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Lecutre7 GSM Security and Addressing Authentication Stop unauthorised access to telecom services via cloning of a valid user identifier GSM anticipated.

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Presentation on theme: "Lecutre7 GSM Security and Addressing Authentication Stop unauthorised access to telecom services via cloning of a valid user identifier GSM anticipated."— Presentation transcript:


2 Lecutre7 GSM Security and Addressing

3 Authentication Stop unauthorised access to telecom services via cloning of a valid user identifier GSM anticipated this and defined an authentication procedure A user is challenged to provide proof of the claimed identity User accesses network and provides the user identifier Network sends a random number (RAND) to the MS Which together with Ki provide a response (SRES)

4 Ciphering MS sends a connection request to the network Among others, this request contains –Ciphering key sequence number (CKSN) –Mobile station class mark Mobile station class mark indicates the available ciphering algorithms (A5/X) in the mobile station

5 Ciphering VLR examines the CKSN and decides whether authentication is necessary Authentication not required a second time during the same network access Multiparty call- an example of second connection while another connection already exists A message sent to the MS in case authentication is necessary Message contains the random number, RAND SIM uses the RAND, value Ki and algorithm A3 to calculate SRES MS sends SRES to the VLR VLR compares this SRES with the one earlier sent by HLR/AuC Auth successful if both values are identical Immed after SRES, the MS calculates ciphering key Kc using RAND, Ki and algorithm A8 To activate ciphering, the VLR sends –Value Kc that the AuC has calculated –A reference to the chosen A5/X algorithm Via the MSC and the BSC to the BTS

6 Calculation of SRES & Kc

7 Ciphering BTS retrieves from the ENCR_CMD message –Kc –Info about the required ciphering algorithm BTS only forwards info about the A5/X algorithm in a CIPH_MOD_CMD message to the MS Which triggers MS to enable –Ciphering of all outgoing data and –Deciphering of all incoming information MS confirms the change to ciphering mode by sending a CIPH_MOD_COM message A5/X uses the current value of the frame number (FN) at the time and Kc as input parameters Output of this operation are ciphering sequences, each 114 bits long, one is needed for ciphering and the other one for deciphering First ciphering sequence and the 114 bits of “useful data” of a burst are XORed –To provide encrypted 114 bits that are actually sent over the Air-interface Ciphering sequences altered with every frame number Which in turn changes the encryption with every frame number Deciphering takes place exactly the same way but in the opposite direction

8 Ciphering

9 De-ciphering

10 Authentication = ? NSS RAND = RANDom number SRES = Signed RESponse Kc = Ciphering Key Ki = Identification Key RAND Kc RAND (128 bits) SIM card G S M Global GSM Mobility Card The Smart Card to use A8 A3 Ki A3 A8 MS AUC (A3 and A8) (RAND, SRES, Kc) SRES SRESm (32 bits) SRESm CIPHER MODE Ki (128 bits) A3A8A3A8 BSS OK Radio Interface Kc

11 A3 Ki RAND SRESm Purpose: Avoid logging of lost, stolen or forgery SIM-Cards. 5 Triplets 3 AUC (A3 and A8) (RAND, SRES, Kc) HLR MSC BTS BSS BSC RAND 4 SRESm RAND 6 SRESm Authentication 7 Ciphering Command 7 CIPHER MODE 3 2 VLR SRESm = SRES ? SRESm RAND

12 Ciphered data MSBTS Radio interface Frame Number (22 bits) Kc (64 bits) + Ciphering : exclusive-or + A5 Frame Number (22 bits) Block (114 bits) Data to transmit Received dataData to transmit Received data Block (114 bits) Block (114 bits) Block (114 bits)

13 BTS BSS BSC VLR (Rand, SRES, Kc) A5 Kc TDMA# + A8 Ki Rand Kc MSC Kc 2 Ciphered data 5 CIPHERING SET CIPHER MODE (Kc) 1 3 CIPHER MODE COMMAND 4 CIPHER MODE COMPLETE CIPHER MODE COMPLETE 6 Purpose: avoid communication to be tapped. !azer?tcyui p?sdr!f? j sdf!? okgqh

