在C++中實現buffer的加密解密功能通常使用加密算法庫，例如OpenSSL或Crypto++。以下是一個使用OpenSSL庫實現buffer的加密解密功能的示例代碼：

#include <openssl/evp.h>
#include <openssl/aes.h>

void encryptBuffer(const unsigned char* buffer, size_t bufferSize, const unsigned char* key, const unsigned char* iv, unsigned char* encryptedBuffer) {
    EVP_CIPHER_CTX* ctx = EVP_CIPHER_CTX_new();
    EVP_EncryptInit_ex(ctx, EVP_aes_256_cbc(), NULL, key, iv);

    int len;
    int encryptedSize;

    EVP_EncryptUpdate(ctx, encryptedBuffer, &len, buffer, bufferSize);
    encryptedSize = len;

    EVP_EncryptFinal_ex(ctx, encryptedBuffer + len, &len);
    encryptedSize += len;

    EVP_CIPHER_CTX_free(ctx);
}

void decryptBuffer(const unsigned char* encryptedBuffer, size_t encryptedBufferSize, const unsigned char* key, const unsigned char* iv, unsigned char* decryptedBuffer) {
    EVP_CIPHER_CTX* ctx = EVP_CIPHER_CTX_new();
    EVP_DecryptInit_ex(ctx, EVP_aes_256_cbc(), NULL, key, iv);

    int len;
    int decryptedSize;

    EVP_DecryptUpdate(ctx, decryptedBuffer, &len, encryptedBuffer, encryptedBufferSize);
    decryptedSize = len;

    EVP_DecryptFinal_ex(ctx, decryptedBuffer + len, &len);
    decryptedSize += len;

    EVP_CIPHER_CTX_free(ctx);
}

int main() {
    unsigned char key[] = "0123456789abcdef";
    unsigned char iv[] = "abcdefghijklmnop";

    unsigned char buffer[] = "Hello, world!";
    size_t bufferSize = strlen((const char*)buffer);
    size_t encryptedBufferSize = bufferSize + AES_BLOCK_SIZE;

    unsigned char encryptedBuffer[encryptedBufferSize];
    unsigned char decryptedBuffer[bufferSize];

    encryptBuffer(buffer, bufferSize, key, iv, encryptedBuffer);
    decryptBuffer(encryptedBuffer, encryptedBufferSize, key, iv, decryptedBuffer);

    decryptedBuffer[bufferSize] = '\0';
    printf("Decrypted buffer: %s\n", decryptedBuffer);

    return 0;
}

在這個示例中，我們使用AES加密算法對buffer進行加密和解密。首先使用encryptBuffer函數對buffer進行加密，然后使用decryptBuffer函數對加密后的buffer進行解密。在主函數中，我們提供了一個簡單的測試，對加密后的buffer進行解密并輸出結果。