Assign and Validate Digital Signatures with JavaScript via C++

A digital signature provides assurance that a workbook file is valid and no one has altered it. You can create a personal digital signature by using the Microsoft Selfcert.exe or any other tool, or you can purchase a digital signature. After you create a digital signature, you must attach it to your workbook. Attaching a digital signature is similar to sealing an envelope. If an envelope arrives sealed, you have some level of assurance that no one has tampered with its contents.

Introduction

Use the Digital Signature dialog to attach a digital signature. The Digital Signature dialog lists valid certificates. You can use the Digital Signature dialog to view certificates and to select the one you want to use. If a workbook has a digital signature, the name of the signature appears in the Certificate Name field. If you click the Remove button in the Digital Signature dialog, Microsoft Excel removes the digital signature as well.

How to Add Digital Signature for Excel

Aspose.Cells provides the DigitalSignature module to perform the job (assign and validate digital signatures). The module has some useful features for adding and validating digital signatures.

Please see the following sample code that describes how you can perform the task using the Aspose.Cells for JavaScript via C++ API.

<!DOCTYPE html>
<html>
    <head>
        <title>Aspose.Cells Example</title>
        <meta charset="utf-8" />
    </head>
    <body>
        <h1>Digital Signature: Sign and Verify Workbook</h1>

        <label for="pfxInput">Select PFX Certificate (.pfx):</label>
        <input type="file" id="pfxInput" accept=".pfx" />
        <br /><br />

        <label for="certPassword">Certificate Password:</label>
        <input type="text" id="certPassword" value="aa" />
        <br /><br />

        <label for="signatureComment">Signature Comment:</label>
        <input type="text" id="signatureComment" value="test for sign" />
        <br /><br />

        <button id="runExample">Sign Workbook and Verify</button>
        <a id="downloadLink" style="display: none;">Download Result</a>
        <div id="result"></div>
    </body>

    <script src="aspose.cells.js.min.js"></script>
    <script type="text/javascript">
        const { Workbook, SaveFormat, DigitalSignature, DigitalSignatureCollection } = AsposeCells;
        
        AsposeCells.onReady({
            license: "/lic/aspose.cells.enc",
            fontPath: "/fonts/",
            fontList: [
                "arial.ttf",
                "NotoSansSC-Regular.ttf"
            ]
        }).then(() => {
            console.log("Aspose.Cells initialized");
        });

        document.getElementById('runExample').addEventListener('click', async () => {
            const pfxInput = document.getElementById('pfxInput');
            if (!pfxInput.files.length) {
                document.getElementById('result').innerHTML = '<p style="color: red;">Please select a .pfx certificate file.</p>';
                return;
            }

            const password = document.getElementById('certPassword').value || "";
            const comment = document.getElementById('signatureComment').value || "";

            // Read the PFX file
            const pfxFile = pfxInput.files[0];
            const pfxArrayBuffer = await pfxFile.arrayBuffer();
            const pfxBytes = new Uint8Array(pfxArrayBuffer);

            // dsc is signature collection that contains one or more signatures needed to sign
            const dsc = new DigitalSignatureCollection();

            // Cert must contain a private key, constructed from the PFX bytes
            const cert = new DigitalSignature(pfxBytes, password, comment, new Date());
            dsc.add(cert);

            // Create a new workbook
            const wb = new Workbook();

            // wb.setDigitalSignature signs all signatures in dsc -> converted to property assignment
            wb.digitalSignature = dsc;

            // Saving the modified Excel file
            const outputData = wb.save(SaveFormat.Xlsx);
            const blob = new Blob([outputData]);
            const downloadLink = document.getElementById('downloadLink');
            downloadLink.href = URL.createObjectURL(blob);
            downloadLink.download = 'newfile_out.xlsx';
            downloadLink.style.display = 'block';
            downloadLink.textContent = 'Download Signed Excel File';

            // Open the file from the generated data to verify signatures
            const wb2 = new Workbook(new Uint8Array(outputData));

            // isDigitallySigned property
            const signedStatus = wb2.isDigitallySigned;
            
            // Get digitalSignature collection from workbook
            const dsc2 = wb2.digitalSignature;
            let detailsHtml = `<p style="color: green;">Workbook signed: ${signedStatus}</p>`;

            if (dsc2 && dsc2.count > 0) {
                detailsHtml += '<h3>Signatures:</h3><ul>';
                for (let i = 0; i < dsc2.count; i++) {
                    const dst = dsc2.get(i);
                    const dstComments = dst.comments;
                    const dstSignTime = dst.signTime;
                    const dstIsValid = dst.isValid ? dst.isValid() : "N/A";
                    detailsHtml += `<li>
                        <strong>Signature ${i + 1}:</strong><br/>
                        Comments: ${dstComments}<br/>
                        Sign Time: ${dstSignTime}<br/>
                        Is Valid: ${dstIsValid}
                    </li>`;
                }
                detailsHtml += '</ul>';
            } else {
                detailsHtml += '<p>No digital signatures found in the workbook.</p>';
            }

            document.getElementById('result').innerHTML = detailsHtml;
        });
    </script>
</html>

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