Patent Publication Number: US-9853812-B2

Title: Secure key management for roaming protected content

Description:
BACKGROUND 
     As computing technology has advanced, many different types of computing devices have become commonplace. Users oftentimes have multiple different computing devices, such as a laptop or desktop computer, a tablet device, a smartphone, and so forth. Networked or cloud services can be used to allow users to share data across these multiple devices, giving the users access to their data from any of their multiple devices. However, providing data to such a service can be troublesome for users because it can result in a user&#39;s data, which he or she thought would be kept private, being made available to the service provider or other entities (e.g., malicious users that hack into or otherwise compromise the service). 
     SUMMARY 
     This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. 
     In accordance with one or more aspects, in a first computing device content is protected using a data protection public key of a data protection public/private key pair corresponding to an identity of a user of the first computing device. The protected content is copied to cloud storage, and a public key of a public/private key pair of a second computing device is obtained, the first and second computing devices being associated with a same user identity. The data protection private key is encrypted using the public key of the second computing device, and the encrypted data protection private key is provided to the second computing device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The detailed description is described 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 instances in the description and the figures may indicate similar or identical items. Entities represented in the figures may be indicative of one or more entities and thus reference may be made interchangeably to single or plural forms of the entities in the discussion. 
         FIG. 1  illustrates an example system implementing the techniques discussed herein in accordance with one or more embodiments. 
         FIG. 2  illustrates another example computing device implementing the techniques discussed herein in accordance with one or more embodiments. 
         FIG. 3  is a flowchart illustrating an example process for implementing secure key management for roaming protected content in accordance with one or more embodiments. 
         FIG. 4  illustrates an example system in which content is roamed and a data protection private key is shared in accordance with one or more embodiments. 
         FIG. 5  illustrates an example system that includes an example computing device that is representative of one or more systems and/or devices that may implement the various techniques described herein. 
     
    
    
     DETAILED DESCRIPTION 
     Secure key management for roaming protected content is discussed herein. A user of a device has a particular identity (e.g., user account) when using the device. Content on the device is encrypted and protected based on a data protection key corresponding to and protected by the particular identity. The content can be encrypted using the data protection key, or the data protection key can be used to encrypt one or more other keys that are used to encrypt the content. The protected content can then be copied to cloud storage, and from the cloud storage the protected content can be transferred to various other ones of the user&#39;s devices, also referred to as roaming the content. 
     The content is encrypted as part of the protection, and the key used to decrypt the content is maintained by the user&#39;s device. The cloud storage does not have access to the key used to decrypt the content, and thus does not have access to the plaintext (unencrypted) content. 
     The key used to decrypt the content can, however, be communicated to other ones of the user&#39;s devices. To transfer the key used to decrypt the content to another of the user&#39;s devices (a target device), the key used to decrypt the content is protected (e.g., encrypted) with a key of the target device. This protection allows the target device to retrieve the key (e.g., decrypt the key), but prevents other devices from accessing the key. Due to the protection, the protected key can be communicated to the target device via the cloud storage and any communication channels to or from the cloud storage, including an untrusted cloud storage and untrusted communication channels. 
     The techniques discussed herein advantageously improve the usability and security of computing devices by allowing the user&#39;s content to remain protected despite being stored on an untrusted cloud storage or transferred via an untrusted communication channel. The techniques discussed herein further advantageously improve the usability and security of computing devices by allowing the user to access his plaintext content on multiple ones of his devices, while at the same time protecting the plaintext content from being accessed by an untrusted cloud storage, an untrusted communication channel, or other users. 
     References are made herein to encrypting and decrypting content, which can be performed using symmetric key cryptography or public key cryptography. Although such key cryptography is well-known to those skilled in the art, a brief overview of such cryptography is included here to assist the reader. In public key cryptography, an entity (such as a user, hardware or software component, a device, a domain, and so forth) has associated with it a public/private key pair. The public key can be made publicly available, but the entity keeps the private key a secret. Data can be decrypted using the private key, but without the private key it is computationally very difficult to decrypt data that is encrypted using the public key. So, data can be encrypted by any entity with the public key and only decrypted by an entity with the corresponding private key. 
     In symmetric key cryptography, on the other hand, a shared key (also referred to as a symmetric key) is known by and kept secret by the two entities. Any entity having the shared key is typically able to encrypt data with that shared key, and decrypt data encrypted with that shared key. Without the shared key it is computationally very difficult to decrypt data that is encrypted with the shared key. So, if two entities both know the shared key, each can encrypt data that can be decrypted by the other, but other entities cannot decrypt the data if the other entities do not know the shared key. Similarly, an entity with a shared key can encrypt data that can be decrypted by that same entity, but other entities cannot decrypt the data if the other entities do not know the shared key. 
       FIG. 1  illustrates an example system  100  implementing the techniques discussed herein in accordance with one or more embodiments. The system  100  includes a computing device  102  that can be a variety of different types of devices. For example, the computing device  102  can be a desktop computer, a server computer, a laptop or netbook computer, a mobile device (e.g., a tablet or phablet device, a cellular or other wireless phone (e.g., a smartphone), a notepad computer, a mobile station), a wearable device (e.g., eyeglasses, watch), an entertainment device (e.g., an entertainment appliance, a set-top box communicatively coupled to a display device, a game console), a television or other display device, an automotive computer, and so forth. Thus, the computing device  102  may range from a full resource device with substantial memory and processor resources (e.g., personal computers, game consoles) to a low-resource device with limited memory and/or processing resources (e.g., traditional set-top boxes, hand-held game consoles). 
