Security Comparison of Mobile OSes - TKK

Security Comparison of Mobile OSes
Arto Kettula
Helsinki University of Technology
Telecommunications Software and Multimedia Laboratory
Arto.Kettula@hut.fi
Abstract
Wireless applications today include all sorts of services. Still consumers want
their mobile operating systems to provide the same security functionality as with
"wired" applications like authentication, data integrity and data privacy.
This paper discusses several mobile operating systems’ security features. EPOC,
PalmOS, Windows CE and Linux on YOPY are chosen as target operating systems.
These OSs differ greatly in architecture so only general comparison of these is presented. This paper concentrates on giving a detailed overview of each one’s security
features and identifying critical weaknesses in them.
1 Introduction
Mobile users have high demand for services, they want rich Internet access despite channel constraints. Consumers prefer integrated devices, rather than carrying multiple separate
units. Handsets are typically updated frequently; many subscribers advance to new technology as it becomes available, but want to preserve their network identities.
Wireless applications today include corporate network access and E-mail, information
searching and browsing capabilities, personalized information displays (news, quotes, weather,
mapping etc), banking, payments, trading, travel, tickets, reservations, parking, tolls etc.
Document and transaction signatures and synchronization across users’ data stores are becoming important applications in the mobile world. Still consumers want their mobile operating systems to provide the same security functionality as "wired" security, like authentication (no forgery), data integrity (no tampering) and data privacy (no eavesdropping).
Consumers want interoperability - ability to connect to existing "wired" infrastructure and
provide end-to-end security. They want to be able to easily access all critical information
and services on the Internet and behind corporate firewalls.
Wireless devices are highly portable and are easily lost or stolen, so authentication of the
user and protection of private stored data is critical. Wireless transmissions are susceptible
to interception and tampering and portable devices with no fixed connection offer tempting wireless access points to hackers. Internet access is the most important new feature
of wireless devices, so security is vital. Interoperability with security standards such as
SSL and WTLS is critical, the change in the export control of cryptography removes an
important development barrier. End-to-end security is the most desirable design principle
in building these new solutions.[17]
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2 Overview of This Paper
Each operating system is discussed in a separate section. Section 3 concentrates on Symbian’s EPOC operating system, section 4 on PalmOS, section 5 on Microsoft’s Windows
CE and section 6 on Linux running on YOPY hardware. Section 7 summarizes security
features discussed on previous sections.
3 EPOC
EPOC is an operating system, application framework and application suite optimized for
the needs of wireless information devices such as smartphones and communicators, and
for handheld, battery-powered, computers. EPOC also includes connectivity software for
synchronization with data on PCs and servers.[15]
3.1 System Features
EPOC’s primary design requirements include[15]:
Reliable handling of user data, requiring very robust software design achieved through
object orientation, effective software re-use and compactness, a client-server architecture allowing most code to run with user privilege, and good software engineering
disciplines.
Integration with other wireless information devices, handportable computers, PCs
and servers, requiring link protocols such as infrared, RS232 and sockets, and suitable higher-level application protocols including industry standards such as vCard
and vCalendar.
Communication using internet and phone protocols, requiring TCP/IP and dial-up
networking support, telephony API with call control and phonebook support, and
integrated contacts database.
EPOC implements these requirements using the following major components: core, communications, languages and applications. EPOC’s application suite includes messaging,
browsing, office, PIM and Connectivity software. With the connectivity sofware, EPOC
Connect, users can synchronize data, manage files, print via PC and install applications
from PC.[14].
EPOC’s core components provide the APIs and runtime environment on which all other
components are built. The core (illustrated on figure 1) includes:
the base, a runtime adaptable to different hardware
engine support, fundamental APIs for data management
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graphics, font and bitmap management, bit-blitting, printing, and the low-level frameworks for user interaction
the EIKON GUI, which forms the basis for all EPOC release 5 application GUIs
EPOC’s communication components provide the API’s, drivers, link and higher-level protocols. Communications infrastructure includes: serial and socket APIs, telephony API,
TCP/IP and dial-up networking, PC connectivity and infrared including IrDA and IrOBEX.[13]
Figure 1: EPOC’s core
Data synchronization via serial link and infrared interface (IrDA) is possible. EPOC defines both an operating system and a JAVA VM (1.1.4) on top. Supported features include:
PC connectivity, telephony protocols and TCP/IP sockets.[13]
EPOC Connect, the PC-based connectivity and data synchronization program, provides
APIs allowing user to implement extra converters, synchronizers and other utility software.
