Comparative studies on authentication and key exchange methods for 802.11 wireless LAN

Comparative studies on authentication and key exchange methods for 802.11wireless LAN

Jun Lei a ,*,Xiaoming Fu a ,Dieter Hogrefe a ,Jianrong Tan b

a Telematics Group,University of Goettingen,Lotzestrasse 16-18,D-37083Goettingen,Germany

b

State Key Laboratory of CAD&CG,Zhejiang University,310027,Hangzhou,Zhejiang Province,China

Keywords:Authentication Key exchange WLAN Security Con?dentiality

a b s t r a c t

IEEE 802.11wireless LAN has become one of the hot topics on the design and development of network access technologies.In particular,its authentication and key exchange (AKE)aspects,which form a vital building block for modern security mechanisms,deserve fur-ther investigation.In this paper we ?rst identify the general requirements used for WLAN authentication and key exchange (AKE)methods,and then classify them into three levels (mandatory,recommended,and additional operational requirements).We present a review of issues and proposed solutions for AKE in 802.11WLANs.Three types of existing methods for addressing AKE issues are identi?ed,namely,the legacy,layered and access control-based AKE methods.Then,we compare these methods against the identi?ed re-quirements.Based on the analysis,a multi-layer AKE framework is proposed,together with a set of design guidelines,which aims at a ?exible,extensible and ef?cient security as well as easy deployment.

a2007Elsevier Ltd.All rights reserved.

1.Introduction

IEEE 802.11,a set of wireless LAN (WLAN)standards developed by the IEEE LAN/MAN standards committee’s working group 11,including the original 802.11and its extensions such as 802.11b,802.11a,and 802.11g,–sometimes denoted as 802.11x –was designed to offer reliable data transmission under diverse and adverse environmental conditions.Re-cently it has become one of the most popular wireless access technologies.In contrast to the 3G (third-generation)net-works,WLAN can offer higher data transmission rate for users and lower cost despite its coverage limitation.

When a user wants to access an 802.11wireless network,two key security aspects are involved:(1)authentication of the wireless user/device;(2)data con?dentiality between the

wireless device and the network,which are usually achieved by the encryption technique based on key exchange mecha-nisms.In this paper,we mainly focus on the authentication and key exchange mechanisms,which form an important building block for modern security mechanisms.

In our view,there are three major issues with today’s authentication mechanisms for wireless networks.The ?rst issue is the lack of mutual authentication between the user and the network.For instance,the identity of a user is usually veri?ed when he logs into a network,but the authentication of the network is often omitted.The second issue is caused by the wireless technology itself since the shared communica-tion channel could be monitored by any malicious user.Thereafter,attackers can easily eavesdrop or even actively modify the message header and data.To ameliorate this issue,

*Corresponding author.Tel.:t495513913578;fax:t495513914403.

E-mail addresses:lei@informatik.uni-goettingen.de ,lei@cs.uni-goettingen.de (J.Lei),fu@cs.uni-goettingen.de (X.Fu),hogrefe@cs.uni-goettingen.de (D.Hogrefe),egi@https://www.sodocs.net/doc/7f14731694.html, (J.

Tan).

c o m p u t e r s &s e c u r i t y 26(2007)401–409

one method is to apply a challenge/response model via hiding password over the air(Falk,2004).However,this method causes a third issue–known as dictionary attacks–where an attacker might?gure out the password by simply observing the pair of challenge and response messages.

To address the second issue as mentioned above,crypto-graphic algorithms are commonly used to protect the infor-mation transmitted between user and network.In order to control the operation of such cryptographic algorithms, some keys used for encryption and decryption need to be established between the communicating entities,thus various key exchange mechanisms have been proposed.For example, Public Key Infrastructure(PKI)(Housley et al.,2002)allows two parties to establish a secure channel for key exchange. The key exchange issue,however,has not been completely solved yet such as in the electronic commerce(Gar?nkel and Spafford,2002).

