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Codec Merit

  • Artikel

Starting in Windows 7, a Media Foundation codec can be assigned a merit value. When codecs are enumerated, codecs with higher merit are preferred over codecs with lower merit. Codecs with any merit value are preferred over codecs without an assigned merit. For details about codec enumeration, see MFTEnumEx.

Merit values are assigned by Microsoft. Currently, only hardware codecs are eligible to receive a merit value. The codec vendor is also issued a digital certificate, which is used to verify the codec's merit value. To obtain a certificate, send an email request to wmla@microsoft.com. The process for obtaining a certificate includes signing a license and providing a set of information files to Microsoft.

Codec merit works as follows:

  1. The codec vendor implements one of the following:
    • An AVStream mini-driver. Media Foundation provides an standard proxy MFT for AVStream drivers. This is the recommended option.
    • A Media Foundation transform (MFT) that acts as a proxy to the hardware. For more information, see Hardware MFTs.
  2. The codec's merit value is stored in the registry for quick lookup.
  3. The MFTEnumEx function sorts codecs in order of merit. Codecs with merit values appear in the list behind locally registered codecs (see MFTRegisterLocal), but ahead of other codecs.
  4. When the MFT is created, the codec's merit is verified using the Output Protection Manager (OPM) API.
  5. For a proxy MFT, the codec implements the IOPMVideoOutput interface. For an AVStream driver, the codec implements the KSPROPSETID_OPMVideoOutput property set.

The following diagram shows how merit is verified in both cases:

diagram showing two processes: one leads through media foundation proxy mft and avstream driver, the other through custom proxy mft

Custom Proxy MFT

If you provide a proxy MFT for the hardware codec, implement the codec merit value as follows:

  1. Implement the IOPMVideoOutput interface in the MFT. Example code is shown in the next section of this topic.

  2. Add the MFT_CODEC_MERIT_Attribute attribute to the registry, as follows:

    1. Call MFCreateAttributes to create a new attribute store.
    2. Call IMFAttributes::SetUINT32 to set the MFT_CODEC_MERIT_Attribute attribute. The value of the attribute is the codec's assigned merit.
    3. Call MFTRegister to register the MFT. Pass the attribute store in the pAttributes parameter.

  3. The application calls MFTEnumEx. This function returns an array of IMFActivate pointers, one for each codec that matches the enumeration criteria.

  4. The application calls IMFActivate::ActivateObject to create the MFT.

  5. The ActivateObject method calls the MFGetMFTMerit function to verify the certificate and the merit value.

  6. The MFGetMFTMerit function calls IOPMVideoOutput::GetInformation and sends an OPM_GET_CODEC_INFO status request. This status request returns the codec's assigned merit value. If this value does not match the registry value, ActivateObject may fail.

The following code shows how to add the merit value to the registry when you register the MFT:

// Shows how to register an MFT with a merit value.

HRESULT RegisterMFTWithMerit()
{
    // The following media types would apply to an H.264 decoder, 
    // and are used here as an example.

    MFT_REGISTER_TYPE_INFO aDecoderInputTypes[] = 
    {
        { MFMediaType_Video, MFVideoFormat_H264 },
    };

    MFT_REGISTER_TYPE_INFO aDecoderOutputTypes[] = 
    {
        { MFMediaType_Video, MFVideoFormat_RGB32 }
    };
    
    // Create an attribute store to hold the merit attribute.
    HRESULT hr = S_OK;
    IMFAttributes *pAttributes = NULL;

    hr = MFCreateAttributes(&pAttributes, 1);

    if (SUCCEEDED(hr))
    {
        hr = pAttributes->SetUINT32(
            MFT_CODEC_MERIT_Attribute, 
            DECODER_MERIT   // Use the codec's assigned merit value.
            );
    }

    // Register the decoder for MFTEnum(Ex).
    if (SUCCEEDED(hr))
    {
        hr = MFTRegister(
            CLSID_MPEG1SampleDecoder,                   // CLSID
            MFT_CATEGORY_VIDEO_DECODER,                 // Category
            const_cast<LPWSTR>(SZ_DECODER_NAME),        // Friendly name
            0,                                          // Flags
            ARRAYSIZE(aDecoderInputTypes),              // Number of input types
            aDecoderInputTypes,                         // Input types
            ARRAYSIZE(aDecoderOutputTypes),             // Number of output types
            aDecoderOutputTypes,                        // Output types
            pAttributes                                 // Attributes 
            );
    }

    SafeRelease(&pAttributes);
    return hr;
}

Implementing IOPMVideoOutput for Codec Merit

The following code shows how to implement IOPMVideoOutput for codec merit. For a more general discussion of the OPM API, see Output Protection Manager.

Note

The code shown here has no obfuscation or other security mechanisms. It is meant to show the basic implementation of the OPM handshake and status request.

