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In the last few years, there has been a dramatic growth of use of wireless local area networks (WLANs), due to cost effective, convenient and easy solution for installing data networks. However, security over WLANs is more complicated compared to that of wired LANs. As a result, maintaining and improving the security of WLANs remains a major concern, especially for critical applications as, for example, the SCADA networks. In order to provide effective protection of data confidentiality and integrity, a variety of standards have been designed so far. The IEEE 802.11 is a family of standards for wireless local area networks (WLANs) created by the IEEE LAN/MAN Standards Committee. Initially, the IEEE 802.11 implemented two basic security mechanisms: entity authentication (open and shared key) and the Wired Equivalent Privacy (WEP) security protocol. However, in order to overcome unveiled weaknesses of WEP and further enhance the level of security of IEEE 802.11, a new standard was developed. More specifically, the IEEE 802.11i standard was published on 24 June 2004 as an amendment of the original IEEE 802.11 providing security mechanisms for wireless networks in MAC layer. IEEE 802.11i replaces the previous WEP security specification that appeared to have much insecurity, basically related to the encryption algorithm and the length of keys used. In addition to key management and establishment, IEEE 802.11i provides encryption and authentication enhancements. This paper gives a historical retrospection in the development of IEEE 802.11 sets of standards, presents the IEEE 802.11i security architecture and mechanisms and provides a comparative evaluation of its security strength.
Keywords: WLAN Security, IEEE 802.11i, WEP, WAP
Wireless local area networks (WLANs) provide data network communication while eliminating or reducing the need for wired connection installations. WLANs support an easy and flexible way for communicating. Therefore their acceptance and use has increased rapidly the last few years, mainly due to flexibility and mobility offered to the users.
The set of standards IEEE 802.11  provide the necessary specifications for WLAN data communications. Their initial form was presented in 1997 by the IEEE LAN/MAN Standards Committee. Since then, a lot of amendments have been added to improve Quality of Service, to satisfy higher data rates and frequency transmissions and to enhance security mechanisms. More specifically, while 802.11 created in 1997 was the first wireless networking standard, 802.11b  presented in 1999 was the first widely accepted one as a result of its more extensive throughput. Furthermore, in 2003 the 802.11g  standard was proposed which was increasingly adopted due to its higher speeds. 802.11n  is a recent amendment, added in 2009, which improves upon the previous standards by adding multiple input - multiple output antennas having a significant increase in the maximum raw data rate. Other standards in the 802.11 family are service amendments and extensions or corrections to the previous specifications.
Security of WLANs was weak until the publication of the 802.11i amendment. Initially, the IEEE 802.11 implemented two basic security mechanisms: entity authentication (open and shared key) and the Wired Equivalent Privacy (WEP) security protocol. However, in order to overcome unveiled weaknesses of WEP and further enhance the level of security of IEEE 802.11, a new standard was developed. More specifically, the IEEE 802.11i  standard was published on 24 June 2004 as an amendment of the original IEEE 802.11 providing security mechanisms for wireless networks in MAC layer. IEEE 802.11i replaces the previous WEP security specification that appeared to have much insecurity, basically related to the encryption algorithm and the length of keys used. In addition to key management and establishment, IEEE 802.11i provides encryption and authentication enhancements. This paper gives a historical retrospection in the development of IEEE 802.11 sets of standards, presents the IEEE 802.11i security architecture and mechanisms and provides a comparative evaluation of its security strength.
2. IEEE 802.11 Network Components
In order to better examine and understand 802.11 WLANs, their two fundamental architectural components are presented .
Station (STA): wireless endpoint device. For example laptop computers, mobile phones, PDAs and generally any devices with IEEE 802.11 capabilities.
Access Point (AP): device that connects wireless STAs with an organization's wired infrastructure, or wireless STAs with each other.
3. History of WLAN security
While WLANs have become more widely deployed, wireless security remains a serious concern for a large number of organizations. Wireless networks add an extra level of security complexity compared to wired networks. Wireless radio signals propagate through the air and pass through exterior walls. The fact that an organization's network could be reached outside of its physical boundaries, has introduced new threats and vulnerabilities.
The three major objectives of communications security are: confidentiality, authentication and integrity. The major security protocol introduced prior to the ratification of 802.11i that attempted to achieve these goals is the Wired Equivalent Privacy (WEP).
3.1 Wired Equivalent Privacy (WEP)
WEP was introduced as part of the 802.11 set of standards in 1999 , in an effort to meet the three objectives of communications security. First of all, WEP uses the RC4 encryption algorithm to support confidentiality. RC4 is a stream cipher that operates expanding a short key into an infinite random key stream. The station XORs this key stream with plaintext and the cipher text is produced.
Furthermore, WEP uses the Integrity Check Value (ICV) with a 32 bit Cyclic Redundancy Check (CRC-32) in order to ensure integrity. ICV and payload are both encrypted with the RC4 algorithm. Finally, at the destination the message is decrypted and the CRC is computed. If the CRC produced by the source and sent with the message is the same with this new recomputed CRC the message is valid. Otherwise the message is discarded.
Fig. 1 WEP confidentiality and integrity procedures
WEP supports two types of authentication, Open and Shared key authentication. In Open authentication, the Access Point accepts every station without identity verification. As a result, Open authentication can be considered as a non-authentication procedure. On the other hand, Shared key authentication requires the station to be aware of a secret key in order to be able to join the network. If the station possesses the key, a four way message exchange begins to achieve authentication. The first message sent from a station declares the authentication request and includes the station's MAC address. The Access Point replies with a generated string as a challenge text. In response, the third message sends the challenge back to the Access Point encrypted with the RC4 algorithm with the ICV. The Access Point de-encapsulates it, checks the decrypted ICV and if it is successful, the AP compares the received decrypted challenge text with the challenge text sent from the Access Point in the second message. Whether the two texts are identical, the AP sends the fourth and last message indicating successful authentication. Otherwise, the Access Point notifies for unsuccessful authentication and the station is rejected.
