Security Models in Cellular / Wireless Networks

William Chipman

Fall 2010

Colorado State University

Abstract

Wireless technologies are continually advancing and becoming more pervasive.

Cellular-based networks have now become the network of choice for businesses and

individuals. With the introduction of multiple operating systems for mobile devices,

security is now becoming a major issue. To combat the risks involved in cellular

networking, several security models have been proposed and implementations of these

models are being tested and deployed continuously.

Because all cellular data transfer is inherently wireless and therefore insecure all

the risks associated with a wireless network must be addressed and ameliorated. Because

sensitive data is being stored and inputted on mobile devices, keeping the device and its

communication secure is a top priority to network and hardware designers. This push to

keep users and their devices mobile at all times drive the need for better and more

comprehensive security models for implementation in the new devices.

This field is very much in its infantile stage. Most security for cellular networks

are handle by four main ideas; authentication, authorization, encryption and physical

security. Newer smart phones also encompass an idea referred to as sandboxing. In this

concept, applications are given an area of the device to perform their purpose. Any

additional access is described to the user before installation and require authorization

prior to moving forward with the install process. A clear knowledge of the risks and

solutions for wireless cellular networks security issues is important to users, businesses

and scientists.

Keywords: GSM, Cellular Security, Android, CDMA, UTMS, Security Models, 3G

Security, Wireless Security.

I. Introduction

Wireless technologies are continually advancing and becoming more pervasive.

Cellular-based networks have now become the network of choice for businesses and

individuals. “The current ubiquity of Global System for Mobile Communications (GSM)

service and anticipated proliferation of last mile Worldwide Interoperability for

Microwave Access (WiMax) networks places intra-cellular wireless users at great risk”

[12]. With the introduction of multiple operating systems for mobile devices, security is

now becoming a major issue. To combat the risks involved in cellular networking, several

security models have been proposed and implementations of these models are being

tested and deployed continuously.

“Wireless security is ... a combination of wireless channel security (security of the

radio transmission) and network security (security of the wired network through with the

data flows)” [1]. Because all cellular data transfer is wireless all the risks associated with

a wireless network must be addressed and ameliorated. Because sensitive data is being

stored and inputted on mobile devices, keeping the device and its communication secure

is a top priority to network and hardware designers. “In addition, mobile devices having

both cellular phone and WLAN capability are available [and] demand to integrate

multiple mobile computing services into a single entity is preeminent” [9]. This push to

keep users and their devices mobile at all times drives the need for better and more

comprehensive security models for implementation in the new devices.

A clear knowledge of the risks and solutions for wireless cellular networks

security issues is important to users, businesses and scientists. This paper will explore

these issues in detail. A brief introduction to current cellular technology will build the

foundation of this paper and will be used to introduce the topics of mobile platforms,

security risks in these configurations and solution models.

II. Related Work

Work in the field of cellular network security has achieved its renaissance in

recent years. A large amount of research has been completed recently due to the

impending release of the 4G cellular network platform. As a part of this research many

security issues have been addressed and solutions to the most glaring issues have been

developed. In the paper, Improving Intra-Cellular Security Using Air Monitoring with RF

Fingerprints, the author shows that researchers have determined that each mobile device

has a unique RF signature that can be determined and cataloged for future verification.

There has been extensive research in the field of cellular to WLAN integration.

Most of it has focused on proposed architecture but security available in the hardware is

always a part of that research. Security across the WLAN/cellular connection has become

vital to the safety of both systems and the devices connected to those networks. With the

recent release of the 3G protocol and the impending release of the 4G protocol, “the 3rd

Generation Partnership Project (3GPP) has identified this [WLAN] interworking as vital

for 3G networks” [4].

The mobile IP working group is developing initiatives to seamlessly merge IP

networks with cellular-based networks. The cellular networks will need to adopt IETF

standardized protocols [2]. Once this adoption has occurred, cellular integration into

wireless networks will be seamless but security issues will still have to be addressed on

both sides of the connection.

III. Intro to Cellular technology

Global System for Mobile Communications

The Global System for Mobile Communications (GSM) is a system introduced in

most countries as the new standard for cellular communications. Approximately 80% of

the world uses the GSM format. “The main objective of the GSM system is to be at least

as secure … as the public switched telephone” [5]. This requirement has forced the GSM

designers to devise a standardized system that address as many of the security issues

surrounding cellular-based communications. The first line of defense in the GSM

network is authentication. It allows subscribers to attach to the network and drives the

encryption system.

