Patent Description:
The Third Generation Partnership Project (3GPP) is currently developing the standards for Fifth Generation (<NUM>) systems. It is expected that <NUM> networks will support many new scenarios and use cases and will be an enabler for the Internet of Things (IoT). It is also expected that <NUM> systems will provide connectivity for a wide range of new devices such as sensors, smart wearables, vehicles, machines, etc. Flexibility will be a key property in <NUM> systems. This new flexibility is reflected in the security requirements for network access that mandate the support of alternative authentication methods and different types of credentials other than the usual Authentication and Key Agreement (AKA) credentials pre-provisioned by the operator and securely stored in the Universal Integrated Circuit Card (UICC). More flexible security features would allow factory owners or enterprises to leverage their own identity and credential management systems for authentication and access network security.

Among the new security features in <NUM> systems is the introduction of a Security Anchor Function (SEAF). The purpose of the SEAF is to cater to the flexibility and dynamicity in the deployment of the <NUM> core network functions, by providing an anchor in a secure location for key storage. In fact, the SEAF is expected to leverage virtualization to achieve the desired flexibility. As a consequence, the Access and Mobility Management Function (AMF), the <NUM> function responsible for access and mobility management, can be deployed in a domain that is potentially less secure than the operator's core network, while the master key remains in the SEAF in a secure location.

The SEAF is intended to establish and share a key denoted KSEAF with the user equipment (UE), that is used for deriving other keys, such as the keys for the control plane protection (e.g., KCN key) and the radio interface protection. These keys generally correspond to the non-access stratum (NAS) keys and the access stratum key (KENB) in Long Term Evolution (LTE) systems. The SEAF is assumed to reside in a secure location and the KSEAF key would never leave the SEAF. The SEAF communicates with the AMFs and provisions the necessary key material (derived from the KSEAF key) for the protection of the control plane (CP) and user plane (UP) traffic with the user equipment (UE). One advantage of this approach is that it avoids re-authentication each time a UE moves from an area served by one AMF to an area served by another AMF. In fact, authentication is a costly procedure, particularly when the UE is roaming.

Recently, a proposal has been introduced to co-locate the SEAF and AMF, which defeats the purpose of the SEAF in the first place. It is worth noting that the security design in LTE systems was conceptually based on the assumption that the mobility management entity (MME), i.e. the node responsible for mobility management in LTE systems, is always located in a secure location within the operator core network. This assumption does not apply to the AMF in <NUM> systems. In dense areas, an AMF could be deployed closer to the edge of the network and thus potentially in exposed locations (e.g., in a shopping mall). Therefore, during an AMF change, it is possible that one of the AMFs is not located in an equally secure domain as the other, and therefore the target or the source AMF might need to shield itself from the other.

The Evolved Packet System (EPS) relied on the assumption that the MME is always located in a secure location. Therefore, during an MME change, the new MME simply fetched the security context of the UE from the previous MME. In addition, an MME may optionally trigger a new authentication for forward security.

With legacy mechanisms, forward security (i.e. the old MME does not know the security context used by the new MME) could be achieved via re-authentication but there was no mechanism for backward security (i.e. the new MME does not know the security context used by the old MME). The new AMF may trigger a new authentication thus eliminating any possibility for the old AMF to determine the new keys. The need for re-authentication could, for example, be based on an operator policy taking into account the location of the different AMFs.

Relying solely on the authentication procedure is not very efficient since, performance wise, it is one of the most costly procedures. Therefore, there remains a need to provide security when changing AMFs without the need for re-authentication.

The <NPL>" discloses a message flow for a New Radio (NR) to LTE inter-RAT S1/NG mode Handover (HO).

The 3GPP document TS23. <NUM> discloses General Packet Radio Service (GPRS) enhancements for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access. Clause <NUM>. <NUM> discloses Tracking Area Update procedure with Serving Gateway change and clause <NUM>. <NUM> discloses NAS Security Mode Command (SMC) procedure in the context of GPRS for E-UTRAN.

The 3GPP document S2-<NUM> discusses the use of Tracking Area Update (TAU) for idle mode mobility between NG Core and Evolved Packet Core (EPC).

The 3GPP specification TS33. <NUM> V <NUM>. <NUM> relates to security architecture in 3GPP System Architecture Evolution (SAE). Clause <NUM> of that document discloses Single Radio Voice Call Continuity (SRVCC) between Evolved Universal Terrestrial Radio Access Network (E-UTRAN) and Circuit Switched UTRAN/GSM EDGE Radio Access Network (GERAN). Clause <NUM> discloses SRVCC from circuit switched UTRAN/GERAN to E-UTRAN for an emergency call.

The 3GPP document R2-<NUM> discloses a proposal from the company Ericsson wherein the company supports a key change in NR at handover when the RAN security context is moved or need to be refreshed. Key change shall be done when ordered by the network.

The present disclosure relates to methods and apparatus for flexible, security context management during AMF changes.

An aspect relates to a method implemented by a user equipment during an idle mode, the method comprising:.

Another aspect relates to a user equipment in a wireless communication network. The user equipment is configured to, during idle mode:.

Another aspect relates to a computer program comprising executable instructions that, when executed by a processing circuit in a user equipment in a wireless communication network, causes the user equipment to perform the method in the preceding paragraph.

Still another aspect relates to a non-transitory computer-readable storage medium which contains a computer program which comprises executable instructions that, when executed by a processing circuit in a user equipment in a wireless communication network causes the user equipment to perform the above method.

Referring now to the drawings, an exemplary embodiment of the disclosure will be described in the context of a <NUM> wireless communication network. Those skilled in the art will appreciate that the methods and apparatus herein described are not limited to use in <NUM> networks, but may also be used in wireless communication networks operating according to other standards.

<FIG> illustrates a wireless communication network <NUM> according to one exemplary embodiment. The wireless communication network <NUM> comprises a radio access network (RAN) <NUM> and a core network <NUM>. The RAN <NUM> comprises one or more base stations <NUM> providing radio access to UEs <NUM> operating within the wireless communication network <NUM>. The base stations <NUM> are also referred to as gNodeBs (gNBs). The core network <NUM> provides a connection between the RAN <NUM> and other packet data networks <NUM>.

In one exemplary embodiment, the core network <NUM> comprises an authentication server function (AUSF) <NUM>, access and mobility management function (AMF) <NUM>, session management function (SMF) <NUM>, policy control function (PCF) <NUM>, unified data management (UDM) function <NUM>, and user plane function (UPF) <NUM>. These components of the wireless communication network <NUM> comprise logical entities that reside in one or more core network nodes. The functions of the logical entities may be implemented by one or more processors, hardware, firmware, or a combination thereof. The functions may reside in a single core network node, or may be distributed among two or more core network nodes.

