Digital signatures: Let Office XP check for revoked certificates - Help and How-to

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Digital signatures: Let Office XP check for revoked certificates

Applies to
Microsoft Word 2002
Microsoft Excel 2002
Microsoft PowerPoint® 2002

In Microsoft Office XP, digital certificates can be used to create digital signatures for macro projects and files, such as documents, presentations, and workbooks. When you work with a signed macro project or file, a digital signature can provide another level of authenticity to the file or project. A digital certificate is like a driver's license or passport: it adds authenticity to a digital signature by providing information about the signer.

In most cases, the certificates are issued by certificate authorities. If this type of certificate becomes compromised, the certificate authority can later revoke the certificate, thus rendering invalid all macros or documents signed after the signature's revocation date. A certificate could become compromised if an unauthorized third-party obtained the private key associated with the certificate. This private key is typically stored on the signer's computer. With the private key, an unauthorized person could essentially forge a signature.

If you receive a file that contains macros signed with a revoked certificate, avoid enabling the macros in the file. To do this, verify that your Macro Security setting is set to High.

Show How?

Open Word, Excel , or PowerPoint.
On the Tools menu, click Options.
Click the Security tab.
Under Macro Security, click Macro Security.
Click the Security Level tab, and then verify that High is selected.

If you receive a document, presentation, or workbook that has been digitally signed with a revoked certificate, you may not be able to trust its contents.

Enabling revocation checking

By default, Office XP does not check whether a certificate has been revoked. In order for Office XP to check for revoked certificates, you must enable revocation checking in Microsoft Internet Explorer 3.0 or later.

Show Enable revocation checking

Start Internet Explorer.
On the Tools menu, click Internet Options.
Click the Advanced tab.
Scroll down to the Security settings section at the bottom, and then select the Check for publisher’s certificate revocation check box.

Note When you enable revocation checking, your computer must be able to connect to the Internet to get the revocation list. Connecting to the Internet and downloading the list may slow the process of checking the certificate.

Once you enable revocation checking, you'll receive warnings when you try to open files that contain macros with revoked certificates.

Looking at revoked certificates

Show For signed files (documents, presentations, or workbooks)

A signed file will contain the signature stamp icon in the status bar, as shown below.

To see the details of the certificate, double-click the image in the status bar, select the certificate you want to view, and then click View Certificate.

Show For signed macros

The warning you receive will depend on your Macro Security setting.

Show Check the security setting

Open Word, Excel, or PowerPoint.
On the Tools menu, click Options.
Click the Security tab.
Under Macro Security, click Macro Security.
Click the Security Level tab, and then verify that High is selected.

High Security

You'll receive the following message:

This file contains macros with an expired or revoked signature. Since you are running under High Security Level, these macros will be disabled.

Medium Security

You'll receive a security warning similar to the following.

Security Warning for Revoked Certificate

If you click Details, you'll see a message that the certificate has been revoked.

Revoked Digital Certificate Details

If you click View Certificate, you'll see another message on the General tab indicating that certificate has been revoked.

Revoked Digital Certificate