Safeguard your Office 2007 files with encryption, document protection, and digital signatures

In today's business environment, we expect more from office productivity software than just the ability to send and receive e-mail, format documents, produce spreadsheets, and create slide presentations. We also want a way to protect those files from unauthorized access, infiltration of malicious code, and even misuse by those with whom we share them.

Many security mechanisms to help protect our files are in play on modern networks. Firewalls help keep intruders from accessing the network on which the files are stored. Antivirus programs check documents and e-mail messages for viruses and other malware. IPSec can be used to encrypt data so that it can't be viewed at the packet level if it's captured while traveling across the network. Access controls, file level permissions, and EFS encryption can be used to keep unauthorized persons inside the organizations from gaining access to the files.

But the best security strategy is a multilayered one, and the more layers there are, the better. Even with network-level protections and the security mechanisms built into modern operating systems, it never hurts to have extra protections built into the applications, as well. Microsoft Office 2007 has a number of data protection features that help to protect the confidentiality and integrity of files created with Microsoft Word, Excel, and PowerPoint. In this article, we'll look at:

• How to use document encryption to password-protect documents.
• How to restrict formatting and editing of documents.
• How to use digital signatures to ensure that documents and messages aren't changed in transit.

In a follow-up article, we'll explain:

• How to use Information Rights Management (IRM) to keep recipients of documents and messages from copying or saving them or forwarding them to others.
• How to check documents for hidden personal information before sending them to others.
• How to use the Trust Center to protect your privacy and protect against harmful content in documents.

Document encryption/password protection

The password protection in Office 2003 was often criticized because of the weakness of its encryption. Although it used the RC4 stream cipher with a 128-bit key, the initialization vector (IV) and same keystream were used to encrypt the document each time you did so. That meant a hacker could crack the password by comparing two versions of a password-protected file. No hacking skills? No problem. Several software products are marketed as "password recovery tools" for cracking Office 2003 passwords.

Office 2007 uses AES (Advanced Encryption Standard) with a 128-bit key and SHA-1 hashing. For stronger protection, you can increase the key length to 256 bits by editing the registry or using Group Policy. This improves the security of password-protected files, especially when long, complex passwords are used.

Compatibility with previous versions of Office

What if you need to share encrypted files with users who are still running older versions of Office? Users running Office XP or Office 2003 can open and read files encrypted with the Office 2007 programs if:

• They are running an operating system that supports AES (Windows XP SP2, Windows Server 2003, or Windows Vista) and
• They install the Microsoft Office Compatibility Pack for Word, Excel, and PowerPoint 2007 File Formats (which can be downloaded from the Microsoft Web site.

How to encrypt a file in Office 2007

To encrypt a file with password protection in Word, Excel, or PowerPoint, start by clicking the Microsoft Office button at the top-left corner of the Office program window. Select Prepare and then Encrypt Document, as shown in Figure A.

Figure A

To encrypt a document, select Prepare from the main Office Logo menu.

Enter a strong password (at least eight characters, containing upper- and lowercase alpha characters, numbers, and symbols) into the Password field, as shown in Figure B. Retype the same password when prompted to do so.

Figure B

Enter a strong password to encrypt the contents of the file.

You can save the file in either the Office 2007 format (.docx, .xlsx, or .pptx) or in the Office 97-2003 format (.doc, .xls, or .ppt). If you attempt to save in the older format, you'll see a dialog box advising you that the XML formats provide stronger encryption and asking if you want to convert to the XML-based format to increase the security of the document, as shown in Figure C.

Figure C

The Office 2007 XML-based formats provide stronger encryption than the old Office 97-2003 formats.

Now when you or anyone else tries to open the document, a dialog box will open, prompting for the password, as shown in Figure D.

Figure D

You must enter the password to open the file.

In addition to the file encryption password you can set on a document, Office 2007 allows you to set a file-sharing password. The encryption option is called the password to open, and the file-sharing password is called the password to modify.

