xref: /docs/en/application-dev/security/CryptoArchitectureKit/crypto-sm4-sym-encrypt-decrypt-gcm-ndk.md

# Encryption and Decryption with an SM4 Symmetric Key (GCM Mode) (C/C++)


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


## Adding the Dynamic Library in the CMake Script
```txt
   target_link_libraries(entry PUBLIC libohcrypto.so)
```

**Encryption**


1. Use [OH_CryptoSymKeyGenerator_Create](../../reference/apis-crypto-architecture-kit/_crypto_sym_key_api.md#oh_cryptosymkeygenerator_create) and [OH_CryptoSymKeyGenerator_Generate](../../reference/apis-crypto-architecture-kit/_crypto_sym_key_api.md#oh_cryptosymkeygenerator_generate) to generate a 128-bit SM4 symmetric key (**OH_CryptoSymKey**).
   
   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 [OH_CryptoSymCipher_Create](../../reference/apis-crypto-architecture-kit/_crypto_sym_cipher_api.md#oh_cryptosymcipher_create) 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 [OH_CryptoSymCipherParams_Create](../../reference/apis-crypto-architecture-kit/_crypto_sym_cipher_api.md#oh_cryptosymcipherparams_create) to create a symmetric cipher parameter instance, and use [OH_CryptoSymCipherParams_SetParams](../../reference/apis-crypto-architecture-kit/_crypto_sym_cipher_api.md#oh_cryptosymcipherparams_setparam) to set cipher parameters.

4. Use [OH_CryptoSymCipher_Init](../../reference/apis-crypto-architecture-kit/_crypto_sym_cipher_api.md#oh_cryptosymcipher_init) to initialize the **Cipher** instance. Specifically, set **mode** to **CRYPTO_ENCRYPT_MODE**, and specify the key for encryption (**OH_CryptoSymKey**) and the encryption parameter instance (**OH_CryptoSymCipherParams**) corresponding to the GCM mode.

5. Use [OH_CryptoSymCipher_Update](../../reference/apis-crypto-architecture-kit/_crypto_sym_cipher_api.md#oh_cryptosymcipher_update) to update the data (plaintext) to be encrypted.
   
   Currently, the amount of data to be passed in by a single **OH_CryptoSymCipher_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 **OH_CryptoSymCipher_Final()** immediately after **OH_CryptoSymCipher_Init()**.
   - If a large amount of data is to be encrypted, you can call **OH_CryptoSymCipher_Update()** multiple times to pass in the data by segment.

6. Use [OH_CryptoSymCipher_Final](../../reference/apis-crypto-architecture-kit/_crypto_sym_cipher_api.md#oh_cryptosymcipher_final) to generate the ciphertext.
   - If data has been passed in by **OH_CryptoSymCipher_Update()**, pass in **null** in the **data** parameter of **OH_CryptoSymCipher_Final**.
   - The output of **OH_CryptoSymCipher_Final** may be **null**. To avoid exceptions, always check whether the result is **null** before accessing specific data.

7. Use [OH_CryptoSymCipherParams_Create](../../reference/apis-crypto-architecture-kit/_crypto_sym_cipher_api.md#oh_cryptosymcipherparams_create) to create a **Params** instance, and use [OH_CryptoSymCipherParams_SetParam](../../reference/apis-crypto-architecture-kit/_crypto_sym_cipher_api.md#oh_cryptosymcipherparams_setparam) to set **authTag** 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.

8. Use [OH_CryptoSymKeyGenerator_Destroy](../../reference/apis-crypto-architecture-kit/_crypto_sym_key_api.md#oh_cryptosymkeygenerator_destroy), [OH_CryptoSymCipher_Destroy](../../reference/apis-crypto-architecture-kit/_crypto_sym_cipher_api.md#oh_cryptosymcipher_destroy), and [OH_CryptoSymCipherParams_Destroy](../../reference/apis-crypto-architecture-kit/_crypto_sym_cipher_api.md#oh_cryptosymcipherparams_destroy) to destroy the instances created.


**Decryption**


1. Use [OH_CryptoSymCipher_Init](../../reference/apis-crypto-architecture-kit/_crypto_sym_cipher_api.md#oh_cryptosymcipher_init) initializes the **Cipher** instance. Specifically, set **mode** to **CRYPTO_DECRYPT_MODE**, and specify the key for decryption (**OH_CryptoSymKey**) and the decryption parameter instance (**OH_CryptoSymCipherParams**) corresponding to the GCM mode.

2. Use [OH_CryptoSymCipher_Update](../../reference/apis-crypto-architecture-kit/_crypto_sym_cipher_api.md#oh_cryptosymcipher_update) to update the data (ciphertext) to be decrypted.

3. Use [OH_CryptoSymCipher_Final](../../reference/apis-crypto-architecture-kit/_crypto_sym_cipher_api.md#oh_cryptosymcipher_final) to generate the plaintext.


