Initial synchronization (preliminary discussion)

All five schemes below begin with an initial synchronization protocol where each receiver compares its local time with that of the sender, and registers the difference. We remark that a rough upper bound on the clock difference is sufficient. In fact, all that the receiver needs is a value $\delta$ such that the sender's clock is no more than $\delta$ time-units ahead of the receiver's clock, where $\delta$ can be on the order of multiple seconds. In section 2.8 we describe a simple protocol and discuss scalability issues related to the initial synchronization.

A basic assumption that underlies the security of our scheme is that the local internal clocks of the sender and recipient do not drift too much during a session.