In light of recent research into practical attacks on biases in the RC4 stream cipher, Microsoft is recommending that customers enable TLS1.2 in their services and take steps to retire and deprecate RC4 as used in their TLS implementations. Microsoft recommends TLS1.2 with AES-GCM as a more secure alternative which will provide similar performance.
Developed in 1987 by Ron Rivest, RC4 was one of the earliest stream ciphers to see broad use. It was initially used in commercial applications and was faster than alternatives when implemented in software and over time became pervasive because of how cheap, fast and easy it was to implement and use.
Stream vs. Block
At a high level, a stream cipher generates a pseudorandom stream of bits of the same length as the plaintext and then XOR's the pseudorandom stream and the plaintext to generate the cipher text. This is different than a block cipher, which chunks plaintext into separate blocks, pads the plaintext to the block size and encrypts the blocks.
A History of Issues
RC4 consists of a Key Scheduling Algorithm (KSA) which feeds into a Psuedo-Random Generator (PRG), both of which need to be robust for use of the cipher to be considered secure. Beyond implementation issues with RC4, such as, document encryption and the 802.11 WEP implementation, there are some significant issues that exist in the KSA which lead to issues in the leading bytes of PRG output.
By definition, a PRG is only secure if the output is indistinguishable from a stream of random data. In 2001, Mantin and Shamir < http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.4.6198 > found a significant bias in RC4 output, specifically that the second byte of output would be ‘0’. Attacks and research have evolved since 2001, the work of T. Isobe, T. Ohigashi, Y. Watanabe, M. Morii of Kobe University in Japan is especially significant when evaluating the risk of RC4 use. Their findings show additional, significant bias in the first 257 bytes of RC4 output as well as practical plaintext recovery attacks on RC4.
The plaintext recovery attacks show a passive attacker collecting ciphertexts encrypted with different keys. Given 2^32 ciphertexts with different keys, the first 257 bytes of the plaintext are recovered with a probability of more than .5 < http://home.hiroshima- u.ac.jp/ohigashi/rc4/Full_Plaintext_Recovery%20Attack_on%20Broadcast_RC4_pre-proceedings.pdf >.
Since early RC4 output cannot be discarded from SSL/TLS implementations without protocol-level changes, this attack demonstrates the practicality of attacks against RC4 in common implementations.
Internet Use of RC4
One of the first steps in evaluating the customer impact of new security research and understanding the risks involved has to do with evaluating the state of public and customer environments. Using a sample size of five million sites, we found that 58% of sites do not use RC4, while approximately 43% do. Of the 43% that utilize RC4, only 3.9% require its use. Therefore disabling RC4 by default has the potential to decrease the use of RC4 by over almost forty percent.
Today's update provides tools for customers to test and disable RC4. The launch of Internet Explorer 11 (IE 11) and Windows 8.1 provide more secure defaults for customers out of the box.
IE 11 enables TLS1.2 by default and no longer uses RC4-based cipher suites during the >TLS handshake.
More detailed information about these changes can be found in the IE 11 blog <http://blogs.msdn.com/b/ie/archive/2013/11/12/ie11-automatically-makes-over-40-of-the-web-more-secure-while-making-sure-sites-continue-to-work.aspx>
For application developers, we have implemented additional options in SChannel which allow for its use without RC4.
Today's update KB 2868725provides support for the Windows 8.1 RC4 changes on Windows 7, Windows 8, Windows RT, Server 2008 R2, and Server 2012. These updates will not change existing settings and customers must implement changes (which are detailed below) to help secure their environments against weaknesses in RC4.
Call to Action
Microsoft strongly encourages customers to evaluate, test and implement the options for disabling RC4 below to increase the security of clients, servers and applications. Microsoft recommends enabling TLS1.2 and AES-GCM. Clients and servers running on Windows with custom SSL/TLS implementations, such as, Mozilla Firefox and Google Chrome will not be affected by changes to SChannel.
How to Completely Disable RC4
Clients and Servers that do not wish to use RC4 ciphersuites, regardless of the other party's supported ciphers, can disable the use of RC4 cipher suites completely by setting the following registry keys. In this manner any server or client that is talking to a client or server that must use RC4, can prevent a connection from happening. Clients that deploy this setting will not be able to connect to sites that require RC4 while servers that deploy this setting will not be able to service clients that must use RC4.
How Other Applications Can Prevent the Use of RC4 based Cipher Suites
RC4 is not turned off by default for all applications. Applications that call into SChannel directly will continue to use RC4 unless they opt-in to the security options. Applications that use SChannel can block the use of RC4 cipher suites for their connections by passing the SCH_USE_STRONG_CRYPTO flag to SChannel in the SCHANNEL_CRED structure. If compatibility needs to be maintained, then they can also implement a fallback that does not pass this flag.
Microsoft recommends that customers upgrade to TLS1.2 and utilize AES-GCM. On modern hardware AES-GCM has similar performance characteristics and is a much more secure alternative to RC4.
- William Peteroy, MSRC
I would like to thank the Windows, Internet Explorer and .NET teams for their work in this effort as well as Ali Rahbar and Suha Can of the MSRC Engineering team for their hard work and input. I would also like to thank Matthew Green for the excellent write-ups he has for this and other applied cryptography issues on his blog.