Fig. 2 Message authentication
Generally, WEP offers a small level of protection and it is not recommended to be used in high- demanding security environments. More specifically:
The 32 bit CRC is not capable of providing data integrity.
The cryptographic key and IV are short.
Manual key management introduces a lot of problems.
Weak key schedule in RC4 , it allows attackers to assemble what is called "decryption dictionaries".
4. The IEEE 802.11i
IEEE 802.11i is an IEEE standard presented in 2004 for 802.11 networks. It was designed to enhance security in the Medium Access Control (MAC) layer. 802.11i supersedes the previous security specification,Â Wired Equivalent PrivacyÂ (WEP), which has shown important security weaknesses.Â Wi-Fi Protected AccessÂ (WPA) was previously introduced by theÂ Wi-Fi AllianceÂ as a solution to WEP vulnerabilities. The Wi-Fi Alliance refers to their approved, interoperable implementation of the full 802.11i asÂ WPA2, also calledÂ RSNÂ (Robust Security Network). IEEE 802.11i makes use of theÂ Advanced Encryption StandardÂ (AES)Â block cipher, whereas WEP and WPA use theÂ RC4Â stream cipher .
4.1 Wi-Fi Protected AccessÂ (WPA)
WPA was introduced as a successor of WEP on the way to the final IEEE 802.11i standard. It uses the Temporal Key Integrity Protocol (TKIP) to support confidentiality and integrity. Until 2008, that the first attack was published , TKIP was believed to be secure. Furthermore, to support authentication, WPA uses the 802.1X protocol.
More specifically, TKIP uses the RC4 encryption algorithm like WEP, but in order to reinforce security it doubles the IV to 48 bits. These 48-bits are used as a TKIP Sequence Counter (TSC) in order to create a sequence during packet transmission and enhance security to replay attacks. Also, the key mixing function is more complicated. A unique encryption key is generated for MAC Protocol Data Unit (MPDU) by combining the transmit address, the temporal key and the TSC.
Concerning integrity, TKIP uses an 8-byte field called Message Integrity Check (MIC) and has a function similar to the older Integrity Check Value (ICV). However, MIC protects both packet payload and header while ICV protects only the packet payload. The algorithm that implements the MIC is called "Michael". MIC enhances security to forgery attacks.
Fig. 3 WPA MPDU Format
Moreover, WPA uses the same authentication methods described in WEP and the 802.1 X authentication method which was introduced by the 802.11i standard. The 802.1X/EAP authentication mechanisms will be described in the WPA2 section.
4.2 Wi-Fi Protected Access 2 (WPA2)
Based on the IEEE 802.11i standard, WPA2 provides stronger security with new mechanisms without the WEP bindings. The IEEE 802.11i defines the Robust Security Network Association (RSNA) and separates security into the pre-RSNA mode (WEP and WAP) and the RSNA mode (WPA2).
WPA2 uses the Counter Mode with Cipher Block Chaining Message Authentication Code ProtocolÂ (CCMP) to support confidentiality and integrity. CCMP uses AES encryption algorithm with a 128 bit key field and 128 bit block field size. The encrypted fields of the WPA2 MPDU are the payload and the MIC. CCMP does not use the WEP ICV anymore.
Fig. 4 WPA2 MPDU Format
Concerning authentication, WPA2 uses the 802.1X method. This method specifies three main entities, an authentication server, an authenticator and a supplicant. A supplicant is a client device that wants to connect to the WLAN, an authenticator is usually a wireless access point and an authentication server is a host supporting the RADIUS and EAP protocols. The authentication procedure that takes place between them is the following: once the authentication server authenticates a supplicant, informs and passes key material to the authenticator. As a result, key material exchange is implemented between the supplicant and the authenticator using the Extensive Authentication Protocol over LAN (EAPOL). In order to create keys for the EAPOL handshaking procedure a key hierarchy is used. There are two key hierarchies in the IEEE 802.11i: the pairwise key hierarchy for unicast traffic protection and the group key hierarchy for multicast and broadcast traffic protection. Furthermore, the key exchange can be implemented in two ways: with the 4-way handshake and the group handshake.
5. WEP, WAP, and WAP2 Security Comparison
In this section, a summary of the security mechanisms implemented in WEP, WAP and WAP2 is presented to support authentication, confidentiality and integrity.
Shared Key Authentication
802.11X authentication with (RADIUS) server
RC4 with 64 bit key
RC4 with 128 bit key
MPDU + ICV
RC4 with 256-bit key
MPDU + MIC + ICV
AES-CCM with 128 bit TK
MPDU + MIC
32 bit ICV with CRC-32
(i) 64 bit Michael MIC
(ii) 32 bit ICV
(i) 64bit CCMMIC for traffic messages
(ii-a) HMAC-MD5 with KCK,
(ii-b) HMAC-SHA1 with 128 bit KCK for
EAPOL 4-way handshake
It is apparent that WEP and WAP, with the use of the RC4 encryption algorithm and shared key authentication cannot provide guaranteed confidentiality, integrity and user-authentication. Therefore, it can be said that only WPA2 implements strong security characteristics and it is a secure solution in a WLAN.