What is a Digital Signature?

A digital signature, like a conventional handwritten signature, identifies the person signing a document.

Unlike traditional signatures on paper, however, each digital signature stores information about the person signing a document.

Signatures help prevent unwanted changes to a PDF document. For example, an author may not want a PDF document with company letterhead to be changed after it's signed.

The first signature in a document is called the author signature. When you add the first signature to a document, you have the option of certifying the document.

Certifying a document lets you attest to its contents and specify the types of changes allowed for the document to remain certified. Changes to the document are detected in the Signatures tab. Subsequent signatures to the document are called ordinary signatures.

To sign a document, you must select a digital ID, which contains the signature information that you can share with other users in a certificate.

You can create a self-signed digital ID, or you can obtain a digital ID from a third-party provider. Using certificates, other users can validate your signatures, and you should validate the signatures of others.

Looking at your Security preferences

You can set the appearance of your digital signature, select your preferred digital signature signing method, and determine how digital signatures are verified in the Security preferences.

First you'll take a look at the default Security preferences.

1. Start Acrobat.

2. Choose Edit > Preferences (Windows) or Acrobat > Preferences (Mac OS), and select Security in the left pane. (See Figure 1.)

Unless you have already defined an appearance for your digital signature, the Appearance window is blank.

3. Click New to open the Configure Signature Appearance dialog box. This is where you will personalize your digital signature later in the lesson by adding a graphic to your signature. For the moment the Preview pane shows the default digital signature appearance. When you have reviewed the information contained in the default signature, click Cancel to return to the Preferences dialog box.

Now you'll specify a default signing method.

4. Click Advanced Preferences. The Digital Signatures Advanced Preferences dialog box opens with the Verification tab selected.

Notice that the option Require That Certificate Revocation Checking Be Done Whenever Possible When Verifying Signatures option is selected. This ensures that certificates are checked against a list of excluded certificates during validation.

5. In the Verification tab of the dialog box, make sure that signatures are verified using the document-specified method when a document is opened. You'll be prompted if you don't have the necessary software when you try to open a document. (See Figure 2.)

You can only change the default method for verifying signatures in this panel if you change this selection. You set the default method to be used when signing and encrypting documents in the Creation tab.

6. Click the Creation tab and check that Adobe Default Security is selected for the Default Method to Use When Signing and Encrypting Documents option.

On Windows, you have a Windows Integration tab where you can specify whether identities from Windows Certificates can be imported, and whether all root certificates in the Windows Certificates can be trusted. We recommend that you leave the defaults in this tab. (See Figure 3.)

7. Choose Cancel and Cancel again to close the Preferences dialog boxes without making any changes, or choose OK if you changed any of the options.

In our next article, we'll create a digital signature and sign a PDF document.

Pages 343-346 excerpted from "Adobe Acrobat 7.0 Classroom in a Book." Copyright © 2005. Used with the permission of Pearson Education, Inc. and Peachpit.

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