Patent Publication Number: US-9425965-B2

Title: Cryptographic certification of secure hosted execution environments

Description:
RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 13/323,465, filed on Dec. 12, 2011, the disclosure of which is hereby incorporated herein by reference in its entirety for all purposes. U.S. patent application Ser. No. 13/323,465 is related to U.S. patent application Ser. No. 13/323,562, filed on Dec. 12, 2011. 
    
    
     BACKGROUND 
     In a conventional computing environment, the user controls physical access to the user&#39;s computing systems. The user trusts, to some degree, the hardware and software in its data centers. This trust, combined with physical control of the devices, provides the user with a certain degree of confidence that their computing systems are secure. 
     In a hosted computing environment, the user typically does not have physical control over the computing systems used to execute the user&#39;s applications. The user, in addition to trusting the hardware and software that executes in the hosted computing environment, has no choice but to trust the hosted computing provider not to tamper with or snoop on the user&#39;s code and data. The user also trusts the hosted computing provider to provide physical security sufficient to prevent unauthorized persons from removing hard disks or tampering with the system. And users place their trust in the hosted computing provider to prevent third parties from tampering with or stealing their data. A hosted computing provider may therefore incur a certain amount of liability, in the form of guarantees and the like, to encourage users to run their software in the provider&#39;s hosted computing environment. 
     BRIEF SUMMARY 
     This Summary is provided in order to introduce simplified concepts of the present disclosure, which are further described below in the Detailed Description. This summary is not intended to identify essential features of the claimed subject matter, nor is it intended for use in determining the scope of the claimed subject matter. 
     Embodiments of the present disclosure enable an application hosting service to cryptographically certify that it provides a secure execution environment that is resistant to snooping and tampering such that it includes, for example, only the user&#39;s trusted code and data. In order to service a request from a client system to establish a secure execution environment, a protected memory area is instantiated by a security-enabled processor. The hosted computing system goes through an attestation protocol to provide verifiable facts about the security-enabled processor and the software and data in the secure execution environment, such as the manufacturer and model of the security-enabled processor and the vendor or code identity of the software. Upon successful completion of the attestation protocol, a cryptographically protected communication channel is established between the client system and the secure execution environment, and one or more applications are executed within the secure execution environment. 
     The application hosting service may use various trust certificates, including certificates from a trusted authority and one or more intermediaries, to establish a chain of trust from the security-enabled processor to the trusted authority. These trust certificates collectively may be used in the attestation protocol to certify the security-enabled processor&#39;s security. The application hosting service may be audited to verify that the security-enabled processors of the application hosting service are physically secured and have not been tampered with. The auditor may provide an auditor certificate that may be used as part of the attestation protocol. Alternatively, the auditor may make the results of the audit available in other ways (e.g. publish them on the internet). The application hosting service may use (in the attestation protocol) cryptographic credentials for the processor, produced by the hardware manufacturer, vouching for the integrity and proper functioning of the security-enabled processor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The Detailed Description is set forth with reference to the accompanying figures. In the figures, the left-most digit(s) of a reference number identifies the figure in which the reference number first appears. The use of the same reference numbers in different figures indicates similar or identical items. 
         FIG. 1  is a schematic diagram of an example system usable to provide a secure execution environment. 
         FIG. 2  is a block diagram of an example computing system usable to provide an application hosting service according to embodiments. 
         FIG. 3  is a block diagram of an example computing system usable to provide a client system according to embodiments. 
         FIG. 4  is a flow diagram showing an example process for instantiating a secure execution environment. 
         FIG. 5  is a flow diagram showing an example process for verifying establishment of a secure execution environment. 
         FIG. 6  shows an environment for migration of a protected memory area according to embodiments. 
         FIG. 7  is a flow diagram showing an example process for migrating a secure execution environment. 
         FIG. 8  is a flow diagram showing an example process for re-creating a secure execution environment. 
     
    
    
     DETAILED DESCRIPTION 
     Overview 
     As discussed above, a user places a certain degree of trust in a conventional hosted computing provider to securely execute the user&#39;s applications and safeguard the user&#39;s data. Embodiments of the present Detailed Description allow the hosted computing service to provide cryptographic certification both that a user&#39;s execution environment is resistant to tampering and snooping, and that the user&#39;s execution environment is established with the content that the client requests and with no untrusted code or data. Providing a secure execution environment that is free from outside snooping and tampering may, by itself, enable a nefarious hosted computing provider to set up the execution environment with untrusted code that is able to snoop on or tamper with the user&#39;s code and data from within. And simply providing an execution environment with nothing but the user&#39;s trusted code and data may, by itself, enable the hosted provider or a third party to tamper with or snoop on the contents of the execution environment from outside the execution environment. But embodiments of the present disclosure enable a hosted computing provider to cryptographically certify that it provides a secure execution environment that is resistant to outside snooping and tampering and that includes no untrusted code and data. 
     Computing systems according to embodiments include a security-enabled processor configured to instantiate, for a client system (such as a computing device controlled by a hosted computing user or consumer), a secure computing environment including a protected memory area. The code and data selected by the client system is stored in a protected memory area and is accessible to code stored in the protected memory area, but inaccessible to all code executing outside the protected memory area. The latter includes code running in other protected memory areas that might exist. The code in the secure execution environment can be chosen by the client system, the service provider, third parties, or a combination of all of them. For example, the client system might choose to execute only its application code (including support libraries) in the secure execution environment. The execution of this code is protected from all other code on the computer. 
     Threads can transition from running code outside the protected memory area to running code inside the protected memory area only through specific entry gate functions mediated by the security-enabled processor. Likewise, threads transition from running code inside the protected memory area to running code outside the protected memory area through specific exit gate functions mediated the security enabled processor. The code that runs in the protected memory area has no special privileges except the ability to access code and data in the protected memory area. For example, the code that runs in the protected memory area does not need to run in the processor&#39;s kernel mode or privileged mode, nor does it need access to instructions, such as I/O instructions, accessible only to the processor&#39;s kernel mode or privileged mode. The hardware-protected memory area is brought to a well-known initial state and then loaded with a loader module and one or more parameters specified by the user&#39;s client system in order to establish a requested activation state of the protected memory area. 
     The trusted execution environment provides a mechanism by which the user-trusted code running within the protected memory area certifies to the client system that it is running within a secure execution environment. The security-enabled processor performs an attestation protocol, involving providing the client system with a certification that a secure execution environment is established, and that in an initial activation state of the secure execution environment, only the software identified (explicitly or implicitly) in a request from the client user is executed. The attestation protocol may involve the client or other parties. The purpose of the attestation protocol is to cryptographically verify to the client system (or other system) that the secure execution environment has particular properties. These properties may include, in various non-limiting examples:
         1. The manufacturer and model of the security-enabled processor.   2. The code and data with which the secure execution environment was initiated.   3. The software provider and other information about the code and data with which the secure execution environment was initiated. For example,
           a. the software was written by (and signed by) a particular software developer/vendor   b. the software is a particular version with security patches as of a particular date. In various non-limiting embodiments, the software provider signs certificates containing digests, such as hashes of the relevant software modules.   
               

     Non-limiting examples of attestation protocols include:
         Direct Anonymous Attestation (reference: E. Brickell, J. Camenish, L. Chen. Direct anonymous attestation. In Proceedings of the 11 th  ACM conference on computer and communications security. Pages 132-145, 2004.   Standard public key protocols involving attestation certificates signed with the private key of the security-enabled processor.       

