US Robotics USR2450 Informations techniques télécharger pdf (Page 41)

V 2.2 Copyright SystemExperts 2001,2002,2003

Problem: Key Scheduling Algorithm of RC4

n Documented by Scott Fluhrer, Itsik Mantin and

Adi Shamir

n Paper indicated that an attacker could gain access to an entire

WLAN in less than 15 minutes

n Requires between 1 million and 8 million packets, and does

not require significant CPU power

n Main problem is a weakness in the way the RC4

encryption algorithm is implemented in WEP

n By having a “known” plaintext prepended on the key (I.e., the

IV), it leads to weak keys that will generate known ciphertext

output from the RC4 engine

n It allows the attacker to go back and "reverse engineer" the

secret key from encrypted packets

V 2.2 Copyright SystemExperts 2001,2002,2003

Problem: Key Scheduling Algorithm of RC4

n Longer keys won’t help because the attack

recovers each key byte individually, rather

than attempting to decrypt the key as a whole

n The attack scales linearly -- not exponentially

-- as key length increases

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US Robotics USR2450 Informations techniques Page 41