用C语言把SM2算法实现

SM2算法是一种椭圆曲线公钥密码算法，其实现需要使用椭圆曲线运算库。以下是一个简单的示例代码，假设使用了OpenSSL库：

#include <openssl/ec.h>
#include <openssl/ecdsa.h>
#include <openssl/err.h>
#include <openssl/rand.h>

int sm2_encrypt(unsigned char *plaintext, int plaintext_len,
                unsigned char *ciphertext, int *ciphertext_len,
                EC_KEY *ec_key) {
    int ret = 0;
    EC_GROUP *group = NULL;
    EC_POINT *pub_key = NULL;
    unsigned char *tmp_ciphertext = NULL;
    int tmp_ciphertext_len;
    EVP_CIPHER_CTX *ctx = NULL;

    group = EC_KEY_get0_group(ec_key);
    pub_key = EC_KEY_get0_public_key(ec_key);

    /* 生成临时公钥 */
    EC_POINT *tmp_pub_key = EC_POINT_new(group);
    if (!tmp_pub_key) {
        ret = -1;
        goto cleanup;
    }
    if (!EC_POINT_copy(tmp_pub_key, pub_key)) {
        ret = -1;
        goto cleanup;
    }
    if (!EC_KEY_generate_key(ec_key)) {
        ret = -1;
        goto cleanup;
    }

    /* 加密 */
    ctx = EVP_CIPHER_CTX_new();
    if (!ctx) {
        ret = -1;
        goto cleanup;
    }
    if (!EVP_EncryptInit_ex(ctx, EVP_sm4_ecb(), NULL, NULL, NULL)) {
        ret = -1;
        goto cleanup;
    }
    if (!EVP_EncryptUpdate(ctx, ciphertext, ciphertext_len, plaintext, plaintext_len)) {
        ret = -1;
        goto cleanup;
    }
    if (!EVP_EncryptFinal_ex(ctx, ciphertext + *ciphertext_len, &tmp_ciphertext_len)) {
        ret = -1;
        goto cleanup;
    }
    *ciphertext_len += tmp_ciphertext_len;

    /* 生成密文 */
    tmp_ciphertext = (unsigned char *)OPENSSL_malloc(*ciphertext_len + 65);
    if (!tmp_ciphertext) {
        ret = -1;
        goto cleanup;
    }
    memcpy(tmp_ciphertext, ciphertext, *ciphertext_len);
    if (!EC_POINT_point2oct(group, tmp_pub_key, POINT_CONVERSION_UNCOMPRESSED,
                            tmp_ciphertext + *ciphertext_len, 65, NULL)) {
        ret = -1;
        goto cleanup;
    }
    *ciphertext_len += 65;
    memcpy(ciphertext, tmp_ciphertext, *ciphertext_len);

cleanup:
    if (tmp_pub_key) EC_POINT_free(tmp_pub_key);
    if (ctx) EVP_CIPHER_CTX_free(ctx);
    if (tmp_ciphertext) OPENSSL_free(tmp_ciphertext);

    return ret;
}

int sm2_decrypt(unsigned char *ciphertext, int ciphertext_len,
                unsigned char *plaintext, int *plaintext_len,
                EC_KEY *ec_key) {
    int ret = 0;
    EC_GROUP *group = NULL;
    EC_POINT *pub_key = NULL;
    EC_POINT *tmp_pub_key = NULL;
    unsigned char *tmp_ciphertext = NULL;
    int tmp_ciphertext_len;
    EVP_CIPHER_CTX *ctx = NULL;

    group = EC_KEY_get0_group(ec_key);
    pub_key = EC_KEY_get0_public_key(ec_key);

    /* 解密 */
    ctx = EVP_CIPHER_CTX_new();
    if (!ctx) {
        ret = -1;
        goto cleanup;
    }
    if (!EVP_DecryptInit_ex(ctx, EVP_sm4_ecb(), NULL, NULL, NULL)) {
        ret = -1;
        goto cleanup;
    }
    if (!EVP_DecryptUpdate(ctx, plaintext, plaintext_len, ciphertext, ciphertext_len - 65)) {
        ret = -1;
        goto cleanup;
    }
    if (!EVP_DecryptFinal_ex(ctx, plaintext + *plaintext_len, &tmp_ciphertext_len)) {
        ret = -1;
        goto cleanup;
    }
    *plaintext_len += tmp_ciphertext_len;

    /* 生成临时公钥 */
    tmp_pub_key = EC_POINT_new(group);
    if (!tmp_pub_key) {
        ret = -1;
        goto cleanup;
    }
    if (!EC_POINT_oct2point(group, tmp_pub_key, ciphertext + ciphertext_len - 65, 65, NULL)) {
        ret = -1;
        goto cleanup;
    }

    /* 验证公钥是否正确 */
    if (!EC_POINT_cmp(group, pub_key, tmp_pub_key, NULL)) {
        ret = -1;
        goto cleanup;
    }

cleanup:
    if (tmp_pub_key) EC_POINT_free(tmp_pub_key);
    if (ctx) EVP_CIPHER_CTX_free(ctx);
    if (tmp_ciphertext) OPENSSL_free(tmp_ciphertext);

    return ret;
}

需要注意的是，SM2算法中使用的椭圆曲线参数需要从安全可信的源获取，并且在实现过程中需要注意安全性和性能。以上代码仅供参考，实际应用时应进行充分的测试和评估。