This document tries to explain basic principles behind security on the SAFE network.
Big part of algorihms behind the SAFE network rely on so called local group consensus. What it basically means is that for a node to act in some way, it needs to be instructed to do so by a quorum of other nodes. The set of these nodes is determined by the network, but in general they are expected to be as close as possible to a certain HASH address within the XOR network.
We generally call such a set a Group(H) where H is the HASH address or simply a Group when H is known from the context.
The key idea is the assumption that it is cryptographically hard for an adversary to enter an arbitrary group of close nodes. The purpose of the Sentinel is then to ensure that the set of these instructing nodes really lives in a Group surrounding the given HASH.