     The computing device  102  includes a content protection system  104 , one or more programs  106 , and a content store  108 . The programs  106  can include various different applications, parts of an operating system, or other programs that can be run on the computing device  102 . The content store  108  is one or more storage devices used to store content by the computing device  102 . Content refers to any type of data, instructions, or other information stored by the computing device  102 . The content store  108  can be implemented using any of a variety of different types of storage devices, such as solid state devices (e.g., Flash memory), magnetic disks, optical discs, and so forth. Although illustrated as being part of the computing device  102 , it should be noted that one or more of the storage devices that make up the content store  108  can be implemented on devices separate from, but communicatively coupled to, the computing device  102  (e.g., external hard drives, removable flash drives). 
     The content protection system  104  manages protection of content on the computing device  102 , including managing the encryption and decryption of content, managing keys used to protect the content, and so forth. The content protection system  104  can be implemented as part of an operating system of the computing device  102 , or alternatively as another component or module of the computing device  102  separate from the operating system. 
     The content protection system  104  includes a key management module  112 , a key transfer module  114 , an encryption module  116 , and a decryption module  118 . The key management module  112  generates keys that are used to encrypt and decrypt content, including other keys. The key management module  112  also stores keys on the computing device  102  in a secure manner and controls access to the keys, allowing access to the keys only by those components or modules of the computing device  102  that are permitted to access the keys. These keys include a public/private key pair for the computing device  102 . In one or more embodiments, the private key of the computing device  102  is protected in hardware of the computing device  102 , such as by wrapping the device private key to a Next Generation Credential, sealing the device private key to a trusted platform module (TPM), sealing the device private key to a smart card, sealing the device private key to a hardware security module (HSM), and so forth. 
     The key transfer module  114  manages securely transferring keys to and from other computing devices. The key transfer module  114  manages protecting keys for transferring to other computing devices, and retrieving keys from protected keys transferred to the computing device  102 . The key transfer module  114  can facilitate encryption of a key to be transferred by providing the key to be transferred to the decryption module  118 . The key transfer module  114  can similarly facilitate decryption of a key being transferred to the computing device  102  by providing a key (decrypted by the decryption module  118 ) to the key management module  112 . 
     The encryption module  116  encrypts content based on one or more keys. This encryption can be performed using any of a variety of different public and/or proprietary encryption techniques or algorithms, and can use symmetric key cryptography or public key cryptography. The decryption module  118  decrypts content based on one or more keys. This decryption can be performed using any of a variety of different public and/or proprietary decryption techniques or algorithms, and can use symmetric key cryptography or public key cryptography. 
     A user of the computing device  102  also has a particular identity when using the device, such as a user name or user account identifier of the user. This user name or user account identifier is, for example, the name or identifier that the user logs into the computing device  102  with, the name or identifier that the user logs into a service (e.g., of a cloud storage service, also referred to as a cloud service) with, and so forth. The computing device  102  accesses a cloud storage  122 , which is a set of components or technologies that allow the short or long term storage of content in one or more locations. The storage of content can be implemented using any of a variety of different storage mechanisms that can be accessed via any of a variety of different data networks (e.g., the Internet, a local area network (LAN), a phone network, an intranet, other public and/or proprietary networks, or combinations thereof) wired connections (e.g., universal serial bus (USB) connections), wireless connections (e.g., wireless USB connections), and so forth. The cloud storage  122  can be a trusted cloud storage that is relied on to keep content secure from other users or devices, or an untrusted cloud storage that is not relied on to keep content secure from other users or devices. The cloud storage  122  is typically one or more storage devices situated remotely from the computing device  102 , but can alternatively be other storage devices (e.g., a removable device (e.g., a flash or magnetic drive coupled to the computing device  102  using a wired or wireless connection)). 
     The cloud storage  122  can be accessed via any of a variety of different communication channels. The communication channel can be a trusted channel or an untrusted channel. A trusted channel refers to a communication channel that is relied on to keep content secure from other users or devices, and an untrusted channel refers to a communication channel that is not relied on to keep content secure from other users or devices. 
     The cloud storage  122  can optionally be implemented as one of multiple different services (also referred to as cloud services). These cloud services can include, for example, content storage services, content editing services, communication (e.g., email or messaging) services, and so forth. These services can be implemented by one or more of a variety of different types of devices, such as any one or more of those discussed above with reference to computing device  102 . 
     The cloud storage  122  allows content to be shared or roamed among the computing device  102  and other computing devices  124 ( 1 ), . . . ,  124 (M). Protected content refers to content that has been encrypted, and protected content can be transferred to the cloud storage  122  by one of computing devices  102  or  124 , as well as received by others of the computing devices  102  or  124 . These computing devices are other computing devices of the user, or other computing devices that the user has logged into. Although the protected content can be transferred to these devices, the protected content cannot be accessed on these devices without the proper keys, as discussed in more detail below. Additionally, it should be noted that the cloud storage  122  need not be trusted by the user of the computing device  102 —protected content is stored on the cloud storage  122 , and the cloud is not provided with the key to unprotect (decrypt) the content, so the cloud cannot access the plaintext (unencrypted) content. 
     The cloud storage  122  also maintains a user key store  126 , which corresponds to the identity of the user. A user can optionally have different identities at different times (e.g., different times logging into the computing device  102 ). In one or more embodiments, the identity of the user at any given time is the identity of the user when he or she is logged into the computing device  102  (and/or logged into the cloud storage  122 ) at that given time. The user key store  126  includes a public key of one or more public/private key pairs, such as a public key of a computing device  102 , a public key of one of computing devices  124 ( 1 ), . . . ,  124 (M), and a data protection public key. These keys are used to protect the content while at the same time allowing the user to access the content from multiple devices as discussed in more detail below. 
       FIG. 2  illustrates another example computing device  202  implementing the techniques discussed herein in accordance with one or more embodiments. The computing device  202  includes a content protection system  104 , one or more programs, and a content store  108  analogous to the computing device  102  of  FIG. 1 . However, the computing device  202  differs from the computing device  102  in that the content protection system  104  is implemented as part of a trusted computing base  204 . 
