GSM architecture and security overview

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.

21. _________
_____
__