WO2001020925A2

WO2001020925A2 - System and method of passing encryption keys after inter-exchange handoff

Info

Publication number: WO2001020925A2
Application number: PCT/SE2000/001685
Authority: WO
Inventors: Richard Stewart; Joseph Howard; Jose Arturo Arreaga Garza
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 1999-09-10
Filing date: 2000-09-01
Publication date: 2001-03-22
Also published as: WO2001020925A3; AU7326400A; AR027858A1

Abstract

A system and method in a radio telecommunications network of passing a mobile station's encryption keys from an anchor Mobile Switching Center (MSC (11)) to a serving MSC (12) after the mobile station (MS (14)) has been handed off (25) from the anchor MSC to the serving MSC. If the need arises to perform an inter-system handoff of the MS before the encryption keys are sent from the Home Location Register/Authentication Center (HLR/AC (17)) to the anchor MSC, the MS may be handed off, and the invention enables the keys to be forwarded to the serving MSC. The anchor MSC modifies an ANSI-41 Information Forward (INFOFWD(27)) Invoke message to carry the encryption keys, and sends the message from the anchor MSC to the serving MSC. The message also includes a Confidentiality Modes (Desired) parameter. The serving MSC sends an activation order (28) to the MS to activate the confidentiality modes desired, and then sends an INFOFWD Return Result message (30) to the anchor MSC reporting the confidentiality modes that were actually activated.

Description

SYSTEM AND METHOD OF PASSING ENCRYPTION KEYS AFTER INTER-EXCHANGE HANDOFF

PRIORITY STATEMENT UNDER 35 U.S.C. § 119(e) & 37 C.F.R. § 1.78 This nonpro visional application claims priority based upon the prior U.S. provisional patent application entitled, "Method of Passing Voice Privacy Masks and Signaling Message Encryption (SME) Masks After Inter-Exchange Handoff, application number 60/153,407, filed September 10, 1999, in the names of Richard Stewart, Joseph Howard, and Jose Arturo Arreaga Garza.

BACKGROUND OF THE INVENTION Technical Field ofthe Invention

This invention relates to radio telecommunication systems and, more particularly, to a system and method of passing encryption keys for voice signals and control signals from an anchor Mobile Switching Center (MSC) to a serving MSC after a mobile station has been handed off from the anchor MSC to the serving MSC.

Description of Related Art

In Time Division Multiple Access (TDMA) radio telecom-munication systems today, there are two types of encryption that occur on the digital traffic channels. One type of encryption is Voice Privacy (VP). VP, as its name implies, encrypts the voice signal that goes over the traffic channel between the mobile station and the serving base station. A second type of encryption, Signaling Message Encryption (SME), encrypts control messages that are sent to the mobile station over the traffic channel to instruct the mobile station to perform certain actions. SME is also used for Analog Voice Channels.

Encryption keys (masks) are needed to encrypt the traffic channels, and the encryption keys are generated on a per-call basis. The mobile station generates the keys at the same time that the network generates the keys during the call setup process. The network generates the keys in the Home Location Register/ Authentication Center (HLR/AC) and then provides them to the anchor MSC and the serving base station.

The same keys are used for the duration of the call, even if the mobile station is handed off to another MSC. Therefore, if the keys have already been provided to the anchor MSC, and the mobile station moves into the service area of another MSC, the keys must be passed in an inter-exchange handoff in order to enable the new serving MSC to encrypt the voice and/or control signals. If the encryption keys are available during handoff, they are passed as part ofthe handoff process in a Facilities Directive

(FACDIR) message.