The AMF <NUM>, among other things, performs mobility management functions similar to the MME in LTE. The AMF and MME are referred to herein generically as mobility management functions. In the exemplary embodiment shown in <FIG>, the AMF <NUM> is the termination point for non-access stratum (NAS) security. The AMF <NUM> shares a key, denoted the core network key (KCN), with the UE <NUM> that is used to derive the NAS lower level protocol keys for integrity and confidentiality protection. The KCN is generally equivalent to the base key named KASME in the Evolved Packet System (EPS). The KCN key is generally equivalent to the KAMF key used in the <NUM> specifications. It is always the case that following authentication, a new KCN is taken into use. How the KCN key is established after authentication is not a material aspect of the present disclosure. The methods and apparatus described herein do not depend on the particular method used for computing KCN after authentication. That is, the security context handling methods work regardless of whether the KCN is derived from a higher level key or is established directly by the authentication procedure similar to the establishment of KASME in EPS.

Once a UE <NUM> is authenticated, the UE <NUM> may move between cells within the network. When a UE <NUM> moves between cells while in a connected mode, a handover is executed. When a UE <NUM> in idle mode moves between cells, a location update procedure may be executed. The AMF <NUM> keeps track of the location of the UE <NUM> in its domain. Typically, the core network <NUM> will have multiple AMFs <NUM>, each providing mobility management services in a respective domain. When a UE <NUM> moves between cells supervised by different AMFs <NUM>, the security context needs to be transferred from the source AMF <NUM> to the target AMF <NUM>.

In LTE systems, the security context is transferred unaltered from a source mobility management entity (MME) to the target MME during an inter-MME handover or location update. Following a AMF change, a NAS security mode command (SMC) procedure may be performed, which takes new NAS and access stratum (AS) keys into use. Generation of NAS and AS keys may be necessary, for example, when an algorithm change is needed at the NAS level. Generally, changing the algorithm used at the NAS protocol layer does not have any effect on the AS keys. However, changing the main NAS context key renders the current AS keys outdated.

One aspect of the disclosure is a mechanism for achieving backward security during AMF changes. Instead of passing the current NAS key to the target AMF <NUM>, the source AMF <NUM> derives a new NAS key, provides the new NAS key to the target AMF <NUM>, and sends a KCI to the UE <NUM>. The UE <NUM> can then derive the new NAS key from the old NAS key. In some embodiments, the source AMF <NUM> may provide a key generation parameter to the UE <NUM> to use in deriving the new NAS key. In other embodiments, the target AMF <NUM> may change one or more security algorithms.

<FIG> illustrates an exemplary procedure for transferring a security context during a handover where the AMF changes. At step <NUM>, the source base station <NUM> (e.g., source gNB) decides to initiate an N2-based handover due, for example, to no Xn connectivity to the target base station <NUM> (e.g. target gNB). The Xn interface is the <NUM> equivalent of the X2 interface in EPS. At step <NUM>, the source base station <NUM> sends a handover required message (or <NUM> equivalent of handover required message) to the source AMF <NUM>. This is the AMF <NUM> currently serving the UE <NUM>, with which it shares a full NAS security context based on a non-access stratum key referred to herin as the KCN key. The KCN key was established possibly following a previous authentication or AMF <NUM> change procedure. At step <NUM>, the source AMF <NUM> selects the target AMF <NUM> and decides to derive a new Kcn key in order to shield itself and all the previous sessions from the target AMF <NUM>. The decision to derive a new key may be based on an operator specific security policy.

As an example, a new KCN key could be taken into use when an AMF set changes. It is generaly assumed that a horizontal key derivation is not needed when an AMF set does not change. The current reasoning behind these two assumptions is that <NUM> security context is stored in the Unstructured Data Storage network function (UDSF) within an AMF set. So, when a UE is assigned a different AMF within the same AMF set, then horizontal derivation of KCN is not necessary. But when a UE is assigned a different AMF in a different AMF set, then the UDSF;is different and a horizontal derivation of KCN is necessary. These assumptions, however, may not hold true for all possible network deployments. First, he UDSF is an optional network function. Further, there is no reason to restrict the network architecture to deployments where there is a shared storage only within an AMF set. Some network deployments could have secure storage across multiple AMF sets. In this case, it is not necessary to mandate horizontal derivation of KCN when the AMF set changes. Similarly, some network deployments could use multiple secure storage within a single AMF set. In this case, horizontal key derivation may be desirable even when the UE <NUM> does not change AMF sets. Therefore, decision to perform horizontal derivation of KCN when changing between AMF should be done according to network policy, rather than mandating/restricting based on AMF set. For example, the network operator may have a policy that a new KCN is required when the UE <NUM> changes from a source AMF <NUM> to a target AMF <NUM> that do not share the same secure storage.

Returning to <FIG>, the source AMF <NUM>, at step <NUM>, sends a forward relocation request message (or <NUM> equivalent) including the new KCN key along with any relevant security parameters, such as the UE capabilities. The target AMF <NUM> uses this KCN key to set up a new security context and derive a new AS key. At step <NUM>, the target AMF <NUM> sends a handover request (or <NUM> equivalent) to the target base station <NUM>. The handover request includes the new AS key and all relevant security parameters, such as the UE capabilities. This establishes the UE <NUM> security context at the target base station <NUM>. At step <NUM>, the target base station <NUM> acknowledges the handover request. Responsive to the acknowledgement, the target AMF <NUM> sends, at step <NUM>, a forward relocation response message (or <NUM> equivalent) including a transparent container to the source AMF <NUM>. This container is forwarded all the way down to the UE <NUM> in steps <NUM> and <NUM>.

At steps <NUM> and <NUM>, the source AMF <NUM> sends a handover command message to the UE <NUM> via the source base station <NUM>, which forwards the handover command to the UE <NUM>. The handover command includes the relevant information from the forward relocation response message and a KCI indicating that a new KCN has been derived. The KCI may comprise an explicit key change indicator flag set to a value indicating that the KCN key has been changed. Responsive to the KCI, the UE <NUM> establishes a new security context and derives a new KCN. The UE <NUM> uses the new KCN key to derive a new AS key for communicating with the target base station <NUM>.

