GSM Mobile SecurityPrepared by:Leen F. Arikat

Introduction 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 FactsNearly 1 billion subscribers worldwideEstimated that worldwide mobile phone fraud will reach $40 billion dollarsUS Law enforcement agents have found that 80% of drug dealers arrested in US using cloned mobile phones.Ironically, Pablo Escobar the top Columbian drug dealer was tracked down by monitoring his mobile phone activity.Two aspects relevant to a Forensic AnalystHas the phone been used for a criminal act?Can the phone be used to secure a conviction?

Some GSM FactsThe European Telecommunication Standards Institute (ETSI) regulates the GSM standard (all 4000 pages of it!).Any equipment used on a GSM network has to have approval by the ETSI.All MSs are independent from any network.

What are the components of a GSM network?

Subscriber Equipment

The Mobile Station (MS) is the user equipment in GSM. It is what the user can see of the GSM system. The station consists of two entities:The Mobile Equipment (the phone itself)The Subscriber Identity Module (SIM)

Components of a GSM network (cont..)The Switching System (SS)

Home Location Register (HLR) - A database which stores data about GSM subscribers, including the Individual Subscriber Authentication Key (Ki) for each Subscriber Identity Module (SIM).Mobile Services Switching Center (MSC) - The network element which performs the telephony switching functions of the GSM network. Visitor Location Register (VLR) - A database which stores temporary information about roaming GSM subscribers.Authentication Center (AUC) - A database which contains the International Mobile Subscriber Identity (IMSI) the Subscriber Authentication key (Ki), and the defined algorithms for encryption.Equipment Identity Register (EIR) - A database which contains information about the identity of mobile equipment in order to prevent calls from stolen, unauthorized, or defective mobile stations.

Components of a GSM network (cont..)The Base Station System (BSS)

Base Station Controller (BSC) - The network element which provides all the control functions and physical links between the MSC and BTS. The BSC provides functions such as handover, cell configuration data, and control of radio frequency (RF) power levels in Base Transceiver Stations.Base Transceiver Station (BTS) - The network element which handles the radio interface to the mobile station. The BTS is the radio equipment (transceivers and antennas) needed to service each cell in the network.

Components of a GSM network (cont..)The Operation and Support System (OSS)

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

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

The Subscriber

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

Electronic Access to the SIMEvery SIM can be protected by a Personal Identification Number (PIN)Set at point of manufactureCan be changed by the SubscriberFour digit codeUsually 3 attempts before phone is blockedBypassing the PIN requires the Pin Unblocking Key (PUK)8 digit codeSet by manufacturerMaximum 10 attempts before phone is permanently blocked

What Can Be Extracted From A SIM?As SIM is a smart card it hasA processorNon-volatile memoryProcessor is used for providing access to the data and security

To access the data we need;Standard smart card readerSIM access SoftwareData stored in binary files

What Can Be Extracted From A SIM?Ideally an Analyst would download an image of the contents and compute a hash value of the contents as a means of validating originality of contentAt present files are downloaded traditionallySoftwareSim Manager ProChipItSimScanCards4Labs only available to Law Enforcement AgenciesProduces a text report of content rather than downloading.29 files stored on a SIM

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

Serial NumberIntegrated Circuit Card IdentifierCorresponds to the number printed on the surface of the SIMIdentifies the SIM

Subscriber IdentifierInternational Mobile Subscriber IdentityAs stored in the HLR/VLRs on the networksUnique ID for every subscription on the Operators network

Phone NumberMobile Station International ISDN number

Text Message Data (SMS)Short Message Service is a popular communication methodMost SIMs have 12 slots for storing messagesModern MSs allow storage on the device as well

Text Message Data (SMS) - StatusWhen user deletes a message only the status flag is changedTherefore, providing the message has not been overwritten any message in a slot can be recovered and translated using software