 

This example assumes that g_TestCert is a byte array that contains the codec's certificate chain, and g_PrivateKey is a byte array that contains the private key from the certificate:

// Byte array that contains the codec's certificate.

const BYTE g_TestCert[] =
{
    // ... (certificate not shown)

// Byte array that contains the private key from the certificate.

const BYTE g_PrivateKey[] = 
{
    // .... (key not shown)

In the IOPMVideoOutput::StartInitialization method, generate a random number for the handshake. Return this number and the certificate to the caller:

STDMETHODIMP CodecMerit::StartInitialization(
    OPM_RANDOM_NUMBER *prnRandomNumber,
    BYTE **ppbCertificate,
    ULONG *pulCertificateLength
    )
{
    HRESULT hr = S_OK;

    DWORD cbCertificate = sizeof(g_TestCert);
    const BYTE *pCertificate = g_TestCert;

    // Generate the random number for the OPM handshake.
    hr = BCryptGenRandom(
        NULL,  
        (PUCHAR)&m_RandomNumber, 
        sizeof(m_RandomNumber),
        BCRYPT_USE_SYSTEM_PREFERRED_RNG
        );

    // Allocate the buffer to copy the certificate.
    if (SUCCEEDED(hr))
    {
        *ppbCertificate = (PBYTE)CoTaskMemAlloc(cbCertificate);

        if (*ppbCertificate == NULL) 
        {
            hr = E_OUTOFMEMORY;
        }
    }

    // Copy the certificate and the random number.
    if (SUCCEEDED(hr))
    {
        *pulCertificateLength = cbCertificate;
        CopyMemory(*ppbCertificate, pCertificate, cbCertificate);
        *prnRandomNumber = m_RandomNumber;
    }
    return hr;
}

The IOPMVideoOutput::FinishInitialization method completes the OPM handshake:

STDMETHODIMP CodecMerit::FinishInitialization(
    const OPM_ENCRYPTED_INITIALIZATION_PARAMETERS *pParameters
    )
{
    HRESULT hr = S_OK;
    BCRYPT_ALG_HANDLE hAlg = NULL;
    BCRYPT_KEY_HANDLE hKey = NULL;
    BCRYPT_OAEP_PADDING_INFO paddingInfo = {0};
    DWORD DecryptedLength = 0;
    PBYTE pbDecryptedParams = NULL;

    // The caller should pass the following structure in
    // pParameters:

    typedef struct {
        GUID  guidCOPPRandom;   // Our random number.
        GUID  guidKDI;          // AES signing key.
        DWORD StatusSeqStart;   // Status sequence number.
        DWORD CommandSeqStart;  // Command sequence number.
    } InitParams;

    paddingInfo.pszAlgId = BCRYPT_SHA512_ALGORITHM;

    //  Decrypt the input using the decoder's private key.

    hr = BCryptOpenAlgorithmProvider(
        &hAlg,
        BCRYPT_RSA_ALGORITHM,
        MS_PRIMITIVE_PROVIDER,
        0
        );

    //  Import the private key.
    if (SUCCEEDED(hr))
    {
        hr = BCryptImportKeyPair(
            hAlg,
            NULL,
            BCRYPT_RSAPRIVATE_BLOB,
            &hKey,
            (PUCHAR)g_PrivateKey, //pbData,
            sizeof(g_PrivateKey), //cbData,
            0
            );
    }

    //  Decrypt the input data.

    if (SUCCEEDED(hr))
    {
        hr = BCryptDecrypt(
            hKey,
            (PBYTE)pParameters,
            OPM_ENCRYPTED_INITIALIZATION_PARAMETERS_SIZE,
            &paddingInfo,
            NULL,
            0,
            NULL,
            0,
            &DecryptedLength,
            BCRYPT_PAD_OAEP
            );
    }

    if (SUCCEEDED(hr))
    {
        pbDecryptedParams = new (std::nothrow) BYTE[DecryptedLength];

        if (pbDecryptedParams == NULL) 
        {
            hr = E_OUTOFMEMORY;
        }
    }

    if (SUCCEEDED(hr))
    {
         hr = BCryptDecrypt(
             hKey,
             (PBYTE)pParameters,
             OPM_ENCRYPTED_INITIALIZATION_PARAMETERS_SIZE,
             &paddingInfo,
             NULL,
             0,
             pbDecryptedParams,
             DecryptedLength,
             &DecryptedLength,
             BCRYPT_PAD_OAEP
             );
    }

    if (SUCCEEDED(hr))
    {
        InitParams *Params = (InitParams *)pbDecryptedParams;
        
        //  Check the random number.
        if (0 != memcmp(&m_RandomNumber, &Params->guidCOPPRandom, sizeof(m_RandomNumber)))
        {
            hr = E_ACCESSDENIED;
        } 
        else 
        {
            //  Save the key and the sequence numbers.

