A METHOD FOR SECURE HANDOVER

Abstract: In a mobile, wireless telecommunications network, communications relating to a mobile terminal can be protected during a handover of the mobile terminal from a first access point to a second access point. This may be accomplished by transmitting a security token from the first access point to the mobile terminal, and then from the mobile terminal to the second access point, over the radio interface. Thereafter, the security token is transmitted from the first access point to the second access point through the fixed network to which both the first and the second access points are connected. The communiations link between the mobile terminal and the second access point needed to achieve secure handover is then established only if the second access point determines that the security token received from the mobile terminal matches the security token received from the first access point.

Description: A METHOD FOR SECURE HANDOVER FIELD OF THE INVENTION The present invention relates to the field of telecommunications. More particularly, the present invention relates to the field of wireless, mobile telecommunications . BACKGROUND FIG. 1 illustrates an exemplary wireless network 100, such as a wireless local area network. As shown, the wireless network 100 includes a fixed network portion 105, wherein fixed network portion 105 typically includes transmission equipment that communicates with various systems (not shown) that are external to the wireless network 100, such as a publically switched telephone network and/or the Internet. The fixed network portion 105 is further connected to a number of fixed radio stations known as base stations or access points, for example, access points AP ] and AP 2 . Each of the access points, in turn, are capable of communicating with one or more mobile terminals, such as mobile terminal MT, over a radio (i.e., a wireless) interface. Further with regard to FIG. 1, each of the access points, API and AP2 primarily communicates with a mobile terminal in a corresponding cell Cl and C2 respectively. Moreover, it will be understood that a mobile terminal MT, communicating through an access point API is generally located within the cell Cl corresponding to the access point API. However, as the mobile terminal MT moves away from access point API, as indicated by the arrow 110, toward another access point, for example AP2 and its corresponding cell C2, the signal quality associated with the communication link between the mobile terminal MT and the access point API tends to decrease. If the mobile terminal MT continues to move away from the access point API, and the signal quality associated with the communication link between the mobile terminal MT and the access point API continues to deteriorate, the communications link will, in all likelihood, be lost if the connection is not passed off or transferred from the access point API to another access point, such as, access point AP2. The process of passing off or transferring the connection from API to AP2 is known as handover or, alternatively, handoff. During handover, the communications link associated with a mobile terminal is highly susceptible to intruders, that is, entities that wish to hijack or simply disrupt the communications link. For instance, an intruding device can, during a handover, present itself to the new access point (i.e., the access point to which the mobile terminal is attempting to establish a communications link). If the new access point accepts the intruding device as the mobile terminal, the access point may begin transmitting information to the intruder that is intended for the mobile terminal. Although providing security for communications between a mobile terminal and one or more access points at all times is an important concern, providing a method and/or system that does so particularly during handover would be highly desirable. SUMMARY The present invention is of particular relevance to mobile telecommunications networks, wherein mobile terminals undergo handover from one radio station (i.e., access point) to another as they move from one cell to another within the telecommunications network. More particularly, the present invention involves protecting communications associated with a mobile terminal against unauthorized intrusion when the mobile terminal undergoes a handover from one access point to another. In accordance with one aspect of the present invention, a method and/or a telecommunications network is provided for achieving secure handover of a mobile terminal from a first access point to a second access point, wherein the first access point and the second access point are physically connected through a fixed network. The method and/or network involves transmitting a security token from the first access point to the mobile terminal, and then from the mobile terminal to the second access point over a radio interface. The security token is then transmitted from the first access point to the second access point through the fixed network. A communication link is then established between the mobile terminal and the second access point, to achieve secure handover, if the second access point determines that the security token received from the mobile terminal matches the security token received from the first access point. In accordance with another aspect of the present invention a method and/or a telecommunications network is provided for achieving secure handover of a mobile terminal from a first access point to a second access point. The method and/or network involves transmitting a first message from the first access point to the mobile terminal over a radio interface, the first message containing an encrypted security token and a hash code. Then, in the mobile terminal, the encrypted security token is