JAILED FOR A TEXT: China’s Censors Are Spying on Mobile Chat Groups.

One night this September, construction supervisor Chen Shouli fired off a joke in a chat group.

“Haha,” he typed on his black iPhone 7, followed by an off-color wisecrack about a rumored love triangle involving a celebrity and one of China’s most senior government officials.

Four days later, he says, the police telephoned, ordering him in for questioning.

“I thought, I haven’t done anything wrong, have I? I’m law-abiding,” recalls Mr. Chen, a wiry 41-year-old. “So I went in. Once I arrived, they wouldn’t let me leave.”

Mr. Chen was locked in a cell for five days, he says. According to the police report, his comment on the WeChat messaging app was deemed “picking quarrels and provoking trouble,” a broad offense that encompasses gang fighting and destruction of public property and is punishable by detention without trial.

In China’s swiftly evolving new world of state surveillance, there are fewer and fewer private spaces. Authorities who once had to use informants to find out what people said in private now rely on a vast web of new technology. They can identify citizens as they walk down the street, monitor their online behavior and snoop on cellphone messaging apps to identify suspected malcontents.

Strong encryption is everyone’s friend — except the tyrant.