for example memory devices or co-processors, on the platform could be verified by enacting fixed challenge/response interactions to ensure consistent results. Where the trusted device 24 is a separable component, some such form of interaction is desirable to provide an appropriate logical binding between the trusted device 24 and the platform. Also, although in the present embodiment the trusted device 24 utilises the data bus as its main means of communication with other parts of the platform, it would be feasible, although not so convenient, to provide alternative communications paths, such as hard-wired paths or optical paths. Further, although in the present embodiment the trusted device 24 instructs the main processor 21 to calculate the integrity metric in other embodiments, the trusted device itself is arranged to measure one or more integrity metrics.
[0080] Preferably, the BIOS boot process includes mechanisms to verify the integrity of the boot process itself. Such mechanisms are already known from, for example, Intel's draft “Wired for Management baseline specification v 2.0-BOOT Integrity Service”, and involve calculating digests of software or firmware before loading that software or firmware. Such a computed digest is compared with a value stored in a certificate provided by a trusted entity, whose public key is known to the BIOS. The software/firmware is then loaded only if the computed value matches the expected value from the certificate, and the certificate has been proven valid by use of the trusted entity's public key. Otherwise, an appropriate exception handling routine is invoked.
[0081] Optionally, after receiving the computed BIOS digest, the trusted device 24 may inspect the proper value of the BIOS digest in the certificate and not pass control to the BIOS if the computed digest does not match the proper value. Additionally, or alternatively, the trusted device 24 may inspect the Boolean value and not pass control back to the BIOS if the trusted device 24 was not the first memory accessed. In either of these cases, an appropriate exception handling routine may be invoked.
[0082] Optionally, as shown in FIG. 6 , to provide control and support to the computer platform 10 a system management counsel (SMC) 60 is coupled to computer platform 10 via connection 62. In one embodiment, SMC 60 includes separate independent processors (not shown), such as standard non-networked personal computers (PCs). Connection 62 can include serial interfaces (e.g., RS-232 and USB), and/or private LAN connections. SMC 60 is primarily employed to authenticate SPK 32 during computer platform 10 initialization. In addition, computer platform 10 is configured via SMC 60. In one embodiment, SMC 60 performs remote debugging for SPK 32 and SPGS 33.
[0083] In one embodiment, GUI interfaces for system control and management are only implemented on SMCs 60. This embodiment permits development and testing of system management interfaces and human factors in parallel with development of the rest of computer platform 10, without having to wait for the entire computer platform 10 to be brought up.
[0084] More than one SMC 60 can be coupled to computer platform 10 via serial interface and/or LAN connection 62. In one embodiment, SMC 60 functions are integrated into SPGS 33 in a computer platform 10 having a single processor, such as a workstation.
[0085] Additionally, the trust device 24 could be located in the SMC and act as the trusted device remotely to the computer platform 10.
[0086] Once the trusted device 24 has initiated a trusted boot-up sequence, as described above, it is still necessary to ensure the chain of trust is maintained through to the initialisation of the operating domains. Therefore, in addition to utilising the trusted device 24 to provide information as to whether the computer platform can be trusted it is necessary to determine that a users operating environment can be trusted.
[0087] Accordingly, once the trusted device 24 has passed control to the BIOS firmware 28 the SPA 31 is arranged to provide a trusted operating environment as described below.
[0088] Initially on passing control to the BIOS firmware 28 the BIOS firmware 28, inter alia, boots up and authenticates the EFI.
[0089] An EFI file system stores a secure platform (SP) loader, a system configuration database (SCD), a SPK image 32, and a SPGS image 33. The EFI loads SP loader from EFI file system into memory 25. The EFI authenticates this image using the processor 21 manufacturer's public key. This authentication requires that SP loader be digitally signed with the processor 21 manufacturer's private key.
[0090] The EFI then transfers control to SP loader stored in memory 25. SP loader is an EFI-based secondary loader which is secure platform specific. SP loader is responsible for loading SP images into memory 25.
[0091] In one embodiment, it is possible for execution to be transferred to an EFI shell prompt to enable initial system installation and other administrative details, which breaks the SP chain of trust. In this case, the EFI recognizes that trust was lost and does not precede with loading SP loader. Instead, computer platform 10 resets so that all processors 21 will again start fetching instructions from trusted device 24.
