It turns out that TiddlyWiki now includes an encryption module as part of the core functionality. This means that an entire wiki full of notes can be encrypted and decrypted with a single password. The current version is v5.1.7, dating from May 2018. The next release, v5.1.8, due quite soon, will include the ability to use https, which means that a password can be used securely to open and save a file remotely.
I set up Joplin for my wife but abandoned it after a couple of days in favour of TiddlyWiki v5 because of the better security. I’ll futz with Joplin a bit longer as it seems quite good apart from this weakness, which I hope will be temporary.