D2D ("Drupal-to-Drupal") is a module to built-up a network among Drupal instances using cryptography and XML-RPC.
An instance holds a public/private key pair allowing messages to be encrypted and/or to be signed. The concept of friendship among Drupal instances is introduced. Instances can send friend requests, accept friendship requests and also terminate friendship.
An instance is identified using a globally unique D2D-id. The aim of this id is to solve the naming problem for D2D instances, i.e. you can uniquely refer to a certain instance by this D2D-id, compare the public keys you have stored for an instance with the ones your friends store etc..
Using public key cryptography, an instance can implement secure XML-RPC methods that can be called by friend instances. Friends are organized in groups to allow privileged access to particular methods.