SMS4

SMS4

General
Designers	Chinese Government
First published	2006 (declassified),(standardized in March,21 2012)
Cipher detail
Key sizes	128 bits
Block sizes	128 bits
Structure	unbalanced Feistel network
Rounds	32
Best public cryptanalysis
linear and differential attacks against 22 rounds

SMS4 is a block cipher used in the Chinese National Standard for Wireless LAN WAPI (Wired Authentication and Privacy Infrastructure).

SMS4 was a proposed cipher to be used in 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 SMS4 algorithm was invented by Prof. LU Shu-wang(吕述望). The algorithm was declassified in January, 2006. A few details of the SMS4 cipher are:

• It has a block size of 128 bits.
• Uses an 8-bit S-box
• The key size is 128 bits.
• The only operations used are XOR, circular shifts and S-Box applications
• Performs 32 rounds to process one block.
• Each round updates a quarter (32 bits) of the internal state.
• A non-linear key schedule is used to produce the round keys.
• Decryption is using the same keys as encryption, but in reversed order.

Terms and Definitions

Word and Byte

Define $Z^e_2$ as a vector set of e bits.

$Z^{32}_2$ is a word.

$Z^8_2$ is a byte.

S-box

S-box is fixed for 8-bit input and 8-bit output, noted as Sbox().

Keys and Key Parameters

The length of encryption keys are 128-bit, represented as $MK=(MK_0,\ MK_1,\ MK_2,\ MK_3)$, in which $MK_i\ (i=0,\ 1,\ 2,\ 3)$ is a word.

A round key is represented as $(rk_0,\ rk_1,\ \ldots,\ rk_{31})$,where each $rk_i(i=0,\ \ldots,\ 31)$ is a word. It is generated by the encryption key.

$FK=(FK_0,\ FK_1,\ FK_2,\ FK_3)$ is a system parameter.

$CK=(CK_0,\ CK_1,\ \ldots,\ CK_{31})$ is a fixed parameter, used to generate $rk_i$.

$FK_i$ and $CK_i$ are words, used for extension of the algorithm.

Remark

From Mar.21, 2012,Chinese government published the industrial standard <GM/T 0002-2012 SM4 Block Cipher Algorithm Standard>, SMS4 was officially renamed to SM4.

External links

• Chinese document describing the SMS4 cipher
• English translation of the Chinese document
• Linear and Differential Cryptanalysis of Reduced SMS4 Block Cipher
• Example of SMS4 implemented as a Spreadsheet
• Page of Prof. LU Shu-wang(吕述望) in Chinese
• Example of SMS4 implemented in ANSI C