CEH.V6.-.Module.20.Hacking.Wireless.Networks

Reproduction is strictly prohibited wired equivalent

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Unformatted text preview: cryption at the data link layer to prevent unauthorized eavesdropping on a network • This is accomplished by encrypting data with the RC4 encryption algorithm EC-Council Copyright © by EC-Council All Rights Reserved. Reproduction is Strictly Prohibited Wired Equivalent Privacy (cont’d) Cryptographic mechanism is used to defend against threats It is developed without : • Academic or public review • Review from cryptologists It has significant vulnerabilities and design flaws Only about a quarter to a third of wireless access points use WEP: • Tam et al. 2002 • Hamilton 2002 • Pickard and Cracknell 2001, 2003 2003 EC-Council Copyright © by EC-Council All Rights Reserved. Reproduction is Strictly Prohibited Wired Equivalent Privacy (cont’d) WEP is a stream cipher: • It uses RC-4 to produce a stream of bytes that are XORed with the plaintext • The input to the stream cipher algorithm is an "initial value" (IV) sent in plaintext and a secret key • IV is 24 bits long • Length of the secret is either 40 or 104 bits, for a total length for the IV and secret of 64 or 128 bits • Marketing publicized the larger number, implying that the secret was a 64 or 128 bit number, in a classical case of deceptive advertising: • How else can you call a protection that is 16.8 million times weaker weaker than advertised? EC-Council Copyright © by EC-Council All Rights Reserved. Reproduction is Strictly Prohibited WEP Issues CRC32 CRC32 is not sufficient to ensure complete cryptographic integrity of a packet • By capturing two packets, an attacker can reliably flip a bit in the encrypted stream, and modify the checksum so that the yp packet is accepted IV’s are 24 bits • An AP broadcasting 1500 byte packets at 11 Mb/s would exhaust the entire IV Space in five hours Known Plaintext Attacks • When there is IV Collision, it becomes possible to reconstruct the RC4 keystream based on the IV and the decrypted payload of the packet EC-Council Copyright © by EC-Council All Rights Reserved. Reproduction is Strictly Prohibited WEP Issues (cont’d) Dictionary Attacks Password • WEP is based on a password Denial of Services • Associate and Disassociate messages are not authenticated Eventually, an attacker can construct a decryption table of reconstructed key streams • With about 24 GB of space, an attacker can use this table an to decrypt WEP Packets in real-time EC-Council Copyright © by EC-Council All Rights Reserved. Reproduction is Strictly Prohibited WEP Issues (cont’d) A lack of centralized key management makes it difficult to change WEP lack keys with any regularity IV IV is a value that is used to randomize the key stream value and each th th packet has IV value • The standard only allows 24 bits, which can be used within hours at a busy AP • IV values will be reused The standard does not dictate that each packet must have a unique IV, so vendors use only a small part of the available 24-bit possibilities ll bit • A mechanism that depends on randomness is not random at all and attackers can easily figure out the key stream and decrypt other messages EC-Council Copyright © by EC-Council All Rights Reserved. Reproduction is Strictly Prohibited WEP - Authentication Phase When a wireless station wants to access a network, it sends a probe request packet on all channels so that any AP in range will respond The AP responds with packets containing the AP’s SSID and other network information • When o...
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This note was uploaded on 02/12/2013 for the course CEH v6 taught by Professor John during the Summer '13 term at West Point.

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