Telecommunications – Radiotelephone system – Security or fraud prevention
Reexamination Certificate
1998-10-30
2002-01-22
Kincaid, Lester G. (Department: 2682)
Telecommunications
Radiotelephone system
Security or fraud prevention
C455S433000, C455S456500, C455S067150
Reexamination Certificate
active
06341220
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a method and apparatus for preventing misuse of a copied subscriber identity in a mobile communication system.
In all telecommunication networks, both the users and the network operator must be protected as well as possible against undesired invasions by third parties. Consequently, there is need in the networks for many types of security functions. The most important features relating to the security of the network are (1) protecting the information conveyed by the network and (2) authenticating the users and controlling their access. The most important safety measure for protecting information in the foreseeable future is some kind of encryption. Authentication is a way by means of which an effort is made to ensure that information originates from the source it is claimed to, originate from. Typically, it is based on passwords and keys. Access rights refer to the ability to transmit and/or receive via a transmission channel. Access mechanisms are based on some kind of a password or key.
As transmission to mobile subscribers takes place by a radio connection, radio connected Public Land Mobile Networks (PLMN) are particularly sensitive to unauthorized users using their resources and eavesdropping on the information exchange on the radio path. This results from the possibility to listen to and to transmit radio signals from anywhere without having to gain access to the equipment of the user or the operator. It can be seen that better standards of security are required in PLMN networks than in conventional communication networks.
The basic structure of the Pan-European digital cellular radio system GSM (Global System for Mobile Communications) is illustrated by FIG.
1
. The structure of the GSM network consists of two parts: a Base Station Subsystem (BSS) and a Network Subsystem (NSS). The BSS and mobile stations MS communicate by radio connections. In the BSS, every cell is served by a Base Transceiver Station (BTS). A group of base stations BTS are connected to a Base Station Controller (BSC) whose function is to control the radio frequencies and channels used by the BTS. The base station controllers BSC are connected to a Mobile Switching Center (MSC). The function of the MSC is to switch calls that involve at least one MS. Some MSCs are coupled to other communication networks, such as a Public Integrated Services Network (PISN), and they comprise switching functions for processing calls to and from these networks. Such mobile switching centers are referred to with the term Gateway MSC (GMSC).
Routing of calls involves two types of databases. In a Home Location Register (HLR) are stored, permanently or semi-permanently, subscriber data of all the subscribers of the network, such data containing information on all the services the subscriber is entitled to, and the subscriber's present location. The second register type is a Visitor Location Register (VLR). The VLR is usually connected to one MSC, but it may also serve a plurality of them. The VLR is commonly integrated in the MSC. Such an integrated network element is referred to as a VMSC (VLR+MSC). When the MS is active (it has registered in the network, and may make or receive a call), the majority of the subscriber data concerning the MS included in the HLR is loaded (copied) into the VLR of the MSC in whose service area the MS is located.
The GSM system comprises a most secure authentication system. This also holds true of the derivatives of the GSM system, such as the DCS. The authentication system is based on a so-called challenge and response principle. Upon drawing up a subscriber contract, a secret subscriber authentication key (Ki) and an International Mobile Subscriber Identity (IMSI) are assigned to the subscriber. The Ki is stored in a network element serving this purpose in the GSM network, referred to as an Authentication Center (AUC), which is associated with or connected to the subscriber's HLR. The AUC also contains a ciphering algorithm known as A
8
, an authentication algorithm known as A
3
, and a generator of random numbers RAND. On the basis of Ki and RAND, a ciphering key Kc is produced by the algorithm A
8
. Similarly on the basis of Ki and RAND, a Signed Response (SRES) is produced by applying the A
3
algorithm. These three parameters, RAND, Kc and SRES form a triplet which is characteristic to the subscriber and which will be employed in later authentication and ciphering.
Referring to
FIG. 2
, the AUC comprises a database
20
to which the authentication key Ki of each subscriber in the GSM network is stored. A subscriber's Ki may be retrieved from the database
20
by using the IMSI as an index.
To avoid computing and transmitting the triplet every time it is needed, the AUC/HLR computes several triplets for each subscriber in advance, and if required supplies them to the VLR to which they are stored. The MSC/VLR has at all times an unused triplet at its disposal for each visiting subscriber. The high level of security sets the condition that a triplet is used once only, for one connection, after which it is destroyed.
FIG. 4
illustrates a few of such subscriber-specific triplets. A security parameter file
40
comprises n triplets for each
1
-n IMSIs. Such a reserve in the security parameter file
40
is established when a subscriber for the first time registers in a visited MSC and a VLR. It is part of the subscriber data that were loaded from the HLR in connection with an INSERT SUBSCRIBER DATA message.
Referring to
FIG. 5
, when a subscriber has used all the triplets at his disposal, a request is made to the AUC/HLR to compute and send back a new series. The supplementing procedure of the triplets consists of two messages: SEND PARAMETERS and a response thereto SEND PARAMETERS RESULT. The former contains the mobile subscriber's IMSI which is used to retrieve the Ki for computing the triplets, as disclosed in connection with FIG.
2
. The triplets computed are sent to the MSC/VLR in the SEND PARAMETERS RESULT message and they are stored in the VLR.
Referring again to
FIG. 4
, the MS transmits an access request to the MSC/VLR. The MSC/VLR retrieves a triplet assigned to the MS from the security parameter file
40
by using the IMSI as an index. On the one hand the MSC/VLR conveys the Kc value to the BSC's channel equipment to be used in traffic channel coding, and on the second hand it conveys the RAND value to the MS in an AUTHENTICATION REQUEST message. This is illustrated by block
41
in FIG.
4
. On the basis of RAND, the MS computes the other values of the triplet (SRES and Kc).
Now referring to
FIG. 3
, the mobile station comprises in its memory a copy of the mobile subscriber's authentication key Ki, as well as the encryption algorithm A
8
and the authentication algorithm A
3
. Upon receiving an AUTHENTICATION REQUEST message, the mobile station MS extracts the RAND from the message and feeds it and the stored Ki to the algorithms A
3
and A
8
, respectively, to compute a signed SRES and the encryption key Kc. The computed SRES is conveyed to the MSC/VLR in an AUTHENTICATION RESULT message to complete the authentication, as illustrated in
FIGS. 4 and 5
.
Referring to
FIG. 4
, the MSC/VLR extracts the value of the SRES from the AUTHENTICATION RESULT message (block
42
) and retrieves the stored value of the SRES from the file
40
(block
43
). Following this, for this connection and prior to other processing, the MSC/VLR authenticates the mobile subscriber by checking that the SRES computed in the AUC/HLR is identical with the SRES (block
44
) computed in the mobile station. If the two values are identical, access is allowed. If not, access is denied (block
46
).
In the exemplary GSM system, the authentication of the mobile subscriber requires an identity unit, which is subscriber-specific. The terminal equipment proper is therefore not confined to a specific subscriber. The subscriber identity module, such as a SIM card, is a functional card or a smart card which is placed in the mobi
Altera Law Group LLC
Kincaid Lester G.
Nokia Telecommunications Oy
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