Unfortunately, if I read https://news.ycombinator.com/item?id=11727870 right, Moxie formulated that more clearly by saying they would only approve of copyleft reimplementations. Unless I'm misreading things, this confirms the concerns and turns it more into something like the Java conformance testing case which the Apache folks ran into. I do get why they want protocol conformance, but expecting everyone to publish their implementation as copyleft is unreasonable and unrealistic. The only acceptable solution, if they want to see widespread use of the protocol, is to have a shared, no strings attached testing framework everyone can use to check their protocol implementation doesn't deviate. We have such things for web standards, why not for a secure chat solution? Their aim is good, but the solution is unacceptable for an industry standard to be.

Quoting

    If you'd like to do the work of developing a strong copyleft
    version of the GPL that you feel is appropriate for use in the
    app store, and get it OSI approved, we'd certainly look at
    using that.

    We'd like to get this better documented [...] It's a priority [...]
    hopefully we'll have more to publish this year.

    We haven't patented any of the concepts here, and we've done a
    lot to explain and popularize them. We're happy for people to
    use these concepts to build their own implementations of similar
    protocols, but we don't want people slapping things together and
    calling that Signal Protocol.

But he's not happy with recompilation of Signal? https://fdroid.eutopia.cz/

> Any other distribution channel we be treated as malware

He isn't fine with anyone distributing unsigned or resigned Signal. What he says is more or less "my app, my signature".

The fdroid people could recompile, check that the binary matches the one Moxie signed, and distribute the one with Moxie's signature.