Patent Publication Number: US-7587595-B2

Title: Method and apparatus for providing software-based security coprocessors

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/681,094, filed on May 13, 2005, and entitled Methods and Apparatus Pertaining to Software-Based Security Coprocessors. 
    
    
     FIELD OF THE INVENTION 
     The present disclosure relates generally to the field of data processing, and more particularly to methods and related apparatus to support secure virtualization. 
     BACKGROUND 
     A conventional processing system may include hardware resources, such as a central processing unit (CPU) and random access memory (RAM), as well as software resources, such as an operating system (OS) and one or more end-user programs or applications. An application is typically developed to run on a particular OS. When a typical conventional computer system is started, it loads the OS before loading the end-user programs or applications. The OS typically serves as an intermediary between software applications and the hardware in a processing system. 
     In addition to RAM and one or more CPUs, a processing system may include a security coprocessor (SC) such as a trusted platform module (TPM). A TPM is a hardware component that resides within a processing system and provides various facilities and services for enhancing the security of the processing system. For example, a TPM may be implemented as an integrated circuit (IC) or semiconductor chip, and it may be used to protect data and to attest to the configuration of a platform. 
     A TPM may be implemented in accordance with specifications such as the Trusted Computing Group (TCG) TPM Specification Version 1.2, dated Oct. 2, 2003 (hereinafter the “TPM specification”), which includes parts such as Design Principles, Structures of the TPM, and TPM Commands. The TPM specification is published by the TCG and is available from the Internet at www.trustedcomputinggroup.org/home. 
     The sub-components of a TPM may include an execution engine and secure non-volatile (NV) memory or storage. The secure NV memory is used to store sensitive information, such as encryption keys, and the execution engine protects the sensitive information according to the security policies dictated by the TPM&#39;s control logic. 
     In general, a TCG-compliant TPM provides security services such as attesting to the identity and/or integrity of the platform, based on characteristics of the platform. The platform characteristics typically considered by a TPM include hardware components of the platform, such as the processor(s) and chipset, as well as the software residing in the platform, such as the firmware and OS. A TPM may also support auditing and logging of software processes, as well as verification of platform boot integrity, file integrity, and software licensing. It may therefore be said that a TPM provides a root of trust for a platform. 
     Accordingly, when a processing system such as a server handles requests from other processing systems such as clients, the server may enforce security policies based on TPM-based attestation. For instance, the server may be configured to deny requests from any client system unless those requests are accompanied by valid, TPM-based platform attestation from that client system. When a conventional processing system uses a TPM, however, that processing system may be able to support only one software environment at a time. 
     Virtualization products provide features for partitioning a processing system into multiple virtual machines (VMs). For instance, virtualization products may partition and manage a processing system&#39;s hardware resources in a way that allows multiple OSs to execute on the same machine concurrently. Specifically, each OS may run in a different VM. Each VM may therefore be considered a substantially independent software environment. An OS running in a VM may be referred to as a guest OS. The VMs may be managed by virtualization products such as a virtual machine monitor (VMM) or hypervisor. 
     As recognized by the present invention, it would be advantageous if a VMM could allow each of the OSs to operate substantially as if that OS were in its own independent physical machine. U.S. patent application Ser. No. 10/876,944 (“the &#39;944 application”), which is assigned to the same entity as the present application, discusses features to support use of TPMs by VMs. The present application discloses additional features and capabilities relating to TPMs and virtualization. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features and advantages of the present invention will become apparent from the appended claims, the following detailed description of one or more example embodiments, and the corresponding figures, in which: 
         FIG. 1  is a block diagram depicting a suitable data processing environment in which certain aspects of an example embodiment of the present invention may be implemented; 
         FIG. 2  presents a block diagram depicting an example TPM, with regard to various structures and keys to support functions such as TPM virtualization; 
         FIG. 3  is a block diagram showing various components of an example generalized virtual TPM framework and related items; 
         FIG. 4  presents a block diagram of an example embodiment of a virtual TPM double wrapped key; and 
         FIG. 5  is a block diagram illustrating example components and operations to prepare a virtual manufacturer authority to provide security attestation for a virtual TPM. 
     
    
    
