Does she really need new powers?
While Amber Rudd is suddenly concerned that a WhatsApp message might theoretically be unavailable for her to read, let’s first remember all the ways in which she can already get information about the likes of Khalid Masood, who carried out the Westminster attacks last week.
She has all the metadata
The UK tries to collect everyone’s metadata, so that they know who people are communicating with and when. Sometimes, as with WhatsApp, the metadata will exist at the company, and she can compel them to hand it over.
Usually the metadata is more important than the message. If Masood was talking to Daesh on WhatsApp, he wasn’t discussing pizza toppings.
The UK stores everyone’s phone records, IP addresses and email records for a year. The government has the power to add web visits as well, and may already be doing so.
She can hack his phone
GCHQ have powers to hack devices, and are well known to be highly skilled in breaking into mobile devices. They can also use their cable taps to interrupt web traffic and inject attacks.
For instance, friend requests on Facebook and LinkedIn were replaced with links that helped GCHQ gain permanent access to targets’ computers.
Once a phone or computer is hacked, WhatsApp messages can be read, just as you can read them by looking at the screen’s display data.
She has all the government’s records to check