Protection of non-access stratum communication in a wireless communication network

1. A method performed by network equipment configured for use in a wireless communication network, wherein the network equipment implements an access and mobility function (AMF), the method comprising:

detecting one or more conditions under which non-access stratum (NAS) keys that protect NAS communication between the network equipment and a wireless device are to be refreshed, wherein the one or more conditions include one or more conditions that are detected before a value of a NAS COUNT for a NAS connection between the network equipment and the wireless device wraps around from a maximum value to an initial value, wherein the one or more conditions include a condition that the value of the NAS COUNT is within a certain threshold from the maximum value, wherein the NAS COUNT counts a number of NAS messages sent in a certain direction over the NAS connection;

responsive to detecting the one or more conditions, deriving, from a base key on which the NAS keys were derived and from the value of the NAS COUNT, a new base key on which fresh NAS keys are to be derived, wherein the new base key is derived using horizontal key derivation without running an authentication and key agreement (AKA)-based primary authentication procedure and without activating a native security context for the wireless device, wherein the base key is a K AMF key, and wherein the new base key is a new K AMF key;

activating the new base key;

deriving fresh NAS keys from the new base key; and

transmitting or receiving NAS communication that is protected with the fresh NAS keys.

2. The method of claim 1 , wherein said deriving comprises computing the new base key as an output of a key derivation function that takes a string and a key as inputs, wherein the base key is input to the key derivation function as said key, wherein a set of parameters concatenated together is input to the key derivation function as said string, wherein the set of parameters includes the value of the NAS COUNT.

3. The method of claim 2 , wherein the key derivation function is a hash-based message authentication code (HMAC) function that uses a Secure Hash Algorithm (SHA) to hash inputs to the HMAC function in the form of a string and a key.

4. The method of claim 1 , wherein said activating comprises performing a NAS security mode command (SMC) procedure between the network equipment and the wireless device to establish between the network equipment and the wireless device a new NAS security context that includes the new base key.

5. The method of claim 4 , wherein performing the NAS SMC procedure comprises transmitting a NAS SMC message to the wireless device indicating the value of the NAS COUNT.

6. The method of claim 1 , wherein deriving the new base key comprises deriving the new base key without running a primary authentication procedure and without activating a native security context.

7. Network equipment configured for use in a wireless communication network, wherein the network equipment implements an access and mobility function (AMF), the network equipment comprising:

communication circuitry;

and processing circuitry configured to:

detect one or more conditions under which non-access stratum (NAS) keys that protect NAS communication between the network equipment and a wireless device are to be refreshed,

wherein the one or more conditions include one or more conditions that are detected before a value of a NAS COUNT for a NAS connection between the network equipment and the wireless device wraps around from a maximum value to an initial value,

wherein the one or more conditions include a condition that the value of the NAS COUNT is within a certain threshold from the maximum value,

wherein the NAS COUNT counts a number of NAS messages sent in a certain direction over the NAS connection;

responsive to detecting the one or more conditions,

derive, from a base key on which the NAS keys were derived and from the value of the NAS COUNT, a new base key on which fresh NAS keys are to be derived,

wherein the new base key is derived using horizontal key derivation without running an authentication and key agreement (AKA)-based primary authentication procedure and without activating a native security context for the wireless device,

wherein the base key is a K AMF key, and wherein the new base key is a new KAMF key;

activate the new base key;

derive fresh NAS keys from the new base key; and

transmit or receive NAS communication that is protected with the fresh NAS keys.

8. The network equipment of claim 7 , wherein the processing circuitry is configured to derive the new base key by computing the new base key as an output of a key derivation function that takes a string and a key as inputs, wherein the base key is input to the key derivation function as said key, wherein a set of parameters concatenated together is input to the key derivation function as said string, wherein the set of parameters includes the value of the NAS COUNT.

9. The network equipment of claim 8 , wherein the key derivation function is a hash-based message authentication code (HMAC) function that uses a Secure Hash Algorithm (SHA) to hash inputs to the HMAC function in the form of a string and a key.

10. The network equipment of claim 7 , wherein the processing circuitry is configured to activate the new base key by performing a NAS security mode command (SMC) procedure between the network equipment and the wireless device to establish between the network equipment and the wireless device a new NAS security context that includes the new base key.

11. The network equipment of claim 10 , wherein the processing circuitry is configured to transmit a NAS SMC message to the wireless device indicating the value of the NAS COUNT.

12. The network equipment of claim 7 , wherein the processing circuitry is configured to derive the new base key by deriving the new base key without running a primary authentication procedure and without activating a native security context.

13. Anon-transitory computer readable medium on which is stored instructions that, when executed by at least one processor of network equipment, causes the network equipment to:

detect one or more conditions under which non-access stratum (NAS) keys that protect NAS communication between the network equipment and a wireless device are to be refreshed,

wherein the one or more conditions include one or more conditions that are detected before a value of a NAS COUNT for a NAS connection between the network equipment and the wireless device wraps around from a maximum value to an initial value,

wherein the one or more conditions include a condition that the value of the NAS COUNT is within a certain threshold from the maximum value,

wherein the NAS COUNT counts a number of NAS messages sent in a certain direction over the NAS connection;

responsive to detecting the one or more conditions,

derive, from a base key on which the NAS keys were derived and from the value of the NAS COUNT, a new base key on which fresh NAS keys are to be derived,

wherein the new base key is derived using horizontal key derivation without running an authentication and key agreement (AKA)-based primary authentication procedure and without activating a native security context for the wireless device,

wherein the base key is a K AMF key, and wherein the new base key is a new KAMF key;

activate the new base key;

derive fresh NAS keys from the new base key; and

transmit or receive NAS communication that is protected with the fresh NAS keys.