Signal-Server VS ContactDiscoveryService

> They could at least BSL the server code and allow others to verify the server code and host but not compete.

This is exactly what they do (except they use AGPL): https://github.com/signalapp/Signal-Server

https://github.com/signalapp/Signal-Server/blob/main/service/src/main/java/org/whispersystems/textsecuregcm/push/APNSender.java for APNs push notification payloads

git clone https://github.com/signalapp/Signal-Server.git

yes it is available here https://github.com/signalapp/Signal-Server/blob/main/service/pom.xml

You could also start your own signal server, but with blackjack, and hookers

The fact that it locks you into using their servers, does not distribute on F-Droid (only Google Play OR an APK with an insecure update mechanism), and has a completely closed-source "abusive message filter" module server side, that could functionally be used for censorship, storing messages for future decryption, or any other number of nefarious purposes - we have no idea since it's not open source (https://github.com/signalapp/Signal-Server/blob/main/.gitmod...).

Additionally, you cannot distribute branded forks or Signal, and if you do fork it, your fork is not allowed to connect to Signal's "official" OWS (open whisper systems) servers - hostility to federation should be viewed with prejudice and suspicion at the very least, it suggests a vested interest in a single point of failure (or control), which goes against user interests.

Further reading: https://drewdevault.com/2018/08/08/Signal.html

It’s actually the signal server that isn’t reproducible because of the abusive message filter that’s in a private repository. It’s clear as day here: https://github.com/signalapp/Signal-Server

[1] https://signal.org/blog/private-contact-discovery/

Signal uses SGX for remote attestation, which presumably lets the client verify that the code running on the server is a build of the OSS code and not a modified version. But I don't know the details or if this is reliable.

SGX and remote attestation described here:

https://signal.org/blog/private-contact-discovery/

Signal does private contact discovery and the effort they've gone to to do this is quite impressive.

TEEs have numerous privacy-enhancing applications that may benefit users. One of them is, as discussed earlier, private contact discovery; the Signal application uses a contact discovery service enhanced using Intel SGX, a TEE technology, to protect its users' privacy. A similar application of TEEs is performing malware analysis in a remote cloud service, so that the service may not identify users by the contents of their devices, such as the applications they have installed, especially important as 98.93% of users may be uniquely identified by the list of applications they have installed.

Signal (or, more accurately, one of its predecessors) used to use client-side private set intersection for contact discovery, but this scales poorly [1].

Now they use a solution based on Intel SGX and server-side trusted computing [2].

[1] https://signal.org/blog/contact-discovery/

[2] https://signal.org/blog/private-contact-discovery/