     The trusted computing base  204  operates as a secure or trusted component of the computing device  202 , generating and protecting keys. Components of the trusted computing base  204  can encrypt or decrypt content in response to requests to do so from a program  106  (including requests from the operating system), but the trusted computing base  204  does not reveal the encryption or decryption keys to any other program of module of the computing device  202  that is not included as part of the trusted computing base  204 . Thus, if a program  106  were to be malware, or a portion of the operating system of computing device  202  were to be untrusted or compromised by malware, the keys remain protected by the trusted computing base  204  and are not revealed to such a program or operating system. The trusted computing base  204  can, however, transfer keys that have been encrypted to other computing devices, as discussed in more detail below. 
     Returning to  FIG. 1 , content on the computing device  102  is protected by the content protection system  104  based on a data protection public/private key pair. The key management module  112  generates the data protection public/private key pair, and keeps the data protection private key secret. The data protection public key is communicated to the cloud storage  122  and stored as part of the user key store  126 . The data protection public/private key pair corresponds to or is tied to the identity of the user. If the user has multiple different identities, then multiple different data protection public/private key pairs can be generated (each corresponding to or tied to a different one of the multiple identities), or alternatively a single data protection public/private key pair can be shared across the multiple different identities. At any particular time, the key management module  112  allows the data protection private key corresponding to the user identity at that particular time to be used to protect content, but does not allow data protection private keys corresponding to other user identities to be used to protect content. The data protection private key is thus also referred to as being protected by the particular identity. 
     The protected content is also stored on the cloud storage  122 . This storage of protected content on the cloud storage  122  can be managed by one of the programs  106 , such as a content synchronization program. As new content is generated on or otherwise added to the content store  108 , the new content is protected by the content protection system  104  and stored on the cloud storage  122  by the content synchronization program. Similarly, as new content is generated on or otherwise added to the content stores of other computing devices  124  (e.g., while the user is logged into such other devices using the same identity) and stored on the cloud storage  122  by the other computing devices  124 , the new content is obtained from the cloud storage  122  by the content synchronization program and stored in the content store  108 . 
     The protected content is protected based on the data protection public/private key pair. The content protection system of a computing device  102  or  124  uses the data protection private key to obtain the plaintext content from the protected content. Thus, in order for multiple different ones of the computing devices  102  and  124  to obtain the plaintext content from protected content, the data protection private key is shared among the different computing devices  102  and  124 . This sharing is done in a manner that protects the data protection private key, allowing the computing devices  102  and  124  to obtain the data protection private key but preventing other users or devices from obtaining the data protection private key. Thus, the sharing of the data protection private key can be done via unsecure cloud storage and/or unsecure communication channels. 
       FIG. 3  is a flowchart illustrating an example process  300  for implementing secure key management for roaming protected content in accordance with one or more embodiments. Process  300  is carried out by a content protection system of a computing device, such as content protection system  104  of  FIG. 1  or  FIG. 2 , and can be implemented in software, firmware, hardware, or combinations thereof. Process  300  is shown as a set of acts and is not limited to the order shown for performing the operations of the various acts. Process  300  is an example process for implementing secure key management for roaming protected content; additional discussions of implementing secure key management for roaming protected content are included herein with reference to different figures. 
     In process  300 , content is protected using a data protection public key corresponding to an identity of the user (act  302 ). The content can be protected using the data protection public key in various different manners. In one or more embodiments, the content is encrypted using a file encryption key (act  304 ). The file encryption key is used to encrypt the content by using the file encryption key as the key of an encryption process (e.g., a symmetric encryption process). The file encryption key is also encrypted with the data protection public key (act  306 ). The data protection public key is used to encrypt the file encryption key by using the data protection public key as the key of a public key cryptography process. Thus, multiple different keys can be used to protect the content—the file encryption key (e.g., a symmetric key) that is used to encrypt the content, and the data protection public/private key pair of which the data protection public key is used to encrypt the file encryption key. 
     Alternatively, rather than using a file encryption key, the content is encrypted with the data protection public key (act  308 ). The data protection public key is used to encrypt the content by using the data protection public key as the key of a public key cryptography process. Thus, the data protection public/private key pair of which the data protection public key is used to encrypt the content can be used to protect the data without any use of a symmetric key. 
     Regardless of the manner in which the content is protected, the protected content is copied to cloud storage (act  310 ). Because the content is protected, the cloud storage itself does not have access to the plaintext (unencrypted) content. The protected content is thus stored on the computing device implementing the process  300 , and due to copying the protected content to the cloud storage the protected content is stored by the cloud storage and roamed to others of the user&#39;s devices. 
     A public key of a target device that is to have access to the content is obtained (act  312 ). The public key of the target device can be obtained in a variety of different manners. In one or more embodiments, each computing device that the user logs into has its own public/private key pair. This device public/private key pair is generated by the computing device (e.g., the key management module of the computing device), and the device private key is kept secret by the computing device (e.g., protected in hardware of the computing device as discussed above). However, in one or more embodiments each computing device stores its device public key in the user key store of the cloud (e.g., the user key store  126  of  FIG. 1 ). Thus, each of the computing devices of a user (e.g., those that the user logs into using the same identity) provides its device public key to the cloud, which makes those device public keys available to the other computing devices of the user. Alternatively, a computing device can make its device public key available to the other computing devices of the user in different manners, such as a direct exchange independent of the user key store  126  and/or the cloud (e.g., transferring the device public key to and reading the device public key from a removable flash memory device or phone, transferring the device public key using another wired or wireless communication channel, and so forth). 
     The data protection private key is encrypted using the target device public key (act  314 ). By encrypting the data protection private key using the target device public key, the target device is able to retrieve the data protection private key using the target device private key, but other devices (without the target device private key), are not able to retrieve the data protection private key. 
