Patent Publication Number: US-11038852-B2

Title: Method and system for preventing data leakage from trusted network to untrusted network

Description:
BACKGROUND 
     Field 
     This disclosure is generally related to data security. More specifically, this disclosure is related to a system and method for facilitating a trusted network in a cloud computing environment. 
     Related Art 
     In recent years, cloud computing has become a highly demanded service or utility due to the advantages of high computing power, cheap cost of services, high performance, scalability, and accessibility, as well as availability. In cloud computing, different services, including servers, storage, and applications, can be delivered by the service provider to a customer&#39;s computers and devices via the Internet. The cloud service providers are responsible for protecting and maintaining the physical environment (e.g., data centers). Cloud computing can be a cost-saving solution for enterprise users because they no longer need to purchase hardware, and can only pay for the services that is actually used. Could computing customers can have immediate access to a broad range of resources and applications hosted in the infrastructure of other organizations, allowing data collaborations among users in different physical domains. However, security is a big concern for cloud computing users. Conventional data security approaches cannot meet the unique security requirements of cloud computing. 
     SUMMARY 
     One embodiment described herein provides a system and method for establishing a secure network that includes a plurality of host computers. During operation, a server computer can distribute at least one symmetric encryption key among the plurality of host computers to enable the plurality of host computers to communicate securely with each other using the distributed symmetric encryption key. Each host computer can include at least a smart network interface card, and a central processing unit (CPU) of each host computer supports remote attestation. Distributing the symmetric encryption key among the plurality of host computers can include performing a remote attestation operation to establish a trusted channel between the server computer and a protected region within the CPU of a respective host computer; and transmitting, over the trusted channel, the symmetric encryption key to the CPU of the respective host computer, which in turn forwards the symmetric encryption key to the smart network interface card of the respective host computer over a secure channel established between the protected region within the CPU and the smart network interface card. 
     In a variation on this embodiment, the smart network interface card of the respective host computer is configured to encrypt a packet to be sent to a second host computer using the symmetric encryption key before transmitting the packet to the second host computer. 
     In a further variation, a corresponding smart network interface card of the second host computer is configured to decrypt the encrypted packet. 
     In a variation on this embodiment, the CPU of each host computer implements at least in part an embedded trusted execution environment (TEE) technology, which can include Intel® Software Guard Extension (SGX) technology, Arm® TrustZone technology, AMD® Secure Technology, and other TEE technologies. 
     In a variation on this embodiment, the server computer is configured to distribute a set of symmetric encryption keys, with each individual symmetric encryption key being used for secure communications between a particular pair of host computers selected from the plurality of host computers. 
     In a further variation, the server computer is further configured to distribute, among the plurality of host computers, a key-host mapping table. 
     In a variation on this embodiment, the smart network interface card can include a processor embedded with a trusted firmware module, and the trusted firmware module facilitates establishing the secure communication channel between the protected region within the CPU and the smart network interface card. 
     In a further variation, establishing the secure communication channel between the protected region within the CPU and the smart network interface card involves a public/private key pair associated with the trusted firmware module. 
     In a variation on this embodiment, the plurality of host computers forms a network group, and a global server computer can be configured to distribute at least one inter-group encryption key among a plurality of network groups to enable the plurality of network groups to communicate securely with each other using the inter-group encryption key. 
     In a further variation, a respective network group can include a gateway host computer, and a smart network interface card associated with the gateway host computer can include an inter-group port for transmitting and receiving inter-group packets. The global server computer can be configured to send the inter-group encryption key to the smart network interface card associated with the gateway host computer. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  presents a diagram illustrating the architecture of an exemplary smart network interface card (SmartNIC), according to one embodiment. 
         FIG. 2  presents a diagram illustrates communications within an exemplary secure network group, according to one embodiment. 
         FIG. 3  presents a flowchart illustrating an exemplary configuration process, according to one embodiment. 
         FIG. 4  presents a time-space diagram illustrating an exemplary process for key distribution, according to one embodiment. 
         FIG. 5  presents a diagram illustrating various types of secure communications within a secure network group, according to one embodiment. 
         FIG. 6  illustrates an exemplary secure network that includes multiple network groups, according to one embodiment. 
         FIG. 7  illustrates the architecture of an exemplary client computer, according to one embodiment. 
         FIG. 8  illustrates the architecture of an exemplary server computer, according to one embodiment. 
         FIG. 9  illustrates an exemplary client-server network environment for implementing the disclosed technology for establishing a secure network, in accordance with some embodiments described herein. 
         FIG. 10  conceptually illustrates an electronic system with which some implementations of the subject technology are implemented 
     
    
    
     In the figures, like reference numerals refer to the same figure elements. 
     DETAILED DESCRIPTION 
     The following description is presented to enable any person skilled in the art to make and use the embodiments, and is provided in the context of a particular application and its requirements. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the present disclosure. Thus, the present invention is not limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein. 
     Overview 
     In this disclosure, a method and system are presented for facilitating a trusted network in a cloud computing setting. More specifically, each host in the trusted network is equipped with a smart network interface card (SmartNIC), which has a built-in processor. The SmartNIC can be configured to perform encryption/decryption operations on all outgoing and incoming packets using a secret key shared among the hosts within the trusted network. To ensure the secrecy of the shared key in order to prevent information leakage, a hardware-based attestation scheme can be used. More specifically, the central processing unit (CPU) of each host can implement the Intel® Software Guard Extension (SGX) technology that can protect sensitive information. 
     Trusted Execution Environment 
     Trusted Computing is an emerging technology developed by the Trusted Computing Group (TCG) with a view toward building trustworthy computer platforms. In trusted computing, the computer will consistently behave in expected ways, and those behaviors will be enforced by computer hardware and software. Enforcing this behavior is achieved by loading the hardware with a unique encryption key inaccessible to the rest of the system. According to the TCG, “trusted component, operation, or process is one whose behavior is predictable under almost any operating condition and which is highly resistant to subversion by application software, viruses, and a given level of physical interference.” 