deciphered using an encryption key that is shared by the mobile terminal and the first access point. The mobile terminal then re- encrypts the security token using an encryption key that it shares with the second access point. Thereafter, a message is transmitted from the mobile terminal to the second access point, this second message containing the re-encrypted security token and the hash code. The second access point then deciphers the re-encrypted security token using the encryption key that it shares with mobile terminal. Finally, a communications link is established between the mobile terminal and the second access point, to achieve secure handover, if the second access point authenticates the mobile terminal based on the deciphered security token and the hash code. BRIEF DESCRIPTION OF THE DRAWINGS The objects and advantages of the invention will be understood by reading the following detailed description in conjunction with the drawings in which: FIG. 1 illustrates an exemplary mobile, wireless telecommunications network; FIG. 2 illustrates a first embodiment of the present invention; FIG. 3 illustrates a second embodiment of the present invention; FIG. 4 illustrates a third embodiment of the present invention; and FIG. 5 illustrates a fourth embodiment of the present invention. DETAILED DESCRIPTION FIG. 2 illustrates a technique for securing communications for a mobile terminal MT during a handover procedure from a first access point (e.g., AP 0LD ) to a second access point (e.g., AP NEW ), in accordance with a first exemplary embodiment of the present invention. As shown in FIG. 2, AP 0LD sends the mobile terminal MT, before the communications link between the mobile terminal MT and AP 0LD is disconnected, a message (1) containing a security token (S/TOKEN). In this first exemplary embodiment, the security token may be any sequence of characters, such as a random number sequence. Then, prior to the establishment of a communications link between the mobile terminal MT and AP NEW tne mobile terminal MT transmits a message (2) to AP NEW , wherein the message (2) contains the security token provided by AP OLD , and one or more mobile identity codes. Upon receiving the message (2) from the mobile terminal MT, AP NEW sends a message (3) to AP 0LD via the fixed network. This message (3) contains a security token request (S/TOKEN/R) as well as one or more of the mobile identity codes provided by the mobile terminal MT. In response, AP 0LD transmits to AP NEW , a message (4), which contains the security token that AP 0LD also provided to the mobile terminal MT in the message (1). Further in accordance with the first exemplary embodiment of the present invention, AP NEW now attempts to authenticate the mobile terminal MT. The AP NEW accomplishes this by comparing the security token it received from the mobile terminal MT in the message (2) with the security token it received from AP 0LD in the message (4). If AP NEW determines that the security token it received from the mobile terminal MT matches the security token it received from AP 0LD , AP NEW authenticates the mobile terminal MT, thereby allowing a communications link to be established between the mobile terminal MT and AP NEW . Alternatively, AP 0LD may store a timestamp associated with the instant of time it transmitted the message (1) to the mobile terminal MT. By storing this timestamp, AP 0LD can determine how much time elapses between the time it transmitted the security token in the message (1) to the mobile terminal MT and the time it received a security token request in the message (3) from AP NEW . If AP 0LD determines that an excessive amount of time has elapsed, AP 0LD may reject AP NEW 's security token request. In so doing, AP 0LD protects against "replay" attempts, that is, an intruder's attempt to use an old security token when trying to access AJ NEW . In another alternative to the first exemplary embodiment of the present invention, AP 0LD transmits data to the mobile terminal MT, wherein the mobile terminal MT uses that data to compute a security token, which is then transmitted to AP NEW in the message (2). The AP 0LD also transmits the data to AP NEW , upon AP NEW transmitting a security token request message to AP 0LD . The AP NEW then computes the security token and compares it with the security token received from the mobile terminal MT in message (2). Again, if the security tokens match, AP NEW authenticates the mobile terminal MT, thereby paving the way for a communication link to be established between the mobile terminal MT and AP NEW . FIG. 3 illustrates a technique to secure communications for a mobile terminal MT during a handover procedure from one access point (e.g., AP 0LD ) to a second access point (e.g., AP NEW ), in accordance with a second exemplary embodiment of the present invention. As shown in FIG. 3, the technique associated with this second exemplary embodiment is similar to the technique associated with the first exemplary embodiment, but for the fact that the security token transmitted from AP 0LD to the mobile terminal MT in message (1), and thereafter, transmitted from the mobile terminal MT to AP NEW in message (2) is encrypted, as indicated by the notation ENC_S/TOKEN. Accordingly, AP NEW must obtain the correct encryption key to decipher the security token. The AP NEW may obtain the encryption key, for example, by accessing a common key database or through transference from AP 0LD . It will be understood that the mobile terminal MT need not maintain that encryption key, as it merely receives the encrypted security token from AP 0LD and then passes it on to AP NEW . Moreover, it will be understood that in order to further protect communications, the encryption key may periodically change. Further in accordance with the second exemplary embodiment, AP NEW