  Vector Stream Cipher (VSC) is a stream cipher which consists of permutation
polynomial over a ring of modulo $2^w$. The algorithm for generating key stream
is very simple and the encryption is very fast. Some theoretical attacks for
VSC have been reported so far since the invention of VSC in 2004. Then, the
authors proposed some improvements and developed "Vector Stream Cipher 2.0 (VSC
2.0)" to be immune against the theoretical attacks. In this paper, we propose
further improvement of VSC 2.0 to publish as a new chaos cipher "Vector Stream
Cipher 2.1 (VSC2.1)". VSC 2.1 is faster and more secure than VSC 2.0. Our
result suggests that permutation polynomials over a ring of modulo $2^w$ are
useful for cryptography.
