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Comprobar firmas de documentos y certificados

Este tema describe cómo comprobar las firmas de un documento XPS y cómo verificar los certificados relacionados con dichas firmas.

Antes de utilizar los siguientes ejemplos de código en su programa, lea la cláusula de exención de responsabilidad en Tareas comunes de programación de firmas digitales.

El siguiente ejemplo de código comprueba las firmas digitales que se encuentran en un documento XPS.

Para comprobar las firmas de un documento XPS, realice los siguientes pasos:

  1. Cargue el documento en un administrador de firmas, como se describe en Inicializar el administrador de firmas.
  2. Obtenga la colección de firmas del administrador de firmas digitales.
  3. Obtenga el número de firmas de la colección.
  4. Para cada firma de la colección, llame al método Comprobar como se muestra en el siguiente ejemplo de código.
HRESULT
VerifyAllDigitalSignaturesAndAuthenticateCertificates(
    IXpsSignatureManager *signatureManager
)
{
    HRESULT                       hr                              = S_OK;
    IXpsSignature                 *signature                      = NULL;
    IXpsSignatureCollection       *signaturesInDocument           = NULL;
    UINT32                        numberOfSignaturesInDocument    = NULL;

    hr = signatureManager->GetSignatures(&signaturesInDocument);
    if (SUCCEEDED(hr)) {
        hr = signaturesInDocument->GetCount(&numberOfSignaturesInDocument);
    }

    if (SUCCEEDED(hr)) {
        // Check each signature in the XPS document that was opened in
        //  the signature manager.
        for (UINT32 index = 0; index < numberOfSignaturesInDocument; index++)
        {
            // Get the signature in the current index of the 
            //  IXpsSignatureCollection object
            hr = signaturesInDocument->GetAt(index, &signature);

            if (SUCCEEDED(hr)) {
                PCCERT_CONTEXT       signingCertificate = NULL;
                XPS_SIGNATURE_STATUS signatureStatus; 

                signatureStatus = XPS_SIGNATURE_STATUS_BROKEN;
                // Verify the signature and authenticate 
                //  its signing certificate
                hr = VerifySignatureAndCertificates (
                        signature,
                        &signingCertificate,
                        &signatureStatus);
                if (FAILED(hr)) {
                    // If a FACILITY_SECURITY error code is returned then 
                    //  the current certificate was not the 
                    //    signing certificate, so continue with 
                    //  the enumeration.
                    if (HRESULT_FACILITY(hr) != FACILITY_SECURITY)
                    {
                        // If the error was not a FACILITY_SECURITY error  
                        //  then exit and return the error
                        break; // out of for loop
                    }
                }
                // release pointers for next loop
                if (NULL != signature) {
                    signature->Release(); 
                    signature = NULL; 
                }
                if (NULL != signingCertificate) {
                    CertFreeCertificateContext (signingCertificate); 
                    signingCertificate = NULL;
                }
            }
        }
    }
    if (NULL != signaturesInDocument) signaturesInDocument->Release();
    
    return hr;
}

Para comprobar una firma digital, primero compruebe la firma creada por el certificado de firma, y luego, valide el certificado de firma. El método de validación utilizado en el siguiente ejemplo de código almacena en caché los certificados en un almacén de certificados temporal, que las funciones de Crypto API utilizan cuando se las llama más adelante en este ejemplo.

Para crear un almacén de certificados temporal, siga estos pasos:

  1. Cree un almacén de certificados temporal para guardar los certificados utilizados por la firma.
  2. Recorra el conjunto de certificados de la firma y cargue cada certificado en el almacén temporal de certificados.
HRESULT VerifySignatureAndCertificates (
    IXpsSignature           *signature,
    PCCERT_CONTEXT          *signingCertificate,
    XPS_SIGNATURE_STATUS    *signatureStatus
)
{
    HRESULT                         hr                        = S_OK;
    BOOL                            moreCertificates          = FALSE;
    IOpcCertificateEnumerator       *certificatesInSignature  = NULL;
    
    HCERTSTORE                      signatureCertificateStore = NULL;
    
    // Create a temporary certificate store.  
    signatureCertificateStore = CertOpenStore(
        CERT_STORE_PROV_MEMORY, 
        X509_ASN_ENCODING, 
        NULL, 
        0, 
        NULL);