[0092] SP loader running from memory 25 loads the SCD from EFI file system into memory 25. SP loader then authenticates SCD employing a public key contained in the SP loader image. SP loader employs SCD to determine which SPK 32 and SPGS 33 images to load from EFI file system into memory. SP loader employs the above public key for authenticating the SPK 32 and SPGS 33 images. SP loader creates a virtual mapping for an entry area of SPK 32 with read and execute only permissions. SP loader then switches to virtual mode and branches to the SPK 32 entry point.
[0093] In the boot sequence for bringing up SPK 32, SPK 32 running from memory 25 on processor 21, initialises privilege state (e.g., interruption vector table (NT), control registers, and some interrupt configuration) and creates any other additional memory mappings required for SPK 32, such as writeable areas for SPK data. SPK 32 then creates any required memory mappings and any additional set up required to run SPGS 33.
[0094] A secure platform (SP) 34 mirrored file system stores two redundant control block images. SPK 32 reads the two redundant control block images from SP mirrored file system into SPK 32 in memory 25 as redundant control block images. The two redundant control block images contain control information initialized at the very first computer platform 10. The redundant control block images are employed to test whether computer platform 10 has already been initialized.
[0095] In one embodiment, the redundant control block images each contain at least three distinct control areas. First control area contains an image that also is signed by the processor 21 manufacturer's public key, which was written when computer platform 10 booted for the first time. First control area is employed to store a root system key (RSK) in second control area. Second control area contains the RSK encrypted under itself. Second control area is employed to validate that a correct RSK has been supplied on subsequent boots. Encrypting the RSK under itself permits validation of the RSK, by comparing the results with the value already stored in second control area. Third control area contains a top-level directory of platform control information, including keys, pseudo random number generator (PRNG) state, and last entropy pool snapshot, all encrypted and integrity checked by the RSK. SPK 32 typically has minimal or no I/O capability. In one embodiment the SP loader performs I/O accesses prior to transfer of control to SPK 32. In another embodiment, SPGS 33 is brought up to an I/O ready state prior to the I/O operation to read from the disk, and returns control to SPK 32. In another embodiment, SPGS 33 loads memory 25 and then a call is made to SPK 32 which performs the above operation.
[0096] SPK 32 determines whether the control areas of the two redundant control block images agree and the digital signature checks. If the control areas disagree, the control areas of the redundant control block image whose integrity checks as valid are used, and the control areas of the other redundant control block whose integrity checks as invalid are restored to match the used control areas of the valid redundant control block image. If the control areas of both redundant control block images are damaged, logs are used to recover, similar to many database systems, and to restore the control areas of both redundant control block images. Once the RSK is obtained, the boot process continues.
[0097] SPK 32 reads and decrypts protection keys from the SP mirrored file system.
[0098] The initial SPGS 33 domain initializes and performs discovery of I/O to include access to SMC 60. The initial SPGS 33 domain loads an encrypted SCD from the SP mirrored file system. The initial SPGS 33 domain requests SPK 32 to decrypt the encrypted SCD. The decrypted SCD specifies the number of SPGS 33 domains to create and which system resources belong to which SPGS 33 domain. The initial SPGS 33 domain then creates each additional SPGS 33 domain specifying the corresponding subset of system resources to include in the processor 21 in which the SPGS 33 domain is run on.
[0099] Each SPGS 33 domain similarly reads the decrypted SCD and creates the specified domains. Each SPGS created domain includes the following. System resources are allocated to each SPGS 33 domain on a per domain basis. A domain initial image (DII) is loaded from EFI file system into memory 25 as DII. DII is typically an operating system specific loader for initiating the loading of an operating system for a specific domain in PL2. If SCD indicates that the given SPGS 33 domain is a secure domain, the self-contained public key of SP loader is employed to authenticate DII. Thus, DIIs which are to run in secure SPGS 33 domains are preferably digitally signed with the SP loader's private key. One use of a non-secure SPGS 33 domain is to allow development and debugging of DIIs.
[0100] On creation of each of the specified domains an associated virtual trusted device is created in the SPK 32.
