CryptAcquireContext using MS_ENHANCED_PROV

From:

ipramod@gmail.com

Newsgroups:

microsoft.public.vc.language

Date:

8 May 2007 05:14:24 -0700

Message-ID:

<1178626464.225652.312430@h2g2000hsg.googlegroups.com>

Hi,

I am facing one problem when decrypting the password using
MS_ENHANCED_PROV. I have the following code:

HRESULT DecryptString(BSTR bstrToDecrypt, LPCTSTR pszSecret, BSTR
*pbstrDecrypted)
{
    HCRYPTPROV hCryptProv;
    HCRYPTKEY hCryptKey;
    BYTE baKeyRandom[10] = {87,194, 253,4,98,23,192,102,176,37};
    HCRYPTHASH hSaveHash;
    USES_CONVERSION;
    HRESULT hr;

    hCryptProv = NULL;
    hCryptKey = NULL;
    hSaveHash = NULL;

    try {
        BOOL bSuccess;

        // Attempt to acquire a context and a key container. The context
will use the default CSP (cryptographic service provider)
        // for the RSA_FULL provider type.
        bSuccess = ::CryptAcquireContext(&hCryptProv, // handle to the CSP
                                        CRYPTCONTEXT, // use default key container(CRYPTCONTEXT)
                                        //MS_DEF_PROV,
                                        MS_ENHANCED_PROV, // use the Enhanced Cryptographic
Provider (128 bit)
                                        PROV_RSA_FULL, // provider type
                                        CRYPT_MACHINE_KEYSET);// flag values, No special
action(CRYPT_MACHINE_KEYSET)

        // Derive a symmetric key from a hash object by performing the
        // following steps:
        // 1. Call CryptCreateHash to retrieve a handle to a hash object.
        // 2. Call CryptHashData to add a text string (password) to the
        // hash object.
        // 3. Call CryptDeriveKey to create the symmetric key from the
        // hashed password derived in step 2.
        // You will use the key later to create an HMAC hash object.

        FAILED_LAST(::CryptCreateHash(hCryptProv, ALG_CLASS_HASH|
ALG_TYPE_ANY|ALG_SID_MD5, 0, 0, &hSaveHash));

        // Hash the random stream
        FAILED_LAST(::CryptHashData(hSaveHash, baKeyRandom,
sizeof(baKeyRandom), 0) );

        // Hash the data
        FAILED_LAST(::CryptHashData(hSaveHash, (LPBYTE)T2CW(pszSecret),
_tcslen(pszSecret) * sizeof(WCHAR), 0));

        // Create a session key based on the hash of the password
        FAILED_LAST(::CryptDeriveKey(hCryptProv, CALG_RC4, hSaveHash, 0,
&hCryptKey) );

        // Now Encrypt the value
        DWORD dwSize;
        LPBYTE pbTemp;

        dwSize = ::SysStringByteLen(bstrToDecrypt);
        if (dwSize)
        {
            pbTemp = new BYTE[dwSize+2];
            memset(pbTemp, 0, dwSize+2);
            memcpy((LPWSTR)pbTemp, bstrToDecrypt, dwSize);
            bSuccess = ::CryptDecrypt(hCryptKey, NULL, TRUE, 0, pbTemp,
&dwSize);
            if (!bSuccess)
            {
                delete [] pbTemp;
                FAILED_LAST(bSuccess);
            }

            *pbstrDecrypted = ::SysAllocStringByteLen((LPCSTR)pbTemp, dwSize);
            delete [] pbTemp;
        }
        else
        {
            *pbstrDecrypted = ::SysAllocString(NULL);
        }

        hr = S_OK;
    }

    catch (HRESULT hrCatch)
    {
        ATRACE(ATRACE_CATEGORY_ERROR, _T("CBasicCrypto::DecryptString Failed
to decrypt the string - %d.\n"), GetLastError());
        hr = hrCatch;
    }

    if (hSaveHash)
        ::CryptDestroyHash(hSaveHash);

    if (hCryptKey)
        ::CryptDestroyKey(hCryptKey);

    if (hCryptProv)
        ::CryptReleaseContext(hCryptProv, 0);

    return hr;
}

Now, when the CryptAcquireContext is called with MS_DEF_PROV (which is
commented in the above code), I get correct value of my password and
if it is called with MS_ENHANCED_PROV, then it returns some garbage
values "?????????". Do I need to change ALG_ID anywhere in the code
while creating the hashkey or deriving the key? Is this code
correct?/?

Please let me know.

Thanks in adv,
PI