Secure communications requires the exchange of keying material, which in general is not trivial problem. A simple solution is to use alternative communication channels to exchange the cryptographic keys, like standard mail services or reciprocal visual inspection of text strings. Here, we propose to use the standard Public Mobile Network (PMN) as an alternative channel, because the use of mobile phones has become pervasive and affordable for most of users. The basic assumption is that the PMN is more secure than other wireless and wired networks. We envision a system for subscribers who wish to exchange their cryptographic keys, which can be used afterwards for sending encrypted messages over other (insecure) communication channels, like Internet. We assume that every user or its mobile phone is able 1) to generate a public/private key pair, and 2) to store it inside his/her mobile phone rubric. The public key is exchanged by sending special requests by means of standard PMN services, like the text messaging system. We analyze the scalability of such a system, by assuming that the subscribers can send group queries, i.e. queries which request the whole (public) keys stored in the rubrics of a subset of the closest neighbors of an user. The performance of such an approach depends on the properties of the graph model of interactions among people. By means of simulations, we show that it is preferable to send few group queries instead of many single requests. This result can be used to dimension the service provided by the PMN.
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