Secure End-to-End Communications in Mobile Networks

Cellular communication has become an important part of our daily life. Besides using cell phones for voice communication, we are now able to access the Internet, conduct monetary transactions, send voice, video and text messages and new services continue to be added. The frequencies over which voice is transmitted are public, so voice encryption is necessary to avoid interception of the signal over the air. But once the signal reaches the operators Base Station (BS), it will be transmitted to the receiver over a wired or wireless mean. In either case, no protection is defined. This does not seem a problem, but this is not true. Along the path across operator network, voice is at risk. It will only be encrypted again, with a different key, from the BS to the receiver if the receiver is herself a mobile user. Moreover, voice encryption is not mandatory. The choice whether or not to accept an unprotected communication is up to the network. When adopted, the same encryption algorithm is used for sending SMS messages between mobile telephones and base stations and for encrypting of calls. Unfortunately, vulnerabilities in this encryption systems were already revealed more than 10 years ago and more continue to be discovered. Currently the most popular communication technologies are the GSM and the UMTS. The UMTS is in use as a successor to GSM. Along with mobile phone services, It provides rapid data communication. The security algorithms in UMTS differs from GSM in two important ways: encryption and mutual authentication. Although security standards have been improved, the endto-end security is not provided. At the time of this writing, the user who makes a call with another, remote, user by means of a mobile network, is subject to several threats: phishing, session hijacking and eavesdropping are such examples. This is mainly due to the lack of either mechanisms for mutual authentication of the end point of the conversation and the end-to-end digital encryption of the content of the conversation. In this Thesis we first give an overview about several generations of cellular networks such as GSM and 3G and discuss their security issues and different types of attacks. Then, we introduce novel end-to-end security systems SPEECH and SEESMS for voice and text communications, respectively. Finally, we move our attention to video-communication over UMTS. In particular, we propose an end-to-end security systems for videotelephony named SECR3T. SPEECH is a software for making secure calls by using a Windows Mobile powered handheld device and the GSM data communication channel. The notion of security implemented by SPEECH is stronger than that available in other secure conversation software, it includes the mutual authentication of the end point of a conversation, the end-to-end digital encryption of the content and the possibility to digitally sign the content for non-repudiation purpose. SEESMS is a software framework written in Java which allows two peers to exchange encrypted and digitally signed SMS messages. The communication between peers is secured by using public-key cryptography. The key-exchange process is implemented by using a novel and simple security protocol which minimizes the number of SMS messages to use. SECR3T is a full fledged secure communication system for mobile devices based on the Circuit Switched Domain (CSD, in brief) of 3G networks. The use of CSD is the most innovative contribute of the project since, to the best of our knowledge, no solution is available for such a channel. Similarly to the SPEECH project, the notion of Security implemented by SECR3T includes the mutual endpoint authentication and the end-to-end channel encryption. A SECR3T packet is recognized by the network as a frame of the native videotelephony protocol which is 3G-324M. This allows a SECR3T client to communicate also with a different video-client in a non-secured mode.