signald - unofficial utility for interacting with Signal

Signal does not offer any sort of official API. Their server software is
open source and anyone can figure out the REST endpoints, but formatting the
payloads turns out to be a little trickier. Signal, unlike traditional
messaging applications, puts a number requirements around encryption and
account data storage on the client software. signald aims to handle all of
the Signal-specific requirements, exposing an API that sends and receives
unencrypted messages with other software on the same machine.

signald does not come with a user friendly frontend. Users should select an
existing client to use signald with.