The Most Ecient Distinguishing Attack on VMPC and RC4A

This paper reports the greater bias found in the output se- quence of VMPC, a modified RC4 stream cipher proposed in 2004. Us- ing the bias with approximately 238 output bytes allows us to distinguish VMPC from truly random sequence. Distinguishing attack can also break RC4A, an algorihm based on RC4, more eciently than any existing at- tacks. With about 223 output bytes, distinguishing attack makes it pos- sible to distinguish RC4A from truly random sequence.