GSM Mobile Security

GSM Mobile SecurityGSM Mobile SecurityPrepared by:Prepared by:

Leen F. ArikatLeen F. Arikat

IntroductionIntroduction With GSM, systems for mobile communication

reached a global scale. In the western world, it seems everyone has their own mobile phone, and GSM has taken more and more of the market.

GSM allows users to roam seamlessly between networks, and separate the user identity from the phone equipment. In addition the GSM system provides the functional basis for the 3rd generation mobile system, UMTS.

Some GSM FactsSome GSM Facts

Nearly 1 billion subscribers worldwideNearly 1 billion subscribers worldwideEstimated that worldwide mobile phone fraud will reach Estimated that worldwide mobile phone fraud will reach $40 billion dollars$40 billion dollarsUS Law enforcement agents have found that 80% of US Law enforcement agents have found that 80% of drug dealers arrested in US using cloned mobile phones.drug dealers arrested in US using cloned mobile phones.Ironically, Pablo Escobar the top Columbian drug dealer Ironically, Pablo Escobar the top Columbian drug dealer was tracked down by monitoring his mobile phone was tracked down by monitoring his mobile phone activity.activity.Two aspects relevant to a Forensic AnalystTwo aspects relevant to a Forensic Analyst

Has the phone been used for a criminal act?Has the phone been used for a criminal act? Can the phone be used to secure a conviction?Can the phone be used to secure a conviction?

Some GSM FactsSome GSM Facts

The European Telecommunication Standards The European Telecommunication Standards Institute (ETSI) regulates the GSM standard Institute (ETSI) regulates the GSM standard (all 4000 pages of it!).(all 4000 pages of it!).

Any equipment used on a GSM network has Any equipment used on a GSM network has to have approval by the ETSI.to have approval by the ETSI.

All MS’s are independent from any network.All MS’s are independent from any network.

What are the components of a What are the components of a GSM network?GSM network?

Subscriber EquipmentSubscriber Equipment

The Mobile StationThe Mobile Station (MS) is the user (MS) is the user equipment in GSM. It is what the user can equipment in GSM. It is what the user can see of the GSM system. see of the GSM system.

The station consists of two entities:The station consists of two entities:

• The Mobile Equipment (the phone itself)The Mobile Equipment (the phone itself)

• The Subscriber Identity Module (SIM)The Subscriber Identity Module (SIM)

Components of a GSM network Components of a GSM network (cont..)(cont..)

The Switching System (SS)The Switching System (SS)

Home Location Register (HLR) - A database which stores data Home Location Register (HLR) - A database which stores data about GSM subscribers, including the Individual Subscriber about GSM subscribers, including the Individual Subscriber Authentication Key (Ki) for each Subscriber Identity Module (SIM).Authentication Key (Ki) for each Subscriber Identity Module (SIM).

Mobile Services Switching Center (MSC) - The network element Mobile Services Switching Center (MSC) - The network element which performs the telephony switching functions of the GSM which performs the telephony switching functions of the GSM network. network.

Visitor Location Register (VLR) - A database which stores Visitor Location Register (VLR) - A database which stores temporary information about roaming GSM subscribers.temporary information about roaming GSM subscribers.

Authentication Center (AUC) - A database which contains the Authentication Center (AUC) - A database which contains the International Mobile Subscriber Identity (IMSI) the Subscriber International Mobile Subscriber Identity (IMSI) the Subscriber Authentication key (Ki), and the defined algorithms for encryption.Authentication key (Ki), and the defined algorithms for encryption.

Equipment Identity Register (EIR) - A database which contains Equipment Identity Register (EIR) - A database which contains information about the identity of mobile equipment in order to information about the identity of mobile equipment in order to prevent calls from stolen, unauthorized, or defective mobile stations.prevent calls from stolen, unauthorized, or defective mobile stations.


The Base Station System (BSS)The Base Station System (BSS)

Base Station Controller (BSC) - The network element Base Station Controller (BSC) - The network element which provides all the control functions and physical links which provides all the control functions and physical links between the MSC and BTS. The BSC provides functions between the MSC and BTS. The BSC provides functions such as handover, cell configuration data, and control of such as handover, cell configuration data, and control of radio frequency (RF) power levels in Base Transceiver radio frequency (RF) power levels in Base Transceiver Stations.Stations.