     The following describes various embodiments of an attestation protocol. Embodiments are not limited to the following embodiments, and the attestation protocol described below may include additional functionality, such as with chains of trust rooted in one or more trusted certificate authorities, without departing from the scope of the present Detailed Description. Once the protected memory area is instantiated with the requested activation state, the security-enabled processor produces an identifier that identifies the initial activation state of the hardware-protected memory area and stores the identifier in a location accessible only to the security-enabled processor. The identifier may include a digest, such as a hash, of the activation state of the protected memory area. The identifier may include a public key that the security-enabled processor used to decrypt the contents placed into the protected memory area in the activation state. The identifier may be some other identifier that identifies the activation state. 
     The loader module is executed and causes the security-enabled processor to create an attestation certificate signed by a private key of the security-enabled processor. The signed attestation certificate is transmitted to the client system and therefore enables the client system to verify, using a known public key of the security-enabled processor that corresponds to the private key of the security-enabled processor, that the attestation certificate is signed by the security-enabled processor. The signed attestation certificate also enables the client system to verify that the client system communicates with a loader module running in a protected memory area created by the security-enabled processor. Thus, a trust relationship is formed between the client system and the security-enabled processor. A chain of trust including additional certificates from a trusted authority, and possibly one or more intermediaries, may be used in embodiments to establish the trust relationship. 
     The attestation certificate includes the identifier of the activation state of the protected memory area. The client system compares the identifier with a known identifier of the requested activation state to determine that the activation state of the protected memory area is the requested activation state, including the loader module and the one or more parameters. Because the attestation certificate with the identifier is signed/encrypted with the security-enabled processor&#39;s private key, and because a trust relationship is established between the client system and the security-enabled processor, the client system is able to rely on the identifier to determine that the activation state of the protected memory area is the requested activation state. 
     Thus, embodiments provide the client system with verification of both that the hosted computing provider establishes a secure execution environment resistant to tampering and snooping, and that the secure execution environment is instantiated with the requested activation state. The signed attestation certificate provides the client system with verification that the secure execution environment is established. And the identifier provides the client system with verification that the secure execution environment is instantiated with the requested activation state. The client system then utilizes the secure execution environment to load and execute requested applications. 
     Although the security-enabled processor may be resistant to casual physical attacks, it may be vulnerable to physical tampering. The hosted computing system is also configured, in various embodiments, to certify to the client system that the security-enabled processor is physically secure, for example in one embodiment the hosted computing system is configured to transmit an auditor certificate, signed by an auditing entity&#39;s private key, declaring that the security-enabled processor has not been physically tampered with during a specific time period. Personnel from the auditing entity may periodically or continually monitor the hosted computing service to determine that the security-enabled processors are physically intact. The auditor certificate therefore provides the client system with additional degrees of confidence in the secure execution environment. In another embodiment, the client system presents the certificate of the security-enabled processor directly to a computer system of the auditing entity requesting verification of the processor&#39;s physical security. The computer system of the auditing entity responds with a certificate verifying that the security-enabled processor has not been physically tampered with. 
     The application hosting service according to various embodiments described herein runs only code in the secure execution environment that has been selected by the client system. An entity associated with the client system may write all of the software that runs within the secure execution environment, or the entity may outsource portions of the software to software providers that the entity trusts. In one non-limiting example, the client system may select an application from a trusted application software vendor and a library operating system from a trusted operating system vendor. The entity associated with the client system is considered the software vendor for the portions of the software created directly by the entity. The software vendors may provide certificates for signed binaries verifying that the software binary files are indeed the ones provided by the software vendors respectively, and that the binaries have not been altered. 
     With these divisions of responsibility, the application hosting service acts as an intermediary, but does not actually certify the integrity of any system component. The security-enabled processor vendor certifies the secure execution environment. The auditing entity certifies the physical security of the secure execution environment. The software providers certify the software running within the secure execution environment. The application hosting service may perform some, none, or all of these roles in various embodiments. In embodiments, the application hosting service provider maintains availability of the computing facility, which includes providing the computing facility, power, and network connectivity, and other entities, such as the hardware providers, software providers, and auditing entities, provide for other various aspects of the security of the applications being hosted. 
     Examples of application hosting services include internet hosting facilities, cloud computing providers, outsourced corporate data centers, corporate data centers operated by contract, and content delivery networks. 
     The processes, systems, and devices described herein may be implemented in a number of ways. Example implementations are provided below with reference to the following figures. 
     Example Environment for Providing a Secure Execution Environment 
       FIG. 1  is a schematic diagram of an example system  100  usable to provide a secure execution environment. Aspects of the system  100  may be implemented on various suitable computing device types that are capable of implementing an application hosting computing system, a client computing system, and so forth. Suitable computing device or devices may include, or be part of, one or more personal computers, servers, server farms, datacenters, special purpose computers, tablet computers, game consoles, smartphones, combinations of these, or any other computing device(s) capable of storing and executing all or part of a secure execution environment. 
     An application hosting service  102  includes a memory  104  and a security-enabled processor  106 . The memory  104  includes a host operating system (OS)  108  and a set-up module  110 . Although the set-up module  110  is shown in  FIG. 1  to be separate from the host OS  108 , the set-up module  110  may be a component of the host OS  108 . Also, the application hosting service  102  may include multiple processors, including multiple security-enabled processors such as the security-enabled processor  106 . The host OS  108  and/or the set-up module  110  may execute on the security-enabled processor  106 , or on one or more other processors not shown in  FIG. 1 . 
     System  100  performs various functions, such as but not limited to one or more of the following: (a) initializing secure execution environments with code and data; (b) receiving client requests to bind an instance of a secure execution environment to a client system and configure it to run the client&#39;s software; (c) binding an instance of a secure execution environment to a client and configuring the secure execution environment to run the client&#39;s software; (d) providing to the client a certified specification of the software that is to run within the secure execution environment. These various functions may be performed in different orders, depending on specific embodiments. Furthermore, specific embodiments may combine some of the functions. 
     In one embodiment, the hosting service may perform function (b) prior to performing other ones of the above-mentioned functions. When a client request arrives, the hosting service initializes a secure execution environment (action a). The hosting service may include code and/or data (e.g. parameters) supplied in the client request in the initialization. Thus, binding (action c) may be performed implicitly as part of action (b). Alternatively, the hosting service may initialize the secure execution environment (action a) with generic code and data (not specific to the client) and bind the secure execution environment to a client (action c) in a separate step. 
     In another embodiment, the hosting service initializes one or more secure execution environments (action a) with generic code and data. This generic code and data could provide a generic run time environment for arbitrary applications. When a client request arrives (action b), the hosting service selects one of the previously initialized secure execution environments and binds it to the client (action c) by sending it code or data from the client request. 
     Actions (c) and (d) can be combined. For example, variants of authenticated key exchange protocols may perform an attestation protocol. The attestation protocol provides to the client verifiable properties about the software and data in the secure execution environment (action d) and establishes a shared cryptographic key between the secure execution environment and the client (action c). 
