Working with Digital Signatures

A digital signature is used to authenticate a document to establish that the sender of the document is who they say they are and the content of the document has not been tampered with.

Aspose.Words supports documents with digital signatures and provides access to them allowing you to detect and validate digital signatures on a document and sign a generated PDF document with a supplied certificate. At the present time digital signatures are supported on DOC, OOXML and ODT documents. Signing of generated documents is supported in PDF format.

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You can try this functionality with our Free online signature.

Digital Signatures are not Preserved on Open and Save

An important point to note is that a document loaded and then saved using Aspose.Words will lose any digital signatures signed on the document. This is by design as a digital signature ensures that the content has not been modified and furthermore authenticates the identify of who signed the document. These principles would be invalidated if the original signatures were carried over to the resulting document.

Due to this, if you process documents uploaded to a server this could potentially mean you may corrupt a document uploaded to your server in this way without knowing. Therefore it is best to check for digital signatures on a document and take the appropriate action if any are found, for example an alert can be sent to the client informing them that the document they are passing contains digital signatures which will be lost if it is processed. You can download template file of this example from here.

The code above uses the FileFormatUtil.detectFileFormat method to detect if a document contains digital signatures without loading the document first. This provides an efficient and safe way to check a document for signatures before processing them. When executed, the method returns a FileFormatInfo object which provides the property FileFormatInfo.hasDigitalSignature. This property returns true if the document contains one or more digital signatures. It’s important to note that this method does not validate the signatures, it only determines if signatures are present. Validating digital signatures is covered in the next section.

You can also check if a document has digital signatures after loading by checking the Count property of the Document.digitalSignatures collection.

Digital Signatures on Macros (VBA Projects)

Digital signatures on macros cannot be accessed or signed. This is because Aspose.Words does not directly deal with macros in a document. However digital signatures on macros are preserved when exporting the document back to any word format. These signatures can be preserved on VBA code because the binary content of the macros are not changed even if the document itself is modified.

Access and Verify Digital Signatures

A document can have multiple digital signatures. These signatures can all be accessed through the Document.digitalSignatures collection. Each object returned is a DigitalSignature which represents a single digital signature belonging to the document. This provides members that allow you to check the validity of the signature.

The most important property to check with digital signatures is the validity of each signature in the document. All signatures in the document can be validated at once by calling the DigitalSignatureCollection.isValid property. This will return true if all signatures in the document are valid or if the document has no signatures and false if at least one digital signature is not valid.

Each signature can also be individually validated by calling DigitalSignature.isValid. A signature can return not valid for several reasons, for instance the document has been changed since signing or the certificate has expired. Additionally extra details of the signature can also be accessed.

The following code example shows how to validate each signature in a document and display basic information about the signature:

You can download template file of this example from here.

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:

Note that you can not remove only one digital signature within your document.