    // Create a certificate enumerator to store the certificates 
    //  that are associated with the current signature.
    hr = signature->GetCertificateEnumerator(&certificatesInSignature);

    if (SUCCEEDED(hr))
    {
    // We need to call the MoveNext method to initialize the enumerator.
        hr = certificatesInSignature->MoveNext(&moreCertificates);
    }
    if (SUCCEEDED(hr))
    {
        // Iterate through the certificates in the signature, 
        //  and add each one to the temporary certificate store.  
        //  This temporary  certificate store simplifies 
        //  authentication of the signing certificate.
        while (moreCertificates)
        {
            PCCERT_CONTEXT certificate  = NULL;
            hr = certificatesInSignature->GetCurrent(&certificate);
            if (SUCCEEDED(hr))
            {
                // got the next certificate so
                // add the current certificate to the temporary certificate store.
                if (!CertAddCertificateContextToStore(signatureCertificateStore,
                    certificate,
                    CERT_STORE_ADD_REPLACE_EXISTING,
                    NULL))
                {
                    hr = E_FAIL;
                    // ERROR: could not add the certificate to the certificate store
                    break; // out of while loop
                }
                CertFreeCertificateContext (certificate);
            }
            else
            {
                // unable to get the certificate so skip
            }

            // move to next certificate in set
            if (FAILED(hr = certificatesInSignature->MoveNext(&moreCertificates)))
            {
                // ERROR: could not move to the next certificate in the enumerator
                break; // out of while loop
            }
            // moreCertificates == FALSE when the end of the set has been reached.
        }//End while
    }
    if (NULL != certificatesInSignature) certificatesInSignature->Release();

Para comprobar la firma digital y el certificado utilizado para firmar el documento, realice los siguientes pasos:

  1. Encuentre el certificado de firma iterando a través de los certificados que utiliza la firma.
  2. Pruebe el certificado comprobando la firma con el certificado. El certificado de firma se encuentra cuando el método Comprobar devuelve un XPS_SIGNATURE_STATUS de XPS_SIGNATURE_STATUS_VALID oXPS_SIGNATURE_STATUS_QUESTIONABLE, y no devuelve un error FACILITY_SECURITY.
    // Reset the enumerator
    hr = signature->GetCertificateEnumerator(&certificatesInSignature);
    if (SUCCEEDED (hr))
    {
        moreCertificates = FALSE;
        hr = certificatesInSignature->MoveNext(&moreCertificates);
    }
    if (SUCCEEDED(hr))
    {
        // Iterate through the certificates in the signature,
        //  and call the IXpsSignature.Verify() method
        //  on each certificate.  
        // A signature can include an entire certificate chain, and so 
        //  only one of the certificates found in this enumeration 
        //  is the certificate that was used to sign the package. 
        //  The signing certificate is the one to authenticate.  
        // To find the signing certificate,  iterate through 
        //  the certificates in the signature and select the certificate that 
        //  returns an XPS_SIGNATURE_STATUS of XPS_SIGNATURE_STATUS_VALID
        //  or XPS_SIGNATURE_STATUS_QUESTIONABLE and does not return a
        //  FACILITY_SECURITY error.
        XPS_SIGNATURE_STATUS localSignatureStatus;
        localSignatureStatus = XPS_SIGNATURE_STATUS_INCOMPLIANT;
        do
        {
            PCCERT_CONTEXT certificate = NULL;
            DWORD certificateStatus = NULL;

            if (FAILED(hr = certificatesInSignature->GetCurrent(&certificate)))
            {
                // We will skip corrupted certificates
                // free this one and move to the next
                CertFreeCertificateContext (certificate);
                hr = certificatesInSignature->MoveNext(&moreCertificates);
                if (FAILED(hr))
                {
                    // ERROR: could not move to the next 
                    //  certificate in the enumerator
                    break; // out of do loop with failed hr
                }
                // continue with next loop iteration
                continue;
            }
            
            // Verify that the signature conforms to the XPS signing policy.
            hr = signature->Verify(certificate, &localSignatureStatus);
            if (FAILED(hr))
            {
                // If a FACILITY_SECURITY error code is returned, then the
                //  current certificate was not the signing certificate,
                //  so continue to the next certificate.
                if (HRESULT_FACILITY(hr) == FACILITY_SECURITY)
                {
                    // free this one and move to the next
                    CertFreeCertificateContext (certificate);
                    hr = certificatesInSignature->MoveNext(&moreCertificates);
                    if (FAILED(hr))
                    {
                        // ERROR: could not move to the next certificate 
                        //  in the enumerator
                        break; // out of do loop with failed hr
                    }
                    continue;
                }
                // ERROR: An attempt to verify the signature has failed
                break; // out of do loop with failed hr
            }
            // if verification was successful, localSignatureStatus will
            //  contain the status of the signature.
            //
            // do loop continues in next code example

Una vez encontrado el certificado de firma, realice los siguientes pasos:

  1. Guardar el estado de la firma devuelta.
  2. Actualice el estado local, si es necesario, para realizar pruebas de certificado posteriores:
    1. Si el estado de la firma es correcto, establezca el estado local en cuestionable para probar los certificados.
    2. Si el estado de la firma es no conforme, deje el estado local como no conforme.
    3. Si el estado de la firma es roto o incompleto, establezca el estado local en roto.

