How to reverse engineer keyexchange.proto ?

[clouedoc]

Hello librespot,

I'm trying to implement my own client based on the mobile protocol for Spotify. This requires me to communicate with Spotify APs.

Apparently, there is a difference between the keyexchange.proto librespot-java uses and the one the mobile app uses.

The mobile app's answer can't be decoded with the keyexchange.proto of this repository.

That's why I'm asking: what's the methodology to reverse engineer a .proto based on the raw messages the phone sends ?

How was the reverse engineering of the current keyexchange.proto achieved ?

Thanks for your attention 🙏

[sashahilton00]

I haven’t had the chance to look at the two to confirm whether they are different or not, but the reverse engineering can be done in a couple of ways:

1. Download the Android app and decompile it. This is the simplest way, as Android apps are not encrypted.
2. Use the iOS app for reverse engineering. This is slightly more involved as you need to remove the FairPlay DRM on the IPA, but if you have a jailbroken iOS device then that is relatively simple.
3. Look around for a Spotify connect speaker, pull the firmware from that, then dig around for a library named libspotify or libspotify-embedded. This can be a slightly more longwinded process, but may be more useful if trying to reverse engineer speaker only APIs.

Once you have your app/binary/library, there are a number of scripts such as PBTK that will attempt to extract the protobuf files, or otherwise you can open it up in something like IDA and transcribe manually if automated extraction is unsuccessful.

You can then use the extracted protos with wire shark to match up live request/responses to the protocol definitions, though this is mostly trial and error as Spotify has a mishmash of JSON, grpc and protobuf endpoints, some of which give hints as to which proto corresponds by sending a header with the proto message name.

I recommend doing it in front of a movie or something, it can take a while :)

[SuisChan]

The mobile app's answer can't be decoded with the keyexchange.proto of this repository.

What message are you talking about? ClientResponsePlaintext/Encrypted?

How was the reverse engineering of the current keyexchange.proto achieved ?

Looks like extracted from libspotify.

[tradertrue]

With wireshark the third packet (which is supposed to be the ClientResponsePlaintext if i understood correctly) looks like this : https://pastebin.com/38rdXyVg
And it can't be decoded with the keyexchange.proto from librespot

[SuisChan]

which is supposed to be the ClientResponsePlaintext if i understood correctly

Yes, but official client actually send two messages at the same time, first is ClientResponsePlaintext, and second (encrypted with Shannon) ClientResponseEncrypted

(From https://pastebin.com/38rdXyVg)
ClientResponsePlaintext -> https://pastebin.com/jsS5283t
ResponseEncrypted -> https://pastebin.com/Cb238zbC

So just read the length of the first message and decode it, you can decrypt second if you know the key, nonce is 0.

If you are familiar with golang, here is an example to read the first package https://pastebin.com/5gv2vVp8

Also you can try https://gchq.github.io/CyberChef/ for protobuf decoding

[Velka-DEV]

@sashahilton00 I'm not sure how to you can get appropriate data in this way, decompilling is easy but for exemple how was find the spotify server key in the app ? Such byte array would be extremly hard to extract. Or was it stored in another file ?

[sashahilton00]

For stuff like the server key, the way to extract it is with a disassembler such as IDA

[Velka-DEV]

Thanks for you reply @sashahilton00. I will try to find the key for playplay and how it's obfuscated (Seems complicated for me, but let's try)

[devgianlu]

Doing it like so will be very hard. Static reverse engineering will be practically impossible as there's too much code. You might get luckier doing some hybrid debugging (eg ghidra + x64dbg), try breaking on some interesting places and hope to see something on the stack.

[SuisChan]

I will try to find the key for playplay and how it's obfuscated

Which app / version are you trying to reverse?

[Velka-DEV]

Doing it like so will be very hard. Static reverse engineering will be practically impossible as there's too much code. You might get luckier doing some hybrid debugging (eg ghidra + x64dbg), try breaking on some interesting places and hope to see something on the stack.

I will give a try to this. But IDA, ghydra and x64dbg have the same utility no ? Disassembler / Debugger

I will try to find the key for playplay and how it's obfuscated

Which app / version are you trying to reverse?

The last version of Spotify Desktop App (Windows). Playplay is the new HTTP based API