Recently,there have been a number of authentication and key exchange approaches proposed for WLANs.Examples include Shared Key Authentication(SKA)(Baek et al.,2004), EAP-TLS(Aboba and Simon,1999),PEAP(Anderson et al., 2003),802.1X(IEEE,2001),and802.11i(He and Mitchell,2004). However,these proposals still contain a number of security weaknesses.For instance,‘‘rouge’’access points deployed by end users pose great security risks since Denial of Service (DoS)and Distributed DoS(DDoS)attacks may occur by?ood-ing such access points.Therefore,it is useful to perform a systematic comparison among different approaches and an investigation of the overall design aspects in the long run. Among previous works,Baek et al.(2004)presented eight de-sired properties of WLAN authentication and reviewed some of recent WLAN authentication protocols.Interlink networks identi?ed the requirements that an authentication method must meet for wireless networks and discussed several EAP authentication methods.They did cover some of existing and emerging authentication and key exchange approaches and did propose some useful metrics for the comparison. However,most of these reviews were made rather from a high level perspective without providing details into the sub-stantial properties of the authentication and key exchange (AKE)methods,in particular the distinction between their common features and scenario-speci?c purposes.Moreover, some other approaches have not been considered.The objec-tive of this paper is to investigate and compare current AKE methods against their substantial properties,and accordingly, to pave the way towards developing the authentication and key exchange framework for802.11WLANs.

The rest of this paper is organized as follows.In Section2, we identify the general requirements used for WLAN AKE methods.Section3reviews an array of selected existing AKE methods by classifying them into three categories.In particu-lar,we describe their properties according to the proposed re-quirements.In Section4,we summarize the advantages and disadvantages introduced by these methods.We conclude that the existing solutions have different de?ciencies,and so far there is no single perfect solution that?ts all requirements. Based on the analysis,a plausible multi-layered framework that can be extracted from the result is presented,where sev-eral critical design guidelines are introduced for AKE method developments.Finally,this paper concludes in Section6.2.Authentication and key exchange(AKE) method requirements for IEEE802.11WLANs

Today’s most802.11networks authenticating users rely on the Extensible Authentication Protocol(EAP),de?ned in Aboba et al.(2004).EAP supports the exchange of various authentica-tion credentials such as digital certi?cates,user names and passwords.Based on Aboba et al.(2004),we identify the re-quirements for WLAN authentication and key exchange (AKE)methods,which are classi?ed into three levels:manda-tory requirements,recommended/desired requirements and additional operational requirements.

AKE methods used in802.11WLAN must satisfy the follow-ing requirements:

(1)Mutual authentication(Aboba et al.,2004)–any AKE method

for wireless networks must provide mutual authentication.

That is,the network must authenticate the user,but the user must be able to authenticate the network as well. (2)Credential security(Stanley et al.,2005)–for the con?den-

tiality and integrity protection of user’s data.

(3)Resistance to dictionary attack(Aboba et al.,2004)–this is

to prevent easy snif?ng onto802.11frames.

(4)Man-in-the-middle attack protection(Aboba et al.,2004)–

802.11provides no authentication functionality to the access point,an attacker can easily fool the network via deploying a rogue access point.

(5)Immune to forgery attacks–an attacker may forge the

public key pair so that he could be validly veri?ed.

(6)Anti-replay(packet forgery)protection–an AKE method

needs to be able to protect the network from replaying pre-vious packets by malicious users(thus limiting the amount of(D)DoS attacks).

(7)Strong session key(Stanley et al.,2005)–it is required to

have a strong session key which offers the message authentication,the con?dentiality,and the integrity pro-tection for the sessions.

Besides,the following requirements are recommended:

(1)Management message authentication–to authenticate all

management messages,preventing Denial of Service(DoS) attacks.

(2)Authenticate users–not only authenticating a user device,

but also authenticating the user can improve the commu-nication reliability.

(3)Key integrity check–to provide a key protection for key

exchanges and session-key generation procedure.

(4)Weak key protection–to address one of the vulnerabilities

from WEP,avoiding weak key attacks.

Lastly,there are three additional operational requirements:

(1)No computational burden–the new requirement for cur-

rent authentication and key exchange mechanisms be-cause some devices may have constrained power and limited hardware support(e.g.PDA).

(2)Ease implementation–one important criterion to evaluate

the deployment of different methods.

c o m p u t e r s&s e c u r i t y26(2007)401–409 402

(3)Fast reconnection(Aboba et al.,2004)–the ability to create

a new or a refreshed security association more ef?ciently if

a security association has been established in the previous

short time.

