Whatsapp is the biggest concern, for good reason they switched to using the Signal encryption algorithm two years ago (Spring 2016) but they store more metadata like date, time stamp, phone numbers... that Signal doesn't have even if ordered to. Unfortunately Whatsapp doesnt encrypt backups locally or to icloud.
But if he had that many messages still on the phone, sounds like he didn't delete them and he may have had to surrender the password or even had it stored.
If he had a laptop or desktop then the signal messages are typically stored in plain text but the drive they are on may be encrypted.
So at the very least they know most of the persons he communicated to and when, at best they have his passwords to decrypt his PC drives & phone, keys to the kingdom. I would guess the NSA was very interested in his communications and stored everything.
Also you have to trust the person on the other end isnt compromised or isnt a man in the middle, manaforts daughter phone got hacked and hundreds of screenshots were leaked to darknet in 2016 including some messages where she said her dad is essentially a thug and has made people dissappear.
Another thing, after WhatsApp switched to using Open Whisper's Signal in 2016, their app continued to function in China, unlike Signal which quickly gets blocked by the great firewall of china's AI, so... makes one wonder why its still allowed if its "secure".
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