US Robotics USR2450 Informacje Techniczne Pobierz pdf (Strona 40)

V 2.2 Copyright SystemExperts 2001,2002,2003

Problem: Keystream Reuse

n The shared key is static and rarely changed

n Randomness of key stream depends on IV

n When IV is reused, then you have two messages

encrypted with same keystream (a collision)

n 2^24 possible IV, so repeated after ~16 million

packets

n Most clients reset IV to 0 and increment by 1 for

each packet

n lots of collisions

V 2.2 Copyright SystemExperts 2001,2002,2003

Problem: Keystream Reuse Attack

n Attacker sends you a known packet (i.e., ping)

n A bunch of them J

n Sees the response: Ciphertext and IV

n Now knows Plaintext and Ciphertext, can get

keystream

n K = P XOR C

n note: the attacker does *not* know the key, but the keystream

n Makes a database indexed with IV

n Now for any IV he/she sees in the future, then have the

keystream needed to decrypt the packet

n Major problem because of shared keys

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US Robotics USR2450 Informacje Techniczne Strona 40