Un estado de firma XPS_SIGNATURE_STATUS_INCOMPLIANT significa que se firmaron partes del documento XPS que no debían firmarse o que no se firmaron partes del documento XPS que debían firmarse. Si Comprobar devuelve este estado de firma, no será necesario seguir comprobando la firma.

            // continuing do loop from previous code example
            *signingCertificate = certificate;
            *signatureStatus = localSignatureStatus;
            
            // note that this test should only downgrade the 
            // signature status, it should not upgrade it.
            switch (localSignatureStatus) {
                case XPS_SIGNATURE_STATUS_VALID:
                case XPS_SIGNATURE_STATUS_QUESTIONABLE:
                    // the signature is valid or questionable so
                    // save the actual status and set the new status
                    // to questionable so the certificates will be checked.
                    localSignatureStatus = XPS_SIGNATURE_STATUS_QUESTIONABLE;
                    break;

                case XPS_SIGNATURE_STATUS_INCOMPLIANT:
                    // the signature is not compliant 
                    break;

                case XPS_SIGNATURE_STATUS_INCOMPLETE:
                case XPS_SIGNATURE_STATUS_BROKEN:
                    // The Windows 7 XPS viewer displays incomplete signatures
                    // and broken signatures as broken.
                    *signatureStatus = XPS_SIGNATURE_STATUS_BROKEN;
                    localSignatureStatus = XPS_SIGNATURE_STATUS_BROKEN;
                    break;

                default:
                    // there should be no other possible status values
                    break;
            }
            // do loop continues in next code example

Para comprobar la confianza del certificado si el estado de la firma era válido o cuestionable, realice los siguientes pasos:

  1. Obtenga el estado de confianza del certificado.
  2. Evalúe el estado de confianza del certificado devuelto.
  3. Devuelve el estado resultante.

El siguiente ejemplo de código no comprueba todos los posibles estados de confianza de los certificados. Para obtener más información sobre los valores de estado que se pueden devolver, consulte CERT_TRUST_STATUS.

            // continuing do loop from previous code example
            //
            // at this point, localSignatureStatus should be less than or 
            // equal to what it was before the test.

            // Check the certificate to see if it is valid
            if ((localSignatureStatus == XPS_SIGNATURE_STATUS_VALID) || 
                (localSignatureStatus == XPS_SIGNATURE_STATUS_QUESTIONABLE))
            {
                // This call builds the certificate trust chain from the 
                //  supplied certificate.  The certificate chain is used to
                //  authenticate the supplied certificate.
                hr = GetCertificateTrustStatus (
                    *signingCertificate, 
                    &signatureCertificateStore,
                    &certificateStatus);
                if (FAILED(hr))
                {
                    // ERROR: An attempt to authenticate the certificate 
                    //  has failed
                    break; // out of do loop with failed hr
                }

                // The Crypt API returns a status that can contain more than
                //  one status value.
                // statusFlagMask is set to test all bits except for the
                //  CERT_TRUST_REVOCATION_STATUS_UNKNOWN
                //  CERT_TRUST_IS_OFFLINE_REVOCATION
                //  CERT_TRUST_IS_NOT_TIME_VALID
                //  values because, for this test, these are not considered
                //  to be error conditions.
                DWORD statusFlagMask = ~(
                    CERT_TRUST_REVOCATION_STATUS_UNKNOWN | 
                    CERT_TRUST_IS_OFFLINE_REVOCATION | 
                    CERT_TRUST_IS_NOT_TIME_VALID);

                if (CERT_TRUST_NO_ERROR == (certificateStatus & statusFlagMask))
                {
                    // If *signatureStatus is already 
                    //    XPS_SIGNATURE_STATUS_VALID then there is no need to 
                    //    change the status as the certificate status has no 
                    //    certificate trust errors.
                    // If *signatureStatus is already 
                    //  XPS_SIGNATURE_STATUS_QUESTIONABLE then we will not
                    //  upgrade the trust status of the signature just 
                    //  because there is no trust issue with the certificate.
                }
                else
                {
                    // If trust errors were detected with the certificate, 
                    //  then this XPS signature is given a status of 
                    //  XPS_SIGNATURE_STATUS_QUESTIONABLE
                    *signatureStatus = XPS_SIGNATURE_STATUS_QUESTIONABLE;
                }

                // Handle additional certificate errors.  
                //  This is not an exhaustive list of possible errors.

