The Great Firewall of China is an interesting thing from a technical point of view. My fiance was on a business trip for 3 days in China and as long as she used her Korean SIM card with roaming everything worked well. But then she bought a Chinese SIM card and suddenly things started going very weird.
- In Matrix she could see my messages - and mind you it's end-to-end encrypted and we are both on my private server running in Germany - but all her messages arrived and couldn't be decrypted.
- In Signal none of her messages arrived in Korea.
- Same with the Korean KakaoTalk, non of the messages arrived.
- SMS didn't arrive either.
I was very worried because for half a day I couldn't get hold of her.
I really wonder how they do it, KakaoTalk, Signal and SMS I have some ideas, but how did they interfere with the communication between her iPhone and my Matrix server in Germany? It's a super small server, only 4 users on it.