"Secrets, how the fuck do they work?"

docs/secrets.md

@@ -1,5 +1,108 @@
 ---
-title: Manually load secrets into Docker swarm
+title: Managing secret data
 ---
 
-TODO.
+# Managing secret data
+
+Co-op Cloud uses [Docker Secrets] to handle sensitive data, like database passwords and API keys, securely:
+
+```
+$ DOCKER_CONTEXT=swarm.example.com docker secret ls
+example_mediawiki_db_password_v1
+example_wordpress_db_password_v1
+```
+
+`abra` includes several commands to make it easier to manage secrets:
+ - `abra app <app> secret generate` -- to auto-generate a single secret, or all secrets defined by the application, and store them in the Docker Swarm store,
+ - `abra app <app> secret insert` -- to insert a single secret value from the Docker Swarm store,
+ - `abra app <app> secret delete` -- to remove a single secret, or all secrets defined in the application, from the Docker Swarm store.
+
+<a id="versions"></a>
+
+## Secret versions
+
+You will notice `v1` in the example secret names above: like Docker Configs, Docker Secrets are [immutable], which means that their values can't be changed after they're set. To accommodate this, Co-op Cloud uses the established convention of "secret versions". Every time you change (rotate) a secret, you will insert it as a new version.
+
+Because secret versions are managed per-instance by the people deploying their applications, secret versions are stored in the `.env` file for each application:
+
+```
+$ find -L ~/.abra/servers/ -name '*.env' -print0 | xargs -0 grep -h SECRET
+OIDC_CLIENT_SECRET_VERSION=v1
+RPC_SECRET_VERSION=v1
+CLIENT_SECRET_VERSION=v1
+...
+```
+
+If you try and add a secret version which already exists, Docker will helpfully complain:
+
+```
+$ abra app example_wordpress secret insert db_password v1 foobar
+Error response from daemon: rpc error: code = AlreadyExists desc = secret example_wordpress_db_password_v1 already exists
+```
+
+By default, new app instances will look for `v1` secrets.
+
+## Generating secrets automatically
+
+You can generate secrets in one of two ways:
+
+1. While running `abra app new <type>`, by passing `--secrets`
+2. At any point once an app instance is defined, by running `abra app <app> secret generate ...` (see `abra help secret generate` for full options)
+
+!!! note "How are secrets generated?"
+
+		Depending on how the application is configured, you will require the `pwqgen` (from `passwdqc`) and `pwgen` binaries by default, although you can specify your own password-generation app when running `abra <app> secret generate` by providing the `<cmd>` argument.
+
+## Inserting secrets manually
+
+For third-party API tokens, like OAuth client secrets, or keys for services like Mailgun, you will be storing values you already have as the appropriately-named Docker secrets. `abra` provides a convenient interface to the underlying `docker secret create` command:
+
+```
+abra app example_wordpress secret insert db_password v2 "your-secret-here"
+```
+
+## Rotating a secret
+
+So, given how [secret versions](#versions) work, here's how you change a secret:
+
+1. Find out the current version number of the secret, e.g. by running `abra app example_wordpress config`, and choose a new one. Let's assume it's currently `v1`, so by convention the new secret will be `v2`.
+2. Generate or insert the new secret:
+	```
+	abra app example_wordpress secret generate db_password v2
+	```
+	or
+	```
+	abra app example_wordpress secret insert db_password v2 "foobar"
+	```
+3. Edit the app configuration to change which secret version the app will use:
+	```
+	abra app example_wordpress config
+	```
+4. Re-reploy the application with the new secret version:
+	```
+	abra app example_wordpress deploy
+	```
+
+## Storing secrets in `pass`
+
+The Co-op Cloud authors use the [UNIX `pass` tool][pass] to share sensitive data, including Co-op Cloud secrets, and `abra <app> secret...` commands include a `--pass` option to automatically manage generated / inserted secrets:
+
+```
+# Store generated secrets in `pass`:
+abra app new wordpress --secrets --pass
+abra app example_wordpress secret generate --all --pass
+# Store inserted secret in `pass`:
+abra app example_wordpress secret insert db_password v2 --pass
+# Remove secrets from Docker, and `pass`:
+abra app example_wordpress secret rm --all --pass
+```
+
+This functionality currently relies on our specific `pass` structure; patches to make that configurable are very welcome!
+
+## What makes secrets secure?
+
+TODO
+
+[Docker Secrets]: https://docs.docker.com/engine/swarm/secrets/
+[immutable]: https://en.wikipedia.org/wiki/Immutable_object
+[pass]: https://www.passwordstore.org