 for LDAP. LDAP is also a base for public key infrastructure (PKI). Almost all PKI systems use LDAP servers to search, verify and revoke certificates. The LDAP transfer must be safe, in particular when handling public keys. What if someone else pretends to be the LDAP directory? LDAP can be encapsulated in SSL or TLS. 6.8 INTEGRITY BY ACCESS CONTROL - AUTHORIZATION IN ENTERPRISES Data that is available in an internal company system must be protected. Only authorized staff must be able to access, read and change such data. Such security functions are built into most network operating systems. There are in principle two common ways to solve the problem: Users must log on with user identification and a password to get access to the network. In addition, different resources can be allocated to different users. The network is open to all, but when you want access to a specific resource you have to use a password which is related to that resource. It is possible to have many levels of security within a company or system. A good idea is to avoid using more security than necessary to avoid unnecessary inconveniences. What is experienced as complications can lead to confusion and less efficiency. However, one of the greatest threats in all computer systems is the company's own staff. Most of the investigations done in this area show that the staff inside the company is one of the most frequent intruders in computer systems. 6.9 INTEGRITY BY ACCESS CONTROL - FIREWALLS 6.9.1 WHAT IS A FIREWALL? In general, a firewall is a node (computer and/or router) often located at the border between the public Internet and servers or a private intranet. See Figure 6.18. The purpose of Public Internet Firewall Figure 6.18 Location of a firewall Understanding Changing Telecommunications - Building a Successful Telecom Business firewall is to block access to the intranet from the public Internet. People outside the intranet must not be able to access information inside the intranet or perform actions such as destroying information. At the 