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Home » Newsletter » September 2007

By Raju Sinha

An encrypted PDF document places certain restrictions on how the document can be used - to open it, to modify it, to print it, to copy content, etc... An encrypted document may require either one of two passwords - owner or user password. If you provide the owner password, you have full access to the document including the ability to specify new password and change document restrictions. If you provide the user password, you will be able to open the document but may be subject to restrictions placed by the owner of the document. If a document creator encrypted a document without specifying a user password, then anyone can open the document without providing a password althought document restrictions would still apply.

This month's tip is about how to encrypt a document and place restrictions on its users.

To encrypt a document, you have to set PDFDocument.Security.Enabled property to true. (The PDFDocument.Security internally uses an object of the PDFSecurity class.) Here is the C# code to encrypt a PDF:

// Create a new document object
PDFDocument doc = new PDFDocument();

// Load an existing PDF
doc.Load("unencrypted_doc.pdf");

// Enable document encryption
doc.Security.Enabled = true;
doc.Security.Level = PDFEncryptionLevel.AES_128bit;

// Set passwords
doc.Security.UserPassword = "abc";
doc.Security.OwnerPassword = "xyz";  

// Specify document restrictions
doc.Security.UserPermissions = 
    PDFUserPermissions.Commenting | 
    PDFUserPermissions.DocAssembly | 
    PDFUserPermissions.HighResPrint;

// Write encrypted document to file stream
doc.Save("encrypted_doc.pdf");

// Dispose the document object
doc.Close()

In the above code, you will note that the PDFSecurity.UserPermissions property can take a combination of values from the PDFUserPermissions enumeration.

To decrypt an encrypted document, the password is supplied by an event handler registered with the PDFDocument.Password event. This event occurs when you call PDFDocument.Load() method with an encrypted document. Here is the C# code to decrypt a PDF:

// Create a new PDFDocument object
PDFDocument doc = new PDFDocument();

// Specify Password event handler
doc.Password += new PasswordEventHandler(password);

// Load encrypted document
doc.Load("encrypted_doc.pdf"); // Calls Password event handler

// Disable document encryption
doc.Security.Enabled = false;

// Write unencrypted document to file stream
doc.Save("Decryption.pdf");

// Dispose the document object
doc.Close();

The event handler can be implemented as below:

public void password(object d, ref string str, ref bool b) {
 // Supply owner password to decrypt document
 str = "xyz";   // "xyz" is the owner password
}