feat: init commit, fork from libsm

docs/sm4.md

@@ -0,0 +1,53 @@
+# SM4
+
+SM4 is a block cipher. CFB mode, OFB mode, CTR mode and CBC mode are implemented in libsm.
+
+Here are their definitions:
+
+```rust
+pub enum CipherMode {
+    Cfb,
+    Ofb,
+    Ctr,
+    Cbc,
+}
+```
+
+## Create Cipher
+
+Choose a mode when creating a cipher. Then call the creating function.
+
+Sample:
+
+```rust
+use libsm::sm4::{Mode, Cipher};
+use rand::RngCore;
+
+fn rand_block() -> [u8; 16] {
+    let mut rng = rand::thread_rng();;
+    let mut block: [u8; 16] = [0; 16];
+    rng.fill_bytes(&mut block[..]);
+    block
+}
+
+let key = rand_block();
+
+let cipher = Cipher::new(&key, Mode::Cfb);
+```
+
+## Encryption and Decryption
+
+Initialize a random IV(Initial Vector), which can be generated using the `rand_block()` function above.
+
+Sample:
+
+```rust
+let iv = rand_block();
+let plain_text = String::from("plain text");
+
+// Encryption
+let cipher_text: Vec<u8> = cipher.encrypt(plain_text.to_bytes(), &iv);
+
+// Decryption
+let plain_text: Vec<u8> = cipher.decrypt(&cipher_text[..], &iv);
+```