A problem occurs in existing networks because congestion in the HLR/AC can delay the generation of the encryption keys. As noted above, the network generates the keys in the HLR/AC, and then provides them to the anchor MSC. By the time the keys are received in the anchor MSC, the mobile station may already be set up in the call. This is not a major problem as long as the call is still in the anchor MSC where it began. In this case, once the keys are received by the anchor MSC, the keys may be sent to the serving base station, and an activation order is sent to the mobile station to enable the encryption of voice and or control signals. A greater problem arises if the system assigns a traffic channel immediately, the MS is already engaged in a call, and there is a need for an inter-system handoff in order to continue providing service. This situation may arise before the keys are generated and passed to the anchor MSC. The problem may be particularly critical when the MS is operating in a border area between the anchor MSC and an adjacent MSC. If the handoff were to be completed, the encryption keys could not be passed from the anchor MSC to the serving MSC. The ANSI-41 inter-system signaling standard includes a section on signaling between MSCs after a handoff, but it does not address this problem or identify anyway to pass encryption keys from the anchor MSC to the serving MSC after an inter-exchange handoff. In order to overcome the disadvantage of existing solutions, it would be advantageous to have a system and method of passing encryption keys for voice signals and control signals from an anchor MSC to a serving MSC after a mobile station has been handed off from the anchor MSC to the serving MSC. The present invention provides such a system and method.

SUMMARY OF THE INVENTION In one aspect, the present invention is a method in a radio telecommunications network of passing a mobile station's encryption keys from an anchor MSC to a serving MSC after the MS has been handed off from the anchor MSC to the serving MSC. When the encryption keys are sent from the HLR/AC to the anchor MSC, the anchor MSC adds the encryption keys to an information message, and sends the information message to the serving MSC. A parameter identifying the confidentiality modes desired may also be added to the information message. The method may also include sending an order from the serving MSC to the MS to activate the confidentiality modes indicated by the parameter in the information message, determining by the serving MSC which confidentiality modes were actually activated, and sending a return information message from the serving MSC to the anchor MSC reporting the confidentiality modes that were actually activated. In the preferred embodiment, the information message is a modified ANSI-41 Information Forward Invoke message, and the return information message is a modified ANSI-41 Information Forward Return Result message.

In another aspect, the present invention is a system in a radio telecommunications network for passing a mobile station's encryption keys from an anchor MSC to a serving MSC after the MS has been handed off from the anchor MSC to the serving MSC. The system includes an HLR/AC for generating the encryption keys and sending the encryption keys to the anchor MSC; means in the anchor MSC for adding the encryption keys to an information message; and means for sending the information message from the anchor MSC to the serving MSC.

In yet another aspect, the present invention is an anchor MSC in a radio telecommunications network which includes means for passing a mobile station's encryption keys to a serving MSC when the encryption keys are received by the anchor

MSC after the MS has been handed off from the anchor MSC to the serving MSC. The anchor MSC receives the encryption keys from the HLR AC after the MS has been handed off, adds the encryption keys to an information message, and sends the information message to the serving MSC.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be better understood and its numerous objects and advantages will become more apparent to those skilled in the art by reference to the following drawings, in conjunction with the accompanying specification, in which:

FIG. 1 is a simplified block diagram of a radio telecommunications network suitable for implementing the present invention;

FIG.2 is a message flow diagram illustrating the flow of messages between the nodes ofthe network of FIG. 1 when performing the method ofthe present invention;

FIG. 3 is a table of parameters for an Information Forward Invoke message modified in accordance with the teachings ofthe present invention; and FIG. 4 is a table of parameters for an Information Forward Return Result message modified in accordance with the teachings ofthe present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

The present invention enables the anchor MSC to pass a mobile station's encryption keys to the serving MSC at any time, even after an inter-exchange handoff has been completed. In the preferred embodiment, the ANSI-41 Information Forward Invoke message is modified to enable the anchor MSC to pass the encryption keys to the serving MSC. The invention uses a message that is sent from the anchor MSC because the anchor MSC always knows which MSC is the current serving MSC for the mobile station. Therefore, if there is an excessive delay in generating the keys in the HLR/AC, and the mobile station performs more than one inter-exchange handoff by the time the keys are provided to the anchor MSC, the anchor MSC can still pass the keys to the current serving MSC when they are received.

