WO2008058841A2

WO2008058841A2 - Bootstrapping method

Info

Publication number: WO2008058841A2
Application number: PCT/EP2007/061442
Authority: WO
Inventors: Matthias Franz; Günther Horn; Wolf-Dietrich Moeller
Original assignee: Nokia Siemens Networks Gmbh & Co. Kg
Priority date: 2006-11-16
Filing date: 2007-10-24
Publication date: 2008-05-22
Also published as: DE102006054091A1; WO2008058841A3; DE102006054091B4

Abstract

Disclosed is a bootstrapping method for supplying a common temporary application-specific code (Ks_NAF) to a terminal (UE) and an application server (NAF). Said method comprises the following steps: a bootstrapping server (BSF) is selected from among several available bootstrapping servers on the basis of a first user ID received from the terminal (UE); the selected bootstrapping server (BSF) assigns a temporary second user ID (B-TID) to said terminal (UE) and forms a temporary basic code (Ks) once the terminal (UE) has been successfully authenticated; and the selected bootstrapping server (BSF) derives the temporary application-specific code (Ks_NAF) from the temporary basic code (Ks) and transmits the same to the application server (NAF) after receiving a request message which originates from the application server (NAF) and has the same temporary second user ID (B-TID).

Description

description

Bootstrapping procedure

The invention relates to a bootstrapping method for providing a common temporarily valid application-specific key for a terminal and an application server.

Figure 1 shows an example of a network architecture according to the prior art. A mobile terminal or a user terminal UE (user equipment) can establish a data connection via different access networks, which each have a plurality of base stations BS and a gateway GW in the mobile radio area, and, for example, via visited networks with a home network of the subscriber. The access networks can have different standardized access technologies. An access network may be, for example, a 3GPP access network. In a GBA (Generic Bootstrapping Architecture), the subscriber's home network has one

Bootstrapping Server BSF and a home subscriber system server HSS.

Figure 2 shows the structure of a GBA network according to the prior art. The bootstrapping server BSF is connected to a user terminal UE via a U _b interface. The user terminal is, for example, a mobile terminal such as a cell phone. The bootstrapping server BSF is connected via a Z _n interface to an application server NAF (Network Application Function) on which an application program runs. In addition, the bootstrapping server BSF is connected via a Z _n interface to the home subscriber system server HSS, in which a long-term key or a long-term valid key is stored. Furthermore, the bootstrapping server BSF can be connected via a D _z interface to a server for locating the associated home subscriber system server HSS. This server for localizing a home subscriber system server HSS for a Subscriber is also referred to as the Subscriber Locator Function (SLF) server. In the case of the GBA network architecture, a long-term security relationship or a long-term valid key that is known jointly to the user terminal UE and the home subscriber system server HSS is used to establish a short-term security relationship between the user terminal UE and the application server NAF build. To secure the data connection between the user terminal UE and the application server NAF a temporarily valid key is derived from the long-term valid key. The application server NAF can be located in the subscriber's home network, in a visited network or at another location (3 ^rd party server).

The bootstrapping server BSF is always located in the subscriber's home network and is the only component within the GBA network architecture that contacts the home subscriber system server HSS.

As soon as the bootstrapping server BSF receives a request from the user terminal UE via the U _b interface, it loads cryptographic intermediate keys or derived keys from the HSS server via the Z _h interface. These cryptographic intermediate keys are derived from the long-term valid common key or the long-term key which is stored in the HSS server and which is also known to the user terminal UE. Subsequently, the bootstrapping server BSF, in response to a request from the application server NAF, which receives it via the Z _n interface, derives an application-specific cryptographic key from the cryptographic intermediate key or base key and transmits these derived application-specific keys to the application server NAF. The user terminal UE also derives the same intermediate key or base key and the same application-specific short-term valid key as the bootstrapping server BSF. This is possible because the user terminal UE the long-term valid key is also known. After the transfer the application-specific and temporarily valid keys to the application server NAF both the application server and the user terminal UE have the same temporary valid application-specific key (Ks NAF) and can use this to secure a data connection via the U _a -

