MicrosoftВ® Office ExcelВ® 2007 Inside Out (Inside Out (Microsoft))

Legions of hackers are seemingly willing to go to any length to gain bragging rights by creating viruses, breaking firewalls, and thwarting password protection. Excel has always had security features, but Office Excel 2007 has added a couple of new features to help make your workbooks a little more secure.

For more about workbook security, see "Hiding and Protecting Workbooks" on page 175.

Removing Personal Information from Your Workbooks

It is increasingly important to keep track of the information you send over the Internet or share with others. Identity theft is a growing problem, and it's important to make sure you don't leave helpful tidbits of information lying around for others to abuse. Although it's up to you to make sure you're not giving away personal information in your worksheets (your password-protected worksheets, of course!), information also lives in places you might not think to look. Excel 2007 includes the new Document Inspector that is designed to look in all the nooks and crannies for you. Click the Microsoft Office Button, click Prepare, and then click Inspect Document to display the dialog box shown in Figure 4-6. (You can also access the Document Inspector by clicking the button of the same name located in the Privacy Options category in the Trust Center dialog box.)

Figure 4-6: The Document Inspector examines the hidden places in your workbooks where personal data can hide.

As you can see in Figure 4-6, data can hide in many places. The Document Inspector looks in them all and reports on what it finds, as shown on the dialog box on the bottom of Figure 4-6. You can then click any Remove All buttons that appear or close the dialog box and edit the area in question. Most of these items will usually be perfectly benign, of course. You probably wouldn't want to purge your headers and footers, for example, unless they contained your address or Social Security number. But if you are sharing data with others, the Document Inspector gives you a quick and easy way to find personal data you might not otherwise have found.

Using Digital Signatures

Digital signatures are similar to handwritten signatures in that both are intended to provide authenticity to documents. However, although the digital version might include a graphic representation of an actual signature, it also uses cryptography to establish not only a document's authenticity but also the integrity of the file and the identity of the signer. You can add signatures to your own documents and to others' as well. One way to use signatures is to verify that others have read a document (or at least opened it) by adding their signatures.

One important fact about digital signatures is that when you digitally sign a workbook, Excel saves it as a read-only document, preventing you from making further changes. A handwritten signature, being typically the last element you add before sending a letter, in this case ensures that it actually is the last step you perform in a workbook. But don't worry-you can remove and reapply a signature if you need to make changes. The point is you can be sure that a document won't change after you apply a signature.

To accomplish all this, you'll need a valid digital signature, which you must create or obtain separately. Fortunately, Excel provides ways to accomplish this. To begin, click the Microsoft Office Button, click Prepare, and then click Add A Digital Signature to display the message shown in Figure 4-7.

Figure 4-7: You can add digital signatures to your workbooks.

If you don't yet have your own digital signature, you can click Signature Services From The Office Marketplace to open a Web page where you can select and order one from a number of third-party vendors. Clicking OK before you have a signature displays a dialog box where you can select a third-party vendor signature or where you can choose to create your own digital signature. Microsoft is careful to point out that creating your own digital signature lets you verify only the authenticity of your own documents, and even then on only your own computer. The upshot is that you might consider purchasing a verifiable signature, if you think you need one. If, despite these drawbacks, you still want to create your own digital signature, select Create Your Own Digital ID, and then click OK. After you create or purchase a digital ID, the dialog box at the top of Figure 4-8 appears when you click Add A Digital Signature. After you click OK, Excel saves the current workbook as read-only and the second dialog box in Figure 4-9 appears, letting you know that your signature will become invalid if you further modify the workbook.

Figure 4-8: For what it's worth, you can create your own digital signature.

Figure 4-9: The Signatures task pane appears when you attempt to edit a workbook after adding a signature.

If after dismissing this last dialog box you try to edit the workbook, you'll see that you can't-the workbook is locked and can't be edited, and a Signatures task pane appears on the right side of the screen, as shown in Figure 4-9.

Also note in Figure 4-9 that [Read-Only] appears in the title bar next to the file name, and a small icon that looks like a certificate ribbon appears next to the word Ready in the status bar. You can click this icon to open and close the Signatures task pane, which you can also open and close by clicking the Microsoft Office Button, clicking Prepare, and clicking View Signatures.

To remove a digital signature, click the small downward-pointing arrow next to the signature you want to remove, and click Remove Signature, which again saves the current workbook, this time without the [Read-Only] designation. You can also view more information about the selected signature by clicking Signature Details.