The GSM network is based on a series of base stations that operate in cells similar

in appearance to the cells in a bee hive. These base stations interact directly with the

mobile devices. The base stations receive the signal from the mobile stations and relay it

to the network and switching subsystem. This subsystem then interacts with the GSM

backbone for communications onto the wired communication system. The General Packet

Radio Service (GPRS) “applies a packet radio principle to transfer user data packets in an

efficient way between mobile stations and external packet data networks” [8].

The “GSM security model is based on a 128-bit shared secret Ki between the

subscriber's SIM and the network” [7]. The subscriber SIM is a small factor data storage

card the carries the subscriber identification with the encryption algorithms used to

authenticate, authorize and encrypt the data communication as hardware in the mobile

device. “Key management [in the system] is independent of equipment” [14]. By

detaching the key management from the subscriber's mobile device, the subscriber

identity is not easily identified, compromised equipment is easily detected and data and

signaling are protected. “GSM uses Gaussian Minimum Shift Keying (GMSK)

modulation with a bandwidth-bit period product of Btbit = 0.3 and Gaussian filter

bandwidth of BW = 81.3KHz” [12].

CDMA/UMTS

“UMTS and CDMA2000 belong to a set of wireless network standards known as

3G, third generation mobile telecommunication standards, which replaced or are

replacing the older 2G networks” [7]. CDMA is code division multiple access and is the

format used in the UMTS system to speed-up data transfer rates to the 3G standard and

will eventually be used for the 4G standardization. UMTS security uses the following

defined security functions in the authentication and key agreement algorithm (AKA):

f0: Random challenge generation function

f1: network authentication function

f2: user challenge-response authentication function

f3: cipher key derivation function

f4: integrity key derivation function

f5: anonymity key deprivation function [7].

These are standardized functions across all mobile devices accessing the UMTS network.

By standardizing the functions, they can be implemented in hardware instead of software,

thus increasing the speed of data transfer and making the communication channel more

secure and more useful for subscribers.

Cellular integration into wireless networks

Based on the interdependence of the WLAN and the cellular network, the

coupling between the two can vary. “In a tight coupling, the 802.11 network appears to

the 3G core network as another 3G access network. In a loose coupling, the 802.11

gateway connects to the Internet and does not have any direct link to the 3G network

elements” [9]. The tying of the two networks together open unique security issues that

have not been seen in the past. In a tight coupling, WLAN networks are vulnerable to the

same attacks as standard cellular networks. The WLAN has to be secured against threats

that affect both types of networks. In a loosely coupled system, only the cellular network

is vulnerable to the discussed security issues.

Digital vs. Analog networks

In June 1999, 70% of the subscribers in the United States still used analog

handsets [13].

Security in an analog system is essentially non-existent. There is no encryption and as

such interception of the signal is all that is needed to perpetrate an attack. Digital phones

use either TDMA or CDMA. CDMA is the standard used by the GSM system which is

used in most of the world. The TDMA system is unique to the United States. Interception

and decryption would begin with the eavesdropper knowing which system was being

used. Once this is determined, an attacker would still need to bypass all other security

measures. Also, in digital systems data is both digitized and compressed. Therefore

would-be interceptors would need to know the compression algorithms.

III. Android Platform

Android is a mostly open-source operating system developed by over 30 industrial

partners with Google as the lead developing group. The OS is built on a Linux framework

and therefore contains many of the core applications. It is delivered on mobile devices

with many basic applications such as a phone dialer and address book. The security

issues revolve around the installation of other applications. The applications run in a

Dalvik virtual machine [10]. The Dalvik VM is a register-based virtual machine. The

applications are developed in Java. There is no “App Store” required like for the iPhone.

A developer can build an application and the user is free to install it on their mobile

device. The applications running on the Android system have an UNIX style UID but

sharing of data is allowed at the application level.

“Manifest files are the technique for describing the contents of an application

package” [3]. The Android manifest file is an XML file that describes all the components

contained in the application. Unlisted components are not allowed to execute. The

manifest file also lists all specific access rules for the application. All of the components

are either public or private. Private components are accessible only by the application but

all public components can be accessed by other applications. This can have devastating

effects if the access is malicious in nature or if the access inadvertently has a negative

effect on the device or the network.

