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 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.

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.

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 authentication is currently broken due to Spotify changing the authentication flow. The details of how the new flow works are detailed in https://github.com/librespot-org/librespot/issues/244 and will be implemented at some point in the future.