then transmits a message (3) containing a security token request to AP 0LD , through the fixed network. Assuming the time associated with any timestamp has not elapsed, AP 0LD then transmits a message (4) containing an un-encrypted version of the security token to AP NEW . Because the message (4) is routed through the fixed network and not over the radio interface, the security token contained therein is not encrypted. The AP NEW can now compare the security token received in message (4) with the security token received in message (2), which it subsequently deciphered. Once again, if AP NEW determines that the security tokens match AP NEW authenticates the mobile terminal MT, thereby allowing a communications link to be established between the mobile terminal MT and AP NEW . FIG. 4 illustrates a technique to secure communications for a mobile terminal during a handover procedure from a first access point (e.g., AP 0LD ) to a second access point (e.g., AP NEW ), in accordance with yet a third exemplary embodiment of the present invention. In accordance with this third exemplary embodiment, a two-step process is employed, wherein all of the information needed by AP NEW to authenticate the mobile terminal MT, prior to the establishment of a communication link between AP NEW and the mobile terminal during handover, is transmitted over the radio interface. The first step in the two-step technique associated with this third exemplary embodiment, as shown in FIG. 4, involves AP 0LD transmitting a message (1) to the mobile terminal MT before the communications link between the mobile terminal MT and AP 0LD is disconnected as part of the handover procedure. The message (1) contains, among other items, an encrypted security token (i.e., ENC MT . APO _S/TOKEN), wherein the security token is encrypted using a key that is shared only by the mobile terminal MT and AP 0LD , as indicated by the notation ENC MT . AP0 . The message (1) also contains an encrypted timestamp value (i.e., ENC APO . APN _T/STAMP) and an encrypted hash code (i.e., ENC AP0 . APN _S/TOKEN), wherein the hash code is a function of the security token (S/TOKEN) which has been encrypted using a key that is shared only by the two access points, as indicated by the notation ENC AP0 . APN . The second step in the two-step technique associated with the third exemplary embodiment, as shown in FIG. 4, involves the mobile terminal MT processing the contents of the message (1), and thereafter transmitting a message (2) to AP NEW , wherein processing the contents of the message (1) is primarily handled by software algorithms stored in and executed by the mobile terminal MT. More particularly, processing involves deciphering the encrypted security token using the encryption key that is shared only by the mobile terminal MT and AP 0LD , as stated above, and then re-encrypting the security token using a second encryption key that it shares only with AP NEW . Accordingly, the message (2) contains, among other items, the re-encrypted security token (i.e., ENC MT . APN _S/TOKEN). As the mobile terminal MT does not possess the encryption keys to decipher the timestamp value and the hash code contained in the message (1), the mobile terminal MT merely transfers these to the message (2) for use by AP NEW , as will be discussed in more detail below. Upon receiving the message (2) from the mobile terminal MT, AP NEW deciphers each of the items contained therein using the various encryption keys. For instance, AP NEW deciphers the security token using the encryption key which is shares with the mobile terminal MT. The AP NEW then deciphers the security token from the hash code by applying the encryption key it shares with AP 0LD . The AP NEW can then authenticate the fact that the message (1) and the message (2) originated from a valid source, if the two security tokens match. The AP NEW will also decipher the timestamp value using yet another encryption key that it shares with AP OLD , and if it is determined therefrom that an excessive amount of time has not elapsed since the AP 0LD transmitted the message (1) to the mobile terminal MT, a communications link can be established between the mobile terminal MT and the AP NEW , as part of the handover procedure. FIG. 5 illustrates an alternative technique, similar to that which is illustrated in FIG. 4. The alternative technique illustrated in FIG. 5 differs from the technique illustrated in FIG. 4 in that the first step in the two-step technique involves the transmission of a message (1) from AP 0LD to the mobile terminal MT, wherein the message (1) contains a plaintext (non-encrypted) timestamp value (i.e., T/STAMP) and a hash code (i.e. , ENC AP0 . APN TVSTAMP, S/TOKEN), wherein the hash code is a function of the timestamp value (T/STAMP), the security token (S/TOKEN), and an encryption key that is shared only by the two access points, as indicated by the notation ENC AP0 . APN . Like the technique illustrated in FIG. 4, the message (1) also contains an encrypted security token (i.e., ENC MT . APO _S/TOKEN). The second step of the alternative two-step technique illustrated in FIG. 5, like the technique illustrated in FIG. 4, involves the mobile terminal processing the contents of the message (1), and thereafter transmitting a message (2) to AP NEW , wherein the processing of the contents of the message (1) involves deciphering the encrypted security token using the encryption key that is shared only by the mobile terminal MT and AP 0LD , and then re-encrypting the security token using a second encryption key that the mobile terminal MT shares only with AP NEW . The re- encrypted security token (i.e., ENC MT-APN _S/TOKEN) is then inserted into the message (2) along with the plaintext timestamp value and the hash code. The message (2) is then transmitted to AP NEW . After