The NSA, the original primary developer of SELinux, released the first version to the open source development community under the GNU GPL on December 22, 2000.[6] The software was merged into the mainline Linux kernel 2.6.0-test3, released on 8 August 2003. Other significant contributors include Red Hat, Network Associates, Secure Computing Corporation, Tresys Technology, and Trusted Computer Solutions.
I had a hard time choosing a link. Searching GitHub for "F-Droid" reveals a long convoluted back-and-forth about meeting F-Droid's requirements for reproducible builds. Signal is not, as of earlier today, listed on F-Droid.
F-Droid's reproducibility rules are meant to cut out the kind of shenanigans that would be necessary to hide a back door in the binaries.
Again, this isn't proof. But it's beyond fishy for an open source security tool.
Edit: And Signal's official statements on the topic are always reasonable - but kind of bullshit.
Reasonable in that I alwould absolutely accept that answer, if it were the first time that Signal rejected a contribution to add it to F-Droid.
Bullshit in that it's been a long time, lots of folks have volunteered to help, and Signal still isn't available on F-Droid.