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  • 8/13/2019 1.11 Authentication and Encryption

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    1.11 Authentication and EncryptionGSM takes lots of measures to protect the safety of system, such as using Temporary MobileSubscriber Identity (TMSI) to protect IMSI, using Personal Identification Number (PIN) to protect SIMcard, authentication through authentication center (AUC) for network access, encryption, and equipmentidentity register.Authentication and encryption require a group of three parameters that generated in AUC. Each client isassigned a Mobile Station International ISDN Number (MSISDN) and IMSI when registers in GSMnetwork. IMSI is preserved onto SIM card through SIM printer and SIM printer will generate acorresponding client authentication value Ki that is stored in SIM card and AUC as permanentinformation. AUC has a pseudo number generator used to generate a random number RAND. GSMdefines algorithm A3, A8, and A5 that are used for authentication and encryption. In AUC, RAND and Kitogether produce a response number SRES through A3 authentication algorithm and a Kc through A8encryption algorithm. RAND, Kc, and SRES form a three-parameter group of client. This group is storedin the data base of this client in HLR. Generally, AUC transfers five groups of parameters to HLR forautomatic storage. HLR can save ten groups of such parameters. When MSC/VLR requests for three-parameter group transfer, HLR sends five groups at the same time for MSC/VLR to use one by one.

    When there are two groups left, MSC/VLR will request for transfer again.1.11.1 AuthenticationAuthentication is the process that GSM network checks whether the IMSI or TMSI from MS at radiointerface is valid or not. The purpose of authentication is to avoid unauthorized access to GSM networkand the theft of private information by illegal users. Authentication also provides parameters for MS tocalculate new encryption key.The network initiates authentication procedure in the following situations:MS requesting for the change of information in VLR or HLR;Service access, including MS originated call, MS terminated call, MS activation and deactivation, andsupplementary services;The first network access after MSC/VLR reboot;Mismatching Cipher key Sequence;Whether to initiate authentication procedure depends on if the Kc value of the last service processingstored in network consistent with that of the present access stored in MS. If consistent, authenticationprocedure can be escaped and this Kc value is used directly for encryption; if not, Kc value needs to berecalculated. MS does not send Kc value to network through radio path for the sake of privacy.

    Therefore, Cipher Key Sequence Number (CKSN) is introduced. CKSN is sent to MS by MSC/VLRthrough authentication request message during the last network access. It is stored in both SIM cardand MSC/VLR. During the initial access of MS, CKSN is sent to MSC/VLR through the initial requestmessage of SABM frame. MSC/VLR compares it with the last CKSN. If they are not consistent,authentication is required before encryption. If CKSN=0, it means no Kc is assigned. Authenticationprocedure is initiates and controls by network. MSC/VLR sends an authentication request message toMS to initiate authentication procedure and T3260.I. Authentication Success2) AUTHENTICATION REQUEST contains a RAND (128 bits) and a CKSN. The Ki and RAND togethergenerate a SERS (32 bits) through algorithm A3 and a Kc (64 bits) through algorithm A8. The new Kcreplaces the former key and is stored in SIM card together with CKSN.3) MS sends AUTHENTICATION RESPONSE to network. After receiving this message, the networkstops T3260 and checks its validity (network compares it with the SERS generated by Ki and RANDthrough algorithm A3 and check whether they are consistent or not), and then enters the subsequentprocedures, such as encryption.II. Authentication RejectIf authentication fails, it means AUTHENTICATION RESPONSE is invalid.If the MS uses TMSI, the network will initiate identity procedure. If the IMSI provided by the MS isdifferent from that in network, the network will restart the authentication procedure; if the IMSI is correct,the network will send AUTHENTICATION REJECT to the MS.If the MS uses IMSI, the network will send AUTHENTICATION REJECT directly to MS. After sendingAUTHENTICATION REJECT message, the network releases all the MM connections underestablishment and restarts the procedure for RR connection release.After receiving AUTHENTICATION REJECT message, MS sets the roaming disabled flag and deletesinformation such as TMSI, LAI, and cipher key.If MS receives AUTHENTICATION REJECT message in IMSI DETACH INITIATED state, it stops T3220after RR connection is released. If possible, MS initiates local release procedure after the normalrelease procedure or T3220 timeout; if not (such as the IMSI detach after switch off), MSRR exitsabnormally.If MS receives AUTHENTICATION REJECT message in other state, it exits all MM connections and callre-establishment procedures, stops T3210 and T3230, sets and starts T3240 to enter WAIT FORNETWORK COMMAND state and wait for the release of RR connection; If RR connection is notreleased after T3240 timeout, MS will exit RR connection abnormally. Under the two conditions above,MS enters MM IDLE and NO IMSI state.1.11.2 EncryptionEncryption occurs in service requests such as location updating, service access, and inter-officehandover. It requires the support of GSM network equipment (especially BTS), as well as the encryptionability of MS.I. Signaling Procedure1) MSC sends BSC a Ciphering Mode CMD that contains encryption algorithm, Kc, and whether the

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    Archives 2009(56)

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    3.8 Network Capacity Comparison For thecomparis...