Most of EPOC Connect’s APIs are delivered in COM format, allowing user to program in
any compatible Windows-oriented language, such as Visual C++, Visual Basic or Delphi.
3.2 Security Features
EPOC’s two fundamental security modules are the cryptography module and the certificate management module. Security includes standard cryptography algorithms, hash key
generation, random number generation, and certificate management. Symbian structures
security components as separate API and implementation sections, because of export restrictions of cryptography-related software.[14]
The cryptography module includes the following components[14, 11]:
raw cryptography algorithms allowing data to be encrypted and decrypted, and supporting symmetric ciphers: DES, 3DES, RC2, RC4, RC5, and asymmetric ciphers:
RSA, DSA, DH
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hash functions, supporting message digests: MD5, SHA (SHA1), HMAC
random number generator (RNG), the basis for the cryptographic key generation
The certificate management module provides the following services[14, 11]:
storage and retrieval of certificates
assignment of trust status to a certificate on an application-by-application basis
certificate chain construction and validation
verification of trust of a certificate
The certificate management module includes an API for use by any client requiring its services like the certificate management control panel applet. This provides a user interface to
configure data used by the certificate management component including trusted root certificates and trust status of each certificate on an application-by-application basis. Support
is initially limited to X.509 certificates along with a PKIX certificate usage profile, but the
architecture allows for other certificate formats and profiles to be added.[14]
EPOC’s kernel runs in privileged mode, owns device drivers, does power management and
allocates memory to itself and user-mode (that is, unprivileged) processes. Applications
in EPOC run in their own protected memory area and are they are protected from other
processes by kernel.[12]
4 PalmOS
The Palm OS(r) is the standard for handheld computing, a new form of computing focused
on helping people manage and access information at any time, in any location. Palm
OS handheld devices are becoming the way that everyone manages personal information,
accesses and enters corporate data, and mines the richness of the web.[10]
4.1 System and Security Features
The Palm OS platform (architecture illustrated on figure 2) consists of four primary components: Palm OS software, data synchronization technology, platform component tools
and software interface capabilities.[10]
Applications in PalmOS share the same dynamic RAM. Palm database is a list of memory
chunks and associated database header information. The records from one database can be
interspered with the records from one or more other databases in memory. So PalmOS is
vulnerable to buffer overflow attacks.
Many Palm users keep all sensitive information on their PDAs. These include accounts
information like credit cards, checking and savings accounts, ATM cards, mutual funds,
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Figure 2: PalmOS Architecture
stock accounts and load accounts. Also login ID’s, email ID’s and other sensitive information can be found there. As more Palm devices are finding their way into the enterprise and
mobile workforce, the greater the danger is to not only to the individual but to the corporate
infrastructure as well.
The primary issues surrounding Palm security are the ability to block unauthorized users
within the device. There are software applications available to help a Palm user solve security problems, such as secure authentication into a corporate network or digital signatures
for business transactions. Built into the Palm is secure power off/on routine known as the
Security Application. The application supports "Private Records" and allows user to hide
data on the Palm when it is selected. At this point user may choose to set a password, which
would then be required to show any private records in the future. If the system password is
unassigned then it is an easy matter for anyone to view all the private records.
The Palm OS built-in security feature allows user to protect the device at startup with
a password, but this can be bypassed by using the "I forgot my password feature". If this
bypass feature is used, any files marked private will be lost, and any files not marked private
are open and will not be protected. Another problem with the standard Palm security
feature is that user files, even those marked "private", can be accessed, read and copied
onto a Mac or PC. While the "private files" are not visible in Palm, they can be accessed
via user data files. This data can be viewed with any text editor.
A typical encryption tool secures data by providing a method to highlight, copy, or cut selected entries and/or complete text. It then encrypts the sensitive information and assigns a
method for retrieval, usually individual passwords. One concern with these solutions is that
if any of the characters in a block of encrypted text is edited or changed, the contents will be
damaged and usually lost forever. Some Palm programs feature a separate secure database.