Threats to SIM DataKnowledgeable criminals will be aware of the properties of the SIM and thus manipulate them.Greater threat is that of cloning SIM data for illicit useTwo key pieces of dataIMSIThe data encryption key (Ki)IMSI can be obtained;Directly from the SIM using a scanning softwareEaves-dropping on the networks for unencrypted transmission of the IMSIKi cannot normally be obtained directly as it is derived from an encryption algorithm stored on the SIMHowever, if the encryption algorithm is weak then it is possible to feed numbers

Threats to SIM Data

Obtaining blank SIMsThese 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 Equipment

Generic PropertiesAll MSs have GSM standards on how they access and communicate with the network and SIM cardEvery MS has a unique ID called the International Mobile Equipment Identity (IMEI)Everything else is manufacturer dependentFile systemFeaturesInterfaceEtc.Have to request the SIM PIN if activatedMay have optional MS PIN

MS DataVery much dependent on the model, may include;IMEIShort Dial NumbersText/Multimedia MessagesSettings (languge, date/time, tone/volume etc)Stored Audio RecordingsStored images/multimediaStored Computer FilesLogged incoming calls and dialled numbersStored Executable Progams (eg J2ME)Stored Calendar EventsGPRS, WAP and Internet settings

Threats to MS DataTools such as Flashers and Data Suites can be used to directly manipulate MS dataCommon threat is removing the Service Provider Lock (SP-Lock) limiting the MS to a single networked.Changing the IMEI on stolen phonesNetworks blacklist stolen IMEIs in the EIR.Can also be used to avoid tracing an MS.

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

The Network

Network Operator DataThe Network Operators can provide detailed data on calls made/received, message traffic, data transferred and connection location/timingThe HLR can provide;Customer name and addressBilling name and address (if other than customer)User name and address (if other than customer)Billing account detailsTelephone Number (MSISDN)IMSISIM serial number (as printed on the SIM-card)PIN/PUK for the SIMSubscriber Services allowed

The Call Data Records (CDRs)Produced in the originating MSC transferred to the OMCEvery callEvery messageEach CDR contains;Originating MSISDNTerminating MSISDNOriginating and terminating IMEIDuration of callType of ServiceInitial serving Base Station (BTS) (not subsequent BTSs after handover)

Threats to Network OperatorGSM not immune to interceptionIt is possible for the network to order the MS to switch on and off encryption at times of high loadingThis signal can be spoofed using a man-in-the-middle attack

Radio tower

Cell phone

Operator BTS

Legitimate Subscriber

1.Attempt to register using encryption

Man-in-the-middle

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 Operation

GSM networks utilize encryption for three purposes:Authentication Encryption Key generation

GSM Security Operation (Cont..) GSM provides authentication of users and encryption of the traffic across the air interface.

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

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

Forensics ToolsSIMIS SIM card Interrogation System is the world's leading forensic tool for examining SIM cards forensically. Used throughout the world since 1997, SIMIS has become an integral tool for law enforcement and digital investigators. The SIMIS desktop software has been evaluated by the DoD, and is complimented by a mobile handheld device for data collection in the field

Forensics Tools

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

Cell Seizure Tool

The main goal of Cell Seizure is to organize and report various types of files.Cell Seizure is able to generate comprehensive HTML reports of acquired data. The software is able to retrieve deleted files and check for file integrity.

Advantages of Cell Seizure

It is designed not to change the data stored on the SIM card or cell phone. In other words, all of the data can be examined while keeping the process undetected. In fact, even some forensic software warns of possible data loss. Cell Seizure does not allow data to be changed on the phone.

Disadvantages of Cell SeizureIt does not support all models of cell phones. However, this application can acquire information from most models made by the following companies: Nokia, LG, Samsung, Siemens, Motorola, Sony-Ericcson, and can also acquire GSM SIM Cards. Another disadvantage would be that the format of acquired data can sometimes be confusing. The data is not organized nice and neat and given to the user in a way that they can easily understand what they are seeing.

Cell Seizure Features

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

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