Secure Communications over Insecure Channels Using an Authenticated Channel

A secure peer-to-peer communication over an insecure channel without any prior exchanged key can be established with the help of an authentication step to exchange a public key. Then, standard methods of public-key cryptography such as RSA can be used to communicate securely. In this work, we concentrate on message authentication protocols which require an extra authenticated channel. We start by describing some possible human communications channels, such as telephone, and by analyzing them according to some properties, in particular authentication properties. Then, we recall some message authentication protocols which use an authenticated extra channel. In addition, we recall different types of authentication. For instance, we recall biometrics-based systems which use the ability of humans to recognize the voice of the distant user. We recall also distance bounding-based systems which assumes that there is no other systems in the “integrity area”. In a second step, we prove the maximal security of a message authentication protocol against adversaries and we show that a protocol using k-bit authenticated strings reaches the maximal security when the distribution among all possible authenticated strings is uniform. More precisely, we sketch three generic attacks against any message authentication protocol. Using these results, we study the security of different authentication protocols, either noninteractive or interactive. In addition, we propose a new protocol which achieves the same security level against offline attacks as that of the one used today in many systems, such as SSH or GPG, but using much less authenticated bits. Finally, we compare interactive and non-interactive authentication protocols and we study their usability in different applications.