dcphr, a crude featureless single file offline OpenPGP manual interface.

Save/copy the dcphr.html somewhere.
Open it on a browser.
Select Encrypt, Decrypt or Generate Keys
And follow the instructions.

dcphr uses/includes the OpenPGP.js (license) library.
The remaining code of dcphr (not already covered by the previous license)
is also released under the GNU Lesser General Public License version 3 license
(see source code lines 207-377).
Special thanks to the Erês for their help.