RC4 is a very widely used stream cipher, developed in the late 1980s. As wikipedia puts it, RC4 is remarkable for its speed and simplicity in software, but it has weaknesses that argue against its use in new systems. Today’s paper demonstrates that it is even weaker than previously suspected.

To understand the paper you need to know how stream ciphers work. The core of a stream cipher is a random number generator. The encryption key is the starting seed for the random number generator, which produces a keystream—a sequence of uniformly random bytes. You then combine the keystream with your message using the mathematical XOR operation, and that’s your ciphertext. On the other end, the recipient knows the same key, so they can generate the same keystream, and they XOR the ciphertext with the keystream and get the message back.

If you XOR a ciphertext with the corresponding plaintext, you learn the keystream. This wouldn’t be a big deal normally, but, the basic problem with RC4 is that its keystream isn’t uniformly random. Some bytes of the keystream are more likely to take specific values than they ought to be. Some bytes are more likely to not take specific values. And some bytes are more likely to take the same value as another byte in the keystream. (The ABSAB bias, which is mentioned often in this paper, is an example of that last case: you have slightly elevated odds of encountering value A, value B, a middle sequence S, and then A and B again.) All of these are referred to as biases in the RC4 keystream.

To use RC4’s biases to decrypt a message, you need to get your attack target to send someone (not you) the same message many times, encrypted with many different keys. You then guess a keystream which exhibits as many of the biases as possible, and you XOR this keystream with all of the messages. Your guess won’t always be right, but it will be right slightly more often than chance, so the correct decryption will also appear slightly more often than chance. It helps that it doesn’t have to be exactly the same message. If there is a chunk that is always the same, and it always appears in the same position, that’s enough. It also helps if you already know some of the message; that lets you weed out bad guesses faster, and exploit more of the biases.

Asking the attack target to send the same message many times might seem ridiculous. But remember that many Internet protocols involve headers that are fixed or nearly so. The plaintext of a TKIP-encrypted message, for instance, will almost always be the WiFi encapsulation of an IPv4 packet. If you know the IP addresses of your target and of the remote host it’s communicating with, that means you know eight bytes of the message already, and they’ll always be the same and in the same position. The paper goes into some detail about how to get a Web browser to make lots and lots of HTTPS requests with a session cookie (always the same, but unknown—the secret you want to steal) in a predictable position, with known plaintext on either side of it.

All this was well-known already. What’s new in this paper is: first, some newly discovered biases in the RC4 keystream, and a statistical technique for finding even more; second, improved attacks on TKIP and TLS. Improved means that they’re easier to execute without anyone noticing, and that they take less time. Concretely, the best known cookie-stealing attack before this paper needed to see $13\cdot {2}^{30}$ HTTPS messages (that’s about 14 billion) and this paper cuts it to $9\cdot {2}^{27}$ messages (1.2 billion), which takes only 75 hours to run. That’s entering the realm of practical in real life, if only for a client computer that is left on all the time, with the web browser running.

It’s a truism in computer security (actually, security in general) that attacks only ever get better. What’s important, when looking at papers like this, is not so much the feasibility of any particular paper’s attack, but the trend. The first-known biases in RC4 keystream were discovered only a year after the algorithm was published, and since then there’s been a steady drumbeat of researchers finding more biases and more effective ways to exploit them. That means RC4 is no good, and everyone needs to stop using it before someone finds an attack that only takes a few minutes. Contrast the situation with AES, where there are no known biases, and fifteen years of people looking for some kind of attack has produced only a tiny improvement over brute force.

Advice to the general public: Does this affect you? Yes. What can you do about it? As always, make sure your Web browsers are fully patched up—current versions of Firefox, Chrome, and IE avoid using RC4, and upcoming versions will probably take it out altogether. Beyond that, the single most important thing for you to do is make sure everything you’ve got that communicates over WiFi—router, computers, tablets, smartphones, etc.—are all set to use WPA2 and CCMP only. (The configuration interface may refer to CCMP as AES-CCMP or just AES; in this context, those are three names for the same thing.) The alternatives WEP, WPA, and TKIP all unavoidably involve RC4 and are known to be broken to some extent. Any WiFi-capable widget manufactured after 2006 has no excuse for not supporting WPA2 with CCMP. It should definitely be possible to set your home router up this way; unfortunately, many client devices don’t expose the necessary knobs.