     Trust technologies have predominantly evolved across diverse computing environments, like smart cards, virtualized platforms, and mobile devices. Moreover, recent development in trusted computing technologies includes the development of a trusted execution environment (TEE), which refers to a secure area inside a main processor (e.g., the central processor unit (CPU)). The TEE runs in parallel with the operating system, in an isolated environment, and can guarantee that the code and data loaded in the TEE are protected with respect to confidentiality and integrity. TEE is more secure than the classic system (called REE, or rich execution environment) in that it uses both hardware and software to protect data and code. 
     Trusted applications running in a TEE have access to the full power of a device&#39;s main processor and memory, whereas hardware isolation protects these components from user-installed applications running in the main operating system. Software and cryptographic isolations inside the TEE can protect the different contained trusted applications from each other. 
     A range of implementations of the TEE have been developed with diverse characteristics. Hardware trusted execution environments have been designed in different ways by different vendors. In a cloud computing environment, the performance is very important and affects end user experience. TEE solutions based on a trusted platform module (TPM) often have low speed and cannot meet the requirements of cloud computing. Although a hardware security module (HSM) can be a better solution, it has to be deployed in the cloud environment beforehand. On the other hand, Intel® SGX technology makes hardware the root of trust built inside a CPU directly without any extra device. Intel SGX can provide secure computing capabilities for both personal computers and cloud computing. More specifically, Intel SGX protects an application&#39;s secrets from malicious software by creating isolated memory regions of code and data called enclaves. These non-addressable memory pages are reserved from the system&#39;s physical RAM and then encrypted, allowing the application to access its secrets without fear of exposure. With Intel SGX, secrets remain secret even if the application, operating system, BIOS, or VMM are compromised. Intel SGX applications can be built to include a trusted part and an untrusted part. When an application needs to work with a secret, it creates an enclave that is placed in trusted memory. It then calls a trusted function to enter the enclave, where it is given a view of its secrets in clear text. 
     With SGX, only the data and code deemed security-critical is relocated to the enclave by defining ‘trusted’ functions in SGX specification language controlled by the developer, while the enclave application itself is written in a subset of C/C++. Enclaves may read/write to memory spaces in the host application, but not from the host application to enclaves outside of these pre-defined functions; enclaves are also unable to access other enclaves&#39; contents arbitrarily. 
     One important function provided by SGX is its remote attestation mechanism. This mechanism is based on the Enhanced Privacy ID (EPID) protocol—a Direct Anonymous Attestation (DAA) protocol that uses a group signature scheme in conjunction with a CPU-bound EPID key. A separate, special enclave (the quoting enclave) is used to measure, sign and certify the state of the enclave application, along with collecting other platform-specific data, which is transmitted back to the attestation challenger. Local attestation is also available for enabling two enclaves to verify the operating state of each other running on the same physical CPU. In this process, the enclaves produce separate operating reports containing their identities. Next, MACs (message authentication codes) are computed over these reports using 128-bit AES-CMACs (cipher-based message authentication codes) under a common report key bound to the underlying CPU. The MACs and common report key are used to verify whether the enclaves are executing on the same platform. In some embodiments, SGX&#39;s remote attestation mechanism can allow provisioning of sensitive data (e.g., encryption keys). 
     Smart Network Interface Card (NIC) 
     A smart or programmable network interface card (SmartNIC) refers to a NIC (e.g., a network adapter) that includes an on-board processor and is capable of offloading processing tasks that the system CPU would normally handle. Using its on-board processor, the SmartNIC can perform any combination of encryption/decryption, firewall, TCP/IP and HTTP processing. 
     There are different implementations of SmartNICs, such as application-specific integrated circuit (ASIC)-, field-programmable gate array (FPGA)-, and system-on-chip (SOC)-based implementations. An ASIC NIC is very cost-effective and may deliver the best price performance, but it suffers from limited flexibility. An FPGA NIC by contrast is highly programmable and, with enough time and effort, can be made to support almost any functionality relatively efficiently (within the constraints of the available gates). However, FPGAs are notoriously difficult to program and expensive. On the other hand, SOC can provide what appears to be the best SmartNIC implementation option: good price performance, easy to program, and highly flexible. In some embodiments of the present invention, each host in the secure network can include an SOC-based SmartNIC, such as a Mellanox® BlueField™ SmartNIC. The Mellanox BlueField SmartNIC is a two-port 25 Gb/s network adapter featuring the BlueField system-on-chip (SOC), which integrates the ConnectX®-5 network controller with Arm® core processors, to deliver an innovative and high-performance programmable platform. The BlueField adapter enables a more efficient use of computing resources and reduces total cost of ownership (TCO) by offloading security, networking and storage functions such that the server CPU can focus on computing and applications. 
       FIG. 1  presents a diagram illustrating the architecture of an exemplary SmartNIC, according to one embodiment. SmartNIC  100  can include a network interface  102 , a computing unit  104 , and a host interface  106 . 
     Network interface  102  can facilitate connectivity to a network, e.g., an Ethernet network, and can be responsible for sending and receiving network packets. In some embodiments, interface  102  can be similar to a conventional network interface card (NIC) and can include at least two network ports. One network port can be used for communication with other in-group hosts or service providers within a secure network group, whereas the other port can be used in a gateway unit for communication with hosts in other network groups or a global service provider (SP) server. 
     Computing unit  104  can include one or more processors, a memory (e.g., DRAM (dynamic random-access memory), and a storage unit (e.g., an eMMC (embedded MultiMediaCard) storage unit or an SSD (solid state drive) storage unit). In some embodiments, computing unit  104  can include an array of multicore processors, such as Arm® multicore processors. Computing unit  104  enables more efficient use of computing resources and reduces TCO by offloading security, networking, and storage functions such that the host CPU can focus on computing and applications. In some embodiments, computing unit  104  can include embedded Arm trusted firmware (ATF) that can facilitate establishing trust between SmartNIC  100  and a remote attestation server. 
     Host interface  106  facilitates the interface between SmartNIC and the host computer. In some embodiments, host interface  106  can be a high speed (e.g., 25 GBE or 100 GBE) peripheral component interconnect express (PCIe) interface that can connect SmartNIC  100  to the bus. 
