Patent Publication Number: US-10785653-B2

Title: Secure short message service over non-access stratum

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The presentation application is a continuation application of U.S. patent application Ser. No. 15/974,394, entitled “SECURE SHORT MESSAGE SERVICE OVER NON-ACCESS STRATUM” and filed on May 8, 2018 the entirety of which is incorporated by reference herein. This application claims priority to India (IN) Patent Application 201741026689, filed in India on Jul. 27, 2017, the entirety of which is incorporated by reference herein. 
    
    
     BACKGROUND 
     The non-access stratum (NAS) is a protocol layer implemented in wireless communication protocol stacks between user equipment and a core network. NAS is the highest stratum of the control plane between user equipment and a mobility management entity (MME) or an access and mobility management function (AMF) at the radio interface. The NAS layer manages the establishment of communication sessions and maintains continuous communication as the user equipment moves through the network. Examples of functions supported by the NAS protocols include mobility of the user equipment and session management procedures that are used to establish and maintain connectivity between the user equipment and a packet data network via the mobile data network. In contrast, an access stratum (AS) layer and corresponding AS protocol are used to convey information over an air interface in the wireless portion of the network. The NAS protocol is transported using radio resource control (RRC) messages in Second Generation (2G), Third Generation (3G), Fourth Generation (4G), and Fifth Generation (5G) networks that operate according to the standards defined by the Third Generation Partnership Project (3GPP). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure may be better understood, and its numerous features and advantages made apparent to those skilled in the art by referencing the accompanying drawings. The use of the same reference symbols in different drawings indicates similar or identical items. 
         FIG. 1  is a block diagram of a first example of a communication system that supports secure short message service (SMS) messaging over NAS according to some embodiments. 
         FIG. 2  is a block diagram of a network function virtualization (NFV) architecture according to some embodiments. 
         FIG. 3  illustrates a message flow that is used to register a user equipment with an AMF and an SMS service function (SMSF) for secure SMS messaging over NAS according to some embodiments. 
         FIG. 4  illustrates a process of generating an integrity-protected NAS message that includes an encrypted SMS message according to some embodiments. 
         FIG. 5  is a block diagram of a communication system that supports secure SMS messaging over multiple NAS links according to some embodiments. 
         FIG. 6  is a block diagram of an NAS context according to some embodiments. 
         FIG. 7  is a message flow for a security procedure for providing mobile-originated SMS messages over NAS according to some embodiments. 
         FIG. 8  is a message flow for a security procedure for providing mobile-terminated SMS messages over NAS according to some embodiments. 
         FIG. 9  is a block diagram of a second example of a communication system that supports secure SMS messaging over NAS according to some embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     User equipment register for a short message service (SMS) over non-access stratum (NAS) by transmitting a request to an access and mobility management function (AMF) that forwards the request to an SMS service function (SMSF). Once registered, the user equipment can exchange secure SMS messages over one or more NAS links with the AMF by encrypting the SMS message using an encryption key (such as an NAS encryption key) and adding the encrypted SMS message to a payload of an NAS message. An NAS header is appended to the payload and integrity protection is applied to the NAS message using an integrity key (such as an NAS integrity key). The encryption key and the integrity key are part of an NAS context that is generated during registration of the user equipment with the AMF and the SMSF. The NAS context also includes addresses of the user equipment and the SMSF. In some embodiments, the user equipment and the AMF establish multiple NAS links that can be used to convey the SMS over NAS messages. The NAS links can be associated with different encryption keys and integrity keys. The NAS header in each NAS message therefore includes information identifying the NAS link. The receiving entity (the user equipment for mobile-terminated SMS and the AMF for mobile-originated SMS) uses the information identifying the NAS link to select the appropriate keys for integrity checking and decryption of the SMS message. 
       FIG. 1  is a block diagram of a first example of a communication system  100  that supports secure SMS messaging over NAS according to some embodiments. The communication system  100  provides support for both mobile and fixed access. As used herein, the term “mobile access” refers to accessing a communication system (e.g., the communication system  100 ) over an air interface. Mobile access can therefore be referred to as wireless access, mobile communication, wireless communication, or other similar terms. The term “fixed access” refers to accessing to a communication system using a device that is physically connected to the communication system, e.g., accessing a communication system such as the communication system  100  via wires, optical fibers, and the like. Fixed access can therefore be referred to as wireline access, wired communication, or other similar terms. The communication system  100  supports hybrid access that allows devices to concurrently access the communication system  100  using mobile access and fixed access. 
