In java using PDFBox, how to create visible digital signature with text

Question

Digital text with text and background imageI am trying to digitally sign pdf file using PDFBox in Java with visible text to appear on page similar to one that gets created when manually created in Acrobat. As shown in the image (one with only snap shot I am looking for and another with details of digital signature too), this example shows signing using image file. How to do that?

Answer 1

This code will be included among the samples in the upcoming 2.0.9 release of PDFBox, and the current update can be found in the repository. See also the discussion in PDFBOX-3198. It is more flexible and can include both text and images, or only one of the two, or vector graphics, whatever you want.

/**
 * This is a second example for visual signing a pdf. It doesn't use the "design pattern" influenced
 * PDVisibleSignDesigner, and doesn't create its complex multilevel forms described in the Adobe
 * document
 * <a href="https://www.adobe.com/content/dam/acom/en/devnet/acrobat/pdfs/PPKAppearances.pdf">Digital
 * Signature Appearances</a>, because this isn't required by the PDF specification. See the
 * discussion in December 2017 in PDFBOX-3198.
 *
 * @author Vakhtang Koroghlishvili
 * @author Tilman Hausherr
 */
public class CreateVisibleSignature2 extends CreateSignatureBase
{
    private SignatureOptions signatureOptions;
    private boolean lateExternalSigning = false;
    private File imageFile;

    /**
     * Initialize the signature creator with a keystore (pkcs12) and pin that
     * should be used for the signature.
     *
     * @param keystore is a pkcs12 keystore.
     * @param pin is the pin for the keystore / private key
     * @throws KeyStoreException if the keystore has not been initialized (loaded)
     * @throws NoSuchAlgorithmException if the algorithm for recovering the key cannot be found
     * @throws UnrecoverableKeyException if the given password is wrong
     * @throws CertificateException if the certificate is not valid as signing time
     * @throws IOException if no certificate could be found
     */
    public CreateVisibleSignature2(KeyStore keystore, char[] pin)
            throws KeyStoreException, UnrecoverableKeyException, NoSuchAlgorithmException, IOException, CertificateException
    {
        super(keystore, pin);
    }

    public File getImageFile()
    {
        return imageFile;
    }

    public void setImageFile(File imageFile)
    {
        this.imageFile = imageFile;
    }

    public boolean isLateExternalSigning()
    {
        return lateExternalSigning;
    }

    /**
     * Set late external signing. Enable this if you want to activate the demo code where the
     * signature is kept and added in an extra step without using PDFBox methods. This is disabled
     * by default.
     *
     * @param lateExternalSigning
     */
    public void setLateExternalSigning(boolean lateExternalSigning)
    {
        this.lateExternalSigning = lateExternalSigning;
    }

    /**
     * Sign pdf file and create new file that ends with "_signed.pdf".
     *
     * @param inputFile The source pdf document file.
     * @param signedFile The file to be signed.
     * @param humanRect rectangle from a human viewpoint (coordinates start at top left)
     * @param tsaUrl optional TSA url
     * @throws IOException
     */
    public void signPDF(File inputFile, File signedFile, Rectangle2D humanRect, String tsaUrl) throws IOException
    {
        this.signPDF(inputFile, signedFile, humanRect, tsaUrl, null);
    }

    /**
     * Sign pdf file and create new file that ends with "_signed.pdf".
     *
     * @param inputFile The source pdf document file.
     * @param signedFile The file to be signed.
     * @param humanRect rectangle from a human viewpoint (coordinates start at top left)
     * @param tsaUrl optional TSA url
     * @param signatureFieldName optional name of an existing (unsigned) signature field
     * @throws IOException
     */
    public void signPDF(File inputFile, File signedFile, Rectangle2D humanRect, String tsaUrl, String signatureFieldName) throws IOException
    {
        if (inputFile == null || !inputFile.exists())
        {
            throw new IOException("Document for signing does not exist");
        }

        setTsaUrl(tsaUrl);

        // creating output document and prepare the IO streams.
        FileOutputStream fos = new FileOutputStream(signedFile);

        try (PDDocument doc = PDDocument.load(inputFile))
        {
            int accessPermissions = SigUtils.getMDPPermission(doc);
            if (accessPermissions == 1)
            {
                throw new IllegalStateException("No changes to the document are permitted due to DocMDP transform parameters dictionary");
            }
            // Note that PDFBox has a bug that visual signing on certified files with permission 2
            // doesn't work properly, see PDFBOX-3699. As long as this issue is open, you may want to
            // be careful with such files.