     The encrypted data protection private key is provided to the target device (act  316 ). The encrypted data protection private key can be provided to the target device using any of a variety of different mechanisms, such as the cloud storage  122  of  FIG. 1 , a removable flash memory device or phone, another wired or wireless communication channel, and so forth. It should be noted that, because the data protection private key is encrypted and the target device is the only device with the target device private key, the encrypted data protection private key can be transferred or otherwise communicated to the target device via any of a variety of trusted or untrusted channels. 
     Once the target device has the encrypted data protection private key, the target device can readily retrieve the data protection private key using the target device private key. The content protection system of the target device keeps the data protection private key secure, and can use the data protection private key to retrieve the plaintext content from the protected content (e.g., by decrypting the file encryption key using the data protection private key and then decrypting the encrypted content using the file encryption key, or by decrypting the encrypted content using the data protection private key). 
     The target device to which the data protection private key is to be provided can be identified in different manners. In one or more embodiments, the target device communicates a notification to the device implementing process  300  that the target device desires the data protection private key. This notification can be via the cloud storage, or alternatively another communication channel. Alternatively, the target device to which the data protection private key is to be provided can be determined in different manners. For example, the device implementing process  300  (or the user key store  126 ) can maintain a record of which other devices the data protection policy private key has been provided to, and can automatically store on the cloud storage the data protection private key encrypted using each device public key in the user key store  126  for which the device implementing process  300  has not yet stored an encrypted data protection private key on the cloud storage. 
       FIG. 4  illustrates an example system  400  in which content is roamed and a data protection private key is shared in accordance with one or more embodiments. A user logs into two different ones of his computing devices  402  and  404  at the same or different times. The user can be, but need not be, logged into both of the computing devices  402  and  404  concurrently in order to roam content and/or share the data protection private key. Each computing device  402  and  404  can be a computing device  102  of  FIG. 1  or a computing device  202  of  FIG. 2 . Each computing device  402  and  404  has a content protection system  406  and  408 , respectively, as discussed above. Protected content  410  is provided to the cloud storage  112  by the computing device  402 , and copied to the computing device  404  from the cloud storage  112 . 
     The computing device  402  provides a device public key of the computing device  402  to the key store  126 , which maintains the key, shown as device  1  public key  412 . The computing device  404  provides a device public key of the computing device  404  to the key store  126 , which maintains the key, shown as device  2  public key  414 . The computing device  402  (or alternatively the computing device  404 ) also provides the data protection public key to the key store  126 , which maintains the key, shown as data protection public key  416 . The data protection public key  416  can be used to share data among different devices or different user identities, as discussed in more detail below. 
     The computing device  402  obtains the device  2  public key  414  from the user key store  126 , and encrypts the data protection private key using the device  2  public key  414 . The encrypted data protection private key is provided  418  to the computing device  404 , allowing the computing device  404  to decrypt the protected content  410 . 
     In one or more embodiments, the data protection public/private key pair is generated by one device, such as the first device that the user logs into using the user identity. The data protection private key is then provided to the other devices of the user as he logs into those other devices using the user identity. Thus, the protected content is protected using the same data protection public/private key pair rather than each device protecting the data using its own data protection public/private key pair. 
     In one or more embodiments, an additional user verification is performed prior to encrypting and providing the data protection private key to the target device. The computing device from which the data protection private key is being transferred (e.g., the computing device  402  in the example of  FIG. 4 ), also referred to as the source device, prompts the user of the source device to consent to the transfer of the data protection private key. This prompt can be a visual display or other presentation to the user of the source device. This prompt can include a user-generated nonce or other value (e.g., received from the user at the target device and provided to the source device) so that the cloud storage or any other malicious device or component cannot impersonate the target device or user. A user input is received indicating whether the transfer is consented to. The encryption and providing of the data protection private key continues if the transfer is consented to, but does not continue and the data protection private key is not provided to the target device if the transfer is not consented to. 
     By encrypting and providing the data protection private key to the target device only in response to user consent to transfer the key, additional security protection is advantageously provided against a man-in-the-middle attack. A malicious device or program may attempt a man-in-the-middle attack by pretending to be the target device. The user of both the source device and the target device is typically the user desiring to have the key transferred to the target device, so the user will readily know whether he or she is requesting the transfer. If a prompt to consent to a transfer is provided to the user when he is not having another device of his being set up for data to be roamed to, the user can assume that such a prompt is due to the actions of a malicious device or program, and decline to give consent to the key transfer. 
     As an added security precaution, an offline verification of the target device can be performed as part of the consent provided by the user. For example, if the data protection private key is to be provided from the device  402  (the source device in this example) to the device  404  (the target device in this example), an identification of the device  404  (e.g., a hash value generated from public key of the device  404 ) can be displayed or otherwise presented by both the device  402  and the device  404 . If the two identifications are the same, then the user can consent to transferring the data protection private key to the device  404 . However, if the two identifications are not the same, then consent can be withheld, preventing the data protection private key from being provided to the device  404 . If the two identifications are not the same, then the user can assume that a problem has occurred, such as another device or program is posing as the device  404  (e.g., using a man-in-the-middle attack), and so forth. 
     Thus, the sharing of a data protection private key can be performed in different manners. In one or more embodiments, the sharing is performed from a convenience-based approach. The user can just add a new device to which content is to be roamed (e.g., by logging into the device with his user identity), and a notification is sent to the device  402 . The notification can be sent from the cloud (e.g., a notification from the user key store  126  that a new device public key has been added to the user key store  126 ) or from the new device. In response to the notification, the device  402  cloud provides the encrypted data protection private key to the new device. 
     Alternatively, the sharing of a data protection private key is performed from a security-based approach. A new device to which content is to be roamed sends a value (e.g., an identifier of the new device or a nonce) to the device  402 , which displays or otherwise presents (e.g., plays back audibly) the value at the device  402 . User input to the device  402  confirms the value (so that the cloud storage cannot impersonate the user) and that the user agrees to release the data protection private key (encrypted with the public key of the new device) to the new device. 