     DETAILED DESCRIPTION 
     Platform partitioning technologies, such as technologies for creating VMs, have recently received increased attention for their potential security value. Virtualization technologies allow a platform to be split into VMs, with each VM possibly running less software than today&#39;s complex platforms. Separately, as indicated above, the TCG has provided a standard for hardware augmented security to facilitate the creation of processing systems that can be classified as trusted platforms. A TPM may provide the root of trust for a trusted platform. 
     It would be advantageous if all software in a given VM could be identified, and if a TPM could meaningfully attest to all software in that VM. However, sharing a TPM for use by multiple VMs is difficult, at least in part because of the stateful and opaque nature of the TPM. 
     A well-written VMM should prevent malicious software running in one VM from tampering with software running in another VM. Additionally, using the TCG trusted boot model, it would be advantageous if a TPM could measure the OS and applications in each VM to provide data protection for the OS and applications as well as to facilitate attestation to remote entities. 
     Unfortunately, the measurement facilities of the TPM are designed to store the measurements of exactly one operating system. Conventional TPMs lack the ability to separately store measurements of multiple concurrently running OSs. Furthermore, due to the closed, smartcard-like nature of the TPM, its state cannot be read or swapped out. As a result, traditional techniques for device sharing or virtualization cannot be used for TPMs. 
       FIG. 1  is a block diagram depicting a suitable data processing environment  12  in which certain aspects of an example embodiment of the present invention may be implemented. Data processing environment  12  includes a processing system  20  that includes one or more processors or central processing units (CPUs)  22  communicatively coupled to various other components via one or more system buses  24  or other communication pathways or mediums. 
     As used herein, the terms “processing system” and “data processing system” are intended to broadly encompass a single machine, or a system of communicatively coupled machines or devices operating together. Example processing systems include, without limitation, distributed computing systems, supercomputers, high-performance computing systems, computing clusters, mainframe computers, mini-computers, client-server systems, personal computers, workstations, servers, portable computers, laptop computers, tablets, telephones, personal digital assistants (PDAs), handheld devices, entertainment devices such as audio and/or video devices, and other devices for processing or transmitting information. 
     Processing system  20  may be controlled, at least in part, by input from conventional input devices, such as keyboards, mice, etc., and/or by directives received from another machine, biometric feedback, or other input sources or signals. Processing system  20  may utilize one or more connections to one or more remote data processing systems  76 - 78 , such as through a network interface controller (NIC), a modem, or other communication ports or couplings. Processing systems may be interconnected by way of a physical and/or logical network  80 , such as a local area network (LAN), a wide area network (WAN), an intranet, the Internet, etc. Communications involving network  80  may utilize various wired and/or wireless short range or long range carriers and protocols, including radio frequency (RF), satellite, microwave, Institute of Electrical and Electronics Engineers (IEEE) 802.11, Bluetooth, optical, infrared, cable, laser, etc. 
     Within processing system  20 , processor  22  may be communicatively coupled to one or more volatile or non-volatile data storage devices, such as random access memory (RAM)  26 , read-only memory (ROM), mass storage devices such as integrated drive electronics (IDE) hard drives, and/or other devices or media, such as floppy disks, optical storage, tapes, flash memory, memory sticks, digital video disks, biological storage, etc. For purposes of this disclosure, the term “ROM” may be used in general to refer to non-volatile memory devices such as erasable programmable ROM (EPROM), electrically erasable programmable ROM (EEPROM), flash ROM, flash memory, etc. Processor  22  may also be communicatively coupled to additional components, such as video controllers, small computer system interface (SCSI) controllers, network controllers, universal serial bus (USB) controllers, input devices such as a keyboard and mouse, etc. Processing system  20  may also include one or more bridges or hubs  34 , such as a memory controller hub, an input/output (I/O) controller hub, a PCI root bridge, etc., for communicatively coupling various system components. As used herein, the term “bus” may be used to refer to shared communication pathways, as well as point-to-point pathways. 
     Some components, such as a NIC for example, may be implemented as adapter cards with interfaces (e.g., a PCI connector) for communicating with a bus. In one embodiment, one or more devices may be implemented as embedded controllers, using components such as programmable or non-programmable logic devices or arrays, application-specific integrated circuits (ASICs), embedded computers, smart cards, and the like. 
     As illustrated, processing system  20  also includes a TPM  30  communicatively coupled to processor  24 . TPM  30  may also be referred to as a physical TPM or hardware TPM (hwTPM)  30 . In one embodiment, TPM  30  is implemented as an embedded device, residing on a system motherboard or backplane of processing system  20 . TPM  30  includes several storage facilities, including volatile platform configuration registers (PCRs) and authorization sessions, as well as persistent data integrity registers (DIRs), authorization digests, and general use persistent storage. Each of these facilities may have a corresponding in-memory data structure. 
     The invention may be described by reference to or in conjunction with associated data including instructions, functions, procedures, data structures, application programs, etc., which, when accessed by a machine, result in the machine performing tasks or defining abstract data types or low-level hardware contexts. The data may be stored in volatile and/or non-volatile data storage. 
     For instance, RAM  26  may include one or more sets of instructions which, when executed, implement a generalized virtual TPM (GVTPM) framework  40  to support secure virtualization of TPM  30 . GVTPM framework  40  may also be referred to as a virtual TPM service. For purposes of this disclosure, a virtual TPM (vTPM) is a logical (i.e., primarily software-implemented) component that provides TPM-like functionality. Likewise, a virtual security coprocessor (vSC) is a logical device which provides functionality like that which could potentially be provided by a hardware security coprocessor. In one example embodiment, GVTPM framework  40  may operate partially or completely from within a VMM  64 . In another embodiment, GVTPM framework  40  resides in one or more service VMs supported by the VMM. The service VMs may be referred to as lightweight VMs, since they may require fewer resources than a VM with a guest OS. In alternative embodiments, some or all of the modules for GVTPM framework  40  may reside in the firmware or any other protected environment. Different embodiments of the GVTPM framework may provide virtual TPM services for a wide variety of VMM architectures. In other embodiments, GVTPM framework  40  may not be part of a VMM at all. 
     In the example embodiment, processing system  20  may load VMM  64  into RAM  26  at boot time or at some later time to support one or more VMs  60 A- 60 C within processing system  20 . VMM  64  may be implemented through execution of software or firmware components such as a micro-kernel and a service OS. The micro-kernel may include a small nucleus of instructions for system management tasks such as instruction scheduling. The service OS may include device drivers and environment virtualization software for creating and maintaining VMs. The device drivers in the service OS may include a TPM driver for communicating with TPM  30 . Alternatively, as illustrated in  FIG. 3 , the TPM driver  130  may be loaded into a different software component, such GVTPM manager  110 . Processing system  20  may load the instructions that implement VMM  64  and GVTPM framework  40  from ROM and/or from one or more local or remote mass storage devices, for instance. Any additional instructions used to support or facilitate TPM virtualization may also be loaded from ROM and/or from one or more local or remote mass storage devices, for instance. In the example embodiment, VMM  64  supports multiple VMs  60 A- 60 C, each running its own independent guest OS. One or more of the VMs may run a trusted software stack or TCG software stack (TSS), in compliance with TCG standards. 
     For purposes of this disclosure, vTPMs, proprietary vSCs, and similar virtual devices may be referred to as device models (DMs). In the example embodiment, such device models are supported by GVTPM framework  40 . In addition, GVTPM framework  40  may support multiple DM designs. For example, as described in greater detail below with regard to  FIG. 3 , GVTPM framework  40  may create vTPMs based on one DM design, and GVTPM framework  40  may create proprietary vSCs based on another DM design. Thus different DM designs may be used to facilitate virtualization of different types of security coprocessors. GVTPM framework  40  may thus support vTPMs and other vSCs with different security and performance tradeoffs. Since the vSCs are not limited to vTPMs, GVTPM framework  40  may also be referred to as a virtual security coprocessor (vSC) framework. 
     As illustrated in  FIG. 1 , in one embodiment, processing system  20  includes a data storage device  28  containing one or more VM definitions  41 ,  51 . In one embodiment, the VM definitions may reside on a hard disk drive (HDD). In alternative embodiments, the VM definitions may reside in other types of storage devices. For instance, VM definitions may be retrieved from a remote system and loaded into RAM  26  or into cache memory of processor  22 . A VM definition may define the attributes to be included in a virtual machine. For instance, when VMM  64  determines that is should create VM  60 A, processing system  20  may treat VM definition  41  like a boot block, with GVTPM framework  40  measuring VM definition  41 , and then VMM  64  passing control to initialization instructions or boot code within VM definition  41 . VM  60 A may be instantiated, at least in part, through execution of that boot code. In particular, vTPM  44 A may be created for VM  60 A based in control logic and/or initialization data within VM definition  41 . As described in greater detail below, that control logic may constitute or include the program code segment (PCS)  45  for implementing vTPMs. Similarly, VM definition  51  with the corresponding PCS  55  may be used to create VM  60 C and the corresponding vSC  44 C. 
     In the example embodiment, GVTPM framework  40  operates from protected host memory. For example, processing system  20  may use technology such as that described in U.S. Pat. Nos. 6,507,904; 6,633,963; and/or 6,678,825 (all assigned to Intel Corporation) to load GVTPM framework  40  into, and execute GVTPM framework  40  from, an isolated area of memory that is protected by hardware from access or tampering from software in other partitions. In alternative embodiments, other techniques may be used to provide protected memory. For instance, an environment may include a system management mode (SMM) that provides protected memory, or a protected execution environment could be created using a tamper-resistant software compiler. Other components (e.g., VMM  64 , the microkernel, etc.) may also reside in protected memory areas. In the example embodiment, the protected memory ensures that the software/instructions can run without interference or observation. 
     The protected memory may also serve to prevent unauthorized programs from accessing or tampering with sensitive information. For example, as described in greater detail below, GVTPM framework  40  may create a virtual TPM  44 A to emulate a hardware TPM for VM  60 A. GVTPM framework  40  may use protected memory to store and protect data stored in structures of vTPM  44 A. 
     As illustrated in  FIG. 3 , GVTPM framework  40  may include a protected storage service (PSS)  100 , and PSS  100  may use TPM  30  to protect vTPMs even when the vTPMs are not running. For example, when a vTPM is not actively operating, persistent data structures for that vTPM may be stored on disk and sealed to the PCRs of the vTPM service with the parent SRK. 
     In the example embodiment, vTPM  44 A is able to transparently provide TPM functionality both from itself and from hwTPM  30  under a single user authorization session. The vTPM  44 A accomplishes this objective by maintaining separate authorization sessions with both the user and the hwTPM. That is, the user will create an authorization session with vTPM  44 A as if vTPM  44   a  were a hwTPM. The vTPM  44 A may complete all the same authorization checks based on this session that a hwTPM would do. If vTPM  44 A can provide a requested function directly, vTPM  44 A may simply update the session nonces and reply back. If vTPM  44 A needs the hwTPM to provide the service, vTPM  44 A will create an authorization session or reuse an existing authorization session with the hwTPM to make the request. Once vTPM  44 A is done using the hwTPM, vTPM  44 A may update the nonces on the user&#39;s session and reply back. 
     In the example embodiment, GVTPM framework  40  provides an execution environment for trustworthy virtual TPMs (vTPMs)  44 A and  44 B, and/or for other TPM-like virtual components, such as vSC  44 C. In order to protect sensitive data used by vSCs  44 A- 44 C, GVTPM framework  40  uses TPM  30  to ensure that the trustworthiness of the vSCs are anchored in hardware that meets the expectation of software that uses a TPM. For purposes of this disclosure, the terms virtual TPM and vTPM are used to refer to software emulations or simulations of physical TPMs, as well as software emulations of similar kinds of security subsystems. 
     GVTPM framework  40  may allow multiple mutually distrustful and/or unaware guests to share the TPM without requiring modifications to guest OSs or to applications running on guest OSs. Additionally, GVTPM framework  40  may include features to provide the necessary environment for creating custom cryptographic subsystems with enhanced proprietary functionality. This disclosure also describes additional TPM features for optimizing virtualization under frameworks such as GVTPM framework  40 . 
     The remainder of this detailed description of one or more example embodiments proceeds as follows: The Security Background section provides background on the TPM. The Generalized Virtual TPM Framework section discussed an example framework. The Example vTPM Device Model Designs section describes two example vTPM designs or design models. The Hardware Optimizations section describes example hardware features that may facilitate TPM virtualization. Next are sections on Remote Deployment and Provisioning of Virtual TPMs and on Migrating Virtual TPMs. 
     1.0—Security Background 
     1.1—TPM Functional Introduction 
     The industry consortium TCG has standardized the TPM as a small cryptographic subsystem that promises to provide a foundation for trust on a platform. To this end, the TPM provides functions to facilitate attestation and protected storage. 
     The core of the TPM&#39;s functionality lies in its ability to store information about the platform&#39;s configuration. This information can then be used for both of the TPM&#39;s primary functions. The platform can provide information to a remote entity necessary to allow the remote entity to make decisions about the trustworthiness of the platform. The platform can also instruct the TPM to ensure that keys or sensitive data are only released while the system is in a known “good” configuration. 
     In order to store the platform state, a conventional TPM uses the PCRs to store measurements in the form of 160-bit SHA1 (secure hashing algorithm 1) hashes of software and configuration information for the platform. These measurements start at the boot block. Each boot component measures the next, records it in the TPM, and then launches that component until the operating system takes over the measurement of its core. Because each write to the PCRs adds a measurement to the register, rather than overwriting the previous measurement, no entity can change the measurement of its code made by the preceding component. Thus, a chain of measurements is made, such that if the beginning of the chain (known as the Root of Trust for Measurement) and each link are trustworthy, the entire chain is trustworthy. 
     1.2—Attestation 
     Attestation refers to the set of TPM functions and protocols that enable the platform to report its configuration to a remote party in a trustworthy manner. For example, the TPM provides the ability to sign the PCRs used to store the platform state. For instance, a platform may use an attestation identity key (AIK) to sign the PCRs. Such signed PCRs may be referred to as a quote. 
     To provide proof to a remote entity that the quote was signed by a real TPM, each TPM has a set of credentials. For instance, an endorsement credential signed by the TPM manufacturer states that the TPM meets the TPM specification. The manufacturer also stores a unique key known as the endorsement key (EK) in the TPM, and the manufacturer uses the EK to sign the endorsement credential. Theoretically, the EK could be used directly to sign a quote of the PCRs. However, since the EK is unique, a third party is used instead, to provide privacy. Specifically, the platform uses a third party known as the privacy certification authority (CA) to create an identity credential for each AIK. TCG has defined a protocol that allows the TPM to prove to the privacy CA that the TPM is a real TPM, using the EK and the endorsement credential. In turn, the privacy CA creates identity credentials for AIKs that the TPM claims it owns. 
     Assuming a remote entity trusts the manufacturer of the TPM, the privacy CA, and the root of trust for measurement, a quote signed by an AIK that is accompanied by an identity credential is cryptographic proof of the current state of the platform. For example, if a conventional attestation approach were used in a network environment similar to that illustrated in  FIG. 1 , a remote processing system such as privacy CA  76  could provide an identity credential for an AIK of a conventional TPM, and processing system  20  could use that identity credential to make a trustworthiness assertion to a remote processing system such as challenger  78 . As described in greater detail below, however, the present disclosure introduces a modified attestation approach to support virtual security processors or virtual TPMs for partitions such as virtual machines  60 A- 60 C in devices such as processing system  20 , for example. 
       FIG. 2  is a block diagram depicting an example TPM, such as TPM  30 , with regard to various structures and keys to support functions such as TPM virtualization. In  FIG. 2 , the EK of TPM  30  is illustrated as hwEK  52 , the storage root key is illustrated as hwSRK  50 , and the endorsement credential provided by the TPM manufacturer is illustrated as EK_Cred  54 .  FIG. 2  also depicts PCRs  32  and DIRs  36  within storage  38 . 
     1.3—Secure Storage 
     Another set of services the TPM provides is the secure storage of keys and other data. The TPM can create Rivest-Shamir-Adleman (RSA) keys, which it will only allow use of once (a) the requestor provides authorization via a secret SHA1 hash and (b) the current configuration, as determined by the PCRs, indicates a “good” state. This powerful function allows the platform to encrypt data such that, if the machine is compromised, booted from external media, or otherwise tampered with, the data will remain inaccessible. 
     To support services such as secure storage, the TPM creates keys with single-purpose types for different operations. The key of type EK is only available for decrypting identity credentials from the privacy CA. AIKs are used to sign other keys and to quote PCRs. Storage keys (SKs) are used to protect other keys or to “seal” data, which is a special encryption of data that protects the data with a password or PCR bindings. Binding keys (BKs) are used to encrypt arbitrary data, and to convert data into a TPM-bound data structure. Signing keys (SigKs) are used for signing arbitrary data. Lastly, legacy keys can sign or encrypt data and do not require that the data be in the form of a TPM-bound data structure. 
     Each TPM has two core keys, an EK and a special type of SK known as the storage root key (SRK). The SRK is the top of the hierarchy of keys that can be created by the system or users. This hierarchy is built on Storage Keys as branches and any of the other types, other than the EK type, as leaves. Of the keys in the hierarchy, only the SRK and EK are required to be loaded in the TPM at all times. Other keys may be stored outside the TPM encrypted as a “wrapped key,” and loaded prior to use. Wrapped keys are described in greater detail below. Once a key is loaded, it can perform any functions, provided that the key type is correct for the operations, proper authorization is given, and the PCRs match any PCR binding specified by the key. 
     1.4—TPM Components 
     TPMs, smartcards, the model 4758 cryptographic coprocessor from International Business Machines Corp., and other similar devices or subsystems are, in essence, small self-contained computing environments which generally contain perimeter protections such as tamper resistance. Consequently, such devices can be trusted to do certain computations without relying on external resources for operation. 
     A typical TPM includes the following four components.
         1. Program code segment (PCS): The code segment of a TPM&#39;s control logic, which is typically in ROM and stored as read-only data.   2. Processor: A small CPU which executes the PCS.   3. Non-volatile memory (NV memory or NVM): The NVM is the storage within the TPM where persistent keys, secrets, and other state of the TPM are stored. It is typically located in tamper-resistant flash, which is preserved across restarts.   4. Active memory: This is the volatile memory used to store non-persistent data that is lost on power off.       

