How does the WhatsApp contacts database hash work?

[Andy Burnelli]

Nobody else but Frank explained how it works (in another thread a few weeks
ago) where I think what happens is something like this (but I'm not sure).

1. Your contacts have to be in your default Android sqlite location
2. The WA app will grab them to create a hash on the phone, I think
3. That one-way (we hope) hash is based on what?
a) the phone number only?
b) the name & phone number?
c) whatever is in the contacts record?
4. That hash of your entire contacts db is uploaded to the WA server
(I can't see how the stripping can be done on the phone, can you?)
6. The WA server compares every hash to known WA accounts
7. If the account isn't known, then the WA server (we hope) deletes it
8. The WA server feeds back to the phone just the accounts that it owns
9. The WA app on the phone will display only those WA accounts inside it

Notice this may not be right, but even if it is, there are unknowns to me.
A. If you change the contact by a single character, does the hash change?
B. Is the hash done on the phone or on the WA servers?
C. Is the hash one way only?