Ensure LTV verification using DSS (CMS Container)

Question

I am working on a solution that allows signing using p12 certificate on a remote server.

First, I have the digest of the document which is calculated on a server and then I send it for signature on another server.

Here are the PDF files, you will find two PDF versions. The "CURRENT_SIGNATURE.pdf" file is the result I got using the code below. And the "TARGER_SIGNATUREPDF.pdf" is the target I want to have. As you can see the difference the TARGET file says "certificate revocation list embedded in signature." on the other hand the CURRENT indicates that "the list of revocation of the certificates incorporated in the document." In addition, the TARGET file there is only one signature and no revision added : https://www.grosfichiers.com/i4fmqCz43is

Result vérification :

My goal now is to add a LTV verification, knowing that I am signing on the server part using: PadesCMSSignedDataBuilder

********************* ON SERVER A **********************

    public class ServerA {
    private static PAdESSignatureParameters signatureParameters;
    private static DSSDocument documentToSign;
    public static ExternalCMSPAdESService service;
    private static final String TSA_URL = "http://dss.nowina.lu/pki-factory/tsa/good-tsa";


    public static void main(String[] args) throws Exception {
        documentToSign = new FileDocument(new File("Doc 2.pdf"));

        signatureParameters = new PAdESSignatureParameters();
        signatureParameters.setSignatureLevel(SignatureLevel.PAdES_BASELINE_B);
        signatureParameters.setLocation("Luxembourg");
        signatureParameters.setReason("DSS testing");
        signatureParameters.setContactInfo("Jira");
        signatureParameters.setGenerateTBSWithoutCertificate(true);
        CommonCertificateVerifier commonCertificateVerifier = new CommonCertificateVerifier();

        commonCertificateVerifier.setCrlSource(new OnlineCRLSource());
        commonCertificateVerifier.setOcspSource(new OnlineOCSPSource());
        commonCertificateVerifier.setCheckRevocationForUntrustedChains(true);
        service = new ExternalCMSPAdESService(commonCertificateVerifier);
        byte[] documentDigest = computeDocumentDigest(documentToSign, signatureParameters);

        // Embedded CAdES is generated by a third party
        byte[] cmsSignedData = ServerB.getSignedCMSignedData(documentDigest);

        service.setCmsSignedData(cmsSignedData);
        DSSDocument finalDoc = service.signDocument(documentToSign, signatureParameters, null);


        PAdESService service = new PAdESService(commonCertificateVerifier);
        TimestampDataLoader timestampDataLoader = new TimestampDataLoader();// uses the specific content-type
        OnlineTSPSource tsa1 = new OnlineTSPSource("http://dss.nowina.lu/pki-factory/tsa/ee-good-tsa");
        tsa1.setDataLoader(timestampDataLoader);
        service.setTspSource(tsa1);
        PAdESSignatureParameters extensionParameters = new PAdESSignatureParameters();
        extensionParameters.setSignatureLevel(SignatureLevel.PAdES_BASELINE_LT);

        DSSDocument extendedDocument = service.extendDocument(finalDoc, extensionParameters);


        save(finalDoc);
        save2(extendedDocument);
    }

    private static void save(DSSDocument signedDocument) {
        try (FileOutputStream fos = new FileOutputStream("DSS.pdf")) {
            Utils.copy(signedDocument.openStream(), fos);
        } catch (Exception e) {
            Alert alert = new Alert(Alert.AlertType.ERROR, "Unable to save file : " + e.getMessage(), ButtonType.CLOSE);
            alert.showAndWait();
            return;
        }
    }
    private static void save2(DSSDocument signedDocument) {
        try (FileOutputStream fos = new FileOutputStream("DSS-2.pdf")) {
            Utils.copy(signedDocument.openStream(), fos);
        } catch (Exception e) {
            Alert alert = new Alert(Alert.AlertType.ERROR, "Unable to save file : " + e.getMessage(), ButtonType.CLOSE);
            alert.showAndWait();
            return;
        }
    }

    public static CertificateVerifier getOfflineCertificateVerifier() {
        CertificateVerifier cv = new CommonCertificateVerifier();
        cv.setDataLoader(new IgnoreDataLoader());
        return cv;
    }

    protected static byte[] computeDocumentDigest(final DSSDocument toSignDocument, final PAdESSignatureParameters parameters) {
        IPdfObjFactory pdfObjFactory = new ServiceLoaderPdfObjFactory();
        final PDFSignatureService pdfSignatureService = pdfObjFactory.newPAdESSignatureService();
        return pdfSignatureService.digest(toSignDocument, parameters);
    }

    private static class ExternalCMSPAdESService extends PAdESService {

        private static final long serialVersionUID = -2003453716888412577L;

        private byte[] cmsSignedData;

        public ExternalCMSPAdESService(CertificateVerifier certificateVerifier) {
            super(certificateVerifier);
        }