Use interface between the user terminal UE and the application server NAF. In a GBA network architecture, several home subscriber system servers HSS can be provided. However, each subscriber has an associated home subscriber system server HSS. In this case, it is possible to assign a subscriber another HSS server by transferring his subscription data without having to reconfigure the user terminal UE. In order for a subscriber or a user terminal UE to be able to find the correct home subscriber system server HSS, the bootstrapping

Server BSF via the D _z interface the SLF (Subscriber Longer Function) server using a long-term valid subscriber identity. In the event that the GBA network has only one home subscriber system server HSS, the request to the SLF server can be dispensed with.

In a conventional GBA network, the user terminal UE and the application server NAF contact the bootstrapping server BSF by deriving a DNS name of the bootstrapping server BSF on the basis of a subscriber identity or a temporarily valid user identifier B-TID to inform the network operator.

Conventional GBA networks have only one bootstrapping server BSF, with this bootstrapping server BSF or the bootstrapping server function being part of the home network. If the number of subscribers for a network provider or network provider increases, it may happen that the processing capacity of the bootstrapping server BSF is no longer sufficient. Now it is desirable for the network operator to continue to operate the previous bootstrapping server BSF and only by an additional bootstrapping server BSF, possibly from a manufacturer to be able to supplement. This is not possible with a traditional GBA network. A similar situation arises when, as is possible in a conventional GBA network, the bootstrapping server BSF is integrated with the HSS server and the storage space in the HSS server is no longer sufficient. Also in this case, it is not possible to extend the conventional prior art GBA network illustrated in Figure 2 by providing additional HSS or Home Subscriber System servers integrated with a bootstrapping server. An extension of the conventional GBA network is thus possible only with a very large technical effort.

It is therefore the object of the present invention to provide a bootstrapping method and a bootstrapping proxy server which allow to flexibly extend a GBA network by providing additional bootstrapping servers BSF.

This object is achieved by a bootstrapping method with the features specified in claim 1.

The invention provides a bootstrapping method for providing a common temporarily valid application-specific key (Ks_NAF) for a terminal (UE) and an application server (NAF) with the following steps:

- selecting a bootstrapping server (BSF) from a plurality of existing bootstrapping servers based on a first user identifier received by the terminal (UE);

- After successful authentication of the terminal (UE) by the selected bootstrapping server (BSF) a temporarily valid second user identifier (B-TID) is assigned to this terminal (UE) and a temporary valid base key (Ks) by the selected bootstrapping server (BSF) is formed; and wherein upon receipt of a request message originating from the application server (NAF) and having the same temporarily valid second user identifier (B-TID), the selected bootstrapping server (BSF) selects the temporary valid application-specific key (Ks_NAF) from the derived temporary base key (Ks) and transmits to the application server (NAF).

In a preferred embodiment of the bootstrapping method according to the invention, the first user identifier is formed by a permanently valid user identifier, if this is advantageous.

This first user identifier is preferably an IMPI (IP Multimedia Private Identity).

In one embodiment of the bootstrapping method according to the invention, the bootstrapping server BSF is selected by a B-SLF (Subscriber Locator Function) server.

In one embodiment of the bootstrapping method according to the invention, the messages are transmitted between the terminal and the selected bootstrapping server via a BSF proxy server.

In one embodiment of the bootstrapping method according to the invention, the messages between the application server NAF and the selected bootstrapping server BSF are also transmitted via a BSF proxy server.

In a preferred embodiment of the bootstrapping method according to the invention, the B-SLF server transmits to the BSF proxy server an address of the selected bootstrapping server.

In one embodiment of the bootstrapping method according to the invention, the BSF proxy server temporarily stores the address of the selected bootstrapping server. In one embodiment of the inventive bootstrapping method, the selected bootstrapping server BSF authenticates the terminal UE with the aid of a home-subscriber-system server HSS.

