Describe Zeroconf authentication.

docs/authentication.md

@@ -2,10 +2,50 @@
 Once the connection is setup, the client can authenticate with the AP. For this, it sends an
 `ClientResponseEncrypted` message, using packet type `0xab`.
 
-A few different authentication methods are available, athough only one has really been tried, the
-traditional user / password based authentication.
+A few different authentication methods are available. They are described below.
 
 The AP will then reply with either a `APWelcome` message using packet type `0xac` if authentication
 is successful, or an `APLoginFailed` with packet type `0xad` otherwise.
 
-TODO: investigate other authentication methods (Zeroconf, facebook, stored credentials)
+## Password based Authentication
+Password authentication is trivial.
+The `ClientResponseEncrypted` message's `LoginCredentials` is simply filled with the username
+and setting the password as the `auth_data`, and type `AUTHENTICATION_USER_PASS`.
+
+## Zeroconf based Authentication
+Rather than relying on the user entering a username and password, devices can use zeroconf based
+authentication. This is especially useful for headless Spotify Connect devices.
+
+In this case, an already authenticated device, a phone or computer for example, discovers Spotify
+Connect receivers on the local network using Zeroconf. The receiver exposes an HTTP server with
+service type `_spotify-connect._tcp`,
+
+Two actions on the HTTP server are exposed, `getInfo` and `addUser`.
+The former returns information about the receiver, including its DH public key, in JSON format.
+The latter is used to send the username, the controller's DH public key, as well as the encrypted
+blob used to authenticate with Spotify's servers.
+
+The blob is decrypted using the following algorithm.
+
+```
+IV = encrypted_blob[:0x10]
+expected_mac = encrypted_blob[-0x14:]
+encrypted = encrypted_blob[0x10:-0x14]
+
+base_key       = SHA1(shared_secret)
+checksum_key   = HMAC-SHA1(base_key, "checksum")
+encryption_key = HMAC-SHA1(base_key, "encryption")[:0x10]
+
+mac = HMAC-SHA1(checksum_key, encrypted)
+assert mac == expected_mac
+
+blob = AES128-CTR-DECRYPT(encryption_key, IV, encrypted)
+```
+
+The blob is then used as described in the next section.
+
+## Blob based Authentication
+TODO
+
+## Facebook based Authentication
+TODO