Consider FFDH for decrepit — Apple Screen Sharing/ARD has no alternative

[sibson]

Apple's Screen Sharing / Remote Desktop (ARD) authentication, part of the RFB/VNC protocol family, mandates classic finite-field Diffie-Hellman for its key exchange — there is no alternative to negotiate into, and Apple controls the protocol, not us. A client that wants to authenticate against ARD has no path other than FFDH.

Our usage: vncdotool/rfb.py (_encryptArd) uses dh.DHParameterNumbers/dh.DHPublicNumbers for exactly this. We're tracking the deprecation on our end (sibson/vncdotool#388).

Following up on #15171, which deprecated FFDH in 50.0.0. @alex noted there:

We weren't planning to migrate to decrepit, just deprecate (for a long time) and then remove.

That thread had no concrete interoperability case on the table. decrepit exists precisely for this: CAST5/SEED/IDEA/Blowfish went there in #6809, and 3DES/ARC4 were kept out of deprecation entirely, "due to their prevalence in existing systems." decrepit's own docs: "algorithms that should not be used unless necessary for backwards compatibility or interoperability with legacy systems."

Given there's no migration target for a protocol that mandates FFDH by spec, please reconsider decrepit as FFDH's eventual home instead of outright removal.

[reaperhulk]

Thanks for letting us know, we definitely want to understand use cases like this. This feels like a scenario where talking to SEAR folks might be helpful.

[alex]

If I understand https://support.apple.com/guide/remote-desktop/encrypt-network-data-apdfe8e386b/mac correctly (which I may not, it's not super clearly written), it looks like more recent macOS support non-FFDH key exchange.