<FIG> illustrates an exemplary procedure for transferring a security context when a UE <NUM> in idle mode changes AMFs <NUM>. In EPS, location update during idle mode is indicated by the UE <NUM> in a Tracking Area Update (TAU) request. In <NUM>, it is expected that the UE <NUM> will use a registration request of type "mobility registration" as specified in TS <NUM>, § <NUM>.

At step <NUM>, the UE <NUM> sends a registration request (Registration type = mobility registration, other parameters) to the new AMF <NUM> (i.e. the target AMF). Those skilled in the art will appreciate that other messages may be sent to initiate a location update. The registration request message includes all the necessary information to enable the new AMF <NUM> to identify the old AMF <NUM> (i.e. the source AMF), which is currently holding the UE <NUM> security context. At step <NUM>, the new AMF <NUM> sends, responsive to the registration request message, a context request message to the old AMF <NUM> to request the security context for the UE <NUM>. At step <NUM>, old AMF <NUM> decides to derive a new KCN key in order to shield itself and all the previous sessions from the target AMF <NUM>. The decision may be based on an operator specific security policy.

At step <NUM>, the old AMF <NUM> sends a context request response message to the new AMF <NUM>. The context request response message contains the necessary UE <NUM> security context information including the new KCN key. The context request response message further includes a KCI indicating that the NAS key, KCN, has been changed. The old KCN key is not sent to the new AMF <NUM>. The new AMF <NUM> uses the new KCN key to establish a new security context and activates the new security context by performing a NAS SMC procedure or similar procedure with the UE <NUM> as specified in TS <NUM>, § <NUM>. At step <NUM>, the UE <NUM> is informed of a key change via a KCI in the first downlink message of the NAS SMC procedure, or other message sent during the NAS SMC procedure.

The NAS security context based on the KCN key is shared between the UE <NUM> and the AMF <NUM> currently serving it. The security context includes security parameters similar to those in LTE systems, such as the NAS counters, key set identifier, etc. In one exemplary embodiment, a horizontal key derivation mechanism is used to generate a new KCN key during AMF <NUM> change. The derivation of the new KCN could be solely based on the previous KCN. From a security perspective, there is no benefit from an additional input in the key derivation step.

<FIG> illustrates a first key derivation procedure. In this embodiment, it is assumed that the key derivation function (KDF) derives the new KCN key based solely on the old KCN key. This key chaining from AMF <NUM> to AMF <NUM> may continue on until a new authentication is performed. It may be left to the operator's policy how to configure the AMF <NUM> in respect to which security mechanism is selected during an AMF <NUM> change. For example, depending on an operator's security requirements, the operator can decide whether to perform re-authentication at the target AMF <NUM>, or whether a key change is needed at the source AMF <NUM>.

<FIG> illustrates another key derivation procedure. This embodiment may be useful in scenarios where an AMF <NUM> needs to prepare keys in advance for more than one potential target AMF <NUM>. In this case, an additional key derivation parameter (KDP) is needed for cryptographic separation, so that different KCN keys are prepared for different potential target AMFs <NUM>. Depending on the parameter type, the UE <NUM> might need to be provided with the chosen KDP in addition to the KCI. In some embodiments, the KDP may also serve as an implicit KCI so that a separate KCI is not required. For example, where the KDP comprises a nonce generated by the source AMF <NUM>, the nonce needs to be provided to the UE <NUM>. Other potential KDPs include a timestamp, a version number, and a freshness parameter. During a handover in connected mode, the KDP could be sent from the source AMF <NUM> to the UE <NUM> via the source base station <NUM> in a handover command. Alternatively, the KDP may be sent to the UE <NUM> via the target AMF <NUM> in a transparent NAS container. During a registration or location update procedure, the KDP could be sent from the target AMF <NUM> in a NAS SMC. However, in scenarios where the KDP is otherwise available to the UE <NUM>, such as an AMF public identifier-like parameter, it may not be necessary to provide the UE <NUM> with the KDP parameter. More generally, any static information, such as a static network configuration parameter or static UE configuration parameter, known to the UE <NUM> and Source AMF <NUM> may be used as a KDP.

<FIG> illustrates a handover procedure where a KDP is used to derive the new KCN key. This procedure is generally the same as the procedure shown in <FIG>. For the sake of brevity, steps that are unchanged are not described. At step <NUM>, the source AMF <NUM> selects the target AMF <NUM> and decides to derive a new KCN key in order to shield itself and all the previous sessions from the target AMF <NUM>. In this embodiment, the source AMF <NUM> generates a KDP (e.g., version number) and uses the KDP to derive the new KCN key. At step <NUM>, the source AMF <NUM> sends a forward relocation request message (or <NUM> equivalent) including the new KCN key along with any relevant security parameters, such as the UE capabilities. The target AMF <NUM> uses this KCN key to set up a new security context and derive a new AS key. The source AMF <NUM> does not provde the KDP to the new AMF <NUM>. Instead, at step <NUM>, the source AMF <NUM> sends a handover command to the source base station <NUM>, wherein the handover command includes the KDP in addtion to or in place of the KCI. As noted above, the KDP may serve as an implicit KCl. Responsive to the KCI and/or KDP, the UE <NUM> establishes a new security context and derives a new KCN using the KDP. The UE <NUM> may use the new KCN key to derive a new AS key for communicaitng with the target base station <NUM>.

In LTE systems, a NAS algorithm change at the target AMF <NUM> can only take effect through a NAS SMC procedure. Since the UE <NUM> capabilities are sent with other UE <NUM> context information to the target AMF <NUM>, it is possible for the target AMF <NUM> to indicate which new NAS algorithms have been selected. <FIG> illustrates an exemplary handover procedure where the target AMF <NUM> selects one or more new NAS security algorithms (e.g., cryptographic algorithms). Steps <NUM> - <NUM> are the same as described in <FIG>. At step <NUM>, the target AMF <NUM> selects one or more new NAS security algorithms. Steps <NUM> and <NUM> are the same as steps <NUM> and <NUM> in <FIG>. At step <NUM>, the target AMF <NUM> includes an indication of the new security algorithms in the transparent container to the source information element of the forward relocation response message sent to the source AMF <NUM>. This container is forwarded all the way down to the UE <NUM> in steps <NUM> and <NUM>. The security algorithm indication may be included with the KCI in the handover command, or in a separate message. As a consequence, the UE <NUM> has all the necessary parameters to activate the NAS security context with the target AMF <NUM> without the need of a NAS SMC procedure. This mechanism works regardless how the KCN key is derived.

