Files which are passed on to third parties can generally also be edited by those third parties. This is not always in the interest of the creator of the files. A form of protection against subsequent changes, the forwarding of extracts or further processing would therefore be desirable.
A problem that is often encountered is that information is made available to third parties via the Internet or other networks, but it may not be intended that it should be printed out hundreds of times or integrated seamlessly into other documents.
There are various solutions to this, which in some cases can also be combined with each other. Examples here are:
Using digital signatures in order to prevent changes from being made to files without this being noticed (see also S 4.34 Using encryption, checksums or digital signatures or S 3.23 Introduction to basic cryptographic terms).
Adding copyright notices to WWW information or files. These can be formulated as follows: "This document and all of its parts is protected by copyright. Any use of the document outside the constraints of the Copyright Act without the approval of the author is inadmissible and punishable." and "Copyright ((c)) 7/1999 by BSI".
The use of file formats which make it more difficult to make subsequent changes or edit extracts in any way. Postscript can be used for this, for example, or the security properties of application programs, such as in the case of PDF files.
PDF documents can be assigned access restrictions when they are created. The opening, printing or copying of PDF files can be inhibited, for example.
Two types of password can be assigned with Acrobat Exchange, i.e. the application with which PDF files can be created and edited. One type of password is required for opening the document, and the other for changing the security attributes. PDF documents that are protected against unauthorised opening are encrypted with RC4. The following functions can be inhibited using the security attributes:
Printing
Modifying the document
Selecting text or graphics
Adding or changing notes and form fields
In this way it is very easy to restrict rights so that no-one can take over the contents of a publication by cutting and pasting. If in extreme cases even printing is prevented, all that can be done is read the file on-line.
Unfortunately this offers only rudimentary protection, because PDF files can also be opened with programs which ignore these security attributes. As long as printing is allowed, for example, the document can even be converted back into a PDF file at any time, with no restrictions whatsoever.
