用c语言写椭圆曲线加密算法

下面是一个简单的椭圆曲线加密算法的C语言实现示例：

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <string.h>
#include <openssl/ec.h>
#include <openssl/evp.h>
#include <openssl/bn.h>

// 椭圆曲线参数
const char *curve_name = "secp256k1";

// 生成ECC密钥对
void generate_keypair(EVP_PKEY **pkey) {
    EC_KEY *ec_key = NULL;
    EVP_PKEY *evp_key = NULL;
    
    // 生成EC_KEY对象
    ec_key = EC_KEY_new_by_curve_name(OBJ_sn2nid(curve_name));
    if (ec_key == NULL) {
        printf("Error: failed to generate EC_KEY object.\n");
        return;
    }

    // 生成密钥对
    if (EC_KEY_generate_key(ec_key) != 1) {
        printf("Error: failed to generate key pair.\n");
        return;
    }

    // 将EC_KEY对象转换为EVP_PKEY对象
    evp_key = EVP_PKEY_new();
    if (EVP_PKEY_assign_EC_KEY(evp_key, ec_key) != 1) {
        printf("Error: failed to assign EC_KEY object to EVP_PKEY.\n");
        return;
    }

    *pkey = evp_key;
}

// 加密数据
void encrypt_data(const EVP_PKEY *pkey, const unsigned char *in, int in_len, unsigned char **out, int *out_len) {
    int len;
    EVP_CIPHER_CTX *ctx = NULL;
    const EVP_CIPHER *cipher = NULL;

    // 获取加密算法
    cipher = EVP_get_cipherbyname("aes-256-cbc");
    if (cipher == NULL) {
        printf("Error: failed to get cipher algorithm.\n");
        return;
    }

    // 创建加密上下文
    ctx = EVP_CIPHER_CTX_new();
    if (ctx == NULL) {
        printf("Error: failed to create cipher context.\n");
        return;
    }

    // 初始化加密上下文
    if (EVP_EncryptInit_ex(ctx, cipher, NULL, NULL, NULL) != 1) {
        printf("Error: failed to initialize cipher context.\n");
        return;
    }

    // 设置密钥
    if (EVP_PKEY_CTX_set_ec_key(ctx, pkey) != 1) {
        printf("Error: failed to set EC key.\n");
        return;
    }

    // 加密数据
    *out = malloc(in_len + EVP_CIPHER_block_size(cipher));
    if (*out == NULL) {
        printf("Error: failed to allocate memory for output buffer.\n");
        return;
    }
    if (EVP_EncryptUpdate(ctx, *out, &len, in, in_len) != 1) {
        printf("Error: failed to encrypt data.\n");
        return;
    }

    // 结束加密
    if (EVP_EncryptFinal_ex(ctx, *out + len, &len) != 1) {
        printf("Error: failed to finalize cipher context.\n");
        return;
    }

    *out_len = len + in_len;
    EVP_CIPHER_CTX_free(ctx);
}

// 解密数据
void decrypt_data(const EVP_PKEY *pkey, const unsigned char *in, int in_len, unsigned char **out, int *out_len) {
    int len;
    EVP_CIPHER_CTX *ctx = NULL;
    const EVP_CIPHER *cipher = NULL;

    // 获取解密算法
    cipher = EVP_get_cipherbyname("aes-256-cbc");
    if (cipher == NULL) {
        printf("Error: failed to get cipher algorithm.\n");
        return;
    }

    // 创建解密上下文
    ctx = EVP_CIPHER_CTX_new();
    if (ctx == NULL) {
        printf("Error: failed to create cipher context.\n");
        return;
    }

    // 初始化解密上下文
    if (EVP_DecryptInit_ex(ctx, cipher, NULL, NULL, NULL) != 1) {
        printf("Error: failed to initialize cipher context.\n");
        return;
    }

    // 设置密钥
    if (EVP_PKEY_CTX_set_ec_key(ctx, pkey) != 1) {
        printf("Error: failed to set EC key.\n");
        return;
    }

    // 解密数据
    *out = malloc(in_len + EVP_CIPHER_block_size(cipher));
    if (*out == NULL) {
        printf("Error: failed to allocate memory for output buffer.\n");
        return;
    }
    if (EVP_DecryptUpdate(ctx, *out, &len, in, in_len) != 1) {
        printf("Error: failed to decrypt data.\n");
        return;
    }

    // 结束解密
    if (EVP_DecryptFinal_ex(ctx, *out + len, &len) != 1) {
        printf("Error: failed to finalize cipher context.\n");
        return;
    }

    *out_len = len + in_len;
    EVP_CIPHER_CTX_free(ctx);
}

int main() {
    EVP_PKEY *pkey = NULL;
    unsigned char *encrypted_text = NULL;
    unsigned char *decrypted_text = NULL;
    int encrypted_len, decrypted_len;
    const unsigned char *plain_text = "Hello, World!";

    // 生成密钥对
    generate_keypair(&pkey);

    // 加密数据
    encrypt_data(pkey, plain_text, strlen(plain_text), &encrypted_text, &encrypted_len);
    printf("Encrypted text: %s\n", encrypted_text);

    // 解密数据
    decrypt_data(pkey, encrypted_text, encrypted_len, &decrypted_text, &decrypted_len);
    printf("Decrypted text: %s\n", decrypted_text);

    // 释放资源
    free(encrypted_text);
    free(decrypted_text);
    EVP_PKEY_free(pkey);

    return 0;
}

需要注意的是，上面的示例程序中使用了OpenSSL库来处理椭圆曲线加密算法。在编译时需要加上`-lssl -lcrypto`参数才能链接上这两个库。另外，由于椭圆曲线加密算法涉及到大数运算，因此需要使用大数运算库来处理相关的计算。在OpenSSL库中，已经内置了BN库用于处理大数运算。