     Secure Network 
       FIG. 2  presents a diagram illustrating communications within an exemplary secure network group, according to one embodiment. In  FIG. 2 , a secure network group (also referred to as trusted network group)  200  includes a remote attestation server (also referred to as a service provider server)  202  and hosts  204  and  206 . Each of attestation server  202  and hosts  204  and  206  can be coupled to a SmartNIC module for interfacing with each other. More specifically, attestation server  202  can be coupled to SmartNIC module  212 , and hosts  204  and  206  can be coupled to SmartNIC modules  214  and  216 , respectively. Attestation server  202  can communicate with host  204  via SmartNIC modules  212  and  214 . Similarly, attestation server  202  can communicate with host  206  via SmartNIC modules  212  and  216 . Moreover, hosts  204  and  206  can communicate with each other via SmartNIC modules  214  and  216 . Each SmartNIC module can include an embedded ATF unit. For example, SmartNIC modules  212 ,  214 , and  216  can include ATF units  222 ,  224 , and  226 , respectively. The embedded ATF units can store initial encryption keys (e.g., a private key of the public/private key pair) used for initial communication between hosts or between a host and a service provider (SP) server. 
     Remote attestation or service provider server  202  can be responsible for providing services to or running applications on a host machine. Before providing services, service provider server  202  needs to perform remote attestation on the software or applications running on the host machine. More specifically, remote attestation server  202  can be responsible for verifying that software (or applications) running on a host are running on a trusted platform (e.g., an SGX-enabled platform) and for verifying the software&#39;s integrity and identity. In some embodiments, these verification operations can be enabled by the remote attestation mechanism provided by Intel SGX. Remote attestation server  202  can also generate a number of encryption keys (e.g., symmetric keys) that can be distributed to the host computer after the successful attestation of the trusted platform as well as the application running on the platform. Such encryption keys can facilitate secure communication among the hosts. In some embodiments, remote attestation server  202  can distribute a set of encryption keys to each host within a trusted network to facilitate secure in-network communication. More specifically, a host can select, based on a key-mapping table, a particular key from the set of received keys for communication with a particular host. Such a key-mapping table can also be distributed by remote attestation server  202  among the hosts in the trusted network. 
     Each host (e.g., host  204  or host  206 ) can be Intel SGX-enabled, meaning that the CPU of the host (not shown in  FIG. 2 ) can include an embedded SGX module and can be configured to enable the SGX functionalities. For example, hosts  204  and  206  can include SGX modules  234  and  236 , respectively. The remote attestation mechanism provided by the SGX module facilitates remote attestation server  202  to verify that an application supported by the SGX is running on a trusted platform and to verify the identity and integrity of the application. 
       FIG. 2  also illustrates the various types of secure communication channels in secure network group  200 . The first type of secure communication channels exists between remote attestation server  202  and the SmartNIC of a host, as indicated by solid lines  242  and  244 . An original static key generated by the ATF module of the SmartNIC can be used to encrypt packets going back and forth between service provider server  202  and the SmartNIC of the host. 
     A second type of secure communication channels exists between the SmartNICs of hosts within secure network group  200  (e.g., hosts  204  and  206 ), as indicated by a dashed line  246 . This secure communication channel allows encrypted packets to be sent between the SmartNICs of the hosts using secret keys provisioned to the hosts by service provider server  202 . In some embodiments, the encryption keys used by the hosts for in-group communications can be time-varying and can be referred to as ephemeral keys. Detailed descriptions regarding the provisioning of the ephemeral keys will follow. 
     A third type of secure communication channels exists between the SmartNIC and the SGX module of each host, as indicated by the dash-dotted lines  248  and  250 . Such secure communication channels can be protected by a symmetric key generated by the SGX module and can be referred to as an SGX-to-NIC (s-n) key. Note that such a symmetric key can be passed from the SGX module to the SmartNIC using a public key of the ATF module within the SmartNIC. A detailed description regarding the distribution of the s-n key will follow. 
       FIG. 2  also illustrates a regular, unprotected communication channel between the SmartNIC and the unprotected region within a host, as indicated by dotted lines  252  and  254 . 
     Before a secure network group can be established, the various components (e.g., the service provider server, the hosts, and their corresponding SmartNICs) within the secure network need to be configured. It is assumed that the configuration operation can be performed in a secure environment, before the hosts within the secure network are brought online.  FIG. 3  presents a flowchart illustrating an exemplary configuration process, according to one embodiment. 
     During configuration, an asymmetric key pair (e.g., an RSA (Rivest-Shamir-Adleman) public/private key pair) can be initialized in the ATF module within the SmartNIC of each host (operation  302 ). Such a key pair can represent the identity of the SmartNIC and is protected by the ATF. More specifically, the private key of the key pair will always remain inside the ATF. Subsequently, the public key of the asymmetric key pair of the SmartNIC can be registered with the service provider (operation  304 ). For example, such an asymmetric key pair can be stored into a key database maintained by the service provider. Registering the public key with the service provider allows the host to establish a secure, ATF-protected communication channel with the service provider server. Various mechanisms can be used to register the public key with the service provider server, including registration via a previously established secure channel or manual registration. 
     Additional steps to configure the SmartNIC of a host can also include initiating an original symmetric and static key in the SmartNIC (operation  306 ) to allow the SmartNIC of the host to communicate securely with the SmartNIC of the service provider. To do so, the original symmetric static key can also be sent to the SmartNIC of the service provider server using the ATF public key of the SmartNIC of the service provider server. It is assumed that the ATF public key of the SmartNIC of the service provider server is known to all hosts, and that the ATF private key of that SmartNIC never leaves the ATF module. In addition to configuring the SmartNICs, the configuration operation can also include configuring the SGX enclave within the host (operation  308 ) and configuring the service provider server (operation  310 ). 