[0101] As the virtual trusted devices 37 are executed in the SPK 32, which runs at the PL0 level the only level that executes privileged instructions, the virtual trusted devices 37 can effectively be isolated from software executed in the other processor privilege levels. Accordingly, as the SPK 32 is trusted code a user can be confident that the virtual trusted devices are shielded from non-trusted software.
[0102] Each virtual trusted device 37 comprises, as shown in FIG. 7 , a central routine 70 for controlling the overall operation of the virtual trusted device; a measurement function 71 for acquiring an integrity metric for an associated operating environment and obtaining the integrity metric acquired by the trusted device 24 and makes measurements on software that is to be executed in the associated operating environment; a cryptographic function 72 for signing, encrypting or decrypting specified data. Additionally, each virtual trusted device 37 is able to verify the integrity metric acquired by the trusted device 24 using the trusted third parties public key. The virtual trusted devices 37 have access to memory associated with the PL0 level. Additionally, each virtual trusted device 37 is arranged to be isolated from any other virtual trusted device 37 that is associated with a separate operating environment.
[0103] On creation of an associated operating environment in PL1 the associated virtual trusted device 37 in PL0 is issued with a certificate that is associated with the user of the operating environment.
[0104] Each virtual trusted devices 37 certificate is stored in local memory in the PL0 level. The certificate contains a public key of the respective virtual trusted device 37 and, optionally, an authenticated value of an integrity metric for measured by a trusted third party to allow verification of the integrity metric acquired by the trusted device 24. The certificate is signed by the trusted third party, using the trusted third parties private key, prior to the certificate being stored in the virtual trusted device 37, thereby confirming that the trusted third party vouches for the virtual trusted device 37. In this embodiment possible trusted third parties could be either the physical trusted device 24 or the SMC 60.
[0105] As described below, a user, on accessing a virtual trusted device 37 associated with the respective operating environment, can obtain the computer platform integrity metric acquired and signed by the trusted device 24 with the trusted device's 24 private key and the integrity metric measured and signed by the virtual trusted device 37 and the virtual trusted device's 37 private key for the respective operating environment. Accordingly, the user is able to obtain all the integrity metric information required to allow verification that the respective operating environment can be trusted from the virtual trusted device 37 without the user needing to access the trusted device 24 directly.
[0106] As virtual trusted devices 37 are created and destroyed on the creation and destruction of operating environments it is necessary to ensure that their transitory existence does not compromise the trustworthiness of either the computer platform 10 or associated operating environments. As such, to ensure that trust can be maintained it is essential that secrets associated with the virtual trusted device(s) 37 do not exist in more than one active trusted device at any given time. This requires that strict and reliable methods in the computer platform 10 ensure that on the creation and destruction of a virtual trusted device 37 only one copy of relevant secrets (e.g. for example private keys) are maintained.
[0107] As such, destruction of a virtual trusted device 37 requires the permanent, safe, secret destruction of the virtual trusted devices secrets. If a virtual trusted device 37 is to be stored for re-use at a later date it secrets must be safely and secretly preserved for future use.
[0108] The secrets belonging to the virtual trusted device 37 could be stored in the physical trusted device 24 or SMC 60 using the protected storage facilities of a trusted platform module, for example. Virtual trusted device 37 secrets can be safely stored using the trusted computer platform association (TPCA) maintenance process.
[0109] For operating environments that need to continue to exist despite the computer platform 10 having to be power down and back up again it is possible to reassemble the stored associated virtual trusted device 37. This allows the same virtual trusted device 37 to be maintained for the same operating environment, despite the temporary closing down of the operating environment.
[0110] However, the method required to reassemble a virtual trusted device 37 depends on the method used to dismantle the initial virtual trusted device 37.
[0111] If a virtual trusted device 37 has been saved using the TCPA maintenance process, as described in section 7.3 of the TCPA specification, a new virtual trusted device 37 and trusted platform (i.e. operating environment) must be created (e.g. new endorsement key, credentials can be provided via the virtual trusted devices certificate). The TCPA maintenance process is used to transfer the appropriate secrets of the virtual trusted device to the new virtual trusted device 37 in the new operating environment. This is a two-step process, requiring first that the owner/user of the new operating environment check that the new virtual trusted device 37 and operating environment have at least the same level of security as the original virtual trusted device 37 and operating environment, such that the existing credentials do not overstate the security properties of the new virtual trusted device 37 and associated operating environment.
