xref: /docs/en/application-dev/security/CryptoArchitectureKit/crypto-sm4-sym-encrypt-decrypt-gcm.md (revision e41f4b71)

# Encryption and Decryption with an SM4 Symmetric Key (GCM Mode) (ArkTS)


For details about the algorithm specifications, see [SM4](crypto-sym-encrypt-decrypt-spec.md#sm4).


**Encryption**


1. Use [cryptoFramework.createSymKeyGenerator](../../reference/apis-crypto-architecture-kit/js-apis-cryptoFramework.md#cryptoframeworkcreatesymkeygenerator) and [SymKeyGenerator.generateSymKey](../../reference/apis-crypto-architecture-kit/js-apis-cryptoFramework.md#generatesymkey-1) to generate a 128-bit AES symmetric key (**SymKey**).
   
   In addition to the example in this topic, [SM4](crypto-sym-key-generation-conversion-spec.md#sm4) and [Randomly Generating a Symmetric Key](crypto-generate-sym-key-randomly.md) may help you better understand how to generate an SM4 symmetric key. Note that the input parameters in the reference documents may be different from those in the example below.

2. Use [cryptoFramework.createCipher](../../reference/apis-crypto-architecture-kit/js-apis-cryptoFramework.md#cryptoframeworkcreatecipher) with the string parameter **'SM4_128|GCM|PKCS7'** to create a **Cipher** instance. The key type is **SM4_128**, block cipher mode is **GCM**, and the padding mode is **PKCS7**.

3. Use [Cipher.init](../../reference/apis-crypto-architecture-kit/js-apis-cryptoFramework.md#init-1) to initialize the **Cipher** instance. In the **Cipher.init** API, set **opMode** to **CryptoMode.ENCRYPT_MODE** (encryption), **key** to **SymKey** (the key for encryption), and **params** to **GcmParamsSpec** corresponding to the GCM mode.

4. Use [Cipher.update](../../reference/apis-crypto-architecture-kit/js-apis-cryptoFramework.md#update-1) to pass in the data to be encrypted (plaintext).
   
   Currently, the amount of data to be passed in by a single **Cipher.update** is not limited. You can determine how to pass in data based on the data volume.

   - If a small amount of data is to be encrypted, you can use **Cipher.doFinal** immediately after **Cipher.init**.
   - If a large amount of data is to be encrypted, you can call **Cipher.update** multiple times to pass in the data by segment.

5. Use [Cipher.doFinal](../../reference/apis-crypto-architecture-kit/js-apis-cryptoFramework.md#dofinal-1) to obtain the encrypted data.
   - If data has been passed in by **Cipher.update**, pass in **null** in the **data** parameter of **Cipher.doFinal**.
   - The output of **Cipher.doFinal** may be **null**. To avoid exceptions, always check whether the result is **null** before accessing specific data.

6. Obtain [GcmParamsSpec.authTag](../../reference/apis-crypto-architecture-kit/js-apis-cryptoFramework.md#gcmparamsspec) as the authentication information for decryption.
   In GCM mode, extract the last 16 bytes from the encrypted data as the authentication information for initializing the **Cipher** instance in decryption. In the example, **authTag** is of 16 bytes.


**Decryption**


1. Use [Cipher.init](../../reference/apis-crypto-architecture-kit/js-apis-cryptoFramework.md#init-1) to initialize the **Cipher** instance. In the **Cipher.init** API, set **opMode** to **CryptoMode.DECRYPT_MODE** (decryption), **key** to **SymKey** (the key for decryption), and **params** to **GcmParamsSpec** corresponding to the GCM mode.

2. Use [Cipher.update](../../reference/apis-crypto-architecture-kit/js-apis-cryptoFramework.md#update-1) to pass in the data to be decrypted (ciphertext).

3. Use [Cipher.doFinal](../../reference/apis-crypto-architecture-kit/js-apis-cryptoFramework.md#dofinal-1) to obtain the decrypted data.