receiving the message (2) from the mobile terminal MT, AP NEW deciphers the contents of the message (2) using the various encryption keys. For example, AP NEW deciphers the security token using the encryption key which it shares with the mobile terminal MT. The AP NEW then deciphers the security token and the timestamp value from the hash code by applying the encryption key it shares with AP 0LD . The AP NEW can then authenticate the fact that message (1) and message (2) originated from a valid source, if the two security tokens match and/or the two timestamp values match. If AP NEW determines, based on the timestamp value, that an excessive amount of time has not elapsed since AP 0LD transmitted the message (1) to the mobile terminal MT, a communications link can be established between the mobile terminal MT and the AP NEW , as part of the handover procedure. The present invention has been described with reference to several exemplary embodiments. However, it will be readily apparent to those skilled in the art that it is possible to embody the invention in specific forms other than those specifically described herein above; furthermore, this may be done without departing from the spirit of the invention. These embodiments are merely illustrative and should not be considered restrictive in any way. The scope of the invention is given by the appended claims, rather than the preceding description, and all variations and equivalents which fall within the range of the claims are intended to be embraced therein.

Claims: WHAT IS CLAIMED IS: 1. In a mobile, wireless telecommunications network, a method for achieving secure handover of a mobile terminal from a first access point to a second access point, wherein the first access point and the second access point are physically connected through a fixed network, said method comprising the steps of: transmitting a security token from the first access point to the mobile terminal over a radio interface; transmitting the security token from the mobile terminal to the second access point over the radio interface; transmitting the security token from the first access point to the second access point through the fixed network; and establishing a communications link between the mobile terminal and the second access point, to achieve secure handover, if the second access point determines that the security token received from the mobile terminal matches the security token received from the first access point. 2. The method of claim 1, wherein the security token is encrypted at the first access point. 3. The method of claim 2 further comprising the step of: at the second access point, obtaining an encryption key from the first access point to decipher the encrypted security token. 4. The method of claim 2 further comprising the step of: at the second access point, obtaining an encryption key from a common key database to decipher the encrypted security token. 5. The method of claim 1 further comprising the steps of: transmitting a mobile terminal identification code, along with the security token, from the mobile terminal to the second access point; and transmitting a security token request and the mobile terminal identification code from the second access point to the first access point, through the fixed network. 6. In a mobile, wireless telecommunications network, a method for achieving secure handover of a mobile terminal from a first access point to a second access point, said method comprising the steps of: transmitting a first message from the first access point to the mobile terminal over a radio interface, wherein the first message contains an encrypted security token and a hash code; in the mobile terminal, deciphering the encrypted security token using an encryption key shared by the mobile terminal and the first access point; in the mobile terminal, re-encrypting the security token using an encryption key that is shared by the mobile terminal and the second access point; transmitting a second message from the mobile terminal to the second access point, wherein the second message contains the re-encrypted security token and the hash code; deciphering the re-encrypted security token at the second access point using the encryption key shared by the mobile terminal and the second access point; and establishing a communications link between the mobile terminal and the second access point, to achieve secure handover, if the second access point authenticates the mobile terminal based on the deciphered security token and the hash code. 7. The method of claim 6, wherein the hash code is a function of the security token and an encryption key that is shared by the first access point and the second access point. 8. The method of claim 7 further comprising the steps of: deciphering the security token at the second access point by applying the encryption key shared by the first access point and the second access point to the hash code; comparing the security token deciphered using the encryption key shared by the mobile terminal and the second access point with the security token deciphered using the encryption key shared by the first access point and the second access point; and authenticating the mobile terminal if the second access point determines that there is a match between the security token deciphered using the encryption key shared by the mobile terminal and the second access point and the security token deciphered using the encryption key shared by the first access point and the second access point. 9. The method of claim 7, wherein the hash code is also a function of a timestamp value. 10. The method of claim 6, wherein the first message and the second message each contain a timestamp value. 