     The communication system  100  includes a core network  105  that is accessible by either mobile or fixed devices using a common user plane access and a control plane that supports common authentication, authorization, and accounting (AAA) and policy control. The core network  105  includes an access and mobility management function (AMF)  110  that manages access control and mobility for devices in the communication system  100 . The core network  105  also includes a session management function (SMF)  115  to set up and manage sessions in the communication system  100  according to network policies. An association between user equipment and the core network  105  can be represented as a packet data unit (PDU) session that can be managed by the SMF  115 . The PDU session supports data connectivity between user equipment and a data network. 
     The core network  105  also includes one or more user plane functions (UPF)  120  that can be deployed in the communication system  100  to provide services to users of the communication system  100 . The core network  105  further includes a unified data manager (UDM)  125  that processes credentials, location management, subscription management, and the like. The UDM  125  stores data including user subscription data, such as subscription identifiers, security credentials, access and mobility related subscription data, and session related subscription data. Some embodiments of the core network  105  include other functionality such as a policy control function and a network function repository function, which are not shown in  FIG. 1  in the interest of clarity. Some embodiments of the core network  105  are implemented using network function virtualization and software defined networking, as discussed herein. For example, different network slices can be used to instantiate different instances of the AMF  110 , the SMF  115 , the UPF  120 , or the UDM  125  for different users or devices. Each PDU session is part of one network slice. 
     In the illustrated embodiment, a device  130  has a wired connection to a residential gateway  135  via a home network  140 . The term “user equipment” can be used to refer to either the device  130  or the residential gateway  135  (or a combination thereof) because both the device  130  and the residential gateway  135  can be owned or operated by the customer on the customer premises. The residential gateway  135  has a wired connection to an Ethernet network  145 . Some embodiments of the wired connection use line termination devices such as a digital subscriber line access multiplexer (DSLAM) or a gigabit passive optical network (GPON). The communication system  100  can therefore provide the user equipment (e.g., the device  130  or the residential gateway  135 ) with fixed access to the core network  105 . 
     The communication system  100  can also provide the user equipment with mobile access to the core network  105 , e.g., via a radio access network  150  that is connected to the AMF  110  over a corresponding interface such as an N2 interface. The radio access network  150  is also connected to the UPF  120  by a corresponding interface such as an N3 interface, which is not shown in  FIG. 1  in the interest of clarity. The radio access network  150  is able to provide wireless connectivity to the device  130  via a wireless connection  155 . The radio access network  150  is also able to provide wireless connectivity to the residential gateway  135  via a wireless connection  160 . 
     An interworking function  165  is disposed between the Ethernet  140  and the core network  105 . The interworking function  165  can also be referred to as a non-3GPP interworking function (N3IWF). The interworking function  165  is configured to modify or translate messages conveyed from the fixed access user equipment to the core network  105  so that the fixed access user equipment appears to be accessing the core network  105  according to mobile access standards or protocols from the perspective of the core network  105 . The interworking function  165  is also configured to modify or translate messages conveyed from the core network  105  to the fixed access user equipment so that the messages received by the fixed access user equipment conform to corresponding fixed access standards or protocols. The interworking function  165  supports interfaces with the AMF  110  and the UPF  120 . 
     One or more NAS links are used to convey NAS messages between the user equipment and the AMF  110 , which are the terminating endpoints of the NAS links. The device  130  (or other user equipment) can therefore communicate with the core network  105  using NAS messages that are conveyed via a first (fixed) NAS link that traverses the residential gateway  135  and the wired connection supported by the home network  140 , a second (mobile) NAS link that traverses the radio access network  150  and the wireless connection  155  or other NAS links such as a Wi-Fi link or other non-3GPP link. Some embodiments of the device  130  (or other user equipment) can support multiple concurrent NAS links with the AMF  110 . For example, the device  130  and the AMF  110  can concurrently support a fixed NAS link, a mobile NAS link, and a non-3GPP NAS link. 
     Prior to establishing an NAS link, the user equipment (such as the device  130 ) transmits an NAS registration message to register with the AMF  110 . The NAS registration message can include information that is used to perform authorization (and, in some cases, authentication) of the user equipment. The registration message can also be used to register the user equipment with an SMS service function (SMSF)  170  for SMS messaging. Registration of the user equipment with the AMF  110  also includes generating or provisioning an encryption key and an integrity key for the user equipment and the AMF  110 . The encryption key and the integrity key are used to provide secure SMS messaging over the NAS link, as discussed herein. 
