Wi-Fi Protected Access

An example of a Wi-Fi Protected Access label found on a consumer device

Wi-Fi Protected Access (WPA) and Wi-Fi Protected Access II (WPA2) are two security protocols and security certification programs developed by the Wi-Fi Alliance to secure wireless computer networks. The Alliance defined these in response to serious weaknesses researchers had found in the previous system, Wired Equivalent Privacy (WEP).^[1]

WPA (sometimes referred to as the draft IEEE 802.11i standard) became available in 2003. The Wi-Fi Alliance intended it as an intermediate measure in anticipation of the availability of the more secure and complex WPA2. WPA2 became available in 2004 and is a common shorthand for the full IEEE 802.11i (or IEEE 802.11i-2004) standard.

A flaw in a feature added to Wi-Fi, called Wi-Fi Protected Setup, allows WPA and WPA2 security to be bypassed and effectively broken in many situations.^[2] The WPA and WPA2 security protocols implemented without using the Wi-Fi Protected Setup feature are unaffected by the security vulnerability.

WPA

The Wi-Fi Alliance intended WPA as an intermediate measure to take the place of WEP pending the availability of the full IEEE 802.11i standard. WPA could be implemented through firmware upgrades on wireless network interface cards designed for WEP that began shipping as far back as 1999. However, since the changes required in the wireless access points (APs) were more extensive than those needed on the network cards, most pre-2003 APs could not be upgraded to support WPA.

The WPA protocol implements much of the IEEE 802.11i standard. Specifically, the Temporal Key Integrity Protocol (TKIP) was adopted for WPA. WEP used a 64-bit or 128-bit encryption key that must be manually entered on wireless access points and devices and does not change. TKIP employs a per-packet key, meaning that it dynamically generates a new 128-bit key for each packet and thus prevents the types of attacks that compromised WEP.^[3]

WPA also includes a message integrity check, which is designed to prevent an attacker from altering and resending data packets. This replaces the cyclic redundancy check (CRC) that was used by the WEP standard. CRC's main flaw was that it did not provide a sufficiently strong data integrity guarantee for the packets it handled.^[4] Well tested message authentication codes existed to solve these problems, but they required too much computation to be used on old network cards. WPA uses a message integrity check algorithm called TKIP to verify the integrity of the packets. TKIP is much stronger than a CRC, but not as strong as the algorithm used in WPA2. Researchers have since discovered a flaw in WPA that relied on older weaknesses in WEP and the limitations of Michael to retrieve the keystream from short packets to use for re-injection and spoofing.^[5]

WPA2 replaced WPA. WPA2, which requires testing and certification by the Wi-Fi Alliance, implements the mandatory elements of IEEE 802.11i. In particular, it includes mandatory support for CCMP, an AES-based encryption mode with strong security.^[6] Certification began in September, 2004; from March 13, 2006, WPA2 certification is mandatory for all new devices to bear the Wi-Fi trademark.^[7]

Hardware support

WPA has been designed specifically to work with wireless hardware produced prior to the introduction of WPA protocol,^[8] which provides inadequate security through WEP. Some of these devices support WPA only after applying firmware upgrades, which are not available for some legacy devices.^[8]

Wi-Fi devices certified since 2006 support both the WPA and WPA2 security protocols. WPA2 may not work with some older network cards.

WPA terminology

Different WPA versions and protection mechanisms can be distinguished based on the target end-user (according to the method of authentication key distribution), and the encryption protocol used.

Target users (authentication key distribution)

WPA-Personal

Also referred to as WPA-PSK (pre-shared key) mode, this is designed for home and small office networks and doesn't require an authentication server.^[9] Each wireless network device encrypts the network traffic using a 256 bit key. This key may be entered either as a string of 64 hexadecimal digits, or as a passphrase of 8 to 63 printable ASCII characters.^[10] If ASCII characters are used, the 256 bit key is calculated by applying the PBKDF2 key derivation function to the passphrase, using the SSID as the salt and 4096 iterations of HMAC-SHA1.^[11] WPA-Personal mode is available with both WPA and WPA2.

