openssl crypto memory leak

Question

I have a problem with openssl crypto library, after doing decryption even after exiting function scope, decrypted string is not removed from memory. My code:

int crypto::aes_decrypt(const crypto::vector<unsigned char> &data, const crypto::string &key, const crypto::string &iv,
                        crypto::vector<unsigned char> &decrypted) {
    try {
        int len;
        int plaintext_len;
        const auto *dataArr = data.data();
        size_t dataLength = data.size();

        if (key.length() != 32 || iv.length() != 16) {
            return -1;
        }

        auto *plaintext = new unsigned char[dataLength + 16];

        EVP_CIPHER_CTX *ctx = EVP_CIPHER_CTX_new();
        if (ctx == nullptr) {
            return -2;
        }

        const auto *keyArr = reinterpret_cast<const unsigned char *>(key.data());
        const auto *ivArr = reinterpret_cast<const unsigned char *>(iv.data());

        /*
         * Initialise the decryption operation. IMPORTANT - ensure you use a key
         * and IV size appropriate for your cipher
         * In this example we are using 256-bit AES (i.e. a 256-bit key). The
         * IV size for *most* modes is the same as the block size. For AES this
         * is 128 bits
         */
        if (1 != EVP_CipherInit_ex(ctx, EVP_aes_256_cbc(), nullptr, keyArr, ivArr, 0)) {
            EVP_CIPHER_CTX_free(ctx);
            return -3;
        }

        /*
        * Provide the message to be decrypted, and obtain the plaintext output.
        * EVP_DecryptUpdate can be called multiple times if necessary.
        */
        if (1 != EVP_DecryptUpdate(ctx, plaintext, &len, dataArr, dataLength)) {
            EVP_CIPHER_CTX_free(ctx);
            return -4;
        }

        plaintext_len = len;

        /*
         * Finalise the decryption. Further plaintext bytes may be written at
         * this stage.
         */

        if (1 != EVP_DecryptFinal_ex(ctx, plaintext + len, &len)) {
            EVP_CIPHER_CTX_free(ctx);
            return -5;
        }

        plaintext_len += len;
        decrypted = crypto::vector<unsigned char>(plaintext, plaintext + plaintext_len);

        /* Clean up */
        EVP_CIPHER_CTX_free(ctx);
        free(plaintext);
        return 0;

    } catch (exception &e) {
#ifdef DEBUG_SHARED_LIBRARIES
        cout << "Error while aes decrypting: " << e.what() << endl;
#endif
        return -99;
    }
}

void test() {
    string key = "abcdefghijklmnopabcdefghijklmnop";
    string iv = "1234567890123456";
    string b64 = "82fgiOvXUXrnkGeRsCjKgA==";
    
    vector<unsigned char> decrypted;
    int i = crypto::aes_decrypt(crypto::base64_decode(b64), key, iv, decrypted);
    if (i != 0) {
        cout << "Error: " << i << endl;
        return;
    }

    string dec = {decrypted.begin(), decrypted.end()};

//    cout << "Decrypted raw: " << string_utils::char_array_to_hex(decrypted.data(), decrypted.size()) << endl;
    cout << "Decrypted: " << dec << endl;

    cout << "Click to clear" << endl;
    string input;
    getline(cin, input);
}

I tried many combinations, but with string and vector still the same - decrypted string is still in memory

Before exiting function scope:

After exiting function scope:

and inside that memory dump:

and another:

Any suggestions how can i deal with it? I think its something wrong with aes decrypt implementation, because in memory dump there are many encryption methods related to openssl crypto library

Answer 1

I found what i was looking for here:

https://wiki.openssl.org/index.php/EVP_Symmetric_Encryption_and_Decryption

typedef std::basic_string<char, std::char_traits<char>, zallocator<char> > secure_string;

that secure_string is wiped out from memory when leaving the function scope.