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Verify Document Signatures and Certificates

This topic describes how to verify the signatures in an XPS document and how to verify the certificates that are related to those signatures.

Before using the following code examples in your program, read the disclaimer in Common Digital Signature Programming Tasks.

The following code example checks the digital signatures that are found in an XPS document.

To check the signatures in an XPS document, perform the following steps:

  1. Load the document into a signature manager, as described in Initialize the Signature Manager.
  2. Get the collection of signatures from the digital signature manager.
  3. Get the number of signatures in the collection.
  4. For each signature in the collection, call the Verify method as shown in the code example that follows.
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;
}

To verify a digital signature, first verify the signature created by the signing certificate, then validate the signing certificate. The validation method used in the following code example caches the certificates in a temporary certificate store, which the Crypto API functions use when they are called later in this example.

To create a temporary certificate store, perform the following steps:

  1. Create a temporary certificate store to hold the certificates used by the signature.
  2. Iterate through the signature's certificate set, and load each certificate into the temporary certificate store.
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();

To verify the digital signature and the certificate used to sign the document, perform the following steps:

  1. Find the signing certificate by iterating through the certificates that are used by the signature.
  2. Test the certificate by verifying the signature against the certificate. The signing certificate is found when the Verify method returns an XPS_SIGNATURE_STATUS of XPS_SIGNATURE_STATUS_VALID or XPS_SIGNATURE_STATUS_QUESTIONABLE, and does not return a FACILITY_SECURITY error.
    // 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

When the signing certificate has been found, perform the following steps:

  1. Save the returned signature status.
  2. Update the local status, if necessary, to perform subsequent certificate tests:
    1. If the signature status is successful, set the local status to questionable in order to test the certificates.
    2. If the signature status is incompliant, leave the local status as incompliant.
    3. If the signature status is broken or incomplete, set the local status to broken.

A signature status of XPS_SIGNATURE_STATUS_INCOMPLIANT means that parts of the XPS document that should not have been signed were signed, or parts of the XPS document that should have been signed were not. If Verify returns this signature status, further checking of the signature will be unnecessary.

            // 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

To verify the certificate trust if the signature status was valid or questionable, perform the following steps:

  1. Get the certificate trust status.
  2. Evaluate the returned certificate trust status.
  3. Return the resulting status.

The next code example does not test for every possible certificate trust status. For additional details on the status values that can be returned, see 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;
}

In the next code example, the certificate trust status is obtained by calling the method shown in the code example that follows.

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;
}

The certificate chain used in the preceding code example is created by calling the method shown in the following code example.

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;
}

Used in This Section

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

For More Information

Embed Certificate Chains in a Document

XPS Digital Signature API Errors

XPS Document Errors

XML Paper Specification