     The SMSF  170 , which can also be referred to as a short message service center (SMSC), provides a “store and forward” mechanism for conveying SMS messages. For example, the SMSF  170  can store mobile-terminated SMS messages that are addressed to the user equipment and attempt to forward the SMS messages to the user equipment. For another example, the SMSF  170  can store mobile-originated SMS messages received from the user equipment and then forward the SMS messages to the appropriate destination in an external network. The payload length of SMS messages is limited by the constraints of the signaling protocol to precisely 140 bytes or 1120 bits. The SMS messages can be encoded using a variety of alphabets: the default GSM 7-bit alphabet, the 8-bit data alphabet, and the 16-bit UCS-2 alphabet, and the like. Depending on which alphabet the user equipment using, the maximum number of characters in an SMS message can be 160 7-bit characters, 140 8-bit characters, 70 16-bit characters, or some other value. 
     Mobile-originated or mobile-terminated SMS messages can be securely transmitted in the wireless communication system  100  by transmitting encrypted SMS messages as payloads in NAS messages that are transmitted over NAS links between the user equipment and the AMF  110 . In some embodiments, SMS messages are encrypted using the encryption key stored at the user equipment and the AMF  110 . The encrypted SMS message is added to a payload of an NAS message that includes an NAS header. Integrity protection is applied to the NAS message using the integrity stored at the user equipment and the AMF  110 . The integrity-protected NAS message is then transmitted, e.g., mobile-originated SMS messages are transmitted from the user equipment to the AMF  110  and mobile-terminated SMS messages are transmitted from the AMF  110  to the user equipment. The NAS message is received via an NAS link between the user equipment and the AMF  110 . An integrity check is performed on the received NAS message using the integrity key. The encrypted SMS message is extracted from a payload of the NAS message in response to the integrity check being successful and the encrypted SMS message is decrypted using the encryption key. 
       FIG. 2  is a block diagram of an NFV architecture  200  according to some embodiments. The NFV architecture  200  is used to implement some embodiments of the communication system  100  shown in  FIG. 1 . The NFV architecture  200  includes hardware resources  201  including computing hardware  202 , storage hardware  203 , and network hardware  204 . A virtualization layer  205  provides an abstract representation of the hardware resources  201 . The abstract representation supported by the virtualization layer  205  can be managed using a virtualized infrastructure manager  210 , which is part of the NFV management and orchestration (M&amp;O) module  215 . Some embodiments of the manager  210  are configured to collect and forward performance measurements and events that may occur in the NFV architecture  200 . For example, performance measurements may be forwarded to an orchestrator (ORCH)  217  implemented in the NFV M&amp;O  215 . The hardware resources  201  and the virtualization layer  205  may be used to implement virtual resources  220  including virtual computing  221 , virtual storage  222 , and virtual networking  223 . 
     Virtual networking functions (VNF1, VNF2, VNF3) run over the NFV infrastructure (e.g., the hardware resources  201 ) and utilize the virtual resources  220 . For example the virtual networking functions (VNF1, VNF2, VNF3) may be implemented using virtual machines supported by the virtual computing resources  221 , virtual memory supported by the virtual storage resources  222 , or virtual networks supported by the virtual network resources  223 . Element management systems (EMS1, EMS2, EMS3) are responsible for managing the virtual networking functions (VNF1, VNF2, VNF3). For example, the element management systems (EMS1, EMS2, EMS3) may be responsible for fault and performance management. In some embodiments, each of the virtual networking functions (VNF1, VNF2, VNF3) is controlled by a corresponding VNF manager  225  that exchanges information and coordinates actions with the manager  210  or the orchestrator  217 . 
     The NFV architecture  200  may include an operation support system (OSS)/business support system (BSS)  230 . The OSS/BSS  230  deals with network management including fault management using the OSS functionality. The OSS/BSS  230  also deals with customer and product management using the BSS functionality. Some embodiments of the NFV architecture  200  use a set of descriptors  235  for storing descriptions of services, virtual network functions, or infrastructure supported by the NFV architecture  200 . Information in the descriptors  235  may be updated or modified by the NFV M&amp;O  215 . 
