Digital Signatures for Macros

When you create macros and share them with others, the availability of those macros depends on the security settings on the user's machine. If the security level is set high enough, the user may not even be able to use the macros at all.

One way to help users utilize your macros is to digitally sign them. This capability was introduced by Microsoft in Excel 2002. A digital signature allows a user to know that a macro comes from a trusted source and that it hasn't been modified since it was originally saved by that trusted source. In other words, it is a way for users to be sure that a macro hasn't been tampered with. (Sort of like the product safety seals on some consumer foods and pharmaceuticals.)

In order to digitally sign a macro, you need to first obtain a digital certificate. A certificate is a "seal of approval" from a trusted third party that you are who you say you are. You can get digital certificates from a variety of commercial certificate authorities, each of which has different requirements of how you go about certifying your identity.

You can also create your own digital certificate for testing purposes using the program SelfCert.exe, which is provided with Microsoft Office 2002 and 2003. This route is great for testing, but it won't help you when you distribute your macros to others; you'll still need the certificate from the third-party authority. You can find more information about the SelfCert.exe program by using Excel's online help and searching for "selfcert."

Once you have a digital certificate, you can digitally sign your macro project in this manner:

1. In the Visual Basic Editor, use the Project Explorer to select the project you want to sign.
2. Choose the Digital Signature option from the Tools menu. Excel displays the Digital Signature dialog box. (Click here to see a related figure.)
3. If there is no digital certificate associated with the workbook, or if you want to use a different digital certificate to sign the macro project than what you used for the workbook, click Choose. You can then select which available certificate you want to use.
4. Click OK to dismiss the Digital Signature dialog box. The certificate you selected (or the certificate used for the workbook) is then used to sign the macro project.

You can find more information about digital signatures in Excel's Help system. You can also find some great information about both certificates and signatures at this page in the Knowledge Base:

http://support.microsoft.com/?kbid=820738