openssl C语言DER SM2私钥转为PKCS#8 的PEM格式

以下是将DER编码的SM2私钥转换为PKCS#8 PEM格式的示例代码：

#include <openssl/pem.h>
#include <openssl/ec.h>
#include <openssl/evp.h>

int main()
{
    // 读取DER编码的SM2私钥
    FILE* fp = fopen("sm2.key", "rb");
    if (!fp) {
        printf("failed to open file\n");
        return 1;
    }
    fseek(fp, 0, SEEK_END);
    long fsize = ftell(fp);
    fseek(fp, 0, SEEK_SET);
    unsigned char* buf = malloc(fsize);
    if (!buf) {
        printf("failed to allocate memory\n");
        return 1;
    }
    fread(buf, fsize, 1, fp);
    fclose(fp);

    // 将DER编码的SM2私钥转为EVP_PKEY
    EVP_PKEY* pkey = EVP_PKEY_new();
    if (!pkey) {
        printf("failed to create EVP_PKEY\n");
        return 1;
    }
    EC_KEY* ec_key = EC_KEY_new();
    if (!ec_key) {
        printf("failed to create EC_KEY\n");
        return 1;
    }
    EC_GROUP* ec_group = EC_GROUP_new_by_curve_name(NID_sm2);
    if (!ec_group) {
        printf("failed to create EC_GROUP\n");
        return 1;
    }
    const unsigned char* p = buf;
    ec_key = d2i_ECPrivateKey(&ec_key, &p, fsize);
    if (!ec_key) {
        printf("failed to parse ECPrivateKey\n");
        return 1;
    }
    if (!EC_KEY_set_group(ec_key, ec_group)) {
        printf("failed to set EC_KEY group\n");
        return 1;
    }
    if (!EVP_PKEY_assign_EC_KEY(pkey, ec_key)) {
        printf("failed to assign EVP_PKEY\n");
        return 1;
    }

    // 将EVP_PKEY转为PKCS#8 PEM格式
    BIO* bio = BIO_new(BIO_s_mem());
    if (!bio) {
        printf("failed to create BIO\n");
        return 1;
    }
    if (!PEM_write_bio_PKCS8PrivateKey(bio, pkey, NULL, NULL, 0, NULL, NULL)) {
        printf("failed to write PKCS8PrivateKey\n");
        return 1;
    }

    // 输出PKCS#8 PEM格式私钥
    BUF_MEM* mem = NULL;
    BIO_get_mem_ptr(bio, &mem);
    printf("%.*s", (int)mem->length, mem->data);

    // 释放资源
    free(buf);
    EVP_PKEY_free(pkey);
    BIO_free_all(bio);
    return 0;
}

其中，sm2.key为DER编码的SM2私钥文件。运行后，将输出PKCS#8 PEM格式的私钥。