     The NFV architecture  200  can be used to implement network slices that provide control plane functions, such as instances of the AMF  110 , the SMF  115 , the UPF  120 , or the UD and  125  shown in  FIG. 1 . A network slice is a complete logical network that provides communication services and network capabilities, which can vary from slice to slice. User equipment can concurrently access multiple slices. Some embodiments of user equipment provide Network Slice Selection Assistance Information (NSSAI) parameters to the network to assist in selection of a slice instance for the user equipment. A single NSSAI may lead to the selection of several slices. The NFV architecture  200  can also use device capabilities, subscription information and local operator policies to do the selection. An NSSAI is a collection of smaller components, Single-NSSAIs (S-NSSAI), which each include a Slice Service Type (SST) and possibly a Slice Differentiator (SD). Slice service type refers to an expected network behavior in terms of features and services (e.g., specialized for broadband or massive IoT), while the slice differentiator can help selecting among several network slice instances of the same type, e.g. to isolate traffic related to different services into different slices. 
       FIG. 3  illustrates a message flow  300  that is used to register a user equipment with an AMF and an SMSF for secure SMS messaging over NAS according to some embodiments. The message flow  300  is implemented in some embodiments of the communication system  100  shown in  FIG. 1 . 
     The user equipment (UE) transmits a registration request  305  to the AMF. The registration request  305  includes information indicating that the user equipment supports SMS messaging over NAS. Some embodiments of the registration request  305  also include information indicating whether the user equipment supports SMS messaging over wired NAS links, wireless NAS links, non-3GPP links, or a combination thereof. 
     In response to receiving the registration request  305 , the AMF can optionally perform an update location procedure  310  with a UDM in the core network. If the registration request  305  includes an indication that the user equipment supports SMS messaging over NAS, the AMF can access SMS subscription information for the user equipment from the UDM. The AMF then determines whether the user equipment is authorized for the SMS service based on the SMS subscription information. If so, the AMF can discover and select an SMSF that is used to provide the SMS service to the user equipment. Discovery of the SMSF can be performed using a preconfigured SMSF address or an address that is provided to the AMF by the UDM. 
     In some embodiments, the AMF provides a registration acceptance  315  in response to determining that the user equipment is authorized for SMS service and selecting the SMSF to provide the SMS service. However, some embodiments of the AMF do not transmit the registration acceptance  315  until after receiving confirmation from the SMSF, as discussed below. 
     The AMF transmits a service activation  320  to the SMSF using the SMSF address. The service activation  320  can include the AMF address. In response to the service activation  320 , the SMSF transmits a subscriber data request  325  to the UDM, which returns the requested subscription information. The SMSF can also create a user equipment context to store the SMS subscription information and the AMF address. The SMSF transmits a context management message  330  to instruct the UDM to store the SMSF address for a short message terminating routing. The SMSF then transmits a service activation  335  to the AMF. 
     In some embodiments, the AMF provides a registration acceptance  340  in response to receiving a confirmation that the SMSF will provide the SMS service to the user equipment. In some embodiments, the registration acceptance  340  can also indicate whether the SMSF supports SMS messaging over wired NAS links, wireless NAS links, non-3GPP links, or a combination thereof. 
     The user equipment and the AMF perform an NAS context procedure  345 . Although the NAS context procedure  345  is depicted as occurring after the registration acceptance  340 , some or all of the NAS context procedure  345  can occur at different points within the registration process indicated by the message flow  300 . The NAS context procedure  345  is used to establish an NAS context that is stored by the user equipment and the AMF. The NAS context can include an address of the user equipment, an address of the SMSF, an identifier of an NAS link, an encryption key associated with the NAS link, and an integrity key associated with the NAS link. In some embodiments, multiple NAS links can be established between the user equipment and the AMF. In that case, the NAS context can include identifiers of the multiple NAS links, as well as associated encryption and integrity keys. 
       FIG. 4  illustrates a process  400  of generating an integrity-protected NAS message that includes an encrypted SMS message according to some embodiments. The process  400  is used in some embodiments of the communication system  100  shown in  FIG. 1 . For example, the device  130  shown in  FIG. 1  can use the process  400  to generate integrity-protected NAS messages to carry encrypted mobile-originated SMS messages. For another example, the AMF  110  shown in  FIG. 1  can use the process to generate integrity-protected NAS message to carry encrypted mobile-terminated SMS messages. 
