The present invention relates to a method for establishing a secured communication session in a communication system between a user using an untrusted device and a server. According to the present invention the user first obtains an authentication algorithm and an encryption algorithm and then creates a session key. Next the user obtains a public key of the server and sends a personal identity number to the server for authentication by using the authentication algorithm, the personal identity number being encrypted by using the encryption algorithm and the public key of the server. The user also sends the session key to the server for encrypting purpose between the user and the server, the session key being encrypted by using the encryption algorithm and the public key of the server.