This document describes the KCipher-2 encryption algorithm. KCipher-2 is a stream cipher with a 128-bit key and a 128-bit initialization vector. Since the algorithm for KCipher-2 was published in 2007, security and efficiency have been rigorously evaluated through academic and industrial studies. No security vulnerability has been found as of the time this document was written. KCipher-2 offers fast encryption and decryption by means of simple operations that enable efficient implementation. KCipher-2 has been used for industrial applications, especially for mobile health monitoring and diagnostic services in Japan.