How to certificate virtual conversations? (WhatsApp, Twitter, etc)

I'm a cybersecurity master's degree student. I have take a course on forensic IT, but we never discussed this topic. How can you certificate Tweets, WhatsApp chats, etc?

Talking to my professor he told me the first thing would be to hash the screenshot, but he really didn't know much else. But this does not guarantee the messages and etc. to have come from the accused device. How you do it?

Which steps are actually followed to keep custody chain …

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