            PDSignature signature = null;
            PDAcroForm acroForm = doc.getDocumentCatalog().getAcroForm();
            PDRectangle rect = null;

            // sign a PDF with an existing empty signature, as created by the CreateEmptySignatureForm example.
            if (acroForm != null)
            {
                signature = findExistingSignature(acroForm, signatureFieldName);
                if (signature != null)
                {
                    rect = acroForm.getField(signatureFieldName).getWidgets().get(0).getRectangle();
                }
            }

            if (signature == null)
            {
                // create signature dictionary
                signature = new PDSignature();
            }

            if (rect == null)
            {
                rect = createSignatureRectangle(doc, humanRect);
            }

            // Optional: certify
            // can be done only if version is at least 1.5 and if not already set
            // doing this on a PDF/A-1b file fails validation by Adobe preflight (PDFBOX-3821)
            // PDF/A-1b requires PDF version 1.4 max, so don't increase the version on such files.
            if (doc.getVersion() >= 1.5f && accessPermissions == 0)
            {
                SigUtils.setMDPPermission(doc, signature, 2);
            }

            if (acroForm != null && acroForm.getNeedAppearances())
            {
                // PDFBOX-3738 NeedAppearances true results in visible signature becoming invisible 
                // with Adobe Reader
                if (acroForm.getFields().isEmpty())
                {
                    // we can safely delete it if there are no fields
                    acroForm.getCOSObject().removeItem(COSName.NEED_APPEARANCES);
                    // note that if you've set MDP permissions, the removal of this item
                    // may result in Adobe Reader claiming that the document has been changed.
                    // and/or that field content won't be displayed properly.
                    // ==> decide what you prefer and adjust your code accordingly.
                }
                else
                {
                    System.out.println("/NeedAppearances is set, signature may be ignored by Adobe Reader");
                }
            }

            // default filter
            signature.setFilter(PDSignature.FILTER_ADOBE_PPKLITE);

            // subfilter for basic and PAdES Part 2 signatures
            signature.setSubFilter(PDSignature.SUBFILTER_ADBE_PKCS7_DETACHED);

            signature.setName("Name");
            signature.setLocation("Location");
            signature.setReason("Reason");

            // the signing date, needed for valid signature
            signature.setSignDate(Calendar.getInstance());

            // do not set SignatureInterface instance, if external signing used
            SignatureInterface signatureInterface = isExternalSigning() ? null : this;

            // register signature dictionary and sign interface
            signatureOptions = new SignatureOptions();
            signatureOptions.setVisualSignature(createVisualSignatureTemplate(doc, 0, rect));
            signatureOptions.setPage(0);
            doc.addSignature(signature, signatureInterface, signatureOptions);

            if (isExternalSigning())
            {
                System.out.println("Signing externally " + signedFile.getName());
                ExternalSigningSupport externalSigning = doc.saveIncrementalForExternalSigning(fos);
                // invoke external signature service
                byte[] cmsSignature = sign(externalSigning.getContent());

                // Explanation of late external signing (off by default):
                // If you want to add the signature in a separate step, then set an empty byte array
                // and call signature.getByteRange() and remember the offset signature.getByteRange()[1]+1.
                // you can write the ascii hex signature at a later time even if you don't have this
                // PDDocument object anymore, with classic java file random access methods.
                // If you can't remember the offset value from ByteRange because your context has changed,
                // then open the file with PDFBox, find the field with findExistingSignature() or
                // PODDocument.getLastSignatureDictionary() and get the ByteRange from there.
                // Close the file and then write the signature as explained earlier in this comment.
                if (isLateExternalSigning())
                {
                    // this saves the file with a 0 signature
                    externalSigning.setSignature(new byte[0]);

                    // remember the offset (add 1 because of "<")
                    int offset = signature.getByteRange()[1] + 1;

                    // now write the signature at the correct offset without any PDFBox methods
                    try (RandomAccessFile raf = new RandomAccessFile(signedFile, "rw"))
                    {
                        raf.seek(offset);
                        raf.write(Hex.getBytes(cmsSignature));
                    }
                }
                else
                {
                    // set signature bytes received from the service and save the file
                    externalSigning.setSignature(cmsSignature);
                }
            }
            else
            {
                // write incremental (only for signing purpose)
                doc.saveIncremental(fos);
            }
        }

