Use AES CBC with NCrypt in windows

Question

I'm trying to use the NCrypt.dll to encrypt some data, in C++, and I'm having trouble handling keys and algorithms.

I would like to use AES with the CBC chainging method but, can't get the NCryptEncrypt function to work (I keep getting an invalid buffer size thrown).

I have created and stored a key (in the key storage provider) using the NCRYPT_AES_ALGORITHM flag but, have no idea how to set the algorithm to use the CBC method.

    secSt = NCryptCreatePersistedKey(phProvider, &keyHndl, NCRYPT_AES_ALGORITHM, keyname, 0, 0);

I've tried a few property settings and had no success so, I would like to know if this is even possible with NCrypt?

I know the Bcrypt encrypt function allows this and tried to convert my NCRYPT_KEY_HANDLE to a BCRYPT_KEY_HANDLE without success (so I reckon this is not possible).

Answer 1

You can apply the chaining mode CBC by using NCryptSetProperty and the BCrypt constant BCRYPT_CHAIN_MODE_CBC.

Note that NCryptEncrypt does not seem to support padding for symmetric keys (see description of parameter dwFlags in NCryptEncrypt). So I had to apply some poor man's padding of the clear text to get a multiple of 16 bytes. Without the padding, I also get the status code 0xc0000206 (STATUS_INVALID_BUFFER_SIZE).

// Clear text for testing
static const char* clearText = "The quick brown fox jumps over the lazy dog. 1234567890.        ";
static const int clearTextLen = 64;

int main()
{
    LPCWSTR keyName = L"NCryptTest";
    SECURITY_STATUS status;
    NCRYPT_PROV_HANDLE hProvider;
    NCRYPT_KEY_HANDLE hKey;

    // Open storage provider
    status = NCryptOpenStorageProvider(&hProvider, NULL, 0);

    // Get stored key
    status = NCryptOpenKey(hProvider, &hKey, keyName, 0, 0);
    if (status == NTE_BAD_KEYSET)
    {
        // Create key if it doesn't exist
        status = NCryptCreatePersistedKey(hProvider, &hKey, BCRYPT_AES_ALGORITHM, keyName, 0, 0);
        status = NCryptFinalizeKey(hKey, 0);
    }

    // Set the chaining mode CBC
    LPCWSTR chainMode = BCRYPT_CHAIN_MODE_CBC;
    status = NCryptSetProperty(hKey, NCRYPT_CHAINING_MODE_PROPERTY, (PBYTE)chainMode, wcslen(chainMode) * 2 + 2, 0);

    // Encrypt the text
    DWORD outlen = -1;
    unsigned char* cipherData = new unsigned char[clearTextLen];
    status = NCryptEncrypt(hKey, (PBYTE)clearText, clearTextLen, NULL, cipherData, clearTextLen, &outlen, 0);

    // Cleanup
    delete[] cipherData;
    NCryptFreeObject(hKey);
    NCryptFreeObject(hProvider);

    return 0;
}