I was just going through whatsapp Terms and Service here: https://www.whatsapp.com/legal/ and found that they may work with Facebook to fight spam etc. How is it possible when Whatsapp servers cannot decrypt any messages.
asked Oct 1, 2016 at 2:46
Not a WhatsApp specific answer (as I don't know the details there...) but you can implement a spamfilter directly on the client, as the client can legitimtely access the content of the message.
This is what local email spamfilters have done for decades now.
It may be impossible to do content-based detection, but still possible to have volume and metadata-based controls. In general, it does seem like with higher quality "language", metadata is increasingly a more reliable indicator of spammers.
Add to this the fact that account creation is non-trivial on WhatsApp and it is possible to block unknown contacts from sending to you, and it may explain why there isn't much of a spam problem on Whatsapp, despite how trivial it is to enumerate all users.
If you define "spam" as exactly the same message sent to many users, this could be detected as supplementing each sent message with its hash (this will also help with integrity checks). The server then can compare hashes with known spam message hashes (such as those reported by the users) and thus detect those spam messages on the server side without knowing the context of the message.
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