     The technical capabilities of devices built from these primitives are limited primarily by their internal resources. In practice, as a result of efforts to avoid high development and deployment costs, these devices typically include only a few simple, general purpose constructs for data protection. For example, in a typical device, simple encryption and decryption functions are available, but sophisticated access control policies are not. The framework disclosed herein alleviates this problem by providing computational areas which do not have the same resource constraints, and which are inexpensive to develop and deploy. 
     2.0—Generalized Virtual TPM Framework 
       FIG. 3  presents a block diagram showing various components of an example GVTPM framework and related items. In the illustrated embodiment, GVTPM framework  40  includes several components which help to provide various functional and security properties of the TPM components. 
     Also, the GVTPM PSS  100  serves as the central repository for the NVM of each DM, while the platform&#39;s CPU  22  and RAM  26  provide the processor and active memory resources. Since PSS  100  provides protected storage for the persistent data of each DM, PSS  100  may also be referred to as protected persistent storage. In the example embodiment, GVTPM framework  40  imposes a security requirement that the platform will isolate the use of CPU  22  and RAM  26  to ensure the framework is protected from the rest of the platform. One way to meet the isolation requirement is to implement the components in a trusted virtual machine monitor (TVMM) or use a TVMM to isolate GVTPM framework  40  in its own VM. In the example embodiment, VMM  64  is generally accepted as a TVMM. Alternative embodiments may not store the NVM within the PSS. The PSS may instead encrypt the NVM similar to that above and may return the NVM to the vSCs. 
     Additionally, a GVTPM manager  110  provides creation, deactivation, and other management functions for vSCs  44 A- 44 C, and virtual manufacture authorities (MAs)  124 ,  126  are used to obtain credentials for vSCs  44 A- 44 C. GVTPM framework  40  also includes a key and session manager  140 , which GVTPM manager  110  uses for tasks such as swapping out keys and authorization sessions when one vTPM is unloaded from processor  22  and another vTPM is ready to become active. For instance, keys belonging to vTPMs which are loaded but not currently scheduled for execution on processor  22  may be removed to make room for the keys needed by a vTPM that is (or will soon be) scheduled to run on processor  22 . 
     In the example embodiment, all components of GVTPM framework  40  are isolated from the rest of the system, in order to ensure the security of the secrets stored in these components. 
     2.1—GVTPM Device Models 
     GVTPM framework  40  may use VM definition  41  to create DMs  44 A and  44 B, and GVTPM framework  40  may use VM definition  51  to create DM  44 C. GVTPM framework  40  may use virtualization events (VEs) when providing or using DMs. For instance, a VE may be triggered when software in VM  60 A attempts to access a TPM. In response to the VE, control may be transferred from VM  60 A to VMM  64 . GVTPM manager  110  may intercept the VE to process the event by reference to vTPM  44 A. In the example embodiment, although VM  60 A may be unaware of any TPM other than vTPM  44 A, GVTPM manager  110  may use hwTPM  30  to support vTPM  44 A. 
     In effect, the DMs extend the GVTPM functionality to the OS partitions, such as VMs  60 A- 60 C. The protection perimeter of each DM is provided by the environment it is executing in, such as the TVMM. By placing the perimeter around each GVTPM component and each DM individually, each DM maintains isolation in the event of another DM being compromised. Accordingly, the data structures implemented within the DM in accordance with the device model design may be considered tamper-resistant structures of the device model. 
     The design of the framework allows for flexibility in the design of device models. For example, any functionality allowed by the VMM may run in a DM, as opposed to the limited functionality supported by a typical hardware SC. In the case of proprietary functionality, the flexibility of the framework allows for a wide variety of encryption algorithms, signature schemes, access control policies, and storage mechanisms. 
     In the example embodiment, for virtual TPMs, each DM manages its own set of TPM structures and resources, including its own EK, SRK, PCRs, DIRs, monotonic counters, a user key hierarchy, general purpose NVM, etc. This enables the vTPM to function identically to a hardware TPM, ensuring that applications may use either hardware or virtual TPMs transparently. 
     In the example embodiment, vTPM  44 A uses software to provide simulated, persistent, monotonic counters. The number of counters may be substantially unlimited. In the example embodiment, vTPM  44 A at least provides the four counters expected from hwTPMs. The vTPM counters may not require any direct link to the hardware TPM counters. 
     Virtual PCRs such as vPCRs  92  do not have the resource constraints of hwTPMs, but instead may have a configurable number of PCRs available to them. In the example embodiment, vPCRs  92  are stored in the memory space of vTPM  44 A in PSS  100 , and vTPM  44 A emulates the standard PCR operations on vPCRs  92 . 
     The framework allows individual DM designs the ability to balance performance and security. Some implementations may enjoy faster encryption operations or enhanced migration by implementing keys in software within the DM, while others may require that all keys always reside in the hardware TPM and that the DM act as a portal to them. Additionally, this approach transparently allows different DM designs to tune their services to match the data protection and cryptography laws of different geographies. 
     As described in greater detail below, GVTPM manager  110  may provide a different virtual manufacturer authority (vMA) for each DM design. For instance, in  FIG. 3 , virtual manufacturer authority  124  services DMs based on the DM design provided by VM definition  41 , and virtual manufacturer authority  126  services DMs based on the DM design provided by VM definition  51 . 
     2.2—GVTPM Manager 
     GVTPM manager  110  is the central management component for GVTPM framework  40 . In the example embodiment, GVTPM manager  110  is the management component responsible for vTPM provisioning, bridging the other framework components, and granting serialized access to TPM  30  for the DMs. In one embodiment, GVTPM manager  110  provisions new DMs by requesting that VMM  64  (a) create the necessary VMs and (b) provide communication channels. During the process of provisioning a new DM, GVTPM manager  110  will collect any information that PSS  100  requires to authenticate the DM, such as measurement of the DM code. For example, GVTPM manager  110  may measure the PCS within the particular VM definition that serves as the basis for instantiating the DM in question. For instance, VM definition  41  may include a PCS  45  for a particular type or model of TPM, while VM definition  51  may include a PCS  55  for a particular type or model of smartcard. In one embodiment, each different DM design to be supported by the processing system is defined completely or primarily by a PCS within a VM definition. 
     GVTPM manager  110  may provide communication channels between each particular OS partition and the respective DM. GVTPM manager  110  may also provide communication channels between itself and each DM. GVTPM manager  110  provides the DM access to other GVTPM components, such as access to a virtual manufacturer authority, access to PSS  100 , and serialized access to hwTPM  30 . GVTPM manager  110  is therefore in charge of sharing hwTPM  30  across multiple DMs. The primary resources under management are the set of loaded keys and authorization sessions. Sharing techniques such as those proposed in the Core Services portion of the TPM specification reference above may be fitting for this task. 
     In the example embodiment, GVTPM manager  110  ensures that only one vTPM accesses TPM  30  at a time. In addition, GVTPM manager  110  swaps keys and authorization sessions in and out of TPM  30  to ensure each vTPM has the resources it requires. 
     2.3—GVTPM Protected Storage Service 
     To maintain flexibility in the DMs, GVTPM framework  40  imposes relatively few requirements on how each DM functions. In one embodiment, the only requirement is that all persistent data (e.g., keys, counters, NVM data, and any other state the vTPM needs in order to function across system reboots) is copied from NVM to active memory on load, and then saved back to NVM when necessary. In accordance with this requirement, the PSS is responsible for protecting the DM&#39;s NVM while the DM is not operating. After the DM loads, it is the responsibility of the TVMM to provide isolation and protection of the data while the DM is executing. 
     In the example embodiment, PSS  100  authenticates the vTPM and ensures that the state of a vTPM is only loaded into the vTPM that stored the state previously. PSS  100  also ensures the integrity of the stored state, and provides anti-replay protection. To protect the offline NVM for each DM, PSS  100  provides strong authentication and protection mechanisms that are rooted in TPM  30 . This authentication identifies the PCS of the DM that saved the NVM previously, and ensures that the PCS has not been tampered with since the NVM was saved. In addition to authenticating the PCS that is loading the NVM, it is also critical that the TPM ensures that the TVMM, GVTPM manager  110 , and any other code with the capability to undermine the component isolation have not been tampered with since the NVM was saved. 
     To do this, in the example embodiment, the hash of the GVTPM components will be stored in a PCR in TPM  30 , as will the hash of all software in the underlying trusted computing base (TCB). In general, the term TCB refers collectively to the components of a processing system that can affect the fundamental security policies of the processing system. For instance, the TCB may include the hardware, the boot code, the kernel, the configuration files that control system operation, and any program that can run with the privilege or access rights to alter the kernel or the configuration files. In the example embodiment, TPM  30  will detect tampering of GVTPM components or the TCB, will prevent the release of secrets to an inappropriate DM, and will ensure that the proper memory protections are still in place. 
     2.3.1—Saving NVM 
     In the example embodiment, whenever an operation changes the NVM of a DM, the DM issues a request to PSS  100  to save its NVM. In alternative implementations, DMs can be designed to delay the saving of their NVMs, to increase performance at the possible cost of lower assurance. 
     When PSS  100  receives the NVM, the NVM arrives as opaque data. The NVM data is considered opaque because PSS  100  does not analyze the NVM data it receives. PSS  100  may simply generate a nonce and then encrypt the NVM data and the nonce. In the example embodiment, PSS  100  uses its own non-migratable TPM key to perform the encryption, and this key has its PCR binds set to the measurements of the platform TCB, GVTPM manager  110 , and PSS  100 . By using PCR bindings, TPM  30  will ensure that this blob can only be decrypted by the key when the TCB and GVTPM manager  110  are unmodified. PSS  100  may then give a copy of the encrypted NVM blob to the DM. 
     Once the NVM blob is saved, PSS  100  measures the DM, and PSS  100  records the measurement of the DM, the NVM blob identity, and the nonce, in a persistent database. The hash of the blob may serve as a good unique identifier. If this DM has a previously saved state, PSS  100  may overwrite the old record with the new record. 
     2.3.2—Restoring NVM 
     In the example embodiment, when a DM starts up, it requests that its previously stored NVM be restored by PSS  100 . In response, PSS  100  first calculates the identifier for the blob, allowing PSS  100  to look up the record in the database. Next, PSS  100  measures the DM and verifies that the DM measurement matches that in the record. Then, PSS  100  uses its TPM key to decrypt the blob, and verifies the nonce inside. 
     In the example embodiment, before restoring the NVM, PSS  100  ensures five conditions:
         NVM corresponds to the requesting DM.   NVM is the most recent NVM for that DM.   NVM is unmodified since storing.   NVM blob was created by PSS  100 .   The TCB has not changed since the NVM was stored.       