Actually,most of the mandatory requirements are moti-vated by Extensible Authentication Protocol(EAP)methods used for WLAN deployments.Other mandatory requirements mentioned in Stanley et al.(2005)are not included because they only associated with EAP methods,and do not apply to the general case.In addition,forgery key,key integrity and weak key protections are proposed to address the possible vul-nerabilities in key exchange mechanisms.Considering the properties of wireless networks and some newly discovered security weaknesses(e.g.DDoS attacks),some recommended requirements are https://www.sodocs.net/doc/7f14731694.html,stly,three operational consid-erations are illustrated as the realistic deployment issues of AKE methods are taken into considerations.

3.Authentication and key exchange(AKE) methods overview

The reason why WLAN security is dif?cult to accomplish is that WLANs can be deployed everywhere and generate differ-ent connection concepts from wired networks.In this section we group several AKE methods into three categories,namely, legacy AKE methods,layered AKE methods and access con-trol-based AKE methods.The classi?cation accords with the structural concerns,such as layered and access control-based features.Furthermore,we evaluate them according to the requirements proposed in the previous section.

3.1.Legacy AKE methods

The simplest and default authentication method for legacy 802.11is Open System Authentication(OSA)which uses two steps to authenticate users.First,the client sends an authen-tication request with its identity to the access point.Second, the access point authenticates the client via validating the identity.The second legacy method is Shared Key Authentica-tion(SKA)which uses a challenge/response mode with shared keys to provide the authentication(Interlink Network,2003). The main difference between OSA and SKA is the latter pro-vides mutual authentication.

Based on two presented methods,Wired Equivalent Pri-vacy(WEP)protocol was introduced in1997to provide authen-tication and data encryption between a host and a wireless access point.It uses pre-shared key(PSK)that are manually exchanged at the both endpoints.However,WEP was found rather weak for the purpose of authentication and is no longer recommended for future use.Several papers have identi?ed the weaknesses existed in WEP security issues(Walker, 2000;Fluhrer et al.,2001;Stubble?eld et al.,2001).These de?-ciencies can be summarized as follows.

1.WEP has no protection to forgery attacks.

2.WEP provides no replay protection.

3.WEP misusing RC4algorithm for the encryption so that the

protocol is extremely weak to key attacks.4.WEP has the security hole that attacker without the encryp-

tion key but reusing IV can decrypt the encrypted code.

In summary,legacy AKE methods for802.11networks have demonstrated many?aws of security protocol design,hence many of them are vanishing from today’s deployment.

https://www.sodocs.net/doc/7f14731694.html,yered AKE methods

It has been found that the security mechanisms offered in a single layer(mostly network layer)would not be suf?cient in many deployment scenarios.Thus,some deployments of IEEE802.11WLANs use layered AKE methods to provide secu-rity,including EAP-TLS,EAP-TTLS(Funk and Blake-Wilson, 2004),PEAP,EAP-SPEKE(Jablon,1997),EAP-FAST(Cam-Winget et al.,2005)and EAP-PSK(Bersani and Tschofenig,2005).EAP was standardized in1998(Blunk and Vollbrecht,1998),serving as a framework offering a basis for carrying other authentica-tion methods.Its main advantage is the independence from any particular authentication algorithm and hence it can be highly extended.

Due to the space limitation,however,in this paper we only choose universal EAP-based methods to address the AKE is-sue.Some proprietary protocols(e.g.Lightweight EAP(LEAP) (Banan,2000)developed by Cisco,which is created only to support the Cisco System Aironet products)will not be dis-cussed here.

3.2.1.TLS embedded protocol

EAP-TLS,EAP-TTLS,PEAP and EAP-FAST all incorporate with TLS(Dierks and Allen,1999)to enable the secure communica-tion between a client and a server,as well as to address inher-ent de?ciencies(e.g.no protection of the user identity;no support for fast reconnections)when EAP is deployed into the WLAN alone.Fig.1presents the layered model of TLS em-bedded protocols.They add a TLS layer on top of the EAP and the corresponding TLS session is established to protect the legacy EAP method.

