Nowadays, instant messaging systems achieve a great success and became the main mean of communication between people via an internet. Thanks to the simplicity and quickness of the message exchanging more and more people over the world start to use instant messengers on daily basis. However, such a great attention forces us to discuss another aspect of these systems, an aspect of the information security and user privacy. The main aim of this thesis is to design and implement an instant messaging system that copes with the required functionalities and satisfies the defined security requirements.