**Example**

```c++
#include "CryptoArchitectureKit/crypto_common.h"
#include "CryptoArchitectureKit/crypto_sym_cipher.h"

static OH_Crypto_ErrCode doTestSm4Gcm()
{
    OH_CryptoSymKeyGenerator *genCtx = nullptr;
    OH_CryptoSymCipher *encCtx = nullptr;
    OH_CryptoSymCipher *decCtx = nullptr;
    OH_CryptoSymKey *keyCtx = nullptr;
    OH_CryptoSymCipherParams *params = nullptr;

    Crypto_DataBlob outUpdate = {.data = nullptr, .len = 0};
    Crypto_DataBlob decUpdate = {.data = nullptr, .len = 0};

    uint8_t aad[8] = {0};
    uint8_t tag[16] = {0};
    uint8_t iv[12] = {0};
    Crypto_DataBlob ivData = {.data = iv, .len = sizeof(iv)};
    Crypto_DataBlob aadData = {.data = aad, .len = sizeof(aad)};
    Crypto_DataBlob tagData = {.data = tag, .len = sizeof(tag)};
    Crypto_DataBlob tagOutPut = {.data = nullptr, .len = 0};
    uint8_t plainText[] = "this is test!";
    Crypto_DataBlob msgBlob = {.data = reinterpret_cast<uint8_t *>(plainText), .len = 13};

    // Generate a symmetric key.
    OH_Crypto_ErrCode ret;
    ret = OH_CryptoSymKeyGenerator_Create("SM4_128", &genCtx);
    if (ret != CRYPTO_SUCCESS) {
        goto end;
    }
    ret = OH_CryptoSymKeyGenerator_Generate(genCtx, &keyCtx);
    if (ret != CRYPTO_SUCCESS) {
        goto end;
    }

    // Set parameters.
    ret = OH_CryptoSymCipherParams_Create(&params);
    if (ret != CRYPTO_SUCCESS) {
        goto end;
    }
    ret = OH_CryptoSymCipherParams_SetParam(params, CRYPTO_IV_DATABLOB, &ivData);
    if (ret != CRYPTO_SUCCESS) {
        goto end;
    }
    ret = OH_CryptoSymCipherParams_SetParam(params, CRYPTO_AAD_DATABLOB, &aadData);
    if (ret != CRYPTO_SUCCESS) {
        goto end;
    }
    ret = OH_CryptoSymCipherParams_SetParam(params, CRYPTO_TAG_DATABLOB, &tagData);
    if (ret != CRYPTO_SUCCESS) {
        goto end;
    }

    // Encrypt data.
    ret = OH_CryptoSymCipher_Create("SM4_128|GCM|PKCS7", &encCtx);
    if (ret != CRYPTO_SUCCESS) {
        goto end;
    }
    ret = OH_CryptoSymCipher_Init(encCtx, CRYPTO_ENCRYPT_MODE, keyCtx, params);
    if (ret != CRYPTO_SUCCESS) {
        goto end;
    }
    ret = OH_CryptoSymCipher_Update(encCtx, &msgBlob, &outUpdate);
    if (ret != CRYPTO_SUCCESS) {
        goto end;
    }
    ret = OH_CryptoSymCipher_Final(encCtx, nullptr, &tagOutPut);
    if (ret != CRYPTO_SUCCESS) {
        goto end;
    }

    // Decrypt data.
    ret = OH_CryptoSymCipher_Create("SM4_128|GCM|PKCS7", &decCtx);
    if (ret != CRYPTO_SUCCESS) {
        goto end;
    }
    ret = OH_CryptoSymCipherParams_SetParam(params, CRYPTO_TAG_DATABLOB, &tagOutPut);
    if (ret != CRYPTO_SUCCESS) {
        goto end;
    }
    ret = OH_CryptoSymCipher_Init(decCtx, CRYPTO_DECRYPT_MODE, keyCtx, params);
    if (ret != CRYPTO_SUCCESS) {
        goto end;
    }
    ret = OH_CryptoSymCipher_Final(decCtx, &outUpdate, &decUpdate);
    if (ret != CRYPTO_SUCCESS) {
        goto end;
    }

    // Release resources.
end:
    OH_CryptoSymCipherParams_Destroy(params);
    OH_CryptoSymCipher_Destroy(encCtx);
    OH_CryptoSymCipher_Destroy(decCtx);
    OH_CryptoSymKeyGenerator_Destroy(genCtx);
    OH_CryptoSymKey_Destroy(keyCtx);
    OH_Crypto_FreeDataBlob(&outUpdate);
    OH_Crypto_FreeDataBlob(&decUpdate);
    OH_Crypto_FreeDataBlob(&tagOutPut);
    return ret;
}
```