  The dynamic establishment of shared information (e.g. secret key) between two
entities is particularly important in networks with no pre-determined structure
such as wireless sensor networks (and in general wireless mobile ad-hoc
networks). In such networks, nodes establish and terminate communication
sessions dynamically with other nodes which may have never been encountered
before, in order to somehow exchange information which will enable them to
subsequently communicate in a secure manner. In this paper we give and
theoretically analyze a series of protocols that enables two entities that have
never encountered each other before to establish a shared piece of information
for use as a key in setting up a secure communication session with the aid of a
shared key encryption algorithm. These protocols do not require previous
pre-distribution of candidate keys or some other piece of information of
specialized form except a small seed value, from which the two entities can
produce arbitrarily long strings with many similarities.
