In practice, Person A uses GPG to encrypt messages or data so that only Person B can read/use them. This is done by using a public key which essentially says that "I am who I say I am."
You need a copy of the public key for the person you want to secure the message/data for. This is conceptually similar to needing a person's phone number before you can call them, or a person's email address before you can send them email.
From Wikipedia:
GnuPG is a hybrid-encryption software program because it uses a combination of conventional symmetric-key cryptography for speed, and public-key cryptography for ease of secure key exchange, typically by using the recipient's public key to encrypt a session key which is only used once. This mode of operation is part of the OpenPGP standard and has been part of PGP from its first version. […]
GnuPG encrypts messages using asymmetric key pairs individually generated by GnuPG users. The resulting public keys may be exchanged with other users in a variety of ways, such as Internet key servers. They must always be exchanged carefully to prevent identity spoofing by corrupting public key ↔ "owner" identity correspondences. It is also possible to add a cryptographic digital signature to a message, so the message integrity and sender can be verified, if a particular correspondence relied upon has not been corrupted.
| Recommended Software | Notes; Alternate software | |
|---|---|---|
| macOS | GPG Tools Suite | Alternatively, you can install the command-line version of GPG using MacPorts or Homebrew. Look for the gpg2 package. |
| Windows | gpg4win, Git Bash | Git Bash includes gnugpg and is available in the command line. |
| Linux | gpg2 comes pre-installed on modern Linux distributions |
Instructions for RHEL, CentOS, Amazon Linux, Oracle Linux, Ubuntu, Debian, Upstream GnuPG project downloads |
| Keybase | Keybase |
# make a key (if you don't already have one)
gpg --gen-key
... RSA and RSA is okay ...
... 2048 bits is okay ...
... 1y is okay (but choose your own duration) ...
... Real name: First Last ...
... Email address: me@email.com ...
# lookie in here
cd ~/.gnugpg
# generate an ASCII output of your public key
gpg --armor --output first_last.asc --export 'First Last'