harmony 鸿蒙Encryption and Decryption by Segment with an AES Symmetric Key (GCM Mode) (C/C++)

2025-06-12
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Encryption and Decryption by Segment with an AES Symmetric Key (GCM Mode) (C/C++)

For details about the algorithm specifications, see AES.

Adding the Dynamic Library in the CMake Script

target_link_libraries(entry PUBLIC libohcrypto.so)

How to Develop

Creating an Object

Call OH_CryptoSymKeyGenerator_Create and OH_CryptoSymKeyGenerator_Generate to generate a 128-bit AES symmetric key (OH_CryptoSymKey).

In addition to the example in this topic, AES and Randomly Generating a Symmetric Key may help you better understand how to generate an AES symmetric key. Note that the input parameters in the reference documents may be different from those in the example below.

Encrypting a Message

Call OH_CryptoSymCipher_Create with the string parameter ‘AES128|GCM|PKCS7’ to create a Cipher instance for encryption. The key type is AES128, block cipher mode is GCM, and the padding mode is PKCS7.
Call OH_CryptoSymCipherParams_Create to create a symmetric cipher parameter instance, and call OH_CryptoSymCipherParams_SetParam to set cipher parameters.
Call 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.
Set the size of the data to be passed in each time to 20 bytes, and call OH_CryptoSymCipher_Update multiple times to pass in 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.
- You are advised to check the result of each OH_CryptoSymCipher_Update(). If the result is not null, obtain the data and combine the data segments into complete ciphertext. The OH_CryptoSymCipher_Update() result may vary with the key specifications.
If a block cipher mode (ECB or CBC) is used, data is encrypted and output based on the block size. That is, if the data of an OH_CryptoSymCipher_Update() operation matches the block size, the ciphertext is output. Otherwise, null is output, and the plaintext will be combined with the input data of the next OH_CryptoSymCipher_Update() to form a block. When OH_CryptoSymCipher_Final() is called, the unencrypted data is padded to the block size based on the specified padding mode, and then encrypted. The OH_CryptoSymCipher_Update() API works in the same way in decryption.

If a stream cipher mode (CTR or OFB) is used, the ciphertext length is usually the same as the plaintext length.
Call 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. > NOTE
  > If GCM mode is used, authTag returned by OH_CryptoSymCipher_Final() will be used to initialize the authentication information during decryption and needs to be saved. > In GCM mode, authTag must be of 16 bytes. It is used as the authentication information during decryption. In the example, authTag is of 16 bytes.

Decrypting a Message

Call OH_CryptoSymCipher_Create with the string parameter ‘AES128|GCM|PKCS7’ to create a Cipher instance for decryption. The key type is AES128, block cipher mode is GCM, and the padding mode is PKCS7.
Call 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.

Call OH_CryptoSymCipher_Init to initialize 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.
Set the size of the data to be passed in each time to 20 bytes, and call OH_CryptoSymCipher_Update multiple times to pass in the data (ciphertext) to be decrypted.
Call OH_CryptoSymCipher_Final to generate the plaintext.

Destroying Objects

Call OH_CryptoSymKeyGenerator_Destroy, OH_CryptoSymCipher_Destroy, and OH_CryptoSymCipherParams_Destroy to destroy the instances created.

Example

#include <string.h>
#include "CryptoArchitectureKit/crypto_common.h"
#include "CryptoArchitectureKit/crypto_sym_cipher.h"

#define OH_CRYPTO_GCM_TAG_LEN 16
#define OH_CRYPTO_MAX_TEST_DATA_LEN 128
static OH_Crypto_ErrCode doTestAesGcmSeg()
{
    OH_CryptoSymKeyGenerator *genCtx = nullptr;
    OH_CryptoSymCipher *encCtx = nullptr;
    OH_CryptoSymCipher *decCtx = nullptr;
    OH_CryptoSymKey *keyCtx = nullptr;
    OH_CryptoSymCipherParams *params = nullptr;

    char *plainText = const_cast<char *>("aaaaa.....bbbbb.....ccccc.....ddddd.....eee");
    Crypto_DataBlob msgBlob = {.data = (uint8_t *)(plainText), .len = strlen(plainText)};

    uint8_t aad[8] = {1, 2, 3, 4, 5, 6, 7, 8};
    uint8_t tagArr[16] = {0};
    uint8_t iv[12] = {1, 2, 4, 12, 3, 4, 2, 3, 3, 2, 0, 4}; // iv is generated from an array of secure random numbers.
    Crypto_DataBlob tag = {.data = nullptr, .len = 0};
    Crypto_DataBlob ivBlob = {.data = iv, .len = sizeof(iv)};
    Crypto_DataBlob aadBlob = {.data = aad, .len = sizeof(aad)};
    Crypto_DataBlob encData = {.data = nullptr, .len = 0};
    Crypto_DataBlob decData = {.data = nullptr, .len = 0};
    Crypto_DataBlob tagInit = {.data = tagArr, .len = sizeof(tagArr)};
    int32_t cipherLen = 0;
    int blockSize = 20;
    int32_t randomLen = strlen(plainText);
    Crypto_DataBlob cipherBlob;
    // Define the encryption variables.
    int cnt = randomLen / blockSize;
    int rem = randomLen % blockSize;
    uint8_t cipherText[OH_CRYPTO_MAX_TEST_DATA_LEN] = {0};