14 IMEI Mobile station equipment identity Not mandatory for the network operator to query the IMEI Purpose of the IMEI is to prevent passive theft protection EIR maintains information on stolen mobile equipment in a “black list,” which makes stolen mobile equipment useless

15 IMEI IMEI comprises following: A 24-bit-long type approval code (TAC) –Before any mobile equipment is brought into service, it undergoes a test to show that it complies with safety regulations and functionality requirements –Process called type approval, and the requirements are specified by GSM An 8-bit-long final assembly code (FAC) identifies the manufacturing facility A 24-bit-long serial number A spare field, currently not used

16 Type Approval Code TACFACSNR SP Final Assembly Code Serial number(SPare) TYPE APPROVED MOBILE IDENTIFICATION

17 IMEISV IMEI plus a software version number (SVN) Which can be modified by the manufacturer in case of a software update

18 IMSI International mobile subscriber identity An identifier for a GSM subscriber Part of the subscriber data stored on (SIM) card Uniquely identifies one subscription worldwide Structure similar to the ISDN number, defined in ITU-T Recommendation E.164

19 IMSI 15-digit number and is composed of : Mobile country code (MCC), Mobile network code (MNC) Mobile subscriber identification number (MSIN) MSIN of the IMSI not used as the subscriber’s telephone number To make tracking more difficult, IMSI used only as an identifier when the temporary mobile subscriber identity (TMSI) not available, e.g., for initial system connections

20 MCC & MNC Mobile country code A three-digit identifier Uniquely identifies a country (not a PLMN) Mobile network code A two-digit identifier Used (like the 3-bit-long NCC) to uniquely identify a PLMN

21 IMSI Attach/Detach IMSI detach informs network that –An MS will go into an inactive state –And is no longer available for incoming calls –For example, due to power down or because the SIM is removed MS sends an IMSI_DET_IND message to the network each time it is powered down VLR keeps track of this state This approach saves radio resources and processing time Call processing can switch to secondary call treatment –without first sending a PAGING message and then waiting for expiration of respective timers Secondary call treatment means initiating –Call forwarding –Voice mail, or –Telling caller that the subscriber currently not reachable Complementary to IMSI detach is IMSI attach It indicates to network that a mobile station is active again IMSI attach is related to periodic location updating The location updating procedure is utilized to perform IMSI attach



24 TMSI Temporary mobile subscriber identity Identifies a mobile subscriber, like the IMSI 4-byte-long Unlike the IMSI, TMSI has only temporary significance VLR assigns a TMSI upon location registration for confidentiality So not required to transfer the IMSI over the Air-interface frequently TMSI can take any value, except FF FF FF FFhex This value reserved in case SIM does not contain a valid TMSI

25 MSISDN Mobile subscriber ISDN Dir No of a mobile subscriber Example: is the directory number of a subscriber to the D1 network in Germany Country code (CC) identifies a country or region (e.g., 49 for Germany, 1 for the United States); National destination code (NDC) identifies the PLMN (e.g., 171 for the operator D1) Subscriber number (SN) is a unique identifier within the PLMN

26 MSRN Mobile station roaming number A temporary identifier used for mobile terminating calls –To route a call from the gateway MSC to the serving MSC/VLR VLR assigns MSRN to a mobile MSRN used solely to route an incoming call and contains no information to identify the caller or the called party Contains following codes: Country code (CC) is the prefix of a country National destination code (NDC) identifies the PLMN (e.g., 172 is the D2 operator of Germany); Temporary subscriber number (temp. SN) assigned by the serving MSC/VLR of the called subscriber

27 NDC National destination code Part of an ISDN number as defined by ITU-T in Recommendation E.164 Typically, the NDC addresses an area May also be used to address a service, just as the NDC 800 addresses free phone service in the United States In Germany, the NDCs 171 and 172 used to address the two GSM 900 operators