- Example (using asynchronous APIs):

  ```ts
  import { cryptoFramework } from '@kit.CryptoArchitectureKit';
  import { buffer } from '@kit.ArkTS';

  function genGcmParamsSpec() {
    let arr = [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]; // 12 bytes
    let dataIv = new Uint8Array(arr);
    let ivBlob: cryptoFramework.DataBlob = { data: dataIv };
    arr = [0, 0, 0, 0, 0, 0, 0, 0]; // 8 bytes
    let dataAad = new Uint8Array(arr);
    let aadBlob: cryptoFramework.DataBlob = { data: dataAad };
    arr = [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]; // 16 bytes
    let dataTag = new Uint8Array(arr);
    let tagBlob: cryptoFramework.DataBlob = {
      data: dataTag
    }; 
    // Obtain the GCM authTag from the doFinal result in encryption and fill it in the params parameter of Cipher.init in decryption.
    let gcmParamsSpec: cryptoFramework.GcmParamsSpec = {
      iv: ivBlob,
      aad: aadBlob,
      authTag: tagBlob,
      algName: "GcmParamsSpec"
    };
    return gcmParamsSpec;
  }

  let gcmParams = genGcmParamsSpec();

  // Encrypt the message.
  async function encryptMessagePromise(symKey: cryptoFramework.SymKey, plainText: cryptoFramework.DataBlob) {
    let cipher = cryptoFramework.createCipher('SM4_128|GCM|PKCS7');
    await cipher.init(cryptoFramework.CryptoMode.ENCRYPT_MODE, symKey, gcmParams);
    let encryptUpdate = await cipher.update(plainText);
    // In GCM mode, pass in null in cipher.doFinal in encryption. Obtain the tag data and fill it in the gcmParams object.
    gcmParams.authTag = await cipher.doFinal(null);
    return encryptUpdate;
  }
  // Decrypt the message.
  async function decryptMessagePromise(symKey: cryptoFramework.SymKey, cipherText: cryptoFramework.DataBlob) {
    let decoder = cryptoFramework.createCipher('SM4_128|GCM|PKCS7');
    await decoder.init(cryptoFramework.CryptoMode.DECRYPT_MODE, symKey, gcmParams);
    let decryptUpdate = await decoder.update(cipherText);
    // In GCM mode, pass in null in cipher.doFinal in decryption. Verify the tag data passed in *Cipher.init. If the verification fails, an exception will be thrown.
    let decryptData = await decoder.doFinal(null);
    if (decryptData == null) {
      console.info('GCM decrypt success, decryptData is null');
    }
    return decryptUpdate;
  }
  async function genSymKeyByData(symKeyData: Uint8Array) {
    let symKeyBlob: cryptoFramework.DataBlob = { data: symKeyData };
    let sm4Generator = cryptoFramework.createSymKeyGenerator('SM4_128');
    let symKey = await sm4Generator.convertKey(symKeyBlob);
    console.info('convertKey success');
    return symKey;
  }
  async function main() {
    let keyData = new Uint8Array([83, 217, 231, 76, 28, 113, 23, 219, 250, 71, 209, 210, 205, 97, 32, 159]);
    let symKey = await genSymKeyByData(keyData);
    let message = "This is a test";
    let plainText: cryptoFramework.DataBlob = { data: new Uint8Array(buffer.from(message, 'utf-8').buffer) };
    let encryptText = await encryptMessagePromise(symKey, plainText);
    let decryptText = await decryptMessagePromise(symKey, encryptText);
    if (plainText.data.toString() === decryptText.data.toString()) {
      console.info('decrypt ok');
      console.info('decrypt plainText: ' + buffer.from(decryptText.data).toString('utf-8'));
    } else {
      console.error('decrypt failed');
    }
  }
  ```

- Example (using synchronous APIs):

  ```ts
  import { cryptoFramework } from '@kit.CryptoArchitectureKit';
  import { buffer } from '@kit.ArkTS';


  function genGcmParamsSpec() {
    let arr = [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]; // 12 bytes
    let dataIv = new Uint8Array(arr);
    let ivBlob: cryptoFramework.DataBlob = { data: dataIv };
    arr = [0, 0, 0, 0, 0, 0, 0, 0]; // 8 bytes
    let dataAad = new Uint8Array(arr);
    let aadBlob: cryptoFramework.DataBlob = { data: dataAad };
    arr = [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]; // 16 bytes
    let dataTag = new Uint8Array(arr);
    let tagBlob: cryptoFramework.DataBlob = {
      data: dataTag
    };
    // Obtain the GCM authTag from the doFinal result in encryption and fill it in the params parameter of init() in decryption.
    let gcmParamsSpec: cryptoFramework.GcmParamsSpec = {
      iv: ivBlob,
      aad: aadBlob,
      authTag: tagBlob,
      algName: "GcmParamsSpec"
    };
    return gcmParamsSpec;
  }

  let gcmParams = genGcmParamsSpec();

  // Encrypt the message.
  function encryptMessage(symKey: cryptoFramework.SymKey, plainText: cryptoFramework.DataBlob) {
    let cipher = cryptoFramework.createCipher('SM4_128|GCM|PKCS7');
    cipher.initSync(cryptoFramework.CryptoMode.ENCRYPT_MODE, symKey, gcmParams);
    let encryptUpdate = cipher.updateSync(plainText);
    // In GCM mode, pass in null in doFinal() in encryption. Obtain the tag data and fill it in the gcmParams object.
    gcmParams.authTag = cipher.doFinalSync(null);
    return encryptUpdate;
  }
  // Decrypt the message.
  function decryptMessage(symKey: cryptoFramework.SymKey, cipherText: cryptoFramework.DataBlob) {
    let decoder = cryptoFramework.createCipher('SM4_128|GCM|PKCS7');
    decoder.initSync(cryptoFramework.CryptoMode.DECRYPT_MODE, symKey, gcmParams);
    let decryptUpdate = decoder.updateSync(cipherText);
    // In GCM mode, pass in null in doFinal() in decryption. Verify the tag data passed in **init**. If the verification fails, an exception will be thrown.
    let decryptData = decoder.doFinalSync(null);
    if (decryptData == null) {
      console.info('GCM decrypt success, decryptData is null');
    }
    return decryptUpdate;
  }
  async function genSymKeyByData(symKeyData: Uint8Array) {
    let symKeyBlob: cryptoFramework.DataBlob = { data: symKeyData };
    let sm4Generator = cryptoFramework.createSymKeyGenerator('SM4_128');
    let symKey = await sm4Generator.convertKey(symKeyBlob);
    console.info('convertKey success');
    return symKey;
  }
  async function main() {
    let keyData = new Uint8Array([83, 217, 231, 76, 28, 113, 23, 219, 250, 71, 209, 210, 205, 97, 32, 159]);
    let symKey = await genSymKeyByData(keyData);
    let message = "This is a test";
    let plainText: cryptoFramework.DataBlob = { data: new Uint8Array(buffer.from(message, 'utf-8').buffer) };
    let encryptText = encryptMessage(symKey, plainText);
    let decryptText = decryptMessage(symKey, encryptText);
    if (plainText.data.toString() === decryptText.data.toString()) {
      console.info('decrypt ok');
      console.info('decrypt plainText: ' + buffer.from(decryptText.data).toString('utf-8'));
    } else {
      console.error('decrypt failed');
    }
  }
  ```