            CopyMemory(m_AESKey.abRandomNumber, &Params->guidKDI, sizeof(m_AESKey));
            m_StatusSequenceNumber = Params->StatusSeqStart;
            m_CommandSequenceNumber = Params->CommandSeqStart;
        }
    }

    //  Clean up.

    if (hKey)
    {
        BCryptDestroyKey(hKey);
    }
    if (hAlg)
    {
        BCryptCloseAlgorithmProvider(hAlg, 0);
    }
    delete [] pbDecryptedParams;

    return hr;
}

In the IOPMVideoOutput::GetInformation method, implement the OPM_GET_CODEC_INFO status request. The input data is an OPM_GET_CODEC_INFO_PARAMETERS structure that contains the CLSID of your MFT. The output data is an OPM_GET_CODEC_INFO_INFORMATION structure that contains the codec merit.

STDMETHODIMP CodecMerit::GetInformation( 
    const OPM_GET_INFO_PARAMETERS *Parameters,
    OPM_REQUESTED_INFORMATION *pRequest
    )
{

    HRESULT hr = S_OK;
    OPM_GET_CODEC_INFO_PARAMETERS *CodecInfoParameters;

    //  Check the MAC.
    OPM_OMAC Tag = { 0 };

    hr = ComputeOMAC(
        m_AESKey, 
        (PBYTE)Parameters + OPM_OMAC_SIZE, 
        sizeof(OPM_GET_INFO_PARAMETERS) - OPM_OMAC_SIZE, 
        &Tag
        );

    if (SUCCEEDED(hr))
    {
        if (0 != memcmp(Tag.abOMAC, &Parameters->omac, OPM_OMAC_SIZE))
        {
            hr = E_ACCESSDENIED;
        }
    }

    // Validate the status sequence number. This must be consecutive
    // from the previous sequence number.

    if (SUCCEEDED(hr))
    {
        if (Parameters->ulSequenceNumber != m_StatusSequenceNumber++)
        {
            hr = E_ACCESSDENIED;
        }
    }

    //  Check the status request.

    if (SUCCEEDED(hr))
    {
        if (Parameters->guidInformation != OPM_GET_CODEC_INFO) 
        {
            hr = E_NOTIMPL;
        }
    }

    //  Check parameters.

    if (SUCCEEDED(hr))
    {
        CodecInfoParameters = (OPM_GET_CODEC_INFO_PARAMETERS *)Parameters->abParameters;

        if (Parameters->cbParametersSize > OPM_GET_INFORMATION_PARAMETERS_SIZE ||
            Parameters->cbParametersSize < sizeof(ULONG) ||
            Parameters->cbParametersSize - sizeof(ULONG) != CodecInfoParameters->cbVerifier
            ) 
        {
            hr = E_INVALIDARG;
        }
    }

    //  The input data should consist of the CLSID of the decoder.

    if (SUCCEEDED(hr))
    {
        CodecInfoParameters = (OPM_GET_CODEC_INFO_PARAMETERS *)Parameters->abParameters;
    
        if (CodecInfoParameters->cbVerifier != sizeof(CLSID) ||
            0 != memcmp(&CLSID_MPEG1SampleDecoder,
                        CodecInfoParameters->Verifier,
                        CodecInfoParameters->cbVerifier)) 
        {
            hr = E_ACCESSDENIED;
        }
    }

    if (SUCCEEDED(hr))
    {
        // Now return the decoder merit to the caller.

        ZeroMemory(pRequest, sizeof(OPM_REQUESTED_INFORMATION));

        OPM_GET_CODEC_INFO_INFORMATION *pInfo = 
            (OPM_GET_CODEC_INFO_INFORMATION *)pRequest->abRequestedInformation;

        pInfo->Merit = DECODER_MERIT;
        pInfo->rnRandomNumber = Parameters->rnRandomNumber;

        pRequest->cbRequestedInformationSize = sizeof(OPM_GET_CODEC_INFO_INFORMATION);

        //  Sign it with the key.

        hr = ComputeOMAC(
            m_AESKey, 
            (PBYTE)pRequest + OPM_OMAC_SIZE, 
            sizeof(OPM_REQUESTED_INFORMATION) - OPM_OMAC_SIZE, 
            &pRequest->omac
            );
    }

    return hr;
}

The GetInformation method must compute a Message Authentication Code (MAC) using the OMAC-1 algorithm; see Computing the OMAC-1 Value.

It is not necessary to support any other OPM status requests.

The IOPMVideoOutput::COPPCompatibleGetInformation and IOPMVideoOutput::Configure methods are not required for codec merit, so these methods can return E_NOTIMPL.

STDMETHODIMP CodecMerit::COPPCompatibleGetInformation( 
    const OPM_COPP_COMPATIBLE_GET_INFO_PARAMETERS *pParameters,
    OPM_REQUESTED_INFORMATION *pRequestedInformation)
{
    return E_NOTIMPL;
}

STDMETHODIMP CodecMerit::Configure( 
    const OPM_CONFIGURE_PARAMETERS *pParameters,
    ULONG ulAdditionalParametersSize,
    const BYTE *pbAdditionalParameters)
{
    return E_NOTIMPL;
}

Media Foundation Transforms

Writing a Custom MFT