In addition,EAP-TTLS and PEAP address the weaknesses of insecure authentication channel during the

authentication

Fig.1–TLS embedded protocol layered model.

c o m p u t e r s&s e c u r i t y26(2007)401–409403

phase,which is established by the TLS Handshake protocol allowing the server to authenticate the client (Rigney et al.,2000).Fig.2shows the network architecture model for EAP-TTLS usage and the types of security it provides.The client and AAA server exchange a sequence of EAP messages which are encrypted and authenticated using TLS session keys.Therefore,EAP-TTLS is no longer venerable to dictionary at-tacks or the relay attack because the attackers have to break the secured EAP-TLS tunnel to mount these attacks on the cli-ent authentication if they want to sniff the tunneled session.

It is noted that EAP-TLS has been widely deployed and can provide a well-formed and reliable mechanism to perform mutual authentication between EAP peer and EAP server.Besides,EAP-TTLS and PEAP enhance the security of legacy EAP methods,especially concerning credential security and anti-relay protection aspects.However,all tunneled authenti-cation protocols are potentially venerable to the Man-in-the-Middle attack,which is discovered by Asokan et al.(2003).The paper identi?ed that the attack can be launched either when the client is authenticated based on the same identity and the same authentication token in different environments,or when the client fails to properly authenticate the server while building the tunnel.Therefore,they proposed a crypto-graphic binding between the client authentication protocol and the protection protocol,which allows the ?exible and se-cure usage of the authentication protocol in multiple authen-tication environments.

https://www.sodocs.net/doc/7f14731694.html,yered method with cryptographic design

Incorporating with cryptographic algorithms strengthens the mutual authentication and provides more protections for WLANs.Different from PEAP or EAP-TTLS,EAP-FAST employs a symmetric cryptography and accordingly provides an en-hancement to mutually authenticate the client and the server during the secure channel establishment phase.EAP-PSK is another AKE layered method with cryptographic design,which alleviates computational burden via not using asym-metric cryptography,but using a single cryptographic algo-rithm AES-128.The basic cryptographic design of EAP-PSK is formulated as Fig.3.Because of its particular key design,EAP-PSK has attracted several attentions,mainly in the scope of IEEE 802.1e (WiMAX).The WiMAX forum has suggested EAP-PSK to perform device authentication between the user’s device and the base station of the access network (WiMAX Forum,2005).

Besides,EAP-SPEKE is a simple password authentication method without performing public key or certi?cate mecha-nisms.It utilizes a series random-shape messages which are

exchanged between devices.Upon the message exchange SPEKE modules compute these received messages to deter-mine whether the password used at the other device is cor-rect.Because of this,EAP-SPEKE can further address the issue of Man-in-the-Middle and off-line dictionary attacks.

In summary,the signi?cant difference between the legacy methods and the layered AKE methods is that the layered methods separate the authentication process from the mes-sage protection.This separation is achieved by EAP which acts as the basis for higher layer authentication protocols (such as TLS,TTLS,FAST,and SPEKE).Although layered design appears as a highly ef?cient,easily deployable authentication framework over 802.11WLANs,it also contains certain vulner-abilities,e.g.no identity protection;no protected ciphersuite negotiation;and no fast reconnection capability.To further address these weaknesses,some researchers have proposed access control-based layered AKE methods which are described as below.

3.3.Access control-based layered AKE method

The advent of 802.1X (IEEE,2004)provides a port-based net-work access control as a part of the IEEE 802group of proto-cols.Only when the authentication server authorizes the supplicant,the 802.1X port will be kept connected and the supplicant will be granted access to the network.

3.3.1.Transitional solution

WPA (Baek et al.,2004)is the transitional security solution for 802.11wireless networks until 802.11i can be widely supported by the common hardware,and is developed in conjunction with the IEEE.It takes basis on 802.1X and EAP for authentica-tion and Temporal Key Integrity Protocol (TKIP)for traf?c https://www.sodocs.net/doc/7f14731694.html,IP addresses all WEP’s well-known vulnerabilities but does not guarantee them completely.Counter Code CBC/MAC (CCMP)is then recommended to replace TKIP.However,TKIP is still in use because most legacy hardware offers no capability of supporting the Advanced Encryption Standard (AES)algorithm,the basis of CCMP.

Speci?cally,WPA utilizes a key mixing function to avoid the weak key attack.

If we set Temporal Key ?T;

Intermediate Key ?I;Per-packet Key ?K;

802MAC address of the local wireless interface ?A .

Access Client

AA A (Home)Server

AAA Server Fig.2–

EAP-TTLS architectural model.

