Method and arrangement in a telecommunication system

Abstract

A method in a User Equipment (UE) of an Evolved Packet System (EPS) establishes a security key (K_eNB) for protecting Radio Resource Control/User Plane (RRC/UP) traffic exchanged with a serving eNodeB. The method comprises sending a Non-Access Stratum (NAS) Service Request to a Mobility Management Entity (MME), the request indicating a NAS uplink sequence number (NAS_U_SEQ). The method further comprises receiving an indication of the NAS_U_SEQ of the NAS Service Request sent to the MME, back from the MME via the eNodeB. The method further comprises deriving the K_eNB from at least the received indication of the NAS_U_SEQ and from a stored Access Security Management Entity-key (K_ASME) shared with said MME.

Claims

1. A method in a base station for establishing a security key for protecting traffic between a User Equipment (UE) and the base station, the method comprising the following steps: receiving a derived security key from a mobility management entity, wherein the derived security key is received in an Initial Context Setup Request message, and wherein the derived security key is derived based on a Non-Access Stratum (NAS) uplink sequence number and Access Security Management Entity-key; and sending, based on the received derived security key, a radio bearer establishment and/or and a security configuration to the UE.

2. The method of claim 1, wherein sending the radio bearer establishment and a security configuration comprises sending the radio bearer establishment and a security configuration in a same message or in different messages.

3. The method of claim 1, further comprising sending an initial context setup complete message to the mobility management entity.

4. A base station for establishing a security key for protecting traffic between a User Equipment (UE) and the base station, the base station comprising: a receiver configured to receive a derived security key from a mobility management entity, wherein the derived security key is received in an Initial Context Setup Request message, and wherein the derived security key is derived based on a Non-Access Stratum (NAS) uplink sequence number and Access Security Management Entity-key; and a transmitter configured to send, based on the received derived security key, a radio bearer establishment and/or and a security configuration to the UE.

5. The base station of claim 4, wherein the transmitter is further configured to send the radio bearer establishment and the security configuration in a same message or in different messages.

6. The base station of claim 4, wherein the transmitter is further configured to send an initial context setup complete message to the mobility management entity.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention will now be described in more detail, and with reference to the accompanying drawings, in which:

(2) FIG. 1 is signalling diagram illustrating a conventional UE triggered Service Request in an EPS;

(3) FIG. 2 is a signalling diagram illustrating the first embodiment of this invention, according to which the UE remembers the NAS_U_SEQ sent to the MME in an NAS SERVICE REQUEST-message;

(4) FIG. 3 is a flow diagram illustrating the derivation of the K_eNB by the UE and the MME;

(5) FIG. 4 is a signalling diagram illustrating a second embodiment of this invention, in which the MME returns the received NAS_U_SEQ to the UE;

(6) FIG. 5 is a flow diagram illustrating the second embodiment depicted in the FIG. 4; and

(7) FIG. 6a illustrates schematically an MME (Mobility Management Entity), and the FIG. 6b illustrates schematically a UE, both provided with means for deriving the security key K_eNB.

DETAILED DESCRIPTION

(8) In the following description, specific details are set forth, such as a particular architecture and sequences of steps in order to provide a thorough understanding of the present invention. However, it is apparent to a person skilled in the art that the present invention may be practised in other embodiments that may depart from these specific details.

(9) Moreover, it is apparent that the described functions may be implemented using software functioning in conjunction with a programmed microprocessor or a general purpose computer, and/or using an application-specific integrated circuit. Where the invention is described in the form of a method, the invention may also be embodied in a computer program product, as well as in a system comprising a computer processor and a memory, wherein the memory is encoded with one or more programs that may perform the described functions.

(10) The concept or the invention is that the security key, K_eNB, is derived from the Access Security Management Entity-key, K_ASME, and from the uplink sequence counter, NAS_U_SEQ, of the NAS SERVICE REQUEST message sent from the UE to the MME, thereby triggering the establishment of the UP/RRC security context in the eNodeB.

(11) When the UE is in IDLE mode, a NAS security context exists and comprises e.g. the above described K NAS_enc, K NAS_int, NAS_U_SEQ and NAS_D_SEQ, and the NAS messages are integrity- and possibly confidentiality protected. The NAS security context thus also contains the security capabilities of the UE, in particular the encryption and integrity algorithms.

(12) The protection of the NAS messages is based on the NAS security keys, K_NAS_enc, K_NAS_int, and the uplink and downlink sequence counters, NAS_U_SEQ or NAS_D_SEQ, for the direction of the message. The full sequence counter is normally not transmitted with the NAS message, only some of the low order bits, and the full sequence number will be reconstructed at the receiving end from a local estimate of the high order bits and the received low order bits.

