The document discusses security in GSM networks. It describes the GSM architecture including mobile stations, base stations, and network switching systems. It then explains the security concerns with GSM and the features used to address them, including subscriber authentication using algorithms A3 and A8, and voice encryption with algorithm A5/1. The document provides details on how these algorithms work to authenticate users and encrypt communications in GSM networks.

1. Presented by :
Abu Sadat Mohammed Yasin
Debotosh Dey
Cryptography and Information Security
Topic: GSM Security Overview

2. 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

3. 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.

4. GSM: Architecture

5. 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

6. 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

7. GSM: Security Concerns
 Operators
 Bills right people
 Avoid fraud
 Protect Services
 Customers
 Privacy
 Anonymity

8. 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

9. 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 Identity
IMSI, 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

10. GSM: Security Model
 Mobile station authentication

11. 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

12. 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)

13. 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)

14. 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.

15. 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.

16. GSM: A5/1 Stream Cipher(Example)
 Majority of clock bit (1,0,1) = 1
 First and third LFSRs will be clocked but not the
second.
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
 


17. 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

18. 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.

19. 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.

20. 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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