In this case, the home subscriber system server HSS transmits an authentication vector AV to the selected bootstrapping server BSF.

This transmitted authentication vector AV preferably has an authentication challenge, an authentication response, a cipher key and an integrity key.

In one embodiment of the inventive bootstrapping method, the selected bootstrapping server BSF forms the temporarily valid base key from the encryption key CK and the integrity key IK.

In one embodiment of the inventive bootstrapping method, the selected bootstrapping server generates the temporarily valid second user identifier B-TID.

In one embodiment of the bootstrapping method according to the invention, the temporarily valid second user identifier B-TID, together with a validity period of the formed base key Ks, is transmitted from the selected bootstrapping server BSF to the bootstrapping proxy server in a message containing the temporarily valid second user identifier B-TID and the validity period of the formed base key Ks caches and then forwards the message to the terminal UE.

In one embodiment of the bootstrapping method according to the invention, when a service is requested from the application server NAF, the terminal UE transmits the data forwarded by the bootstrapping proxy server to the terminal UE temporarily valid second user identifier B-TID to the application server NAF in a service request message.

In one embodiment of the bootstrapping method according to the invention, the application server NAF sends a

Service request message with the temporarily valid second user identifier B-TID to the bootstrapping proxy server, which forwards the service request message to the selected bootstrapping server BSF.

In one embodiment of the bootstrapping method according to the invention, the terminal UE forms the temporary valid base key.

In one embodiment of the bootstrapping method according to the invention, the terminal UE derives the temporarily valid application-specific key Ks NAF from the temporarily valid base key Ks.

In one embodiment of the bootstrapping method according to the invention, the selected bootstrapping server BSF generates the temporarily valid second user identifier B-TID and encodes therein from which bootstrapping server BSF the generated second user identifier B-TID originates.

The invention further provides a bootstrapping proxy server for a generic bootstrapping architecture (GBA) network, which forwards messages between a terminal UE and a bootstrapping server BSF selected on the basis of a permanent user identifier.

In one embodiment of the bootstrapping proxy server according to the invention, the bootstrapping proxy server stores a temporarily valid user identifier B-TID, which is assigned by a bootstrapping server BSF and assigned to the respective bootstrapping server BSF. In one embodiment of the bootstrapping proxy server according to the invention, the bootstrapping proxy server forwards messages between an application server NAF and the selected bootstrapping server BSF.

In the following, preferred embodiments of the bootstrapping method according to the invention and of the bootstrapping proxy server used in this case will be described with reference to the attached figures to explain features essential to the invention.

Show it:

Figure 1: a network according to the prior art;

Figure 2: the network architecture of a GBA network according to the prior art;

FIG. 3 is a signal diagram for explaining a possible embodiment of the bootstrapping system according to the invention;

process;

FIG. 4 shows a first embodiment of a GBA network extended according to the invention with a BSF proxy server according to the invention;

Figure 5: a GBA network architecture for the embodiment shown in Figure 4;

FIG. 6 shows a further embodiment of an extended network according to the invention with a BSF proxy server;

Figure 7: a GBA network architecture for the embodiment shown in Figure 6;

FIG. 8 shows a further embodiment of an extended network according to the invention with a BSF proxy server. The bootstrapping method according to the invention serves to provide a common, temporarily valid, application-specific key Ks_NAF for a terminal UE and an application server NAF. In the signal diagram shown in FIG. 3, two different examples are used

Bootstrapping server BSF-A and BSF-B provided. A BSF proxy server is provided in the bootstrapping method according to the invention for the transmission of messages between the terminal UE and a selected bootstrapping server BSF. Messages between the application server NAF and the selected bootstrapping server are also transmitted via the BSF proxy server. The selection of the bootstrapping server BSF is performed by a subscriber locator function server B-SLF. In the example shown in FIG. 3, the B-SLF server reports the first bootstrapping server BSF-A in order to localize the bootstrapping server BSF.