        @Override
        protected byte[] generateCMSSignedData(final DSSDocument toSignDocument, final PAdESSignatureParameters parameters,
                                               final SignatureValue signatureValue) {
            if (this.cmsSignedData == null) {
                throw new NullPointerException("A CMS signed data must be provided");
            }
            return this.cmsSignedData;
        }

        public void setCmsSignedData(final byte[] cmsSignedData) {
            this.cmsSignedData = cmsSignedData;
        }

    }
}

And to be able to sign the calculated hash :

********************* ON SERVER B **********************

public class ServerB {

private static PAdESSignatureParameters signatureParameters;
private static DSSDocument documentToSign;
public static ExternalCMSPAdESService service;

/**
 * Computes a CAdES with specific things for PAdES
 */
public static byte[] getSignedCMSignedData(byte[] documentDigest) throws Exception {
    signatureParameters = new PAdESSignatureParameters();
    signatureParameters.setSigningCertificate(getSigningCert());
    signatureParameters.setCertificateChain(getCertificateChain());
    signatureParameters.setSignatureLevel(SignatureLevel.PAdES_BASELINE_B);
    signatureParameters.setLocation("Luxembourg");
    signatureParameters.setReason("DSS testing");
    signatureParameters.setContactInfo("Jira");

    CMSProcessableByteArray content = new CMSProcessableByteArray(documentDigest);

    PadesCMSSignedDataBuilder padesCMSSignedDataBuilder = new PadesCMSSignedDataBuilder(getOfflineCertificateVerifier());
    SignatureAlgorithm signatureAlgorithm = signatureParameters.getSignatureAlgorithm();

    CustomContentSigner customContentSigner = new CustomContentSigner(signatureAlgorithm.getJCEId());
    SignerInfoGeneratorBuilder signerInfoGeneratorBuilder = padesCMSSignedDataBuilder.getSignerInfoGeneratorBuilder(signatureParameters, documentDigest);

    CMSSignedDataGenerator generator = padesCMSSignedDataBuilder.createCMSSignedDataGenerator(signatureParameters, customContentSigner,
            signerInfoGeneratorBuilder, null);

    CMSUtils.generateDetachedCMSSignedData(generator, content);

    SignatureTokenConnection signingToken = new Pkcs12SignatureToken("certificate.p12",
            new KeyStore.PasswordProtection("123456".toCharArray()));
    DSSPrivateKeyEntry privateKey = getKey("certificate.p12","123456");

    SignatureValue signatureValue = signingToken.sign(new ToBeSigned(customContentSigner.getOutputStream().toByteArray()),
            signatureParameters.getDigestAlgorithm(), privateKey);

    customContentSigner = new CustomContentSigner(signatureAlgorithm.getJCEId(), signatureValue.getValue());
    generator = padesCMSSignedDataBuilder.createCMSSignedDataGenerator(signatureParameters, customContentSigner, signerInfoGeneratorBuilder, null);

    CMSSignedData cmsSignedData = CMSUtils.generateDetachedCMSSignedData(generator, content);
    return DSSASN1Utils.getDEREncoded(cmsSignedData);
}

public static CertificateVerifier getOfflineCertificateVerifier() {
    CertificateVerifier cv = new CommonCertificateVerifier();
    cv.setDataLoader(new IgnoreDataLoader());
    return cv;
}

public static List<CertificateToken> getCertificateChain() throws Exception {
    List<CertificateToken> list = new ArrayList<>();
    CertificateToken[] l = getKey("certificate.p12","123456").getCertificateChain();
    for (int i = 0; i < l.length; i++) {
        list.add(l[i]);
    }
    return list;
}

public static CertificateToken getSigningCert() throws Exception {
    return getKey("certificate.p12","123456").getCertificate();
}

public static DSSPrivateKeyEntry getKey(String certificate, String pin) throws Exception {
    try (Pkcs12SignatureToken signatureToken = new Pkcs12SignatureToken("certificate.p12",
            new KeyStore.PasswordProtection("123456".toCharArray()))) {
        List<DSSPrivateKeyEntry> keys = signatureToken.getKeys();
        KSPrivateKeyEntry dssPrivateKeyEntry = (KSPrivateKeyEntry) keys.get(0);
        DSSPrivateKeyEntry entry = signatureToken.getKey(dssPrivateKeyEntry.getAlias(),
                new KeyStore.PasswordProtection("123456".toCharArray()));
        return entry;
    }
}
private static class ExternalCMSPAdESService extends PAdESService {

    private static final long serialVersionUID = -2003453716888412577L;

    private byte[] cmsSignedData;

    public ExternalCMSPAdESService(CertificateVerifier certificateVerifier) {
        super(certificateVerifier);
    }