     The following example is a detailed description of one class of embodiments. The set-up module  110  receives a request from the client system  112 , via network  114 , to establish a secure execution environment on the application hosting service  102 . The network  114  may be the public Internet, or some other wired or wireless network type. Embodiments are not limited to any type or types of networks. The request is accompanied by an indication of a loader module  116  and one or more parameters  118 . The indication of the loader module  116  may be an identifier for the loader module  116 , or it may be an application binary of the loader module  116  itself, or some other indicator. In embodiments where the indication of the loader module  116  is an identifier, it may be a uniform resource identifier (URI), such as a uniform resource locator (URL), identifying the loader module  116  and possibly a location where the loader module  116  can be found. 
     The set-up module  110  causes, in response to receipt of the request, the security-enabled processor  106  to instantiate a protected memory area  120 , which is a hardware-protected memory area, within memory  104 . The set-up module  110  provides the security-enabled processor  106  with pointers to the loader module  116  and the parameters  118 , and instructs the security-enabled processor  106  to bring the protected memory area  120  to a well-known initial state (such as all memory addresses within the protected memory area  120  and all appropriate registers within the security-enabled processor  106  zeroed-out, or to some other well-known initial state), and to load the loader module  116  and the parameters  118  into the protected memory area  120  after bringing the protected memory area  120  to the well-known initial state. The instantiation of the protected memory area  120  first into the well-known initial state and then loaded with the loader module  116  and the parameters  118  represents a requested activation state of the protected memory area  120 . In other words, it represents the state of the secure execution environment that the client device specifies in its request to set up a secure execution environment. 
     The combination of the protected memory area  120  and execution of code therein by the security-enabled processor  106  represents the secure execution environment. Although the protected memory area  120  is shown as being part of a contiguous memory area that also includes the host OS  108  and the set-up module  110 , the protected memory area  120  may in alternative embodiments be part of a separate memory area, such as a memory area on the same integrated circuit as the security-enabled processor  106  that physically isolates the protected memory area  120  from the rest of the application hosting service  102 . 
     The security-enabled processor  106  may be configured to encrypt and decrypt all data written to and read from, respectively, the protected memory area  120  in order to prevent outside snooping on the protected memory area  120 . The security-enabled processor  106  may also be configured to produce hashes, or other digests, of the data written to the protected memory area  120  in order to verify, upon a read of the contents of the protected memory area  120 , that the contents have not been altered. 
     As part of the instantiation process, the security-enabled processor  106  produces an identifier  122  identifying the activation state of the protected memory area  120 . The identifier may be, in various embodiments, a digest—such as a hash—of the contents of the protected memory area  120  in the activation state. The identifier may be a public key corresponding to a private key that was used to sign the software stored in the protected memory area  120 . The activation state includes the loader module  116 , the parameters  118 , and any other code or data placed into the protected memory area  120  upon instantiation. The identifier  122  may be stored in a location that is accessible only to the security-enabled processor  106 , such as in a register or memory location within the security-enabled processor  106  that is inaccessible except by the security-enabled processor  106 , or perhaps encrypted in an area of memory  104 . In embodiments wherein the identifier is a hash of the contents of the protected memory area  104  in the activation state, the security-enabled processor produces the hash using a cryptographic hash function, such as the MD5 Message-Digest Algorithm, Secure Hash Algorithms SHA-0, SHA-1, SHA-2, or other hash function. 
     Upon instantiation of the protected memory area  120  with the loader module  116  and the parameters  118 , the set-up module  110  instructs the security-enabled processor  106  to execute the loader module  116 , such as through an entry function or gate which the security-enabled processor  106  uses in order to enable the secure execution environment to receive communications from outside the secure execution environment. An instance of the loader module  116  executes on the security-enabled processor and causes the processor to create an attestation certificate  124  signed by a security-enabled processor (SEP) private key  126 . The SEP private key  126  is permanently stored on the security-enabled processor  106  in a way that is accessible only to the security-enabled processor  106 . Thus, so long as the security-enabled processor  106  is physically intact, an entity receiving the attestation certificate  124  can have a high degree of confidence that the signed attestation certificate  124  is signed by the security-enabled processor  106 . 
     The attestation certificate  124  may include, among other things, the identifier  122 . The loader module  116  then transmits the attestation certificate to the client system  112 , via an exit function or exit gate employed by the security-enabled processor to enable the secure execution environment to communicate with the outside world. Alternatively, the identifier  122  may be encrypted with the SEP private key  126  and transmitted to the client system  112  separately from the attestation certificate  124  (which would also be signed/encrypted using the SEP private key  126 ). 
     Upon receipt of the attestation certificate  124 , the client system  112  decrypts it using a SEP public key  128  that corresponds to the SEP private key  126  of the security-enabled processor. The means by which the client system  112  obtains the SEP public key  128  is described below. 
     The client system  112  compares the identifier  122  contained within the decrypted attestation certificate  124  with a known identifier  130  of the requested activation state of the secure execution environment. A determination that the known identifier  130  matches the identifier  122  provides the client system  112  with a high degree of confidence that the actual activation state of the protected memory area  120  matches the requested activation state of the protected memory area  120 . In various non-limiting examples, successful verification of the identifier  122  provides the client system  112  with confidence that the protected memory area  120  includes the loader module  116 , the parameters  118 , any other code or data implicitly or explicitly specified in the request to establish a secure execution environment, and nothing else. As noted above, the attestation certificate  124  is signed/encrypted by using the SEP private key that is accessible only by the security-enabled processor  106 . The attestation certificate  124  includes the identifier  122  of the activation state of the protected memory area  120 , and the identifier  122  is produced by the security-enabled processor  106  and may be stored securely in a way that makes the identifier  122  accessible only to the security-enabled processor  106 . 
     Because the loader module  116  or other code (such as some malicious code surreptitiously loaded within the protected memory area  120 ) cannot access the SEP private key  126 , the loader module  116  or other code cannot alter the identifier  122  without also altering the attestation certificate  124  and invalidating the security-enabled processor&#39;s  106  signature. Thus, upon successfully using the SEP public key  128  to verify that the attestation certificate  124  is properly signed using the SEP private key  126 , and upon successfully verifying that the identifier  122  contained therein matches the known identifier  130 , the client system  112  can have a high degree of confidence that it communicates with a secure execution environment that was instantiated with the requested activation state. 
     To achieve this high degree of confidence, the client system  112  forms a trust relationship with the security-enabled processor  106 . Merely possessing the SEP public key  128  may be insufficient to establish that the security-enabled processor  106  is a true security-enabled processor that is properly configured to provide a secure execution environment on the application hosting service  102 . A chain of trust is therefore provided to vouch for the authenticity of the security-enabled processor  106 . 
     The loader module  116 , or the set-up module  110 , may transmit one or more trust certificates, such as a trusted authority (TA) certificate  132  and possibly one or more intermediary certificates  134 . The TA certificate  132  is signed using a private key of a trusted authority  136 . The TA certificate  132  identifies either the security-enabled processor  106 , or possibly one or more intermediaries, and provides the public key of either the security-enabled processor  106  (i.e., the SEP public key  128 ), or public keys of the intermediate authority directly below it. The client system  112  may obtain the trusted authority (TA) public key  138  and use it to decrypt the TA certificate  132  and then obtain the public key published therein. Any intermediate certificates  134  are decrypted, and the public keys of any underlying intermediaries are extracted from the intermediate certificates  134 . Ultimately, the client system  112  is able to obtain—either from the TA certificate  132  or from one of the intermediate certificates  134 , the SEP public key  128 . 