(13) The concept of the invention may be explained in the context of the signalling diagram for UE-triggered service requests, as depicted in the above-described FIG. 1:

(14) In S1 and S2 of the conventional signalling diagram in FIG. 1, a NAS SERVICE REQUEST, comprising an uplink sequence counter, NAS_U_SEQ, is forwarded from the UE to the MME, and the NAS SERVICE REQUEST-message is integrity protected based on said NAS_U_SEQ. The MME checks the integrity of the message and accepts it if it is not a replay, and this guarantees that the NAS_U_SEQ is fresh and has not been used before.

(15) Thereafter, according to this invention, the MME derives the K_eNB based at least on the received uplink sequence counter NAS_U_SEQ and on the K_ASME, using a conventional key derivation function, and this is not included in the conventional signalling diagram illustrated en FIG. 1. Consequently, the sequence counter may only be reset at the authentication. The MME will send the derived K_eNB down to the eNodeB in, or piggybacked to, the message of the signal S4, the Initial Context Setup Request (S1-AP).

(16) In signal S5, the eNodeB sends a Radio Bearer Establishment and a security configuration message (Security Mode Command) to the UE. These messages may be sent as two separate messages or combined in one message, as in FIG. 1, and the reception of these messages by the UE will implicitly be a confirmation of the UEs NAS SERVICE REQUEST, in signal S1. The Security Mode Command will determine e.g. when the protection should start and which algorithm to use.

(17) According to this invention, the UE derives the K_eNB based at least on the NAS_U_SEQ and the K_ASME, using a conventional key derivation function, upon reception of the message in signal S5, if not performed before. Thereafter, the eNodeB and the UE will establish the UP/RRC security contexts, and this is not illustrated in the conventional signalling diagram in FIG. 1.

(18) According to a first embodiment of this invention, the UE stores the uplink sequence counter, NAS_U_SEQ, included in the initial NAS SERVICE REQUEST in signal S1, and uses the stored NAS_U_SEQ for the derivation of the K_eNB.

(19) However, according to a second embodiment, the MME includes the uplink sequence counter, NAS_U_SEQ, or only low order bits indicating the NAS_U_SEQ, in the S1-AP set-up message, in signal S4, sent to the eNodeB, in which case this information is also forwarded to the UE from the eNodeB during the RRC/UP context establishment. In this case, the UE will be able to retrieve the indication of the NAS_U_SEQ from the eNodeB for the derivation of the K_eNB, and does not have to keep the NAS_U_SEQ of the NAS NAS SERVICE REQUEST-message sent to the MME in the signals S1 and S2.

(20) FIG. 2 illustrates the first embodiment of this invention, in which the UE keeps the NAS_U_SEQ of the initial NAS SERVICE REQUEST-message, in the signal S21, for the derivation of K_eNB in the signal S24. The MME will receive the NAS_U_SEQ from the UE in the signal S21, or only low order bits indicating the NAS_U_SEQ, and derive the K_eNB based on the NAS_U_SEQ and the K_ASME in S22. The MME forwards the derived K_eNB to the eNodeB in the signal S23.

(21) Thereafter, not illustrated in FIG. 2, the eNodeB and the UE will establish the UP/RRC security context using the K_eNB, the UP/RRC security contexts comprising the encryption key, K_eNB_UP_enc for protecting the UP-traffic, as well as the encryption key and the integrity protection key, K_eNB_RRC_enc and K_eNB_RRC_int, respectively, for protecting the RRC-traffic, thereby enabling a secure UP/RRC traffic, in signal S25.

(22) The derivation of the K_eNB is performed by a conventional key derivation function, e.g. by a Pseudo-Random Function; K_eNB=PRF(K_ASME, NAS_U_SEQ, . . . ).

(23) Further, as illustrated by the dots in the above-described PRF-function, the K_eNB-deriving function may have additional conventional input values, such as e.g. the eNodeB-identity.

(24) FIG. 3 is a flow diagram illustrating the method according to this invention, and in step 31, the UE 11 sends the initial NAS SERVICE REQUEST-message to the MME 13, the message indicating the NAS uplink sequence counter, NAS_U_SEQ, normally only by sending the low order bits of the counter. In step 32, the MME receives the NAS SERVICE REQUEST-message from the UE, obtaining the NAS_U_SEQ, and reconstructing the full sequence from the received low order bits. In step 33, the MME derives the security key, K_eNB, from at least the received NAS_U_SEQ and the K_ASME from the ASME (Access Security Mobility Entity), using a suitable key derivation function, e.g. a Pseudo-Random Function.

(25) Thereafter, the MME forwards the derived K_eNB to the eNodeB 12, in step 34, to be used by the eNodeB to establish the complete UP/RRC security context shared with the UE. In step 35, said UE will derive the same K_eNB from at least the stored K_ASME and from the NAS_U_SEQ of the initial NAS SERVICE REQUEST-message transmitted from the UE to the MME in step 31, and establish the UP/RRC security context from the derived K_eNB.

(26) In the first embodiment of this invention, the UE stores the NAS_U_SEQ transmitted to the MME in the initial NAS NAS SERVICE REQUEST-message, and uses the stored sequence number to derive the K_eNB.

