RC4 is the most popular stream cipher in the world. It is used to protect as many as 30 percent of SSL traffic today, probably
summing up to billions of TLS connections every day.
In this paper, we revisit the Invariance Weakness – a 13-year-old vulnerability of RC4 that is based on huge classes of RC4 weak
keys, which was first published in the FMS paper in 2001. We show how this vulnerability can be used to mount several partial
plaintext recovery attacks on SSL-protected data when RC4 is the cipher of choice, recovering part of secrets such as session
cookies, passwords, and credit card numbers. This paper will describe the Invariance Weakness in detail, explain its impacts, and
recommend some mitigating actions.
by Imperva