In most cases each database has a "password set" that controls data entry. Secure database
utilities do not interface with other applications, they are entirely self-contained.[10]
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4.2 Palm and SSL
SSL (Secure Sockets Layer) is impractical to run over a low bandwidth wireless network
because it is quite verbose. Secure transmissions increase the size of the data packet, slowing its transmission over the network relative to unsecure transmissions. Palm implemented
a level of security for the wireless portion of the network that is equivalent to the 128-bit
SSL encryption algorithms, but optimized for use on a wireless network. The wireless part
of the network is protected by a security system that includes encryption, message integrity
checking, and server authentication.
Message encryption is done via an elliptic curve cryptography engine supplied by Certicom
Corporation. Message integrity checking protects against transmission errors or message
manipulation. Server authentication prevents the wireless session between the Palm device
and the proxy server from being hijacked or spoofed.
4.3 PalmOS Password Retrieval and Decoding
All basic built-in applications offer hiding private records from unauthorized users by
means of password. These records can be accessed only if the correct password is entered.
Palm device sends an encoded form of the password over the serial, IR, or network ports
to the HotSync Manager or HotSync Server on the desktop during the HotSync process.
The encoded password (XOR’ed against a constant block of data) can easily be decoded
into the actual ASCII version of the password. The encoded block can also be found in
the Unsaved Preferences database on the Palm device. This database is readable by any
application.
This threat can be avoided by using the "turn off and lock device" functionality of the
Security Application, or by using some third-party encryption solution.
Several proof-of-concept tools have been written to demonstrate obtaining the encoded
bassword block from the Palm device and decoding encoded password blocks to ASCII
passwords.[4]
4.4 Viruses and Trojans
Several malicious applications for Palm OS has been reported. These include LibertyCrack
(Trojan), Phage (Virus), Vapor (Trojan).[1, 2, 3]
PalmOS/LibertyCrack is the first known trojan to target the Palm operating system.
It attemts to delete all add-on applications from the handheld device. The trojan
is generally installed to a PalmOS device from a host computer during a HotSync
operation, or it can be beamed from one PDA device to another via infrared.[1]
PalmOS/Phage is the first real virus for the PalmOS. It overwrites the beginning of
Palm executables and spreads from one Palm to another during a HotSync operation.[2]
Vapor is a trojan for PalmOS. It hides the installed applications, but does not destroy
the applications themselves. When user tries to execute the application, the trojan
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is activated and it modifies the application attributes so that they are hidden from
view.[3]
5 Windows CE
Windows CE is the modular real-time embedded operating system for small footprint and
mobile 32-bit intelligent and connected devices. Windows CE combines Windows compatibility and advanced application services with support for multiple CPU architectures and
built-in networking and communications options to deliver a rich, scalable open foundation for building a wide variety of products. Windows CE powers consumer electronic devices, Web terminals, Internet access appliances, specialized industrial controllers, mobile
data acquisition handhelds, and embedded communication devices. This highly modular
platform allows developers to flexibly and reliably build the new generation of small footprint and mobile 32-bit devices that integrate seamlessly with Windows and the Internet.[7]
5.1 Windows CE System and Security Features
Applications running in Windows CE are protected from interfering with each other by
separate Memory Management Unit (MMU). Windows CE can run up to 32 processes at
one time, each running in their own threads. So Windows CE can be concidered threadsafe.[9]
Main security technologies of Windows CE-based devices include Security Support Provider
Interface (SSPI), cryptography, digital certificate handling and smart card support.[8]
SSPI provides a common interface between transport-level applications and security providers. With SSPI a transport application can call one of several security
providers and obtain an authentic connection without knowing the details of the security protocol. Windows NT LAN Manager (NTLM), Secure Sockets Layer (SSL)
versions 2 and 3 and Private Communication Technology (PCT) version 1.0 are included with Windows CE.
Windows CE supports the Microsoft Cryptographic API (CAPI) for secure communication.
For digital ceriticates management a subset of CAPI version 2.0 is supported.
The Windows CE smart card subsystem supports the Cryptography API and the
device driver model for developing smart card readers. Additional PC/SC support
facilitates the porting of existing smart card reader drivers and service providers.