     This process creates a chain of trust from the trusted authority  136  to the security-enabled processor  106 . Essentially, the trusted authority vouches for the most immediate intermediary, the intermediaries vouch for any lower-level intermediaries, and ultimately one of the intermediaries (or the trusted authority  136  if there are no intermediaries) vouches for the security-enabled processor  106  and provides the SEP public key  128 . By following the chain of trust in this way, the client system  112  is able to establish a trust relationship with the security-enabled processor  106 . 
     The trusted authority  136  may be the hardware manufacturer that manufactured the security-enabled processor  106 . Alternatively, the trusted authority  136  may be some other entity that provides assurances that an intermediary—which may be the hardware manufacturer—is trustworthy. 
     In the same or different embodiments, the set-up module  110  is further configured to transmit, to the client system  112 , an auditing certificate  140 , signed by a private key of an auditor entity, indicating that the security-enabled processor has not been tampered with. In the same or alternative embodiments, the client system  112  provides the auditor entity with an identity of the security-enabled processor  106  (which may be included in the attestation certificate) and requests that the auditor entity provide the client system  112  with the auditing certificate  140 . The auditor entity may employ one or many mechanisms for verifying the physical security of the security-enabled processor. For example, personnel of the auditor entity may periodically visit the data center(s) that house the application hosting service  102 , physically inspect the computing devices, and verify that the security-enabled processor  106  is physically uncompromised, has not been tampered with, and is otherwise intact. In other embodiments, personnel of the auditor entity may continuously monitor the data center(s) that house the application hosting service  102  using closed circuit cameras, or personnel of the auditor entity may conduct random inspections of randomly-chosen computing devices within the data center(s) that house the application hosting service  102 . Depending on the auditing processes employed, the auditor entity may offer different levels or degrees of certification of physical security to suit the business needs of various clients. In various embodiments, the auditing certificate  140  may be part of the chain of trust described above. Alternatively, the auditing certificate may be a stand-alone certificate (perhaps backed by its own chain of trust) used by the client system  112  to further verify that the facilities provided by the application hosting service  102  are secure. 
     Once the client system  112  verifies that the attestation certificate  124  is properly signed by the security-enabled processor  106  and that the identifier  122  contained therein matches the known identifier  130  (and possibly after verifying the chain of trust via the TA certificate  132  and the intermediate certificate(s)  134  and any other certificates such as the auditing certificate  140 ), the client system  112  and the loader module  116  may establish an encrypted communication channel. In one embodiment, to establish an encrypted communication channel, the client system  112  produces a session key, encrypts the session key with the SEP public key  128 , and transmits the encrypted session key to the loader module  116 . The loader module  116  receives the session key (such as through an entry function or gate of the security-enabled processor  106 ). The loader module  116  causes the security-enabled processor  106  to decrypt the session key using the SEP private key  126 , and the loader module  116  establishes communications with the client system  112  using the decrypted session key The client system  112  and the loader module  116  use the session key to cryptographically protect communications between them. 
     The attestation protocol described above may be protected from various state replay attacks using various methods, such as those described in U.S. Pat. No. 7,421,579, issued to England et al. on Sep. 2, 2008 and entitled “Multiplexing a secure counter to implement second level secure counters”; U.S. Pat. No. 7,065,607, issued to England et al. on Jun. 20, 2006 and entitled “System and method for implementing a counter”; and as described in “Memoir: Practical State Continuity for Protected Modules”, by Bryan Parno, Jacob R. Lorch, John R. Douceur, James Mickens, and Jonathan M. McCune, and published in Proceedings of the IEEE Symposium on Security and Privacy, IEEE, May 2011. 
     The client system  112  instructs the loader module  116  to load and execute an application  142 . The loader module  116  then loads and executes the application  142  into the protected memory area  120 . Alternatively, the application  142  is pre-identified by the parameters  118  and is loaded by the loader module  116  upon establishment of the encrypted channel. In still other embodiments, the parameters  118  include an application binary of the application  142 , and the loader module  116  receives a command via the encrypted channel to execute the application  142 . In other embodiments, the application  142  is loaded as part of the activation state of the protected memory area  120 . In other embodiments, the application  142  is loaded over the Network  114  from an application vendor. Other variations are possible without departing from the scope of the present disclosure. 
     The application  142  may include an operating system subsystem (sometimes referred to as a “library OS”) such as is described in U.S. patent application Ser. No. 12/834,895, filed Jul. 13, 2010 and entitled “ULTRA-LOW COST SANDBOXING FOR APPLICATION APPLIANCES.” The operating system subsystem provides various operating system elements within the application process. The operating system subsystem also utilizes a small subset of application programming interfaces (APIs) to communicate with a host operating system, via an operating system platform adaptation layer (PAL), in order to provide the application  141  with basic computation services. 
     In any event, loading and executing the application  142  that is specified by the client system  112  in the secure execution environment may be the ultimate aim of the attestation protocol described above. The attestation protocol thus provides a user associated with the client system  112  with a high degree of confidence that the application  142  executes within a secure execution environment that is free of snooping and tampering from the outside, and that is loaded with no untrusted content. 
     Example Computing System for Providing an Application Hosting Service 
       FIG. 2  is a block diagram of an example computing system  200  usable to provide an application hosting service according to embodiments. The computing system  200  may be configured as any suitable computing device capable of implementing an application hosting service. According to various non-limiting examples, suitable computing devices may include personal computers (PCs), servers, server farms, datacenters, special purpose computers, tablet computers, game consoles, smartphones, combinations of these, or any other computing device(s) capable of storing and executing all or part of an application hosting service. 
     In one example configuration, the computing system  200  comprises one or more processors  202  and memory  204 . The processors  202  include one or more security-enabled processors that are the same as or similar to security-enabled processor  106 . The processors  202  may include one or more general-purpose or special-purpose processors other than a security-enabled processor. The computing system  200  may also contain communication connection(s)  206  that allow communications with various other systems. The computing system  200  may also include one or more input devices  208 , such as a keyboard, mouse, pen, voice input device, touch input device, etc., and one or more output devices  210 , such as a display, speakers, printer, etc. coupled communicatively to the processor(s)  202  and memory  204 . 
     Memory  204  may store program instructions that are loadable and executable on the processor(s)  202 , as well as data generated during execution of, and/or usable in conjunction with, these programs. In the illustrated example, memory  204  stores an operating system  212 , which provides basic system functionality of the computing system  200  and, among other things, provides for operation of the other programs and modules of the computing system  200 . The operating system  212  may be the same as or similar to the host OS  108 . 
     Memory  204  includes a set-up module  214 , which may be the same as or similar to the set-up module  110 . Memory  204  includes a protected memory area  216 , established by the security-enabled processor. The protected memory area  216  may be the same as or similar to the protected memory area  120 . 
     Memory  204  includes a loader module  218 , which may be the same as or similar to the loader module  116 . The loader module  218  may be loaded into the protected memory area  216  upon request by a client system. Memory  204  includes a TA certificate  220  and one or more intermediate certificates  222 , which may be the same as or similar to the TA certificate  132  and the intermediate certificate(s)  134 , respectively. 
     Memory  204  includes an auditing certificate  224 , which may be the same as or similar to the auditing certificate  140 . Memory  204  includes a persistence module  226 , which may be the same as or similar to the persistence module  614 . 