28 CKSN Ciphering key sequence number References to a ciphering key, Kc When a particular Kc is stored in the MS and the MSC/VLR, a CKSN is assigned as well Allows MS and network a negotiation of the Kc without compromising security by transmitting the value of Kc over the air Particularly when an MS tries to establish an additional or subsequent operation with the network In such a case, when the MS requests a connection, it sends its last valid CKSN to the VLR VLR then decides, based on the CKSN, if ciphering can start immediately or if another authentication is required VLR may decide to request another authentication, even if the CKSN matches the VLR’s entry

29 LMSI Local mobile subscriber identity VLR assigns it to a subscriber on a temporary basis Purpose is to expedite queries in the VLR Although no use for the LMSI in the HLR, but it still must be stored in the HLR HLR required to send the LMSI whenever data between the two databases exchanged

30 CI Cell identity A 2-byte-long hexadecimal identifier CI together with the location area (LAI) uniquely identifies a cell within a PLMN

31 Location area (LA) LA comprises at least one but typically several BTSs Defined for the following purpose: –An MS that changes the serving cell in the same location area does not need to perform a location update –When network tries to establish a connection to an MS for a mobile terminating call, PAGING message is sent to only those BTSs that belong to the current location area of the MS LA therefore, serves mainly one purpose –Reduction of signalling load Every BTS broadcasts the LA via the parameter location area identity (LAI)

32 Location area Identity (LAI) Even during an active call, LA communicated to the MS (particularly important in a handover) Shaded, one-digit field is a filler (1111bin) and Extends three-digit MCC to 2 bytes Actual location area code (LAC) is four digits long LAC is an identifier that can be assigned by the network operator All values, except 0000hex and FFFE hex allowed Those two values reserved for cases when the LAI on a SIM has been deleted

33 Registration: The Very First Location Updat 1. Channel allocation (Connection request procedure): –MS sends (on RACH) a CHANNEL REQUEST message –Network responds with IMMEDIATE ASSIGNMENT (on dedicated channel) 2. MS sends to BSS a LOCATION UPDATING REQUEST message with IMSI 3. VLR triggers and monitors the Authentication procedure and can also activate Ciphering procedure 4. VLR stores the LA of the MS and informs the HLR which: –stores VLR identity –downloads the subscriber profile, if the MS is allowed to roam 5. VLR may assign a TMSI and sends it to the MS in the LOCATION UPDATING ACCEPT message 6. MSC releases the connection

34 LAI HLR IMSI VLR id TMSI IMSI TMSI Release VLR IMSI TMSI LAI MSC BTS BSS BSC Registration: the Very First Location Update TMSI 5

35 BSIC Base station identity code An identifier for a BTS Does not uniquely identify a single BTS, since it is reused several times per PLMN Purpose of the BSIC is to allow the MS to identify and distinguish among neighbor cells, even when neighbor cells use the same BCCH frequency Since BSIC is broadcast within SCH of a BTS, MS need not even have to establish a connection to a BTS to retrieve the BSIC

36 BSIC Consists of the –Network color code (NCC), which identifies the PLMN –Base station color code (BCC)

37 NCC Network color code 3-bit-long code Identifies the PLMN Is part of the BSIC and Is broadcast in the synchronization channel

38 BCC Base station color code 3-bit-long parameter Part of the BSIC Used to distinguish among the eight different training sequence codes (TSCs) The BCC is used by the MS (Mobile Station) to distinguish between cells using the same frequencies, when the MS is deciding on which cell to select and to lock- on to.MSMobile Station

39 PIN Personal identification number A four- to eight-digit number Provides limited protection against unauthorized use. Can be changed by the user and is stored on the SIM. Optional and can be disabled When enabled, the PIN needs to be entered at power up When the wrong PIN entered three consecutive times, the SIM is blocked and Only the PIN unblocking key (PUK) can release the Pin

40 PUK PIN unblocking key A 10-digit code stored on the SIM Cannot be altered by the user Unblocks a SIM that was blocked due to wrong PIN entry three consecutive times