FIG. 1 is a simplified block diagram of a radio telecommunications network 10 suitable for implementing the present invention. An anchor MSC 11 and a serving

MSC 12 have service areas either side of an exchange boundary 13. A mobile station (MS) 14 initially operates within a cell generated by a first base station (BS-1) 15 controlled by the anchor MSC, and then moves into a cell generated by a second base station (BS-2) 16 controlled by the serving MSC. The anchor MSC is connected to a A Home Location Register/ Authentication Center (HLR/AC) 17 provides subscriber profile information and location information to the anchor MSC via a Visitor Location Register (VLR) 18. The HLR/AC generates encryption keys for encrypting voice signals and control signals sent over the air interface to the MS. The encryption keys may include, for example, a Voice Privacy (VP) Mask and a Signaling Message Encryption (SME) key. The VLR 18 can also generate the encryption keys if Shared Secret Data (SSD) is shared between the HLR/AC and the VLR.

FIG.2 is a message flow diagram illustrating the flow of messages between the nodes ofthe network of FIG. 1 when performing the method ofthe present invention. When the MS 14 originates a call at 21, the anchor MSC 11 sends an Authentication Request (AuthReq) Invoke message 22 to the VLR 18 (if supplied with SSD) or the HLR/AC 17. The encryption keys are then generated at step 23. The anchor MSC then completes the normal call setup at 24. When authentication is finished, the encryption keys are sent back to the anchor MSC in an AuthReq Return Result message 26.

In the example illustrated, the need arises for an inter-system handoff the MS 14 before the HLR/AC completes the encryption keys and forwards the keys to the anchor MSC at 26. Under the existing ANSI-41 standard, this is not possible since the ANSI-41 inter-system signaling standard does not identify any way to pass encryption keys from the anchor MSC to the serving MSC after an inter-exchange handoff. With the present invention, however, the MS maybe handed off at 25 from the anchor MSC 11 to the serving MSC 12 before the encryption keys are sent to the anchor MSC, thus preventing the call from being dropped.

The anchor MSC knows that the MS has been handed off to the serving MSC, and also knows whether the MS has been subsequently handed off from the serving MSC to a third MSC (not shown). Therefore, the anchor MSC constructs an information message such as, for example, a modified Information Forward

(INFOFWD) Invoke message 27 and sends it to the current serving MSC. The INFOFWD Invoke message includes the encryption keys and a parameter indicating the Confidentiality Modes (C-modes) desired. The C-modes parameter may indicate, for example, whether VP and SME are to be on or off. The serving MSC 12 receives the INFOFWD Invoke message and sends an activation order 28 to the MS 14 to activate the desired C-modes. The MS returns an acknowledgment message 29 and indicates whether the desired modes are activated. The serving MSC then constructs a modified INFOFWD Return Result message 30 and sends it to the anchor MSC 11. The INFOFWD Return Result message includes a parameter indicating the C-modes actually activated. FIG. 3 is a table of parameters for the Information Forward Invoke message 29 modified from ANSI-41.5-D in accordance with the teachings ofthe present invention. The Voice Privacy Mask parameter 35 and the Signaling Message Encryption parameter 36 are the actual keys being passed. The Voice Privacy Mask parameter is provided if it is available and has not been provided earlier to the serving MSC, and the MS supports TDMA and is authorized to have VP. The Signaling Message

Encryption parameter is provided if it is available and has not been provided earlier to the serving MSC, and the MS supports SME or VP. If present, the Confidentiality Modes (Desired) parameter 37 must also be present.

The Confidentiality Modes (Desired) parameter 37 tells the serving MSC which encryption modes should be on or off. For example, SME on and VP off. This parameter is provided if it is available and has not been provided earlier to the serving MSC, and the MS supports SME or VP. Some Code Division Multiple Access (CDMA) systems also use ANSI-41 signaling, so a CDMA Private Long Code Mask parameter 38 is provided for those systems. This parameter is provided if it is available and has not been provided earlier to the serving MSC, and the MS supports

CDMA and is authorized to have VP.

FIG. 4 is a table of parameters for the Information Forward Return Result message 33 modified from ANSI-41.5-D in accordance with the teachings of the present invention. A Confidentiality Modes (Actual) parameter 39 is provided to reflect the actual assignment of C-modes by the serving MSC if the Confidentiality

Modes (Desired) parameter 37 was included in the INFOFWD Invoke message.

