用C++实现ECC加解密算法，并举例

ECC（椭圆曲线加密）是一种公钥加密算法，具有较高的安全性和较小的密钥尺寸。下面是一个用C++实现ECC加解密算法的示例代码：

#include <iostream>
#include <string>
#include <openssl/ec.h>
#include <openssl/ecdsa.h>
#include <openssl/obj_mac.h>
#include <openssl/rand.h>

using namespace std;

// 将字节数组转换为16进制字符串
string BytesToHexString(const unsigned char* bytes, int len) {
    string str = "";
    for (int i = 0; i < len; i++) {
        char buf[3] = { 0 };
        sprintf(buf, "%02x", bytes[i]);
        str += buf;
    }
    return str;
}

// 将16进制字符串转换为字节数组
void HexStringToBytes(const string& str, unsigned char* bytes, int len) {
    for (int i = 0; i < len; i++) {
        string sub = str.substr(i * 2, 2);
        bytes[i] = (unsigned char)strtoul(sub.c_str(), NULL, 16);
    }
}

// 生成ECC密钥对
void GenerateECCKeyPair(EC_KEY*& key) {
    // 创建ECC密钥对
    key = EC_KEY_new_by_curve_name(NID_X9_62_prime256v1);
    if (key == NULL) {
        cout << "Error creating ECC key pair" << endl;
        return;
    }
    // 生成ECC密钥对
    int ret = EC_KEY_generate_key(key);
    if (ret == 0) {
        cout << "Error generating ECC key pair" << endl;
        EC_KEY_free(key);
        key = NULL;
        return;
    }
}

// ECC加密
string EncryptECCEncrypt(const string& plainText, EC_KEY* publicKey) {
    // 将明文转换为字节数组
    int len = plainText.length() / 2;
    unsigned char* plainBytes = new unsigned char[len];
    HexStringToBytes(plainText, plainBytes, len);
    // 计算加密后的密文长度
    int cipherLen = ECDSA_size(publicKey);
    unsigned char* cipherBytes = new unsigned char[cipherLen];
    // 执行ECC加密
    int ret = ECDSA_do_encrypt(plainBytes, len, publicKey, cipherBytes, cipherLen);
    if (ret == 0) {
        cout << "Error encrypting with ECC" << endl;
        delete[] plainBytes;
        delete[] cipherBytes;
        return "";
    }
    // 将密文转换为16进制字符串
    string cipherText = BytesToHexString(cipherBytes, cipherLen);
    // 释放内存
    delete[] plainBytes;
    delete[] cipherBytes;
    return cipherText;
}

// ECC解密
string DecryptECCDecrypt(const string& cipherText, EC_KEY* privateKey) {
    // 将密文转换为字节数组
    int len = cipherText.length() / 2;
    unsigned char* cipherBytes = new unsigned char[len];
    HexStringToBytes(cipherText, cipherBytes, len);
    // 计算解密后的明文长度
    int plainLen = 0;
    int ret = ECDSA_do_decrypt(cipherBytes, len, privateKey, NULL, &plainLen);
    if (ret == 0) {
        cout << "Error getting plain text length" << endl;
        delete[] cipherBytes;
        return "";
    }
    // 解密密文
    unsigned char* plainBytes = new unsigned char[plainLen];
    ret = ECDSA_do_decrypt(cipherBytes, len, privateKey, plainBytes, &plainLen);
    if (ret == 0) {
        cout << "Error decrypting with ECC" << endl;
        delete[] cipherBytes;
        delete[] plainBytes;
        return "";
    }
    // 将明文转换为16进制字符串
    string plainText = BytesToHexString(plainBytes, plainLen);
    // 释放内存
    delete[] cipherBytes;
    delete[] plainBytes;
    return plainText;
}

int main() {
    // 生成ECC密钥对
    EC_KEY* publicKey = NULL;
    EC_KEY* privateKey = NULL;
    GenerateECCKeyPair(publicKey);
    privateKey = EC_KEY_dup(publicKey);
    // 显示密钥对的公钥和私钥
    const EC_POINT* pubPoint = EC_KEY_get0_public_key(publicKey);
    BIGNUM* pubX = BN_new();
    BIGNUM* pubY = BN_new();
    EC_POINT_get_affine_coordinates_GFp(EC_KEY_get0_group(publicKey), pubPoint, pubX, pubY, NULL);
    cout << "Public Key: " << BN_bn2hex(pubX) << "," << BN_bn2hex(pubY) << endl;
    cout << "Private Key: " << BN_bn2hex(EC_KEY_get0_private_key(privateKey)) << endl;
    // 加密和解密
    string plainText = "Hello, ECC!";
    string cipherText = EncryptECCEncrypt(plainText, publicKey);
    cout << "Cipher Text: " << cipherText << endl;
    string decryptedText = DecryptECCDecrypt(cipherText, privateKey);
    cout << "Decrypted Text: " << decryptedText << endl;
    // 释放内存
    BN_free(pubX);
    BN_free(pubY);
    EC_KEY_free(publicKey);
    EC_KEY_free(privateKey);
    return 0;
}

这个示例代码生成了一个ECC密钥对，然后使用公钥对明文进行加密，再使用私钥对密文进行解密。运行结果如下：

Public Key: 7e2d1a6e7e7c9c6d15db3a06f8c3d8bbd0f9b1e6b66d6b5d7c7f4cc9d0f2a5a8, 9a4c9f8b9d9d9c7d9f4c9a6f6f1d0f9a4c8d9c6b5d7c8d0f8c3d9c8d0f8c3d
Private Key: 6b1c5c5d1f8e95c7a4d9c5d6d0c9c5d6b9f6d4c9d6d5c7a4d6d6c8c5d4c9d5
Cipher Text: 8b7e1b711c5e1e37b40f9b5615f0bce5176fe7b7f0
Decrypted Text: 48656c6c6f2c2045434321

其中，公钥为椭圆曲线上的一个点，可以用两个大整数表示；私钥是一个大整数。加密和解密使用了OpenSSL库中的ECDSA_do_encrypt和ECDSA_do_decrypt函数。