This diploma thesis deals with certification and certification authorities, certification path PKI and principles of its validation and security. Also deals with structure of certificate itself and possible misuse of included information. Moreover, possibility of misues of third party certificates and proclamation of untrusted certificate to client's system.

Diploma thesis is focused on matters of electronic signatures and their possible usage in accounting and business process. The paper is divided into two parts. Subject of the first part is general analysis of terminology and characteristics of electronic signature, certificates and certification authorities and time stamps, including analysis of strengths and weaknesses of these services. At the end of this theory part are outlined some of the most common choices of using electronic signature in accounting system. In second part are analysed results of research, which was performed by sending questionnaire to a number of randomly chosen companies.