Base Transceiver Station (BTS) - The network element Base Transceiver Station (BTS) - The network element which handles the radio interface to the mobile station. which handles the radio interface to the mobile station. The BTS is the radio equipment (transceivers and The BTS is the radio equipment (transceivers and antennas) needed to service each cell in the network.antennas) needed to service each cell in the network.


The Operation and Support System (OSS)The Operation and Support System (OSS)

Message Center (MXE) - A network element Message Center (MXE) - A network element which provides Short Message Service (SMS), which provides Short Message Service (SMS), voice mail, fax mail, email, and paging.voice mail, fax mail, email, and paging.

Gateway Mobile Services Switching Center Gateway Mobile Services Switching Center (GMSC) - A network element used to (GMSC) - A network element used to interconnect two GSM networks. interconnect two GSM networks.

The SubscriberThe Subscriber

How to Identify a SubscriberHow to Identify a SubscriberEvery mobile subscriber Every mobile subscriber is issued with a smart is issued with a smart card called a Subscriber card called a Subscriber Identity Module (SIM)Identity Module (SIM)As physical evidence As physical evidence the SIM provides details the SIM provides details printed on the surface printed on the surface of;of;

Name of the Network Name of the Network ProviderProvider

Unique ID NumberUnique ID Number

Electronic Access to the SIMElectronic Access to the SIM

Every SIM can be protected by a Personal Every SIM can be protected by a Personal Identification Number (PIN)Identification Number (PIN) Set at point of manufactureSet at point of manufacture Can be changed by the SubscriberCan be changed by the Subscriber Four digit codeFour digit code Usually 3 attempts before phone is blockedUsually 3 attempts before phone is blocked

Bypassing the PIN requires the Pin Unblocking Bypassing the PIN requires the Pin Unblocking Key (PUK)Key (PUK) 8 digit code8 digit code Set by manufacturerSet by manufacturer Maximum 10 attempts before phone is permanently Maximum 10 attempts before phone is permanently

blockedblocked

What Can Be Extracted From A What Can Be Extracted From A SIM?SIM?

As SIM is a smart card it hasAs SIM is a smart card it has A processorA processor Non-volatile memoryNon-volatile memory

Processor is used for providing access to the Processor is used for providing access to the data and securitydata and security

To access the data we need;To access the data we need; Standard smart card readerStandard smart card reader SIM access SoftwareSIM access Software

Data stored in binary filesData stored in binary files

What Can Be Extracted From A What Can Be Extracted From A SIM?SIM?

Ideally an Analyst would download an image of the Ideally an Analyst would download an image of the contents and compute a hash value of the contents as a contents and compute a hash value of the contents as a means of validating originality of contentmeans of validating originality of contentAt present files are downloaded At present files are downloaded traditionallytraditionally

SoftwareSoftwareSim Manager ProSim Manager ProChipItChipItSimScanSimScan

Cards4Labs only available to Law Enforcement AgenciesCards4Labs only available to Law Enforcement AgenciesProduces a text report of content rather than downloading.Produces a text report of content rather than downloading.

29 files stored on a SIM29 files stored on a SIM

Location Information FileLocation Information File

The bytes 5-9 of the LOCI contain the network The bytes 5-9 of the LOCI contain the network Location Area Identifier (LAI) codeLocation Area Identifier (LAI) codeNetwork Operator specificNetwork Operator specificThis data is retained when the MS is powered downThis data is retained when the MS is powered downUpdated as MS moves from one location to anotherUpdated as MS moves from one location to anotherAnalyst can determine which location the MS was Analyst can determine which location the MS was present in when last used.present in when last used.Location Areas can contain many cells.Location Areas can contain many cells.LOCI DOES NOT DETAIL WHICH CELL!LOCI DOES NOT DETAIL WHICH CELL!Cell data not stored on SIM.Cell data not stored on SIM.