The smart card subsystem provides a link between smart card reader hardware and applications that are smart card-aware. This link consists of DLLs, the smart card resource
manager API, and the smart card reader hardware device drivers. The Windows CE security model is illustrated on figure 3.
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Figure 3: Windows CE security model
The cryptographic functions supported exists as an integral part of CAPI. With these services users can add encryption to their CE-based applications. The algorithms and standards used by CAPI are implemented through cryptographic service providers (CSPs),
CAPI functions are available through the Coredll.dll module.
5.2 Security Support Provider Interface
To provide security in Intranets, client applications, such as Web browsers and e-mail applications, and their servers become more complex. Applications require different security
options depending on the use case (user authentication and data encryption). To increase
modularity of these functions, Windows CE provides the SSPI, which enables applications
to access dynamic-link libraries (DLLs) containing common authentication and cryptographic data schemes, the Security Support Providers (SSPs). The relationship of the SSP
DLLs to the SSPI Secur32.dll, Winsock, and WinInet is presented in figure 3.
SSPs provide a common method for applications to support security features. These security packages map various SSPI functions to the security protocols specified in the package. An application implementing the SSPI doesn’t need to know details about the security
protocols that the security package implements. The application programming interfaces
(APIs) contained in the SSPI are divided into package management, credential management, context management and message support.[8]
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5.3 Pocket Internet Explorer, SSL, and Encryption Pack
Windows CE’s Pocket Internet Explorer supports various security technologies used on the
Web. From e-commerce point of view, there are two kinds of security used on the Web:
1. Authorization against a server account by using a logon, and
2. Encryption via secure socket layer (SSL) using the HTTPS protocol.
While the first method is used mainly for corporate extranets, the second method is widely
used by e-commerce and Internet shopping sites. Many corporate extranet sites on the
Internet are using a password-protected entry page to protect themselves against intruders.
This feature is supported also by Pocket Internet Explorer.
Most e-commerce and shopping sites on the Web use SSL to encrypt data transfer and
increase the security of their merchant transactions. Pocket Internet Explorer can access
these sites if the SSL layer is using 40 bits or 128 bits. According to Microsoft, Pocket
Internet Explorer can be concidered as secure as its desktop counterpart.[6]
The Pocket Internet Explorer High Encryption Pack provides user with 128-bit encryption,
which can be considered the highest level of protection on the Web. Recent changes to US
export laws now allow Microsoft to distribute high encryption products worldwide.[5]
6 YOPY and PocketLinux
YOPY is a state-of-art gadget for the new network generation of the 21st information age.
It is designed and developed for personal information management, easy internet access,
E-mail, and various entertainment functions like colorful graphical games. YOPY runs on
the open-sourced Linux operating system with kernel version 2.2.14. The Linux operating
system in YOPY is developed, modified and streamlined by G.Mate,Inc to be used efficiently
for a portable device.[18]
PocketLinux is a complete platform that provides an end-to-end solution for building unified, standardized and open information communications infrastructure across the entire
spectrum of computer systems. PocketLinux is the first Open Source framework to leverage
a common software architecture to deliver consistent services to all users. PocketLinux is
unique in its ability to provide all these elements.[16]
The PocketLinux platform is ushering in a new era of information technology by refocusing users away from devices, platforms and networks and directing their attention toward
personalized information. At PocketLinux we call this a CIE (Customised Information
Exchange) - the ability to provide and access synchronized and "themed" information customized for each user’s requirements, regardless of what devices are being used.[16]
6.1 System Features
PocketLinux is built on four key pieces of technology[16]:
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Linux 2.4.x - the latest incarnation of the Linux kernel reengineered for small devices
such as PDAs, cellphones and TVs.
Kaffe - Open Source Java implementation. It enables PocketLinux to provide a
uniform programming engine on any device, regardless of hardware.
XML - used to represent all data in PocketLinux. This enables maximal interoperation between devices.
The Web - Webserver and data proxy can deliver a consistent interface to web,
whether you’re using a desktop machine or another device.
PocketLinux can run on different kinds of hardware, and because XML and Java are used,
the same applications can be run nearly anywhere. Currently PocketLinux is available on
two PDA devices - the "Helio" (vTech) and the "iPaq" (Compaq). With these the shipped
software is replaced with the PocketLinux platform and new XML and Java software can
be run on the platform unchanged. PocketLinux is distributed as Open Source under the
GNU General Public License.[16]
YOPY is based on Linux Kernel version 2.2.14 and is optimized for a handheld device.