        // Do not close signatureOptions before saving, because some COSStream objects within
        // are transferred to the signed document.
        // Do not allow signatureOptions get out of scope before saving, because then the COSDocument
        // in signature options might by closed by gc, which would close COSStream objects prematurely.
        // See https://issues.apache.org/jira/browse/PDFBOX-3743
        IOUtils.closeQuietly(signatureOptions);
    }

    private PDRectangle createSignatureRectangle(PDDocument doc, Rectangle2D humanRect)
    {
        float x = (float) humanRect.getX();
        float y = (float) humanRect.getY();
        float width = (float) humanRect.getWidth();
        float height = (float) humanRect.getHeight();
        PDPage page = doc.getPage(0);
        PDRectangle pageRect = page.getCropBox();
        PDRectangle rect = new PDRectangle();
        // signing should be at the same position regardless of page rotation.
        switch (page.getRotation())
        {
            case 90:
                rect.setLowerLeftY(x);
                rect.setUpperRightY(x + width);
                rect.setLowerLeftX(y);
                rect.setUpperRightX(y + height);
                break;
            case 180:
                rect.setUpperRightX(pageRect.getWidth() - x);
                rect.setLowerLeftX(pageRect.getWidth() - x - width);
                rect.setLowerLeftY(y);
                rect.setUpperRightY(y + height);
                break;
            case 270:
                rect.setLowerLeftY(pageRect.getHeight() - x - width);
                rect.setUpperRightY(pageRect.getHeight() - x);
                rect.setLowerLeftX(pageRect.getWidth() - y - height);
                rect.setUpperRightX(pageRect.getWidth() - y);
                break;
            case 0:
            default:
                rect.setLowerLeftX(x);
                rect.setUpperRightX(x + width);
                rect.setLowerLeftY(pageRect.getHeight() - y - height);
                rect.setUpperRightY(pageRect.getHeight() - y);
                break;
        }
        return rect;
    }

    // create a template PDF document with empty signature and return it as a stream.
    private InputStream createVisualSignatureTemplate(PDDocument srcDoc, int pageNum, PDRectangle rect) throws IOException
    {
        try (PDDocument doc = new PDDocument())
        {
            PDPage page = new PDPage(srcDoc.getPage(pageNum).getMediaBox());
            doc.addPage(page);
            PDAcroForm acroForm = new PDAcroForm(doc);
            doc.getDocumentCatalog().setAcroForm(acroForm);
            PDSignatureField signatureField = new PDSignatureField(acroForm);
            PDAnnotationWidget widget = signatureField.getWidgets().get(0);
            List<PDField> acroFormFields = acroForm.getFields();
            acroForm.setSignaturesExist(true);
            acroForm.setAppendOnly(true);
            acroForm.getCOSObject().setDirect(true);
            acroFormFields.add(signatureField);

            widget.setRectangle(rect);

            // from PDVisualSigBuilder.createHolderForm()
            PDStream stream = new PDStream(doc);
            PDFormXObject form = new PDFormXObject(stream);
            PDResources res = new PDResources();
            form.setResources(res);
            form.setFormType(1);
            PDRectangle bbox = new PDRectangle(rect.getWidth(), rect.getHeight());
            float height = bbox.getHeight();
            Matrix initialScale = null;
            switch (srcDoc.getPage(pageNum).getRotation())
            {
                case 90:
                    form.setMatrix(AffineTransform.getQuadrantRotateInstance(1));
                    initialScale = Matrix.getScaleInstance(bbox.getWidth() / bbox.getHeight(), bbox.getHeight() / bbox.getWidth());
                    height = bbox.getWidth();
                    break;
                case 180:
                    form.setMatrix(AffineTransform.getQuadrantRotateInstance(2)); 
                    break;
                case 270:
                    form.setMatrix(AffineTransform.getQuadrantRotateInstance(3));
                    initialScale = Matrix.getScaleInstance(bbox.getWidth() / bbox.getHeight(), bbox.getHeight() / bbox.getWidth());
                    height = bbox.getWidth();
                    break;
                case 0:
                default:
                    break;
            }
            form.setBBox(bbox);
            PDFont font = PDType1Font.HELVETICA_BOLD;

            // from PDVisualSigBuilder.createAppearanceDictionary()
            PDAppearanceDictionary appearance = new PDAppearanceDictionary();
            appearance.getCOSObject().setDirect(true);
            PDAppearanceStream appearanceStream = new PDAppearanceStream(form.getCOSObject());
            appearance.setNormalAppearance(appearanceStream);
            widget.setAppearance(appearance);

            try (PDPageContentStream cs = new PDPageContentStream(doc, appearanceStream))
            {
                // for 90° and 270° scale ratio of width / height
                // not really sure about this
                // why does scale have no effect when done in the form matrix???
                if (initialScale != null)
                {
                    cs.transform(initialScale);
                }

                // show background (just for debugging, to see the rect size + position)
                cs.setNonStrokingColor(Color.yellow);
                cs.addRect(-5000, -5000, 10000, 10000);
                cs.fill();