     Comparing the DM measurements ensures meeting condition 1. The combination of a successful lookup and a successful decryption indicates that conditions 2 and 3 are also true. Inclusion of the secret nonce indicates that the PSS created this blob and it is not a forgery using the PSS public key, hence ensuring condition 4. Lastly, the successful use of the PSS TPM key to do the decryption indicates that the TPM has verified that the GVTPM framework and the TCB are both in the same state as they were before. 
     Once all five conditions are verified, the opaque NVM is returned to the DM, and the DM proceeds through its initialization. 
     2.4—Virtual Manufacturer Authority 
     In many cases, it is valuable to have evidence that a given key resides in a DM, which in turn resides in a given GVTPM environment. In the case of a hardware TPM, the TPM manufacturer signs the endorsement credential to state that the EK is protected inside the TPM, and the platform manufacturer signs the platform credential to state that the TPM resides in a TCG compliant platform. In GVTPM framework  40 , each virtual manufacturer authority acts in a similar fashion, certifying that keys reside in a DM which complies with a particular device model within GVTPM framework  40  within a TCG compliant platform. 
     In effect, GVTPM framework  40  allows a trusted certificate authority to delegate TPM manufacturer and platform manufacturer status to a virtual manufacturer authority. In an example embodiment, the virtual manufacturer authority is a piece of measurable software on the platform acting as the TPM manufacturer, the platform manufacturer, or both. The virtual manufacturer authority allows platforms using virtualization to securely create new vTPMs with the appropriate credentials required for attestation. 
       FIG. 5  is a block diagram illustrating example components and operations to prepare a virtual manufacturer authority to provide security attestation for a vTPM. In  FIG. 5 , the hardware of processing system  20  is depicted collectively as platform hardware  252 . That hardware includes TPM  30 . Running on top of the hardware are one or more trusted partitions. In one embodiment, those partitions include VM  60 A and a vMA partition  254 . VMM  64  may operate in third distinct partition. One or more of the partitions may be implemented as virtual machines, for instance. In alternative embodiments, one or more of the components can share a protected partition. 
     In the example embodiment, the virtual manufacturer authorities reside locally on processing system  20  within a trusted component. For example, virtual manufacturer authority  124  may reside within a trusted VMM  64  or in a trusted partition  254  (e.g., a trusted VM) supported by VMM  64 . However, in alternative embodiments, virtual manufacturer authorities may reside in firmware or any other protect partition where they can be measured and that measurement can be stored in a platform&#39;s TPM. The vTPMs also reside locally on processing system  20  within a trusted component. For instance, vTPM  44 A resides in VM  60 A in the example embodiment. 
     Each virtual manufacturer authority creates a signing key  250  within TPM  30 , to be used for signing TPM credentials. In the example embodiment, in order to make the certification from virtual manufacturer authority  124  meaningful, virtual manufacturer authority  124  first convinces a third party (e.g., an external CA) that the configuration of virtual manufacturer authority  124  is trustworthy and that the signing key of virtual manufacturer authority  124  is protected by a TPM. This third party may be considered a virtual manufacturer certifying authority (VMCA). In essence, the VMCA is an entity trusted by privacy CAs to determine which GVTPM environments are trustworthy enough to manufacture reliable virtual TPMs. The same entity can serve as the privacy CA and the VMCA, or, as depicted in  FIG. 1 , privacy CA  76  and VMCA  77  may be separate entities, with privacy CA  76  trusting VMCA  77  to accurately assess vSC frameworks and DMs. 
     In the example embodiment, when processing system  20  launches VMM  64 , and when VMM  64  creates partition  254  and loads virtual manufacturer authority  24  into that partition, the values in PCRs  32  are modified to reflect the evolution of the platform, as indicated by arrows  270  and  271 . 
     As indicated by arrow  272 , virtual manufacturer authority  124  then creates a TPM signing key  250  that is bound to the state of virtual manufacturer authority  124  as well as any software that affects the integrity of the virtual manufacturer authority  124 , such as GVTPM framework  40 , VMM  64 , and any other components in the TCB, as reflected in PCRs  32 . Virtual manufacturer authority  124  then proves to VMCA  77  that TPM  30  will not allow any entity other than virtual manufacturer authority  124  to access to that TPM signing key. 
     The bindings to the state of the virtual manufacturer authority and the TCB prove that virtual manufacturer authority  124  will operate in accordance with the policies embodied in GVTPM framework  40 . Virtual manufacturer authority  124  thus proves that it is controlled by a specified platform and software configuration. Signing key  250  therefore implicitly verifies that any virtual TPMs created by GVTPM framework  40  will operate in accordance with the policies embodied in GVTPM framework  40 . VMCA  77  can then decide whether that environment is trustworthy, for instance by reference to a list of approved environments. 
     For instance, virtual manufacturer authority  124  in  FIG. 3  may use a protocol to prove to VMCA  77  that the signing key  250  for virtual manufacturer authority  124  resides in TPM  30  and is only available to virtual manufacturer authority  124  when processing system  20  is in the same configuration as when the signing key was created. One example of such a protocol is for virtual manufacturer authority  124  to create an AIK within the TPM  30 . Through standard channels, virtual manufacturer authority  124  then acquires an identity credential for this key, which is signed by a privacy CA and is proof that any claims made by the identity key are made by a trusted TPM. This identity key is then used to certify signing key  250 , which is a process by which TPM  30  uses the identity key to sign a statement that the signing key is located in the same TPM as the identity key, and the signing key is bound to the specified configuration that ensures trust for the environment supporting virtual manufacturer authority  124 . When this statement is combined with the identity credential, which says that identity is in a real TPM, it is derived that the signing key  250  of virtual manufacturer authority  124  is protected by a legitimate TPM and only available for use in the specified configuration that provides trust for the operations of virtual manufacturer authority  124 . 
     Once virtual manufacturer authority  124  convinces VMCA  77  that virtual manufacturer authority  124  has a TPM signing key bound to a particular configuration, VMCA  77  makes an informed decision on whether that configuration is safe enough to be trusted. If this configuration is deemed sufficiently trustworthy, VMCA  77  creates a certificate with the public portion of the signing key which states that VMCA  77  has delegated power to sign endorsement credentials and platform credentials to the virtual manufacturer authority&#39;s signing key. This certificate is pushed through the network of other CAs that trust the decisions of VMCA  77 . 
     The validity period on the credential of a virtual manufacturer authority and on credentials signed by the virtual manufacturer authority will likely be proportional to how extensive was the review of the GVTPM framework and its TCB. The discovery of a vulnerability in the GVTPM framework or the underlying TCB should result in removal of trust in the GVTPM DMs. Under such circumstances, VMCA  77  would wish to revoke the delegation to the virtual manufacturer authority, and would typically inform the network of other CAs that VMCA  77  no longer trusts that virtual manufacturer authority. For instance, after delegating authority to virtual manufacturer authority  124 , if VMCA  77  subsequently determines that the configuration of virtual manufacturer authority  124  and/or its vTPM architecture are vulnerable, VMCA  77  can revoke this delegation. 
     After VMCA  77  accepts the proof from virtual manufacturer authority  124 , virtual manufacturer authority  124  may use signing key  250  to attest to the trustworthiness of virtual TPMs such as vTPM  44 A. Specifically, in the example embodiment, virtual manufacturer authority  124  generates new endorsement and platform credentials  260  based on information about vTPM  44 A, as indicated by arrow  274 , and virtual manufacturer authority  124  uses signing key  250  from TPM  30  to sign those credentials without further interaction with VMCA  77 . That new signature  262  is therefore appended to certificates  260  to create the finished certificate  280 , as indicated by arrow  276 . Certificates such as finished certificate  280  that have been signed by a virtual manufacturer authority may be referred to as vMA certificates or vMA credentials. 
     Virtual TPM  44 A may then use vMA credentials  280  in the same manner as if they were credentials for a hardware TPM signed by a TPM manufacturer and a platform manufacturer. Processing system  20  can therefore create virtual security coprocessors such as vTPM  44 A in real time (i.e., without the delay associated with obtaining credentials from an external processing system), while still providing high security assurance. 
     As indicated above, in the example embodiment, at least each type of DM has its own virtual manufacturer authority. For example, a particular GVTPM framework may include one virtual manufacturer authority for all TPM DMs, and different virtual manufacturer authorities for each proprietary DM design. Accordingly, virtual manufacturer authority  124  may service vTPMs  44 A and  44 B, for instance. In the example embodiment, when processing system  20  creates a new vTPM, such as vTPM  44 A, GVTPM manager  110  provides virtual manufacturer authority  124  with the endorsement key for the new vTPM, along with any other information necessary for virtual manufacturer authority  124  to create an endorsement credential and platform credential that properly identify the software extension to TPM  30  and the software platform which the vTPM architecture resides in. Virtual manufacturer authority  124  is then responsible for signing those endorsement and platform credentials, using signing keys endorsed by a trusted CA. 
     The paragraphs above describe an example embodiment in which a virtual manufacturer authority uses signing keys and identity keys from a TPM to attest to the virtual manufacturer authority&#39;s configuration. Other types of security subsystems, such as proprietary security coprocessors, and other types of keys may be used in alternative embodiments. Similarly, other approaches may be used to verify that the configuration for the virtual manufacturer authority is acceptable to an external CA serving, in effect, as a VMCA. For example, the external CA may seal a secret to a key in such a manner that the secret can be unsealed by a virtual manufacturer authority only if the virtual manufacturer authority has a predetermined configuration. After the external CA transmits the secret to the virtual manufacturer authority, if the configuration of the virtual manufacturer authority matches the predetermined configuration, the virtual manufacturer authority will be able to unseal the secret. The virtual manufacturer authority may then send proof to the external CA that the virtual manufacturer authority was able to unseal the secret. In response, the external CA may delegate the requested authority to the virtual manufacturer authority. The virtual manufacturer authority may then use the delegated authority to sign credentials for the vTPM. 
     The following paragraphs describe example protocols that may be used by a virtual manufacturer authority to obtain acknowledgement from a VMCA, as well as example methods for creating such credentials. Acknowledgement from the VMCA may provide the virtual manufacturer authority with the credentials needed to sign TPM and non-TPM credentials. 
     2.5—vTPM Credentials 
     As indicated above, each virtual manufacturer authority may first create a TPM identity key (AIK) and acquire an identity credential from a trusted third-party CA (TTPCA) using the process outlined in the TCG specification. The TTPCA may be a TCG privacy CA, for instance. The virtual manufacturer authority then creates its signing key, bound to the current platform configuration. For example, virtual manufacturer authority  124  may use the TPM_CertifyKey operation to have TPM  30  use the virtual manufacturer authority&#39;s AIK to sign the properties of the signing key, including its migration abilities and the PCRs to which it is bound. Virtual manufacturer authority  124  may then send the CertifyKey results along with the identity credential to VMCA  77 . 
     Generally, VMCA  77  should trust the identity credential, which was signed by a TCG privacy CA. The credential indicates that the identity key resides in a valid TPM, and the CertifyKey certificate indicates that the signing key will only be available to that specific virtual manufacturer authority and TCB. 
     Once VMCA  77  is convinced that virtual manufacturer authority  124  is trustworthy, VMCA  77  will delegate TPM manufacturer status to virtual manufacturer authority  124 . For instance, VMCA  77  may create a delegation certificate and distribute that certificate to TCG privacy CAs. 
     Virtual manufacturer authority  124  may then use the delegated TPM manufacturer status to create an endorsement credential and a platform credential for a DM (e.g., vTPM  44 A). The model fields in these credentials may indicate both the hardware and software platform on which vTPM  44 A resides. In one embodiment, for the endorsement credential, the model field would indicate the hardware TPM model and the GVTPM manager  110 . For the platform credential, the model field would indicate the hardware platform and the TCB, which includes VMM  64  and similar software. 
     2.6—Non-vTPM Credentials 
     A proprietary DM (e.g., DM  44 C) that does not adhere to TCG specifications should not have an endorsement or platform credential; though, it can still benefit from similar credentials. The developer of a proprietary DM might operate its own evaluator service, similar to a privacy CA, for the express purpose of issuing credentials, so that the developer can guarantee a key is in an instance of the developer&#39;s DM before data is encrypted to that key. The precise protocol for doing so is completely up to the entity that designed the proprietary DM and the virtual manufacturer authority that corresponds to that DM. 
     3.0—Example vTPM Device Model Designs 
