security: Update README.md

This adds more information into the `security/README.md` file detailing
what exactly the OPI Security APIs are currently targeting, and the
proposed architecture around the IPsec portion of the APIs.

I also collapsed the security-spec.md file into the README.md file.

Signed-off-by: Kyle Mestery <mestery@mestery.com>

security/README.md

@@ -1,5 +1,46 @@
 # OPI Security APIs
 
+## Documentation for Reference
+
+* [strongSwan VICI Protocol](https://www.strongswan.org/apidoc/md_src_libcharon_plugins_vici_README.html)
+* [vici API](https://github.com/strongswan/strongswan/blob/master/src/libcharon/plugins/vici/README.md)
+* [govici](https://github.com/strongswan/govici)
+
+## Terminology
+
+| Term              | Definition                                       |
+|-------------------|--------------------------------------------------|
+| IKE               | Internet Key Exchange is the protocol used to setup security associations in the IPsec suite. |
+| ESP               | Encapsulating Security Payload provides origin authenticity through source authentication, data integrity through hash functions and confidentiality through encryption protection for IP packets. |
+
+## Objective
+
+To define an industry standard "OPI Security Interface" for DPUs and IPUs that
+will enable vendors to use the protobuf files from the security API, and expose
+those externally and work across a number of orchestration systems. The Storage
+solution is one part of a higher-level architecture API defined for DPUs and
+IPUs as shown in the following image:
+
+![OPI Common APIs and SHIM abstraction layer](../doc/images/API-GW-Layers.png/)
+
+This document focuses specifically on the **OPI Security API Service**, and
+even more specifically. currently on the IPsec portion of that API.
+
+## Architecture
+
+The OPI Security APIs are currently focusing on implementing an IPsec API which
+maps on top of the [strongSwan](https://www.strongswan.org)
+[vici Plugin](https://docs.strongswan.org/docs/5.9/plugins/vici.html). For
+more details on the vici API, you can look at the strongSwan documentation
+found [here](https://github.com/strongswan/strongswan/blob/master/src/libcharon/plugins/vici/README.md).
+
+The architecture is seen in the diagram below.
+
+![OPI IPsec Security Architecture](https://github.com/opiproject/opi-poc/blob/main/security/sec-architecture.drawio.png)
+
+The idea here is that DPU and IPU vendors will implement strongSwan plugins to
+offload the tunnels into hardware.
+
 ## OPI Security API Spec
 
 The [Security Spec](security-spec.md) is the best place for up to date information on the OPI