CSecureChannelClient::MACUpdate

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The MACUpdate method adds a value to a message authentication code (MAC).

Syntax

HRESULT MACUpdate(HMAChMAC,
  BYTE*pbData,
  DWORDdwDataLen);

Parameters

hMAC

[in] Handle to the array specifying the MAC for the current parameter data. This handle is returned from the MACInit method. This datatype is declared in Sac.h installed with the Windows Media Format SDK.

pbData

[in] Pointer to the parameter data to add to the MAC value.

dwDataLen

DWORD specifying the length of the data to which pbData points.

Return Values

The method returns an HRESULT. All the interface methods in Windows Media Device Manager can return any of the following classes of error codes:

  • Standard COM error codes
  • Windows error codes converted to HRESULT values
  • Windows Media Device Manager error codes

For an extenstive list of possible error codes, see Error Codes.

Possible values include, but are not limited to, those in the following table.

Return code Description
S_OK The method succeeded.
E_INVALIDARG A parameter is invalid or is a NULL pointer.
E_FAIL An unspecified error occurred.

Remarks

An application calls MACUpdate repeatedly with each piece of data to add to the MAC. MACInit must always be called before MACUpdate, and MACFinal must always be called after MACUpdate. MACInit acquires the MAC handle, phMAC, to be used by the MACUpdate and MACFinal methods.

Example Code

The following example code checks the MAC received by a call to IWMDMStorage::GetRights to verify that the data has not been tampered with.

HRESULT hr;
CSecureChannelClient *pSPClient = NULL;
hr = m_pStorage->GetRights(ppRights, pnRightsCount, abTempMAC);
if (SUCCEEDED(hr))
{
    // Verify MAC returned by GetRights on the service provider.
    pSPClient->MACInit(&hMAC);
    pSPClient->MACUpdate(hMAC, (BYTE*)(*ppRights),
                       sizeof(WMDMRIGHTS) * (*pnRightsCount));
    pSPClient->MACUpdate(hMAC, (BYTE*)(pnRightsCount),
                        sizeof(*pnRightsCount));
    pSPClient->MACFinal(hMAC, abMACVerify);
    if (memcmp(abMACVerify, abTempMAC, WMDM_MAC_LENGTH) != 0)
    {
        hr = WMDM_E_MAC_CHECK_FAILED;
        goto exit;
    }
}

Requirements

Header: Include scclient.h.

Library: mssachlp.lib

See Also