                // show background image
                // save and restore graphics if the image is too large and needs to be scaled
                cs.saveGraphicsState();
                cs.transform(Matrix.getScaleInstance(0.25f, 0.25f));
                PDImageXObject img = PDImageXObject.createFromFileByExtension(imageFile, doc);
                cs.drawImage(img, 0, 0);
                cs.restoreGraphicsState();

                // show text
                float fontSize = 10;
                float leading = fontSize * 1.5f;
                cs.beginText();
                cs.setFont(font, fontSize);
                cs.setNonStrokingColor(Color.black);
                cs.newLineAtOffset(fontSize, height - leading);
                cs.setLeading(leading);
                cs.showText("(Signature very wide line 1)");
                cs.newLine();
                cs.showText("(Signature very wide line 2)");
                cs.newLine();
                cs.showText("(Signature very wide line 3)");
                cs.endText();
            }

            // no need to set annotations and /P entry

            ByteArrayOutputStream baos = new ByteArrayOutputStream();
            doc.save(baos);
            return new ByteArrayInputStream(baos.toByteArray());
        }
    }

    // Find an existing signature (assumed to be empty). You will usually not need this.
    private PDSignature findExistingSignature(PDAcroForm acroForm, String sigFieldName)
    {
        PDSignature signature = null;
        PDSignatureField signatureField;
        if (acroForm != null)
        {
            signatureField = (PDSignatureField) acroForm.getField(sigFieldName);
            if (signatureField != null)
            {
                // retrieve signature dictionary
                signature = signatureField.getSignature();
                if (signature == null)
                {
                    signature = new PDSignature();
                    // after solving PDFBOX-3524
                    // signatureField.setValue(signature)
                    // until then:
                    signatureField.getCOSObject().setItem(COSName.V, signature);
                }
                else
                {
                    throw new IllegalStateException("The signature field " + sigFieldName + " is already signed.");
                }
            }
        }
        return signature;
    }

    /**
     * Arguments are
     * [0] key store
     * [1] pin
     * [2] document that will be signed
     * [3] image of visible signature
     *
     * @param args
     * @throws java.security.KeyStoreException
     * @throws java.security.cert.CertificateException
     * @throws java.io.IOException
     * @throws java.security.NoSuchAlgorithmException
     * @throws java.security.UnrecoverableKeyException
     */
    public static void main(String[] args) throws KeyStoreException, CertificateException,
            IOException, NoSuchAlgorithmException, UnrecoverableKeyException
    {
        // generate with
        // keytool -storepass 123456 -storetype PKCS12 -keystore file.p12 -genkey -alias client -keyalg RSA
        if (args.length < 4)
        {
            usage();
            System.exit(1);
        }

        String tsaUrl = null;
        // External signing is needed if you are using an external signing service, e.g. to sign
        // several files at once.
        boolean externalSig = false;
        for (int i = 0; i < args.length; i++)
        {
            if (args[i].equals("-tsa"))
            {
                i++;
                if (i >= args.length)
                {
                    usage();
                    System.exit(1);
                }
                tsaUrl = args[i];
            }
            if (args[i].equals("-e"))
            {
                externalSig = true;
            }
        }

        File ksFile = new File(args[0]);
        KeyStore keystore = KeyStore.getInstance("PKCS12");
        char[] pin = args[1].toCharArray();
        keystore.load(new FileInputStream(ksFile), pin);

        File documentFile = new File(args[2]);

        CreateVisibleSignature2 signing = new CreateVisibleSignature2(keystore, pin.clone());

        signing.setImageFile(new File(args[3]));

        File signedDocumentFile;
        String name = documentFile.getName();
        String substring = name.substring(0, name.lastIndexOf('.'));
        signedDocumentFile = new File(documentFile.getParent(), substring + "_signed.pdf");

        signing.setExternalSigning(externalSig);

        // Set the signature rectangle
        // Although PDF coordinates start from the bottom, humans start from the top.
        // So a human would want to position a signature (x,y) units from the
        // top left of the displayed page, and the field has a horizontal width and a vertical height
        // regardless of page rotation.
        Rectangle2D humanRect = new Rectangle2D.Float(100, 200, 150, 50);

        signing.signPDF(documentFile, signedDocumentFile, humanRect, tsaUrl, "Signature1");
    }

    /**
     * This will print the usage for this program.
     */
    private static void usage()
    {
        System.err.println("Usage: java " + CreateVisibleSignature2.class.getName()
                + " <pkcs12-keystore-file> <pin> <input-pdf> <sign-image>\n" + "" +
                           "options:\n" +
                           "  -tsa <url>    sign timestamp using the given TSA server\n"+
                           "  -e            sign using external signature creation scenario");
    }

}