US Robotics USR2450 Informacje Techniczne Strona 41

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V 2.2 Copyright SystemExperts 2001,2002,2003
81
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
82
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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