<FIG> illustrates an exemplary procedure for transferring a security context when a UE <NUM> in idle mode changes AMFs <NUM>. This procedure is similar to the procedure shown in <FIG>. In EPS, location update during idle mode is indicated by the UE <NUM> in a Tracking Area Update (TAU) request. In <NUM>, it is expected that the UE <NUM> will use a registration request of type "mobility registration" as specified in TS <NUM>, § <NUM>.

At step <NUM>, the UE <NUM> sends a registration request (Registration type = mobility registration, other parameters) to the new AMF <NUM> (i.e. target AMF). Those skilled in the art will appreciate that other messages may be sent to initiate a location update. The registration request message includes all the necessary information to enable the new AMF <NUM> to identify the old AMF <NUM> (i.e. source AMF), which is currently holding the UE <NUM> security context. At step <NUM>, the new AMF <NUM> sends, responsive to the registration request message, a context request message to the old AMF <NUM> to request the security context for the UE <NUM>. At step <NUM>, old AMF <NUM> decides to derive a new KCN key in order to shield itself and all the previous sessions from the target AMF <NUM>. The decision may be based on an operator specific security policy.

In one embodiment denoted Alternative <NUM>, the old AMF <NUM> sends, at step 4A, a context request response message to the new AMF <NUM>. The context request response message contains the necessary UE <NUM> security context information including the new KCN key. The context request response message further includes a KCI indicating that the NAS key, KCN, has been changed and a KDP used to derive the new KCN key. The old KCN key is not sent to the new AMF <NUM>. The new AMF <NUM> uses the new KCN key to establish a new security context and activates the new security context by performing a NAS SMC procedure or similar procedure with the UE <NUM> as specified in TS <NUM>, § <NUM>. At step 5A, the KCI and KDP (e.g. a freshness parameter or nonce) is sent to the UE <NUM> in the first downlink message of the NAS SMC procedure, or other downlink message in the NAS SMC procedure. The KCI indicates to the UE <NUM> that the KCN key has been changed. The KDP is a security parameter that is used by the UE <NUM> to derive the new KCN key. In this embodiment, the KCI and KDP are separate parameters.

In another embodiment denoted Alternative <NUM>, the old AMF <NUM> sends, at step 4B, a context request response message to the new AMF <NUM>. The context request response message contains the necessary UE <NUM> security context information including the new KCN key. The context request response message further includes a KDP implicitly indicating that the NAS key, KCN, has been changed. The old KCN key is not sent to the new AMF <NUM>. The new AMF <NUM> uses the new KCN key to establish a new security context and activates the new security context by performing a NAS SMC or similar procedure with the UE <NUM> as specified in TS <NUM>, § <NUM>. At step 5B, the new AMF <NUM> sends the KDP (e.g. a freshness parameter or nonce) to the UE <NUM> in the first downlink message of the NAS SMC procedure, or some other downlink message in the NAS SMC procedure. The KDP functions as a key change indication to indicate to the UE <NUM> that the NAS key has been changed. The UE <NUM> uses the KDP and its old KCN key to derive the new KCN key.

<FIG> illustrates an exemplary method <NUM> implemented during a handover by a source base station <NUM> in an access network of a wireless communication network <NUM>. The source base station <NUM> sends a first handover message to a source AMF <NUM> in a core network <NUM> of the wireless communication network <NUM> to initiate a handover of a UE <NUM> (block <NUM>). Subsequently, the source base station <NUM> receives, responsive to the first handover message, a second handover message from the source AMF <NUM> (block <NUM>). The second handover message includes a KCI indicating that a non-access stratum key (e.g. KCN) has been changed. The source base station <NUM> forwards the second handover message with the KCl to the UE <NUM> (block <NUM>).

In some embodiments of the method <NUM>, the KCI comprises a key change indicator flag set to a value indicating that the non-access stratum key has been changed. In other embodiments, the KCI comprises a security parameter implicitly indicating that the non-access stratum key has been changed. The security parameter comprises one of a nonce, timestamp, freshness parameter and version number.

Some embodiments of the method <NUM> further comprise receiving, from the source AMF <NUM>, a KDP needed by the UE <NUM> to generate a new non-access stratum key, and forwarding the KDP to the UE <NUM>. In some examples, the KDP is received with the KCI in the second handover message. The KDP comprises, for example, one of a nonce, timestamp, freshness parameter and version number. In some embodiments, the key derivation serves as an implicit KCI.

Some embodiments of the method <NUM> further comprise receiving, from the source AMF <NUM>, a security algorithm parameter indicating at least one security algorithm to be used by the UE <NUM>, and forwarding the security algorithm parameter to the UE <NUM>. In one example, the security algorithm parameter is received with the KCI in the second handover message.

In one embodiment of the method <NUM>, the first handover message comprises a handover required message indicating a need for a handover of the UE <NUM>.

In one embodiment of the method <NUM>, the second handover message comprises a handover command including a KCI.

In one embodiment of the method <NUM>, the non-access stratum key comprises a core network key (KCN).

<FIG> is an exemplary base station <NUM> configured to perform the method <NUM> shown in <FIG>. The base station <NUM> comprises a sending unit <NUM>, a receiving unit <NUM> and a forwarding unit <NUM>. The sending unit <NUM> is configured to send a first handover message to a source AMF <NUM> in a core network <NUM> of the wireless communication network <NUM> to initiate a handover of a UE <NUM>. The receiving unit <NUM> is configured to receive, responsive to the first handover message, a second handover message from the source AMF <NUM>. The forwarding unit <NUM> is configured to forward the second handover message with the KCI to the UE <NUM>. The KCI indicates a change of the non-access stratum key (e.g. KCN). The sending unit <NUM>, receiving unit <NUM> and forwarding unit <NUM> may comprise hardware circuits, microprocessors, and/or software configured to perform the method shown in <FIG>. In some embodiments, the sending unit <NUM>, receiving unit <NUM> and forwarding unit <NUM> are implemented by a single microprocessor. In other embodiments, the sending unit <NUM>, receiving unit <NUM> and forwarding unit <NUM> may be implemented by two or more microprocessors.

<FIG> illustrates an exemplary method <NUM> implemented during a handover by a source AMF <NUM> in a core network <NUM> of a wireless communication network <NUM>. The source AMF <NUM> receives, from the source base station <NUM>, a first handover message indicating that a handover of the UE <NUM> is needed (block <NUM>). The source AMF generates a new non-access stratum key (e.g. KCN) (block <NUM>), and sends the new non-access stratum key to a target AMF <NUM> in the core network <NUM> of the wireless communication network <NUM> (block <NUM>). The source AMF <NUM> also sends a KCI to the UE <NUM> in a second handover message (block <NUM>). The KCI indicates a change of the non-access stratum key.