PSK

Plain Text Block

Cipher Text Block

Fig.3–Cryptographic design of EAP-PSK.

c o m p u t e r s &s e c u r i t y 26(2007)401–409

404

Step1:T4A?I.This way,combing with MAC address causes different results of I even thought it generates from the

same temporal key T;

Step2:b4I?K,where b is a tiny cipher to encrypt the packet sequence number;

Step3:streamkey?RC4(IV,K),to overcome the one of the vulnerabilities in RC4cryptography and protecting

from the weak key https://www.sodocs.net/doc/7f14731694.html,IP uses an extended Ini-

tialization Vector(IV)as a packet sequence number

and requires the receiver to enforce proper IV sequence

of arriving packets.Here,the IV sequence enforce-

ment is a strategy of defeating replay attacks.

Since2004,the second generation of WPA(WPA2)has been standardized to provide a scalable and long-term wireless se-curity system.WPA2uses the Advanced Encryption Standard (AES)for data encryption.However,new hardware is neces-sary for running in the WPA2mode(Wi-Fi Alliance,2005).

3.3.2.Long-term scheme

The802.11architecture consists of three parts:802.1X for authentication;Robust Security Network(RSN)for keeping the track of associations;and Advanced Encryption Stan-dard-based Counter Mode CBC-MAC Protocol(AES-CCMP)to provide integrity,replay protection and con?dentiality(Prasad and Schoo,2002).The important element of802.11i authenti-cation process consists of four-way handshake as depicted in Fig.4.Upon a successful handshake,a secure communication channel will be established to protect the subsequent data transmission.

802.11i offers crucial security enhancements to802.11,in-cluding a complete protection of the Layer2packet,i.e.both header and payload and also prepares the framework with strong,mutual authentication between the client and the ac-cess point(security server).Besides,802.11i can provide a com-plicated authentication based on802.1X and the AES-CCMP encryption protocol which is seen as the long-term solution for data transfer over WLANs.On the one side,CCMP provides con?dentiality and privacy by encoding the plaintext before encrypting it.On the other side,AES has been proven to be an extremely solid encryption algorithm(Prasad and Schoo, 2002).Unfortunately,802.11i still contains unavoidably weak-nesses and is complicated to implement in a certain extent (He and Mitchell,2005)https://www.sodocs.net/doc/7f14731694.html,parison results

Table1gives a comparison of aforementioned methods against the proposed requirements in the three levels as dis-cussed in Section2.Firstly,WEP can satisfy few of them; whereas most EAP layered methods can satisfy all mandatory requirements.Secondly,TLS embedded protocols(e.g.EAP-TLS,PEAP,etc.)can be immune to forgery attacks and protect themselves from the anti-relay attack because TLS provides a secured tunnel for the communication between the wireless client and the server.Once incorporated with cryptographic techniques,some layered AKE methods(e.g.EAP-FAST)have some indications of vulnerabilities,such as the dictionary attack,which is understandable because password-based methods are suffering from the weaknesses of numeric de-cryptions.Moreover,access control-based AKE methods(e.g. WPA,802.11i)rely on a layered model which takes EAP as the basis for authentication,and some complicated crypto-graphic mechanisms for encryption.As a result,they have elementary capability of defending common attacks despite their complicated and costly implementation.

We can further conclude that mutual authentication is not equal to simply authenticate a user and a server since the con-cept of‘‘a user’’includes‘‘a client’’and‘‘a device’’(e.g.tele-phone,and PDA).For example,EAP-TTLS and PEAP show a strong capability of mutual authentication.Nevertheless, they cannot completely satisfy the requirement of authenti-cating users.Only after authenticating both the client and the access device,the authentication for a user is successful. Based on the table of comparison,we conclude that there is no existing solution immune to any attack and it is hard to identify which is the most appropriate method for any 802.11WLAN.

5.Towards a multi-layer AKE framework

and its architectural considerations

Upon the above analysis,we propose a multi-layered AKE framework for802.11WLANs.As shown in Fig.5,it comprises three components:802.1X for access control,the combination of EAP and TLS for mutual authentication and key distribu-tion,and other high-layer protocols based on TLS-EAP for other new functionalities,such as PSK(Blunk and Vollbrecht, 1998).

