The Joint Declaration was agreed upon at an informal meeting of the European Chiefs of Police in London hosted by the National Crime Agency on 18 April.
Police Chiefs of all EU Member States and Schengen Associated Countries were invited, alongside Europol’s Executive Director.
Here is the declaration (pdf).
Once you have a tool that uses pgp with keys you provide, and encrypts messages in normal chats, changing the actuall message format would probably be easy, so there’s plenty of room for adittional steganography. Images would make for a perfect cover, with something like last-bit steganography.
But, I hope it won’t come to that.
They will have tools to scan for steganography. I bet Palantir or some other dodgy tech bro company is more than happy to sell something like that to them. And with PGP’s not only strong encryption, but just as strong authentication, they’ll have no problem whatsoever proving that it was you who sent those illegal encrypted messages.