The fight against terrorism makes the encryption of messages on services such as Facebook-owned internet messaging service WhatsApp "completely unacceptable," according to Home Secretary Amber Rudd.
She told BBC's Andrew Marr Show on Sunday that "there should be no place for terrorists to hide."
"We need to make sure that organisations like WhatsApp, and there are plenty of others like that, don't provide a secret place for terrorists to communicate with each other," sbe said.
Her statements come after reports from Sky News and MailOnline said that Westminster attacker Khalid Masood was active on WhatsApp just minutes before the attack on Westminster Bridge.
Screenshots show he had "seen" his messages at 2:37 p.m. on Wednesday, just three minutes before he drove a vehicle through pedestrians on the bridge. His messages were encrypted, but the police are believed to have seized his cellphone.
According to International Business Times, encrypted platforms like WhatsApp allow users to send private messages which are "heavily encoded, using end-to-end encryption," meaning that governments and hackers are unable to eavesdrop on messages and voice calls.
"It used to be that people would steam-open envelopes or just listen in on phones when they wanted to find out what people were doing, legally, through warrantry," she said.
"But on this situation we need to make sure that our intelligence services have the ability to get into situations like encrypted WhatsApp."
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