Aristide Tsemo In this paper, we study cryptography from a geometrical viewpoint. Let N be a network, we endow N with a natural Grothendieck topology. We use geometric representations of cohomological classes to define encryptions protocols. Link to link encryption is related to the notion of torsors. We use the notion of connective structure defined on a gerbe to define public encryption. We study statistical properties of data conveyed in a network, and define an entropy cocycle. https://arxiv.org/abs/math/0608790