The Android security model centers on an access control policy. An access

permission label is assigned to each component and each application requests a list of

permission labels. The manifest file assigns the access permissions but if the permissions

are not expressly set on public components then any application can access it. These

access permissions are fixed at install time. Access permissions are also used to control

access to resource APIs in the operating system.

IV. Security Risks

Interception

Interception is the hijacking of the mobile subscriber's signal. Interception of the

signal in and of itself is not a security risk. The cellular signal is available to all that wish

to retrieve it. The security of the signal comes in issue when the intercepted signal is

decrypted and the data is available to a non-authorized third party. Current cellular

systems use the A5 encryption algorithm. Brute force decryption of the A5 algorithm is

“infeasible in real-time” [7]. Interception and off-line decryption is possible even with a

keyspace of 2^54 bits. To make the offline decryption also infeasible, signaling protection

in the form of secure signaling is required. Secure encryption of the data inside of the

cellular transmission protects communication and makes interception useless to a would-

be attacker.

Jamming

Jamming of cellular signals is similar to a denial of service attack but focused on a

specific physical location. The jammer sends a blanket signal to an region that disrupts

the cellular traffic in the area with no regard for what the specific traffic is carrying. For

a minimal amount of money, using readily available equipment, an RF engineer could

easily build a jamming device that works in a small directed area. Jamming attacks lower

the availability of the cellular network resource. This can be directed at a individual

subscriber or at an entire network. “An attack capable of preventing the large majority of

voice communication in a metropolitan area is indeed possible with the bandwidth

available to a single cable modem” [15].

Virus introduction and propagation

The fact that users will install almost any application opens devices and networks

to exploitation through hidden virus and then propagation of those viruses. Stopping or at

least delaying the propagation can be handled through sandboxing of applications and

through limiting access privileges of applications. The closer coupling of the cellular

network and the WLAN that is being seen in the current configurations opens security

holes that would allow the propagation of these viruses across networks and into devices

that have not been the classic targets of such attacks.

Solution Models

Solution models designed for cellular traffic are all very similar and address four

basic issues. These are anonymity, authentication, encryption and integrity. A fifth

solution model, sandboxing, is being used in many newer mobile smart devices.

Anonymity

Anonymity is what protects the subscriber's identity in the case of interception.

Security models for both major network implementations use anonymity in some fashion

to protect subscribers. The identity of the subscriber is not readily available to potential

intruders. This makes intercepted data signals less useful to attackers as interception of a

specific user data stream is very difficult in a busy network. Anonymity is provided

through the use of temporary identifiers [8]. These temporary identifiers are setup and

exchanged during the initialization of the cellular communication. To bypass this factor,

the originating setup signals would have to be intercepted. These temporary identifiers

are only available in digital devices; analog cellular security has no anonymity.

Authentication

Authentication in a mobile network proves that the device sending the data signal

is who it claims to be and is provided through the use of challenge-response that is based

on the designated encryption model. The GSM network uses a standard authentication

and key agreement algorithm (AKA). [7] It uses a universal smart card that performs the

initiation, credential transfer and challenge-response sequence. The use of strong client

authentication is important to avoiding billing fraud from the operator [14]. Once a

device is authorized on the network, all communication between the device and the

network is confirmed to be from the authorized device. “Authentication in GSM proceeds

as follows: [mobile subscriber]MS receives the RAND from MSC, calculates the SRES

with A3 algorithm using RAND and Ki and sends it back to MSC” [7]. If the SRES

matches the one on file at the MSC then authentication succeeds. The corresponding Kc

is used to encrypt further communications between the device and the network.

Encryption

Encryption in the network is the most significant portion of the cellular security

model. “Most network operators use COMP128 algorithm for A3 implementation” [7].

The COMP128 algorithm uses two 128-bit inputs and produces a 128-bit output. These

are Ki and RAND in the case of A3 algorithm implementations and the first 32 bits are

the SRES output. There is a session key, Kc, that is used for encryption over-the-air after

authentication. Session key Kc is generated using the A8 algorithm. The A8 algorithm is

the voice privacy key generation algorithm and has never been made public. Combining

the A8 and COMP128 increases the level of encryption to a point that it is infeasible to

decrypt using current conventional methods.

Over the air encryption is handled using the A5 algorithm. A5 is a hardware based

cipher that uses linear feedback shift registers [7]. Export restriction of encryption

technology forced the A5 algorithm to be split into two versions: A5/1 which is stronger

but is only available in Europe and the US and A5/2 which has a weaker encryption but

can be exported worldwide [7]. Both versions of the A5 algorithm are strong enough to

halt most decryption attempts. The A5 algorithm has never been made public though

recently individuals have claimed to be able to decrypt messages using rainbow tables.

