java - 在使用 PDFBox 的 java 中，如何使用文本创建可见的数字签名

Question

Digital text with text and background image我正在尝试使用 Java 中的 PDFBox 对 pdf 文件进行数字签名，并在页面上显示可见文本，类似于在 Acrobat 中手动创建时创建的文本。如图所示(一张只有我正在寻找的快照，另一张也有数字签名的详细信息)，此示例显示使用图像文件进行签名。如何做到这一点？

Answer 1

此代码将包含在即将发布的 PDFBox 2.0.9 版本中的示例中，当前的更新可以在 in the repository 中找到。另请参阅 PDFBOX-3198 中的讨论。它更灵活，可以包含文本和图像，或仅包含两者之一，或 vector 图形，随您所愿。

/**
 * 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");
    }

}