11. The method of claim 10 further comprising the step of: determining, as a function of the timestamp value, whether an excessive period of time elapsed between the time the first message was transmitted from the first access point to the mobile terminal and the time the second message was received by the second access point. 12. The method of claim 11 further comprising the step of: preventing the establishment of a communications link between the mobile terminal and the second access point, if it is determined that an excessive amount of time has elapsed. 13. The method of claim 11 further comprising the step of: encrypting the timestamp value using an encryption key that is shared by the first access point and the second access point. 14. In a mobile, wireless telecommunications network, a method for achieving secure handover of a mobile terminal from a first access point to a second access point, wherein the first access point and the second access point are physically connected through a fixed network, said method comprising the steps of: transmitting a security token from the first access point to the mobile terminal over a radio interface; transmitting the security token from the mobile terminal to the second access point over the radio interface; transmitting the security token from the first access point to the second access point through the fixed network; and establishing a communications link between the mobile terminal and the second access point, to achieve secure handover, if the second access point determines that the security token received from the mobile terminal matches the security token received from the first access point. 15. The method of claim 14, wherein the security token is encrypted at the first access point. 16. The method of claim 15 further comprising the step of: at the second access point, obtaining an encryption key from the first access point to decipher the encrypted security token. 17. The method of claim 15 further comprising the step of: at the second access point, obtaining an encryption key from a common key database to decipher the encrypted security token. 18. The method of claim 14 further comprising the steps of: transmitting a mobile terminal identification code, along with the security token, from the mobile terminal to the second access point; and transmitting a security token request and the mobile terminal identification code from the second access point to the first access point, through the fixed network. 19. In a mobile, wireless telecommunications network, a method for achieving secure handover of a mobile terminal from a first access point to a second access point, said method comprising the steps of: transmitting a first message from the first access point to the mobile terminal over a radio interface, wherein the first message contains an encrypted security token and a hash code; in the mobile terminal, deciphering the encrypted security token using an encryption key shared by the mobile terminal and the first access point; in the mobile terminal, re-encrypting the security token using an encryption key that is shared by the mobile terminal and the second access point; transmitting a second message from the mobile terminal to the second access point, wherein the second message contains the re-encrypted security token and the hash code; deciphering the re-encrypted security token at the second access point using the encryption key shared by the mobile terminal and the second access point; and establishing a communications link between the mobile terminal and the second access point, to achieve secure handover, if the second access point authenticates the mobile terminal based on the deciphered security token and the hash code. 20. The method of claim 19, wherein the hash code is a function of the security token and an encryption key that is shared by the first access point and the second access point. 21. The method of claim 20 further comprising the steps of: deciphering the security token at the second access point by applying the encryption key shared by the first access point and the second access point to the hash code; comparing the security token deciphered using the encryption key shared by the mobile terminal and the second access point with the security token deciphered using the encryption key shared by the first access point and the second access point; and authenticating the mobile terminal if the second access point determines that there is a match between the security token deciphered using the encryption key shared by the mobile terminal and the second access point and the security token deciphered using the encryption key shared by the first access point and the second access point. 22. The method of claim 20, wherein the hash code is also a function of a timestamp value. 23. The method of claim 19, wherein the first message and the second message each contain a timestamp value. 24. The method of claim 23 further comprising the step of: determining, as a function of the timestamp value, whether an excessive period of time elapsed between the time the first message was transmitted from the first access point to the mobile terminal and the time the second message was received by the second access point. 25. The method of claim 24 further comprising the step of: preventing the establishment of a communications link between the mobile terminal and the second access point, if it is determined that an excessive amount of time has elapsed. 26. The method of claim 24 further comprising the step of: encrypting the timestamp value using an encryption key that is shared by the first access point and the second access point.

Inventors: ALMEHAG LORENS, CHENG YI, JERRESTAM DAN, NOREFORS ARNE, ALMEHAG, LORENS, CHENG, YI, JERRESTAM, DAN, NOREFORS, ARNE
Applicants: ERICSSON TELEFON AB L M, TELEFONAKTIEBOLAGET LM ERICSSON (PUBL)
Application year: 2000
Main classification: H04Q7
IPC codes: H04L29, H04W12, H04W36