The unfortunate reality of exchanging files over networks,
especially over the Internet, is that you need to be sure you know the
origin of the files you’re working with. One way an organization can
guard against files with viruses or substitute data is to authenticate
every workbook using a digital signature. A digital signature is a
character string created by combining a user’s unique secret digital
signature file mathematically with the contents of the workbook, which
programs such as Excel can recognize and use to verify the identity of
the user who signed the file. A good analogy for a digital signature is
a wax seal, which was used for thousands of years to verify the
integrity and origin of a document.
Tip
The technical details of and procedure for managing digital
certificates are beyond the scope of this book, but your network
administrator should be able to create a digital certificate for you.
You can also directly purchase a digital signature from a third party,
which can usually be renewed annually for a small fee. For the
purposes of this book, you’ll use the selfcert.exe Microsoft Office
accessory program to generate a certificate with which to perform the
exercise at the end of this topic. This type of certificate is useful
for certifying a document as part of a demonstration, but other users
will not accept it as a valid certificate to verify that the contents
of the document haven’t changed since it was signed.
To create a digital certificate that you can use as a
demonstration, open the Start menu, click All Programs, click Microsoft
Office, click Microsoft Office 2010 Tools, and then click Digital
Certificate For VBA Projects. In the Create Digital Certificate dialog
box, type a name for your certificate and click OK to have the program
create your trial certificate. Then, in Excel, click the File tab and on
the Info page of the Backstage view, click Protect Workbook and then
click Add A Digital Signature. In the Sign dialog box, type your purpose
for signing the document, and then click Sign to sign your
workbook.
Tip
After you click Add A Digital Signature, Excel displays a dialog
box, indicating that you can buy digital signatures from third-party
providers. To get information about those services, click the
Signature Services From The Office Marketplace button. To bypass the
message, click OK; to prevent the dialog box from appearing again,
select the Don’t Show This Message Again check box, and then click
OK.
If you have several certificates from which to choose, and the
desired certificate doesn’t appear in the Sign dialog box, you can click
Change to display the Select Certificate dialog box. In the Select
Certificate dialog box, click the certificate with which you want to
sign the workbook, and then click OK. The Select Certificate dialog box
closes, and the certificate with which you signed the workbook appears
in the Sign dialog box. As before, click Sign to sign your document by
using the digital certificate.
In this exercise, you’ll create a digital certificate and
digitally sign a workbook by using the certificate.
Set Up
-
On the Start menu, click
All Programs, click Microsoft Office, click Microsoft Office 2010 Tools, and then click
Digital Certificate for VBA
Projects.
The Create Digital Certificate dialog box opens.
-
In the Your certificate’s
name box, type Excel2010SBS, and then click OK.
A message box indicates that the program created your
certificate successfully.
-
Click OK.
The message box closes.
-
Click the File tab and
then, if necessary, click Info.
Click Protect Workbook and then
click Add a Digital
Signature.
A message box appears, offering the opportunity to view
signature services on Office Marketplace.
-
Click OK.
The message box closes, and the Sign dialog box opens.
-
In the Purpose for signing this
document box, type Testing.
-
Verify that the Excel2010SBS certificate appears in the
Signing as area of the dialog
box, and then click Sign.
The Signature Confirmation dialog box opens.
-
Click OK.
The Signatures task pane opens and the workbook is marked as
final. If you edit the workbook, it will invalidate the digital
signature, which is based on the workbook’s contents at the time you
signed it.
Clean Up
Save the ProjectionsSigned workbook, and then close it.
