document agenix integration

Co-authored-by: David Arnold <dar@xoe.solutions>

doc/secrets.md

@@ -1,18 +1,110 @@
 # Secrets
-Secrets are managed using [git-crypt][git-crypt] so you can keep your flake in
+Secrets are managed using [git-crypt][git-crypt] and [agenix][agenix]
-a public repository like GitHub without exposing your password or other
+so you can keep your flake in a public repository like GitHub without
-sensitive data.
+exposing your password or other sensitive data.
 
 By default, everything in the secrets folder is automatically encrypted. Just
 be sure to run `git-crypt init` before putting anything in here.
 
+## Agenix
+Currently, there is [no mechanism][secrets-issue] in nix itself to deploy secrets
+within the nix store because it is world-readable.
+
+Most NixOS modules have the ability to set options to files in the system, outside
+the nix store, that contain sensitive information. You can use [agenix][agenix]
+to easily setup those secret files declaratively.
+
+[agenix][agenix] encrypts secrets and stores them as .age files in your repository.
+Age files are encrypted with multiple ssh public keys, so any host or user with a
+matching ssh private key can read the data. The [age module][age module] will add those
+encrypted files to the nix store and decrypt them on activation to `/run/secrets`.
+
+### Setup
+All hosts must have openssh enabled, this is done by default in the core profile.
+
+You need to populate your `secrets/secrets.nix` with the proper ssh public keys.
+Be extra careful to make sure you only add public keys, you should never share a
+private key!!
+
+secrets/secrets.nix:
+```nix
+let
+  system = "<system ssh key>";
+  user = "<user ssh key>";
+  allKeys = [ system user ];
+in
+```
+
+On most systems, you can get your systems ssh public key from `/etc/ssh/ssh_host_ed25519_key.pub`. If
+this file doesn't exist you likely need to enable openssh and rebuild your system.
+
+Your users ssh public key is probably stored in `~/.ssh/id_ed25519.pub` or
+`~/.ssh/id_rsa.pub`. If you haven't generated a ssh key yet, be sure do so:
+```sh
+ssh-keygen -t ed25519
+```
+
 > ##### _Note:_
-> Currently, there is [no mechanism][secrets-issue] in nix to deploy secrets
+> The underlying tool used by agenix, rage, doesn't work well with password protected
-> within the nix/store so, if they end up in the nix/store after deployment, they
+> ssh keys. So if you have lots of secrets you might have to type in your password many
-> will be world readable on that machine.
+> times.
->
+
-> The author of devos intends to implement a workaround for this situation in
+
-> the near future, but for the time being, simple be aware of this.
+### Secrets
+You will need the `agenix` command to create secrets. DevOS conveniently provides that
+in the devShell, so just run `nix develop` whenever you want to edit secrets. Make sure
+to always run `agenix` while in the `secrets/` folder, so it can pick up your `secrets.nix`.
+
+To create secrets, simply add lines to your `secrets/secrets.nix`:
+```
+let
+  ...
+  allKeys = [ system user ];
+in
+{
+  "secret.age".publicKeys = allKeys;
+}
+```
+That would tell agenix to create a `secret.age` file that is encrypted with the `system`
+and `user` ssh public key.
+
+Then go into the `secrets` folder and run:
+```sh
+agenix -e secret.age
+```
+This will create the `secret.age`, if it doesn't already exist, and allow you to edit it.
+
+If you ever change the `publicKeys` entry of any secret make sure to rekey the secrets:
+```sh
+agenix --rekey
+```
+
+### Usage
+Once you have your secret file encrypted and ready to use, you can utilize the [age module][age module]
+to ensure that your secrets end up in `/run/secrets`.
+
+In any profile that uses a NixOS module that requires a secret you can enable a particular secret like so:
+
+```nix
+{ self, ... }:
+{
+  age.secrets.mysecret.file = "${self}/secrets/mysecret.age";
+}
+```
+
+
+Then you can just pass the path `/run/secrets/mysecret` to the module.
+
+You can make use of the many options provided by the age module to customize where and how
+secrets get decrypted. You can learn about them by looking at the
+[age module][age module].
+
+
+> ##### _Note:_
+> You can take a look at the [agenix repository][agenix] for more information
+> about the tool.
 
 [git-crypt]: https://github.com/AGWA/git-crypt
+[agenix]: https://github.com/ryantm/agenix
+[age module]: https://github.com/ryantm/agenix/blob/master/modules/age.nix
 [secrets-issue]: https://github.com/NixOS/nix/issues/8