Turn on package-check-signature and install ELPA packages on Mac OS X Emacs

I had my GUI emacs installed by brew cask install emacs.

I wanted to strictly check the downloaded package, by its signature, so I put following (as for testing, solely) in ~/.emacs.d/init.el:

(require 'package) (setq package-check-signature t) 

When try to install any package (from elpa) by the package interface of M-x list-packages, it results in message of epg-context--make: GPG error: "no usable configuration", OpenPGP, and cannot install.

Question

• How can I configure the init.el so that I can turn on package-check-signature and install packages?

Env

• GNU Emacs 25.3.1 (x86_64-apple-darwin13.4.0, NS appkit-1265.21 Version 10.9.5 (Build 13F1911))
• OS X El Capitan