     This section describes two example DM designs for creating vTPMs. For GVTPM framework  40 , these correspond roughly to opposite ends of the spectrum from strict security to greater performance and flexibility. The first model, referred to as the software-based DM, recognizes that once the vTPM is anchored in TPM hardware, software can provide complete TPM functionality to the OS in a VM. All private keys are stored in the device model&#39;s memory, as is all other data stored in the virtual TPM. In the second model, referred to as the hardware-based DM, all keys are stored in the hardware TPM. In the second model, when a key is used, the device model issues a request to the TPM to use the key. In the first model, the vTPM is not hindered by the performance limitations of the TPM, while the second still relies on the hardware TPM to service most requests. The security difference between the first model and the second model during normal functioning is identical. However, the resulting state of the system and its secrets after a compromise has occurred is different between the models. 
     If the DM or the VMM of the platform is compromised, all data stored in the DM&#39;s memory may be compromised. During the compromise, both models may allow the attacker to use keys in that vTPM. Once the vulnerability is patched and the compromise is ended, access to those keys is terminated in the hardware-based DM. However, in the case of the software-based DM, the private keys may have been permanently compromised since they were available in memory. 
     In most environments, the software-based approach can achieve an adequate level of security. However, in environments in which it is more expensive to revoke and regenerate compromised keys, the hardware-based approach may be appropriate, since compromises would be temporary. These types of environments might include a server where high performance, highly tamper-resistant TPMs are used to protect important corporate keys. The proposed framework leaves these choices to the implementers, who can decide on the preferred approach for meeting the assurance needs of a particular implementation. For example, a hybrid approach that uses the TPM to house some keys and/or data structures may be appropriate for certain implementations. 
     3.1—Software-Based vTPM Device Model 
     A completely software-based vTPM DM uses few or no hardware TPM resources for providing TPM functionality. Once the PSS and the hardware TPM have guaranteed that the DM and the TCB are the same as those reflected in the credentials of the DM, the DM is left to function independently of the hardware TPM. All virtual PCRs, monotonic counters, non-volatile storage, and other TPM resources are stored and managed in the memory of the DM. 
     The benefit of this design is that the functionality exposed by the device model is not hampered in any way by the functionality or performance provided by the hardware TPM. Stronger keys, larger numbers of key slots and more PCRs are all easily supported. In addition, typical hardware TPMs are very resource constrained and are not very high performance devices. However, the performance of a software-based DM is not bound by that of the hardware TPM. For example, a software-based DM can support bulk encryption, whereas bulk encryption with a conventional hardware TPM is extremely slow. 
     3.2—Hardware-Based vTPM Device Model 
     The second example vTPM device model attempts to maximize the use of the protected processing within the hardware TPM. The main use of the hardware TPM resources is that all keys for each DM are stored in the hardware TPM, and private keys are never stored in main memory. 
     3.2.1—Key Hierarchy 
     This section describes the overall key hierarchy utilized in an example embodiment of a hardware-based device model. This example embodiment is described in the context of DM  44 A serving as a vTPM for VM  60 A, as illustrated in  FIG. 3 . 
     Referring again to  FIG. 2 , TPM  30 , as usual, maintains a standard endorsement key (hwEK)  52  and a storage root key (hwSRK)  50 . Additionally, an AIK known as a key binding key (KBK)  160  is used to protect vTPM keys. 
     Also, each DM may have appropriate keys and data structures to emulate a hardware TPM for each respective VM. For instance, in the example embodiment, DM  44 A has a virtual EK (vEK)  150  and a virtual SRK (vSRK)  152  whose parent key is hwSRK  50 . Parented in the vSRK, there are virtual signing keys (vSigKs)  154 , virtual storage/encryption keys (vEncKs)  156 , and virtual identity keys (vAIKs)  158 . Additional structures in each DM for emulating a hardware TPM may include virtual PCRs (vPCRs)  92  and virtual DIRs (vDIRs)  94 . Each DM may also contain data such as a virtual EK credential (vEK_Cred)  96  and one or more virtual AIK credentials (vAIK_Cred)  98 . 
     As indicated by the legend in the lower right corner of  FIG. 2 , storage keys are illustrated as ovals with no fill, attestation identity keys (AIKs) are illustrated as ovals filled with horizontal lines, and signing keys are illustrated as ovals filled with a pattern of dots. In addition, bolded ovals represent keys that are bound to PCRs  32  of TPM  30 . Lines between keys indicate parent/child relationships among the keys. For example, those lines indicate that SRK  50  is a parent key for certain virtual keys within each DM. Credentials are represented by parallelograms. 
     In one embodiment, the virtual keys and other structures or objects within a vTPM may have the same structure as hardware TPM keys or objects, but the virtual objects within a virtual TPM are not mere references to the standard objects within TPM  30 , such as EK  52 , SRK  50 , and PCRs  32 . Instead, as described in greater detail below, each virtual TPM gets its own distinct objects, such as vEK  150 , etc. Those virtual objects may be based on or derived from the objects of the hardware TPM. For example, in the example embodiment, the virtual SRKs and virtual EKs are children of the hardware SRK or, in the case of nested vTPMs, a virtual SRK ultimately based on the hardware SRK. By allowing for vTPM keys to be rooted in vSRKs, this model allows for vTPM nesting. 
     Virtual TPM objects such as vEK  150 , vSRK  152 , and vPCRs  92  may in turn serve as the basis for additional virtual objects within DM  44 A, such as vSigKs  154 , virtual AIKs (vAIKs)  158 , and virtual storage/encryption keys (vEncKs)  156 . In the example embodiment, each DM may provide all of the functions provided by a corresponding hardware device, with the same application program interfaces (APIs). For example, DM  44 A may include its own vDIRs  94 , vPCRs  92 , vAIKs  158 , etc. Consequently, the guest OS in each VM may be completely unaware that the corresponding vTPM is not a hwTPM. The VMs may therefore use legacy OS code. In addition, according to the example embodiment, a processing system with a conventional hwTPM may be configured to provide vTPMs without requiring any modifications to the hwTPM. 
     The virtual machine architecture may leverage the hardware TPM to protect the virtual keys and related data. In one embodiment, the vTPM key hierarchies and related data are protected within a standard hwTPM. For example, the virtual TPM keys may be stored in, and never released from, the hardware TPM, unless the data is first encrypted. Consequently, if a virtual TPM is compromised, the public portions of the associated vTPM keys may possibly be subject to unauthorized use, but only for the duration of the compromise. In the example hardware-based embodiment, all keys will remain inside the hardware TPM, and the private keys therefore cannot be stolen or used once the compromise has ended. 
     A processing system according to the present invention may also provide an attestation protocol architecture that allows vTPMs to provide conventional TPM attestation services. Remote challengers with no awareness of virtual TPMs may participate fully in the attestation process. Moreover, remote challengers with vTPM awareness may be capable, without additional protocols, of distinguishing hwTPMs from vTPMs, and may then decide whether or not to trust a platform hosting a vTPM. Remote challengers may include, without limitation, entities that provide data only to verifiably safe clients. Such a challenger may be referred to as a third party data provider. 
     As indicated above, in the example hardware-based model, all keys for each DM are stored in TPM  30 . Depending on the capabilities of TPM  30  and the particular security requirements of a particular implementation, one or more of other structures and data items, such as credentials  96  and  98 , vPCRs  92 , vDIRs  94 , etc., may also be stored in the hardware TPM, or they may be stored in PSS  100  of GVTPM framework  40 . 
     When a privacy CA creates an identity credential for a platform, the privacy CA encrypts the identity credential to the platform&#39;s EK before transmitting the identity credential to the platform. In one embodiment, VM  60 A operates as a platform interacting with privacy CA  76 , and VM  60 A uses a hardware-based vTPM device model (e.g., DM  44 A). Accordingly, privacy CA  76  will encrypt the identity credential (e.g., vAIK_Cred  98 ) to the VM&#39;s vEK  150 . This encryption protects the credential in transit. When VM  60 A receives the identity credential from the privacy CA, VM  60 A uses the vEK&#39;s private key from DM  44 A to decrypt this credential. Since vAIK_Cred  98  is not in a TCG bound data structure, vEK  150  must be a TPM legacy key in order to support decrypting this credential. 
     In the example embodiment, vSRK  152  and vEncKs  156  are traditional TPM storage keys and require no special attention. Similarly, the vSigKs  154  are traditional TPM signing keys and require no special attention. Identity keys, however, are only able to sign quoted PCR values. This means the vAIK, if implemented as an AIK, would be unable to sign virtual PCRs stored in DM memory, since they are data that is external to hwTPM  30 . Therefore, vAIKs  158  may be implemented as TPM signing keys. Quote structures may be constructed for virtual PCRs  92  in DM  44 A and then signed with a vAIK. Lastly, KBK  160  is a traditional binding key. 
     3.2.2—Enforcing Virtual PCRs on TPM Keys 
     The hardware TPM (e.g., TPM  30 ) and the VM using the vTPM (e.g., VM  60 A using DM  44 A) have different notions of what the current PCR values are. Care must be taken to ensure that information flow between the vTPM and the TPM remains consistent. When a VM requests that a key be created in the vTPM, that request is accompanied by vPCR bindings, though the guest may not realize that the binding are virtual. When this request is forwarded to the hardware TPM, the PCR field in the request must be translated into correct hwPCR bindings. DM&#39;s may use the GVTPM+TCB as bindings or omit them for performance. The resulting newly created wrapped key returned from the hardware TPM will thus not contain the vPCR bindings requested by the VM. Since the wrapped key does not contain those bindings, it may be possible for an attacker to bypass the vTPM, if attacker were to obtain the original wrapped key and load it directly into the hardware TPM. 
     Additionally, the wrapped key returned by a hardware TPM is a TCG_KEY structure containing the TPM version, PCR bindings, public key, encrypted private key, and other information that is returned to the requestor. The version and PCR binding information is that of the hardware TPM, not the vTPM. In order to preserve transparency, the TPM_KEY structure returned by the vTPM should have the vPCR bindings and the vTPM version information. In one embodiment, to address this issue, the wrapped key returned by the vTPM will be a modified form of the wrapped key returned by the hardware TPM. For purposes of this disclosure, the wrapped key returned by the vTPM may be referred to as a vTPM double wrapped key. 
       FIG. 4  presents a block diagram of an example embodiment of a vTPM double wrapped key. In  FIG. 4 , vTPM wrapped key  210  is a modified form of the wrapped key  200  returned by TPM  30 . In one embodiment, the operations below may be performed whenever VM  60 A requests that a new key be created by vTPM  44 A, such as when DM  44 A in VM 60 A creates a new vAIK, and processing system  20  proceeds to store that key in TPM  30 . 
     In vTPM wrapped key  210 , all public portions of the structure remain intact to ensure transparency. Also, as illustrated at block  214 , the encrypted private key section  216  of the TCG_KEY structure will be expanded to include the hwPCR bindings, authorization to use this key, the hardware TPM&#39;s version, a digest of the public portions of the key, and the original encrypted private key section, which is unreadable to the vTPM device model (e.g., DM  44 A). As indicated at block  212 , the PCR bindings in the public portion of the TCG_KEY structure will be replaced with the vPCR bindings, and the version will be set to that of the vTPM. The digest stored in the private portion will reflect these modifications. Lastly, the expanded private key section  216  will be encrypted with KBK  160  (described above). The result is a TCG_KEY  210  with the expected version and vPCR bindings, and an unreadable encrypted section, which, if passed to the hardware TPM, will not decrypt properly. 
     For use, the key returned to the user must be loaded into the virtual TPM (e.g., DM  44 A), which will decrypt the private key section  216 , verify the virtual PCR binding  212 , and reconstruct the original wrapped key  200 . Once the vPCR and authorization are verified, the DM loads the original wrapped key  200  into the TPM, including a copy  204  of private key section  216 . The original wrapped key  200  will similarly be decrypted by the hardware TPM and the hardware PCR binding  202  will be verified before the key  200  is fully loaded and available for use. 
     3.2.3—Other Device Model Resources 
     The virtual TPM device model may be unable to share most of the other TPM resources across many DMs. Monotonic counters typically cannot be shared without modifying applications to expect non-exclusive counter usage, and therefore may either be permanently allocated to a specific DM or be implemented in software similar to that of the software-based device model. The vTPM non-volatile storage can be stored in the hardware TPM, as long as it does not exceed the storage of the hardware TPM. If the hardware TPM&#39;s storage is inadequate, non-volatile storage can also be virtualized similarly to that of the software-based DM. 
     The VM must be able to create authorization sessions to use many of the TPM functions; however, it typically should not differentiate between functions handled by the vTPM device model directly and those passed on to the hardware TPM. In one embodiment, the DM transparently provides TPM functionality, from both itself and the hardware TPM, under a single user authorization session. 
     To accomplish this, the DM maintains separate authorization sessions with both the VM and the hardware TPM. That is, the user will create an authorization session with the DM as normal. The DM may do all the same authorization checks based on this session that a hardware TPM would do. If the DM provides the requested function directly, the DM may simply update the session nonces and reply. If the DM needs the hardware TPM to provide the service, the DM may create or reuse an existing authorization session it has with the hardware TPM and make the request. Once the DM is done using the hardware TPM, it may update the nonces on the user&#39;s session and reply. 
     4.0—Hardware Optimizations 
     The framework discussed above may provide TPM capabilities to multiple guests using a single conventional TPM. The framework may use either software or the hardware TPM to provide the TPM functionality; however, using the TPM to enforce virtual PCRs may be cumbersome. This section describes TPM features to optimize and simplify the hardware-based device model. Those features include the following:
         Virtual PCRs   Virtual Attestation Identity Keys   Virtual EK       

