GSM Security Feature, Threats and Solution

Global system for Mobile communication in short, GSM. It is a digital mobile telephony system and it uses a variation of time division multiple access (TDMA). It is mostly implemented in the three digital wireless telephony technologies (TDMA, GSM and CDMA). CDMA also known as Code Division Multiple Access and it refers to any protocol used in second-generation(2G) and the third-generation (3G) wireless communications.

GSM converts the data into digital form before compression it and finally sending them down a channel with two other streams of user data, each having its own time allocation. It operates at either 900 MHz or 1800 MHz frequency band.

GSM Security Objectives

Concerns

·         Operators – Billing in the right people, prevention of duplicity and protection of services

·         Customers - Privacy and a state of being anonymous.

·         Making the system at least secure as PSTN

 Security Goals

• Prevention of operators from compromising of each others’ security which might cause any thoughtless and competition pressure
• Confidentiality and Anonymity on the radio path
• Strong client authentication in order to protect the operator against any billing fraud

Security Design Requirements

The security mechanism must not

·         Increase error rate

·         Add important overhead on call set up

·         Increase bandwidth of the channel

·         Add any unnecessary misunderstanding to the system.

Instead it should be a cost effective scheme and be able to define security procedures such as

·         Confidentiality of algorithms

·         Generation and distribution of keys

·         Exchanging of information between operators

GSM Security Mechanisms

Features

·         Key management is independency of equipment. An example is when subscribers wants to change physical parts of the equipment, they do not have to worry about the settings they have set earlier  

·         Subscriber identity protection is about the difficulty of  identifying the user of the system intercepting a user data

·         Detection of compromised equipment such as whether a mobile device was being compromised or not

·         Subscriber authentication meaning that the operator must have the authority to know the billing purposes  and also who is using the system

·         Signaling and user data channels must be protected over the radio path

GSM Mobile Station

·         Mobile Equipment (ME)

       Physical mobile device and Identifiers (IMEI – International Mobile Equipment Identity)

·         Subscriber Identity Module (SIM)

       Smart Card which comprises of keys, identifiers and algorithms

       Examples of Identifiers are:

•    K_i – Subscriber Authentication Key
•  IMSI – International Mobile Subscriber Identity
• TMSI – Temporary Mobile Subscriber Identity
•  MSISDN – Mobile Station International Service Digital Network
• PIN – Personal Identity Number protecting a SIM
• LAI – location area identity
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Subscriber Identity Protection

TMSI – Temporary Mobile Subscriber Identity

Goals

• TMSI is used instead of IMSI as an a temporary subscriber identifier
• TMSI prevents an eavesdropper from identifying of subscribe

Usage

• TMSI is assigned when IMSI is transmitted to AuC on the first phone switch on
• Every time a location update (new MSC) occur the networks assigns  a new TMSI
• TMSI is used by the MS to report to the network or during a call initialization
• Network uses TMSI to communicate with MS
• On MS switch off TMSI is stored on SIM card to be reused next

The Visitor Location Register (VLR) performs assignment, administration and update of the TMSI

Authentication

 Goal

     Subscriber (SIM holder) authentication

     Protection of the network against the unauthorized use

     Create a session key

A3 – MS Authentication Algorithm

A8 – Voice Privacy Key Generation Algorithm

      Logical Implementation of A3 and A8

             Both A3 and A8 algorithms are implemented on the SIM

•           Operator can decides which algorithm to us
• • Algorithms implementation is independent of hardware manufacturers and network    operators
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COMP128 is used for both A3 and A8 in most GSM networks

COMP128 is a keyed hash function

A5 – Encryption Algorithm

    A5 is a stream cipher and is being implemented very efficiently on hardware and the design was never made public

    Variants:

A5/1 – the strong version

A5/2 – the weak version

A5/3 - GSM Association Security Group and 3GPP design, b

ased on Kasumi algorithm used in 3G mobile systems

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  References: 
• www.gsm-security.net/gsm-security-papers.shtml
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• www.blackhat.com/presentations/bh-asia-01/gadiax.ppt
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en.wikipedia.org/wiki/GSM