'You can't have a situation where you have terrorists talking to each other ... and it can't be accessed'
The British government said on Sunday that its security services must have access to encrypted messaging applications such as WhatsApp, as it revealed that the service was used by the man behind the parliament attack.
Khalid Masood, the 52-year-old British man who killed four people in a rampage in Westminster on Wednesday before being shot dead, reportedly used the Facebook-owned service moments before the assault.
Home Secretary Amber Rudd told Sky News it was "completely unacceptable" that police and security services had not been able to crack the heavily encrypted service.
"You can't have a situation where you have terrorists talking to each other - where this terrorist sent a WhatsApp message - and it can't be accessed," she said.
Police said on Saturday that they still did not know why Masood, a Muslim convert with a violent criminal past, carried out the attack and that he probably acted alone, despite a claim of responsibility by the Islamic State group.
"There should be no place for terrorists to hide," Rudd said in a separate interview with the BBC.
"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."
She said end-to-end encryption was vital to cyber security, to ensure that business, banking and other transactions were safe - but said it must also be accessible.
— BBC News (UK) (@BBCNews) March 26, 2017
"We need to make sure that our intelligence services have the ability to get into situations like encrypted WhatsApp."
According to technology magazine Wired, end-to-end encryption means messages can only be decoded by the recipient and not by anyone in between, including the company providing the service.
The attack on Wednesday looks set to reignite the privacy-versus-secrecy debate in Europe, especially after warnings from security officials that Western countries will be increasingly targeted as Islamic State loses ground in the Middle East.
US authorities last year fought a legal battle with tech giant Apple to get it to unlock an iPhone used by one of the shooters in a terror attack last year in San Bernadino, California.
The FBI's own experts ended up breaking into the device.
Rudd, appointed home secretary shortly after Britain voted to leave the EU, said the British case was different when asked about Apple's opposition to helping the FBI.
"This is something completely different. We're not saying open up, we don't want to go into the Cloud, we don't want to do all sorts of things like that," she said.
"But we do want them to recognise that they have a responsibility to engage with government, to engage with law enforcement agencies when there is a terrorist situation."
But we do want them to recognise that they have a responsibility to engage with government, to engage with law enforcement agencies when there is a terrorist situation.
-Home Secretary Amber Rudd
She said she wanted to see an industry-wide board set up in Britain to allow technology companies to better police their sites and stop letting "their sites, their platforms, their publishing enterprises ... being used by terrorists".
Rudd said she did not yet intend to force the industry's hand with new legislation, but would meet key players on Thursday to discuss this issue, as well as the "constant battle" against extremist videos posted online.
Social media giants under pressure
WhatsApp said it was working with British authorities investigating the Westminster attack, but did not specify whether it would change its policy on encrypted messaging.
"We are horrified at the attack carried out in London earlier this week and are cooperating with law enforcement as they continue their investigations," a company spokeswoman told AFP.
Social media giants are also coming under pressure over extremist content being posted on their sites.
Germany this month proposed imposing fines on social networks such as Facebook if they fail to remove illegal hate speech from their sites.
Google, meanwhile, has faced a boycott by companies whose adverts appeared alongside extremist content on its internet platforms, particularly its video-sharing site YouTube.