     Subsequent to the initial configuration, the hosts and the service provider server can exchange encryption/decryption keys in order to establish secure communication channels. More specifically, to ensure security, the key exchange process can be facilitated by the remote attestation mechanism provided by the Intel SGX. More specifically, through remote attestation, the service provider server can establish a trusted channel to the SGX enclave within the host, and can subsequently send encryption keys used among the hosts to the SGX enclave. The SGX enclave can then pass such encryption keys to the corresponding SmartNIC to allow the SmartNIC to encrypt packages exchanged among the hosts. 
       FIG. 4  presents a time-space diagram illustrating an exemplary process for key distribution, according to one embodiment. More specifically, the key-distribution process can involve SGX enclave  402  of a client, SmartNIC module  404  of the client, service provider server  406 , and SmartNIC module  408  of the service provider server  406 . Note that, after the configuration operations, SmartNIC module  404  of the client and SmartNIC module  408  of service provider server  406  each holds the original static key and can perform secure communication using the original static key. 
     During key distribution, a remote attestation operation (operation  412 ) can be performed between SGX enclave  402  and service provider server  406 . Note that, during remote attestation, messages exchanged between SGX enclave  402  and service provider server  406  are protected, in the application layer, by a specific attestation key, which is a secret key embedded in SGX enclave  402 . Moreover, packets exchanged between the SmartNIC modules  404  and  408  are also encrypted, in the TCP (transport communication protocol) layer, using the original static key. More specifically, SmartNIC modules  404  and  408  are responsible for encryption and decryption of the packets. Because a packet is first encrypted at one SmartNIC module and decrypted at the other, such an encryption/decryption operation can be transparent to SGX enclave  402  and service provide server  406 . 
     Subsequent to the remote attestation, service provider server  406  verifies that SGX enclave  402  is trustworthy and can provision encryption keys for the client machine via SGX enclave  402 . More specifically, service provider server  406  can distribute a set of secret keys among a group of clients to allow clients within the group to communicate with each other in a secure manner. The key provision process starts with service provider server  406  sending a plurality of encryption keys to SGX enclave  402  via SmartNIC module  404  and SmartNIC module  408  (operation  414 ). In some embodiments, the encryption keys can include a set of ephemeral keys along with a host-key mapping table, and the ATF public key of SmartNIC module  404 . In a further embodiment, a rotated version of the original static key can also be included. To ensure security, the message including all these keys can be protected (e.g., encrypted) using the attestation key that is protected by SGX enclave  402 . Moreover, SmartNICs  404  and  408  can perform the standard encryption/decryption operation in the TCP layer using the original static key. As discussed previously, such operations are transparent to SGX enclave  402  and service provider server  406 . Note that although the trust between SGX enclave  402  and service provider server  406  has been established, the trust or a secure channel between SGX module  402  and SmartNIC module  404  is not yet established. 
     To establish a secure channel to SmartNIC module  404 , upon receiving the set of keys (which includes the ATF public key of SmartNIC module  404 ) from service provider server  406 , SGX enclave  402  can generate and send an SGX-to-NIC (s-n) key to SmartNIC module  404  (operation  416 ). Such a message can be protected using the ATF public key of SmartNIC module  404 . SmartNIC module  404  can obtain the s-n key by decrypting the message using its own private key. The s-n key can be a symmetric key, thus enabling more efficient secure communication between SGX enclave  402  and SmartNIC module  404 . In other words, the first communication between SGX enclave  402  and SmartNIC module  404  can be protected using the asymmetric key provided by the ATF module, and subsequent communications can be protected by the s-n symmetric key in order to increase efficiency. 
     Subsequently, SGX enclave  402  can send the set of keys received from service provider server  406  to SmartNIC module  404  using the s-n key (operation  418 ). In some embodiments, SGX enclave  402  can send the set of ephemeral keys and the corresponding host-key mapping table to SmartNIC module  404 . The set of ephemeral keys allows SmartNIC module  404  to establish secure communication channels with other SmartNIC modules of other hosts. In some embodiments, the number of ephemeral keys matches the number of other hosts within the secure group. For example, if there are n hosts in the secure group, there will be n−1 encryption keys to allow a particular host to communicate with every other host using a different key. A respective entry in the host-key mapping table specifies the particular key that should be used by SmartNIC module  404  for encrypting/decrypting packets exchanged between SmartNIC module  404  and a corresponding SmartNIC module of a particular host. For example, such an entry can map a key identifier (ID) to a host ID. When the host coupled to SmartNIC module  404  needs to communicate with a different host within the secure group, SmartNIC module  404  can look up the host-key mapping table to obtain a key corresponding to the different host. The set of ephemeral keys and the key-host mapping table can be sent periodically or on demand by service provider server  406  to SGX enclave  402 , which then forwards the set of ephemeral keys and the mapping table to SmartNIC module  404 . It is also possible for service provider server  406  to send a single key to SmartNIC module  404 , meaning that SmartNIC module  404  is required to use the same key to establish secure communication channels with all other hosts that also receive the same key from service provider server  406 . 
     In the event of SGX enclave  402  also receiving a rotated static key, SGX enclave  402  can also send the rotated static key to SmartNIC module  404 . Rotating the static key can enhance the security of the communication between SmartNIC modules  404  and  408 . In addition to key rotation, other key ratcheting mechanisms can also be used to update the encryption key used between the SmartNIC of the host and the SmartNIC of the service provider server. For example, an update of the key can include a hash function of the original static key. 
     Upon receiving encrypted information from SGX enclave  402 , SmartNIC module  404  performs a decryption operation using the previously received s-n key to obtain the set of ephemeral keys and the mapping table (operation  420 ). This operation ends the key-provisioning operation. More specifically, after receiving the set of ephemeral keys and the key-host mapping table, each host can now communicate with other hosts securely using the set of received ephemeral keys. As one can see from  FIG. 4 , the secrecy or security of the ephemeral keys is protected by the SGX module embedded in the host CPU, as well as by the ATF module embedded in the SmartNIC of the host. Such hardware-assist key-provisioning can provide a much higher level of security than conventional key-provisioning schemes. 