In addition, in the example illustrated in FIG. 3, the network has two home-subscriber-system servers HSS-X and HSS-Y, which are located by means of another subscriber location-function server H-SLF. The H-SLF function performs the function of a conventional SLF server.

In the method according to the invention is first a

Bootstrapping server, for example, the bootstrapping server BSF-A, from a group of existing bootstrapping server, for example, one consisting of the bootstrap servers BSF-A, BSF-B selected group, the selection of the bootstrapping server BSF preferably by a corresponding SLF server B-SLF takes place. The selection of the bootstrapping server BSF takes place on the basis of a first user identifier received by the terminal UE. This first user ID is a permanently valid or long-term user ID. This first long-term user identifier may be, for example, an IMPI. After successful authentication of the terminal UE, a temporarily valid second user identifier B-TID is assigned to this terminal UE by the selected bootstrapping server BSF and a temporarily valid base key is formed by the selected bootstrapping server BSF.

After receiving a request message received from the application server NAF, which has the same temporarily valid second user identifier B-TID, by the selected bootstrapping server BSF, this bootstrapping server BSF derives the temporarily valid application-specific key Ks NAF from the temporarily valid base key Ks and ü transmits this temporary valid application-specific key Ks NAF to the application server NAF.

In the following, the exact sequence of a possible embodiment of the bootstrapping method according to the invention will be described with reference to FIG.

In a first step S1, the user terminal UE transmits a long-term valid user identifier in a message. This first user identifier is a permanent or long-term valid user identifier, for example an IMPI. The user terminal UE thus initiates the bootstrapping process, for example by determining a DNS name of the BSF server. A DNS server provides the IP address of the BSF proxy server, and the user terminal UE then sends, for example, an http request via the U _b interface to the BSF proxy server. For a 2G GBA network, the user terminal UE first sets up a TLS tunnel to the BSF proxy server.

As soon as the BSF proxy server has received the long-term user ID or the permanently valid first user ID from the user terminal in step S1, the BSF proxy server in step S2 transmits a message containing this permanently valid first user ID, to the Subscriber Location Function server B-SLF via the D _z ^* interface. at In one embodiment, the BSF proxy server merely forwards the received http request message to the B-SLF server. The D _z interface, as specified in the 3GPP standard, will be extended in this regard.

The subscriber location function server B-SLF sends in step S3 a message back to the BSF proxy server, which contains the name of the selected bootstrapping server BSF or an address of the selected bootstrapping server BSF. In the example shown in FIG. 3, the address of the bootstrapping server BSF-A is transmitted to the BSF proxy server. To determine the correct name of the BSF server or the correct address of the BSF server, the Subscriber Location Function server B-SLF proceeds as follows. The B-SLF server first determines the subscribers HSS belonging to the subscriber. The B-SLF server then uses a stored table to determine the registered BSF server associated with the determined HSS server and sends a corresponding message to the BSF proxy server, for example using an http redirect function.

In one possible embodiment, the BSF proxy server stores the received BSF name or the address of the selected BSF server, wherein the BSF name or the BSF address assigned to the received long-term valid first user ID of the subscriber is stored , Storing the BSF name is not mandatory, but avoids repeated transmission of requests from the BSF proxy server via the D _z ^* interface. The BSF proxy server forwards the request for the desired service received from the UE to the selected BSF server in step S4. In the example shown in FIG. 3, the request is forwarded by the BSF proxy server to the BSF server BSF-A in step S4.

The selected BSF server BSF-A processes the received message according to the 3GPP standard, whereby the BSF Server replaces its own DNS name with the DNS name of the BSF proxy server.

In a further step S5, the selected BSF server transmits an HSS request message to a subscriber location function server H-SLF. The H-SLF server supplies the requesting BSF server BSF-A with the address of the home subscriber system server HSS in step S6. In the illustrated example, this is the address of the home subscriber server HSS-Y. Following a request transmitted in step S7, the HSS-Y sends an authentication vector AV to the requesting bootstrapping server BSF-A in step S8. In one embodiment, the authentication vector AV transmitted to the selected bootstrapping server BSF-A has an authentication scheme, an authentication response or an authentication response.

