SM4

SM4 is a symmetric-key block cipher designed in China for general-purpose encryption. It was standardized as GB/T 32918-2016 and is part of the country’s family of domestic cryptographic algorithms known as the SM (ShangMi) series. The cipher is widely referenced in Chinese security standards and implementations for government, industry, and consumer applications.

Technically, SM4 operates on 128-bit blocks and uses a 128-bit key. The algorithm performs 32 rounds of processing. Each round applies a nonlinear substitution using an 8-bit S-box, followed by a linear diffusion transformation. A key schedule expands the 128-bit key into 32 32-bit round keys using a fixed set of constants. Encryption and decryption share the same round structure; decryption uses the round keys in reverse order.

Security and evaluation of SM4 are aligned with contemporary block ciphers: no practical cryptanalytic break is known that would compromise 128-bit security under standard models. Research has explored reduced-round variants and related-key scenarios, but these do not translate into practical attacks on the full 32-round cipher. As with any cipher, proper implementation is important to avoid side-channel weaknesses; constant-time code and resistance to timing and cache-based attacks are commonly recommended.

Implementation and usage of SM4 are widespread in China, appearing in numerous standards, protocols, and software libraries. It is available in reference implementations and optimized versions for various platforms, including support for modes such as SM4-GCM and SM4-CTR in modern cryptographic stacks. SM4 remains the primary Chinese block cipher for domestic encryption needs and interoperability within national standards.

See also: SM2, SM3, SM4 in Chinese encryption standards.