FileFile PurposePurpose SizeSize

LOCILOCI Location InformationLocation Information 11 bytes11 bytes

Serial NumberSerial Number

Integrated Circuit Card IdentifierIntegrated Circuit Card Identifier

Corresponds to the number printed on the Corresponds to the number printed on the surface of the SIMsurface of the SIM

Identifies the SIMIdentifies the SIM


ICCIDICCID Serial NumberSerial Number 10 bytes10 bytes

Subscriber IdentifierSubscriber Identifier

International Mobile Subscriber IdentityInternational Mobile Subscriber Identity

As stored in the HLR/VLR’s on the networksAs stored in the HLR/VLR’s on the networks

Unique ID for every subscription on the Unique ID for every subscription on the Operator’s networkOperator’s network

FileFile PurposePurpose SizeSize IMSIIMSI Subscriber IDSubscriber ID 9 bytes9 bytes

Phone NumberPhone Number

Mobile Station International ISDN numberMobile Station International ISDN number


MSISDNMSISDN Phone NumberPhone Number variablevariable

Text Message Data (SMS)Text Message Data (SMS)

Short Message Service is a popular Short Message Service is a popular communication methodcommunication method

Most SIM’s have 12 slots for storing messagesMost SIM’s have 12 slots for storing messages Modern MS’s allow storage on the device as wellModern MS’s allow storage on the device as well


SMSSMS The text messagesThe text messages n * 176 bytesn * 176 bytes

SMSPSMSP Message parametersMessage parameters variablevariable

SMSSSMSS Status of the messageStatus of the message variablevariable

Text Message Data (SMS) - Text Message Data (SMS) - StatusStatus

When user deletes a message only the status flag is changed

Therefore, providing the message has not been overwritten any message in a slot can be recovered and translated using software

ValueValue InterpretationInterpretation

0000000000000000 UnusedUnused

0000000100000001 Mobile terminated message, readMobile terminated message, read

0000001100000011 Mobile terminated message, unreadMobile terminated message, unread

0000010100000101 Mobile originated message, sentMobile originated message, sent

0000011100000111 Mobile originated message, not sentMobile originated message, not sent

Threats to SIM DataThreats to SIM DataKnowledgeable criminals will be aware of the properties of the SIM Knowledgeable criminals will be aware of the properties of the SIM and thus manipulate them.and thus manipulate them.Greater threat is that of cloning SIM data for illicit useGreater threat is that of cloning SIM data for illicit use Two key pieces of dataTwo key pieces of data

IMSIIMSIThe data encryption key (Ki)The data encryption key (Ki)

IMSI can be obtained;IMSI can be obtained;Directly from the SIM using a scanning softwareDirectly from the SIM using a scanning softwareEaves-dropping on the networks for unencrypted Eaves-dropping on the networks for unencrypted transmission of the IMSItransmission of the IMSI

Ki cannot normally be obtained directly as it is derived from an Ki cannot normally be obtained directly as it is derived from an encryption algorithm stored on the SIMencryption algorithm stored on the SIM

However, if the encryption algorithm is weak then it is However, if the encryption algorithm is weak then it is possible to feed numberspossible to feed numbers

Threats to SIM DataThreats to SIM Data

Obtaining blank SIMsObtaining blank SIMs These cards can be ordered from the same

source where network providers get their cards.

The card must then be programmed with a special tool for programming of fresh cards. Such a tool is distributed together with the Sim-Scan package.

An attacker could also get hold of a generic smart card and smart card programmer, and then program the card to act as a SIM.

The EquipmentThe Equipment

Generic PropertiesGeneric Properties

All MS’s have GSM standards on how they access and All MS’s have GSM standards on how they access and communicate with the network and SIM cardcommunicate with the network and SIM cardEvery MS has a unique ID called the International Mobile Every MS has a unique ID called the International Mobile Equipment Identity (IMEI)Equipment Identity (IMEI)Everything else is manufacturer dependentEverything else is manufacturer dependent

File systemFile system FeaturesFeatures InterfaceInterface Etc.Etc.

Have to request the SIM PIN if activatedHave to request the SIM PIN if activatedMay have optional MS PINMay have optional MS PIN

MS DataMS Data

Very much dependent on the model, may include;Very much dependent on the model, may include; IMEIIMEI Short Dial NumbersShort Dial Numbers Text/Multimedia MessagesText/Multimedia Messages Settings (languge, date/time, tone/volume etc)Settings (languge, date/time, tone/volume etc) Stored Audio RecordingsStored Audio Recordings Stored images/multimediaStored images/multimedia Stored Computer FilesStored Computer Files Logged incoming calls and dialled numbersLogged incoming calls and dialled numbers Stored Executable Progams (eg J2ME)Stored Executable Progams (eg J2ME) Stored Calendar EventsStored Calendar Events GPRS, WAP and Internet settingsGPRS, WAP and Internet settings