     A typical conventional TPM may be unable to store vPCRs, to allow the TPM to enforce key bindings and provide vPCR quotes. This leads to the need for double wrapped keys, with the DM enforcing and managing vPCRs. A modified TPM that is capable of storing vPCRs removes a great deal of complexity and provides increased performance. U.S. patent application Ser. No. 11/095,034, assigned to the same assignee as the present application, discusses technology to supporting PCRs and/or vPCRs for virtual machines. 
     Once vPCRs are available in the modified TPM, AIKs should be able to quote them. In one embodiment, the modified TPM has the ability to create virtual AIKs (vAIKs) which quote the vPCRs rather than the main PCRs. 
     Lastly, in order to effectively quote PCRs, a vAIK requires an identity credential. As noted earlier, identity credentials are encrypted to the EK. In one embodiment, the modified TPM includes support for vEKs that can decrypt the credentials for the vAIK. 
     With these modifications, the performance of the hardware-based DM within GVTPM framework  40  may be increased, while simultaneously reducing the framework&#39;s complexity and therefore increasing its trustworthiness. 
     5.0—Remote Deployment and Provisioning of Virtual TPMs 
     The use of TPMs on platforms may create new challenges for software and data distribution. For example, an information technology (IT) department within an organization may create a software build or patch, and then push the build or patch to hundreds or thousands of machines across the organization. TPMs add a new dynamic to this process, due to the way TPMs are used to protect keys and seal data. To distribute a new software package to a machine, the IT department may find that any keys which are needed by that software need to be distributed to the machine&#39;s TPM. 
     The following paragraphs describe how vTPMs can be provisioned on one machine (e.g., a server), and then distributed to other machines (e.g., clients) with the software that is expected to make use of the vTPMs. An example usage model for the approach described below is in a business enterprise; however, that approach or variations thereof can be used in any environment where the consumer of the services of the vTPM has a sufficiently close relationship with the entity providing the services, such as when the client system runs software from a particular entity, and the client system uses that software to access data provided by the same entity. For example, a client system in a physician&#39;s office could obtain, from an insurance company, a vTPM along with a content viewer to use that vTPM. The client system could then use that content viewer to access protected insurance records from the insurance company. In an example embodiment, the only entity that needs to trust the vTPM is the insurance company, which is the same entity that provided the vTPM. 
     A remotely provisioned vTPM, unlike a normal vTPM, is created externally to the platform in which it will operate. This means that the virtual manufacturer authority will no longer sign both the endorsement credential and the platform credential for the vTPM. In an example provisioning environment, an IT department manufactures the vTPM, generates the endorsement key for that vTPM, signs the endorsement credential, and then sends the vTPM and endorsement credential to a destination machine. Once the vTPM is inserted into the destination machine, the virtual manufacturer authority will create a platform credential for the new vTPM and sign it. The signatures on these two credentials identify two entities a challenger must trust. The IT department had access to the private endorsement key as well as any other keys that it preloaded into the vTPM, so the IT department signs the endorsement credential. The vTPM architecture components (e.g., GVTPM framework  40 ) and the platform&#39;s isolation mechanism integrate the vTPM into the software platform. They potentially have access to the secrets stored in the vTPM. Therefore, these entities must also be trusted by the challenger. Consequently, the platform credential is signed by the virtual manufacturer authority. 
     If a challenger does not trust the entity that signs the endorsement credential and the entity that signs the platform credential, the challenger may reject attestations from this vTPM. In the case of an enterprise, the IT department will likely control the manufacturing of the vTPM, the vTPM platform, and the challenger software. Thus, there is an inherent trust among these entities. Similarly, a content provider may control manufacturing of vTPMs and related challenger software. Therefore, such a content provider may only need to trust the vTPM platform. 
     On example mechanism for deployment is to transfer the vTPM to the PSS of the destination. How this is accomplished is dependent on the implementation of the PSS and the GVTPM manager. In an example implementation, the PSS maintains a storage key which it can prove to external entities is protected by a hardware TPM and bound to the specific vTPM architecture. While this key is used to store state, it can also be used to receive state. The following steps exemplify how this transmission may occur.
         1) The provisioner generates the new vTPM and all keys needed within the vTPM. Also, the provisioner generates and signs the new endorsement credential for that vTPM.   2) The provisioner requests that the destination PSS provide an anti-replay nonce.   3) PSS sends the provisioner a nonce and optionally the public key of the PSS. This key may already be known from a prior communication.   4) The provisioner then encrypts the state of the new vTPM and the nonce from step 3 to the storage key of the PSS.   5) The provisioner sends the encrypted blob to the PSS, along with the endorsement credential for that vTPM.   6) The PSS receives the vTPM state, decrypts it, seals it like it does all offline vTPMs, and records it as a known vTPM. This vTPM is now officially part of the PSS&#39;s platform.   7) A virtual manufacturer authority for the PSS creates a platform credential for the vTPM.
 
