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librespot/docs/authentication.md
Paul Lietar 2c88787658 Add Facebook authentication protocol docs.
2016-03-16 00:23:03 +00:00

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Authentication

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. 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.

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.

# encrypted_blob is the blob sent by the controller, decoded using base64
# shared_secret is the result of the DH key exchange

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

data = b64_decode(blob)
base_key = PBKDF2(SHA1(deviceID), username, 0x100, 1)
key = SHA1(base_key) || htonl(len(base_key))
login_data = AES192-DECRYPT(key, data)

Facebook based Authentication

The client starts an HTTPS server, and makes the user visit https://login.spotify.com/login-facebook-sso/?csrf=CSRF&port=PORT in their browser, where CSRF is a random token, and PORT is the HTTPS server's port.

This will redirect to Facebook, where the user must login and authorize Spotify, and finally make a GET request to https://login.spotilocal.com:PORT/login/facebook_login_sso.json?csrf=CSRF&access_token=TOKEN, where CSRF is the same string sent earlier, and TOKEN is the facebook authentication token.

Since login.spotilocal.com resolves the 127.0.0.1, the request is received by the client.

The client must then contact Facebook's API at https://graph.facebook.com/me?fields=id&access_token=TOKEN in order to retrieve the user's Facebook ID.

The Facebook ID is the username, the TOKEN the auth_data, and auth_type is set to AUTHENTICATION_FACEBOOK_TOKEN.