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HOWTO Add or remove a digital signature in Office documents

What is a digital signature?

Digital signatures is used to authenticate digital information — such as documents, e-mail messages, and macros — by using computer cryptography. Digital signatures help to establish the following assurances:
  • Authenticity The digital signature helps to assure that the signer is who he or she claims to be.
  • Integrity The digital signature helps to assure that the content has not been changed or tampered with since it was digitally signed.
  • Non-repudiation The digital signature helps to prove to all parties the origin of the signed content. "Repudiation" refers to the act of a signer's denying any association with the signed content.

To make these assurances, the content creator must digitally sign the content by using a signature that satisfies the following criteria:

  • The digital signature is valid.
  • The certificate associated with the digital signature is current (not expired).
  • The signing person or organization, known as the publisher, is trusted.
  • The certificate associated with the digital signature is issued to the signing publisher by a reputable certificate authority (CA).

The 2007 Microsoft Office system programs detect these criteria for you and alert you if there is a problem with the digital signature.

How can digital signatures be used in Office documents?
There are two different ways you can use digital signatures to sign Office documents. You can either:

  • Add visible signature lines to a document to capture one or more digital signatures.
  • Add an invisible digital signature to a document.

The differences between these two methods and details about how to digitally sign a document in either of these two ways are described in the following sections.

Add one or more signature lines to a document

The 2007 Microsoft Office system introduces the ability to insert a signature line into a document. You can insert signatures lines only into Word documents and Excel workbooks.

A signature line looks like a typical signature placeholder that might appear in a print document, but it works differently. When a signature line is inserted into an Office document, the document author can specify information about the intended signer, as well as instructions for the signer. When an electronic copy of the document is sent to the intended signer, this person sees the signature line and a notification that his or her signature is requested. The signer can click the signature line to digitally sign the document. The signer can then either type a signature, select a digital image of his or her signature, or write a signature by using the inking feature of the Tablet PC. When the signer adds a visible representation of his or her signature to the document, a digital signature is added simultaneously to authenticate the identity of the signer. After a document is digitally signed, it will become read-only to prevent modifications to its content.

The ability to capture digital signatures by using signature lines in Office documents makes it possible for organizations to use paperless signing processes for documents like contracts or other agreements. Unlike signatures on paper, digital signatures can provide a record of exactly what was signed and they allow the signature to be verified in the future.

To add a signature line to a document

1. Place your pointer in the location in your document where you want to add a signature line.
2. On the Insert tab, in the Text group, point to the arrow next to Signature Line, and then click Microsoft Office Signature Line.
3. In the Signature Setup dialog box, type information about the person who will be signing on this signature line. This information is displayed directly beneath the signature line in the document. Do any of the following:
  • Type the signer's name in the Suggested signer box.
  • Type the signer's organizational title (if any) in the Suggested signer's title box.
  • Type the signer's e-mail address (if any) in the Suggested signer's e-mail address box.
4. If you want to provider the signer with any instructions, type these instructions in the Instructions to signer box. These instructions are displayed in the Signature dialog box that the signer uses to sign the document.
5. If you want the signer to be able to add comments along with the signature, select the Allow the signer to add comments in the Sign dialog check box.
6. If you want to show the date when the signature is added in the signature line, select the Show sign date in signature line check box.
7. Click OK.
8. To add additional signature lines, repeat steps 1 through 7.

Sign the signature line in a document

When you sign the signature line in an Office document, you add both a visible representation of your signature and a digital signature.

  1. In the document, double-click the signature line where your signature is requested.
  2. In the Sign dialog box, do one of the following:
  • To add a printed version of your signature, type your name in the box next to the X.
  • To select an image of your written signature, click Select Image. In the Select Signature Image dialog box, find the location of your signature image file, select the file that you want, and then click Select.
  • To add a handwritten signature (Tablet PC users only), sign your name in the box next to the X by using the inking feature.
  • Click Sign.

Add an invisible digital signature to a document

If you do not need to insert visible signature lines into a document, but you still want to provide assurance as to the authenticity, integrity, and origin of a document, you can add an invisible digital signature to the document. You can add invisible digital signatures to Word documents, Excel workbooks, and PowerPoint presentations.

Unlike an Office signature line, an invisible digital signature is not visible within the contents of the document itself, but recipients of the document can determine that the document has been digitally signed by viewing the document's digital signature or by looking for the Signatures button on the status bar at the bottom of the screen.

After a document has been digitally signed, it becomes read-only to prevent modifications.

  1. Click the Microsoft Office Button, point to Prepare, and then click Add a Digital Signature.
  2. If you want to state your purpose for signing the document, type this information in the box under Purpose for signing this document in the Sign dialog box.
  3. Click Sign.

Source: Microsoft Office Online


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