Now, the sender's app has been completely blocked. Users can not even send text messages, according to Nadim Kobeissi, a cryptographer at Symbolic Software, a Paris-based research firm that also monitors digital censorship in China.
"Basically, what we initially thought was censorship of photos, videos and audio in WhatsApp in July has now turned into blocking messages across China," Kobeissi said.
Kobeissi found that recently, China has fixed their firewall so they can detect and block the NoiseSocket protocol that WhatsApp uses to send text messages.
The Chinese firewall has also been able to block HTTPS / TLS that WhatsApp uses to transmit photos and videos.
He said, "I think it takes time for the Chinese firewall to adapt this new protocol, so that it can also target written messages." His company began to realize the interruption of WhatsApp on Wednesday last week.
This blocking will further complicate Facebook, WhatsApp owners whose social media services have been blocked in China since 2009.
The only Facebook application that operates in China is the Colorful Balloons app they released quietly last month.
Censorship in China is becoming increasingly strict because the 19th National Congress of the Chinese Communist Party will be held in the near future.
The reason WhatsApp is blocked is because the app offers end-to-end encryption, which ensures that messages sent by users are inaccessible to anyone. In contrast, local applications such as WeChat provide all user personal data to the government.
WeChat, which now has an active user of 963 million, will benefit from the blocking of WhatsApp, which is the only competitor in China. WhatsApp declined to comment.