     Returning to  FIG. 1 , the techniques discussed herein allow the encrypted content to be transferred to (roamed to) various other ones of the user&#39;s devices. Each of the user&#39;s devices can retrieve the plaintext content after obtaining the data protection private key, as discussed above. Furthermore, the techniques discussed herein allow a user to share protected content with other users or other user identities of the same user. 
     In one or more embodiments, one or more keys of the user key store  126  can be configured to be readable by additional user identities, such as all other user identities, specific user identities (e.g., as specified by the user of the computing device  102 ), and so forth. These different user identities can be user identities corresponding to different users or the same user. Thus, the data protection public key in the user key store  126  can be made available to other user identities. If a user logged into a computing device  124  desires to communicate content to a user with a particular user identity, the computing device  124  can obtain the data protection public key of the particular user identity from the user key store  126 , protect the content using the obtained data protection public key, and store the protected content on the cloud storage. The content can be protected by encrypting the content with the data protection public key of the particular user identity, or by encrypting a file encryption key (that was or is used to encrypt the content) with the data protection public key of the particular user identity. The plaintext content can thus be retrieved from the protected content only by a device with the data protection private key, and the protected content can be communicated to the computing device  102  via the cloud storage  122  or other untrusted cloud storage or communication channel. 
     As an added security precaution, an offline verification of the data protection public key obtained from the user key store  126  can be performed prior to protecting the content with the obtained data protection public key. For example, if a user A logged into the computing device  124 ( 1 ) with user identity A desires to share protected content with a user B logged into the computing device  102  with user identity B, the computing device  124 ( 1 ) obtains the data protection public key of user identity B from the user key store  126 . An identification of this obtained data protection public key (e.g., a hash value generated from the obtained data protection public key) can be displayed or otherwise presented to the user A of the computing device  124 ( 1 ). Furthermore, an identification of the data protection public key (e.g., a hash value generated from the data protection public key) of user identity B can be displayed or otherwise presented to the user B of the computing device  102 . These two displayed (or otherwise presented) identifications of the data protection public key can be compared (e.g., by either or both users) to verify the identifications are the same. If the two identifications are the same, then user A (and/or user B) can authorize the computing device  124 ( 1 ) to proceed with protecting the content with the data protection public key of user identity B. However, if the two identifications are not the same, then user A (and/or user B) can provide an input indicating to the computing device  124 ( 1 ) to cancel the sharing of the content, and to not protect the content with the data protection public key of user identity B. If the two identifications are not the same, then user A (and/or user B) can assume that a problem has occurred, that another device is posing as the computing device  102  (e.g., using a man-in-the-middle attack), and so forth. 
     It should be noted that the data protection private key is maintained by the content protection system  104  and is not revealed to the cloud storage  122 . There is no centralized key management facility for the various devices where the data protection private key is stored (e.g., no key escrow service or similar service is provided by the cloud storage  122 ). In one or more embodiments, one or more measures are taken to provide a backup of the data protection private key, allowing seamless recovery of the data protection private key in the event the data protection private key is no longer available from the computing device  102  (e.g., due to the computing device  102  malfunctioning, the computing device  102  being lost or stolen, etc.). 
     Various different measures can be taken to protect the data protection private key for recovery, such as protecting the data protection private key using biometrics, protecting the data protection private key using a phone, protecting the data protection private key using a secret-question-secret-answer technique, combinations thereof, and so forth. The measures can be taken by the content protection system  104  of  FIG. 1  (e.g., the key management module  112 ). 
     Protecting the data protection private key using biometrics refers to collecting biometric data regarding the user and deriving a key from the biometric data. The biometric data can take various different forms, such as a fingerprint data, eye scan (e.g., retina scan) data, face scan (e.g., face recognition) data, voice data, and so forth. This biometric data can be converted into a key using any of a variety of public and/or proprietary techniques, such as deriving a key based on the entropy from the biometric data. The key derived from the biometric data is used to encrypt the data protection private key (e.g., using any of a variety of symmetric key cryptography techniques). The encrypted data protection private key can then be stored at a location external to the computing device  102 , including the cloud storage  122 . 
     If the data protection private key were to be lost from the computing device  102  (or the computing device  102  were no longer available or usable), the data protection private key can be retrieved by the user. Biometric data is again obtained from the user, and if the biometric data is the same as was used when encrypting the data protection private key, a key derived from the newly obtained biometric data can be used to decrypt the data protection private key. 
     Protecting the data protection private key using a phone refers to storing the data protection private key on a phone (e.g., a smartphone) of the user&#39;s. Although discussed herein as a phone, the data protection private key can alternatively be stored on various other devices that are trusted by the user. The data protection private key can be transferred to the phone in a variety of different manners, including manual entry of the key by the user (e.g., the data protection private key is displayed by the computing device  102 , and the user manually enters the key into his or her phone). The data protection private key can alternatively be transferred to the phone in other manners, such as based on automatic identification of a display of the key or a representation of the key by the computing device  102 . For example, the data protection private key can be displayed by the computing device  102  and the display captured by a camera of the phone. Optical character recognition can optionally be performed on the captured image to determine the key. By way of another example, a quick response (QR) code that encodes the data protection private key can be displayed by the computing device  102  and captured by a camera of the phone. The data protection private key can alternatively be transferred to the phone in various other wired or wireless manners, such as using a USB connection, a wireless USB connection, infrared communication, NFC (near-field communication), and so forth. 
     If the data protection private key were to be lost from the computing device  102  (or the computing device  102  were no longer available or usable), the data protection private key can be retrieved from the phone. The key can be provided from the phone to the computing device  102  (or a new computing device) using any of a variety of techniques analogous to those discussed in storing the data protection private key on the phone. 