Response, an encryption key CK (Cipher Key) and an integrity key IK (Integrity Key).

The authentication noise received in the authentication vector AV is transmitted from the bootstrapping server BSF-A to the BSF proxy server in step S9, and from there to the user terminal UE in step S10. The user terminal UE calculates a response or an authentication response and transmits it in step Sil to the BSF proxy server, which forwards this message to the BSF server BSF-A in step S12. The BSF / Bootstrapping server BSF-A checks whether the authentication response received from the user terminal UE is correct. Subsequently, the selected bootstrapping server generates a temporarily valid second user identifier B-TID and transmits it to the BSF proxy server in step S13. In addition, the selected bootstrapping server BSF-A forms a temporary valid base key BSF from the encryption key CK and from the integrity key IK of the authentication vector AV, which it has received in step S8 from the home-subscriber system server HSS-Y. The bootstrapping proxy server BSF proxy receives in step S13 the temporarily valid second user identifier B-TID and the validity period (key lifetime) of the base key Ks formed by the selected bootstrapping server BSF-A and forwards the temporarily valid second user identifier B-TID. TID together with the validity period to the user terminal UE in step S14 on. In one possible embodiment, the bootstrapping proxy server temporarily stores the temporarily valid second user identifier B-TID and the validity period of the basic key Ks formed before it is used in the

Forward step S14. In this case, the bootstrapping proxy server preferably also stores the BSF names for the validity period of the user identifier B-TID and of the base key K.

If the user terminal UE wishes to use an application or service, it sends a service request message to the application server NAF in step S15. The application server then transmits a request for a temporary valid application-specific key Ks NAF in

Step S16 to the BSF proxy server, which forwards this request to the selected bootstrapping server BSF-A in step S17. The bootstrapping server BSF-A derives the temporarily valid application-specific key Ks NAF from the temporarily valid base key Ks and transmits this temporary valid application-specific key Ks NAF in step S18 to the BSF proxy server, which provides this temporary valid application-specific Key Ks_NAF in step S19 to the application server NAF forwards. After the mutual authentication in the

Step S20 in step S21 a cryptographically secured data exchange between the user terminal UE and the application server NAF via the interface U _a using the temporary valid application-specific key Ks_NAF, which is known to both the user terminal UE and the application server NAF. The application server NAF receives the temporarily valid application-specific key Ks NAF from the selected bootstrapping server. Server BSF-A. The user terminal UE derives the temporary valid application-specific key Ks NAF from the temporarily valid base key Ks itself.

FIG. 4 shows a possible embodiment of a GBA network extended according to the invention, which contains a BSF proxy server. In the embodiment illustrated in FIG. 4, a plurality of bootstrapping servers BSF are provided, each of which has an associated home subscriber system server HSS. For example, the bootstrapping server function BSF is integrated in the HSS server.

FIG. 5 shows a GBA network architecture for the embodiment shown in FIG.

FIG. 6 shows a further embodiment of a GBA network extended according to the invention, wherein in this embodiment also several BSF servers are provided which communicate with a common BSF proxy server. In the embodiment shown in FIG. 6, each BSF or bootstrapping server can exchange HSS data with different home-subscriber servers. Each BSF server may be connected to multiple HSS servers and each HSS server may be connected to different BSF servers. Since each BSF server can exchange data with different HSS servers in the embodiment shown in FIG. 6, it requires a subscriber location function H-SLF for finding the associated HSS server.

