They can’t …prove… you know a key to decrypt data, but in the UK you can be charged under the Regulation of Investigatory Powers Act.

“RIPA regulates the manner in which certain public bodies may conduct surveillance and access a person's electronic communications. The Act:

enables certain public bodies to demand that an ISP provide access to a customer's communications in secret;

enables mass surveillance of communications in transit;

enables certain public bodies to demand ISPs fit equipment to facilitate surveillance;

enables certain public bodies to demand that someone hand over keys to protected information;

allows certain public bodies to monitor people's Internet activities;

prevents the existence of interception warrants and any data collected with them from being revealed in court.”

https://en.m.wikipedia.org/wiki/Regulation_of_Investigatory_...