SM4 (formerly SMS4) is a block cipher used in the Chinese National Standard for Wireless LAN WAPI (Wired Authentication and Privacy Infrastructure). SM4 (formerly SMS4) is a block cipher used in the Chinese National Standard for Wireless LAN WAPI (Wired Authentication and Privacy Infrastructure). SM4 was a cipher proposed to for the IEEE 802.11i standard, but has so far been rejected by ISO. One of the reasons for the rejection has been opposition to the WAPI fast-track proposal by the IEEE. The SM4 algorithm was invented by Lu Shuwang (Chinese: 吕述望). The algorithm was declassified in January, 2006, and it became a national standard (GB/T 32907-2016) in August 2016. A few details of the SM4 cipher are: Define Z 2 e {displaystyle Z_{2}^{e}} as a vector set of e bits. Z 2 32 {displaystyle Z_{2}^{32}} is a word. Z 2 8 {displaystyle Z_{2}^{8}} is a byte.