Encryption Using Certificates

PDF authors can encrypt documents with selective permissions and distribute the files to a number of different Adobe Reader users. Using a user's public certificate, authors can encrypt a PDF document with password protection that allows only the intended party to view the file and denies all other users access to the document. PDF authors can protect the file against unauthorized access and prevent other users from accessing the document and any file attachments contained in the document.

To open files that have been encrypted with public certificates:

1.	Open a file that has been encrypted using your public certificate. If you don't have such a file, contact a PDF author who can encrypt a document for you. You need to create your personal Digital ID and send the PDF author your public certificate. The PDF author then adds your certificate to his or her list of trusted identities, and, when preparing the PDF file for your use, and using your public certificate, selects permissions options that are saved with the file.
2.	Click the Open tool in Adobe Reader and select a PDF document encrypted using your public certificate.
3.	The first thing that opens is a dialog prompting you for your password (Figure 17.26). Type the password you used when you created your Digital ID and click OK. Figure 17.26. Type the same password you used when you created your Digital ID. NOTE Passwords are case-sensitive. Be certain to type your password using the same letter case as when you first created your Digital ID.
4.	If you type your password correctly, the document opens. PDF authors can assign different permissions to the file. Files can be protected against editing and printing. To determine what permissions are assigned to encrypted files, press Ctrl/Command+D to open the Document Properties dialog.
5.	Click the Security tab. In the Document Restrictions Summary, you can review the permissions assigned to the document. In Figure 17.27, notice that printing the PDF document and filling in form fields are not allowed. Figure 17.27. Click the Security tab in the Document Properties dialog to review a restrictions summary.
6.	Click OK in the Document Properties to return to the file in the Document pane.

Do PDF authors who encrypt PDF documents always need to acquire public certificates?

PDF authors have several options for protecting PDF documents. A PDF author can use a Password Security option and add security to a PDF file using a password of the PDF author's choice. Any user knowing the password can open the PDF document.

When Certificate Security is used, PDF documents are opened only when a password used with your Digital ID is supplied in the opening dialog that prompts you for a password. This form of encryption has several benefits. First, the PDF author does not need to send you a password to open the document; not distributing passwords provides you and the PDF author with greater protection against unauthorized viewing. Second, the same PDF document can be encrypted with different permissions rights for different users. For example, a PDF author can grant you permission to fill in form fields and print a document. The same file sent to another user can be assigned permissions that prevent printing. Again, the same document can be sent to a third user preventing that user from filling in form fields. Each user opens the document using personal passwords derived from Digital IDs and public certificates.