It Supports various devices necessary for a handheld device (LCD, Compact Flash Card,
IrDA, Audio CODEC, Touch Screen, LED, vibrator, USB, UART, 28 General Purpose
I/O ports, Extended GPIO ports, Power Management, Flash ROM/SDRAM Control, and
Bluetooth).[18]
6.2 Security Features
Based on the discussion found in the Pocketlinux-devel mailing list, there is no securityrelated detailed information available concerning PDA running some distribution of Linux
OS. Linux offers features not usually found on PDAs. Pocketlinux is booted currently to
single user mode, but multi-user mode should be functional as well. Evaluating standard
Linux multi-user features on a PDA, one also has to weight the possiblities of destructive
programs getting on a PDA. Since PDAs are so ingerently different from PCs, a new OS
would be needed to handle the new security scheme.[16]
PocketLinux’s developers aren’t planning on providing a mechanism whereby untrusted
Java code would be running on the device in a sandbox because Pocketlinux doesn’t have a
full bytecode verifier. According to developers, primary threat would come from malicious
data sent to the device that would exploit bugs on the PocketLinux framework or Kaffe
itself (eg. buffer overruns). Java code tends to be more secure than similar C code (due to
the lack of pointers, etc).[16]
YOPY gadget uses Linux kernel version 2.2.14 optimized for handheld devices. Initialization System in YOPY is composed of following three parts[18]:
Boot Loader - designed to be operated only in YOPY hardware.
Kernel
Root File System - default file system like any other Linux machine.
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6.3 Linux Security Overview
Linux offers a comprehensive security support that has been part of the operating system
from the very beginning. In detail, Linux contains the following features[11]:
User identification and authentication
User rights profiles
Access control on files and directories based on owner principle (user/group/all)
Logging of security-relevant activities
Object reuse
Various levels of file system encryption available (loopback encryption, EFS etc.)
Various levels of network encryption available (PPTP,IPSEC,SSH etc.)
Access control under Linux cannot be as granular as with ACL-based systems. UNIX
(Linux) resource owner principle that assigns access rights for the owner, the owner group
and the rest of the world. Also, the user hierarchy doesn’t allow the granular delegation
of administrational rights. The overall security of Linux is monitored and improved all
the time, mainly because the disclosure of the Linux source code and the matureness of
Linux with its UNIX ancestors. The system is under permanent inspection of the Internet
community, so that discovered security leaks are published and addressed/fixed in a quite
short time frame.[11]
Processes running in Linux are protected from interfering with other processes running on
the same machine.
7 Comparative Review, Conclusion
EPOC, PalmOS, Windows CE and PocketLinux differ greatly in architecture and features
which makes it difficult to compare their security with each other. In this section we present
a short review of their features and finally collect all the comparable information into one
table.
EPOC offers great amount of functionality including a full application suite, connectivity
software and various software development kits. Naturally, the bigger the system is, the
harder it is to handle the security of that system. EPOC’s security features are basically
limited to cryptography module and certificate management module. These two modules
can’t handle all the security-related functionality required.
Processes in EPOC are protected from interfering with other processes running in the same
memory.
PalmOS can be considered the market-leader on PDAs. As demonstrated on chapter 4.1,
security features of Palm OS concentrate on securing user data and authenticating users.
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All the mentioned features have some weaknesses in their security. Passwords can be
retrieved and decoded, private data can be accessed and encrypted data can be changed
so that it becomes useless. Beginning with Palm OS 3.2, a strong security system for the
"Web Clipping" technology has been implemented, containing the following features:
Elliptic Curve-based key management
SSL authentication and encryption
DESX data encryption (DESX is a variant of DES that uses 128 bit of additional
keying material to strenghten DES against brute force attacks)
Message Integrity Check (MIC)
With PalmOS all the applications share the same dynamic RAM and can therefore interfere with each others data. Buffer overflow attacks are also easily implemented against
PalmOS.
Until Windows CE version 3, security hasn’t been much different from Windows 95/98.
