tc_sm4

The SM4 block cipher, the Chinese national standard GB/T 32907-2016, with a 128-bit key and a 128-bit block. The engine implements the tc_block_cipher traits.

The crate is no_std, needs no allocator and contains no unsafe code. It depends on tc_block_cipher and tc_zeroize.

Requires Rust 1.85 or later (edition 2024).

Types

Sm4Engine — SM4 with a 16-byte key and a 16-byte block; variable time.

The engine takes its key through any KeyParams, such as KeyRef. Another key length returns InitError::InvalidKeyLength and keeps the previous key. process_block transforms the first block and returns its length, returning BlockError::NotInitialised before init and BlockError::BufferTooShort for a buffer shorter than a block, without touching the output. Display writes "SM4" without inspecting key material.

The constants BLOCK_BYTES and KEY_BYTES, both 16, give the block and key lengths, and ALGO_NAME holds the name. The engine implements Default and has const fn new.

Usage

[dependencies]
tc_sm4 = "0.1.0"
tc_block_cipher = "0.1.0"

use tc_block_cipher::{BlockCipher, BlockCipherInit, CipherDirection, KeyRef};
use tc_sm4::Sm4Engine;

let key = [0x42; 16];
let mut engine = Sm4Engine::new();
engine
    .init(CipherDirection::Encrypt, &KeyRef::new(&key))
    .expect("SM4 accepts a 16-byte key");
let mut ciphertext = [0; 16];
assert_eq!(engine.process_block(&[0x11; 16], &mut ciphertext), Ok(16));

The crate and engine documentation carry executable examples.

Security

The engine is variable time: key setup and block processing look up the S-box with secret data, so cache timing can leak the key. Use it only where cache-timing leakage is outside the threat model; this crate provides no constant-time engine.

The engine keeps an expanded key schedule rather than borrowing the caller's key, and wipes it when it is replaced and on drop. Wiping does not reach the caller's key buffer or copies left in registers and on the stack.

This is a block-cipher primitive, not a message-encryption format. It supplies no padding, mode of operation or authentication.

Validation

The engine is tested against the Bouncy Castle vectors, the published single block and the million-iteration chain, in both directions, and against RustCrypto's sm4 on random keys and blocks. Contract tests cover error state, untouched output on errors and preserved output tails. A test requires every engine API to document whether it is constant or variable time.

Missing public documentation and unsafe code are rejected by crate-level lints.

Run these commands from the workspace root:

cargo test -p tc_sm4 --locked
cargo clippy -p tc_sm4 --all-targets --locked -- -D warnings
cargo fmt -p tc_sm4 --check
cargo doc -p tc_sm4 --no-deps --locked

Before a release, check the archive contents and run publication validation from a committed checkout:

cargo package -p tc_sm4 --list --locked
cargo publish -p tc_sm4 --dry-run --locked

The archive includes both license texts, this README, the changelog, the source and the integration tests. It must not include target/ or other build artifacts.

License