To set a password to modify, you use the Tools in the Save As dialog box (this is also another way to set a password to open). Just click the Office Logo button and select Save As. Then, in the Save As dialog box, enter a name for the file, click the down arrow beside Tools, and select General Options, as shown in Figure E.

Figure E

A second way to set a password is through the General Options selection in the Save As dialog box.

In the General Options dialog box, you can type a password to open and/or a password to modify, as shown in Figure F.

Figure F

You can set both a password to open and a password to modify.

Note that the file-sharing password does not encrypt the document and is not a security measure. The General Options dialog box also allows you to open the Macro Settings section of the Trust Center, which we'll discuss later in this article.

Formatting and editing restrictions

Clicking the Protect Document button in the General Options dialog box enables restriction of formatting and editing (which can also be done from the Protect Document selection on the Review tab of the Office ribbon). Sometimes, you want to distribute a document to others but don't want them to make changes to the formatting -- or want to limit them to only certain types of editing (such as inserting comments).

You select and apply formatting and editing settings through the task pane shown in Figure G.

Figure G

Enabling formatting and editing restrictions with the Protect Document button opens the Restrict Formatting And Editing task pane.

You can limit formatting to only the style(s) you used, as shown in Figure H.

Figure H

You can restrict formatting to selected styles only.

This prevents others from changing the styles or applying direct formatting to the document. You can also choose whether to allow AutoFormat to override the formatting restrictions, to block theme or scheme switching, and/or to block Quick Style Set switching. (By default, none of these options is enabled.)

If the document contains formatting or styles you have disallowed, you'll be asked if you want to remove them.

You can restrict editing to:

• No changes (read only)
• Comments
• Filling forms
• Tracked changes

If there are parts of the document you want others to be able to edit freely, you can designate those selections as unrestricted or you can allow only specified persons to change those parts of the document. This is done with the Exceptions option in the Formatting And Editing task pane.

Select the block of the document where you want to permit editing and either select Everyone from the Exceptions list or click More Users and enter the user names of those who should be permitted to edit the selection.

With this feature, you can allow one person to edit one section of the document, another person to edit the next section, and so forth. For example, you might allow someone from the legal department to edit a section that deals with legal issues but prevent that person from editing the sections that deal with technical issues.

Once you've set the restrictions and exceptions you want, click the Yes, Start Enforcing Protection button. Then, in the Start Enforcing Protection dialog box, shown in Figure I, choose the option to protect with a password only (no encryption) or with user authentication (document is encrypted and Restricted Access is enabled).

Figure I

You can enforce formatting and editing protection with a password only or with encryption

Digital signatures

Digital signatures are used to verify the authenticity of the creator or sender of a document or message and to ensure that the message hasn't been intercepted, with its content surreptitiously changed after it left the sender. (This is called document integrity.) Signing a document does not encrypt it.

How digital signatures work

Digital signatures use digital certificates that are issued by a trusted third party called a certification authority (CA). The CA is a server running certificate services software, such as the Certificate Services included in Windows 2000 Server and Windows Server 2003. The CA can be internal, running on a computer on a company's local area network, or it can be an external or public CA, such as those run by VeriSign, Thawte, and other companies. Either way, the CA verifies and vouches for the identity of the user or computer to which it issues a digital certificate.

The digital certificate is based on asymmetric or public key cryptography. The certificate contains the user's name and a public key that is associated with the private key the user uses to sign messages, as well as a serial number, expiration date, and the digital signature of the CA.

Compatibility with previous versions of Office

Although older versions of Office also support digital signing, they use a different format for digital signatures. Office 2007 programs use the XMLDSig format, which is not compatible with previous versions. Thus if a user running Word 2003 attempts to open a document that was signed in Office 2007, a dialog box will appear, informing the user that the digital signature has been lost.

How to add digital signatures to Office 2007 messages and documents

You can add digital signatures to Word, Excel, and PowerPoint 2007 documents and Outlook 2007 e-mail messages.

In Outlook, you can digitally sign individual messages or you can configure Outlook to digitally sign all outgoing messages. To sign a message individually, create the message and then, on the Message tab, click beside Options to open the Message Options dialog box shown in Figure J.

