GNU General Public License or GPL. Now, several other vendors also have developed other OpenPGP-compliant software.

So, to summarize, PGP is a software program that was first developed by Phil Zimmermann in 1991. OpenPGP is the standard proposed by IETF. And GnuPG is a freely available software under the GNU General Public License.

How to use PGP?

PGP is based on public key cryptography. A user has to create a public-private key pair. The private key is kept secret to the user. The public key can be distributed to others. This key pair can later be used to sign, encrypt, or decrypt documents, texts, emails, etc.

Digital Signature

A digital signature is done to ensure the authenticity and integrity of a document. A user has to make digital signatures using his private key. Any user who has the public key of the sender will be able to verify that the document is indeed sent by the particular sender. And the document has been unmodified since then. (How to digitally sign a document using GPG?)

Encryption and Decryption

To send someone an encrypted document or email securely (How to sign and encrypt emails using GPG?), one has to encrypt it using the public key of the recipient. The recipient needs to decrypt the document or email using his private key. As the private key is secret to the recipient, only the recipient would be able to decrypt and read the message. (How to encrypt and decrypt documents using GPG?)

Signing and Encryption

If a sender wants to send a secret message to a recipient and ensure the authenticity and integrity of the message at the same time, then the sender has to sign as well as encrypt the message.

For that purpose, the sender needs to sign the message using his private key and encrypt it with the public key of the recipient. As the private keys are kept secret to users, only the recipient would be able to decrypt the message. At the same time, no one else other than the sender will be able to modify the message.

Interested readers can find more information on PGP and how to use it here: PGP and GPG Course

I hope this helps. Interested readers who want to know more about how different cryptographic algorithms and Public Key Infrastructure work may want to refer to the book “Cryptography And Public Key Infrastructure.”