[0112] If the previous virtual trusted device 37 has been saved in full, a blank virtual trusted device 37 and associated operating environment are created in PL0 and PL1 respectively and the original secrets stored from the original virtual trusted device 37 are loaded into the new virtual trusted device. As above, the new operating environment must be checked that the new virtual trusted device 37 and operating environment have at least the same level of security as the original virtual trusted device 37 and associated operating environment, such that the existing credentials do not overstate the security properties of the new virtual trusted device 37 and operating environment. If a SMC 60 holds the secrets, some separate security service is required to confidentially communicate the secrets from the SMC 60 to the computer platform 10. This will require a key distribution service, as is well known to a person skilled in the art.
[0113] This allow multiple operating environments to be created, where each operating environment has its own associated virtual trusted device 37 such that each virtual trusted device 37 derives the integrity metric for the computer platform 10 from the trusted device 24 and additionally measures an integrity metric for the associated operating environment. This allows a computer platform 10 to have multiple users, each with their own respective operating environment, where each operating environment is isolated from each other and each operating environment can provide an integrity metric for both itself and the computer platform 10. This allows a user of an operating environment to determine whether his respective operating environment can be trusted without requiring any information as to whether any other operating environment is running on the computer platform 10.
[0114] Additionally, as each domain is isolated and the virtual trusted devices 37 are executed in a privileged processor level PL0 rouge software executed in one domain can not attack software executed in another domain.
[0115] FIG. 8 illustrates a computer platform 10 having a trusted device 24 with BIOS and SPK code installed on processor 21. The computer platform 10 is acting as a server having three operating environments 80′, 80″, 80′″ executed in privilege level 1 where each user would typically communicate with the operating environment 80′, 80″, 80′″ via a network connection. Each of the operating environments 80′, 80″, 80′″ has their own respective virtual trusted device 37′, 37″, 37′″ executed in the SPK 32 at privilege level PL0. Each virtual trusted device 37′, 37″, 37′″ has their own unique certificate (not shown) for their respective operating environment. When a user for each operating environment 80′, 80″, 80′″ wishes to communicate with their respective operating environment 80′, 80″, 80′″ they create a nonce (not shown), such as a random number, and, issue a challenge to their respective virtual trusted device 37′, 37″, 37′″. The nonce is used to protect the user from deception caused by replay of old but genuine signatures (called a ‘replay attack’) by untrustworthy platforms. The process of providing a nonce and verifying the response is an example of the well-known ‘challenge/response’ process.
[0116] The respective virtual trusted device 37′, 37″, 37′″ receives the challenge and creates an appropriate response. This may be a digest of the measured integrity metric of the computer platform integrity metric received from the trusted device 24 and signed with the trusted device's 24 private key and the measured integrity metric for the respective operating environment 80′, 80″, 80′″ signed with the respective virtual trusted device's 37 private key and the nonce, and optionally its ID label. The respective trusted device 37′, 37″, 37′″ return the signed integrity metric, accompanied by the respective virtual trusted devices 37′, 37″, 37′″ certificate and the trusted device's 24 certificate 350, to the user.
[0117] The user receives the challenge response and verifies the certificate using the well known public key of the TP(s). The user then extracts the virtual trusted device's 37′, 37″, 37′″ public key and the trusted device's 24 public key from the certificate and uses them to decrypt the signed integrity metrics from the challenge response. Then the user verifies the nonce inside the challenge response. Next the user compares the computed integrity metrics, which it extracts from the challenge response, with the proper platform integrity metrics, which in this embodiment are extracted from the certificates. If any of the foregoing verification steps fails the whole process ends with no further communications taking place.
[0118] Assuming all is well the user and the trusted platform use other protocols to set up secure communications for other data, where the data from the platform is preferably signed by the trusted device 37′, 37″, 37′″ without any knowledge of the other two operating environments installed on the computer platform 10.