In some embodiments of the method <NUM>, generating the new non-access stratum key comprises generating the new non-access stratum key from a previous non-access stratum key. In other embodiments, generating the new non-access stratum key comprises generating the new non-access stratum key from a previous non-access stratum key and the KDP. In some embodiments, the source AMF sends the KDP to the UE <NUM> along with the KCI in the second handover message.

Some embodiments of the method <NUM> further comprise selecting the target AMF <NUM>, and generating the new non-access stratum key depending on the selection of the target AMF <NUM>.

Some embodiments of the method <NUM> further comprise generating two or more non-access stratum keys, each for different target AMFs <NUM>. In one example, the two or more non-access stratum keys are generated using different KDPs.

Some embodiments of the method <NUM> further comprise sending one or more security parameters to the target AMF <NUM>. In one example, the one or more security parameters are transmitted to the target AMF <NUM> in the second handover message. In one example, the one or more security parameters include UE capability information.

Some embodiments of the method <NUM> further comprise receiving, from the target AMF <NUM>, a security algorithm parameter indicating at least one security algorithm, and forwarding the security algorithm parameter to the UE <NUM>. In another example, the security algorithm parameter is received from the target AMF <NUM> in a forward relocation response message.

In one embodiment of the method <NUM>, the second handover message comprises a handover command including the KCI.

In one embodiment of the method <NUM>, the new non-access stratum key is sent to the target AMF (<NUM>) in a forward relocation request message.

<FIG> is an exemplary source AMF <NUM> configured to perform the method <NUM> shown in <FIG>. The source AMF <NUM> comprises a receiving unit <NUM>, a key generating unit <NUM>, a first sending unit <NUM> and second sending unit <NUM>. The receiving unit <NUM> is configured to receive, from a source base station <NUM>, a first handover message indicating that a handover of the UE <NUM> is needed. The key generating unit <NUM> is configured to generate a new non-access stratum key (e.g. KCN) as herein described. The first sending unit <NUM> is configured to send the new non-access stratum key to a target AMF <NUM> in the core network <NUM> of the wireless communication network <NUM>. The second sending unit <NUM> is configured to send a KCI to the UE <NUM> in a second handover message. The KCI indicates a change of the non-access stratum key. The receiving unit <NUM>, a key generating unit <NUM>, first sending unit <NUM> and second sending unit <NUM> may comprise hardware circuits, microprocessors, and/or software configured to perform the method shown in <FIG>. In some embodiments, the receiving unit <NUM>, key generating unit <NUM>, first sending unit <NUM> and second sending unit <NUM> are implemented by a single microprocessor. In other embodiments, the receiving unit <NUM>, key generating unit <NUM>, first sending unit <NUM> and second sending unit <NUM> may be implemented by two or more microprocessors.

<FIG> illustrates an exemplary method <NUM> implemented during a handover by a target AMF <NUM> in a core network <NUM> of a wireless communication network <NUM>. The target AMF <NUM> receives, from the source AMF <NUM>, a new non-access stratum key (e.g. KCN) (block <NUM>). The target AMF establishes a new security context including a new access stratum key derived from the new non-access stratum key (block <NUM>), and sends the new access stratum key to a target base station <NUM> (block <NUM>).

Some embodiments of method <NUM> further comprise receiving one or more security parameters from the source mobility management function. In one example, the one or more security parameters include UE capability information. In one embodiment, the security parameters are received with the new non-access stratum key.

In some embodiments of method <NUM>, establishing the new security context comprises selecting one or more security algorithms. In one example, at least one of the security algorithms is selected based on the UE capability information.

Some embodiments of method <NUM> further comprise sending to the source mobility management function, a security algorithm parameter indicating at least one security algorithm for the new security context.

In some embodiments of method <NUM>, the new non-access stratum key is received from the source mobility management function in a forward relocation request message.

In some embodiments of method <NUM>, the new access stratum key is sent to the target base station in a handover request.

In some embodiments of method <NUM>, the security algorithm parameter is sent to the source mobility management function in a forward relocation response message.

In some embodiments of method <NUM>, the non-access strum key comprises a core network key (KCN).

<FIG> is an exemplary target AMF <NUM> configured to perform the method <NUM> shown in <FIG>. The target AMF <NUM> comprises a receiving unit <NUM>, a security unit <NUM> and a sending unit <NUM>. The receiving unit <NUM> is configured to receive, from a source AMF <NUM>, a new non-access stratum key (e.g. KCN). The security unit <NUM> is configured to establish a new security context including a new access stratum key derived from the new non-access stratum key. The sending unit <NUM> is configured to send the new access stratum key to a target base station <NUM>. The receiving unit <NUM>, security unit <NUM> and sending unit <NUM> may comprise hardware circuits, microprocessors, and/or software configured to perform the method shown in <FIG>. In some embodiments, the receiving unit <NUM>, security unit <NUM> and sending unit <NUM> are implemented by a single microprocessor. In other embodiments, the receiving unit <NUM>, security unit <NUM> and sending unit <NUM> may be implemented by two or more microprocessors.

<FIG> illustrates an exemplary method <NUM> implemented by a UE <NUM> in a wireless communication network <NUM> during a handover. The UE <NUM> receives a handover message including a KCI from a source base station <NUM> in the domain of a source AMF <NUM> of the wireless communication network <NUM> (block <NUM>). The KCI indicates to the UE <NUM> that a non-access stratum key (e.g. KCN) has been changed. The UE <NUM> performs a handover from the source base station <NUM> to a target base station <NUM> in a domain of a target AMF <NUM> (block <NUM>). The UE <NUM> establishes, responsive to the KCI, a new security context with the target AMF <NUM> (block <NUM>). The new security context includes a new non-access stratum key. The UE <NUM> may optionally communicate with the target AMF <NUM> using the new non-access stratum key (block <NUM>).

In some embodiments of the method <NUM>, the KCI comprises a key change indicator flag set to a value indicating that the non-access stratum key has been changed. In other embodiments, the KCI comprises a security parameter implicitly indicating that the non-access stratum key has been changed. The security parameter comprises a KDP used to generate the new non-access stratum key.

Some embodiments of the method <NUM> further comprise generating the new non-access stratum key using the KDP. In one example, the KDP comprises one of a nonce, timestamp, freshness parameter, version number and static information known to the UE <NUM> and the source AMF. In some embodiments, the KDP is received with the KCI in the second handover message. In some embodiments, the KDP serves as an implicit KCI.

