http://nksecur5aoxbew7x4t2jebk7ordomil3a4xubeamno76zexicppwhiad.onion/how-to-pgp.html
Alternatively, Whonix because of it's transparent torification using the "Gateway" virtual machine. While the messages may be encrypted or signed by a particular key, there is no way to know that someone hasn't handed their keys over to LE . It common for LE to offer a "minimal discount" for "assistance" as a part of a plea bargain.