Decrypt problem using Openssl aes-256-cbc

[lwmo...@gmail.com]

hi,all:

I'm doing Encrypt and Decrypt work with aes-256-cbc + Base64Encode; But with openssl library, I have encountered the following problem:

1\ Encrypt works:  string "1000" is encrypted to "Vz3XJBIVoIM0UcTuiA49JQ=="

2\ Decrypt seems bad: 

    The plaintext length decrypted by  "Vz3XJBIVoIM0UcTuiA49JQ=="  is always 16,  but the real length is 4. 

The following is my code of Decryption:

std::string SimpleAES::Decrypt(const std::string &cipherstr_b64)

{

    unsigned char *ciphertext = new unsigned char[CalcDecodeLength(cipherstr_b64.c_str())]();

    unsigned char *cipherstr_b64_data = new unsigned char[cipherstr_b64.size() + 1]();

    std::copy(cipherstr_b64.begin(), cipherstr_b64.end(), cipherstr_b64_data);

    cipherstr_b64_data[cipherstr_b64.size()] = '\0';

    //int ciphertext_len = Comm::DecodeBase64(cipherstr_b64_data, ciphertext, cipherstr_b64.size());

    //int ciphertext_len = Base64Decode((const char *)cipherstr_b64_data, cipherstr_b64.size(), ciphertext);

    //int ciphertext_len = Base64Decode((const char *)ciphertext, cipherstr_b64.size(), cipherstr_b64_data);

    // 原始串通过Base64 decode 到ciphertext 中

    int ciphertext_len = Base64Decode((const char *)cipherstr_b64_data, cipherstr_b64.size(), (char *)ciphertext);

    int len = 0, plaintext_len = 0;

    unsigned char *plaintext = new unsigned char[cipherstr_b64.size()]();

    //unsigned char plaintext[cipherstr_b64.size() + AES_BLOCK_SIZE]{};

    //unsigned char plaintext[ciphertext_len + AES_BLOCK_SIZE]{};

    EVP_DecryptInit_ex(&decrypt_ctx, EVP_aes_256_cbc(), NULL, key, iv);

    EVP_DecryptUpdate(&decrypt_ctx, plaintext, &len, ciphertext, ciphertext_len);

    plaintext_len = len;

    cout << "plaintext_len1:" << plaintext_len << endl;

    EVP_DecryptFinal_ex(&decrypt_ctx, plaintext + len, &len);

    plaintext_len += len;

    cout << "plaintext_len2:" << plaintext_len << endl;

    std::string plaintext_str(reinterpret_cast<const char*>(plaintext), plaintext_len);

    //std::string plaintext_str((char *)plaintext);

    delete [] ciphertext;

    delete [] cipherstr_b64_data;

    delete [] plaintext;

    return plaintext_str;

}

And the result is :

Any clue??

Best wishes