    // Define the decryption variables.
    int decCnt = cipherLen / blockSize;
    int decRem = cipherLen % blockSize;
    int32_t plantLen = 0;
    uint8_t plantText[OH_CRYPTO_MAX_TEST_DATA_LEN] = {0};
    
    // Generate a key.
    OH_Crypto_ErrCode ret;
    ret = OH_CryptoSymKeyGenerator_Create("AES128", &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, &ivBlob);
    if (ret != CRYPTO_SUCCESS) {
        goto end;
    }
    ret = OH_CryptoSymCipherParams_SetParam(params, CRYPTO_AAD_DATABLOB, &aadBlob);
    if (ret != CRYPTO_SUCCESS) {
        goto end;
    }
    ret = OH_CryptoSymCipherParams_SetParam(params, CRYPTO_TAG_DATABLOB, &tagInit);
    if (ret != CRYPTO_SUCCESS) {
        goto end;
    }
    
    // Encrypt the message.
    ret = OH_CryptoSymCipher_Create("AES128|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;
    }
    
    for (int i = 0; i < cnt; i++) {
        msgBlob.len = blockSize;
        ret = OH_CryptoSymCipher_Update(encCtx, &msgBlob, &encData);
        if (ret != CRYPTO_SUCCESS) {
            goto end;
        }
        msgBlob.data += blockSize;
        memcpy(&cipherText[cipherLen], encData.data, encData.len);
        cipherLen += encData.len;
    }
    if (rem > 0) {
        msgBlob.len = rem;
        ret = OH_CryptoSymCipher_Update(encCtx, (Crypto_DataBlob *)&msgBlob, &encData);
        if (ret != CRYPTO_SUCCESS) {
            goto end;
        }
        memcpy(&cipherText[cipherLen], encData.data, encData.len);
        cipherLen += encData.len;
    }
    ret = OH_CryptoSymCipher_Final(encCtx, nullptr, &tag);
    if (ret != CRYPTO_SUCCESS) {
        goto end;
    }

    // Decrypt the message.
    cipherBlob = {.data = reinterpret_cast<uint8_t *>(cipherText), .len = (size_t)cipherLen};
    ret = OH_CryptoSymCipher_Create("AES128|GCM|PKCS7", &decCtx);
    if (ret != CRYPTO_SUCCESS) {
        goto end;
    }
    ret = OH_CryptoSymCipherParams_SetParam(params, CRYPTO_TAG_DATABLOB, &tag);
    if (ret != CRYPTO_SUCCESS) {
        goto end;
    }
    ret = OH_CryptoSymCipher_Init(decCtx, CRYPTO_DECRYPT_MODE, keyCtx, params);
    if (ret != CRYPTO_SUCCESS) {
        goto end;
    }
    for (int i = 0; i < decCnt; i++) {
        cipherBlob.len = blockSize;
        ret = OH_CryptoSymCipher_Update(decCtx, &cipherBlob, &decData);
        if (ret != CRYPTO_SUCCESS) {
            goto end;
        }
        cipherBlob.data += blockSize;
        memcpy(&plantText[plantLen], decData.data, decData.len);
        plantLen += decData.len;
    }
    if (decRem > 0) {
        cipherBlob.len = decRem;
        ret = OH_CryptoSymCipher_Update(decCtx, &cipherBlob, &decData);
        if (ret != CRYPTO_SUCCESS) {
            goto end;
        }
        memcpy(&plantText[plantLen], decData.data, decData.len);
        plantLen += decData.len;
    }
    ret = OH_CryptoSymCipher_Final(decCtx, nullptr, &decData);
    if (ret != CRYPTO_SUCCESS) {
        goto end;
    }

end:
    OH_CryptoSymCipherParams_Destroy(params);
    OH_CryptoSymCipher_Destroy(encCtx);
    OH_CryptoSymCipher_Destroy(decCtx);
    OH_CryptoSymKeyGenerator_Destroy(genCtx);
    OH_CryptoSymKey_Destroy(keyCtx);
    OH_Crypto_FreeDataBlob(&encData);
    OH_Crypto_FreeDataBlob(&tag);
    OH_Crypto_FreeDataBlob(&decData);
    return ret;
}