Some embodiments of the method <NUM> further comprise generating a new access stratum key from the new non-access stratum key, and communicating with a target base station <NUM> using the new access stratum key.

Some embodiments of the method <NUM> further comprise receiving a security algorithm parameter from the source base station <NUM> identifying one or more security algorithms used in the new security context. In one example, the security algorithm parameter is received in the handover message along with the KCI.

In some embodiments of the method <NUM>, the handover message comprises a handover command.

In some embodiments of the method <NUM>, the non-access stratum key comprises a core network key (KCN).

<FIG> is an exemplary UE <NUM> configured to perform the method <NUM> shown in <FIG>. The UE <NUM> comprises a receiving unit <NUM>, a handover unit <NUM> and a security unit <NUM>. The receiving unit <NUM> is configured to receive a handover message including a KCI from a source base station <NUM> in the domain of a source AMF <NUM> of the wireless communication network <NUM>. The KCI indicates to the UE <NUM> that a non-access stratum key (e.g. KCN) has been changed. The handover unit <NUM> is configured to perform a handover from the source base station <NUM> to a target base station <NUM> in a domain of a target AMF <NUM>. The security unit <NUM> is configured to establish, responsive to the KCI, a new security context with the target AMF <NUM>. The UE <NUM> may also optionally include and a communication unit <NUM> configured to communicate with the target AMF <NUM> using the new non-access stratum key. The receiving unit <NUM>, handover unit <NUM>, security unit <NUM> and communication unit <NUM> may comprise hardware circuits, microprocessors, and/or software configured to perform the method shown in <FIG>. In some embodiments, the receiving unit <NUM>, handover unit <NUM>, security unit <NUM> and communication unit <NUM> are implemented by a single microprocessor. In other embodiments, the receiving unit <NUM>, handover unit <NUM>, security unit <NUM> and communication unit <NUM> may be implemented by two or more microprocessors.

<FIG> illustrates an exemplary method <NUM> implemented by a source AMF <NUM> in a core network <NUM> of the communication network <NUM> when a UE <NUM> in idle mode changes AMFs <NUM>. The source AMF <NUM> receives a request for a security context for the UE <NUM> from a target AMF <NUM> (block <NUM>). The source AMF <NUM> generates a new non-access stratum key (e.g. KCN) (block <NUM>), and sends, responsive to the request, the new non-access stratum key and a KCI to the target AMF <NUM> (block <NUM>). The KCI indicates a change of the non-access stratum key.

In some embodiments of the method <NUM>, generating a new non-access stratum key comprises generating the new non-access stratum key from the old non-access stratum key. In other embodiments, generating a KDP, and generating the new non-access stratum key from an old non-access stratum key and the KDP.

In some embodiments of the method <NUM>, the key change indication comprises a key change indicator flag set to a value indicating that the non-access stratum key has been changed. In other embodiments, the KCI comprises a security parameter implicitly indicating that the non-access stratum key has been changed. The security parameter may comprise, for example, a KDP used to generate the new non-access stratum key.

Some embodiments of the method <NUM> further comprise sending, responsive to the request, a KDP used to generate the new non-access stratum key. The KDP comprises one of a nonce, timestamp, freshness parameter and version number.

Some embodiments of the method <NUM> further comprise selecting the target AMF <NUM>, and generating a new non-access stratum key depending on the selection of the target AMF <NUM>.

In some embodiments of the method <NUM>, generating a new non-access stratum key comprises generating two or more non-access stratum keys, each for a different target AMF <NUM>. In one example, the two or more non-access stratum keys are generated using different KDPs.

Some embodiments of the method <NUM> further comprise sending one or more security parameters with the new non-access stratum key to the target AMF <NUM>. In one example, the one or more security parameters include UE capability information.

In some embodiments of the method <NUM>, the request for a security context is received from the target AMF <NUM> in a context request message.

In some embodiments of the method <NUM>, the new non-access stratum key is sent to the target AMF <NUM> in a context request response message.

<FIG> is an exemplary source AMF <NUM> configured to perform the method <NUM> shown in <FIG>. The source AMF <NUM> comprises a receiving unit <NUM>, a key generating unit <NUM> and a sending unit <NUM>. The receiving unit <NUM> is configured to receive a request for a security context for the UE <NUM> from a target AMF <NUM>. The key generating unit <NUM> is configured to generate a new non-access stratum key (e.g. KCN). The sending unit <NUM> is configured to send, responsive to the request, the new non-access stratum key and a KCI to the target AMF <NUM>. The receiving unit <NUM>, a key generating unit <NUM> and a sending unit <NUM> may comprise hardware circuits, microprocessors, and/or software configured to perform the method shown in <FIG>. In some embodiments, the receiving unit <NUM>, key generating unit <NUM> and sending unit <NUM> are implemented by a single microprocessor. In other embodiments, the receiving unit <NUM>, key generating unit <NUM> and sending unit <NUM> may be implemented by two or more microprocessors.

<FIG> illustrates an exemplary method <NUM> implemented by a target AMF <NUM> in a core network <NUM> of a wireless communication network <NUM> when a UE <NUM> in idle mode changes AMFs <NUM>. The target AMF <NUM> receives, from the UE <NUM>, a registration message or other control message indicating an AMF change (block <NUM>). The target AMF <NUM> requests a security context from a source AMF <NUM> in the wireless communication network (block <NUM>). Responsive to the request, the target AMF <NUM> receives a new non-access stratum key (e.g. KCN) and a KCI indicating the non-access stratum key has been changed (block <NUM>). The target AMF <NUM> sends the KCI to the UE <NUM> (block <NUM>) and optionally establishes a new security context for the UE <NUM> including the new non-access stratum key (block <NUM>).

Some embodiments of the method <NUM> further comprise establishing a new security context including the new non-access stratum key.

Some embodiments of the method <NUM> further comprise receiving one or more security parameters from the source AMF <NUM>. In an example, the one or more security parameters include UE capability information. In another example, the security parameters are received along with the KCI.

In some embodiments of the method <NUM>, the key change indication comprises a key change indicator flag set to a value indicating that the non-access stratum key has been changed. In other embodiments, the key change indication comprises a security parameter implicitly indicating that the non-access stratum key has been changed. The security parameter may comprise, for example, a KDP used to generate the new non-access stratum key.

Some embodiments of the method <NUM> further comprise receiving, responsive to the request, a KDP used to generate the new non-access stratum key. In one example KDP comprises one of a nonce, timestamp, freshness parameter and version number. In some embodiments, the target AMF <NUM> sends the KDP to the UE <NUM> along with the KCI in a NAS SMC message.