It is thus believed that the operation and construction ofthe present invention will be apparent from the foregoing description. While the system and method shown and described has been characterized as being preferred, it will be readily apparent that various changes and modifications could be made therein without departing from the scope ofthe invention as defined in the following claims.

Claims

WHAT IS CLAIMED IS:

1. A method in a radio telecommunications network of passing a mobile station's encryption keys from an anchor Mobile Switching Center (MSC) to a serving MSC after the mobile station (MS) has been handed off from the anchor MSC to the serving MSC, said method comprising the steps of: sending the encryption keys from a Home Location Register/ Authentication Center (HLR/AC) to the anchor MSC after the MS has been handed off; adding the encryption keys to an information message; and sending the information message from the anchor MSC to the serving MSC.

2. The method of passing encryption keys of claim 1 wherein the encryption keys enable a first confidentiality mode for the encryption of voice signals and a second confidentiality mode for the encryption of control signals transmitted over an air interface to and from the MS.

3. The method of passing encryption keys of claim 2 wherein the step of adding the encryption keys to an information message also includes adding a parameter that indicates which confidentiality modes are desired to be activated.

4. The method of passing encryption keys of claim 1 further comprising the steps of: sending an order from the serving MSC to the MS to activate the confidentiality modes indicated by the parameter in the information message; determining by the serving MSC which confidentiality modes were actually activated; and sending a return information message from the serving MSC to the anchor MSC reporting the confidentiality modes that were actually activated.

5. The method of passing encryption keys of claim 4 wherein the information message is an ANSI-41 Information Forward Invoke message, and the return information message is an ANSI-41 Information Forward Return Result message.

6. In an anchor Mobile Switching Center (MSC) in a radio telecommunications network, a method of passing a mobile station's encryption keys from the anchor MSC to a serving MSC when the encryption keys are received by the anchor MSC after the mobile station (MS) has been handed off from the anchor MSC to the serving MSC, said method comprising the steps of: receiving the encryption keys from a Home Location Register/ Authentication Center (HLR/AC) in the anchor MSC after the MS has been handed off; adding the encryption keys to an information message; and sending the information message from the anchor MSC to the serving MSC.

7. A system in a radio telecommunications network for passing a mobile station's encryption keys from an anchor Mobile Switching Center (MSC) to a serving MSC after the mobile station (MS) has been handed off from the anchor MSC to the serving MSC, said system comprising: a Home Location Register/ Authentication Center (HLR/AC) for generating the encryption keys and sending the encryption keys to the anchor MSC; means in the anchor MSC for adding the encryption keys to an information message; and means for sending the information message from the anchor MSC to the serving MSC.

8. The system for passing encryption keys of claim 7 wherein the encryption keys enable a first confidentiality mode for the encryption of voice signals and a second confidentiality mode for the encryption of control signals transmitted over an air interface to and from the MS.

9. The system for passing encryption keys of claim 8 wherein the means for adding the encryption keys to an information message also includes means for adding a parameter that indicates which confidentiality modes are desired to be activated.

10. The system for passing encryption keys of claim 7 further comprising: means for sending an order from the serving MSC to the MS to activate the confidentiality modes indicated by the parameter in the information message; means for determining by the serving MSC which confidentiality modes were actually activated; and means for sending a return information message from the serving MSC to the anchor MSC reporting the confidentiality modes that were actually activated.

11. The system for passing encryption keys of claim 10 wherein the information message is an ANSI-41 Information Forward Invoke message, and the return information message is an ANSI-41 Information Forward Return Result message.

12. An anchor Mobile Switching Center (MSC) in a radio telecommunications network for passing a mobile station's encryption keys from the anchor MSC to a serving MSC when the encryption keys are received by the anchor MSC after the mobile station (MS) has been handed off from the anchor MSC to the serving MSC, said anchor MSC comprising: means for receiving the encryption keys from a Home Location Register/ Authentication Center (HLR/AC) after the MS has been handed off; means for adding the encryption keys to an information message; and means for sending the information message from the anchor MSC to the serving MSC.