Amber Rudd has complained that apps like WhatsApp are not doing enough to help the government catch offenders. They must do more to allow the government to break into people's messages and read what they are saying, she said.
A large part of Ms Rudd's problem with WhatsApp is that it uses end-to-end encryption to keep messages safe from being viewed by criminals, but which also keeps the government from reading people's private conversations. Technology companies say they need to use that technology to keep people safe – but Ms Rudd said it must be banned, while at the same time admitting that she didn't understand it.
The latest intervention comes after Ms Rudd has repeatedly complained, following recent terror attacks, that tech companies aren't doing enough to stop extremism. Theresa May has also repeatedly called for the regulation of the internet so that it can't be used for crime.
Such a sharp public reprimand for WhatsApp, which hasn't publicly responded, indicates frustration within the government.
"We also know that end-to-end encryption services like WhatsApp, are being used by paedophiles," Home Secretary Amber Rudd told party activists in Manchester.
"I do not accept it is right that companies should allow them and other criminals to operate beyond the reach of law enforcement," Rudd said. "We must require the industry to move faster and more aggressively. They have the resources and there must be greater urgency."
WhatsApp, which is owned by Facebook, and similar rival chat services let users communicate with pictures, video and text, using "end-to-end" encryption that can be read only with a key held on the user's device. Without access to the devices, security services cannot read the messages.
Rudd also called on technology giants such as Facebook , Google, Microsoft and Twitter to go further and faster to counter extremist material.
The industry says it wants to help governments remove extremist or criminal material but also has to balance the demands of state security with the freedoms enshrined in democratic societies.
Britain's MI5 security service has said it needs access to encrypted communications to foil attacks. In the United States, the Federal Bureau of Investigation has pushed for full access to encrypted communications and devices but Congress has so far refused.
Security Minister Ben Wallace, who oversees British counter-terrorism and communications data legislation, said he was disturbed to see that end-to-end encryption services prevented security services from tracking criminal messaging.
"We see on a daily basis that this end-to-end encryption protects peadophiles, it protects organised crime too, but it is very disturbing in my job when I know that we know two peadophiles are talking and we think they are doing something about snatching a child but we don't know and we can't get into these communications," Wallace said.
"There are other ways I can't talk about which we think they can help us more without necessarily entering into end-to-end encryption. So we think they can do more."
Additional reporting by Reuters
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