    3.7 Multiple Reuse Pattern Technology3.7.1 Basic...

    3.6 Concentric Cell Technology 3.6.1 Concept I...

    3.5 Aggressive Frequency Reuse Technology3.5.1 ...

    3.4 Normal Frequency Reuse Technology 3.4.1 C...

    3.3 Frequency Planning Principle Generally, w hen ...

    3.2 Frequency Division and C/I Requirement 3.2.1 ...

    3 GSMFrequency Planning 3.1 Overview Frequency...

    2.13 Conclusion Network planning is the foundatio...2.12 Repeater Planning 2.12.1 Application Backg...

    2.11 Tunnel Coverage 2.11.1 Characteristic of T...

    2.10 Design of Indoor Coverage System2.10.1 Ch...

    2.9 Dual-Band Network Design 2.9.1 Necessity for...

    2.8 Location Area Design 2.8.1 Definition of Loc...

    2.7 Design of Base Station Address 2.7.1 Addressd...

    2.6 Base Station Number Decision After traffic an...

    2.5 Traffic Analysis 2.5.1 Traffic Prediction an...

    2.4 Network Structure Analysis When consideringt...

    2.3 Coverage Analysis 2.3.1 Area Division I. Typ...

    2.2 Planning Foundation 2.2.1 Coverage andCapacit...

    2 GSMRadio Network Planning 2.1 Overview Thede...

    1.17 CBS Cell Broadcast Service (CBS) is similar ...

    1.16 Call Re-Establishment 1.16.1 Introduction ...

    1.15 HO As a key technology in the cellular mobil...

    1.14 MS Originated Call Flow 1.14.1 Enquiry Af te...

    1.13 MS Originating Call Flow The MS needs to set ...

    1.12 Location Update In GSM, the paging informati...

    1.11 Authentication and Encryption GSMtakes lots...

    1.10 Immediate Assignment Procedure The purposeo...

    1.9 Power Control 1.9.1 Power Control Overview P...

    1.8 Discontinuous Reception and DiscontinuousTra...

    1.7 Frequency Hopping With the ever growing traff...

    1.6 Cell Selection and Re-Selection 1.6.1 Cell S...

    1.5 SystemInformation Systeminformation is sent ...

    1.4 Timing advance Signal transmission has a dela...

    1.3 Data Transmission Radio channel has totally d...1.2 Multiple Access Technology and Logical

    Channel...

    1 GSMPrinciples and Call Flow 1.1 GSMFrequency...

    Radio Network Planning Optimization The objective ...