     Protecting the data protection private key using a secret-question-secret-answer technique refers to collecting one or more answers to one or more questions. The questions can take various forms, such as questions regarding knowledge of the user&#39;s past, questions regarding secret information (e.g., a personal identification number (PIN) or password created by the user), and so forth. The answers are designed to be secret answers that only the user is expected to know. The one or more answers can be converted into a key using any of a variety of public and/or proprietary techniques, such as a one-way hash. The key derived from the one or more answers is used to encrypt the data protection private key (e.g., using any of a variety of symmetric key cryptography techniques). The encrypted data protection private key can then be stored at a location external to the computing device  102 , including the cloud storage  122 . 
     If the data protection private key were to be lost from the computing device  102  (or the computing device  102  were no longer available or usable), the data protection private key can be retrieved by the user. The one or more answers are again obtained from the user, and if the one or more answers are the same as the one or more answers used when encrypting the data protection private key, a key derived from the one or more answers can be used to decrypt the data protection private key. 
     A single one of these different measures (e.g., biometric protection, phone protection, secret-question-secret-answer protection) can be used to protect the data protection private key, or a combination of these different measures can be used to protect the data protection private key. For example, both biometric protection and secret-question-secret-answer protection can be used, so the data protection private key can be retrieved by the user only if biometric data obtained from the user is the same as was used when encrypting the data protection private key, and if one or more answers obtained from the user to one or more questions are the same as the one or more answers used when encrypting the data protection private key. 
     In the discussions herein, reference is made to a single data protection public/private key pair that is used by all of the computing devices. Alternatively, multiple data protection public/private key pairs can be used, such as a separate data protection public/private key pair for each computing device, multiple data protection public/private key pairs for each of one or more computing devices, and so forth. The data protection private keys for each of these multiple data protection public/private key pairs is provided to the other computing devices using the techniques discussed herein. 
     It should also be noted that although reference is made to one cloud storage (e.g., cloud storage  122  of  FIG. 1 ), alternatively multiple different cloud storages or cloud services can be used, whether concurrently, sequentially, or even temporarily. For example, different services could be for redundancy of the user key store  126 , to have targeted per-service user key stores such as to share securely with more and/or different users, to store the user key store  126  temporarily on a different cloud storage or cloud service for the purpose of sharing with a user on a different cloud storage or cloud service, and so forth. 
     Although particular functionality is discussed herein with reference to particular modules, it should be noted that the functionality of individual modules discussed herein can be separated into multiple modules, and/or at least some functionality of multiple modules can be combined into a single module. Additionally, a particular module discussed herein as performing an action includes that particular module itself performing the action, or alternatively that particular module invoking or otherwise accessing another component or module that performs the action (or performs the action in conjunction with that particular module). Thus, a particular module performing an action includes that particular module itself performing the action and/or another module invoked or otherwise accessed by that particular module performing the action. 
       FIG. 5  illustrates an example system generally at  500  that includes an example computing device  502  that is representative of one or more systems and/or devices that may implement the various techniques described herein. The computing device  502  may be, for example, a server of a service provider, a device associated with a client (e.g., a client device), an on-chip system, and/or any other suitable computing device or computing system. 
     The example computing device  502  as illustrated includes a processing system  504 , one or more computer-readable media  506 , and one or more I/O Interfaces  508  that are communicatively coupled, one to another. Although not shown, the computing device  502  may further include a system bus or other data and command transfer system that couples the various components, one to another. A system bus can include any one or combination of different bus structures, such as a memory bus or memory controller, a peripheral bus, a universal serial bus, and/or a processor or local bus that utilizes any of a variety of bus architectures. A variety of other examples are also contemplated, such as control and data lines. 
     The processing system  504  is representative of functionality to perform one or more operations using hardware. Accordingly, the processing system  504  is illustrated as including hardware elements  510  that may be configured as processors, functional blocks, and so forth. This may include implementation in hardware as an application specific integrated circuit or other logic device formed using one or more semiconductors. The hardware elements  510  are not limited by the materials from which they are formed or the processing mechanisms employed therein. For example, processors may be comprised of semiconductor(s) and/or transistors (e.g., electronic integrated circuits (ICs)). In such a context, processor-executable instructions may be electronically-executable instructions. 
     The computer-readable media  506  is illustrated as including memory/storage  512 . The memory/storage  512  represents memory/storage capacity associated with one or more computer-readable media. The memory/storage  512  may include volatile media (such as random access memory (RAM)) and/or nonvolatile media (such as read only memory (ROM), Flash memory, optical disks, magnetic disks, and so forth). The memory/storage  512  may include fixed media (e.g., RAM, ROM, a fixed hard drive, and so on) as well as removable media (e.g., Flash memory, a removable hard drive, an optical disc, and so forth). The computer-readable media  506  may be configured in a variety of other ways as further described below. 
     The one or more input/output interface(s)  508  are representative of functionality to allow a user to enter commands and information to computing device  502 , and also allow information to be presented to the user and/or other components or devices using various input/output devices. Examples of input devices include a keyboard, a cursor control device (e.g., a mouse), a microphone (e.g., for voice inputs), a scanner, touch functionality (e.g., capacitive or other sensors that are configured to detect physical touch), a camera (e.g., which may employ visible or non-visible wavelengths such as infrared frequencies to detect movement that does not involve touch as gestures), and so forth. Examples of output devices include a display device (e.g., a monitor or projector), speakers, a printer, a network card, tactile-response device, and so forth. Thus, the computing device  502  may be configured in a variety of ways as further described below to support user interaction. 
     Computing device  502  also includes a content protection system  514 . The content protection system  514  provides various protection of content, including keys, as discussed above. The content protection system  514  can implement, for example, the content protection system  104  of  FIG. 1  or  FIG. 2 . 
     Various techniques may be described herein in the general context of software, hardware elements, or program modules. Generally, such modules include routines, programs, objects, elements, components, data structures, and so forth that perform particular tasks or implement particular abstract data types. The terms “module,” “functionality,” and “component” as used herein generally represent software, firmware, hardware, or a combination thereof. The features of the techniques described herein are platform-independent, meaning that the techniques may be implemented on a variety of computing platforms having a variety of processors. 