41 The Abis-Interface Interface between the BTS and the BSC A PCM30 interface, like all the other terrestrial interfaces in GSM Specified by ITU in the G-series of recommendations Transmission rate is Mbps Partitioned into 32 channels of 64 Kbps each GSM compression techniques pack up to 8 GSM traffic channels into a single 64-Kbps channel (for half rate channles) and 4 GSM channles to a single 64- Kbps channels for full rate channels. Like B interface GSM never specified the Abis-interface in every detail Abis-interface regarded as proprietary leading to –Variations in the Layer 2 protocol between manufacturers –Different channel configurations Consequence - normally, a BTS from manufacturer A cannot be used with a BSC from manufacturer B

42 Channel Configurations Two possible channel configurations of the Abis- interface ahead Fixed mapping of the air-interface traffic channels (Air0, Air1, …) of a BTS onto a TS of the Abis- interface Advantage-possible to determine which Abis TS will be used when a particular air-interface channel is assigned

43 Star configuration and Serial connection (4x BTSs with two TRX each)

44 Alternatives for Connecting the BTS to the BSC Line resources on the Abis-interface usually not used efficiently As a BTS, typically, has only a few TRXs resulting small traffic volume capability Consequently, the line between the BTS and the BSC used only to a fraction of its capacity Star configuration- a case of a BTS with four TRXs, in which only 47% of the 2 Mbps actually needed Shaded areas mark the unused channels. For BTS with only one TRX, value goes down to 16% Such waste of resources has a historical background

45 Alternatives for Connecting the BTS to the BSC GSM specifications allows for a BTS to have up to 16 TRXs Since a single 2-Mbps link able to support only up to 10 TRXs incl O&M signaling So two 2-Mbps links req to connect such a BTS to the BSC Even fewer resources req on the Abis-interface for a BTS with a smaller number of TRXs Remaining resources cannot easily be used Practically- optimum for a BTS is in the range of one to four TRXs This compromise reflects parameters : –Capacity: How many traffic and signalling channels does a BTS need to provide, on average and during busy hours to avoid overload –Available frequency range: What is the minimum distance between BTSs after which frequencies can be reused

46 Alternatives for Connecting the BTS to the BSC N/W operators worldwide-bad experiences particularly with the latter point With the Introduction of Digital radio it was assumed that ACI and CCI impact will reduce However, the assumption was soon proved wrong as more and more interference problems b/w BTSs degraded the QoS Hence, in urban areas use more cells with fewer TRXs and smaller output power rather fewer cells with more TRXs and high output power Such a configuration req a larger no of BTSs to cover any given area Connecting larger no of BTSs to BSCs in turn req a larger no of Abis interfaces Serial connection configuration introduced –Because of above stated trend –High costs of links b/w BTS and BSC –Low efficiency when using such links

47 BTS Connection in a Serial Configuration BTSs connected in a line or a ring topology Only one BTS, for the line topology or two BTSs, for the ring topology, physically connected to the BSC For network operator, serial approach saves line cost in comparison to star configuration Serial connection- more efficient use of when co located or sectored BTSs used Disadvantage- a single link failure causes loss of connection to a large number of BTSs

48 BTS Connection in a Serial & Star Configuration

49 Signaling on the Abis-Interface OSI Protocol Stack on the Abis-Interface Abis-interface utilizes Layers 1 through 3 of the OSI protocol stack Layer 1 forms the D-channel LAPD is in Layer 2 Layer 3 is divided into –TRX management (TRXM) –Common channel management (CCM) –Radio link management (RLM) –Dedicated channel management (DCM)

50 OSI Protocol Stack on the Abis Interface D channel (data) is a telecommunications term which refers to the ISDN channel in which the control and signalling information is carried.telecommunicationsISDNsignalling

51 Radio Link Layer Management (RLM): Procedures to establish, modify and release connection of link layer (LAPDm) to MS at the air interface Um. Dedicated Channel Management (DCM): Procedures to start ciphering, transfer of channel measurement reports of a MS, transmitter power control of MS and BTS e.t.c Common Channel Management (CCM): Procedure for transferring channel requests from MS (received on RACH), modification of BCCH broadcast information, channel assignment to MS e.t.c TRX management (TRXM): Procedure for the transfer of measurement of free traffic channels of a TRX to the BSC e.t.c. Four groups of Layer 3, Traffic management messages

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