Figure 7 shows a GBA network architecture for the embodiment shown in Figure 6. As can be seen from FIG. 7, the BSF or bootstrapping server is connected via its own interface D _z to a subscriber location function server H-SLF for locating the home subscriber server HSS. The BSF proxy server in turn is connected to a B-SLF server via a D _z ^* interface to find the correct BSF server. In one embodiment of the bootstrapping method according to the invention, the selected bootstrapping server BSF generates the temporarily valid second user identifier B-TID and encodes therein from which bootstrapping server BSF the generated second user identifier B-TID originates. This enables the application server NAF to be able to access the BSF server directly and that the BSF proxy server does not have to store the temporarily valid second user identifier B-TID and the associated validity periods.

FIG. 8 shows a further embodiment of the inventive extended GBA network, wherein each BSF or bootstrapping server can be connected to more than one home subscriber server. All BSF servers and the BSF proxy server are locally configured with a BSF name, whereby the name of the BSF proxy server is standardized according to today's GBA specifications. All DNS servers provide the IP address of the proxy server on request. Upon request, the B-SLF server will provide the DNS name or IP address of the selected BSF server for a given subscriber, for example, given the given IMPI. The supplied DNS name is different from the BSF name standardized in today's GBA specifications. In the embodiment illustrated in FIG. 8, the user terminal UE initiates the bootstrapping process and determines the DNS name of the bootstrapping server BSF, which in this embodiment refers to the BSF proxy, which, however, need not be known to the user terminal. Subsequently, the user terminal UE sends a first http request via the U _b interface. In the case of a 2G GBA network architecture, the user terminal UE sets up a TLS tunnel to the BSF proxy server. The http request reaches the BSF proxy server because the DNS name of the bootstrapping server BSF is translated into the IP address of the BSF proxy server. The BSF proxy server transmits a message with the long-term valid first user ID IMPI via a D _z ^* interface to the B-SLF server. The B-SLF server sends a message back to the BSF proxy server stating the DNS name or IP address of the server for the participant valid BSF server. This DNS message or the IP address associated with the subscriber identity may be stored by the BSF proxy server.

The BSF proxy server forwards the original HTTP request received from the user terminal to the BSF or bootstrapping server specified by the B-SLF server. If the request is for a 2G GBA network and the BSF server requests a TLS tunnel, it will be established. The transmitted message is processed by the BSF or bootstrapping server and a response is transmitted to the BSF proxy server, which forwards this message without changes to the user terminal UE. The user terminal UE processes the received message and sends another http request to the user

BSF proxy server, which forwards these to the correct bootstrapping server BSF. The BSF server processes the received message and sends back a response to the BSF proxy server, which stores the temporarily valid second user ID B-TID and the associated validity period together with the IP address of the BSF server. The BSF proxy server then forwards the message unchanged to the user terminal UE, wherein after expiration of the validity period all entries belonging to the temporarily valid second user identifier B-TID are deleted by the BSF proxy server.

In another embodiment, the DNS server that responds to the DNS requests is configured to translate the DNS request for the standardized BSF name into the IP address of the respective BSF server rather than into the IP address of the BSF server. Address of the BSF proxy server.

In the network according to the invention, it is possible to provide a plurality of BSF servers or bootstrapping servers, without this being noticeable for the user terminal UE and the application server NAF. The network according to the invention is downwardly compatible, since the user terminals UE and the application terminals Server NAF can use the interfaces U _a , U _b , Z _n in the usual way.

Claims

claims

A bootstrapping method for providing a common temporary application specific key (Ks_NAF) for a UE and an application server (NAF), comprising the steps of:

(a) selecting a bootstrapping server (BSF) from a plurality of existing bootstrapping servers based on a first user identifier received from the terminal (UE); (b) after successful authentication of the terminal (UE) by the selected bootstrapping server (BSF), a temporarily valid second user identifier (B-TID) is assigned to this terminal (UE) and a temporary valid base key (Ks) by the selected bootstrapping -Server (BSF) and the terminal (UE) is formed;

(c) and wherein upon receipt of a request message originating from the application server (NAF) and having the same temporarily valid second user identifier (B-TID), the selected bootstrapping server (BSF) assigns the temporary valid application-specific key (Ks NAF) from the temporary base key (Ks) and transfers to the application server (NAF).