     An implementation of the described modules and techniques may be stored on or transmitted across some form of computer-readable media. The computer-readable media may include a variety of media that may be accessed by the computing device  502 . By way of example, and not limitation, computer-readable media may include “computer-readable storage media” and “computer-readable signal media.” 
     “Computer-readable storage media” refers to media and/or devices that enable persistent storage of information and/or storage that is tangible, in contrast to mere signal transmission, carrier waves, or signals per se. Thus, computer-readable storage media refers to non-signal bearing media. The computer-readable storage media includes hardware such as volatile and non-volatile, removable and non-removable media and/or storage devices implemented in a method or technology suitable for storage of information such as computer readable instructions, data structures, program modules, logic elements/circuits, or other data. Examples of computer-readable storage media may include, but are not limited to, RAM, ROM, EEPROM, flash memory or other memory technology, CD-ROM, digital versatile disks (DVD) or other optical storage, hard disks, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, or other storage device, tangible media, or article of manufacture suitable to store the desired information and which may be accessed by a computer. 
     “Computer-readable signal media” refers to a signal-bearing medium that is configured to transmit instructions to the hardware of the computing device  502 , such as via a network. Signal media typically may embody computer readable instructions, data structures, program modules, or other data in a modulated data signal, such as carrier waves, data signals, or other transport mechanism. Signal media also include any information delivery media. The term “modulated data signal” means a signal that has one or more of its characteristics set or changed in such a manner as to encode information in the signal. By way of example, and not limitation, communication media include wired media such as a wired network or direct-wired connection, and wireless media such as acoustic, RF, infrared, and other wireless media. 
     As previously described, the hardware elements  510  and computer-readable media  506  are representative of instructions, modules, programmable device logic and/or fixed device logic implemented in a hardware form that may be employed in some embodiments to implement at least some aspects of the techniques described herein. Hardware elements may include components of an integrated circuit or on-chip system, an application-specific integrated circuit (ASIC), a field-programmable gate array (FPGA), a complex programmable logic device (CPLD), and other implementations in silicon or other hardware devices. In this context, a hardware element may operate as a processing device that performs program tasks defined by instructions, modules, and/or logic embodied by the hardware element as well as a hardware device utilized to store instructions for execution, e.g., the computer-readable storage media described previously. 
     Combinations of the foregoing may also be employed to implement various techniques and modules described herein. Accordingly, software, hardware, or program modules and other program modules may be implemented as one or more instructions and/or logic embodied on some form of computer-readable storage media and/or by one or more hardware elements  510 . The computing device  502  may be configured to implement particular instructions and/or functions corresponding to the software and/or hardware modules. Accordingly, implementation of modules as a module that is executable by the computing device  502  as software may be achieved at least partially in hardware, e.g., through use of computer-readable storage media and/or hardware elements  510  of the processing system. The instructions and/or functions may be executable/operable by one or more articles of manufacture (for example, one or more computing devices  502  and/or processing systems  504 ) to implement techniques, modules, and examples described herein. 
     As further illustrated in  FIG. 5 , the example system  500  enables ubiquitous environments for a seamless user experience when running applications on a personal computer (PC), a television device, and/or a mobile device. Services and applications run substantially similar in all three environments for a common user experience when transitioning from one device to the next while utilizing an application, playing a video game, watching a video, and so on. 
     In the example system  500 , multiple devices are interconnected through a central computing device. The central computing device may be local to the multiple devices or may be located remotely from the multiple devices. In one or more embodiments, the central computing device may be a cloud of one or more server computers that are connected to the multiple devices through a network, the Internet, or other data communication link. 
     In one or more embodiments, this interconnection architecture enables functionality to be delivered across multiple devices to provide a common and seamless experience to a user of the multiple devices. Each of the multiple devices may have different physical requirements and capabilities, and the central computing device uses a platform to enable the delivery of an experience to the device that is both tailored to the device and yet common to all devices. In one or more embodiments, a class of target devices is created and experiences are tailored to the generic class of devices. A class of devices may be defined by physical features, types of usage, or other common characteristics of the devices. 
     In various implementations, the computing device  502  may assume a variety of different configurations, such as for computer  516 , mobile  518 , and television  520  uses. Each of these configurations includes devices that may have generally different constructs and capabilities, and thus the computing device  502  may be configured according to one or more of the different device classes. For instance, the computing device  502  may be implemented as the computer  516  class of a device that includes a personal computer, desktop computer, a multi-screen computer, laptop computer, netbook, and so on. 
     The computing device  502  may also be implemented as the mobile  518  class of device that includes mobile devices, such as a mobile phone, portable music player, portable gaming device, a tablet computer, a multi-screen computer, and so on. The computing device  502  may also be implemented as the television  520  class of device that includes devices having or connected to generally larger screens in casual viewing environments. These devices include televisions, set-top boxes, gaming consoles, and so on. 
     The techniques described herein may be supported by these various configurations of the computing device  502  and are not limited to the specific examples of the techniques described herein. This functionality may also be implemented all or in part through use of a distributed system, such as over a cloud  522  via a platform  524  as described below. 
     The cloud  522  includes and/or is representative of a platform  524  for resources  526 . The platform  524  abstracts underlying functionality of hardware (e.g., servers) and software resources of the cloud  522 . The resources  526  may include applications and/or data that can be utilized while computer processing is executed on servers that are remote from the computing device  502 . Resources  526  can also include services provided over the Internet and/or through a subscriber network, such as a cellular or Wi-Fi network. 