Claims
What is claimed:
1. A computer apparatus for creating a trusted environment comprising:
a trusted device arranged to acquire a first integrity metric to allow determination as to whether the computer apparatus is operating in a trusted manner, said trusted device being a physical device which binds an identity of the computer apparatus to first reliably measured data by means of said first integrity metric;
a software trust routine which binds an identity of at least a first operating system environment to second reliably measured data by means of at least a second integrity metric;
a processor arranged to allow execution of said software trust routine and said at least a first operating environment, and
means for restricting a privilege level of the at least a first operating system environment to a lower privilege level than that assigned to the software trust routine, wherein the software trust routine is arranged to acquire (i) the first integrity metric of said trusted device as an indicator of whether a user of said computer apparatus should trust that said computer apparatus is operating consistently with said first reliably measured data and (ii) the second integrity metric to allow determination by the user as to whether the at least a first operating system environment is operating in a trusted manner consistently with said second reliably measured data.
2. The computer apparatus of claim 1 wherein the trusted device includes means for controlling at least an initiation of a boot process when said computer apparatus is rebooted.
3. The computer apparatus of claim 1 wherein the trusted device is arranged on a motherboard of said computer apparatus along with at least said processor.
4. The computer apparatus of claim 3 wherein the trusted device includes a controller programmed to control its overall operation and to interact with other devices on said motherboard.
5. The computer apparatus of claim 1 wherein the trusted device comprises an integrated circuit device.
6. A computer apparatus for creating a trusted environment, the computer apparatus comprising:
a trusted device arranged to acquire a first integrity metric to allow determination as to whether the computer apparatus is operating in a trusted manner, said trusted device being a physical device which binds an identity of the computer apparatus to reliably measured data by means of said first integrity metric, and
a processor arranged to allow execution of a first trust routine and an associated first operating environment, the processor is arranged to restrict access of the first operating environment from resources available to the trust routine, wherein the trust routine is arranged to acquire the first integrity metric and is further arranged to acquire a second integrity metric to allow determination as to whether the first operating environment is operating in a trusted manner,
wherein the trust routine, in response to acguiring the first integrity metric of said trusted device, indicates to a user of said computer apparatus whether the user should trust that said computer apparatus is operating in its trusted manner, and
wherein the trust routine, in response to acquiring the second integrity metric, indicates to the user whether the user should trust that the first operating system environment is operating in its trusted manner.
7. A computer apparatus for creating a trusted environment, the computer apparatus comprising:
a trusted device arranged to acquire a first integrity metric to allow determination as to whether the computer apparatus is operating in a trusted manner, said trusted device being a physical device which binds an identity of the computer apparatus to reliably measured data by means of said first integrity metric, and
a processor arranged to allow execution of a plurality trust routines in a first privilege level of the processor and to allow execution of respective associated operating environments in a second privilege level of the processor such that access to resources available to code executed in the first privilege level is restricted to code executed in the second privilege level, wherein each trust routine is arranged to acquire the first integrity metric and an integrity metric associated with the respective associated operating environment to allow determination as to whether the respective operating environment is operating in a trusted manner.
8. A computer apparatus according to claim 7, wherein the trusted device is a tamper resistant device.
9. A computer apparatus according to claim 7, wherein the trust routine is arranged to incorporate cryptographical functionality for restricting access to data associated with the trust routine.
10. A computer apparatus according to claim 7, wherein the trusted device is arranged to store one or more secrets.
11. A computer apparatus according to claim 10, wherein at least one secret is a private asymmetric encryption key.
12. A computer apparatus according to claim 7, wherein one or more secrets are associated with the trust routine.
13. A computer apparatus according to claim 12, wherein the trusted device is arranged to store secrets, on powering down of the computer apparatus, utilised by a trust routine.
14. A computer apparatus according to claim 7, wherein the code executed in the second privilege level has restricted access to data associated with a trust routine executed in the first privilege level.
15. A computer apparatus according to claim 7, wherein data associated with the trust routine is protected from alteration from code executed in the second privilege level.
16. A computer apparatus according to claim 7, wherein secrets associated with a trust routine are not accessible to code executed in the second privilege level.