The PSS may now load and use the vTPM like it would any other.
       

     When software requiring TPM keys is to be distributed to a client, if the vTPM were to be created on the client system instead of being created on a server and then transferred to the client, the server would typically need to instruct the client to create a vTPM, and then wait while the client generates each key required by the software. The client could then send the public key portions of the generated keys to the server, and the server would then use those public keys to generate the application and data to deploy. If hundreds or thousands of clients are to be updated, this process could take a long time to complete. Moreover, the server does not have control of the environment generating the keys. 
     When the entity that creates keys will also be the challenger (i.e., the entity that will be requesting attestation or otherwise relying on the keys), the approach introduced by this disclosure may provide a faster and more efficient method for creating the necessary keys and vTPMs to support those keys. 
     As reflected in  FIG. 2 , like TPMs, vTPMs includes structures for storing data that constitutes the state of the vTPM. For instance, some of the state for a vTPM is stored in PCRs and DIRs. In addition, the state of a vTPM may include various keys, credentials, counters, etc. For purposes of this disclosure, generating a significant portion of the state data for a vTPM may be considered creating a vTPM. 
     In particular, for purposes of this disclosure, the generation of an EK constitutes the creation of a vTPM. As described above, once a vTPM has been created, it may be transferred to a target system. The rest of the state for that vTPM may be generated when a platform, VMM, or VM takes ownership of the vTPM. The process of creating a vTPM and transferring it to another processing system may be referred to in general as remote provisioning and deployment of the vTPM. 
     6.0—Migrating Virtual TPMs 
     In some environments, the fixed nature of the TPM is important. Some environments, however, could benefit from controlled mobility of the TPM across platforms. The TCG has approved a mechanism by which a single key can, with third party intervention, be migrated from one TPM to another. This mechanism can be cumbersome, however, particularly when more than one key needs to be migrated. The paragraphs below introduce a new way to migrate vTPMs from platform to platform in a controlled manner. In the example embodiment, all keys are migrated at once, without requiring third party intervention for each key being migrated. 
     The method of migration described below provides important assurances. For instance, if a vTPM is advertised to have specific security properties, when a key is created in the vTPM, all stakeholders for that key must be assured that, if that vTPM is migrated to another platform, those security properties will also be present in the new platform. Also, when a vTPM is migrated, the vTPM state must be moved, not copied, from the source platform to the destination platform. 
     A migration policy is used to support guarantees of advertised security properties. This policy determines what criteria a vTPM architecture and the platform mechanism protecting this architecture must meet in order for a particular vTPM to be migrated to that platform. Additionally, this policy is strictly enforced. In an example embodiment, a virtual manufacturer authority serves the purpose of enforcing the migration policy, and the virtual manufacturer authority maintains one key per migration policy (e.g., a signing key). When the virtual manufacturer authority registers with the trusted CA in order to gain manufacturer status, the virtual manufacturer authority also sends data to identify the migration policy that will be enforced by the virtual manufacturer authority for any vTPM with credentials that the virtual manufacturer authority signs with this signing key. 
     In an example embodiment, to support the creation of migratable vTPMs, in addition to the vTPM management functions that GVTPM manager  110  is normally required to handle, GVTPM manager  110  will declare vTPMs as either migratable or non-migratable during vTPM creation. Migratable vTPMs may require further specification to indicate which available migration policy will be used. For example, when the virtual manufacturer authority creates an endorsement credential and a platform credential, the virtual manufacturer authority may use a model number indicating the vTPM is migratable, and may sign the credentials with its migratable vTPM manufacturer key. That is, the virtual manufacturer authority may sign the credentials with a signing key recognized as belonging to a manufacturer of migratable vTPMs. 
     Thus, in one embodiment, migratable vTPMs get their endorsement credentials signed by one signing key, while non-migratable vTPMs get their endorsement credentials signed by a different signing key. This methodology allows better identification of migratable vTPMs. In some embodiments, one virtual manufacturer authority signs credentials for migratable vTPMs, and a different virtual manufacturer authority signs credentials for non-migratable vTPMs. 
     An example mechanism for migration is that the PSS on a source platform transfers the state of a migratable vTPM to the PSS in a destination platform. How this transfer is accomplished is dependent on the implementation of the PSS and the GVTPM manager. 
     In an example implementation, the PSS maintains a storage key which the PSS can prove to external entities is protected by a hardware TPM and bound to the specific vTPM architecture. While this key is used to store state, it can also be used to receive state. If the PSS in a first platform (PSS  1 ) wants to migrate a vTPM to a PSS in a second platform (PSS  2 ), the following steps may transpire.
         1. PSS  1  requests that PSS  2  transmit its storage public key and proof of the bindings of this key.   2. PSS  2  sends its storage public key and a nonce to PSS  1 .   3. PSS  1  evaluates the policy for the vTPM on PSS  2 &#39;s state. PSS  1  should only migrate the vTPM if the policy declares PSS  2 &#39;s state to be trustworthy to provide safe operation of the vTPM and to continue to uphold this policy for the next migration.   4. If PSS  2  passes, PSS  1  ensures that the vTPM is not running. PSS  1  then encrypts the following to PSS  2 &#39;s storage key: the persistent state of the vTPM, the nonce from step 2, and the measurement of the correct vTPM in which this state should run.   5. PSS  1  deletes the record for this vTPM from the list of known vTPMs. This ensures that this state cannot be reloaded into this service, and that when the migration is over, this vTPM will only exist under PSS  2 .   6. PSS  1  sends PSS  2  the encrypted blob, along with the endorsement credential for the vTPM.   7. PSS  2  receives the vTPM state, decrypts it, seals it like it does all offline vTPMs, and records it as a known vTPM. This vTPM is now officially part of PSS  2 &#39;s platform.   8. (optional) PSS  2 &#39;s virtual manufacturer authority creates a platform credential for the vTPM. It is inaccurate for the vTPM to continue using PSS  1 &#39;s platform credential, since the vTPM no longer resides on that platform. However, since the trustworthiness of the vTPM is only that of the weakest configuration that will pass the policy, it is not critical for security that this new credential be used.
 