In some embodiments of the method <NUM>, establishing a new security context comprises, in part, selecting one or more security algorithms. In one example, at least one of the security algorithms is selected based on UE capability information.

Some embodiments of the method <NUM> further comprise sending the UE <NUM> a security algorithm parameter indicating at least one security algorithm for the new security context.

In some embodiments of the method <NUM>, the KCI is received from a source AMF <NUM> in a context request response message.

In some embodiments of the method <NUM>, the KCI is sent to the UE <NUM> in a security establishment message.

<FIG> is an exemplary target AMF <NUM> configured to perform the method <NUM> shown in <FIG>. The target AMF <NUM> comprises a first receiving unit <NUM>, a requesting unit <NUM>, a second receiving unit <NUM>, and a sending unit <NUM>. The first receiving unit <NUM> is configured to receive, from the UE <NUM>, a registration message or other control message indicating an AMF change. The requesting unit <NUM> is configured to request, responsive to the registration message, a security context from a source AMF <NUM> in the wireless communication network. The second receiving unit <NUM> is configured to receive, from the source AMF <NUM> responsive to the security context request, a new non-access stratum key and a KCI indicating that the non-access stratum key (e.g. KCN) has been changed. The sending unit <NUM> is configured to send the KCI to the UE <NUM>. The target AMF <NUM> may also optionally include a security unit <NUM> configured to establish a new security context for the UE <NUM> including the new non-access stratum key. The first receiving unit <NUM>, requesting unit <NUM>, second receiving unit <NUM>, sending unit <NUM> and security unit <NUM> may comprise hardware circuits, microprocessors, and/or software configured to perform the method shown in <FIG>. In some embodiments, the first receiving unit <NUM>, requesting unit <NUM>, second receiving unit <NUM>, sending unit <NUM> and security unit <NUM> are implemented by a single microprocessor. In other embodiments, the first receiving unit <NUM>, requesting unit <NUM>, second receiving unit <NUM>, sending unit <NUM> and security unit <NUM> may be implemented by two or more microprocessors.

<FIG> illustrates an exemplary method <NUM> implemented by an idle mode UE <NUM> in a wireless communication network <NUM> when the UE <NUM> changes AMFs <NUM>. The UE <NUM> sends a registration message or other control message to a target AMF <NUM> in the wireless communication network (block <NUM>). The UE <NUM> receives, responsive to the registration message or other control message, a KCI indicating that a non-access stratum key (e.g. KCN) has been changed (block <NUM>). Responsive to the KCl, the UE <NUM> generates a new non-access stratum key (block <NUM>). After generating the new non-access stratum key, the UE <NUM> may optionally establish a new security context with the target AMF <NUM> (block <NUM>), where the new security context includes the new non-access stratum key and thereafter communicate with the target AMF <NUM> using the new non-access stratum key (block <NUM>).

Some embodiments of the method <NUM> further comprise establishing, a new security context with the target AMF <NUM>, the new security context including the new non-access stratum key, and communicating with the target AMF <NUM> using the new non-access stratum key.

In some embodiments of the method <NUM>, the KCI comprises a key change indicator flag set to a value indicating that the non-access stratum key has been changed. In other embodiments, the KCI comprises a security parameter implicitly indicating that the non-access stratum key has been changed. In one example, the security parameter comprises one of a nonce, timestamp, freshness parameter and version number.

Some embodiments of the method <NUM> further comprise receiving a KDP from the target AMF <NUM>, and generating the new non-access stratum key using the KDP. In an example, the KDP comprises one of a nonce, timestamp, freshness parameter and version number. In another example, the KDP is received with the KCI. In some embodiments, the KDP serves as an implicit KCI.

In some embodiments of the method <NUM>, generating the new non-access stratum key comprises generating the new non-access stratum key from the previous non-access stratum key. In other embodiments of the method <NUM>, generating the new non-access stratum key comprises generating the new non-access stratum key from the previous non-access stratum key and a KDP. The various embodiments, the KDP comprises at least one of a nonce, timestamp, freshness parameter and version number. In other embodiments, the KDP comprises static information that is known to the UE <NUM> and the source AMF <NUM>.

Some embodiments of the method <NUM> further comprise receiving a security algorithm parameter from the target AMF <NUM> identifying one or more security algorithms used in the new security context. In one example, the security algorithm parameter is received with the KCI.

In some embodiments of the method <NUM>, the new non-access stratum key is received in a security establishment message.

<FIG> is an exemplary UE <NUM> configured to perform the method <NUM> shown in <FIG>. The UE <NUM> comprises a sending unit <NUM>, a receiving unit <NUM> and a key generating unit <NUM>. The sending unit <NUM> is configured to send a registration message or other control message to a target AMF <NUM> in the wireless communication network. The receiving unit <NUM> is configured to receive, responsive to the registration message or other control message, a KCI indicating that a non-access stratum key has been changed. The key generating unit <NUM> is configured to generate, responsive to the KCI, a new non-access stratum key. The UE <NUM> may also optionally include security unit <NUM> configured to establish a new security context with the target AMF <NUM>, and a communication unit <NUM> configured to communicate with the target AMF <NUM> using the new non-access stratum key. The sending unit <NUM>, receiving unit <NUM>, key generating unit <NUM>, security unit <NUM> and communication unit <NUM> may comprise hardware circuits, microprocessors, and/or software configured to perform the method shown in <FIG>. In some embodiments, the sending unit <NUM>, receiving unit <NUM>, key generating unit <NUM>, security unit <NUM> and communication unit <NUM> are implemented by a single microprocessor. In other embodiments, the sending unit <NUM>, receiving unit <NUM>, key generating unit <NUM>, security unit <NUM> and communication unit <NUM> may be implemented by two or more microprocessors.

<FIG> illustrates the main functional components of base station <NUM> configured to implement the security context handling methods as herein described. The base station <NUM> comprises a processing circuit <NUM>, a memory <NUM>, and an interface circuit <NUM>.

The interface circuit <NUM> includes a radio frequency (RF) interface circuit <NUM> coupled to one or more antennas <NUM>. The RF interface circuit <NUM> comprises the radio frequency (RF) components needed for communicating with the UEs <NUM> over a wireless communication channel. Typically, the RF components include a transmitter and receiver adapted for communications according to the <NUM> standards or other Radio Access Technology (RAT). The interface circuit <NUM> further includes a network interface circuit <NUM> for communicating with core network nodes in the wireless communication network <NUM>.