       FIG. 5  presents a diagram illustrating various types of secure communications within a secure network group, according to one embodiment. In  FIG. 5 , a secure network group can include a first client comprising an SGX enclave  502 , a host  504 , and a SmartNIC module  506 ; a second client comprising an SGX enclave  512 , a host  514 , and a SmartNIC module  516 ; a service provider server  510 , and a SmartNIC module  508  of service provider server  510 . Note that here hosts  504  and  514  refer to the unprotected regions (i.e., regions not protected by the SGX) within the CPUs of the first and second clients, respectively. 
     During operation, SGX enclave  502  of the first client can communicate with SGX enclave  512  of the second client by sending an encrypted packet to SmartNIC module  506  using the s-n key (operation  522 ). For example, an application running in SGX enclave  502  can communicate with an application running in SGX enclave  512 . SmartNIC module  506  can decrypt the packet, identify the second client as the recipient of the packet, and look up in the host-key mapping table to obtain the corresponding ephemeral key (operation  524 ). SmartNIC module  506  can then encrypt the packet using the corresponding ephemeral key and send the encrypted packet to SmartNIC module  516  (operation  526 ). SmartNIC module  516  can then decrypt the packet using the corresponding ephemeral key (operation  528 ). More specifically, SmartNIC module  516  can also perform a table lookup to find the corresponding ephemeral key based on the identity of the sender of the packet. 
     Subsequently, SmartNIC module  516  can then encrypt the packet using an s-n key shared between SmartNIC module  508  and SGX enclave  512  (operation  530 ) and forward the packet to SGX enclave  512  (operation  532 ). SGX enclave  512  can decrypt the packet using the shared s-n key to obtain the secret message sent by SGX enclave  502  (operation  534 ). 
     In addition to the communication between the SGX enclaves, applications running in the unsecure area within the CPUs of the first and second clients may also communicate with each other. In such a scenario, there is no longer a need to encrypt the packets using the s-n key. In fact, packets can be sent from the unsecured area within the client CPU directly to the corresponding SmartNIC modules. On the other hand, packets exchanged between SmartNIC modules are still encrypted using a corresponding ephemeral key shared between the SmartNIC modules. Note that the s-n key protects secure content within the SGX enclave from being accessible to untrusted applications running outside the SGX enclave when such content leaves the SGX enclave while being transmitted to the corresponding SmartNIC. 
     In  FIG. 5 , host  504  can communicate with host  514  by sending an unencrypted packet to SmartNIC module  506  (operation  536 ). SmartNIC module  506  identifies the receiving host, performs a table lookup to obtain an ephemeral key, and encrypts the packet using the ephemeral key (operation  538 ). SmartNIC module  506  then sends the encrypted packet to SmartNIC module  516  (operation  540 ). SmartNIC module  516  decrypts the packet using a corresponding ephemeral key (operation  542 ) and sends the decrypted packet to host  514  (operation  544 ). 
       FIG. 5  also shows the secure communication between the SGX enclave of a client and the service provider server. More specifically, SGX enclave  502  can communicate with server provider server  510  by sending an encrypted packet to SmartNIC module  506  (operation  546 ). The packet can be encrypted using the s-n key shared between SGX enclave  502  and SmartNIC module  506 . SmartNIC module  506  decrypts the packet using the shared s-n key (operation  548 ) and then encrypts the packet using a symmetric key shared between SmartNIC modules  504  and  508  (operation  550 ). In some embodiments, such a shared key can be a static key or a rotated version of the static key. SmartNIC module  506  can then send the encrypted packet to SmartNIC module  508  of service provider server  510  (operation  552 ). SmartNIC module  508  also has a copy of the currently active rotated static key and can decrypt the received packet using such a key (operation  554 ). SmartNIC module  508  can then forward the decrypted packet to service provider server  510  (operation  556 ). 
     It is also possible for an application running in the unsecured region within the CPU of a client machine to communicate with the service provider server. In such a scenario, there is no longer a need to send the encrypted packet to the SmartNIC module of the client. However, the SmartNIC module of the client can still encrypt the packet using the static key or the rotated static key. 
     Cascaded Secure Network 
     In the examples shown in  FIGS. 2-5 , all host computers belong to a single secure network. In practice, in addition to the single layer secure network, it is also possible to construct a secure network that includes multiple sub-networks, with each sub-network including a service provider server and a set of host computers, and the host computers within each sub-network can securely communicate with each other using a set of keys distributed by the service provider server among the set of host computers. Moreover, a global service provider server can distribute a different set of keys among the different sub-networks to facilitate inter-sub-network secure communication. Each sub-network can also be referred to as a network group, with the communication within the sub-network referred to as in-group communication and the communication between sub-networks referred to as inter-group communication. 
       FIG. 6  illustrates an exemplary secure network that includes multiple network groups, according to one embodiment. Secure network  600  can include a global service provider (SP) server  602  and a plurality of network groups (i.e., network groups  610  and  620 ). Each network group can also include a local service provider server and a number of host computers. For example, network group  610  can include local SP server  612  and hosts  614  and  616 , and network group  620  can include local SP server  622  and hosts  624  and  626 . Moreover, a particular host in each network group can act as a gateway to facilitate inter-group communication. In the example shown in  FIG. 6 , host  616  can act as a gateway for network group  610 , while host  626  can act as a gateway for network group  620 . 
     Global SP server  602  can be responsible for distributing various encryption keys (referred to as inter-group keys) to the gateway of each network group, thus allowing the different network groups to communicate with each other using the inter-group keys. The distribution of the inter-group keys can use a process similar to what is shown in  FIG. 4 . More specifically, each gateway host can be SGX-enabled, and global SP server  602  can use the SGX remote attestation mechanism to build a trusted channel to the SGX enclave of the gateway host. The gateway host can include at least two SmartNIC interfaces (e.g., two ports on a SmartNIC module), with one SmartNIC interface assigned for the inter-group communication and one SmartNIC interface assigned for the in-group communication. The SGX enclave can securely transfer the inter-group keys to the SmartNIC interface assigned for the inter-group communication. 