PSS  2  may now load and use the vTPM like it would any other.
       

     The safety of the migration protocol may be validated to prove that it ensures the following: for a given migratable vTPM, the vTPM has never been compromised during its movement. By an inductive proof methodology, this assurance may be established if two following propositions can be proven to a challenger:
         1. The vTPM state was originally created in a safe vTPM.   2. If the vTPM is in a safe vTPM, the vTPM architecture will only migrate the vTPM state to another safe vTPM.
 
The assurance of these two statements is found in the signatures on the credentials. When the vTPM was created, its endorsement credential was created and signed by a virtual manufacturer authority. Prior to this, the virtual manufacturer authority created its signing key and exchanged it, along with the migration policy, with a certificate authority. This CA will not sign an endorsement credentials for a virtual manufacturer authority unless the CA recognizes the virtual manufacturer authority as providing safe vTPMs and enforcing a safe migration policy. A virtual manufacturer authority with credentials signed by the CA therefore will never migrate a vTPM to a vTPM that is not safe.
       

     Typically, a challenger receives a set of PCRs signed by an AIK, and an accompanying identity credential signed by a privacy CA. The challenger may base its assessment of the vTPM on this information. By seeing this credential, the challenger may safely conclude that the vTPM showed the privacy CA an endorsement credential signed by a trustworthy signing key. The privacy CA only trusts signatures that itself or another CA has vouched for. The only way the signature on the endorsement credential will have been vouched for by another CA is if the signing key was in a safe virtual manufacturer authority and the signing key corresponds to a migration policy which only allows for migration to other vTPMs that are safe. 
     Therefore, the existence of the identity credential should convince the challenger that this vTPM was created in a legitimate vTPM, and that it has never resided in a rogue vTPM before giving this attestation. 
     Similarly, the above migration approach may be initiated from the platform to receive the vTPM, rather than the platform to provide the vTPM. 
     The disclosed migration methodology may be useful for a wide range of applications, including usage models in which individuals desire to access data from multiple machines. For example, a physician could conveniently migrate vTPMs between a home computer and an office computer, to allow the physician to access protected medical records from more than one location. The types of protected data that could be accessed include, without limitation, content protected by digital right management (DRM) protocols, proprietary content from the same entity that provides the software for accessing the content, and personal, sensitive, and/or confidential information (e.g., medical records, financial data, etc.) that should only be available to certain entities. 
     Similarly, the above migration approach could support usage models that, in effect, liberate an individual&#39;s computing environment from any particular workstation. For example, the Internet Suspend/Resume (ISR) project described at http://info.pittsburgh.intel-research.net/project/isr/ pertains to an “approach to mobile computing in which a user&#39;s computing environment follows the user [across workstations] as he or she travels.” The ISR project describes one example deployment of a transportable computing environment as follows:
         For example, imagine a telecommuter who works from home in the morning and at the office in the afternoon. After completing a morning&#39;s work, the user clicks “suspend” on the home machine and begins to travel to the office. While the user is en route, the state of the user&#39;s computing environment is also en route, through the network, to the machine in the user&#39;s office. When the telecommuter arrives at the office, the office machine is presenting the same environment that the user left at home: the same applications and files are open, the windows are all in the expected places, and the cursor is in the appropriate location.
 
Such a model and similar models could use vTPM migration to support transportable, TPM-protected computing environments, such as an environment that includes a TPM-protected OS, TPM-protected applications, and/or TPM-protected data.
 
8.0—Conclusion
       

     This disclosure describes a generalized framework for virtualization of security coprocessors such as TPMs. An example embodiment uses a TPM to enable secure virtual TPM operation. Disclosed embodiments allow multiple VMs to use TPM functionality without requiring multiple dedicated hardware TPMs, without requiring modification to the software within a VM, and without requiring modification to remote entities that interact with a TPM or vTPM-protected system. According to the present disclosure, a virtual TPM can measure the OS and applications in a VM to provide attestation to remote entities. Moreover, a virtual TPM can attest to a VM&#39;s state for a hardware TPM challenger, even though the hardware TPM and the challenger may utilize only the functionality described in the current TPM specifications, such as the TPM Version 1.2 Design Specification referenced above. The guest OS in a VM may remain unaware that a hardware TPM is being shared, and trust relationships are not required between the VMs within a system. 
     The disclosed framework may also facilitate the development of secure, software custom cryptographic subsystems which, if implemented in hardware, would be cost prohibitive. Virtual TPMs enable the combination of isolation created by VM technology with TPM functionality that provides hardware-based secure storage and attestation. Custom cryptographic subsystems enable richer functionality than that of the TPM for providing access controls and cryptographic protocols. This disclosure includes embodiments illustrating examples of how the framework can be used in accordance with different security and performance tradeoffs, while ensuring virtualization transparency. This means that applications do not need to treat TPM access from within VMs differently than TPM access on platforms without virtualization. For instance, applications may use the same APIs to communicate with DMs as they would to communicate with physical security coprocessors. 
     This disclosure also describes additional hardware TPM features for enabling simpler, optimized TPM virtualization. In addition, this disclosure describes mechanisms for remotely provisioning and deploying virtual TPMs, and for migrating virtual TPMs between platforms. 
     As indicated above, different embodiments may rely more or less on a hardware TPM to protect data. For instance, all keys may be stored in a hardware TPM, or for increased flexibility and/or performance, virtual keys can be created and used by the vTPM software, and the virtual keys may not be stored in or directly protected by the hwTPM. Private keys belonging to or generated by the virtual TPM may not be operated on by the hardware TPM, in that the hardware TPM may not use those private keys to perform cryptographic operations. Instead, the virtual TPM may use the host processor and cryptographic software to perform cryptographic operations with its private keys. To do this, the virtual TPM service may store its private keys in protected host memory. However, while the private key is not in use, the virtual TPM service may use hardware TPM features to wrap the key to its software configuration. 
     These options may allow the vTPM to encrypt, decrypt, sign, and verify objects in the vTPM software with much higher performance than may be provided by a hardware TPM. These options may thus be preferred for bulk encryption or use in performance-sensitive server environments, for instance. However, a tradeoff for added performance is that virtual keys may be permanently compromised if a vTPM is compromised. 
     In light of the principles and example embodiments described and illustrated herein, it will be recognized that the illustrated embodiments can be modified in arrangement and detail without departing from such principles. For example, virtual TPMs have been described in connection with VMs, but alternative embodiments also include vTPMs used in connection with other types of system subdivisions, such as partitions within a server or group of servers that share a hardware TPM. For instance, virtual TPMs may be used in a four-processor system that is partitioned into two logical two-processor systems. The teachings herein could also be used to provide a logical TPM to one or more service coprocessors, or to one or more other types of independent processing elements on a hardware platform. 
     Furthermore, alternative embodiments include vTPM services that do not emulate a hardware TPM, but do extend and/or amplify the capabilities of a hardware TPM (e.g., by providing more PCRs, more storage, etc.). Alternative embodiments also include a virtual TPM service running on top of a secure OS, on top of a managed run-time environment (MRTE), in a service processor or coprocessor, in a system management mode (SMM) of a platform, etc. Instead of or in addition to providing virtual TPMs, additional embodiments provide other kinds of emulated security coprocessors. 
     Also, the foregoing discussion has focused on particular embodiments, but other configurations are contemplated. In particular, even though expressions such as “in one embodiment,” “in another embodiment,” or the like are used herein, these phrases are meant to generally reference embodiment possibilities, and are not intended to limit the invention to particular embodiment configurations. As used herein, these terms may reference the same or different embodiments that are combinable into other embodiments. 
     Similarly, although example processes have been described with regard to particular operations performed in a particular sequence, numerous modifications could be applied to those processes to derive numerous alternative embodiments of the present invention. For example, alternative embodiments may include processes that use fewer than all of the disclosed operations, processes that use additional operations, processes that use the same operations in a different sequence, and processes in which the individual operations disclosed herein are combined, subdivided, or otherwise altered. 
     Alternative embodiments of the invention also include machine accessible media encoding instructions for performing the operations of the invention. Such embodiments may also be referred to as program products. Such machine accessible media may include, without limitation, storage media such as floppy disks, hard disks, CD-ROMs, ROM, and RAM; as well as communications media such antennas, wires, optical fibers, microwaves, radio waves, and other electromagnetic or optical carriers. Accordingly, instructions and other data may be delivered over transmission environments or networks in the form of packets, serial data, parallel data, propagated signals, etc., and may be used in a distributed environment and stored locally and/or remotely for access by single or multi-processor machines. 
     It should also be understood that the hardware and software components depicted herein represent functional elements that are reasonably self-contained so that each can be designed, constructed, or updated substantially independently of the others. In alternative embodiments, many of the components may be implemented as hardware, software, or combinations of hardware and software for providing the functionality described and illustrated herein. 
     In view of the wide variety of useful permutations that may be readily derived from the example embodiments described herein, this detailed description is intended to be illustrative only, and should not be taken as limiting the scope of the invention. What is claimed as the invention, therefore, is all implementations that come within the scope and spirit of the following claims and all equivalents to such implementations.