Presented by :

Abu Sadat Mohammed Yasin

Debotosh Dey

Cryptography and Information Security

Topic: GSM Security Overview

Content

Introduction

Architecture

Security Concern

Security Features

Security model

Security Algorithms

A3, The MS Authentication Algorithm

A8, The Voice-Privacy Key Generation Algorithm

A5/1 Stream Cipher

A5/1 (Key Generation)

Conclusion

GSM: Introduction

GSM - Group Special Mobile or General System for

Mobile Communications.

GSM is the Pan-European standard for digital cellular

communications.

GSM was established in 1982 within the European

Conference of Post and Telecommunication

Administrations (CEPT).

In 1991 the first GSM based networks commenced

operations.

GSM: Architecture

GSM: Architecture

A GSM network is made up of three subsystems:

The Mobile Station (MS)

Mobile Equipment (ME) Physical mobile device

Identifiers

IMEI – International Mobile Equipment Identity

Subscriber Identity Module (SIM) Smart Card containing keys, identifiers and algorithms

Identifiers

Ki – 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

GSM: Architecture

The Base Station Sub-system (BSS)

Comprising a Base Station Controller (BSC) and

Several Base Transceiver Stations (BTS)s

The Network and Switching Sub-system (NSS)

comprising an Mobile services Switching Center (MSC) and

associated registers Home Location Register (HLR),

Authentication Center (AuC), Visitor Location Register (VLR)

The interfaces defined between each of these sub

systems include:

'A' interface between NSS and BSS

'Abis' interface between BSC and BTS (within the BSS)

'Um' air interface between the BSS and the MS

GSM: Security Concerns

Operators

Bills right people

Avoid fraud

Protect Services

Customers

Privacy

Anonymity

GSM: Security Features

Key management is independent of equipment Subscribers can change handsets without compromising

security

Subscriber identity protection not easy to identify the user of the system intercepting a

user data

Detection of compromised equipment Detection mechanism whether a mobile device was

compromised or not

Subscriber authentication The operator knows for billing purposes who is using the

system

Signaling and user data protection Signaling and data channels are protected over the radio

path

GSM: Security Features

The security mechanisms of GSM are implemented

in three different system elements:

The Subscriber Identity Module (SIM) - The SIM

contains the International Mobile Subscriber IdentityIMSI, the individual subscriber authentication key (Ki), the

ciphering key generating algorithm (A8), the

authentication algorithm (A3), as well as a Personal

Identification Number(PIN).

The GSM handset - The GSM handset contains the

ciphering algorithm (A5).

The GSM network - The encryption algorithms (A3, A5,

A8) are present in the GSM network

GSM: Security Model

Mobile station authentication

GSM: Security Algorithms.

A3, The MS Authentication Algorithm

A8, The Voice-Privacy Key Generation Algorithm

A5/1, The Strong Over-the-Air Voice-Privacy

Algorithm

GSM: A3, The MS Authentication Algorithm

Inputs are

the RAND from the MSC

the secret key Ki from the SIM

generates a 32-bit output, which is the SRES

response.

A3

RAND (128 bit)

Ki (128 bit)

SRES (32 bit)

GSM: A8, The Voice-Privacy Key Generation Algorithm

Inputs are

the RAND from the MSC

the secret key Ki from the SIM

generates a 64-bit output, which is Session Key

Kc.

A8

RAND (128 bit)

Ki (128 bit)

KC (64 bit)

GSM: A5/1 Stream Cipher

Combination of 3 linear feedback shift registers

(LFSRs)

Each register has an associated clocking bit.

The registers are clocked in a stop/go fashion

using a majority rule.

GSM: A5/1 Stream Cipher

At each cycle, the clocking bit of all three registers is examined

and the majority bit is determined.

A register is clocked if the clocking bit agrees with the majority

bit.

At each step at least

two or three registers

are clocked.

GSM: A5/1 Stream Cipher(Example)

Majority of clock bit (1,0,1) = 1

First and third LFSRs will be clocked but not thesecond.

1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 0 1

1 1 1 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 0 0 0 1

1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1

GSM: A5/1 Stream Cipher(Example)

1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 0 1

0 1 1 1 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 0 0 0

0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0

Keystream bit will be 0 1 0 = 1

GSM: A5/1 Stream Cipher (Initialization)

64-bit Session Key loaded in to the LFSR(bit by bit)

The majority clocking rule is disabled

22-bit frame number is also loaded into the register

The majority clocking rule applies from now on.

The registers are clocked one hundred times.

Generated keystream bits are discarder.

In order to mix the frame number and keying material

together.

GSM: A5/1 (Key Generation)

228 bits of keystream output are generated.

The first 114 bits are used to encrypt the frame from

MS(Mobile Station) to BTS(base transceiver station).

The next 114 bits are used to encrypt the frame from BTS

to MS.

The same Session Key is used throughout the call,

but the 22-bit frame number changes during the

call, that is why, A5 algorithm is initialized again

with the same session key and the number of the

next frame.

Conclusion

GSM is the most widely used cellular network

standard.

Security mechanism specified in the GSM make it a

secure cellular telecommunication available.

The use of authentication, encryptions ensures the

privacy and anonymity of the system’s users.

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