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.To protect data, WEP requires the use of the RC4 cipher, which is a symmetric (secret­key) stream cipher.RC4 shares a number of properties with all stream ciphers.Generallyspeaking, a stream cipher uses a stream of bits, called the keystream.The keystream isthen combined with the message to produce the ciphertext.To recover the originalmessage, the receiver processes the ciphertext with an identical keystream.RC4 uses theexclusive OR (XOR) operation to combine the keystream and the ciphertext.Figure 5-1illustrates the process. Figure 5-1.Generic stream cipher operationMost stream ciphers operate by taking a relatively short secret key and expanding it into apseudorandom keystream the same length as the message.This process is illustrated inFigure 5-2.The pseudorandom number generator (PRNG) is a set of rules used to expandthe key into a keystream.To recover the data, both sides must share the same secret keyand use the same algorithm to expand the key into a pseudorandom sequence.Figure 5-2.Keyed stream cipher operationBecause the security of a stream cipher rests entirely on the randomness of the keystream,the design of the key-to-keystream expansion is of the utmost importance.When RC4was selected by the 802.11 working group, it appeared to be quite secure.But once RC4was selected as the ciphering engine of WEP, it spurred research that ultimately found anexploitable flaw in the RC4 cipher that will be discussed later.5.1.1 Stream Cipher SecurityA totally random keystream is called a one-time pad and is the only known encryptionscheme that is mathematically proven to protect against certain types of attacks.One-timepads are not commonly used because the keystream must be perfectly random and thesame length as the data that will be protected, and it can never be reused. Attackers are not limited to attacking the underlying cipher.They can choose to exploitany weak point in a cryptographic system.One famous Western intelligence effort, code-named VENONA, broke Soviet messages encrypted with one-time pads that were reused.The National Security Agency has made some information on the project public athttp://www.nsa.gov/docs/venona.It is easy to understand the temptation to reuse the one­time pads.Huge volumes of keying material are necessary to protect even a small amountof data, and those keying pads must be securely distributed, which in practice proves tobe a major challenge.Stream ciphers are a compromise between security and practicality.The perfectrandomness (and perfect security) of a one-time pad is attractive, but the practicaldifficulties and cost incurred in generating and distributing the keying material isworthwhile only for short messages that require the utmost security.Stream ciphers use aless random keystream but one that is random enough for most applications.5.1.2 Cryptographic PoliticsThree major nontechnical concerns may impact the use of WEP:1.RC4 is the intellectual property of RSA Security, Inc., and must be licensed.RSAwould almost certainly file suit against any unlicensed RC4 implementation.Formost end users, this is a minor point because wireless LAN equipment vendorswould need to license RC4.In the past, this has been a problem for Linux usersbecause some early wireless cards didn't include WEP on the card, and patentsprevented open source developers from implementing it in the device driver.Thelatest generation of wireless cards solves this problem by implementing WEP onthe card itself; all the device driver has to do is load the card with the keys.2.Products must be exportable from U.S.locations to compete across the world.The802.11 project committee specifically designed WEP to meet with approval fromthe U.S.export regulations at the time; as a consequence, WEP implementationswere restricted to a maximum key length of 40 bits.Rules have been relaxed sincethen, and longer keys are allowed.Unfortunately, longer key lengths were neverformally specified and may not be interoperable between products from differentvendors.3.Some governments impose restrictions on the importation of cryptographichardware and software, which may prevent the use of encryption to protect thewireless LAN link.Without even the minimal protection provided by WEP, itmay not be worth the risk to use wireless LAN technology in such locations.5.2 WEP Cryptographic OperationsCommunications security has three major objectives.Any protocol that attempts to securedata as it travels across a network must help network managers to achieve these goals.Confidentiality is the term used to describe data that is protected against interception byunauthorized parties.Integrity means that the data has not been modified.Authenticationunderpins any security strategy because part of the reliability of data is based on its origin.Users must ensure that data comes from the source it purports to come from.Systems must use authentication to protect data appropriately.Authorization and accesscontrol are both implemented on top of authentication.Before granting access to a pieceof data, systems must find out who the user is (authentication) and whether the accessoperation is allowed (authorization).WEP provides operations that attempt to help meet these objectives.Frame bodyencryption supports confidentiality.An integrity check sequence protects data in transitand allows receivers to validate that the received data was not altered in transit.WEP alsoenables stronger shared-key authentication of stations for access points, a featurediscussed in Chapter 7.In practice, WEP falls short in all of these areas.Confidentialityis compromised by flaws in the RC4 cipher; the integrity check was poorly designed; andauthentication is of users' MAC addresses, not users themselves [ Pobierz caÅ‚ość w formacie PDF ]
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