Analog and digital plain old telephony service (POTS) is increasingly replaced by technologies that integrate voice, data, and video communication services. The integrated services digital network (ISDN) offers the embedding of this advanced multimedia communication into the subscriber loop. Nowadays widely available, ISDN builds a promising approach to improve company’s, public authority’s, or individual’s telecommunication facilities. However, a secure means of transferring sensitive information in any publicly accessible communication infrastructure is a major concern. In particular, authenticated and confidential communication is of paramount importance.