    History of GSM1 GSMDevelopment Mobile

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    MS is required to add IMEI in Ciphering Mode CMP.2) BSC decides the final algorithm based on the encryption algorithm in Ciphering Mode CMD, theencryption algorithm that BSC allows, and the encryption algorithm that MS supports, and then informBTS.3) BSC sends MS Ciphering Mode CMD to inform MS of the selected encryption algorithm.4) After receiving Ciphering Mode CMD, MS starts the transmission of ciphering mode and sendsCiphering Mode CMP to the system.5) After receiving the Ciphering Mode CMP from MS, BSC transfer it to MSC.II. Procedure DescriptionA5 algorithmGSM protocol specifies eight kinds of encryption algorithm from A5/0 to A5/7. A5/0 stands for noencryption. The encryption procedure is initiated by the network. The encryption information of CipherMode CMD specifies the required encryption algorithm. The algorithm that generates encrypted code iscalled A5 algorithm. It calculates by using the Kc (64 bits) and the current frame number (22 bits) togenerate a 114-bit encryption sequence and then implements XOR operation with the 114-bit burst. Twoencryption sequences are used for uplink and downlink. For each burst, one sequence is used for MSencryption and BTS decryption, the other sequence is used for BTS encryption and MS decryption.Encryption algorithm selectionWhen MS initiates call request, the SABM frame carries Classmark 1 or 2 to indicate whether the MSsupports algorithm A5/1, A5/2, or A5/3, and reports Classmark 3 in CLASS MARK CHANGE to furtherindicate whether the MS supports Algorithm A5/4, A5/5, A5/6, or A5/7(In system information, ifECSC=1, MS reports Classmark 3 immediately; if ECSC = 0, the Classmark 3 is reported afterCLASSMARK ENQUIRY is initiated by the network. Therefore, the configuration of ECSC = 1 isrecommended when the encryption is used). MSC sends encryption command based on theconfiguration of secret data. BSC chooses the intersection of the encryption algorithm allowed in thecommand sent by MSC, the encryption algorithm allowed in BSC data configuration, and the encryptionalgorithm supported in the MS report. In the intersection, BSC selects a proper algorithm based on thepriority level of A5/7 > A5/6 > A5/5 > A5/4 > A5/4 > A5/3 > A5/2 > A5/1 > A5/0.Encryption in handoverThe HANDOVER REQUEST contains the encryption information unit that indicates the requiredencryption algorithm and key. If one of the two A interfaces of BSS is in PHASE I, due to the limitationof ETSIGSM PHASE I protocol (no ciphering mode setting information unit in handover command), thetwo A interfaces match only when they share the same encryption algorithm (such as A5/2) to ensurethe normal inter-BSC handover. Otherwise, special treatment has to be made to the target MSC ortarget BSC (or the source MSC or source BSC) to change the handover command for inter-BSChandover.For the interconnection of A-interfaces when the encryption is used, whether special data configurationis required for BSC and MSC must be considered.1.11.3 TMSI ReallocationAfter authentication and encryption, the system sends CM SERVICE ACCEPT or TMSI reallocationcommand to MS and initiates T3250.When MS registers in the location area for the first time, the network allocates a TMSI to it. When theMS leaves this location area, it releases the TMSI. When the MS receives the TMSI reallocationcommand, it saves the TMSI and LAI and sends TMSI reallocation complete message. After receivingthis message, the network stops T3250.

    If the system cannot identify TMSI of the MS, for example, when the data base error occurs, the MSmust provide its IMSI. The identification program is initiated before the TMSI reallocation to request forthe IMSI.The identification program sends identity request message to the MS, after receiving this message, theMS provides its IMSI by sending identity response message to the network. When this procedure isover, authentication, encryption, and IMSI reallocation are implemented if required.1.11.4 Exceptional SituationsI. AuthenticationRR connection failureIf the network detects RR connection failure before receiving AUTHENTICATION RESPONSE, itreleases all the MM connections and terminates all the active MM procedures.T3260 timeoutT3260 is started when MSC sends authentication request to BSC and stops when MSC receivesAUTHENTICATION RESPONSE. If the T3260 times out before the AUTHENTICATION RESPONSE isreceived, the network releases RR connection, terminates the authentication procedure and all theactive MM procedures, and then releases all the MM connections and initiates RR connection release

    procedure.Unregistered SIM cardIf the SIM card of the MS is not registered, the network sends AUTHENTICATION REJECT messagedirectly to the MS.II. EncryptionEncryption rejectIf BSS does not support the encryption algorithm specified in CIPHERING MODE CMD, it sendsCIPHER MODE REJECT message to MSC.If the encryption is initiated in BSS before MSC requests for the change of encryption algorithm, BSSalso sends CIPHER MODE REJECT message to MSC.Un-encrypted MSThe CIPHERING MODE COMMAND message is valid when:The un-encrypted MS receives CIPHERING MODE COMMMAND message that requires encryption.The un-encrypted MS receives CIPHERING MODE COMMMAND message that requires non-encryption.The encrypted MS receives CIPHERING MODE COMMMAND message that requires non-encryption.

    In other cases, CIPHERING MODE COMMAND is considered wrong. The MS sends RR STATUSmessage with the cause of protocol error and performs no action.III. TMSI ReallocationRR connection failureIf RR connection fails before TMSI reallocation complete message is received, all the MM connectionsare released and both the old and new TMSIs are saved during a certain recovery time.T3250 timeoutT3250 is started when MSC sends TMSI_ REALL_ CMD message or LOC UPD ACC message with the

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    new TMSI and stops when MSC receives TMSI _REALL_COM. If T3250 times out before the TMSI_REALL_COM is received, MSC sends CLEAR COM message to release RR connection and terminateTMSI reallocation.: ourdot 1:06

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