     The process  400  is performed on SMS data  405 . In the case of a mobile-originated SMS message, the SMS data  405  is generated by user equipment. In the case of a mobile-terminated SMS message, the SMS data  405  is generated by an SMSF and provided to an AMF, which performs the process  400 . The SMS data  405  and an encryption key  410  are provided to an encryption module  415 . The encryption key  410  is stored in an NAS context associated with the user equipment and the AMF and copies of the encryption key  410  are stored at the user equipment and the AMF. Some embodiments of the encryption key  410  are NAS encryption keys. The encryption module  415  encrypts the SMS data  405  using the encryption key  410 . 
     An NAS message  420  is formed by adding the encrypted SMS data  425  to a payload of the NAS message and appending an NAS header  430 . Some embodiments of the NAS header  430  include information identifying an NAS link that is used to convey the NAS message  420 . As discussed herein, the information identifying the NAS link can be used to identify the encryption key  410  in an NAS context and an integrity key  435  that is used to provide integrity protection for the NAS message  420 . 
     An integrity protection module  440  uses the integrity key  435  to generate an integrity-protected NAS message  445 . For example, information  450  generated based on the integrity key  435  can be appended to the NAS message  422  generate the integrity-protected NAS message  445 . 
       FIG. 5  is a block diagram of a communication system  500  that supports secure SMS messaging over multiple NAS links according to some embodiments. The communication system  500  includes user equipment  505  and an AMF  510 , which are used to implement some embodiments of the user equipment and the AMF  110  shown in  FIG. 1 . The user equipment  505  and the AMF  510  implement NAS layers  515 ,  520 , respectively, which are configured to establish and maintain NAS links between the user equipment  505  and the AMF  510 . In the illustrated embodiment, the user equipment  505  and the AMF  510  have established multiple NAS links including a mobile NAS link that is established via a radio access network that includes a node-B  525 , a fixed NAS link that is established via fixed hardware  530  that includes a wired connection to the user equipment  505 , and a non-3GPP NAS link that is established via a Wi-Fi access point  535 . The user equipment  505  and the AMF  510  store NAS contexts  540 ,  545 , respectively, which include information such as addresses of the user equipment  505  and an SMSF that provides an SMS service to the user equipment  505 , encryption keys for the NAS links, integrity keys for the NAS links, and the like. 
     An NAS message  550  can be transmitted between the NAS layers  515 ,  520  over a selected one of the NAS links. In some embodiments, one of the NAS links is selected by the transmitting node (either the user equipment  505  or the AMF  510 ) based on selection criteria such as latencies, congestion, usage costs, priorities, and the like. The NAS message  550  includes an NAS header  555 , an NAS payload  560 , and integrity check information  565  that is formed using an integrity key. In the illustrated embodiment, the NAS payload  560  includes an SMS payload  570 , which includes an encrypted SMS message. For example, the SMS payload  570  can be formed by encrypting an SMS message using the encryption key for the NAS link that is used to convey the NAS message  550 . The NAS header  555  includes information identifying the NAS link that is used to convey the NAS message  550 . The receiving node can then use this information to access the context information for the NAS link used to convey the NAS message  550 , e.g., the receiving node can access the correct encryption key and integrity key that are used to perform the integrity check of the NAS message  550  and decryption of the SMS payload  570 . 
       FIG. 6  is a block diagram of an NAS context  600  according to some embodiments. The NAS context  600  is used to implement some embodiments of the NAS contexts  540 ,  545  shown in  FIG. 5 . As discussed herein, the NAS context  600  can be configured during a registration process such as the registration process implemented in some embodiments of the method flows  300  shown in  FIG. 3 . The NAS context  600  is stored in some embodiments of the user equipment and the AMF  110  shown in  FIG. 1  and in some embodiments of the user equipment  505  and the AMF  510  shown in  FIG. 5 . 
     The NAS context  600  includes an address  605  of the user equipment and an address  610  of an SMSF that provides an SMS service to the user equipment indicated by the address  605 . The NAS context  600  also includes link-specific information  615 ,  620  that is associated with the NAS links that have been established between the user equipment and the AMF that stores the NAS context  600 . For example, the link-specific information  615  includes an identifier  625  of a first NAS link between the user equipment and the AMF, an encryption key  630  that is used to encrypt or decrypt portions of the NAS messages that are transmitted over the first NAS link, and an integrity key  635  that is used perform integrity checks on the NAS messages that are transmitted over the first NAS link. For another example, the link-specific information  620  includes an identifier  640  of a second NAS link between the user equipment and the AMF, an encryption key  645  that is used to encrypt or decrypt portions of the NAS messages that are transmitted over the second NAS link, and an integrity key  650  that is used perform integrity checks on the NAS messages that are transmitted over the second NAS link. 
