Start work on documenting authentication/security

docs/security.md

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+## Security Notes for Kubernetes
+
+## SSH Access
+
+SSH is allowed to the masters and the nodes, by default from anywhere.
+
+To change the CIDR allowed to access SSH (and HTTPS), set AdminAccess on the cluster spec.
+
+When using the default images, the SSH username will be `admin`, and the SSH private key is be
+the private key corresponding to the public key in `kops get secrets --type sshpublickey admin`.  When
+creating a new cluster, the SSH public key can be specified with the `--ssh-public-key` option, and it
+defaults to `~/.ssh/id_rsa.pub`.
+
+To change the SSH public key on an existing cluster:
+
+* `kops delete secret --name <clustername> sshpublickey admin`
+* `kops create secret --name <clustername> sshpublickey admin -i ~/.ssh/newkey.pub`
+* `kops update cluster --yes` to reconfigure the auto-scaling groups
+* `kops rolling-update --yes` to immediately roll all the machines so they have the new key (optional)
+
+
+## Kubernetes API
+
+(this section is a work in progress)
+
+Kubernetes has a number of authentication mechanisms:
+
+### API Bearer Token
+
+The API bearer token is a secret named 'admin'.
+
+`kops get secrets admin -oplaintext` will show it
+
+### Admin Access
+
+Access to the administrative API is stored in a secret named 'kube':
+
+`kops get secrets kube -oplaintext` or `kubectl config view --minify` to reveal