Encryption
Encryption alters the bits of each data packet to guard
eavesdroppers from decoding data, such as credit card numbers. Before encryption
the data is called plaintext, which is easy to decode by using sniffing tools.
The encryption converts the plaintext into ciphertext, which someone can decode
only through the use of a proper secret key.
Many encryption methods, such as the 802.11 Wired Equivalent Privacy (WEP), are symmetric—that is, the same key
that does the encryption is also the one that performs the decryption. Figure 8-4 illustrates this process.
For example, the radio NIC uses key xyz to encrypt a data
packet, and an access point uses key xyz to perform the decryption. This
requires both the sending and receiving stations to trust each other, as is the
case with a private wireless network application such as an enterprise wireless
LAN. It's not practical to use symmetric keys in a public application, however,
because anyone, including hackers, could obtain the key.
For symmetric encryption to be effective, the function must
minimize the reuse of encryption keys by changing them often, possibly every
frame transmission. This decreases the time available for a hacker to break into
the network and makes it difficult—if not impossible—to compromise the security
of the network. As a result, symmetric encryption mechanisms must have effective
key distribution methods.
Public key cryptography uses asymmetric keys, with one that is
private and another one that is public. As the name applies, the private key is
secret; however, anyone can know the public key. This enables more effective
encryption and authentication mechanisms because it simplifies key
distribution.
An important requirement of public key encryption is that a set
of public and private keys must match from a cryptographic standpoint. For
example, the sending station can encrypt data using the public key, and the
receiver uses the private key for decryption. The opposite is also true. The
sending station can encrypt data using the private key, and the receiving
station decrypts the data using the public key.
If the goal is to encrypt data, the sending station will use a
public key to encrypt the data before transmission; this is shown in Figure 8-5. The receiving station uses the
matching private key to decrypt the data upon reception. Each station keeps its
private key hidden in order to avoid compromising encrypted information. As a
result, the process allows any station to use a publicly known key to send
encrypted data to any other station.
Public key cryptography works effectively for encrypting data
because the public key can be made freely available to anyone wanting to send
encrypted data to a particular station. A station that generates a new private
key can distribute the corresponding public key over the network to everyone
without worry of compromise. The public key can be posted on a website or sent
unencrypted across the network.
WEP
WEP is 802.11's optional encryption and authentication standard
implemented in the MAC Layer that most radio NIC and access point vendors
support. When deploying a wireless network, you need to fully understand the
ability of WEP to improve security.
WEP Operation
If a user activates WEP, the NIC encrypts the payload (frame
body and cyclic redundancy check [CRC]) of each 802.11 frame before transmission
using an RC4 stream cipher provided by RSA security. The receiving station, such
as an access point or another radio NIC, performs decryption upon arrival of the
frame. As a result, 802.11 WEP only encrypts data between 802.11 stations. Once
the frame enters the wired side of the network, such as between access points,
WEP no longer applies.
As part of the encryption process, WEP prepares a key schedule
(seed) by linking the shared secret key supplied by the user of the sending
station with a randomly generated 24-bit initialization vector (IV). The IV
lengthens the life of the secret key because the station can change the IV for
each frame transmission. WEP inputs the resulting seed into a pseudo-random
number generator that produces a key stream equal to the length of the frame's
payload plus a 32-bit integrity check value (ICV).
The ICV is a checksum that the receiving station recalculates
and compares to the one sent by the sending station. It determines whether the
transmitted data underwent any form of tampering while in transit. If the
receiving station calculates an ICV that doesn't match the one found in the
frame, the receiving station can reject the frame or flag the user.
WEP specifies a shared secret key to encrypt and decrypt the
data. With WEP, the receiving station must use the same key for decryption. Each
radio NIC and access point, therefore, must be manually configured with the same
key.
Before transmission takes place, WEP combines the key stream
with the payload/ICV through a bitwise XOR process, which produces ciphertext
(encrypted data). WEP includes the IV in the clear (unencrypted) within the
first few bytes of the frame body. The receiving station uses this IV along with
the shared secret key supplied by the receiving station user to decrypt the
payload portion of the frame body.
In most cases, the sending station will use a different IV for
each frame (this is not required by the 802.11 standard). When transmitting
messages having a common beginning, such as the sender's address in an e-mail,
the beginning of each encrypted payload will be equivalent when using the same
key. After encrypting the data, the beginnings of these frames would be the
same, offering a pattern that can aid hackers in cracking the encryption
algorithm. Since the IV is different for most frames, WEP guards against this
type of attack. The frequent changing of IVs also improves the ability of WEP to
safeguard against someone compromising the data.
