1# Encryption and Decryption with an SM4 Symmetric Key (CBC Mode) (C/C++)
2
3
4For details about the algorithm specifications, see [SM4](crypto-sym-encrypt-decrypt-spec.md#sm4).
5
6## Adding the Dynamic Library in the CMake Script
7```txt
8   target_link_libraries(entry PUBLIC libohcrypto.so)
9```
10
11**Encryption**
12
13
141. 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**).
15
16   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-ndk.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.
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182. Use [OH_CryptoSymCipher_Create](../../reference/apis-crypto-architecture-kit/_crypto_sym_cipher_api.md#oh_cryptosymcipher_create) with the string parameter **'SM4_128|CBC|PKCS7'** to create a **Cipher** instance. The key type is **SM4_128**, block cipher mode is **CBC**, and the padding mode is **PKCS7**.
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203. 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_SetParam](../../reference/apis-crypto-architecture-kit/_crypto_sym_cipher_api.md#oh_cryptosymcipherparams_setparam) to set cipher parameters.
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224. 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 CBC mode.
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245. 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.
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26   - If a small amount of data is to be encrypted, you can use **OH_CryptoSymCipher_Final()** immediately after **OH_CryptoSymCipher_Init()**.
27   - 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.
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296. Use [OH_CryptoSymCipher_Final](../../reference/apis-crypto-architecture-kit/_crypto_sym_cipher_api.md#oh_cryptosymcipher_final) to generate the ciphertext.
30
31   - If data has been passed in by **OH_CryptoSymCipher_Update()**, pass in **null** in the **data** parameter of **OH_CryptoSymCipher_Final**.
32   - The output of **OH_CryptoSymCipher_Final** may be **null**. To avoid exceptions, always check whether the result is **null** before accessing specific data.
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347. 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.
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36
37**Decryption**
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39
401. 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 CBC mode.
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422. 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.
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443. Use [OH_CryptoSymCipher_Final](../../reference/apis-crypto-architecture-kit/_crypto_sym_cipher_api.md#oh_cryptosymcipher_final) to generate the plaintext.
45
46**Example**
47
48```c++
49#include "CryptoArchitectureKit/crypto_common.h"
50#include "CryptoArchitectureKit/crypto_sym_cipher.h"
51#include <string.h>
52
53static OH_Crypto_ErrCode doTestSm4Cbc()
54{
55    OH_CryptoSymKeyGenerator *genCtx = nullptr;
56    OH_CryptoSymCipher *encCtx = nullptr;
57    OH_CryptoSymCipher *decCtx = nullptr;
58    OH_CryptoSymKey *keyCtx = nullptr;
59    OH_CryptoSymCipherParams *params = nullptr;
60    Crypto_DataBlob outUpdate = {.data = nullptr, .len = 0};
61    Crypto_DataBlob decUpdate = {.data = nullptr, .len = 0};
62
63    char *plainText = const_cast<char *>("this is test!");
64    Crypto_DataBlob msgBlob = {.data = (uint8_t *)(plainText), .len = strlen(plainText)};
65    uint8_t iv[16] = {1, 2, 4, 12, 3, 4, 2, 3, 3, 2, 0, 4, 3, 1, 0, 10}; // iv is generated from an array of secure random numbers.
66    Crypto_DataBlob ivBlob = {.data = iv, .len = sizeof(iv)};
67    // Generate a symmetric key.
68    OH_Crypto_ErrCode ret;
69    ret = OH_CryptoSymKeyGenerator_Create("SM4_128", &genCtx);
70    if (ret != CRYPTO_SUCCESS) {
71        goto end;
72    }
73    ret = OH_CryptoSymKeyGenerator_Generate(genCtx, &keyCtx);
74    if (ret != CRYPTO_SUCCESS) {
75        goto end;
76    }
77
78    // Set parameters.
79    ret = OH_CryptoSymCipherParams_Create(&params);
80    if (ret != CRYPTO_SUCCESS) {
81        goto end;
82    }
83    ret = OH_CryptoSymCipherParams_SetParam(params, CRYPTO_IV_DATABLOB, &ivBlob);
84    if (ret != CRYPTO_SUCCESS) {
85        goto end;
86    }
87
88    // Encrypt data.
89    ret = OH_CryptoSymCipher_Create("SM4_128|CBC|PKCS7", &encCtx);
90    if (ret != CRYPTO_SUCCESS) {
91        goto end;
92    }
93    ret = OH_CryptoSymCipher_Init(encCtx, CRYPTO_ENCRYPT_MODE, keyCtx, params);
94    if (ret != CRYPTO_SUCCESS) {
95        goto end;
96    }
97    ret = OH_CryptoSymCipher_Final(encCtx, &msgBlob, &outUpdate);
98    if (ret != CRYPTO_SUCCESS) {
99        goto end;
100    }
101
102    // Decrypt data.
103    ret = OH_CryptoSymCipher_Create("SM4_128|CBC|PKCS7", &decCtx);
104    if (ret != CRYPTO_SUCCESS) {
105        goto end;
106    }
107    ret = OH_CryptoSymCipher_Init(decCtx, CRYPTO_DECRYPT_MODE, keyCtx, params);
108    if (ret != CRYPTO_SUCCESS) {
109        goto end;
110    }
111    ret = OH_CryptoSymCipher_Final(decCtx, &outUpdate, &decUpdate);
112    if (ret != CRYPTO_SUCCESS) {
113        goto end;
114    }
115
116    // Release resources.
117end:
118    OH_CryptoSymCipherParams_Destroy(params);
119    OH_CryptoSymCipher_Destroy(encCtx);
120    OH_CryptoSymCipher_Destroy(decCtx);
121    OH_CryptoSymKeyGenerator_Destroy(genCtx);
122    OH_CryptoSymKey_Destroy(keyCtx);
123    OH_Crypto_FreeDataBlob(&outUpdate);
124    OH_Crypto_FreeDataBlob(&decUpdate);
125    return ret;
126}
127```
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