Figure J

Click Security Settings to add a digital signature to an e-mail message.

Click the Security Settings button and then select the Add Digital Signature To This Message check box, as shown in Figure K.

Figure K

You can send the signed message as clear text and/or request a Security MIME receipt.

If you don't already have a digital certificate associated with the e-mail address from which you're sending the message, you'll see an Invalid Certificate warning, which tells you how to get a digital ID to use with the account, as shown in Figure L.

Figure L

If you don't have a digital ID to use with the e-mail account, you'll get an Invalid Certificate message.

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Tip

You can get a free digital ID for personal e-mail from Thawte.

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To digitally sign all outgoing messages automatically, you use the Trust Center, which we will discuss in our follow-up to this article.

You can digitally sign a Word, Excel, or PowerPoint file in one of two ways:

• Transparently
• With a signature line

A transparent signature provides assurance of the authenticity and integrity of the document without being visible within the document itself. A Signatures button shows up on the status bar at the bottom of the application window. After you add the signature, the document becomes Read Only so that it can't be changed.

To add a transparent signature, click the Microsoft Office button at the top-left corner of the application window and select Prepare, then Add A Digital Signature, as shown in Figure M.

Figure M

Add a digital signature to a Word, Excel, or PowerPoint file from the Microsoft Office menu.

If you haven't yet saved the document, you'll be notified that you must save it before you can sign it, and the Save As dialog box will appear to allow you to do so. Then, the Sign dialog box appears, showing the user name that will be used to sign the document and providing a space where you can enter a reason for signing the document, as shown in Figure N. You can leave the Purpose field blank if you wish.

Figure N

Click the Sign button to sign the document.

You can change the user by clicking the Change button. Only user names that have certificates will be available. To sign the file, click the Sign button. A Signature Confirmation dialog box will appear, notifying you that the signature has been saved with the document. Now if the document is changed from this moment forward, the signature will become invalid and anyone who opens the document will be notified to that effect. If there are problems with a signature, the Signatures task pane will display, indicating the problem.

A small red ribbon icon in the status bar of the application indicates that a document has been signed.

You can also add digital signature lines to a document, so that signers can add their own digital signatures. To do so, click the Insert tab on the Office ribbon, click the Signature Line button, and then select Microsoft Office Signature Line. This will open the Signature Setup dialog box, shown in Figure O.

Figure O

You can insert signature lines where you and/or others can sign the document.

Here, you insert the signer's name, title, and e-mail address, as well as any special instructions to the signer(s). You can insert multiple signature lines in a document.

You can allow the signer to add comments in the Sign dialog if you want (this is disabled by default) and you can have the Office application automatically insert the date in the signature line (this is enabled by default).

The signature line will be inserted into the document as shown in Figure P.

Figure P

The signature line is inserted into the document and awaits the user's signature.

When you double-click the signature line, the Sign dialog box will appear, as shown in Figure Q.

Figure Q

The Sign dialog box allows the signer to sign the signature line.

The document signer signs the document by doing one of the following:

• Typing his/her name into the signature field
• Handwriting his/her name with a pen (in a Tablet PC)
• Inserting a graphic file containing an image of his/her handwritten signature

After the document is signed, a dialog box notifies you that the signature has been saved with the document and the date is inserted (if you checked that option) above the typed or handwritten signature in the document, as shown in Figure R.

Figure R

The typed or handwritten signature appears in the document and the digitally signature is saved with the document.

The Signatures task pane will appear on the right side when the document is opened in Office 2007, notifying you that the document is signed. Any changes will invalidate the signature and the red ribbon icon will appear in the status bar, as shown in Figure S.

Figure S

The Signatures task pane indicates that the signature is valid.

Summary
A number of mechanisms are built into Office 2007 that can be used to protect your documents and e-mail messages. In this article, we looked at three of them: document encryption, formatting and editing restrictions, and digital signatures. In our next article, we'll discuss Information Rights Management (IRM), checking documents for personal information before sending them, and using the Trust Center