Excel 2003 Programming: A Developers Notebook (Developers Notebook)

6.1. Dress in Layers

When it's cold you dress in layers, and security works the same way (Figure 6-1). The outer layer is a firewall, preventing attacks from the Internet. Next, virus detection software scans permitted attachments and other files from bringing in malicious code. Then, operating system security defines users and their permissions. Finally, Excel provides its own security layer.

Data most at risk is that which is shared outside of these layers, such as a workbook posted on a public server. In that case, Excel becomes the primary security layer. Of course, not all data needs the same level (or type) of protection. Therefore, Excel itself provides layers through the following security approaches:

• Password-protection and encryption controls read and write access to workbooks.

• Worksheet protection password protects items within a workbook and, alternately, can authorize changes to the base of user lists.

• User-based permissions allow authors to limit the rights of others to read, change, print, copy, or distribute a document. Permissions can also set an expiration date for a document.

• Digital signatures identify the author of a document ensuring that a document is the authentic originalnot a modified or spoof copy. Signatures can also be applied to macros and ActiveX controls to ensure their code is from a trusted source.

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  Note: Code used in this chapter and additional samples are available in ch06.xls.

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• Macro security levels determine what level of trust is required before Excel will run code included in worksheets, templates, add-ins, or Smart Documents.

• ActiveX control security levels similarly limit which controls Excel will trust.

• Office Anti Virus API provides an interface for anti-virus software to scan documents for malicious code before they are opened.

• Custom installation wizard permits administrators to configure which security options are enabled during installation on user's machines.

These security approaches can be combined to provide a high level of assurance while still allowing files to be shared, macros to be run, and (ultimately) work to be done. The rest of this chapter discusses each of these approaches, along with Windows file security, then provides a list of common security tasks and describes how you complete those tasks by combining Excel security features.