     Example Computing System for Providing a Client System 
       FIG. 3  is a block diagram of an example computing system  300  usable to provide a client system according to embodiments. The computing system  300  may be configured as any suitable computing device capable of implementing a client system. According to various non-limiting examples, suitable computing devices may include personal computers (PCs), servers, server farms, datacenters, special purpose computers, tablet computers, game consoles, smartphones, combinations of these, or any other computing device(s) capable of storing and executing all or part of a client system. 
     In one example configuration, the computing system  300  comprises one or more processors  302  and memory  304 . The computing system  300  may also contain communication connection(s)  306  that allow communications with various other systems. The computing system  300  may also include one or more input devices  308 , such as a keyboard, mouse, pen, voice input device, touch input device, etc., and one or more output devices  310 , such as a display, speakers, printer, etc. coupled communicatively to the processor(s)  302  and memory  304 . 
     Memory  304  may store program instructions that are loadable and executable on the processor(s)  302 , as well as data generated during execution of, and/or usable in conjunction with, these programs. In the illustrated example, memory  304  stores an operating system  312 , which provides basic system functionality of the computing system  300  and, among other things, provides for operation of the other programs and modules of the computing system  300 . 
     Memory  204  includes an establishment module  314  configured to transmit a request to an application hosting service—such as the application hosting service  102 —to establish a secure execution environment within the application hosting service. The request includes an indication of a requested activation state of the secure execution environment, such as a loader module  316  and one or more parameters that are to be loaded into the secure execution environment. 
     The verification module  318  is configured to receive, from an instance of the loader module  316  executing in a protected memory area of the application hosting service, an encrypted attestation certificate. The encrypted attestation certificate is encrypted/signed with a private key of a security-enabled processor of the application hosting service. The verification module  318  is, in various embodiments, configured to decrypt the attestation certificate using a SEP public key  320  of the security-enabled processor. Successful decryption of the attestation certificate with the SEP public key  320  indicates that the attestation certificate was encrypted/signed by the security-enabled processor. The verification module  318  is configured, in various embodiments, to receive one or more trust certificates, such as a trusted authority certificate and one or more intermediate certificates, to establish a chain of trust between the trusted authority and the security-enabled processor, as is described elsewhere within this Detailed Description. The one or more trust certificates may collectively vouch for the identity of the security-enabled processor and/or to indicate that the security-enabled processor is secure. 
     The verification module  318  is configured, in various embodiments, as part of or in addition to establishment of the chain of trust, to receive an auditing certificate signed by a private key of an auditor entity indicating that the security-enabled processor is physically uncompromised. The auditing certificate may be provided by the application hosting service, or by some other entity. 
     The verification module  318  is configured, in various embodiments, as part of or in addition to establishment of the chain of trust, to receive a processor certificate—such as from a manufacturer of the security-enabled processor—that indicates that the security-enabled processor is secure. 
     Upon successfully establishing a chain of trust, and verifying that any other certificates such as the auditing certificate and/or a processor certificate are valid (such as by decrypting such certificates using public keys of their issuers), the verification module  318  accepts that the security-enabled processor is a legitimate security-enabled processor. 
     The verification module  318  is configured to extract an identifier from the attestation certificate and compare it to known identifier  322 . The known identifier  322  represents the requested activation state of the secure execution environment, as identified in the request transmitted by the establishment module. The establishment module  314  is configured to establish, in response to verification by the verification module  318  that the legitimacy of the security-enabled processor is verified and that the digest matches the known identifier  322  of the requested activation state, an encrypted connection with the instance of the loader module executing in the secure execution environment. The known identifier  322  may include a digest, such as a hash, of the requested activation state of the protected memory area. The known identifier  322  may include a public key matching a private key that was used to sign the contents placed into the protected memory area in the requested activation state. The known identifier  322  may be some other identifier that identifies the initial activation state. In some embodiments, the establishment module  314  produces a session key for the encrypted connection, encrypts the session key using the SEP public key  320  of the security-enabled processor, and transmits the encrypted session key to the instance of the loader module executing in the secure execution environment of the application hosting service. The encrypted connection utilizes the session key to send and receive data to and from the secure execution environment. Other embodiments of establishing an encrypted connection are possible without departing from the scope of this present Detailed Description 
     The establishment module  314  instructs the instance of the loader module executing in the secure execution environment to load an application  324  for execution within the secure execution environment. Neither the application  324  nor the loader module need be included in the computing system  300 . Rather, the establishment module  314  may instruct the loader module to download the application  324  from some other location, such as by providing a URI or URL for the application  324 . The one or more parameters provided by the establishment module  314  in the request sent to the application hosting service may identify the application  324  for execution within the secure execution environment. The parameters may include a URI, URL, or other identifier of the application  324 . Alternatively, the parameters may include an application binary for the application  324  that is directly loaded into the secure execution environment upon instantiation. 
     Memory  304  may also include a client-side persistence module  326  configured to perform one or more functions associated with persisting the secure execution environment such as, for example, in order to migrate the secure execution environment between computers in an application hosting service, or in order to re-create the secure execution environment on the same computer in the application hosting service, as is described in more detail with respect to  FIGS. 6-8 . Such client-side persistence functions include receiving a persistence key as part of a migration of a secure execution environment, decrypting the persistence key with a private key of the computing system  300 , and transmitting the unencrypted persistence key to a persistence module, such as persistence module  614 , residing on a migration computer of an application hosting service, such as migration computer  604 . 
     Example Operations for Instantiating a Secure Execution Environment 
       FIG. 4  is a flow diagram showing an example process  400  performed by an application hosting service for instantiating a secure execution environment. At  402 , a set-up module, such as the set-up module  110 , receives a request from a client system to establish a secure execution environment on an application hosting service, such as the application hosting service  102 . The request includes an indication of a loader module, such as the loader module  116 , and one or more parameters, such as parameters  118 . The indication of the loader module may be a URI or URL (or other identifier type) for the loader module, or it may be an application binary of the loader module, application package, and so forth. The parameters may indicate an application requested to be run in the secure execution environment. The parameters may be an application binary of the application requested to be run in the secure execution environment. The parameters may be some other parameter that the client system requests to be placed into a protected memory area of the secure execution environment. In various embodiments, the parameters may be omitted from the request. 
     At  404 , the host operating system places the loader module and the parameters into an area of memory to be protected. 
     At  406 , the set-up module instructs a security-enabled processor (SEP) of the application hosting service, such as the security-enabled processor  106 , to instantiate, in response to the request, a protected memory area that includes the loader module and one more parameters identified by the request. 
     At  408 , the security-enabled processor establishes a protected memory area by putting the area of memory that includes the loader module and the parameters into a protected state. The protected memory area is placed into a well-known initial state. The well-known initial state may be all memory cells of the protected memory area, and all processor registers, are all written to zero, to one, or to some other predetermined value or pattern of values. As described elsewhere within this Detailed Description, data stored in the protected memory area is inaccessible to code stored and executed outside the protected memory area once the security-enabled processor puts the protected memory area into the well-known initial state. It is the security-enabled processor that governs this access. 
     At  410 , the set-up module instructs the loader module to execute within the protected memory area. The set-up module may be able to pass instruction to the loader module through an entry gate or function provided by the security-enabled processor that enables the secure execution environment to receive communication from the outside execution environment. 