(27) FIG. 4 is a signalling diagram illustrating a second embodiment of this invention, in which the UE does not have to store the NAS_U_SEQ. Instead, the MME will return an indication of the received NAS_U_SEQ back to the UE, via the eNodeB. In signal S41, corresponding to the signal S21 in FIG. 2, the UE 11 transmits an initial NAS SERVICE REQUEST to the MME 13, indicating an uplink sequence number, NAS_U_SEQ, and MME will receive the NAS_U_SEQ and derive the K_eNB based on at least the NAS_U_SEQ and the K_ASME, in S42. However, according to this second embodiment, the MME will include an indication of said received NAS_U_SEQ in signal S43 transmitted to the eNodeB 12 together with the derived K_eNB, and the eNodeB will forward the NAS_U_SEQ to the UE, in signal S44. Thereafter, the UE will derive the K_eNB from at least the K_ASME and from the NAS_U_SEQ returned by the MME, in signal S45. From the derived security key, K_eNB, the eNodeB and the UE will establish the UP/RRC security context, thereby enabling secure UP/RRC traffic, in signal S46.

(28) FIG. 5 is a flow diagram illustrating the above-described method according to a second embodiment of this invention, in which an indication of the NAS_U_SEQ is returned to the UE by the MME. In step 41, the UE 11 sends the initial NAS SERVICE REQUEST-message to the MME 13, the message indicating the NAS uplink sequence counter, NAS_U_SEQ, normally the low order bits. In step 52, the MME receives the NAS SERVICE REQUEST-message from the UE, thereby obtaining the NAS_U_SEQ, and, if necessary, reconstructing the full NAS_U_SEQ from the received low order bits. In step 53, the MME derives the security key, K_eNB, from at least the received NAS_U_SEQ and the K_ASME, using a suitable key derivation function.

(29) Thereafter, the MME includes an indication of the NAS uplink sequence counter, NAS_U_SEQ, in the message forwarding the derived K_eNB to the eNodeB 12, in step 54, and the eNodeB uses the received security key, K_eNB, for establishing a UP/RRC security context. The received NAS_U_SEQ is forwarded to the UE 11 by the eNodeB, in step 55, and in step 56, the UE derives the security key, K_eNB, from at least the K_ASME and from said received NAS_U_SEQ, in order to establish the UP/RRC security context shared with the eNodeB.

(30) The derivation of the K_eNB by the MME, in the step 53, and by the UE, in the step 56, is performed by a suitable conventional key derivation function, e.g. a Pseudo-Random Function; K_eNB=PRF(K_ASME, NAS_U_SEQ, . . . ). Normally, the key derivation function will have additional conventional input values, e.g. the eNodeB-identity.

(31) FIG. 6a illustrates an MME 13 (Mobility Management Entity) for an EPS, according to this invention, further arranged to establish a security key, K_eNB, for a security context for the protection of UP/RRC traffic between a UE and a serving eNodeB. The MME is provided with conventional communication means, not illustrated in the figure, for communicating with the nodes in the EPS, e.g. with the eNodeBs via an S1-MME interface. Further, in the MME of FIG. 1, an ASME (Access Security Management Entity) 61 is illustrated by hatched lines, since this functional entity of an EPS may be co-located with the MME.

(32) The means of the MME 13 illustrated in FIG. 6a for establishing the security key, K_eNB, comprises reception means 62 for receiving an NAS SERVICE REQUEST-message including an NAS_U-SEQ from a UE (via its serving eNodeB, key derivation means 63 for deriving a security key, K_eNB based on at least the received NAS_U-SEQ and a stored K_ASME, using a conventional key derivation function; and sending means 64 for sending the derived K_eNB to the eNodeB serving the UE.

(33) FIG. 6b illustrates a UE 11 (User Entity) according to this invention, the UE adapted for an EPS, and further arranged to establish a security key, K_eNB, for a security context for the protection of UP/RRC traffic exchanged with its serving eNodeB. The UE is provided with conventional communication means, not illustrated in the figure, for communicating with the nodes in the EPS via an LTE-Uu interface to its serving eNodeB.

(34) The means of the UE 11 illustrated in the FIG. 6b for establishing the security key, K_eNB, comprises sending means 66 for sending an NAS SERVICE REQUEST-message to the MME, via the serving eNodeB, the request indicating an uplink sequence number, NAS_U-SEQ, and the means for establishing a security key, K_eNB, comprises key derivation means 67 for deriving a security key, K_eNB based on at least the NAS_U-SEQ and a stored K_ASME, using a conventional key derivation function.

(35) The above-described means of the MME and the UE, as illustrated in the FIGS. 6a and 6b, implement the described functions using a suitable combination of software and hardware, e.g. a programmed microprocessor or an application-specific integrated circuit, as well as conventional radio transmitters and receivers.

(36) While the invention has been described with reference to specific exemplary embodiments, the description is in general only intended to illustrate the inventive concept and should not be taken as limiting the scope of the invention.