We argue this multi-layer framework is advantageous over existing systems.First,it allows an EAP peer to take advantage of the protected ciphersuite negotiation,mutual authentica-tion and key management capabilities of the TLS protocol (Aboba and Simon,1999).Note that EAP-TLS has been one of only available EAP methods for a long time.EAP-TLS-PSK (Otto and Tschofenig,2006)is currently being standardized, which takes advantages from both the EAP-TLS and TLS-PSK.Second,802.1X is a framework for the user authentica-tion and key distribution.It utilizes many authentication methods,including passwords,certi?cates and smart cards to control the network access.Therefore,the multi-layered AKE framework is?exible and extensible due to its intrinsic support from EAP and802.11X.As having considered today’s

Fig.4–Four-way handshake in802.11i.

c o m p u t e r s&s e c u r i t y26(2007)401–409405

AKE requirements,the proposed framework intends to be applicable for future deployment scenarios.Fourth,new functionalities could be easily incorporated into the frame-work,and hence the framework can be augmented to ad-dress threats caused new security concerns or development challenges of wireless technologies.It is also noted tradeoffs within this framework can be made between the security as-surance level,high ef?ciency and the required implementa-tion and development costs.

This framework implies a set of key design guidelines to improve the ef?ciency of AKE methods,and also to meet the security requirements identi?ed in Section2.They are identi-?ed as follows.

A To select a proper method for a certain security purpose is

a good choice.Generally,we should conduct a risk analysis

to determine the level of protection a certain WLAN re-quires and then?nd the most cost-effective protection against attacks.For instance,the EAP-SPEKE method espe-cially?ts for SOHO users and public hot spots where client distribution can be controlled and managed.The advan-tages of easy implementation and low cost as well as?ex-ible infrastructure are the main reasons why EAP-SPEKE is chosen.

A AKE methods should consider preventing from some types

of(D)DoS attacks.First,wireless networks are extremely vulnerable to(D)DoS attacks since its data transmission happens in the open air.Second,(D)DoS attacks may result in other attacks.However,today’s methods involve little in

(D)DoS attacks protection.Wan et al.(2005)propose a new

Public Key Cryptosystem(PKC)-based protocol to provide the DoS resistance as well as the identity anonymity for the users.

A Decision on how to?nd the tradeoff between easy imple-

mentation and strong security is another consideration for AKE https://www.sodocs.net/doc/7f14731694.html,ers prefer an easy and ef?cient AKE implementation,while service providers pay more atten-tion to the stability and reliability.One example is that the widespread deployment of802.11i has been cumbered until now because the use of AES requires new?rmware and driver supports which are both expensive and time consuming.

A A combination of existing mechanisms or with some new

technique together to overcome existing problems may be

a feasible solution,since deploying a new security protocol

is expensive and time-consumable.For instance,Mobile IPv6(Johnson et al.,2004)combines AAA architecture.IPsec operates at the IP layer,which can support the system authentication and authorization.But if using IPsec alone to protect the wireless LAN,the structure will fall under a weak environment.Just using AAA for a wireless environment,the part of wireless may also cause a loophole (Engelstad et al.,2003).In this case,the use of Secure Sockets Layer(SSL)may overcome the problem.Currently,

a Diameter AAA architecture for Mobile IP users has been

implemented at the University of Stuttgart(Zhang,2004).

With the continuous development of wireless technolo-gies,new attacks and security concerns will increase.As a re-sult,the extensibility property for future developments should be taken into account when designing new AKE solu-tions.It is therefore necessary to devise an ef?cient and strong AKE framework.The development of this framework may fol-low the above-mentioned design guidelines,which we believe will result in a compromised solution being both practical and secure in the near future.

6.Conclusion

In this paper we have identi?ed the AKE requirements for 802.11WLAN.We reviewed11desired802.11WLAN authenti-cation and key exchange methods,and compared most of them:WEP,EAP-TLS,EAP-TTLS,EAP-PEAP,EAP-FAST,EAP-SPEKE,EAP-PSK,WPA and802.11i(WPA2)against the identi-?ed requirements.It seems that among them EAP-based layered AKE methods are more promising since they can pro-vide the strong security by EAP-TLS as well as some comple-mentary features,e.g.high ef?ciency and easy deployment. In particular,WPA2and802.11i provide the fundamental capability of defending common attacks despite their compli-cated and costly implementation.Based on the analysis,we proposed a multi-layered AKE framework,and as well as a set of design guidelines.

