The paper presents security issues and architectures for mobile applications and GSM infrastructure. The article also introduces the idea of a new secure architecture for an inter-sector electronic wallet used in payments - STP4EW (Secure Transmission Protocol for Electronic Wallet)secure architecture, m-application, smart-cards, 3G Mobile

Cristian TOMA

English

Research Papers in Economics

Revista Informatica Economică, nr. 4 (44)/2007  89
Secure Architectures for Mobile Applications 
 
Cristian TOMA, Bucharest, Romania, cristian.toma@ie.ase.ro  
 
The paper presents security issues and architectures for mobile applications and GSM infra-
structure. The article also introduces the idea of a new secure architecture for an inter-sector 
electronic wallet used in payments – STP4EW (Secure Transmission Protocol for Electronic 
Wallet).  
Keywords: secure architecture, m-application, smart-cards, 3G Mobile. 
 
SM Overview 
Th
present
e basic GSM network architecture is 
ed in fig. 1. The main components in-
volved in voice and data transmission are the 
following: 
  BSS – Base Station Subsystem. It con-
trols the quality of the links from GSM radio 
interface and contains BTS and BCS. 
  BTS – Base Transceiver Station. Controls 
the “antennas” and maintains the communi-
cation through a duplex radio channel. It 
supports configurations for: electro-magnetic 
power, radio channel used for broadcasting, 
BSIC – Base Station Identity Code. Its main 
functionalities are: message encryption, 
channel coding and modularization. 
  BCS – Base Station Controller. Adminis-
trates and controls base stations and radio 
channels. It provides the coding implementa-
tion for voices messages and manages the 
data localization. 
  NSS – Network Sub System. It contains 
MSCs, Databases such as VLR, HLR, EIR 
and AuC and adaptation modules such as 
XC, IWF, EC. NSS provides the following 
functionalities: management of communica-
tion link with other mobiles, land and satel-
lite networks, management of mobile sub-
scribers from other BSCs, and the manage-
ment of the subscribers using data from AuC, 
EIR, VLR and HLR databases 
  MSC – Mobile Switching Center. Con-
tains switching subsystems (e.g. for PBX 
signaling and for communication signaling 
over SS7 with other MSCs) and control sub-
systems 
  HLR – Home Location Register. Stores 
the subscribers’ parameters including the 
MSISDN and the service type (e.g. 10 SMS, 
80 minutes for 1 month). 
  VLR – Visitors Location Register. It is a 
mirroring database of HLR for temporary 
subscribers of another VLR area. 
  EIR – Equipment Identity Register. It is 
the centralized database with IMEI (unique 
number for each provider-device) for each 
mobile device. 
  AuC – Authentication Center. The fol-
lowing functionalities and responsibilities are 
included: authorization process for the sub-
scriber access into mobile network for en-
crypting transmission on radio path and for 
assign of the temporary identity - TMSI 
(Temporary Mobile Subscriber). 
Fig. 2 presents the main concept used in 
GSM for end-user device: the mobile is a 2 in 
1 computer. The first computer is represented 
by the SIM – Subscriber Identity Module. 
Actually, the SIM is a smart card with a mi-
crocontroller, three types of memory area 
(ROM, EEPROM and RAM) and I/O ports 
for outside communication (usually in half 
duplex mode). The mobile device itself is the 
second computer. It also includes a micro-
processor, different types of memory areas 
and an operating system. 
 
G Revista Informatica Economică, nr. 4 (44)/2007  90 
BSS = Base Station Subsystem 
BTS = Base Transceiver Station 
BSC = Base Station Controller 
 
NSS = Network Sub System 
MSC = Mobile Switching Center 
GMSC = Gateway MSC 
EIR = Equipment Identity Register 
HLR = Home Location Register 
VLR = Visitor Location Register 
 
 
Fig. 1. GSM Network Architecture 
 
 
Fig. 2. GSM Mobile Equipment Structure 
 
The GSM evolution is based on this infra-
structure and the security of the entire system 
depends on many factors. 
2.  Architectures for Voice and Data Se-
curity in GSM 
This chapter focuses on the security mecha-
nisms involved within mobile data and voice 
transmissions. Fig. 3. presents the overall 
processes for authentication and confidential-
ity of mobile communication. 
Fig. 4 and 5 present a more detailed technical 
view of the systems’ security process. Fig. 4 
shows the main network items involved in 
voice and data security, and fig. 5 presents 
the logical flow of the data. 
 
  
Fig. 3. Processes for authentication and confidentiality 
of mobile voice and data transmission 
 Revista Informatica Economică, nr. 4 (44)/2007  91
 
Fig. 4. The details of the security mechanisms 
 
AuC – Authentication Center plays the main 
part in voice and data authentication and in-
tegrity. Its main functionalities and responsi-
bilities consist in: authorizing the subscrib-
ers’ access into the mobile network; encrypt-
ing transmission on radio path and assign of 
the temporary identity - TMSI (Temporary 
Mobile Subscriber Identity). The authentica-
tion process of the mobile subscriber is de-
scribed as it follows: 
  The mobile station sends the SIM’s 
IMEI to the HLR through the “Net Attach” 
(or GPRS Attach). 
  A triplets request is sent to the AuC 
from HLR (all HLR, AuC and SIM suppose 
to store same Ki) through the “Send Auth. 
Info” message (contains SIM’s IMSI) 
  The AuC generates a response that 
contains: 
  RAND (random number – 
challenge)  
  Kc – encryption key that is a 
result of the A8 algorithm. The A8 algorithm 
uses the stored identity key – Ki from AuC 
corresponding to the received IMEI. 
  SRES – Signed RESponse 
generated through A3 Authentication Algo-
rithm with RAND and Ki as input 
  The HLR receives the triplets and 
sends to the mobile only RAND 
  The mobile device must be enabled 
using Ki from SIM, A3 and A8 algorithms in 
order to reconstruct Kc and SRES. It then 
sends the SRES to the HLR via MSC or 
SGSN (in GPRS only). 
  If the SRES received by the HLR 
from the mobile device is the same with the 
one that is received from the AuC, then the 
authentication is done and Kc will be used 
for ENCRYPTION. 
 
