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Is RC4 safe for use in SSL?

August 28, 2009

I received a rather interesting and informative couple of emails last week from Paweł Krawczyk, who wanted to make a point how RC4 is still safe to use in SSL. I thought that his comments would be of interest to a wider audience, so here they are (with permission):

[...] TLS_RSA_WITH_RC4_128_MD5 [...] is extremely widespread among commercial servers. Main reasons are 1) it's default preferred cipher in most versions of IIS, 2) these two algorithms are absolutely fastest and least CPU-intensive.

However, because there are known cryptoanalytic attacks against both RC4 and MD5, this ciphersuite is notoriously reported as "weak" by some pentesting tools and teams.

This is not true or at least not accurate, as the specific usage of RC4 and MD5 - in SSLv3 with 128 bit key - has no known and working attacks. That's one reason why PCI-DSS v1.2 now doesn't list any specific algorithms for SSL but instead just says you should use "strong" ones. And widespread usage of TLS_RSA_WITH_RC4_128_MD5 among financial organisations seems to confirm the interpretation that it's still considered a strong ciphersuite.

On the other hand NIST SP 800-52 doesn't allow neither RC4 or MD5 because they're not FIPS-approved algorithms, with one exception - connecting in client mode to external, commercial systems with this specific ciphersuite enabled. Which seems to set the balance quite even, because SP is only binding for US federal agencies.


There are two reasons why the new attacks do not apply to RC4-based SSL. First, SSL generates the encryption keys it uses for RC4 by hashing (using both MD5 and SHA1), so that different sessions have unrelated keys. Second, SSL does not re-key RC4 for each packet, but uses the RC4 algorithm state from the end of one packet to begin encryption with the next packet. The recent techniques of Fluhrer, Mantis and Shamir thus do not apply to SSL.

Further resources: