Non-uniformities in the RC4 Stream Cipher

The RC4 stream cipher is used to protect messages from eavesdroppers in many settings, including some of the Transport Layer Security (TLS) protocols used to secure much internet traffic. Nonuniformities in the output of a stream cipher are a weakness that an eavesdropper can exploit to gain information about the encrypted messages. In this report we verify the size of some nonuniformities of RC4 in TLS that were recently reported based on experimental observations by Al Fardan et al. (2013). We extend this work to report on several newly discovered non-uniformities. We also report methods and results quantifying the vulnerability to eavesdropping of messages encrypted by RC4 in TLS as a result of these non-uniformities. Based on the resulting analysis, and in conjunction of the work of others, we add our voice to those urging that RC4 no longer be used.