Online censorship

Limits on Bolton's China revelations go beyond not covering the matter: there is evidence of discussion of the book being censored on two of the biggest platforms on the Chinese internet.

Users on Weibo -- a Twitter-like service -- complained they were unable to comment on the book or share passages from it, while on WeChat, China's largest messaging app, posts about Bolton appeared to be being hidden or deleted. CNN was able to upload a screenshot of the book cover to WeChat, but no contacts could see the resulting post.

It is unclear what exactly triggers the censorship, as some posts about the book, including reports in Chinese media, are permitted.

Yan Duan, an office worker in Beijing, said she was locked out of her WeChat account after sharing a PDF of the book in a group message. She received the notification: "As this WeChat account is suspected of disseminating false information, the current login is disabled."

"A friend of mine expressed interest in the book, so I thought I could directly forward the file," she said, having received it from another contact on WeChat herself. "It seems that there's a window of censorship. The file was sharable earlier. But afterward, I heard many incidents where the messages got hidden or the sender got kicked out like me."

Representatives for Sina and Tencent, which operate Weibo and WeChat respectively, did not respond to a request for comment.

'Ask the US'

The Great Firewall -- and Chinese censorship in general -- is a black box, and the exact purpose of any particular block can be hard to ascertain, whether it's the work of individual censors or a top-down order to control coverage.

This is especially the case on a topic where some discussion is permitted, but certain elements are off limits.

According to Tetsushi Takahashi, China bureau chief for the newspaper Nikkei, a report on Japanese public broadcaster NHK about Bolton was blacked out in China. Foreign television channels, including CNN, are often censored in this fashion.