ShāngMì 4 (SM4, 商密4) (formerly SMS4) is a block cipher, standardised for commercial cryptography in China. It is used in the Chinese National Standard for Wireless LAN WAPI (WLAN Authentication and Privacy Infrastructure), and with Transport Layer Security.

SM4 was a cipher proposed for the IEEE 802.11i standard, but it has so far been rejected. One of the reasons for the rejection has been opposition to the WAPI fast-track proposal by the IEEE.^{[citation needed]}

SM4 was published as ISO/IEC 18033-3/Amd 1 in 2021.

The SM4 algorithm was drafted by Data Assurance & Communication Security Center, Chinese Academy of Sciences (CAS), and Commercial Cryptography Testing Center, National Cryptography Administration. It is mainly developed by Lü 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.

The SM4 cipher has a key size and a block size of 128 bits each. Encryption or decryption of one block of data is composed of 32 rounds. A non-linear key schedule is used to produce the round keys and the decryption uses the same round keys as for encryption, except that they are in reversed order.

The length of encryption keys is 128 bits, represented as ${\displaystyle MK=(MK_{0},\ MK_{1},\ MK_{2},\ MK_{3})}$, in which ${\displaystyle MK_{i}\ (i=0,\ 1,\ 2,\ 3)}$ is a 32-bit word. The round keys are represented by ${\displaystyle (rk_{0},\ rk_{1},\ \ldots ,\ rk_{31})}$, where each ${\displaystyle rk_{i}(i=0,\ \ldots ,\ 31)}$ is a word. It is generated by the encryption key and the following parameters:

${\displaystyle FK_{i}}$ and ${\displaystyle CK_{i}}$ are words, used to generate the round keys.

Each round are computed from the four previous round outputs ${\displaystyle X_{i},X_{i+1},X_{i+2},X_{i+3}}$ such that: ${\displaystyle X_{i+4}=X_{i}\oplus F(X_{i+1}\oplus X_{i+2}\oplus X_{i+3}\oplus rk_{i})}$