     At  412 , the security-enabled processor produces an identifier of the contents of the protected memory area. The identifier may be, in various embodiments, a digest—such as a hash—of the contents of the protected memory area in the activation state. The identifier may be a public key matching a private key used to sign the software stored in the protected memory area. At the point where the identifier is created, the contents of the protected memory area includes the loader module and the one or more parameters included in the request, but no other data or code. Thus, the content of the protected memory area represents the requested activation state implicitly or explicitly identified by the request. In embodiments where the identifier is a digest, the digest may be produced using one of various hash functions, or other similar cryptographic functions. 
     At  414 , the security-enabled processor stores the identifier in a manner that is accessible only to the security-enabled processor. For example, the security-enabled processor may store the identifier in a secure register of the security-enabled processor. The security-enabled processor may store the identifier encrypted in a memory location. But in any event, the identifier is stored in a way that makes it accessible only to the security-enabled processor. The identifier may be created upon instruction by the loader module. In alternative embodiments, the identifier may be created upon instantiation of the protected memory area without instruction from the loader module. 
     At  416 , an instance of the loader module executing in the secure execution environment instructs the security-enabled processor to produce an attestation certificate including the identifier and signed by a private key of the security-enabled processor. The private key of the security-enabled processor is securely stored in the security-enabled processor in a way that is accessible only to the security-enabled processor. The attestation certificate may include other information besides the identifier, such as a time-stamp, the one or more parameters, or other data. 
     At  418 , the security-enabled processor encrypts/signs the attestation certificate using the private key of the security-enabled processor and provides the signed attestation certificate to the loader module executing in the secure execution environment. 
     At  420 , the loader module provides the client system with certification that the secure execution environment is established in an initial activation state to execute only that software identified by the client system request. The certification may include a signed attestation certificate. The loader module may transmit the attestation certificate via an exit gate or function of the security-enabled processor that enables the secure execution environment to communicate with code outside of the secure execution environment. 
     At  422 , the loader module and/or the application hosting service transmits one or more trust certificates, such as TA certificate  132  and intermediate certificates  134  that collectively vouch for the identity of the security-enabled processor and/or indicate that the security-enabled processor is secure. This may also include an auditing certificate, signed by a private key of an auditor entity, indicating that the security-enabled processor is physically intact. This may include a processor certificate from a manufacturer of the security-enabled processor indicating that the security-enabled processor is secure. In alternative embodiments, the auditing certificate and/or the processor certificate are delivered separately to the client system, such as directly from the auditing entity, a hardware manufacturer, or third party. In one non-limiting example, the attestation certificate may include a URI identifying where the client system can retrieve the auditing certificate. 
     At  424 , the loader module receives an authorization message from the client system. 
     At  426 , the loader module obtains one or more application components to be executed in the secure execution environment. Obtaining one or more application components may include retrieving the one or more application components from a persistent storage of the application hosting service, from a remote location (such as via a URI identified in the parameters of the initial activation state or the authorization message, or in some other location). Obtaining one or more application components may include retrieving the one or more application components from the client system, such as via a cryptographically protected communication channel. 
     The one or more application components are selected by the client system, either by transmitting a URI, URL, other identifier, or an application binary, application package, or file-system image. Transmission of the URI, URL, other identifier, application binary, application package, file-system image, and so forth may be done via the request received at  402 , in the authorization message received at  424 , or received in some other manner such as via a secure communication connection established between the loader module and the client system channel (using for example secure socket layer (SSL) protocol or other protocol). 
     In various non-limiting embodiments, the client system provides the loader module with an encryption key. In these embodiments, obtaining the one or more application components may include decrypting the one or more components using the encryption key (received through a secure communication channel). In various embodiments, the encryption key may be transported in the clear over a secure communication channel. In other embodiments the encryption key may be encrypted with the public key of the security-enabled processor and decrypted by the security-enabled processor on behalf of the loader module. In various embodiments, obtaining the one or more application components at  426  may occur in a different sequence than is shown in  FIG. 4  without departing from the scope of the present disclosure. In one non-limiting example of such alternative embodiments, the one or more application components may have been obtained and loaded into the protected memory area as part of the initial activation state. At  428 , the loader module causes the security-enabled application to execute the obtained one or more application components within the protected memory area. 
     Example Operations for Verifying Secure Execution Environment Establishment 
       FIG. 5  is a flow diagram showing an example process  500  for verifying establishment of a secure execution environment. At  502 , a client system, such as client system  112 , transmits a request to an application hosting service, such as the application hosting service  102 , to establish a secure execution environment. The request includes an indication of a requested activation state of the secure execution environment, such as a requested loader module and one or more parameters to be loaded into a protected memory area of the secure execution environment. 
     At  504 , the client system receives, from an instance of the loader module executing in a protected memory area of the application hosting service, a certification, such as an attestation certificate, that the secure execution environment is established to execute in the activation state only the software identified by the request. The attestation certificate includes an identifier. Receipt of the attestation certificate indicates that the application hosting service purports to have established a secure execution environment. The identifier identifies the contents of a protected memory area of the secure execution environment upon its instantiation. The identifier may be, in various embodiments, a digest—such as a hash—of the contents of the protected memory area in the activation state. The identifier may be a public key matching a private key used to sign the software stored in the protected memory area. 
     At  506 , the client system receives one or more trust certificates, such as the TA certificate  132  and/or the intermediate certificates  134 . As described elsewhere within this Detailed Description, the one or more trust certificates verifiably and collectively establish a chain of trust between a trusted authority and the security-enabled processor of the application hosting service to indicate that the security-enabled processor is secure. 
     At  508 , the client system receives an auditing certificate, signed by a private key of an auditor entity, indicating that the security-enabled processor is physically uncompromised. The auditor entity periodically inspects the security-enabled processors of the application hosting service as is described elsewhere within this Detailed Description. 
     At  510 , the client system receives a processor certificate from a manufacturer of the security-enabled processor indicating that the security-enabled processor is secure. The processor certificate may be one of the trust certificates received at  506 . Alternatively, the processor certificate may be separately received from the hardware manufacturer of the security-enabled processor—or third party—vouching for the security and proper functioning of the security-enabled processor. 
     At  512 , the client system verifies that the various certificates received at  506 ,  508 , and  510  are proper, such as by verifying their authenticity using the various issuers&#39; public keys. If one or more of the certificates are invalid, then the client system may reject the secure execution environment as invalid. 
     At  514 , the client system obtains a public key of the security-enabled processor. The client system may obtain the public key from one of the trust certificates received at  506 . Alternatively, the client system may have pre-stored the public key of the security-enabled processor. 
     At  516 , the client system verifies the attestation certificate using the known public key of a security-enabled processor of the application hosting service. The known public key corresponds to a private key of the security-enabled processor. Successful decryption with the public key therefore indicates that the attestation certificate is verifiably from the security-enabled processor. 
     At  518 , the client system extracts the identifier from the decrypted attestation certificate. And at  520 , the client system compares the identifier with an expected value of a known identifier of the requested activation state of the secure execution environment. A successful match indicates that the activation state of the secure execution environment instantiated by the security-enabled processor of the application hosting service is as specified in the request transmitted at  502 . 
     At  522 , the client system determines whether the identifiers match. If the identifiers do not match, then the client system rejects the secure execution environment for not having the requested activation state. Identifiers that do not match indicate that the actual activation state of the secure execution environment has less, more, or different code and data than the requested activation state. A successful match of the identifiers, along with verification that the attestation and trust certificates are valid, permits the client system to have a high degree of confidence that a secure execution environment has been established with no untrusted code, such that for example the secure execution environment is established with only code and data that is trusted by the client system. 
