In general, you can perform all tests with both unprotected and password protected PDF documents. The syntax differs slightly in the instantiation method, a second parameter is required representing the password:

// Access to password protected PDF
AssertThat.document(filename, ownerPassword)  

// Test methods:
.hasEncryptionLength(..) 
.hasOwnerPassword(..) 
.hasUserPassword(..)

This syntax is the same for “user password” and “owner password”.

Opening a document with a password is already a test. But you can verify the second password with the methods hasOwnerPassword(..) or hasUserPassword(..):

// Verify the owner-password of the document:
@Test
public void hasOwnerPassword() throws Exception {
  String filename = "documentUnderTest.pdf";
  String userPassword = "user-password";
  
  AssertThat.document(filename, userPassword)    
            .hasOwnerPassword("owner-password")  
  ;
}

// Verify the user-password of the document:
@Test
public void hasUserPassword() throws Exception {
  String filename = "documentUnderTest.pdf";
  String ownerPassword = "owner-password";
  
  AssertThat.document(filename, ownerPassword)   
            .hasUserPassword("user-password")    
  ;
}

Usually it's bad practice to hard code passwords in the source code, but it's OK for test passwords in test environments.

This example shows how to verify the encryption length:

@Test
public void hasEncryptionLength() throws Exception {
  String filename = "documentUnderTest.pdf";
  String userPassword = "user-password";
  
  AssertThat.document(filename, userPassword)
            .hasEncryptionLength(128)
  ;
}