     The platform  524  may abstract resources and functions to connect the computing device  502  with other computing devices. The platform  524  may also serve to abstract scaling of resources to provide a corresponding level of scale to encountered demand for the resources  526  that are implemented via the platform  524 . Accordingly, in an interconnected device embodiment, implementation of functionality described herein may be distributed throughout the system  500 . For example, the functionality may be implemented in part on the computing device  502  as well as via the platform  524  that abstracts the functionality of the cloud  522 . 
     In the discussions herein, various different embodiments are described. It is to be appreciated and understood that each embodiment described herein can be used on its own or in connection with one or more other embodiments described herein. Further aspects of the techniques discussed herein relate to one or more of the following embodiments. 
     A method implemented in a first computing device comprises: protecting content using a data protection public key of a data protection public/private key pair corresponding to an identity of a user of the first computing device; copying the protected content to cloud storage; obtaining a public key of a public/private key pair of a second computing device, the first and second computing devices being associated with a same user identity; encrypting the data protection private key using the public key of the second computing device; and providing the encrypted data protection private key to the second computing device. 
     Alternatively or in addition to the above described method, any one or combination of: the providing comprising providing the encrypted data protection private key to the second computing device via a removable drive; the protecting the content comprising encrypting the content with a file encryption key, and encrypting the file encryption key with the data protection public key; the protecting the content comprising encrypting the content with the data protection public key; the providing comprising storing the encrypted data protection private key to the cloud storage; the cloud storage being an untrusted cloud storage that is not relied on to keep the protected content or the encrypted data protection private key secure; receiving additional protected data from the cloud storage, the additional protected data having been protected by a third computing device using the data protection public key, and storing the additional protected data in a content store of the first computing device; displaying a prompt for user consent to transfer the data protection private key to the second computing device, and performing the encrypting and providing only in response to a user input indicating the transfer is consented to; the displaying the prompt for user consent including displaying, at the first computing device, an identification of the second computing device; protecting the data protection private key for recovery, and recovering the data protection private key in response to the data protection private key no longer being available from the first computing device; the protecting further comprising encrypting the data protection private key based on one or both of biometric data of the user and answers from a secret-question-secret-answer technique, and storing the encrypted data protection private key to the cloud storage. 
     A first computing device comprises an encryption module configured to protect content by encrypting, using a data protection public key of a data protection public/private key pair corresponding to an identity of a user of the first computing device, the content or a file encryption key that is used to encrypt the content; one or more programs configured to copy the protected content to a cloud storage; and a key transfer module configured to: obtain a public key of a public/private key pair of a second computing device, the first and second computing devices being associated with a same user identity on the cloud storage; facilitate the encryption module encrypting the data protection private key using the public key of the second computing device; and provide the encrypted data protection private key to the second computing device. 
     Alternatively or in addition to the above described computing device, any one or combination of: the cloud storage comprising a cloud service from which the data protection private key is protected; the key transfer module being further configured to display a prompt for user consent to transfer the data protection private key to the second computing device, and facilitate the encryption module encrypting the data protection private key and provide the encrypted data protection private key to the second computing device only in response to a user input indicating the transfer is consented to; the key transfer module being further configured to protect the data protection private key for recovery using biometric data of the user, and subsequently recover the data protection private key, using newly obtained biometric data of the user, in response to the data protection private key no longer being available from the first computing device; display a QR code that encodes the data protection private key for capture by a camera of a phone of the user. 
     A computer-readable storage medium having stored thereon multiple instructions that, responsive to execution by one or more processors of a computing device, cause the one or more processors to perform operations comprising: protecting content by encrypting a file encryption key using a data protection public key of a data protection public/private key pair corresponding to an identity of a user of the computing device, the content being encrypted using the file encryption key; copying the protected content to a cloud storage; obtaining a public key of a public/private key pair of an additional computing device, the computing device and the additional computing device both accessing the cloud storage for protected content stored to the cloud storage by the other; encrypting the data protection private key using the public key of the additional computing device; and providing the encrypted data protection private key to the additional computing device. 
     Alternatively or in addition to the above described computer-readable storage medium, any one or combination of: the providing comprising storing the encrypted data protection private key to the cloud storage; the cloud storage being an untrusted cloud storage that is not relied on to keep the protected content or the encrypted data protection private key secure; protecting the data protection private key for recovery, and recovering the data protection private key in response to the data protection private key no longer being available from the computing device. 
     A first computing device comprises: means for protecting content using a data protection public key of a data protection public/private key pair corresponding to an identity of a user of the first computing device; means for copying the protected content to cloud storage; means for obtaining a public key of a public/private key pair of a second computing device, the first and second computing devices being associated with a same user identity; means for encrypting the data protection private key using the public key of the second computing device; and means for providing the encrypted data protection private key to the second computing device. 
     Alternatively or in addition to the above described method, any one or combination of: the means for providing comprising means for providing the encrypted data protection private key to the second computing device via a removable drive; the means for protecting the content comprising means for encrypting the content with a file encryption key, and means for encrypting the file encryption key with the data protection public key; the means for protecting the content comprising means for encrypting the content with the data protection public key; the means for providing comprising means for storing the encrypted data protection private key to the cloud storage; the cloud storage being an untrusted cloud storage that is not relied on to keep the protected content or the encrypted data protection private key secure; means for receiving additional protected data from the cloud storage, the additional protected data having been protected by a third computing device using the data protection public key, and means for storing the additional protected data in a content store of the first computing device; means for displaying a prompt for user consent to transfer the data protection private key to the second computing device, and means for performing the encrypting and providing only in response to a user input indicating the transfer is consented to; the means for displaying the prompt for user consent including means for displaying, at the first computing device, an identification of the second computing device; means for protecting the data protection private key for recovery, and means for recovering the data protection private key in response to the data protection private key no longer being available from the first computing device; the means for protecting further comprising means for encrypting the data protection private key based on one or both of biometric data of the user and answers from a secret-question-secret-answer technique, and means for storing the encrypted data protection private key to the cloud storage. 
     Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims.