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67 lines
2.6 KiB
Markdown
67 lines
2.6 KiB
Markdown
# Connection Setup
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## Access point Connection
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The first step to connecting to Spotify's servers is finding an Access Point (AP) to do so.
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Clients make an HTTP GET request to `http://apresolve.spotify.com` to retrieve a list of hostname an port combination in JSON format.
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An AP is randomly picked from that list to connect to.
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The connection is done using a bare TCP socket. Despite many APs using ports 80 and 443, neither HTTP nor TLS are used to connect.
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If `http://apresolve.spotify.com` is unresponsive, `ap.spotify.com:443` is used as a fallback.
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## Connection Hello
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The first 3 packets exchanged are unencrypted, and have the following format :
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header | length | payload
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---------|--------|---------
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variable | 32 | variable
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Length is a 32 bit, big endian encoded, integer.
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It is the length of the entire packet, ie `len(header) + 4 + len(payload)`.
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The header is only present in the very first packet sent by the client, and is two bytes long, `[0, 4]`.
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It probably corresponds to the protocol version used.
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The payload is a protobuf encoded message.
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The client starts by sending a `ClientHello` message, describing the client info, a random nonce and client's Diffie Hellman public key.
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The AP replies by a `APResponseMessage` message, containing a random nonce and the server's DH key.
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The client solves a challenge based on these two packets, and sends it back using a `ClientResponsePlaintext`.
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It also computes the shared keys used to encrypt the rest of the communication.
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## Login challenge and cipher key computation.
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The client starts by computing the DH shared secret using it's private key and the server's public key.
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HMAC-SHA1 is then used to compute the send and receive keys, as well as the login challenge.
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```
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data = []
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for i in 1..6 {
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data += HMAC(client_hello || ap_response || [ i ], shared)
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}
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challenge = HMAC(client_hello || ap_response, data[:20])
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send_key = data[20:52]
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recv_key = data[52:84]
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```
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`client_hello` and `ap_response` are the first packets sent respectively by the client and the AP.
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These include the header and length fields.
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## Encrypted packets
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Every packet after ClientResponsePlaintext is encrypted using a Shannon cipher.
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The cipher is setup with 4 bytes big endian nonce, incremented after each packet, starting at zero.
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Two independent ciphers and accompanying nonces are used, one for transmission and one for reception,
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using respectively `send_key` and `recv_key` as keys.
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The packet format is as followed :
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cmd | length | payload | mac
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----|--------|----------|----
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8 | 16 | variable | 32
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Each packet has a type identified by the 8 bit `cmd` field.
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The 16 bit big endian length only includes the length of the payload.
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