 
Fig. 5. Overview of data workflow for authentication and confidentiality 
 Revista Informatica Economică, nr. 4 (44)/2007  92 
The described architecture had been consid-
ered at the beginnings of the GSM and has 
been used since (including in GPRS, EDGE, 
UMTS networks) [WOLF04]. 
 
3.  A new proposed architecture for in-
ter-sector electronic wallet 
According to EN 1546 of CCE [EN 1546] an 
inter-sector electronic wallet for payments in 
retail business and e-commerce can be im-
plemented. The standard provides specifica-
tions for a series of data elements (in bold), 
which allow the data used in the electronic 
wallet system to be referred unambiguously. 
All files that are necessary for proper opera-
tion are contained by a DF – dedicated file – 
within the smart card. The files and the 
commands used by the electronic wallet be-
long to the ISO/IEC 7816-4 standard. The 
commands are: SELECT FILE, READ BI-
NARY and READ RECORD. The specific 
commands used for electronic wallets are de-
fined in EN 1546: INITIALIZE IEP – initial-
izes a subsequent purse command; LOAD 
IEP – loads the purse, cancels the previous 
payment and provides error recovery; DEBIT 
IEP – provides the payment using the wallet 
and confirms payment; CONVERT IEP 
CURRENCY – converts currencies; UP-
DATE IEP PARAMETER – modifies gen-
eral parameters of the wallet. State diagrams 
are used to define the necessary states and 
state transitions within the application, corre-
sponding to a sequence of commands. De-
pending on its current state, the card will ac-
cept or reject various commands. The cryp-
tographic algorithms create the context for 
the entire security of the system. They repre-
sent the only message protection. Some of 
the most commonly used algorithms are DES 
and Rijndael for symmetric key encryption 
and RSA or DSS for asymmetric key algo-
rithms. The standard also describes in the de-
tailed manner the procedures representing 
the main functions of the wallet. 
The new proposed architecture for an elec-
tronic purse improves the standard, binding 
the smart card nature of the SIM with a 
new transmission protocol – STP4EW (Se-
cure Transmission Protocol for Electronic 
Wallet) – for the message exchange 
[TOMA06b]. 
 
4.  Conclusions 
Part of the STP4EW (Secure Transmission 
Protocol for Electronic Wallet) is in devel-
opment phase within mSQE (Evaluation Sys-
tem for the Quality of Services Generated by 
the Mobile Applications in the Field of E-
Business – Contract 104 between Academy 
of Economic Studies and MATNANTECH) 
and eServEval (Quality Evaluation System 
for Online Public Services used within the 
Business Environment and by the Citizens) 
excellence research projects (CEEX). Also, 
some parts of the STP4EW are developed by 
the Software Development Department that 
organizes the Informatics Security Master. 
The proposed secure architecture for elec-
tronic wallet relays on GSM security mecha-
nisms and on the author’s personal ideas. The 
main references for the secure architecture 
used in inter-sector electronic wallet for 
payments are [WOLF04], [EN 1546] and 
[TOMA06b]. 
 
References 
 [EN1546], BS EN 1546-3:2000 – Identifica-
tion Card Systems. Inter-sector Electronic 
Purse. Data Elements and Interchanges. 
http://www.standardsdirect.org
[TOMA06a], Cristian TOMA, “Tutorial on 
Java Smart Card electronic Wallet Applica-
tion”, Informatics Security Handbook, AES 
Publishing House, Romania 2006 
[TOMA06b], Cristian TOMA, “Secure Pat-
terns and Smart-card Technologies used in e-
Commerce, e-Payment and e-Government”, 
Informatics Security Handbook, AES Pub-
lishing House, Romania 2006 
[WOLF04], Wolfgang Rankl & Effing, 
“Smart Card Handbook 3
rd Edition”, John 
Wiley & Sons Publishing House, USA 2004 
[ZHIQ04], Zhiqun Chen, “Java Card Tech-
nology for Smart Cards – Architectures and 
Programmer’s Guide”, Addison Wesley, 
2004 
 

EN 1546-3:2000 – Identification Card Systems. Inter-sector Electronic Purse. Data Elements and Interchanges.

Java Card Technology for Smart Cards – Architectures and Programmer’s Guide”,

Rankl & Effing, “Smart Card Handbook 3 rd Edition”,

Secure Patterns and Smart-card Technologies used in eCommerce, e-Payment and e-Government”, Informatics Security Handbook,

Tutorial on Java Smart Card electronic Wallet Application”, Informatics Security Handbook,

Secure Architectures for Mobile Applications

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Secure Architectures for Mobile Applications

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