     Some embodiments of the NAS context  600  can include more or less link-specific information depending on the number of links that are concurrently established between the user equipment and the AMF. The amount of link-specific information can also increase or decrease in response to changes in the number of links that are concurrently established between the user equipment and the AMF. 
     Secure SMS messaging over NAS can be implemented in standardized communication systems. For example, in 3GPP Technical Specification (TS) 23.502, which is incorporated herein by reference in its entirety, an insecure SMS over NAS procedure is described in clause 4.13.3. A baseline procedure referred to as “MO SMS over NAS in CM-IDLE (baseline)” is described in clause 4.13.3.3 and another optimized procedure referred to as “MO SMS using one step approach in CM-IDLE” is described in clause 4.13.3.4. Both procedures use NAS messages as the transport to carry SMS messages between a UE and an AMF, both in the mobile-originated and mobile-terminated cases. Some embodiments of the secure SMS messaging techniques disclosed herein can be implemented in the standardized SMS over NAS solutions. 
     To perform secure SMS messaging over NAS, the UE first establishes an NAS context with an AMF (such as the NAS context  600  shown in  FIG. 6 ) and then UE registers with an SMSF to send or receive SMS messages over NAS. If the UE is already registered with the SMSF and if the UE goes in to Idle mode, it is possible to send SMS messages in a one-step procedure using NAS as the transport and using NAS security to protect the SMS message. If the UE is already in Idle mode and the SMSF receives an SMS message that is addressed to the UE, the UE is paged. When the UE connects in response to the page, the SMS message can be delivered to the UE in a one-step NAS message. If the UE is not registered with the SMSF, the UE is triggered to register and then the same NAS transport and NAS security can be used. The SMS over NAS is secured by encrypting the SMS message part using an encryption key such as an NAS encryption key. The NAS message is integrity protected using an integrity key such as an NAS integrity key. If multiple NAS instances are active between the UE and the network (AMF), the security context associated with the chosen NAS link is used to partially cipher the SMS message content and integrity protect the whole NAS message. 
     Once the registration procedure for SMS over NAS (e.g., the registration procedure illustrated in the method  300  shown in  FIG. 3 ) is completed, UE can send or receive SMS using the baseline procedure “MO SMS over NAS in CM-IDLE (baseline)” described under 4.13.3.3 of 3GPP TS 23.502 or the optimized procedure “MO SMS using one step approach in CM-IDLE” described under 4.13.3.4 of 3GPP TS 23.502. When the UE is in connected mode CM-CONNECTED, the UE may send or receive SMS via either 3GPP transport or via untrusted 3GPP access. The secure SMS over NAS procedures can be applied across 3GPP and non-3GPP access. 
     At successful completion of the registration procedure, the AMF has the address of the SMSF in the NAS context of the UE. Once registered, the UE is able to send and receive SMS over NAS either in CM-CONNECTED or in CM-IDLE states. The SMS packet data unit (PDU) is encrypted using an encryption key such as the 5G NAS ciphering key, KNAS enc, as specified in clause 8.2 of 3GPP TS 33.401, which is incorporated herein by reference in its entirety. The whole NAS message is integrity protected using an integrity key such as the 5G NAS integrity key, KNAS int, as specified in clause 8.1 of TS 33.401. If there are multiple NAS connections active between the UE and AMF, any one of the links could be used to transport the SMS and protect the data using the keys for the selected NAS link. It is assumed here the key selection inherently goes along with the chosen link. 
     If the UE is in CM-IDLE state, the UE is able to send mobile originated SMS over NAS in one step, protected as mentioned above. To receive mobile terminated SMS over NAS in this state, UE is paged to make the UE connect to the network. Both in the CM-CONNECTED and CM-IDLE states UE need to be registered with SMSF to send and receive SMS messages. 
       FIG. 7  is a message flow  700  for a security procedure for providing mobile-originated SMS messages over NAS according to some embodiments. The message flow  700  is implemented in some embodiments of the communication system  100  shown in  FIG. 1  and the communication system  500  shown in  FIG. 5 . 
