xt challenge, he can ask the right response in advance. Access restrictions by means of authentication is nothing new. An interesting possibility deployed by the old Egyptians and Greeks, who did not have identity cards, is to use biometrical methods. 'Biometry' means 'measurements of humans' with original measures such as length, eye colour, skin colour and scars. A true personal identification can only be achieved using unique personal properties. A key, a personal code or a photo can all be misused. Two fairly cheap alternatives are voice recognition and personal signatures. Voice recognition is tailored to voice communication, and therefore easy to introduce for voice supporting media. This type of verification requires storage of voice prints. The problem with voice recognition is that any type of background noise, or voice changes because of a cold, can cause legitimate calls to be blocked. A more futuristic alternative is scanning of the iris. See biometrics below. Even simpler is a system that is used for personal verification of roamers, who are routed to a call centre, where they verify their identities by answering one or more short questions. However, some people consider the system inconvenient, which may lead to churn. Some common authentication methods use RADIUS (remote authentication dial-in user service) and LDAP (lightweight directory access protocol). See below. It is important to understand that the right response is not a guarantee that we are directly communicating with the right party. There is a very serious attack called 'man in the middle'. Let us take an example from the real world. Mallory claims that he is a master chess player although he is not. If some chess master challenges him to play, Mallory arranges the game to happen in the computer network. Then Mallory challenges still another chess master. When the first master makes a move, Mallory mediates it to another master. They both think that they are playing with Mallory - but actually they Security are playing against each othe