WEP Issues
WEP is vulnerable because of relatively short IVs and keys that
remain static. The issues with WEP don't really have much to do with the RC4
encryption algorithm. With only 24 bits, WEP eventually uses the same IV for
different data packets. For a large, busy network, this reoccurrence of IVs can
happen within an hour or so.
This results in the transmission of frames having key streams
that are too similar. If a hacker collects enough frames based on the same IV,
the individual can determine the shared values among them—that is, the key
stream or the shared secret key. This, of course, leads to the hacker decrypting
any of the 802.11 frames.
The static nature of the shared secret keys emphasizes this
problem. 802.11 doesn't provide any functions that support the exchange of keys
among stations. As a result, system administrators and users generally use the
same keys for weeks, months, and even years. This gives mischievous culprits
plenty of time to monitor and hack into WEP-enabled networks.
When to Use WEP
Despite its flaws, you should enable WEP as a minimum level of
security. Many people have discovered wireless networks that use protocol
analyzers, such as AiroPeek and AirMagnet. Most of these people are capable of
detecting wireless networks where WEP is not in use and then use a laptop to
gain access to resources located on the associated network.
By activating WEP, however, you significantly minimize this
from happening, especially if you have a home or small business network. WEP
does a good job of keeping most people out. Beware: There are true hackers
around who can exploit the weaknesses of WEP and access WEP-enabled networks,
especially those with high utilization.
Temporal Key Integrity Protocol
The 802.11i standard includes improvements to wireless LAN
security. One of the upgrades is the Temporal Key Integrity Protocol (TKIP),
initially referred to as WEP2. TKIP is an interim solution that fixes WEP's key
reuse problem. In fact, many wireless LAN products already have TKIP as an
option.
The TKIP process begins with a 128-bit temporal key shared
among clients and access points. TKIP combines the temporal key with the
client's MAC address and then adds a relatively large 16-octet IV to produce the
key that will encrypt the data. This procedure ensures that each station uses
different key streams to encrypt the data.
TKIP uses RC4 to perform the encryption, which is the same as
WEP. A major difference from WEP, however, is that TKIP changes temporal keys
every 10,000 packets. This provides a dynamic distribution method that
significantly enhances the security of the network.
An advantage of using TKIP is that companies having existing
WEP-based access points and radio NICs can upgrade to TKIP through relatively
simple firmware patches. In addition, WEP-only equipment will still interoperate
with TKIP-enabled devices using WEP. TKIP is a temporary solution, and most
experts believe that stronger encryption is still needed.
In addition to the TKIP solution, the 802.11i standard includes
the Advanced Encryption Standard (AES) protocol. AES offers much stronger
encryption. AES uses the Rine Dale encryption algorithm, which is a tremendously
strong encryption that replaces RC4. Most cryptographers feel that AES is
uncrackable. In addition, the 802.11i standard will include AES as an option
over TKIP. In fact, the U.S. Commerce Department's National Institutes of
Standards and Technology (NIST) organization chose AES to replace the aging Data
Encryption Standard (DES). AES is now a Federal Information Processing Standard,
which defines a cryptographic algorithm for use by U.S. government organizations
to protect sensitive but unclassified information. The Secretary of Commerce
approved the adoption of AES as an official government standard in May 2002.
The problem with AES is that it requires more processing power
than what most access points on the market today can support. As a result, the
implementation of AES will require companies to upgrade their existing wireless
LAN hardware to support the performance demands of AES. An issue, however, is
that AES requires a coprocessor (additional hardware) to operate. This means
that companies need to replace existing access points and client NICs to
implement AES.
Wi-Fi Protected Access
The Wi-Fi Protocol Access (WPA) standard provided by the Wi-Fi
Alliance provides an upgrade to WEP that offers dynamic key encryption and
mutual authentication. Most wireless vendors now support WPA. WPA clients
utilize different encryption keys that change periodically. This makes it more
difficult to crack the encryption.
WPA 1.0 is actually a snapshot of the current version of
802.11i, which includes TKIP and 802.1x
mechanisms. The combination of these two mechanisms provides dynamic key
encryption and mutual authentication, something needed in wireless LANs. WPA 2.0
offers full compliance with the 802.11i standard.
Virtual Private Networks
If wireless users will be roaming into public areas, such as
airports and hotels, strongly consider virtual
private network (VPN) solutions. Even though VPNs are not foolproof, they
provide an effective means of end-to-end encryption. VPNs are also effective
when clients roam across different types of wireless networks because they
operate above the dissimilar network connection levels. | 
Sections
Archive
Syndication