     At block  705 , a UE establishes an NAS context and registers with an SMSF to receive SMS services via a radio access network (RAN). An AMF has the address of the SMSF in the NAS context for the UE. In some embodiments, a one-step procedure can be performed in which the SMSF and the UE keep the registration and thus the AMF retrieves the SMSF address from the UE NAS context. In some embodiments, a generic SMS over NAS procedure is performed in which the AMF accesses the SMSF address in the UE context prior to sending or receiving the SMS message. The NAS context in the UE and the AMF determines whether the one-step procedure or the generic procedure is performed. 
     At step  710 , a mobile-originated SMS message is generated at the UE. The UE sends the NAS message containing the SMS PDU. The SMS PDU is encrypted using an encryption key such as the 5G NAS encryption key, e.g. similar to KNAS enc as specified in clause 8.2 of TS 33.401. The NAS message is integrity protected using an integrity key such as the 5G NAS integrity key, e.g. similar to KNAS int as specified in clause 8.1 of TS 33.401. After AMF has checked the integrity of the NAS message, the SMS PDU is retrieved by using the encryption key to decrypt the SMS PDU at block  715 . At step  720 , the AMF forwards the SMS PDU to the SMSF if the NAS integrity verification succeeds, e.g., if the integrity check verifies integrity of the NAS message and the SMS PDU is successfully decrypted. 
       FIG. 8  is a message flow  800  for a security procedure for providing mobile-terminated SMS messages over NAS according to some embodiments. The message flow  800  is implemented in some embodiments of the communication system  100  shown in  FIG. 1  and the communication system  500  shown in  FIG. 5 . 
     At block  805 , a UE establishes an NAS context and registers with SMSF to receive SMS services. An AMF has the address of the SMSF in the UE NAS context. In some embodiments, a one-step procedure is performed in which the SMSF and the UE keep the registration and thus the AMF retrieves the SMSF address from the UE NAS context. In some embodiments, a generic SMS over NAS procedure is performed in which the AMF accesses the SMSF address in the UE context prior to sending or receiving the SMS message. The NAS context in the UE and the AMF determines whether the generic procedure or the one-step procedure is performed. 
     At step  810 , the SMSF provides an SMS message containing an SMS PDU to the AMF. The SMS message is addressed to the UE. In response to receiving the SMS message, the AMF determines the connection mode of the UE, e.g., the AMF determines whether the UE is in a connected mode (CM-CONNECTED) or an idle mode (CM-IDLE). 
     If the UE is in CM-IDLE, AMF initiates paging the UE (at block  815 ) to make the UE connect to the network. If the UE is already in CM-CONNECTED, paging of the UE at block  815  is bypassed and the message flow  800  flows directly from step  810  to block  820 . 
     At block  820 , the AMF encrypts the SMS PDU using an encryption key and performs integrity protection on the NAS message including the encrypted SMS PDU using an integrity key. For example, the AMF has the NAS context for the recipient UE available and can encrypt the SMS PDU using an encryption key such as the 5G NAS encryption key, e.g. similar to KNAS enc as specified in clause 8.2 of TS 33.401. The AMF subsequently uses an integrity key such as the 5G NAS integrity key, e.g. similar to KNAS int as specified in clause 8.1 of TS 33.401, to form an integrity-protected NAS message to be sent to the UE. At step  825 , the AMF forwards the integrity-protected NAS message including the encrypted SMS PDU to the UE, which verifies the correctness of the message using a local copy of the integrity key and retrieves the SMS PDU by using decryption based on a local copy of the encryption key. 
     Embodiments of the techniques disclosed herein use NAS security to do partial ciphering of SMS messages that are carried over NAS transport and apply integrity protection to the NAS message. The secure SMS over NAS technique can be used in standardized procedures such as “MO SMS over NAS in CM-IDLE (baseline),” “MO SMS using one step approach in CM-IDLE,” “MO SMS over NAS in CM-CONNECTED,” “MT SMS over NAS in CM-IDLE via 3GPP access,” “MT SMS over NAS in CM-CONNECTED,” and “MT SMS over NAS via non 3GPP access.” 
       FIG. 9  is a block diagram of a second example of a communication system  900  that supports secure SMS messaging over NAS according to some embodiments. The communication system  900  includes a user equipment  905 , an AMF  910 , and an SMSF  915 . The communication system  900  can therefore be implemented in some embodiments of the communication system  100  shown in  FIG. 1  and the communication system  500  shown in  FIG. 5 . 
