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RC4 crypto: Get RID of it already, say boffins

This one simple attack busts WPA-TKIP in less than an hour

Remember how many times the crypto world has shouted that the RC4 crypto algorithm needs to be wiped from the face of the Earth? It just got even worse, with researchers demonstrating an attack that can be executed in 52 hours.

Belgian researchers preparing for August's Usenix Security Symposium in Washington, DC, reckon they can decrypt cookies encrypted with RC4 within 75 hours (PDF here).

Mathy Vanhoef and Frank Piessens of the University of Leuven worked on sites running TLS with RC4, and Wi-Fi's also-known-to-be-weak WPA-TKIP.

As they explain here, the weakness of RC4 (inherited by systems using it) is based on biases in the RC4 keystream. The bias was already known, and is why vendors like Microsoft are working to deprecate it. What's different in the new work is the acceleration of the cryptanalysis Vanhoef and Piessens carry out.

For HTTPS sessions secured by TLS and using RC4 for encryption, the authors reckon they can “decrypt a secure cookie with a success rate of 94 per cent using 9x227 cyphertexts”.

This is comparable to work published in March which claimed success using 226 encryptions of a password.

To break TLS/HTTPS, the Belgian researchers injected known data around the cookie, “abusing this by using Mantin's ABSAB bias, and brute-forcing the cookie by traversing the plain-text candidates … we are able to execute the attack in merely 75 hours”.

(That is, with 75 hours' worth of sample data, the researchers were able to complete an attack, compared to the “2,000 hours”required for previous attacks.)

If JavaScript code is added into the victim's browser, “we were able to execute the attack … within merely 52 hours”.

For WPA-TKIP it's even worse – the researchers' attack can be “executed within an hour”:

“To break WPA-TKIP we introduce a method to generate a large number of identical packets. This packet is decrypted by generating its plaintext candidate list, and using redundant packet structure to prune bad candidates. From the decrypted packet we derive the TKIP MIC key, which can be used to inject and decrypt packets”.

In practice, the problem is that it's hard to get rid of something so widely used. Back in March, around 30 per cent of all TLS traffic still used RC4. ®

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