Threats to MS DataThreats to MS Data

Tools such as Flashers and Data Suites can be Tools such as Flashers and Data Suites can be used to directly manipulate MS dataused to directly manipulate MS dataCommon threat is removing the Service Provider Common threat is removing the Service Provider Lock (SP-Lock) limiting the MS to a single Lock (SP-Lock) limiting the MS to a single networked.networked.Changing the IMEI on stolen phonesChanging the IMEI on stolen phones

Networks blacklist stolen IMEI’s in the EIR.Networks blacklist stolen IMEI’s in the EIR. Can also be used to avoid tracing an MS.Can also be used to avoid tracing an MS.

Detecting changes to the IMEIDetecting changes to the IMEICompare the electronic IMEI with that printed on the inside of Compare the electronic IMEI with that printed on the inside of the devicethe device

The NetworkThe Network

Network Operator DataNetwork Operator Data

The Network Operators can provide detailed data on The Network Operators can provide detailed data on calls made/received, message traffic, data transferred calls made/received, message traffic, data transferred and connection location/timingand connection location/timingThe HLR can provide;The HLR can provide;

Customer name and addressCustomer name and address Billing name and address (if other than customer)Billing name and address (if other than customer) User name and address (if other than customer)User name and address (if other than customer) Billing account detailsBilling account details Telephone Number (MSISDN)Telephone Number (MSISDN) IMSIIMSI SIM serial number (as printed on the SIM-card)SIM serial number (as printed on the SIM-card) PIN/PUK for the SIMPIN/PUK for the SIM Subscriber Services allowedSubscriber Services allowed

The Call Data Records (CDR’s)The Call Data Records (CDR’s)

Produced in the originating MSC transferred to Produced in the originating MSC transferred to the OMCthe OMC Every callEvery call Every messageEvery message

Each CDR contains;Each CDR contains; Originating MSISDNOriginating MSISDN Terminating MSISDNTerminating MSISDN Originating and terminating IMEIOriginating and terminating IMEI Duration of callDuration of call Type of ServiceType of Service Initial serving Base Station (BTS) (not subsequent Initial serving Base Station (BTS) (not subsequent

BTSs after handover)BTSs after handover)

Threats to Network OperatorThreats to Network Operator

GSM not immune to interceptionGSM not immune to interceptionIt is possible for the network to order the MS to switch on and off It is possible for the network to order the MS to switch on and off encryption at times of high loadingencryption at times of high loading

This signal can be spoofed using a man-in-the-middle attackThis signal can be spoofed using a man-in-the-middle attack

Operator BTSLegitimate Subscriber Man-in-the-middle

1. Attempt to register using encryption

3. Authenticates

Spoofing BTS

Spoofing MS

2. Passes on the registration request

4. Passes on authentication

5. Dials a number

6. Requests MS switch off encryption 7. Encrypts then passes on the call request

8. Call proceeds wit MIM eavesdropping

GSM Security OperationGSM Security Operation

GSM networks utilize encryption for three GSM networks utilize encryption for three purposes:purposes:

Authentication Authentication Encryption Encryption Key generation Key generation

GSM Security Operation (Cont..)GSM Security Operation (Cont..)

GSM provides authentication of users and encryption of GSM provides authentication of users and encryption of the traffic across the air interface. the traffic across the air interface.

This is accomplished by giving the user and network a This is accomplished by giving the user and network a shared secret, called Ki. This 128-bit number is stored shared secret, called Ki. This 128-bit number is stored on the SIM-card, and is not directly accessible to the on the SIM-card, and is not directly accessible to the user. user. Each time the mobile connects to the network, the Each time the mobile connects to the network, the network authenticates the user by sending a random network authenticates the user by sending a random number (challenge) to the mobile. number (challenge) to the mobile. The SIM then uses an authentication algorithm to The SIM then uses an authentication algorithm to compute an authentication token SRES using the compute an authentication token SRES using the random number and Ki. random number and Ki.

GSM Security Operation (Cont..)GSM Security Operation (Cont..)