     In the example shown in  FIG. 6 , gateway host  616  can include a SmartNIC interface  615  for the in-group communication and a SmartNIC interface  617  for the inter-group communication. In other words, packets destined to hosts within network group  610  will be transmitted via SmartNIC interface  615 , whereas packets destined to hosts outside of network group  610  will be transmitted via SmartNIC interface  617 . Similarly, gateway host  626  can include a SmartNIC interface  625  for the in-group communication and a SmartNIC interface  627  for the inter-group communication. Gateway hosts  616  and  626  can communicate with each other in a secure manner using the inter-group keys distributed by global SP server  602 . 
     A local SP server (e.g., local SP server  612  or  622 ) can be responsible for distributing the encryption keys that can be used for the in-group communication, thus allowing the different hosts within the network group to communicate with each other securely using the distributed in-group keys. The key-distribution process of the in-group keys can be similar to the one shown in  FIG. 4 . More specifically, each host within the network group can be SGX-enabled, and the local SP server can use the SGX remote attestation mechanism to build a trusted channel to the SGX enclave of each host and can send the in-group keys to the SGX enclave of each host. Note that, although not shown in  FIG. 6 , the hosts and the local SP server each can include a SmartNIC module. The SGX enclave of a host can transfer the in-group key securely to the corresponding SmartNIC interface. In the case of a gateway host, the SGX enclave of the gateway host can transfer the in-group key to the SmartNIC interface assigned for the in-group communication. For example, when local SP server  612  sends the in-group key to the SGX enclave of gateway host  616 , the SGX enclave of the gateway host  616  can transfer the in-group key to SmartNIC interface  615 , thus facilitating communication between hosts  616  and  614  via SmartNIC interface  615 . 
     When a first host (e.g., host  614 ) in a first network group (e.g., network group  610 ) communicates with a second host (e.g., host  624 ) in a second network group (e.g., network group  620 ), host  614  can send a packet to gateway host  616  using the in-group key of network group  610 . Gateway host  616  receives the packet via SmartNIC interface  615  and decrypts the packet using the in-group key of network group  610 . In response to determining that the packet is destined to the second network group, gateway host  616  encrypts the packet using the inter-group key maintained by SmartNIC interface  617  and sends the encrypted packet to gateway host  626  via SmartNIC interface  617 . 
     Gateway host  626  receives the encrypted packet on SmartNIC interface  627  and decrypts the packet using the inter-group key maintained by SmartNIC interface  627 . In response to determining that the packet is for a different host (e.g., host  624 ) inside network group  620 , gateway host  626  can encrypt the packet using the in-group key of network group  620  maintained by SmartNIC interface  625 . Gateway host  626  can then send the encrypted packet to host  624  via SmartNIC interface  625 . Upon receiving the packet, a corresponding SmartNIC interface on host  624  can decrypt the packet to obtain the content. 
     In this example, it is assumed that the applications involved in the inter-group communication run outside of the SGX enclave of each host. In the event of the application running inside the SGX enclave, additional data encryption/decryption may be needed between the SGX enclave of the SmartNIC interface, similar to the example shown in  FIG. 5 . 
     The Apparatus 
       FIG. 7  illustrates the architecture of an exemplary client computer, according to one embodiment. A client  700  can include a SmartNIC unit  702  and a CPU  704 . CPU  704  can include an SGX enclave  706  and unprotected region  708 . 
     More specially, SmartNIC unit  702  can include an ATF module  710 , an encryption/decryption module  712 , a key-receiving module  714 , a key database  716 , a table-lookup module  718 , and a packet transmitting/receiving module  720 . SGX-enclave  706  can include a remote-attestation module  722 , an encryption/decryption module  724 , a key-generation module  726 , and a packet transmitting/receiving module  728 . Unprotected region  708  within CPU  704  can include a packet transmitting/receiving module  730 . 
     ATF module  710  can ensure the security of the ATF private key. Moreover, by registering the ATF public key with a service provider server, ATF module  710  can facilitate the secure communicate between SmartNIC unit  702  and the service provider server. Encryption/decryption module  712  can be responsible for encrypting the outgoing packets and decrypting the incoming packets. Depending on the recipient or sender of a packet, encryption/decryption module  712  may select an appropriate key for the encryption or decryption operation. For example, if a packet is destined to a service provider server, encryption/decryption module  712  can select a current rotated version of an original static key shared between SmartNIC unit  702  and the service provider server. On the other hand, if the packet is destined to a different client machine within the same network group, encryption/decryption module  712  can select an ephemeral key previously distributed by the service provider server. Moreover, if the packet is destined to SGX-enclave  706 , encryption/decryption module  712  can select the s-n key shared between SmartNIC unit  702  and SGX-enclave  706 . 
     Key-receiving module  714  can be responsible for receiving encryption/decryption keys distributed by the service provider server or SGX-enclave  706 . Key database  716  can be responsible for storing these received encryption keys. Table-lookup module  718  can be responsible for performing a table lookup when selecting a particular ephemeral key to encrypt a packet that is to be sent to a different client machine. More specifically, depending on the identity of the destination client machine, a particular key can be select from the set of ephemeral keys distributed to client  700  by the service provider server. Packet transmitting/receiving module  720  can be responsible for transmitting and receiving packets. Note that, if client  700  is gateway machine for a network group within a cascaded secure network, SmartNIC unit  702  can include at least two ports, one for the in-group communication and one for the inter-group communication. Moreover, in such a scenario, each module within SmartNIC unit  702  needs to be able to handle both the in-group and the inter-group communication. For example, depending on whether a packet belongs to an in-group or inter-group communication, encryption/decryption module  712  may select an in-group key or inter-group encryption key. Similarly, transmitting/receiving module  730  may transmit the packet from a different port depending on whether the packet belongs to the in-group or inter-group communication. 