    @Override
    protected byte[] generateCMSSignedData(final DSSDocument toSignDocument, final PAdESSignatureParameters parameters,
                                           final SignatureValue signatureValue) {
        if (this.cmsSignedData == null) {
            throw new NullPointerException("A CMS signed data must be provided");
        }
        return this.cmsSignedData;
    }

    public void setCmsSignedData(final byte[] cmsSignedData) {
        this.cmsSignedData = cmsSignedData;
    }

}
}

Answer 1

In comments to an answer to another question a discussion started in which you pointed to this question and asked for help. In that discussion it became apparent that you do not exactly know yet what you are trying to achieve. Thus, let's clarify that a bit.

LTV Verification

You say you want to add LTV verification to your signatures. Let's first take a look at what that means.

LTV is short for Long Term Validation. The goal it stands for is ensuring that a signature can still be verified in a number of years.

The challenges this goal tries to overcome are that information a verifier needs will not be available online in the long run and that algorithms used will eventually not be considered secure anymore.

The means are to retrieve the required information once and bundle them with the signature in a trustable way, and to apply digital time stamps to attest that a certain set of data, signature, and extra information existed and was bundled at a given time (e.g. when the used signing algorithm still was considered strong).

So far, so good.

Adobe early on (before PDF had become an ISO standard) defined one mechanism to achieve LTV: They specified a specific signed attribute into which one should collect the data required for validation right before signing and they recommended applying a time stamp to the embedded signature container.

Since then, though, this mechanism has turned out to be too simple and static. Depending on the validation model in use information collected before signing are not good enough: To check whether a given certificate is valid at signing time, one strictly speaking needs information generated after signing time. And to deal with algorithms getting weak one may need to time stamp the whole document again and again.

To deal with this ETSI (an European norming organization) specified alternative ways to add validation related information to a document and a way to add extra time stamps covering the whole document (and not only the embedded signature container). These mechanisms do not change the original signature container but add the information in incremental updates to the original document. Meanwhile these mechanisms have been added to the international PDF standard in ISO 32000-2. They are subsumed under the term PAdES.

ETSI also defined standard schemes how to use these new mechanisms to enhance signatures in an interoperable way, the PAdES BASELINE profiles:

• The B level only contains a basic signature container profiled to in particular include an ESS certificate ID attribute.
• The T level is based on the B level but additionally requires an after-signing time stamp. This time stamp may be applied as signature time stamp to the original signature container or as document time stamp in an extra incremental update of the document.
• The LT level is based on the T level and requires missing intermediate certificates and required revocation information to be added in a new incremental update.
• The LTA level is based on the LT level and requires another document time stamp added in yet another incremental update.

To achieve long term validation the additions as for LT and LTA may be repeated to provide validation information for the previous time stamp and to document that the used algorithms were applied at a time when they were still strong.

Adobe has established their own "LTV-enabled" profile which assumes less strict requirements on the validation data (no time stamps needed) and does not care about algorithms becoming weak. They essentially collect all the validation related information they find in the document and use them as-is. (More exactly this is the behavior for the standard settings of Adobe Acrobat. You can configure Acrobat differently to change requirements, e.g. to make certain time stamps do matter. Thus, when talking about "LTV-enabled" signatures always make sure you have the same set of settings in mind as your discussion partner...)

Extending Using eSig DSS

If you want to extend a PDF signature using eSig DSS on server A, simply take the finalDoc and

PAdESService service = new PAdESService(certificateVerifier);
service.setTspSource(tspSource);
PAdESSignatureParameters extensionParameters = new PAdESSignatureParameters();
extensionParameters.setSignatureLevel(extensionLevel);

DSSDocument extendedDocument = service.extendDocument(finalDoc, extensionParameters);

where

• certificateVerifier is a CommonCertificateVerifier initialized for online resources,
• tspSource is a OnlineTSPSource initialized for your time stamp service of choice, and
• extensionLevel is the desired level, e.g. SignatureLevel.PAdES_BASELINE_LT.

The result in extendedDocument should contain the required validation related information.