Add summary

Author: JimMadge

docs/architecture/defence_in_depth.md

@@ -1,15 +1,34 @@
 (arch-defence)=
 # Defence in Depth
 
-- Defence in depth
-  - at K8s
-    - PSS
-    - Network (Cilium)
-    - RBAC (and namespaces)
-    - Data-at-rest encryption (protects from bad actors, compromise at infrastructure provider)
-  - at infrastructure
-    - network (vnet) isolation (out of band!)
-  - Compromising the host is very unlikely
-    - And even if it does happen, and privilege escalation occurs, there is no access to other networks
-    - No access to data from other projects
-    - The worst that can happen is a researcher trashes their own environment
+A FRIDGE instance is a {term}`TRE <Trusted Research Environment>` and so needs to be designed to provide a high level of protection.
+Some aspects of security are enforced by the [Kubernetes clusters](#arch-arch-tenancy-network) and others at the [infrastructure level](#arch-arch-tenancy-requirements).
+Therefore, responsibility for the security of FRIDGE is [shared](#arch-shared-responsibility) between the {term}`TRE Operator Organisation` and {term}`FRIDGE Hosting Organisation`.
+
+A principle in the design of FRIDGE was defence in depth.
+That is, that no single failure should make it possible for sensitive data to be exfiltrated from the TRE in an unauthorised manner.
+Furthermore, the goal should be for security features to be enforced by two independent systems or roles so that a compromise of one is not sufficient to break security.
+
+## Networking
+
+- Network (Cilium)
+- at infrastructure network (vnet) isolation (out of band!)
+
+## Privilege Escalation
+
+### In Kubernetes
+
+- RBAC (and namespaces)
+
+### On Cluster Nodes
+
+- PSS
+- Compromising the host is very unlikely
+  - And even if it does happen, and privilege escalation occurs, there is no access to other networks
+  - No access to data from other projects
+  - The worst that can happen is a researcher trashes their own environment
+
+## Data
+
+- Data-at-rest encryption (protects from bad actors, compromise at infrastructure provider)
+  _e.g._ access from host

docs/architecture/shared_responsibility.md

@@ -1,3 +1,4 @@
+(arch-shared-responsibility)=
 # Shared Responsibility
 
 - Governance remains with the TRE