     Remote-attestation module  722  residing in SGX-enclave  706  can be responsible for remote attestation of SGX-enclave  706 . Encryption/decryption module  724  can be responsible for encrypting and decrypting packets going out of and coming into SGX-enclave  706 , respectively. Key-generation module  726  can be responsible for generating a symmetric key shared between SGX-enclave  706  and SmartNIC unit  702 , i.e., the s-n key. Packet transmitting/receiving module  728  can be responsible for transmitting packets going out of and receiving packets coming into SGX-enclave  706 , respectively. Similarly, packet transmitting/receiving module  730  residing in unprotected region  708  of CPU  704  can be responsible for transmitting packets going out of and receiving packets coming into unprotected region  708 . 
       FIG. 8  illustrates the architecture of an exemplary server computer, according to one embodiment of the present invention. A server  800  can include a SmartNIC unit  802  and a CPU  804 . Similar to SmartNIC  702  shown in  FIG. 7 , SmartNIC unit  802  can include an ATF module  810  and an encryption/decryption module  812 . CPU  804  can include a key-generation module  814 , a table-generation module  816 , a static-key-rotation module  818 , a packet transmitting/receiving module  820 , and a remote attestation module  822 . 
     ATF module  810  can be similar to ATF module  710 . ATF module  810  can maintain the ATF private key, which facilitates secure communication between SmartNIC unit  802  and a client machine. Encryption/decryption module  812  can be responsible for encrypting and decrypting outgoing and incoming packets, respectively, with respect to server  800 . 
     Key-generation module  814  can be responsible for generating the set of ephemeral keys that are distributed among hosts within a network group. In some embodiments, key-generation module  814  can periodically generate the ephemeral keys. Note that, in order to ensure that each individual pair of hosts within a network group can communicate with each other using a unique key, key-generation module  814  can generate a unique key for each individual pair of hosts within in the network group. Table-generation module  816  can be responsible for generating a key-host mapping table that maps a particular key to a particular pair of hosts. Both the generated ephemeral keys and the key-host mapping table can be transmitted to the hosts. In some embodiments, the ephemeral keys and the key-host mapping table may be organized based on the identity of the hosts. A subset of ephemeral keys that pertains to a particular host can be organized together along with the corresponding mapping table entries. This subset of ephemeral keys will then be sent to this particular host along with the corresponding entries in the matching table. In other words, each host will only receive keys that can be used for the secure communication with other hosts, but will not receive keys that are used between other hosts. 
     Static-key-rotation module  818  can be responsible for rotating an original static key shared between SmartNIC unit  802  of server  800  and the SmartNIC unit of a host. The original static key can be rotated periodically or on demand to ensure the communication security. Packet transmitting/receiving module  820  can be responsible for transmitting and receiving packets. Remote attention module  822  can be responsible for performing remote attestation on a remote entity. 
     In general, embodiments of the present invention provide a method and system for establishing trust among a group of host computers in a cloud computing environment. Such a method and system can involve a CPU-based remote attestation scheme as well as SmartNIC interfaces. The CPU-based attestation scheme (e.g., Intel SGX) can ensure that the identity of the remote entity is endorsed by the CPU, which has a much smaller attack surface, thus resulting in a safer and more robust secure network. In addition to Intel SGX, the host CPU may implement other types of TEE technology, such as Arm® TrustZone, AMD® Secure Technology, etc. The SmartNIC interfaces offload the encryption/decryption operations from the CPU, thus significantly increases the network speed. Moreover, the ATF module embedded in each SmartNIC can also provide hardware-assist trust. In addition to a physical host computers, in some embodiments, the host computers can also include one or more virtual machines (VMs) running on a cloud ECS (elastic compute service). As long as the VMs can enable remote attestation, a trusted network group can be established among the VMs. 
       FIG. 9  illustrates an exemplary client-server network environment for implementing the disclosed technology for establishing a secure network, in accordance with some embodiments described herein. A network environment  900  includes a number of electronic devices  902 ,  904  and  906  communicably connected to a server  910  by a network  908 . One or more remote servers  920  are further coupled to the server  910  and/or the one or more electronic devices  902 ,  904  and  906 . 
     In some exemplary embodiments, electronic devices  902 ,  904  and  906  can be computing devices such as laptop or desktop computers, smartphones, PDAs, portable media players, tablet computers, televisions or other displays with one or more processors coupled thereto or embedded therein, or other appropriate computing devices that can be used for displaying a web page or web application. In one example, the electronic devices  902 ,  904  and  906  store a user agent such as a browser or application. In the example of  FIG. 9 , electronic device  902  is depicted as a smartphone, electronic device  904  is depicted as a desktop computer, and electronic device  906  is depicted as a PDA. 
     Server  910  includes a processing device  912  and a data store  914 . Processing device  912  executes computer instructions stored in data store  914 , for example, to assist in scheduling a customer-initiated service or a service-provider-initiated service between a service provider and a customer at electronic devices  902 ,  904  and  906  during a service scheduling process. 
     In some exemplary aspects, server  910  can be a single computing device such as a computer server. In other embodiments, server  910  can represent more than one computing device working together to perform the actions of a server computer (e.g., cloud computing). The server  910  may host the web server communicably coupled to the browser at the client device (e.g., electronic devices  902 ,  904  or  906 ) via network  908 . In one example, the server  910  may host a client application for scheduling a customer-initiated service or a service-provider-initiated service between a service provider and a customer during a service scheduling process. Server  910  may further be in communication with one or more remote servers  920  either through the network  908  or through another network or communication means. 
     The one or more remote servers  920  may perform various functionalities and/or storage capabilities described herein with regard to the server  910  either alone or in combination with server  910 . Each of the one or more remote servers  920  may host various services. For example, servers  920  may host services providing information regarding one or more suggested locations such as web pages or websites associated with the suggested locations, services for determining the location of one or more users, or establishments, search engines for identifying results for a user query, one or more user review or query services, or one or more other services providing information regarding one or more establishments, customers and/or review or feedback regarding the establishments. 
     Server  910  may further maintain or be in communication with social networking services hosted on one or more remote servers  920 . The one or more social networking services may provide various services and may enable users to create a profile and associate themselves with other users at a remote social networking service. The server  910  and/or the one or more remote servers  920  may further facilitate the generation and maintenance of a social graph including the user-created associations. The social graphs may include, for example, a list of all users of the remote social networking service and their associations with other users of a remote social networking service. 