17. A computer apparatus according to claim 7, wherein the trusted device vouches for a public key associated with the trust module using a private key of the trusted device.
18. A computer apparatus according to claim 7, wherein a trusted third party vouches for a public key associated with the trust module using a private key of the trusted parties.
19. A computer apparatus according to claim 7, wherein the trusted device is arranged to transfer boot-up sequence instructions to allow initiation of boot-up of the computer apparatus.
20. A computer apparatus according to claim 19, wherein the processor is arranged to cause the boot-up sequence instructions to be the first instructions executed by the processor after release from reset.
21. The method of claim 7,
wherein the trust routine, in response to acquiring the first integrity metric of said trusted device, indicates to a user of said computer apparatus whether the user should trust that said computer apparatus is operating in its trusted manner, and
wherein the trust routine, in response to acquiring the second integrity metric, indicates to the user whether the user should trust that the first operating system environment is operating in its trusted manner.
22. A method for creating a trusted environment in a computer apparatus, the method comprising
acquiring a first integrity metric to allow determination as to whether a computer apparatus has a trusted device binding an identity of the computer apparatus to first reliably measured data by means of said first integrity metric;
acquiring a second integrity metric to allow determination as to whether a computer apparatus has a first trusted operating environment binding an identity of the first trusted operating environment to second reliably measured data by means of said second integrity metric;
executing a first trust routine which acquires (i) the first integrity metric as an indicator of whether a user of said computer apparatus should trust that said computer apparatus is operating consistently with said first reliably measured data and (ii) acquires the second integrity metric to allow determination by the user as to whether the at least a first operating system environment is operating in a trusted manner consistently with said second reliably measured data, and
restricting a privilege level of the first operating environment to a lower privilege level than that assigned to the trust routine.
23. A method for creating a trusted environment in a computer apparatus, the method comprising:
acquiring a first integrity metric to allow determination by a user of the computer apparatus as to whether the computer apparatus is operating in a trusted manner based upon externally obtained reliable data and executing a first trust routine and an associated first operating environment,
restricting the first operating environment's access from resources available to the trust routine,
acquiring the first integrity metric and a second integrity metric to allow determination by said user as to whether the first operating environment is operating in a trusted manner also based upon externally obtained reliable data.
24. A computer system for creating a trusted environment comprising:
a physical trusted device which is installed in said computer system and is arranged to acquire a first integrity metric to allow determination as to whether the computer system is operating in a trusted manner;
a processor arranged to allow execution of a first trust routine and an associated first operating environment, and
means for restricting access of the first operating environment from resources available to the trust routine, wherein the trust routine is arranged to acquire (i) the first integrity metric to allow a determination to be made by a user of the computer system whether the hardware of the computer system may be utilized in a trusted manner based upon externally obtained reliable data and (ii) a second integrity metric to allow a determination by said user whether the first operating environment is operating in a trusted manner based upon externally obtained reliable data.
25. A computer apparatus for creating a trusted environment comprising: a physical trusted device arranged to acquire a first integrity metric to allow determination as to whether the computer apparatus is operating in a trusted manner, said physical trusted device binding an identity of the computer apparatus to reliably measured data by means of said first integrity metric;
a virtual trusted device arranged to acquire a second integrity metric to allow determination as to whether the computer apparatus has an operating system capable of operating in a trusted manner, said virtual trusted device binding an identity of the operating system to reliably measured data by means of said second integrity metric;
a processor arranged to allow execution of said operating system and at least one application program, said processor having at least four different privilege levels of operation, PL0, PL1, PL2 and PL3, with PL0 being the highest of these four privilege levels and PL4 being the lowest of these four privilege levels, the processor also having certain privileged instructions which may be used only at PL0;
a security kernel which runs at privilege level PL0, the security kernel having code allowing execution of the virtual trusted device and for accessing the physical trusted device, code for trap and interrupt handling as well as code for memory management;
a global service module which runs at privilege level PL1, the global service module having code which does need to respond to trap and interrupt handling but which handles interfacing between the operating system and the security kernel; and
means for restricting the at least one application program to a privilege level no higher than privilege level PL3, and means for restricting the operating system to a privilege level no higher than PL2.
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