GnuPG is a complete and free implementation of the OpenPGP standard as defined by RFC4880 (also known as PGP). GnuPG allows to encrypt and sign your data and communication, features a versatile key management system as well as access modules for all kinds of public key directories. GnuPG, also known as GPG, is a command line tool with features for easy integration with other applications. A wealth of frontend applications and libraries are available. Version 2 of GnuPG also provides support for S/MIME and Secure Shell (ssh).

Download GnuPG: https://gnupg.org/download/index.html

On macOS, you can install GPG Suite: https://gpgtools.org/
On Windows, you can install Gpg4in: https://gpg4win.org/

GnuPG help:

$ gpg -h

Delete Key

Delete public key:

$ gpg --delete-key KEY_ID

Delete private key:

$ gpg --delete-secret-key KEY_ID

Export Key

Export public key:

$ gpg --export -a KEY_ID

Export private key:

$ gpg --export-secret-key -a KEY_ID

Import Key

Import public key:

$ gpg --import public.key

Import private key:

$ gpg --allow-secret-key-import --import private.key

List Keys

List all keys:

$ gpg --list-keys

$ gpg -k

List private keys:

$ gpg --list-secret-keys

$ gpg -K

Sign/Verify

Sign

$ gpg -s -b -a --default-key 6CB58EB6976AA756A61196023A24F80BD0386B7F example.file.txt

Verify

$ gpg --verify example.file.txt.asc example.file.txt
gpg: Signature made Sat Nov 24 12:03:02 2018 CST
gpg:                using RSA key 6CB58EB6976AA756A61196023A24F80BD0386B7F
gpg: Good signature from "HatterJ/L2 (Hatter Jiang's L2 PGP Key) <jht****@gmail.com>" [ultimate]

The OpenPGP Card is a specification of an ISO 7816-4,-8 compatible smartcard and also an actually available implementation of this specification as a standard sized card.

https://openpgpcard.org/

Make Card

https://openpgpcard.org/makecard/

SSH

https://openpgpcard.org/ssh/