SM4 (cipher)
SM4 (formerly SMS4) is a block cipher used in the Chinese National Standard for Wireless LAN WAPI (WLAN 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 drafted by Data Assurance & Communication Security Center, CAS, and Commercial Cryptogrophy Testing Center, National Cryptography Administration. It is mainly developed by Lü Shuwang (). The algorithm was declassified in January, 2006, and it became a national standard (GB/T 32907-2016) in August 2016.

Cipher detail
A few details of the SM4 cipher are:
 It has a block size of 128 bits.
 It uses an 8-bit S-box.
 The key size is 128 bits.
 The only operations used are 32-bit bitwise XOR, 32-bit circular shifts and S-box applications.
 Encryption or decryption of one block of data is composed of 32 rounds.
 Each round updates a quarter (i.e., 32 bits) of the internal state.
 A non-linear key schedule is used to produce the round keys.
 Decryption uses the same round keys as for encryption, except that they are in reversed order.

Terms and definitions

Word and byte
Define  as a vector set of e bits.

 is a word.

 is a byte.

S-box
S-box is fixed for 8-bit input and 8-bit output, noted as Sbox(). As with AES, the S-box is based on the multiplicative inverse over . The affine transforms and polynomial bases are different from that of AES, but due to affine isomorphism it can be calculated efficiently given an AES Rijndael S-box.

Keys and key parameters
The length of encryption keys is 128 bits, represented as , in which  is a word.

A round key is represented as , where each  is a word. It is generated by the encryption key.

 is a system parameter.

 is a fixed parameter, used to generate .

 and  are words, used for extension of the algorithm.

Remark
On March 21, 2012, the Chinese government published the industrial standard "GM/T 0002-2012 SM4 Block Cipher Algorithm", officially renaming SMS4 to SM4.

A description of SM4 in English is available as an Internet Draft. It contains a reference implementation in ANSI C.

SM4 is part of the ARMv8.4-A expansion to the ARM architecture.

References

External links
 Linear and Differential Cryptanalysis of Reduced SMS4 Block Cipher
 Example of SMS4 implemented as a Spreadsheet
 Page of Lu Shu-wang (吕述望) (in Chinese)
 The GmSSL Project (OpenSSL fork with GuoMi algorithms)

Block ciphers