The mobile sends the SRES back to the network The mobile sends the SRES back to the network which compares the value with an independently which compares the value with an independently computed SRES.computed SRES. At the same time, an encryption key Kc is At the same time, an encryption key Kc is computed. This key is used for encryption of computed. This key is used for encryption of subsequent traffic across the air interface. subsequent traffic across the air interface. Thus, even if an attacker listening to the air Thus, even if an attacker listening to the air traffic could crack the encryption key Kc, the traffic could crack the encryption key Kc, the attack would be of little value, since this key attack would be of little value, since this key changes each time the authentication procedure changes each time the authentication procedure is performedis performed

Forensics ToolsForensics Tools

SIMISSIMIS SIM card Interrogation System is the world's SIM card Interrogation System is the world's

leading forensic tool for examining SIM cards leading forensic tool for examining SIM cards forensically. forensically.

Used throughout the world since 1997, SIMIS Used throughout the world since 1997, SIMIS has become an integral tool for law enforcement has become an integral tool for law enforcement and digital investigators. and digital investigators.

The SIMIS desktop software has been evaluated The SIMIS desktop software has been evaluated by the DoD, and is complimented by a mobile by the DoD, and is complimented by a mobile handheld device for data collection in the fieldhandheld device for data collection in the field

Forensics ToolsForensics Tools

Cell Seizure:Cell Seizure:

ParabenParaben Cell SeizureCell Seizure is a piece of is a piece of software that serves the main purpose of software that serves the main purpose of collection and examining data pulled from collection and examining data pulled from various types of various types of cell phonescell phones. .

Cell Seizure ToolCell Seizure Tool

The main goal of Cell Seizure is to The main goal of Cell Seizure is to organize and report various types of files.organize and report various types of files.

Cell Seizure is able to generate Cell Seizure is able to generate comprehensive comprehensive HTMLHTML reports of acquired reports of acquired data. data.

The software is able to retrieve The software is able to retrieve deleted filesdeleted files and check for and check for file integrityfile integrity. .

Advantages of Cell SeizureAdvantages of Cell Seizure

It is designed not to change the data It is designed not to change the data stored on the stored on the SIM cardSIM card or or cell phonecell phone. In . In other words, all of the data can be other words, all of the data can be examined while keeping the process examined while keeping the process undetected. undetected.

In fact, even some forensic software warns In fact, even some forensic software warns of possible data loss. Cell Seizure does of possible data loss. Cell Seizure does not allow data to be changed on the not allow data to be changed on the phonephone. .

Disadvantages of Cell SeizureDisadvantages of Cell Seizure It does not support all models of cell phones. It does not support all models of cell phones.

However, this application can acquire However, this application can acquire information from most models made by the information from most models made by the following companies: following companies: NokiaNokia, , LGLG, Samsung, , Samsung, Siemens, Motorola, Sony-Ericcson, and can Siemens, Motorola, Sony-Ericcson, and can also acquire GSM SIM Cards. also acquire GSM SIM Cards.

Another disadvantage would be that the Another disadvantage would be that the format of acquired data can sometimes be format of acquired data can sometimes be confusing. The data is not organized nice confusing. The data is not organized nice and neat and given to the user in a way that and neat and given to the user in a way that they can easily understand what they are they can easily understand what they are seeing. seeing.

Cell Seizure FeaturesCell Seizure Features

Supports GSM, TDMA, and CDMA cell phones Supports GSM, TDMA, and CDMA cell phones Acquires text messages, address books, call logs, etc. Acquires text messages, address books, call logs, etc. Acquires complete GSM SIM card Acquires complete GSM SIM card Recovers deleted data and full flash downloads Recovers deleted data and full flash downloads Supports multiple languages Supports multiple languages Contains comprehensive HTML reporting and other Contains comprehensive HTML reporting and other reporting formats reporting formats Provides advanced searching including text & hex values Provides advanced searching including text & hex values Contains viewers for proprietary media file formatsContains viewers for proprietary media file formatsAllows viewing of multiple workspaces at one time Allows viewing of multiple workspaces at one time

ConclusionConclusionThe sources of evidenceThe sources of evidence The subscriberThe subscriber The mobile stationThe mobile station The networkThe network

Since GSM is the worlds largest system for Since GSM is the worlds largest system for mobile communication today and also lay the mobile communication today and also lay the foundation for the future UMTS, it is important foundation for the future UMTS, it is important to recognize the need to study the methods and to recognize the need to study the methods and tools for forensic analysis of the GSM system. tools for forensic analysis of the GSM system.

GSM Mobile Security

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