Well, I dunno about a proper solution - I wouldn't expect cryptography to enter into emailing photos, but perhaps it does.
Can you email pictures from outside Lightroom?
Which Lr version, and OS? Which email client?
I mean, one option for now, is to go back to the old way of doing it:
* If Mac: setup Apple Mail in post-processing.
* If Windows: use mapimailer.
If such would work..
I have windows 7 Pro and LR 5.6. What is mapimailer?
It's a plugin - the author (Steve Sutherland) has been incommunicado for a while but the plugin still works (I still use it).
If you send me your address, I'll send you a copy.
(I don't feel comfortable distributing publicly, since it's not my plugin).