                if (certificateStatus & CERT_TRUST_IS_NOT_TIME_VALID)
                {
                    // The XPS Viewer considers signatures with 
                    //  expired certificates as valid.
                }
                if (certificateStatus & CERT_TRUST_IS_PARTIAL_CHAIN)
                {
                    // This test ensures that we only degrade the 
                    //  trust status and never upgrade it
                    if (XPS_SIGNATURE_STATUS_VALID == *signatureStatus)
                    {
                        *signatureStatus = XPS_SIGNATURE_STATUS_QUESTIONABLE;
                    }
                }

                if (certificateStatus & CERT_TRUST_IS_NOT_SIGNATURE_VALID)
                {
                    // This test ensures that we only degrade the 
                    //  trust status and never upgrade it
                    if (XPS_SIGNATURE_STATUS_VALID == *signatureStatus)
                    {
                        *signatureStatus = XPS_SIGNATURE_STATUS_QUESTIONABLE;
                    }
                }

                if (certificateStatus & CERT_TRUST_IS_SELF_SIGNED)
                {
                    // This test ensures that we only degrade the 
                    //  trust status and never upgrade it
                    if (XPS_SIGNATURE_STATUS_VALID == *signatureStatus)
                    {
                        *signatureStatus = XPS_SIGNATURE_STATUS_QUESTIONABLE;
                    }
                }
                
                if (certificateStatus & CERT_TRUST_IS_UNTRUSTED_ROOT)
                {
                    // This test ensures that we only degrade the 
                    //  trust status and never upgrade it
                    if (XPS_SIGNATURE_STATUS_VALID == *signatureStatus)
                    {
                        *signatureStatus = XPS_SIGNATURE_STATUS_QUESTIONABLE;
                    }
                }
            }//End if

            hr = certificatesInSignature->MoveNext(&moreCertificates);
            if (FAILED(hr))
            {
                // ERROR: could not move to the next 
                //  certificate in the enumerator
                break; // out of do loop with failed hr
            }
        } while((*signatureStatus != XPS_SIGNATURE_STATUS_VALID) && 
                    moreCertificates);
    } // end if successful

    if (NULL != certificatesInSignature) certificatesInSignature->Release();

    return hr;
}

En el siguiente ejemplo de código, el estado de confianza del certificado se obtiene llamando al método que se muestra en el siguiente ejemplo de código.

HRESULT GetCertificateTrustStatus(
    __in PCCERT_CONTEXT certificate,
    __in HCERTSTORE* certificateStore,
    __out DWORD* certificateStatus
)
{
    HRESULT    hr = S_OK;

    // The certificate chain that will be created from 
    //  the PCCERT_CONTEXT object passed in.  
    PCCERT_CHAIN_CONTEXT    certificateChain =    NULL;

    hr = CreateCertificateChain(
        certificate, 
        *certificateStore, 
        &certificateChain);

    if (SUCCEEDED(hr)) { 
        *certificateStatus = 
            certificateChain->TrustStatus.dwErrorStatus;
    }

    return hr;
}

La cadena de certificados utilizada en el ejemplo de código anterior se crea llamando al método que se muestra en el siguiente ejemplo de código.

HRESULT 
CreateCertificateChain (
    __in PCCERT_CONTEXT            certificate,
    __in HCERTSTORE                certificateStore,
    __out PCCERT_CHAIN_CONTEXT* certificateChain
)
{
    HRESULT  hr = S_OK;

    CERT_CHAIN_PARA certificateChainParameters = {0};

    certificateChainParameters.cbSize = sizeof(CERT_CHAIN_PARA);
    certificateChainParameters.RequestedUsage.dwType = USAGE_MATCH_TYPE_AND;

    // CertGetCertificateChain builds a certificate chain that starts 
    //  from the PCCERT_CONTEXT structure provided by the caller.
    //  After the certificate chain has been successfully created, 
    //  then the authenticity of the certificate can be determined 
    //  by examining the errors, if any, that occurred while the chain
    //  was created.
    BOOL successCreatingCertChain = CertGetCertificateChain (
        NULL,
        certificate,
        NULL,
        certificateStore,
        &certificateChainParameters,
        CERT_CHAIN_REVOCATION_CHECK_CHAIN_EXCLUDE_ROOT,
        NULL,
        certificateChain);

    if (!successCreatingCertChain)
    {
        hr = HRESULT_FROM_WIN32(GetLastError());
    }
    return hr;
}

Usado en esta sección

CERT_CHAIN_CONTEXT

CERT_CONTEXT

CERT_TRUST_STATUS

CertAddCertificateContextToStore

CertOpenStore

CertGetCertificateChain

IOpcCertificateEnumerator

IOpcCertificateEnumerator::GetCurrent

IOpcCertificateEnumerator::MoveNext

IXpsSignature

IXpsSignature::GetCertificateEnumerator

IXpsSignature::Verify

IXpsSignatureCollection

IXpsSignatureCollection::GetAt

IXpsSignatureCollection::GetCount

IXpsSignatureManager

IXpsSignatureManager::GetSignatures

XPS_SIGNATURE_STATUS

Para obtener más información

Incrustar cadenas de certificados en un documento

Errores de la API de firma digital XPS

Errores en documentos XPS

XML Paper Specification