2. Bootstrapping method according to claim 1, wherein the first user identifier is formed by a permanently valid user identifier.

3. Bootstrapping method according to claim 2, wherein the first user identifier is formed by an IMPI (IP Multimedia Private Identity).

4. Bootstrapping method according to claim 1, wherein the selection of the bootstrapping server (BSF) is performed by a B-SLF (Subscriber Location Function) server.

5. Bootstrapping method according to claim 1, wherein messages are transmitted between the terminal (UE) and the selected bootstrapping server (BSF) via a BSF proxy server.

6. Bootstrapping method according to claim 1, wherein messages between the application server (NAF) and the selected bootstrapping server (BSF) are transmitted via a BSF proxy server.

The bootstrapping method of claim 4 to 6, wherein the B-SLF server transmits to the BSF proxy server an address of the selected bootstrapping server (BSF).

The bootstrapping method of claim 7, wherein the BSF proxy server caches the address of the selected bootstrapping server (BSF).

9. The bootstrapping method according to claim 1, wherein the selected bootstrapping server (BSF) authenticated the terminal (UE) by means of a home subscriber system server (HSS).

10. Bootstrapping method according to claim 9, wherein the home subscriber system server (HSS) transmits an authentication vector (AV) to the selected bootstrapping server (BSF).

11. Bootstrapping method according to claim 10, wherein the authentication vector (AV) transmitted to the selected bootstrapping server (BSF) comprises an authentication challenge, an authentication response, a cipher key (CK) and an integrity key (IK). having.

12. Bootstrapping method according to claim 11, wherein the selected bootstrapping server (BSF) forms the temporary valid base key (Ks) from the cipher key (CK) and the integrity key (IK).

13. Bootstrapping method according to claim 1, wherein the selected bootstrapping server (BSF) generates the temporarily valid second user identifier (B-TID).

The bootstrapping method according to claim 13, wherein the temporarily valid second user identifier (B-TID), together with a validity period of the formed base key (Ks), is transmitted from the selected bootstrapping server (BSF) to the bootstrapping proxy server in a message which temporarily stores the temporarily valid second user identification (B-TID) and the validity period of the basic key (Ks) formed, and then forwards the message to the terminal (UE).

15. Bootstrapping method according to claim 14, wherein the terminal (UE) upon request of a service from the application server (NAF) forwarded by the bootstrapping proxy server to the terminal (UE) temporarily valid second user identifier (B-TID) transmits the application server (NAF) in a service request message.

16. The bootstrapping method according to claim 15, wherein the application server (NAF) sends the service request message with the temporarily valid second user identifier (B-TID) to the bootstrapping proxy server, which forwards the service request message to the selected bootstrapping server (BSF) ,

17. Bootstrapping method according to claim 1, wherein the terminal (UE) forms the temporary valid base key (Ks).

18. Bootstrapping method according to claim 17, wherein the terminal (UE) derives the temporarily valid application-specific key (Ks_NAF) from the temporarily valid base key (Ks).

19. Bootstrapping method according to claim 1, wherein the selected bootstrapping server (BSF) generates the temporarily valid second user identifier (B-TID) and encodes therein, from which bootstrapping server (BSF) the generated second user identifier (B-TID) comes.

A bootstrapping proxy server for a generic bootstrapping architecture (GBA) network which forwards messages between a terminal (UE) and a bootstrapping server (BSF) selected on the basis of a permanent user identifier.

21. The bootstrapping proxy server according to claim 20, wherein the bootstrapping proxy server temporarily stores a temporarily valid user identifier (B-TID) assigned to a respective bootstrapping server (BSF) received from a bootstrapping server (BSF).

22. The bootstrapping proxy server of claim 20, wherein the bootstrapping proxy server forwards messages between an application server (NAF) and the selected bootstrapping server (BSF).