     At  524 , the client system authorizes the instance of the loader module executing in the protected memory area to execute one or more application components within the secure execution environment. In embodiments, authorizing the loader module may include transmitting an indicator of the one or more application components to be executed in the secure execution environment. The indicator may be a URI, URL, other identifier, or an application binary, application package, or file-system image of the one or more application components to be executed. In various embodiments, transmission of the indicator may be part of transmitting the request at  502 . In other embodiments, transmitting the indicator may utilize a separate message (such as for example a message sent over a secure connection established using SSL or other protocol), or a message transmitted at some previous time in order to “pre-stage” the one or more application components to be executed in the secure execution environment. The client system may generate and transmit to the loader module an encryption key to be used to decrypt the various messages and/or the indicator. 
     Example Environment for Persistence of a Secure Execution Environment 
     In conventional hosted computing, migration or re-creation of a hosted consumer&#39;s execution is handled by a virtual machine monitor. In conventional hosted computing migration, a host virtual machine monitor inspects the state of the execution, copies all of the memory pages, writes the memory pages out to a disk or transfers them via a network, and starts up the execution on the same or a different machine. But because code and data stored within a protected memory area of a secure execution environment according to embodiments of the present disclosure are inaccessible to code executing outside the protected memory area, a host OS is not able to inspect the state of execution or copy the state information to a disk in order to migrate to another machine. Instead, the code running inside the protected memory area handles various aspects of the re-creation and migration processes. 
       FIG. 6  shows an environment for migration of a protected memory area according to embodiments. Environment  600  includes a host computer  602  and a migration computer  604 . A secure execution environment, including a protected memory area  606  and a security-enabled processor  608 , is established on host computer  602 . The protected memory area  606  is instantiated in a manner that is described elsewhere within this Detailed Description, in particular in the descriptions of  FIGS. 1-5 . The protected memory area  606  includes a loader module  610  and an application  612  that executes within the secure execution environment. The protected memory area  606  also includes a persistence module  614 , which may be a subcomponent of the loader module  610  or a subcomponent of the application  612 , including a subcomponent of a library OS subcomponent of the application  612 . 
     The host OS  616  determines that there is a need to persist the secure execution environment. Persisting the secure execution environment may be for the purpose of migrating the secure execution environment from the host computer  602  to the migration computer  604 . Alternatively, persisting the secure execution environment may be for the purpose of re-creating the execution on the host computer  602 . In any event, the host OS  616  is configured to call an entry gate or function provided by the security-enabled processor  608  which permits the host OS  616  to instruct the secure execution environment to persist its present execution state to persistent storage. 
     The persistence module  614  receives the persistence instruction via the entry gate and, in response, creates an encrypted checkpoint  618 . To accomplish this, the persistence module  614  shuts down execution of the loader module  610  and the application  612  (including for example causing the threads to quiesce) and writes the processor registers to protected memory. At that point, only one thread—the suspend thread of the persistence module  614 —may be left running. The various memory pages are enumerated and stored, along with the processor registers, as state information  620 . The contents of the protected memory area  606 , including the application  612  and the state information  620 , are stored as the encrypted checkpoint  618  on persistent storage  622 . The encrypted checkpoint  618  is encrypted with a persistence key that is generated by the persistence module  614 . The persistence module  614  encrypts the persistence key using a public key  624  of the client system  626 . Alternatively, the persistence key is encrypted using a public key of the migration security-enabled processor  634  or a public key of the security-enabled processor  608 . The encrypted persistence key is stored as sealed persistence key  628  in persistent storage  622 . 
     In embodiments that utilize the persistence of the execution state to migrate the execution to the migration computer  604 , a migration host OS  630  of the migration computer  604  causes a migration protected memory area  632  to be established. In embodiments that utilize the persistence of the execution state to re-create the execution on the host computer  602  in a new protected memory area (due to, for example, a reboot of the host computer  602  or for other reason), the host OS  616  causes a new protected memory area to be established on host computer  602 . The migration protected memory area  632 , or the new protected memory area to be established on host computer  602 , is instantiated in a manner that is described elsewhere within this Detailed Description, in particular in the descriptions of  FIGS. 1-5 . 
     In embodiments where the persistence of the execution state is for the purpose of migrating it to migration computer  604 , the migration host OS  630  calls an entry gate or function of the migration security-enabled processor  634  to cause the loader module  610  to launch or execute the persistence module  614  within the migration protected memory area  632 . The persistence module  614  copies the sealed persistence key  628  into the migration protected memory area  632  and transmits the sealed persistence key  628  to the client system  626 . The client system  626  decrypts the sealed persistence key  628  using the private key of the client system  626  and transmits it back to the persistence module  614  via the encrypted connection established during the initialization of the secure execution environment on the migration computer  604 . Alternatively, in embodiments that encrypt the persistence key with the public key of the migration security-enabled processor  634 , the sealed persistence key is unsealed using the private key of the migration security-enabled processor  634 . 
     In embodiments that utilize the persistence of the execution state for the purpose of re-creating the execution on the host computer  602 , the host OS  616  calls an entry gate or function of the security-enabled processor  608  to cause the loader module  610  to launch or execute the persistence module  614  within a newly re-created protected memory area  606 . The persistence module  614  copies the sealed persistence key  628  into the newly-recreated protected memory area  606  and transmits the sealed persistence key  628  to the client system  626 . The client system  626  decrypts the sealed persistence key  628  using the private key of the client system  626  and transmits it back to the persistence module  614  via the encrypted connection established during the initialization of the secure execution environment on the migration computer  604 . Alternatively, in embodiments that encrypt the persistence key with the public key of the security-enabled processor  608 , the sealed persistence key is unsealed using the private key of the security-enabled processor  608 . 
     The persistence module  614  copies the encrypted checkpoint  618  into the migration protected memory area  632  or the newly-recreated protected memory area  606  and uses the unsealed persistence key to decrypt the encrypted checkpoint  618 . The persistence module  614  uses the state information  620  from the encrypted checkpoint  618  to repopulate the memory pages associated with the executing threads from the loader module  610  and the application  612 , and to repopulate the registers in the migration security-enabled processor  634  or the security-enabled processor  608 . Thus, in various embodiments, the execution of the secure execution environment on the host computer  602  is either migrated to the migration computer  604 , or re-created on the host computer  602 . 
     In various embodiments, the secure execution environment on migration computer  604  can be instantiated before migration processes begin, such as at the same time that the secure execution environment on host computer  602  is initialized. The persistence module  614  on the migration protected memory area  632  can be pre-populated with the private key of the client system  626  to enable it to decrypt the sealed persistence key  628  without transmitting it to the client system  626  for unsealing. In some embodiments, instead of using the public key of the client system  626  to encrypt the persistence key, the sealed persistence key  628  is encrypted using a public key of the migration security-enabled processor  634  so that it can be decrypted using the private key of the migration security-enabled processor  634 . 
     Example Operations for Migrating a Secure Execution Environment 
       FIG. 7  is a flow diagram showing an example process  700  for migrating a secure execution environment. At  702 , a loader module or a persistence module executing in a secure execution environment receives a command, via an entry gate, to migrate to a migration computer, such as migration computer  604 . 