The framework proposed in this paper has a reasonable set of features;fairly strong security,?exibility and extensibility. In the following steps,we will continue to work on evaluation of such a design,and detailed analysis of each component in terms of its supposed functionalities.However,there are many other areas for interesting future work,for instance, the investigation on new functionalities provided by other high-layer protocols,and possible extensions to the proposed framework for the purpose of ef?ciency.Moreover,it is not sure whether these mentioned AKE methods can support suf-?ciently fast handovers among access points since their fast connection feature is not speci?cally discussed in this paper. It will be,therefore,interesting and useful to characterize how to handle fast-roaming users by these AKE methods.

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Fig.5–A multi-layered AKE framework for802.11WLANs.

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March2005.

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Jun Lei received the B.S degree in Mathematics from Hangzhou Teachers College,China,in2001,the M.S degree in Computer En-gineering from the Research Institute of Engineering&CG,Zhe-jiang University,China,in2004.She is currently working toward the Ph.D.degree in computer science from the Institute for Infor-matics,University of Goettingen,Germany.

Her research interests are overlay networks with emphasis on enabling multicast over the Internet,security issues in wireless networks,mobile networking and QoS-constrained multimedia transmission over MPLS.She is a member of the IEEE,and has served on technical reviewer committees of several journals and conferences.

Xiaoming Fu received his Ph.D.in Computer Science from Tsing-hua University,China in2000.During2000–2002,he was a member of research staff at Telecommunication Networks Group,Techni-cal University Berlin,Germany.Since September2002,he is an as-sistant professor at the Institute for Informatics,University of Goettingen,Germany,leading a research team working on net-work architectures,protocol design,validation and performance evaluation,Internet QoS and signaling,security,mobile networks beyond3G,overlay and multimedia networking.In these aspects he has actively contributed to EU projects ENABLE,DIADALOS II, VIDIOS and MING-T,as well as in other international collaborations.

He is a member of the IEEE and the ACM,and has served on technical program committees of several conferences,such as IEEE INFOCOM,ICNP,ICDCS,GLOBECOM and ICC,and the pro-gram chair of the International Workshop on Mobility in the Evolving Internet Architecture(MobiArch)2006–07.

Dieter Hogrefe graduated at Philips Exeter Academy,USA,in 1976and studied Computer Science and Mathematics at the University of Hannover,Germany,where he graduated with a diploma degree and Ph.D.His research activities are directed towards Computer Networks and Communication Software Engineering.

He is full professor(C4)for Telematics at the University of Goettingen since2002.He published numerous papers and two books on Internet technology,analysis,simulation and testing of formally speci?ed communication systems.Prof.Hog-refe held full professor positions at the Universities of Dort-mund,Bern,Luebeck,and Goettingen and visiting positions at UC Berkeley and Hamilton University.From1983to1986he was with the SIEMENS research centre in Munich and worked in the area of analysis of telecommunication systems.He was responsible for the protocol simulation and analysis of the CCS No.7.

Prof.Hogrefe represents the IITB(Fraunhofer Institute for in-formation and data processing)in the European Telecommunica-tion Standards Institute,ETSI,where he is chairman of the Technical Committee Methods for Testing and Speci?cation.

c o m p u t e r s&s e c u r i t y26(2007)401–409 408

Jiangrong Tan received his Ph.D.degree in Mathematics from Zhejiang University,China,in1992.Now he is a professor and doctoral supervisor of Zhejiang University,China.His research in-terests include CAD,CIMS,virtual reality and scienti?c visualiza-tion,etc.He has published several papers and three books on CAD and CIMS.He is currently also the vice director of‘‘state key lab of CAD&CG,Zhejiang University’’.

Professor Tan received‘‘the State Foundation for Excellent Young Scholars’’in1994and‘‘National Foundation on Graphics Science for Excellent Young Scholars’’in1995.He is also con-?rmed as‘‘The leader of key study of Zhejiang Province’’in1997 and received‘‘Top Grade Award in Excellent Teacher of Baogang’’in2003.He has also received the‘‘State second award of science and technology’’in2004.

c o m p u t e r s&s e c u r i t y26(2007)401–409409

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