The UMTS has a defined function f8 that is used for encryption. This function is similar

in functionality to the A5 algorithm used in the GSM network.

Integrity

Integrity in a cellular network centers on assurance that the same mobile device is

transmitting data throughout the length of the connection. Random challenge vectors are

used in both GSM and UMTS to provide integrity for the length of the connection. These

challenge vectors are similar to the initial challenge-response authentication but with the

added benefit of the encryption of the message.

Sandboxing

The Android platform uses self-signed certificates with no certificate authority.

All application permissions are expressly defined by the developer and approved by the

user during the installation process. “Each app[lication] [is] isolated with its own process,

user [and] data” [10]. This sandbox forces all applications to expressly request access to

other applications and their public components. This access is governed by the manifest

file that is delivered with all applications but is managed under the sandbox protocol to

provide system stability even in an adverse environment. While the sandbox concept can

be effective, the granularity of control is not fine enough to completely protect the device

or the network [10]. Combining of the sandbox protocols with the other four models

allows for protection of the smart device and security of the mobile network being

accessed by the device.

VI. Solution Implementations Examples

Statistical RF Fingerprinting

“Physical (PHY) layer security using RF signal characteristics is one alternative

[as] such characteristics are nearly impossible to mimic” [12]. These RF fingerprints can

be used to establish a device's identity and be used to detect malicious network activity.

Device classification is performed using collective statistical metrics of the signal to

identify and authorize. The statistics “include standard deviation, variance, skewness and

kurtosis” [12]. Elements of the RF fingerprint are formed by dividing the N contiguous

equal subsequences and then calculating the four metrics across all elements. The

elements are then arranged and a parametric analysis is used to generate the unique RF

fingerprints.

PhonePrint is an application that makes use of the RF fingerprints in a commercial

setting. “PhonePrint is a combination of hardware and software that cellular operators

install in base stations in high fraud area” [13]. All handsets that request service in the

PhonePrint domain have the RF signature determined and then stored in a database along

with the mobile device and subscriber information. When future requests are made the

RF signature is compared to those stored in the database and if a match is not made, the

connection is disabled. The major use of this is to prevent cloning of mobile devices.

Physical Security

Physical security of the mobile device is the first line of defense and is the most

important single method to prevent attacks. “Cellular phones and other handheld devices

were designed to be small and mobile, but this also means that they are more likely than

other pieces of technology to get lost or stolen, and thieves can easily conceal them” [1].

In a GSM network, the physical security is achieved through the use of a subscriber SIM

card coupled with a PIN code to unlock the card [8]. Combining the physical card

(something you have) with a PIN code (something you know), makes this combination

highly effective against attacks. The physical security of the device is susceptible to

social engineering as well as device theft and bandwidth hijacking. Social engineering

can be as simple as asking someone their PIN code to use their phone. While this is

simple, it is highly effective. Protection of the device is critical to any security

implementation.

VII. Conclusion

Security in any network is a top priority. In a cellular network where all data

traffic is wireless, it becomes even more of a priority. Security in the network has to be

both complete and must not impact performance. “The security mechanism must not add

significant overhead on call setup, increase bandwidth of the channel, increase error rate

or add expensive complexity to the system” [14].

According to Ahmad, any security implementation needs five elements; it must be

complete, efficient, effective, extensible and user-friendly. Security implementations that

meet all of these requirements will be embraced by the network users and thus will

actually be used. The current and extensive research in the field of cellular to WLAN

integration has yielded an insight to the security issues . Most security is handled in the

hardware to make it seamless and efficient.

The cellular community has backed a single security model that addresses most

issues that arise in the current configurations found in cellular/WLAN networks. This

model includes anonymity, authentication, encryption and integrity. In many of the newer

cellular device that run interactive software, the concept of sandboxing has taken hold.

This adds to the overall protection of the network by protecting the device and the

network from malicious attacks hidden in delivered software. Implementations of this

security model include statistical RF fingerprinting and biometric hardware interfaces but

the most important protection of cellular networks begin with physical security of the

devices on the network and proper training of the users. As new platforms and devices are

developed, all of these concepts will need to be included during the development phase

so that cellular networks can remain safe.

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