I never wanted this. Whatsapp and its parent company are some of the things I most
hate about what tech has become, showing the utter lack of ethics in a industry
that has too much impact on the rest of the planet. Just
go and read all the reports about groups abusing these platforms (and them
allowing them to) to change politics all over the world, or all the shitty security
a lot of IoT stuff has, and how they're used to attack services on the Internet.

But reality is more complex than that. In my home country, Facebook and Whatsapp
specifically are very popular, to the point that, due to our lack of
net-neutrality laws, phone companies offer cheap contracts where those two
application's data usage do not count as such, becoming completely free. This
means people almost stopped using the phone or SMSs, but instead send text,
pictures and voice messages via these platforms. That includes my whole family,
which also almost stopped using email.

So for the moment I left my principles aside and installed the app. My last
attempt at this had failed because you can't install it on a tablet; the device
has to have phone capabilities. Even more, when you try to register it, it
forces you to use a cellular phone number; Signal at least has the decency to let
you register it to a land line too (if it can't send you an SMS, it gives you
the option of being called and the registering code is spelled to you). Luckily
I had a spare number from a throwaway line I bought in my last trip to
homeland, so I used that number instead of the real one. I know it's a useless
step, it's equivalent to giving the finger to someone's back.

Once installed, I tried to send a message to my wife. The app denies you to do
so if you don't give it in exchange access to your contacts. Again luckily for
me, this phone was mostly empty, but I still took steps to avoid giving it all my
contacts. The few I had were already sync'ed to my owncloud instance back home.

First, I exported all my contacts locally and deleted them all. I reimported
them after I got the app running. Then I created a new, empty owncloud account,
so when Whatsapp asked me which 'account' to use to get/sync the contacts, I
gave it that one. This way, when you add contacs, they go to this 'honeypot' and
it doesn't have access to your real Contacts. If you don't have a owncloud or similar
service you control, you can simply create a bogus Google account and use that
instead. The only downside is that you will get dupe'd contacts, but once you
sent them a message, you can safely delete the contact and even completely disable
sync'ing the account. You can also revoque the permission to access Contacts,
but that means you're back to square one, except for the conversations you
already have started.

I'm sorry  can't give you the exact steps I did, I was on the bus, and with all
the failing attempts I lost track. Of course, removing all the contacts means
that you only see phone numbers and their photos, but after a while you can
recognize them by that. Right now I only have my wife and my family's group,
and I hope I can keep it like that for a long, long time.

One last thing: Whatsapp asks you for your contacts, but you can't nicely ask
them back: the phone numbers of new contacts are very difficult to extract. You
either export them to the Contacts Account if you still have around (I didn't) or
you copy them by hand (which I did). Last but not least, I still have the nagging
sensation that Whatsapp would have been able to read the contacts; I really whish
that Android would gives us more fine grained firewall capabilities. Also,
remember that Whatsapp has no option to store media in an SD card, only the
phone's internal storage (WTF, people, seriously!), and it's a pain in the ass to clean up the stuff you
don't want. So for the moment I haven't gave it access to Photos, Media and Files.