There is a power-on password that must be entered correctly when the device has been
switched on. Beginning with version 2.0, there is a password protection for Pocket Word
and Pocket Excel files. Also, connection to a remote (desktop) computer requires a password. Currently Windows CE has, in addition to cryptography and digital certificates,
cryptography API -aware smart card subsystem and Pocket Internet Explorer supporting
SSL with 128-bit encryption.
Windows CE 3.0 introduces several new, enhanced security features:
The WinInet API offers 3 network-layer security protocols: SSLv2, SSLv3, and
Microsoft’s proprietary "Private Communication Technology" (PCT), an extension
to SSL.
The "Security Support Provider Interface" (SSPI) facilitates access to arbitrary security providers in a GSSAPI (RFC 1508) fashion.
The Crypto API facilitates access to arbitrary crypto provider DLLs (just like in
recent WinNT platforms).
Applications running in Windows CE are protected from interfering with each other by
separate Memory Management Unit (MMU).
Linux offers a comprehensive security support that has been part of the operating system
from the very beginning. The matureness of Linux with its UNIX ancestors as well as the
disclosure of the Linux source code substantially contribute to the overall security level
of Linux. Generally Linux supports all the above security features, but it requires more
detailed knowledge on configuring Linux to take advantage on them.
On table 4 we compare the security features of our target operating systems.
The security risks on the PDA are real - there aren’t a whole lot of "moving parts" to
attack, but there’s always going to be some avenue for attack for anybody who’s clever
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Figure 4: Security Comparison
enough. Mobile OSes are pretty compact in size compared to traditional desktop OSes.
As security requirements are compatible with OS’s size, making PDAs more secure can be
considered fairly easy. Unfortunately security has not been a primary issue when designing
these systems. Adding security features afterwards is much harder than taking security as
a component in the design phase. Many of us trust PDAs blindly because we can decide
when to transfer data between handheld and PC or some other device. We should also
consider that anybody can read our private data when we leave our PDA on table.
Eventually, I believe that people will feel safest keeping the master copies of their password
lists, credit card numbers, etc. on their PDAs, and they will even use their PDAs to conduct
financial transactions. When things get to that point, I believe that people are going to want
more than just passwords on their PDAs.
References
[1] F-Secure F-Secure Virus Descriptions - Liberty. F-Secure 30.8.2000. [referred
2.10.2000] <http://www.europe.F-Secure.com/v-descs/lib-palm.htm>
[2] F-Secure F-Secure Virus Descriptions - Phage. F-Secure September,2000. [referred
2.10.2000] <http://www.europe.F-Secure.com/v-descs/phage.htm>
[3] F-Secure F-Secure Virus Descriptions - Vapor. F-Secure September,2000. [referred
2.10.2000] <http://www.europe.F-Secure.com/v-descs/vapor.htm>
[4] Kinkpin PalmOS Password Retrieval and Decoding. @Stake, Inc., 26.9.2000.
<http://www.atstake.com/research/advisories/2000/a092600-1.txt>
[5] Microsoft Microsoft 128-bit Encryption Pack for Pocket PCs. Microsoft 2000 [referred
22.10.2000] <http://www.microsoft.com/POCKETPC/downloads/ssl128.asp>
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[6] Microsoft
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22.10.2000] <http://www.microsoft.com/POCKETPC/columns/piesecurity.asp>
[7] Microsoft
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<http://msdn.microsoft.com/library/wcedoc/wcesecur/overview.htm>
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<http://www.nue.et-inf.uni-siegen.de/ schmidt/tcsecurity/analsec.htmlPSAnal>
[12] Symbian Approaches to memory management. Symbian Oct 1999 Revision 1.0(002)
<http://www.symbiandevnet.com/techlib/techcomms/techpapers/papers/memmanc/memmanc.htm>
[13] Symbian
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[14] Symbian
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[15] Symbian
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<http://www.symbiandevnet.com/techlib/techcomms/techpapers/papers/v6/over/sp0/index.html>
[16] Transvirtual Technologies PocketLinux. Transvirtual Technologies 2000 [referred
11.11.2000] <http://www.pocketlinux.com/>
[17] Vergara, Michael - RSA Security Security in a Wireless World. Conference presentation
[18] YOPY Yopy System Architecture Overview. jhw@gmate.co.kr 2000 [referred
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