If you visit a Web site you have never visited before, you may find an unusual dialog box asking if you want to download a control. Dialog boxes of this kind are actually certificates that let you know if the control has been digitally signed by someone you may be able to trust.

Digital signatures

A digital signature helps you know that a file or program you are about to download has come from a trusted source. For example, if you have created a program file that will be distributed via a vast network (such as the Internet) and you want people to know that the program file is indeed from you, you can digitally sign the file. When you digitally sign a program file, the executable data in the file is not altered. Instead, a special digital signature string is encoded with the data itself. Basically, the signature certifies that you did indeed create the file.

How does this work? By using special software such as Microsoft Authenticode™ technology, a special set of software key algorithms is bundled with the data file. The trick is that when a user decides to download the file, a digital signature appears to help inform the user whether or not the file has been signed.

Digital certificates

Digital certificates are typically issued by trusted third parties known as certification authorities (CAs), such as VeriSign, Inc. When you download a file, the certificate helps verify that the keys used to sign the file are valid. So, if a digital certificate verifies the signature, then the program file was encrypted with a matching software key and the file has not been tampered with.

What about Office 97?

Office 97 applications do not work with digital signatures and certificates. However, with Office 2000 you are able to sign documents that contain Visual Basic® for Applications projects. Since Office 2000 programs use the same file formats as Office 97 programs, you can open Office 2000 documents in Office 97. If a document contains a signed Visual Basic macro, you will be able to view the macro project in Office 97 but you will not be able to change the macro. This will help protect the project as it is round-tripped between Office 97 and Office 2000.