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1# Exporting a Key (C/C++)
2
3<!--Kit: Universal Keystore Kit-->
4<!--Subsystem: Security-->
5<!--Owner: @wutiantian-gitee-->
6<!--Designer: @HighLowWorld-->
7<!--Tester: @wxy1234564846-->
8<!--Adviser: @zengyawen-->
9
10This topic walks you through on how to export the public key of a persistently stored asymmetric key. Currently, HUKS supports export of the ECC, RSA, Ed25519, X25519, and SM2 public keys.
11
12>**NOTE**
13> <!--RP1-->Mini-system devices<!--RP1End--> support export of only the RSA public keys.
14
15## Add the dynamic library in the CMake script.
16```txt
17target_link_libraries(entry PUBLIC libhuks_ndk.z.so)
18```
19
20## How to Develop
21
221. Set parameters.
23   - **keyAlias**: key alias encapsulated in an [OH_Huks_Blob](../../reference/apis-universal-keystore-kit/capi-hukstypeapi-oh-huks-blob.md) struct. The maximum length of the key alias is 128 bytes.
24   - **paramSetIn**: This parameter is reserved. Leave it empty.
25   - **key**: [OH_Huks_Blob](../../reference/apis-universal-keystore-kit/capi-hukstypeapi-oh-huks-blob.md) object used to hold the key exported. Ensure that there is enough memory for storing the key exported.
26
272. Call [OH_Huks_GetKeyItemParamSet](../../reference/apis-universal-keystore-kit/capi-native-huks-api-h.md#oh_huks_getkeyitemparamset) to pass in the preceding parameters.
28
293. Check the return value. If the operation is successful, the exported key is in the **key** field in the DER format defined in X.509. For details about the format, see [Public Key Material Format](huks-concepts.md#public-key-material-format). If the operation fails, an error code is returned.
30
31```c++
32#include "huks/native_huks_api.h"
33#include "huks/native_huks_param.h"
34#include "napi/native_api.h"
35#include <cstring>
36
37/* Generate an ECC key. */
38OH_Huks_Result InitParamSet(struct OH_Huks_ParamSet **paramSet, const struct OH_Huks_Param *params,
39                            uint32_t paramCount)
40{
41    OH_Huks_Result ret = OH_Huks_InitParamSet(paramSet);
42    if (ret.errorCode != OH_HUKS_SUCCESS) {
43        return ret;
44    }
45    ret = OH_Huks_AddParams(*paramSet, params, paramCount);
46    if (ret.errorCode != OH_HUKS_SUCCESS) {
47        OH_Huks_FreeParamSet(paramSet);
48        return ret;
49    }
50    ret = OH_Huks_BuildParamSet(paramSet);
51    if (ret.errorCode != OH_HUKS_SUCCESS) {
52        OH_Huks_FreeParamSet(paramSet);
53        return ret;
54    }
55    return ret;
56}
57
58struct OH_Huks_Param g_testGenerateKeyParam[] = {{.tag = OH_HUKS_TAG_ALGORITHM, .uint32Param = OH_HUKS_ALG_ECC},
59                                                 {.tag = OH_HUKS_TAG_PURPOSE, .uint32Param = OH_HUKS_KEY_PURPOSE_AGREE},
60                                                 {.tag = OH_HUKS_TAG_KEY_SIZE, .uint32Param = OH_HUKS_ECC_KEY_SIZE_256},
61                                                 {.tag = OH_HUKS_TAG_DIGEST, .uint32Param = OH_HUKS_DIGEST_NONE}};
62
63static OH_Huks_Result GenerateKeyHelper(const char *alias)
64{
65    struct OH_Huks_Blob aliasBlob = {.size = (uint32_t)strlen(alias), .data = (uint8_t *)alias};
66    struct OH_Huks_ParamSet *testGenerateKeyParamSet = nullptr;
67    struct OH_Huks_Result ohResult;
68    do {
69        /* 1. Initialize the key property set. */
70        ohResult = InitParamSet(&testGenerateKeyParamSet, g_testGenerateKeyParam,
71                                sizeof(g_testGenerateKeyParam) / sizeof(OH_Huks_Param));
72        if (ohResult.errorCode != OH_HUKS_SUCCESS) {
73            break;
74        }
75        /* 2. Generate a key. */
76        ohResult = OH_Huks_GenerateKeyItem(&aliasBlob, testGenerateKeyParamSet, nullptr);
77    } while (0);
78    OH_Huks_FreeParamSet(&testGenerateKeyParamSet);
79    return ohResult;
80}
81
82static napi_value ExportKey(napi_env env, napi_callback_info info)
83{
84    /* 1. Set the key alias. */
85    const char *alias = "test_key";
86    struct OH_Huks_Blob aliasBlob = { .size = (uint32_t)strlen(alias), .data = (uint8_t *)alias };
87
88    /* Generate a key. */
89    OH_Huks_Result genResult = GenerateKeyHelper(alias);
90    if (genResult.errorCode != OH_HUKS_SUCCESS) {
91        napi_value ret;
92        napi_create_int32(env, genResult.errorCode, &ret);
93        return ret;
94    }
95
96    /* Request the memory for holding the public key to be exported. */
97    uint8_t *pubKey = (uint8_t *)malloc(512); // Request memory based on the key size.
98    if (pubKey == nullptr) {
99        return nullptr;
100    }
101    struct OH_Huks_Blob keyBlob = { 256, pubKey };
102    struct OH_Huks_Result ohResult;
103    do {
104        ohResult = OH_Huks_ExportPublicKeyItem(&aliasBlob, nullptr, &keyBlob);
105        if (ohResult.errorCode != OH_HUKS_SUCCESS) {
106            break;
107        }
108    } while (0);
109    free(pubKey);
110    napi_value ret;
111    napi_create_int32(env, ohResult.errorCode, &ret);
112    return ret;
113}
114```
115