The processing circuit <NUM> processes the signals transmitted to or received by the base station <NUM>. Such processing includes coding and modulation of transmitted signals, and the demodulation and decoding of received signals. The processing circuit <NUM> may comprise one or more microprocessors, hardware, firmware, or a combination thereof. The processing circuit <NUM> includes a mobility unit <NUM> for performing handover-related functions. The mobility unit <NUM> comprises the processing circuitry dedicated to mobility-related functions. The mobility unit <NUM> is configured to perform the methods and procedures as herein described, including the methods shown in <FIG>, <FIG>, <FIG>, and <FIG>.

Memory <NUM> comprises both volatile and non-volatile memory for storing computer program code and data needed by the processing circuit <NUM> for operation. Memory <NUM> may comprise any tangible, non-transitory computer-readable storage medium for storing data including electronic, magnetic, optical, electromagnetic, or semiconductor data storage. Memory <NUM> stores a computer program <NUM> comprising executable instructions that configure the processing circuit <NUM> to implement the methods and procedures described herein including method <NUM> according to <FIG>, <FIG>, <FIG>, and <FIG>. In general, computer program instructions and configuration information are stored in a non-volatile memory, such as a read only memory (ROM), erasable programmable read only memory (EPROM) or flash memory. Temporary data generated during operation may be stored in a volatile memory, such as a random access memory (RAM). In some embodiments, computer program <NUM> for configuring the processing circuit <NUM> as herein described may be stored in a removable memory, such as a portable compact disc, portable digital video disc, or other removable media. The computer program <NUM> may also be embodied in a carrier such as an electronic signal, optical signal, radio signal, or computer readable storage medium.

<FIG> illustrates the main functional components of a core network node <NUM> in the wireless communication network <NUM> configured to implement the security context handling procedure as herein described. The core network node <NUM> may be used to implement core network functions, such as the source AMF <NUM> and target AMF <NUM> as herein described. Those skilled in the art will appreciate that a core network function, such as the AMF <NUM>, may be implemented by a single core network node, or may be distributed among two or more core network nodes.

The core network node <NUM> comprises a processing circuit <NUM>, a memory <NUM>, and an interface circuit <NUM>. The interface circuit <NUM> includes a network interface circuit <NUM> to enable communication with other core network nodes and with base stations <NUM> in the RAN.

The processing circuit <NUM> controls the operation of the core network node <NUM>. The processing circuit <NUM> may comprise one or more microprocessors, hardware, firmware, or a combination thereof. The processing circuit <NUM> may include a NAS security unit <NUM> to handle NAS-related security functions and a mobility management unit <NUM> to handle mobility management functions. Generally, the NAS security unit <NUM> is responsible for deriving security keys, establishing a security context, and other related security functions. The mobility management unit <NUM> is responsible for handling mobility management functions and related signaling. As described previously, the NAS security unit <NUM> may provide the mobility management unit <NUM> with information, such as NAS keys, KDPs, and other security parameters to be sent to the UE <NUM>. In some embodiments, the NAS security unit <NUM> and the mobility management unit <NUM> may reside in the same core network node. In other embodiments, they may reside in different core network nodes. In one exemplary embodiment, the NAS security unit <NUM> and the mobility management unit <NUM> are configured to perform the methods and procedures as herein described, including the methods shown in <FIG>, <FIG>, <FIG>, <FIG>, <FIG>, <FIG>, and <FIG>.

Memory <NUM> comprises both volatile and non-volatile memory for storing computer program code and data needed by the processing circuit <NUM> for operation. Memory <NUM> may comprise any tangible, non-transitory computer-readable storage medium for storing data including electronic, magnetic, optical, electromagnetic, or semiconductor data storage. Memory <NUM> stores a computer program <NUM> comprising executable instructions that configure the processing circuit <NUM> to implement the methods and procedures described herein including methods according to <FIG>, <FIG>, <FIG>, <FIG>, <FIG>, <FIG>, and <FIG>. In general, computer program instructions and configuration information are stored in a non-volatile memory, such as a read only memory (ROM), erasable programmable read only memory (EPROM) or flash memory. Temporary data generated during operation may be stored in a volatile memory, such as a random access memory (RAM). In some embodiments, a computer program <NUM> for configuring the processing circuit <NUM> as herein described may be stored in a removable memory, such as a portable compact disc, portable digital video disc, or other removable media. The computer program <NUM> may also be embodied in a carrier such as an electronic signal, optical signal, radio signal, or computer readable storage medium.

<FIG> illustrates the main functional components of UE <NUM> configured to implement the security context handling methods as herein described. The UE <NUM> comprises a processing circuit <NUM>, a memory <NUM>, and an interface circuit <NUM>.

The interface circuit <NUM> includes a radio frequency (RF) interface circuit <NUM> coupled to one or more antennas <NUM>. The RF interface circuit <NUM> comprises the radio frequency (RF) components needed for communicating with the UEs <NUM> over a wireless communication channel. Typically, the RF components include a transmitter and receiver adapted for communications according to the <NUM> standards or other Radio Access Technology (RAT).

The processing circuit <NUM> processes the signals transmitted to or received by the UE <NUM>. Such processing includes coding and modulation of transmitted signals, and the demodulation and decoding of received signals. The processing circuit <NUM> may comprise one or more microprocessors, hardware, firmware, or a combination thereof. The processing circuit <NUM> may include a NAS security unit <NUM> to handle NAS-related security functions and a mobility management unit <NUM> to handle mobility management functions. Generally, the NAS security unit <NUM> is responsible for deriving security keys, establishing a security context, and other security functions as herein described. The mobility management unit <NUM> is responsible for handling mobility management functions and related signaling. In one exemplary embodiment, the NAS security unit <NUM> and the mobility management unit <NUM> are configured to perform the methods and procedures as herein described, including the methods shown in <FIG>, <FIG>, <FIG>, <FIG> and <FIG>.

Claim 1:
A method (<NUM>) implemented by a user equipment (<NUM>, <NUM>, <NUM>) during an idle mode, the method comprising:
sending (<NUM>) a registration message to a target Access and Mobility Management Function, AMF (<NUM>), in a wireless communication network (<NUM>);
receiving (<NUM>) from the target AMF(<NUM>), responsive to the sent registration message, a freshness parameter and a key change indication comprising a key change indicator flag which has a value indicating that a non-access stratum key has been changed by a source AMF; and
generating (<NUM>), responsive to the key change indication, a new non-access stratum key using a non-access stratum key and the freshness parameter.