WPA-Enterprise

Also referred to as WPA-802.1X mode, and sometimes just WPA (as opposed to WPA-PSK), this is designed for enterprise networks and requires a RADIUS authentication server. This requires a more complicated setup, but provides additional security (e.g. protection against dictionary attacks on short passwords). Various kinds of the Extensible Authentication Protocol (EAP) are used for authentication. WPA-Enterprise mode is available with both WPA and WPA2.

Wi-Fi Protected Setup (WPS)

This is an alternative authentication key distribution method intended to simplify and strengthen the process, but which, as widely implemented, creates a major security hole via WPS PIN recovery.

Encryption protocol

TKIP (Temporal Key Integrity Protocol)

The RC4 stream cipher is used with a 128-bit per-packet key, meaning that it dynamically generates a new key for each packet. Used by WPA.

CCMP (CTR mode with CBC-MAC Protocol)

The protocol used by WPA2, based on the Advanced Encryption Standard (AES) cipher along with strong message authenticity and integrity checking that is significantly stronger in protection for both privacy and integrity than the RC4-based TKIP used by WPA. Among informal names are "AES" and "AES-CCMP". According to the 802.11n specification, this encryption protocol must be used to achieve the fast 802.11n high bitrate schemes, though not all implementations enforce this.^[12] Otherwise, the data rate will not exceed 54 MBit/s.

EAP extensions under WPA and WPA2 Enterprise

Originally, only EAP-TLS (Extensible Authentication Protocol - Transport Layer Security) was certified by the Wi-Fi alliance. In April 2010, the Wi-Fi Alliance announced the inclusion of additional EAP^[13] types to its WPA- and WPA2- Enterprise certification programs.^[14] This was to ensure that WPA-Enterprise certified products can interoperate with one another.

As of 2010 the certification program includes the following EAP types:

• EAP-TLS (previously tested)
• EAP-TTLS/MSCHAPv2 (April 2005 ^[15])
• PEAPv0/EAP-MSCHAPv2 (April 2005)
• PEAPv1/EAP-GTC (April 2005)
• PEAP-TLS
• EAP-SIM (April 2005)
• EAP-AKA (April 2009 ^[16])
• EAP-FAST (April 2009)

802.1X clients and servers developed by specific firms may support other EAP types. This certification is an attempt for popular EAP types to interoperate; their failure to do so as of 2013 is one of the major issues preventing rollout of 802.1X on heterogeneous networks.

Commercial 802.1X servers include Microsoft Internet Authentication Service and Juniper Networks Steelbelted RADIUS as well as Aradial Radius server.^[17] FreeRADIUS is an open source 802.1X server.

Security issues

Weak password

Pre-shared key WPA and WPA2 remain vulnerable to password cracking attacks if users rely on a weak password or passphrase. To protect against a brute force attack, a truly random passphrase of 20 characters (selected from the set of 95 permitted characters) is probably sufficient.^[18]

Brute forcing of simple passwords can be attempted using the Aircrack Suite starting from the four-way authentication handshake exchanged during association or periodic re-authentication.^{[19][20][21][22][23]}

To further protect against intrusion, the network's SSID should not match any entry in the top 1,000 SSIDs^[24] as downloadable rainbow tables have been pre-generated for them and a multitude of common passwords.^[25]

WPA packet spoofing and decryption

Mathy Vanhoef and Frank Piessens^[26] significantly improved upon the WPA-TKIP attacks of Erik Tews and Martin Beck.^[27][28] They demonstrated how to inject an arbitrary amount of packets, with each packet containing at most 112 bytes of payload. This was demonstrated by implementing a port scanner, which can be executed against any client using WPA-TKIP. Additionally they showed how to decrypt arbitrary packets sent to a client. They mentioned this can be used to hijack a TCP connection, allowing an attacker to inject malicious JavaScript when the victim visits a website. In contrast, the Beck-Tews attack could only decrypt short packets with mostly known content, such as ARP messages, and only allowed injection of 3 to 7 packets of at most 28 bytes. The Beck-Tews attack also requires Quality of Service (as defined in 802.11e) to be enabled, while the Vanhoef-Piessens attack does not. Neither attack leads to recovery of the shared session key between the client and Access Point. The authors say using a short rekeying interval can prevent some attacks but not all, and strongly recommend switching from TKIP to AES-based CCMP.

