Improve background section

Author: marc

v2.1/encryption.md

@@ -9,15 +9,36 @@ Encryption At Rest provides transparent encryption for node data on local disk.
 
 <div id="toc"></div>
 
-## Terminology
+## Background
 
-* store key: user-provided key, used to encrypt data keys
-* data key: automatically-generated key, used to encrypt data
-TODO(mberhault): add more
+Encryption at rest allows encryption of all files on disk using AES in counter mode, with all key
+sizes allowed.
 
-## Background
+Encryption is performed in the [storage layer](architecture/storage-layer.html) and configured per store.
+All files used by the store, regarless of contents, are encrypted with the desired algorithm.
+
+To allow arbitrary rotation schedules and ensure security of the keys, we use two layers of keys:
+
+| Level | Description |
+|-|-|
+| Store keys | Store keys are provided by the user in a file. They are used to encrypt the list of data keys (see below).
+This is known as a **key encryption key**: it's only purpose is to encrypt other keys.
+Since very little data is encrypted using this key, it can have a very long lifetime without risk of reuse. |
+| Data keys | Data keys are automatically generated by CockroachDB. They are used to encrypt all files on disk.
+This is known as a **data encryption key**.
+The key has a short lifetime to avoid key/IV reuse. |
+
+Store keys are specified by passing a path to a locally readable file. The file must contain 32 bytes (the key ID)
+followed by the key (16, 24, or 32 bytes). The size of the key dictates the version of AES to use (AES-128, AES-192, or AES-256).
+
+At startup, CockroachDB uses a data key with the same length as the store key. If encryption has just been enabled,
+the size has changed, of the data key is too old (default lifetime is one week), CockroachDB generates a new data key.
+
+Any new file created by the store uses the currently-active data key. Previous data keys are stored in a key registry file and encrypted with the active store key.
+
+CockroachDB does not currently force re-encryption of older files but instead relies on normal RocksDB churn to slowly rewrite all files with the desired encryption.
 
-TODO(mberhault): explain store vs data keys, rotation, etc...
+TODO(mberhault): flesh out key rotation description.
 
 ## Example