     At  704 , a persistence module, such as the persistence module  614 , generates a persistence key. The persistence key is used to encrypt a checkpoint of the secure execution environment. 
     At  706 , the persistence module encrypts the persistence key. In embodiments, the persistence module encrypts the persistence key using a public key of a client system, such as the client system  626 . In alternative embodiments, where the identity of the host migration computer is known, the persistence module encrypts the persistence key using a public key of a security-enabled processor of the migration computer. 
     At  708 , the persistence module writes the encrypted persistence key to persistent storage. Alternatively, the persistence module transmits the persistence key to the client system, or to a pre-established secure execution environment on the migration computer. 
     At  710 , the persistence module shuts down execution of the processes and threads executing in the secure execution environment and writes state information to the protected memory area. 
     At  712 , the persistence module encrypts the contents of the protected memory area using the persistence key to generate a checkpoint. The contents of the protected memory area include the state information, such as the page files and register data associated with the secure execution environment. 
     At  714 , the encrypted checkpoint is stored on a persistent storage, such as a hard disk drive of the application hosting service or some other persistent storage. Alternatively, the encrypted checkpoint is loaded directly into a pre-established protected memory area of the migration computer, such as over an encrypted communication channel to the protected memory area of the migration computer. 
     At  716 , the protected memory area is initialized on the migration computer. The host operating system of the migration computer causes the loader module in the protected memory area of the migration computer to load and execute the persistence module within the secure execution environment of the migration computer. 
     At  718 , the persistence module executing within a secure execution environment of the migration computer transmits the encrypted persistence key to the client system. In alternative embodiments, where the persistence key is encrypted using the public key of the security-enabled processor of the migration computer, the persistence key is not transmitted to the client system; instead, the persistence module requests that the security-enabled processor decrypt the persistence key. In still another alternative embodiment, where the secure execution environment is pre-established on the migration computer, the pre-established secure execution environment on the migration computer may already have the private key of the client system, obviating the need to transmit the encrypted persistence key to the client system. At  720 , the secure execution environment on the migration computer receives the unencrypted persistence key. 
     At  722 , the persistence module decrypts the encrypted checkpoint using the persistence key, and loads the state information in the checkpoint into the page files and registers of the new secure execution environment to restore the state of execution, and the migration process completes. 
     Example Operations for Re-Creating a Secure Execution Environment 
       FIG. 8  is a flow diagram showing an example process for re-creating a secure execution environment. At  802 , a loader module or a persistence module executing in a secure execution environment receives a command, via an entry gate, to persist the present state of the secure execution environment to re-create the execution state in a new secure execution environment (such as on the same or different host computer). 
     At  804 , a persistence module generates a persistence key. The persistence key is used to encrypt a checkpoint of the secure execution environment as described in more detail below. 
     At  806 , the persistence module encrypts the persistence key. In embodiments, the persistence module encrypts the persistence key using a public key of a client system, such as the client system  626 . In alternative embodiments, the persistence module encrypts the persistence key using a public key of a security-enabled processor of the host computer. 
     At  808 , the persistence module writes the encrypted persistence key to persistent storage. Alternatively, the persistence module transmits the persistence key to the client system, or to a pre-established secure execution environment on the host computer, such as over a secure communication channel to the newly-created protected memory area of the pre-established secure execution environment. 
     At  810 , the persistence module shuts down execution of the secure execution environment and writes the state information to the protected memory area. The state information may include the virtual memory pages and register context associated with the secure execution environment. At  812 , the persistence module encrypts the state information using the persistence key to generate a checkpoint. 
     At  814 , the encrypted checkpoint is stored on a persistent storage, such as a hard disk drive of the application hosting service or some other persistent storage. Alternatively, the encrypted checkpoint is loaded directly into a newly-established protected memory area, such as over a communication channel to the newly-created protected memory area. 
     At  816 , a new protected memory area is initialized on the host computer. At  818 , the host operating system of the host computer places the loader module and one or more parameters into the memory area to be protected. 
     At  820 , the persistence module executing within the newly-created secure execution environment of the host computer reads the sealed persistence key from persistent storage. 
     At  822 , the persistence module in the newly-created secure execution environment unseals the persistence key. In embodiments where the persistence key is encrypted using the public key of the security-enabled processor of the host computer, the persistence module requests that the security-enabled processor decrypt the persistence key. In other embodiments where the secure execution environment is pre-established on the host computer, the private key of the security-enabled processor of the host computer is used to unseal the persistence key. 
     At  824 , the persistence module reads the encrypted checkpoint from the persistent storage. At  826 , the persistence module decrypts the encrypted checkpoint using the persistence key, and loads the state information in the checkpoint into the virtual memory and register context of the new secure execution environment to restore the state of execution, and the re-creation process completes. 
       FIGS. 4, 5, 7, and 8  depict flow graphs that show example processes in accordance with various embodiments. The operations of these processes are illustrated in individual blocks and summarized with reference to those blocks. These processes are illustrated as logical flow graphs, each operation of which may represent a set of operations that can be implemented in hardware, software, or a combination thereof. In the context of software, the operations represent computer-executable instructions stored on one or more computer storage media that, when executed by one or more processors, enable the one or more processors to perform the recited operations. Generally, computer-executable instructions include routines, programs, objects, modules, components, data structures, and the like that perform particular functions or implement particular abstract data types. The order in which the operations are described is not intended to be construed as a limitation, and any number of the described operations can be combined in any order, separated into sub-operations, and/or performed in parallel to implement the process. Processes according to various embodiments of the present disclosure may include only some or all of the operations depicted in the logical flow graph. 
     Computer-Readable Media 
     Depending on the configuration and type of computing device used, memories  204  and  304  of the computing systems  200  and  300  in  FIGS. 2 and 3 , respectively, may include volatile memory (such as random access memory (RAM)) and/or non-volatile memory (such as read-only memory (ROM), flash memory, etc.). Memories  204  and  304  may also include additional removable storage and/or non-removable storage including, but not limited to, flash memory, magnetic storage, optical storage, and/or tape storage that may provide non-volatile storage of computer-readable instructions, data structures, program modules, and other data for computing systems  200  and  300 . 
     Memories  204  and  304  are examples of computer-readable media. Computer-readable media includes at least two types of computer-readable media, namely computer storage media and communications media. 
     Computer storage media includes volatile and non-volatile, removable and non-removable media implemented in any process or technology for storage of information such as computer-readable instructions, data structures, program modules, or other data. Computer storage media includes, but is not limited to, phase change memory (PRAM), static random-access memory (SRAM), dynamic random-access memory (DRAM), other types of random-access memory (RAM), read-only memory (ROM), electrically erasable programmable read-only memory (EEPROM), flash memory or other memory technology, compact disk read-only memory (CD-ROM), digital versatile disks (DVD) or other optical storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, or any other non-transmission medium that can be used to store information for access by a computing device. 
     In contrast, communication media may embody computer-readable instructions, data structures, program modules, or other data in a modulated data signal, such as a carrier wave, or other transmission mechanism. As defined herein, computer storage media does not include communication media. 
     CONCLUSION 
     Although the disclosure uses language that is specific to structural features and/or methodological acts, the invention is not limited to the specific features or acts described. Rather, the specific features and acts are disclosed as illustrative forms of implementing the invention.