On the Security of RC4 in TLS


This page is about the security of RC4 encryption in TLS. For details of the Lucky 13 attack on CBC-mode encryption in TLS, click here.
The Transport Layer Security (TLS) protocol aims to provide confidentiality and integrity of data in transit across untrusted networks like the Internet. It is widely used to secure web traffic and e-commerce transactions on the Internet. Around 50% of all TLS traffic is currently protected using the RC4 algorithm. It has become increasingly popular because of recent attacks on CBC-mode encryption on TLS, and is now recommended by many commentators.
We have found a new attack against TLS that allows an attacker to recover a limited amount of plaintext from a TLS connection when RC4 encryption is used. The attacks arise from statistical flaws in the keystream generated by the RC4 algorithm which become apparent in TLS ciphertexts when the same plaintext is repeatedly encrypted at a fixed location across many TLS sessions.
We have carried out experiments to demonstrate the feasibility of the attacks.
The most effective countermeasure against our attack is to stop using RC4 in TLS. There are other, less-effective countermeasures against our attacks and we are working with a number of TLS software developers to prepare patches and security advisories.

Who are we?

The team behind this research comprises Nadhem AlFardan, Dan Bernstein, Kenny Paterson, Bertram Poettering and Jacob Schuldt. Nadhem is a PhD student in the Information Security Group at Royal Holloway, University of London. Dan is a Research Professor at the University of Illinois at Chicago and a Professor at the Eindhoven University of Technology. Kenny is a Professor of Information Security and an EPSRC Leadership Fellow in the Information Security Group at Royal Holloway, University of London. Bertram and Jacob are postdocs in the Information Security Group.

What is affected?

Which versions of SSL and TLS are affected?

The attack applies to all versions of SSL and TLS that support the RC4 algorithm.

Which TLS ciphersuites are affected?

All TLS ciphersuites which include RC4 encryption are vulnerable to our attack.

Which TLS implementations are affected?

All TLS implementations which support RC4 are affected.

How severe are the attacks?

The attack is a multi-session attack, which means that we require a target plaintext to be repeatedly sent in the same position in the plaintext stream in multiple TLS sessions. The attack currently only targets the first 256 bytes of the plaintext stream in sessions. Since the first 36 bytes of plaintext are formed from an unpredictable Finished message when SHA-1 is the selected hashing algorithm in the TLS Record Protocol, these first 36 bytes cannot be recovered. This means that the attack can recover 220 bytes of TLS-encrypted plaintext.
The number of sessions needed to reliably recover these plaintext bytes is around 230, but already with only 224 sessions, certain bytes can be recovered reliably. In contrast to the recent Lucky 13 attack, there is no need for sophisticated timing of error messages, and the attacker can be located anywhere on the network path between client and server.
The sessions needed for our attack can be generated in various ways. The attacker could cause the TLS session to be terminated, and some applications running over TLS then automatically reconnect and retransmit a cookie or password. In a web environment, the sessions may also be generated by client-side malware, in a similar way to the BEAST attack.

How does this work relate to known attacks, like BEAST, CRIME and Lucky 13?

TLS in CBC-mode has been the subject of several attacks over the years, most notably padding oracle attacks, the BEAST attack and the recent Lucky 13 attack. For more details of prior attacks, see the Lucky 13 research paper. There are now countermeasures for the BEAST and Lucky 13 attacks, and TLS in CBC-mode is believed to be secure against them once these countermeasures are applied. By contrast, the new attack targets the RC4 algorithm in TLS.

But isn't RC4 already broken?

There have been many attacks on RC4 over the years, most notably against RC4 in the WEP protocol. There, the known attacks crucially exploit the way in which the algorithm's secret key is combined with public information (the WEP IV) during the algorithm's initialisation step. These attacks do not apply to RC4 in TLS, and new attack ideas are needed. Certain bytes of the RC4 keystream are already known to have biases that assist cryptanalysis; in our work, we identify the complete set of biases in the first 256 keystream bytes and combine these using a particular statistical procedure to extract plaintext from ciphertext.

How do the attacks relate to BEAST, CRIME and Lucky 13?

The attacks are quite different from BEAST, CRIME and Lucky 13. BEAST exploits the inadvisable use of chained IVs in CBC-mode in SSL and TLS 1.0. CRIME cleverly exploits the use of compression in TLS. Lucky 13 defeats existing RFC-recommended countermeasures for padding oracle attacks against CBC-mode. Our attacks are against the RC4 algorithm and are based on analysing statistical weaknesses in the RC4 keystream. However, our attacks can be mounted using BEAST-style techniques.

Why doesn't the attack have a cool name?

In Western culture, naming one's attacks after obscure Neil Young albums is now considered passé.

What are the countermeasures?

There are several possible countermeasures against our attacks, some of which are more effective than others:
  • Switch to using CBC-mode ciphersuites. This is a suitable countermeasure provided previous CBC-mode attacks such as BEAST and Lucky 13 have been patched. Many implementations of TLS 1.0 and 1.1 now do have patches against these attacks.
  • Switch to using AEAD ciphersuites, such as AES-GCM. Support for AEAD ciphersuites was specified in TLS 1.2, but this version of TLS is not yet widely supported. We hope that our research will continue to spur support for TLS 1.2 in client and server implementations.
  • Patch TLS's use of RC4. For example, one could discard the first output bytes of the RC4 keystream before commencing encryption/decryption. However, this would need to be carried out in every client and server implementation of TLS in a consistent manner. This solution is not practically deployable given the large base of legacy implementations and the lack of a facility to negotiate such a byte discarding procedure. Furthermore, this will not provide security against potential future improvements to our attack. Our recommendation for the long term is to avoid using RC4 in TLS and to switch to using AEAD algorithms.
  • Modify browser behaviour. There are ways to modify the manner in which a browser using TLS handles HTTP GET requests to make the attack less effective. However, care is needed to avoid potential future improvements to our attack. Our recommendation for the long term is to avoid using RC4 in TLS and to switch to using AEAD algorithms.

Patches, advisories and press

We are working with the IETF TLS Working Group and affected vendors to prepare and test patches. We will update the information here as this process continues.
The US NIST National Vulnerability Database has accorded this CVE-2013-2566.


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