Work with Digital Signatures

A digital signature is a technological implementation of electronic signatures to sign documents and authenticate the signer to guarantee that a document has not been modified since it was signed. Each digital signature is unique for each signer because of following the PKI protocol to generate both public and private keys. Signing a document digitally means creating a signature using the signer’s private key where a mathematical algorithm is used to encrypt the generated hash.

Aspose.Words allows you to detect, count, or verify existing digital signatures, and also add a new signature to your document to find out any tampering in it. You can also remove all digital signatures from a document. Use the DigitalSignatureUtil class to work with digital signatures.

This article explains how to do all of the above to validate the authenticity and integrity of a digital document.

Supported Formats

Aspose.Words allows you to work with digital signatures on DOC, OOXML, and ODT documents and to sign the generated document in PDF or XPS format.

Limitations of Digital Signatures

The table below describes a few limitations that you may face while working with digital signatures through Aspose.Words, as well as some alternative options.

Limitation Alternative Option
Loss of digital signatures on a document after loading and saving it. Therefore, processing a document to a server may cause the loss of all digital signatures without a notice. Check if a document has digital signatures and take the appropriate action if any are found. For example, send an alert to the clients informing them that the document they are uploading contains digital signatures that will be lost if it is processed.
Aspose.Words supports working with macros in a document. But Aspose.Words does not yet support digital signatures on macros. Export the document back to any Word format, and use Microsoft Word to add a digital signature to macros.

Detect, Count, and Verify Digital Signatures

Aspose.Words allows you to detect digital signature in a document using the the DetectFileFormat method and the HasDigitalSignature property. It is worth noting that such a check will only detect the fact of the signature, but not its validity.

A document can be signed more than once, and this can be done by different users. To check the validity of digital signatures, you need to load them from the document using the LoadSignatures method and use the IsValid property. Also Aspose.Words allows you to count a set of all digital signatures within a document using the Count property.

All of this provides an efficient and safe way to check a document for signatures before processing it.

The following code example shows how to detect the presence of digital signatures and verify them:

Create a Digital Signature

To create a digital signature, you will require to load a signing certificate that confirms identity. When you send a digitally signed document, you also send your certificate and public key.

Aspose.Words allows you to create X.509 certificate, a digital certificate that uses the internationally accepted X.509 PKI standard to verify that a public key belongs to the signer included inside the certificate. To do this, use the Create method within the CertificateHolder class.

The next sections explain how to add a digital signature, signature line, and how to sign a generated PDF document.

Sign a Document

Aspose.Words allows you to sign a DOC, DOCX, XPS, or ODT document digitally using the Sign method and SignOptions properties.

The following code example shows how to sign documents using a certificate holder and sign options:

Add a Signature Line

A signature line is a visual representation of a digital signature in a document. Aspose.Words allows you to insert a signature line using the DocumentBuilder.InsertSignatureLine method. You can also set the parameters for this representation using the SignatureLineOptions class.

For example, the picture below shows how valid and invalid signatures can be displayed.

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Also, if a document contains a signature line and no digital signature, there is a feature to ask the user to add a signature.

The following code example shows how to sign a document with a personal certificate and a specific signature line:

Sign a Generated PDF Document

Aspose.Words allows you to sign and get all details of a PDF document using the PdfDigitalSignatureDetails properties.

The following code example shows how to sign a generated PDF:

Retrieve the Digital Signature Value

Aspose.Words also provides the ability to retrieve the digital signature value from a digitally signed document as a byte array using the SignatureValue property.

The following code example shows how to obtain the digital signature value as a byte array from a document:

Remove Digital Signatures

Aspose.Words allows you to remove all digital signatures from a signed document using the RemoveAllSignatures method.

The following code example shows how to load and remove digital signatures from a document:

FAQ

  1. Q: How can I check whether a document contains any digital signatures?
    A: Use FileFormatUtil.DetectFileFormat to identify the file type and then read the HasDigitalSignature property from the FileFormatInfo object. This returns true if at least one digital signature is present.

  2. Q: How do I verify the validity of the digital signatures in a document?
    A: Load the signatures with DigitalSignatureUtil.LoadSignatures, then iterate through the returned DigitalSignatureCollection and examine each signature’s IsValid property. A value of true indicates a valid, untampered signature.

  3. Q: How can I count the number of digital signatures in a document?
    A: After loading the signatures using DigitalSignatureUtil.LoadSignatures, read the Count property of the resulting DigitalSignatureCollection. This gives the total number of signatures attached to the document.

  4. Q: What is the recommended way to add a digital signature to a Word document?
    A: Create a CertificateHolder with your X.509 certificate (CertificateHolder.Create), configure SignOptions as needed, and call DigitalSignatureUtil.Sign passing the document, the certificate holder, and the sign options.

  5. Q: Can I remove a single digital signature from a document?
    A: No. Aspose.Words currently supports only the removal of all signatures at once via DigitalSignatureUtil.RemoveAllSignatures. To remove a specific signature you would need to recreate the document without that signature using external tools.

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