     Each of the one or more remote servers  920  can be a single computing device such as a computer server or can represent more than one computing device working together to perform the actions of a server computer (e.g., cloud computing). In one embodiment server  910  and one or more remote servers  920  may be implemented as a single server or a cluster of servers. In one example, server  910  and one or more remote servers  920  may communicate through the user agent at the client device (e.g., electronic devices  902 ,  904  or  906 ) via network  908 . 
     Users may interact with the system hosted by server  910 , and/or one or more services hosted by remote servers  920 , through a client application installed at the electronic devices  902 ,  904 , and  906 . Alternatively, the user may interact with the system and the one or more social networking services through a web-based browser application at the electronic devices  902 ,  904 ,  906 . Communication among client devices  902 ,  904 ,  906  and the system, and/or one or more services, may be facilitated through a network (e.g., network  908 ). 
     Communications among the client devices  902 ,  904 ,  906 , server  910  and/or one or more remote servers  920  may be facilitated through various communication protocols. In some aspects, client devices  902 ,  904 ,  906 , server  910  and/or one or more remote servers  920  may communicate wirelessly through a communication interface (not shown), which may include digital signal processing circuitry where necessary. The communication interface may provide for communications under various modes or protocols, including Global System for Mobile communication (GSM) voice calls; Short Message Service (SMS), Enhanced Messaging Service (EMS), or Multimedia Messaging Service (MMS) messaging; Code Division Multiple Access (CDMA), Time Division Multiple Access (TDMA), Personal Digital Cellular (PDC), Wideband Code Division Multiple Access (WCDMA), CDMA2000, or General Packet Radio System (GPRS), among others. For example, the communication may occur through a radio-frequency transceiver (not shown). In addition, short-range communication may occur, including via the use of a Bluetooth-enable device, WiFi, or other such transceiver. 
     Network  908  can include, for example, any one or more of a personal area network (PAN), a local area network (LAN), a campus area network (CAN), a metropolitan area network (MAN), a wide area network (WAN), a broadband network (BBN), the Internet, and the like. Further, network  908  can include, but is not limited to, any one or more of the following network topologies, including a bus network, a star network, a ring network, a mesh network, a star-bus network, a tree or hierarchical network, and the like. 
       FIG. 10  conceptually illustrates an electronic system with which some implementations of the subject technology are implemented. Electronic system  1000  can be a client, a server, a computer, a smartphone, a PDA, a laptop, or a tablet computer with one or more processors embedded therein or coupled thereto, or any other sort of electronic device. Such an electronic system includes various types of computer-readable media and interfaces for various other types of computer-readable media. Electronic system  1000  includes a bus  1008 , processing unit(s)  1012 , a system memory  1004 , a read-only memory (ROM)  1010 , a permanent storage device  1002 , an input device interface  1014 , an output device interface  1006 , and a network interface  1016 . 
     Bus  1008  collectively represents all system, peripheral, and chipset buses that communicatively connect the numerous internal devices of electronic system  1000 . For instance, bus  1008  communicatively connects processing unit(s)  1012  with ROM  1010 , system memory  1004 , and permanent storage device  1002 . 
     From these various memory units, processing unit(s)  1012  retrieves instructions to execute and data to process in order to execute the processes of the subject disclosure. The processing unit(s) can be a single processor or a multicore processor in different implementations. 
     ROM  1010  stores static data and instructions that are needed by processing unit(s)  1012  and other modules of electronic system  1000 . Permanent storage device  1002 , on the other hand, is a read-and-write memory device. This device is a non-volatile memory unit that stores instructions and data even when electronic system  1000  is off. Some implementations of the subject disclosure use a mass-storage device (such as a magnetic or optical disk and its corresponding disk drive) as permanent storage device  1002 . 
     Other implementations use a removable storage device (such as a floppy disk, flash drive, and its corresponding disk drive) as permanent storage device  1002 . Like permanent storage device  1002 , system memory  1004  is a read-and-write memory device. However, unlike storage device  1002 , system memory  1004  is a volatile read-and-write memory, such a random access memory. System memory  1004  stores some of the instructions and data that the processor needs at runtime. In some implementations, the processes of the subject disclosure are stored in system memory  1004 , permanent storage device  1002 , and/or ROM  1010 . From these various memory units, processing unit(s)  1012  retrieves instructions to execute and data to process in order to execute the processes of some implementations. 
     Bus  1008  also connects to input and output device interfaces  1014  and  1006 . Input device interface  1014  enables the user to communicate information and select commands to the electronic system. Input devices used with input device interface  1014  include, for example, alphanumeric keyboards and pointing devices (also called “cursor control devices”). Output device interface  1006  enables, for example, the display of images generated by electronic system  1000 . Output devices used with output device interface  1006  include, for example, printers and display devices, such as cathode ray tubes (CRT) or liquid crystal displays (LCD). Some implementations include devices such as a touchscreen that functions as both input and output devices. 
     Finally, as shown in  FIG. 10 , bus  1008  also couples electronic system  1000  to a network (not shown) through a network interface  1016 . In this manner, the computer can be a part of a network of computers (such as a local area network (“LAN”), a wide area network (“WAN”), or an Intranet, or a network of networks, such as the Internet. Any or all components of electronic system  1000  can be used in conjunction with the subject disclosure. 
     These functions described above can be implemented in digital electronic circuitry; or in computer software, firmware or hardware. The techniques can be implemented using one or more computer program products. Programmable processors and computers can be included in or packaged as mobile devices. The processes and logic flows can be performed by one or more programmable processors or by one or more programmable logic circuitry. General and special purpose computing devices and storage devices can be interconnected through communication networks. 
     The foregoing descriptions of various embodiments have been presented only for purposes of illustration and description. They are not intended to be exhaustive or to limit the present invention to the forms disclosed. Accordingly, many modifications and variations will be apparent to practitioners skilled in the art. Additionally, the above disclosure is not intended to limit the present invention.