Halvorsen and others show how to modify the Beck-Tews attack to allow injection of 3 to 7 packets having a size of at most 596 bytes.^[29] The downside is that their attack requires substantially more time to execute: approximately 18 minutes and 25 seconds. In other work Vanhoef and Piessens showed that, when WPA is used to encrypt broadcast packets, their original attack can also be executed.^[30] This is an important extension, as substantially more networks use WPA to protect broadcast packets, than to protect unicast packets. The execution time of this attack is on average around 7 minutes, compared to the 14 minutes of the original Vanhoef-Piessens and Beck-Tews attack.

The vulnerabilities of TKIP are significant in that WPA-TKIP had been held to be an extremely safe combination; indeed, WPA-TKIP is still a configuration option upon a wide variety of wireless routing devices provided by many hardware vendors. A survey in 2013 showed that 71% still allow usage of TKIP, and 19% exclusively support TKIP.^[26]

WPS PIN recovery

A more serious security flaw was revealed in December 2011 by Stefan Viehböck that affects wireless routers with the Wi-Fi Protected Setup (WPS) feature, regardless of which encryption method they use. Most recent models have this feature and enable it by default. Many consumer Wi-Fi device manufacturers had taken steps to eliminate the potential of weak passphrase choices by promoting alternative methods of automatically generating and distributing strong keys when users add a new wireless adapter or appliance to a network. These methods include pushing buttons on the devices or entering an 8-digit PIN.

The Wi-Fi Alliance standardized these methods as Wi-Fi Protected Setup; however the PIN feature as widely implemented introduced a major new security flaw. The flaw allows a remote attacker to recover the WPS PIN and, with it, the router's WPA/WPA2 password in a few hours.^[2] Users have been urged to turn off the WPS feature,^[31] although this may not be possible on some router models. Also, the PIN is written on a label on most Wi-Fi routers with WPS, and cannot be changed if compromised.

MS-CHAPv2

Several weaknesses have been found in MS-CHAPv2, some of which severely reduce the complexity of brute-force attacks making them feasible with modern hardware. In 2012 the complexity of breaking MS-CHAPv2 was reduced to that of breaking a single DES key, work by Moxie Marlinspike and Marsh Ray. Moxie advised: "Enterprises who are depending on the mutual authentication properties of MS-CHAPv2 for connection to their WPA2 Radius servers should immediately start migrating to something else."^[32]

Hole196

Hole196 is a vulnerability in the WPA2 protocol that abuses the shared Group Temporal Key (GTK). It can be used to conduct man-in-the-middle and denial-of-service attacks. However, it assumes that the attacker is already authenticated against Access Point and thus in possession of the GTK.^[33][34]

Lack of forward secrecy

WPA doesn't provide forward secrecy, meaning that once an adverse person discovers the pre-shared key, they can decrypt all encrypted Wi-Fi packets transmitted in the future and even past, which could be passively and silently collected by the attacker. This also means an attacker can silently capture and decrypt others' packets if a WPA-protected access point is provided free of charge at a public place, because its password is usually shared to anyone in that place. In other words, WPA only protects from attackers who don't have access to the password. Because of that, it's safer to use Transport Layer Security (TLS) or similar on top of that for the transfer of any sensitive data.

References

External links

• Official standards document: "IEEE Std 802.11i-2004" (PDF). IEEE (The Institute of Electrical and Electronics Engineers, Inc.). 23 July 2004. ISBN 0-7381-4074-0. 
• Wi-Fi at DMOZ
• Wi-Fi Alliance's Interoperability Certificate page
• Weakness in Passphrase Choice in WPA Interface, by Robert Moskowitz. Retrieved March 2, 2004.
• The Evolution of 802.11 Wireless Security, by Kevin Benton, April 18th 2010