     The user equipment  905  includes a transceiver  920  for transmitting and receiving signals over an interface  915  with the AMF  910 . The user equipment  905  also includes a processor  930  and a memory  935 . The processor  930  can be used to execute instructions stored in the memory  935  and to store information in the memory  935  such as the results of the executed instructions. The memory  935  can also be configured to store NAS contexts such as the NAS context  600  shown in  FIG. 6 . The transceiver  920 , the processor  930 , and the memory  935  can be configured to perform some embodiments of the message flow  300  shown in  FIG. 3 , the process  400  shown in  FIG. 4 , the message flow  700  shown in  FIG. 7 , and the message flow  800  shown in  FIG. 8 . 
     The AMF  910  includes a transceiver  940  for transmitting and receiving signals over the interface  915  with the AMF  910  and over an interface  945  with the SMSF  915 . The AMF  910  also includes a processor  950  and a memory  955 . The processor  950  can be used to execute instructions stored in the memory  955  and to store information in the memory  955  such as the results of the executed instructions. The memory  955  can also be configured to store NAS contexts such as the NAS context  600  shown in  FIG. 6 . The transceiver  940 , the processor  940 , and the memory  945  can be configured to perform some embodiments of the message flow  300  shown in  FIG. 3 , the process  400  shown in  FIG. 4 , the message flow  700  shown in  FIG. 7 , and the message flow  800  shown in  FIG. 8 . 
     The SMSF  915  includes a transceiver  960  for transmitting and receiving signals over the interface  945  with the AMF  910 . The SMSF  915  also includes a processor  965  and a memory  970 . The processor  965  can be used to execute instructions stored in the memory  970  and to store information in the memory  970  such as the results of the executed instructions. The transceiver  960 , the processor  965 , and the memory  970  can be configured to perform some embodiments of the message flow  300  shown in  FIG. 3 , the message flow  700  shown in  FIG. 7 , and the message flow  800  shown in  FIG. 8 . 
     In some embodiments, certain aspects of the techniques described above may implemented by one or more processors of a processing system executing software. The software comprises one or more sets of executable instructions stored or otherwise tangibly embodied on a non-transitory computer readable storage medium. The software can include the instructions and certain data that, when executed by the one or more processors, manipulate the one or more processors to perform one or more aspects of the techniques described above. The non-transitory computer readable storage medium can include, for example, a magnetic or optical disk storage device, solid state storage devices such as Flash memory, a cache, random access memory (RAM) or other non-volatile memory device or devices, and the like. The executable instructions stored on the non-transitory computer readable storage medium may be in source code, assembly language code, object code, or other instruction format that is interpreted or otherwise executable by one or more processors. 
     A computer readable storage medium may include any storage medium, or combination of storage media, accessible by a computer system during use to provide instructions and/or data to the computer system. Such storage media can include, but is not limited to, optical media (e.g., compact disc (CD), digital versatile disc (DVD), Blu-Ray disc), magnetic media (e.g., floppy disc, magnetic tape, or magnetic hard drive), volatile memory (e.g., random access memory (RAM) or cache), non-volatile memory (e.g., read-only memory (ROM) or Flash memory), or microelectromechanical systems (MEMS)-based storage media. The computer readable storage medium may be embedded in the computing system (e.g., system RAM or ROM), fixedly attached to the computing system (e.g., a magnetic hard drive), removably attached to the computing system (e.g., an optical disc or Universal Serial Bus (USB)-based Flash memory), or coupled to the computer system via a wired or wireless network (e.g., network accessible storage (NAS)). 
     Note that not all of the activities or elements described above in the general description are required, that a portion of a specific activity or device may not be required, and that one or more further activities may be performed, or elements included, in addition to those described. Still further, the order in which activities are listed are not necessarily the order in which they are performed. Also, the concepts have been described with reference to specific embodiments. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the present disclosure as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of the present disclosure. 
     Benefits, other advantages, and solutions to problems have been described above with regard to specific embodiments. However, the benefits, advantages, solutions to problems, and any feature(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential feature of any or all the claims. Moreover, the particular embodiments disclosed above are illustrative only, as the disclosed subject matter may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. No limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope of the disclosed subject matter. Accordingly, the protection sought herein is as set forth in the claims below.