When a user generates a GPG key pair, he generates one public key and one private key. When the user wants to send someone an encrypted document, he encrypts the document using the recipient’s public key. And when someone sends the user a document encrypted with the user’s public key, the user has to decrypt it using his private key. In this way, an encrypted document is shared using GPG.

So, private keys are mainly used for decryption and signing, and public keys are used for encryption and verification.

We can use GPG in server-client communication also. In a server-client communication, when we generate a server key, it is used for encryption and decryption during secure file transfers using FTPS, SFTP (What is the difference between FTPS and SFTP ?) or HTTPS (How does HTTPS work ?). This server key has a public key and a private key (What is public-key encryption ?). If the public key is used to encrypt files on the client side, then the corresponding private key is used to decrypt files on the server side.

On the other hand, the client generates a client key, which is used for signing and verification or for authentication purposes at the start of secure file transfers. This client key also has a public key and a private key. The user can use the private key to sign on the client side, and the server can use the corresponding public key to verify the digital signature.

Let’s take a small example. If Alex wants to send some data to the server, a client key is generated, and the public key is transferred to the server. The user can then use his private key to sign the data, and the signed data is sent to the server. The server can verify the data with the client’s public key.

On the other hand, the server can send its public key to the client. If the client uses the public key to encrypt any data and send it to the server, the server can decrypt the data with its private key.

The advantage of GPG is that GPG key generation is more versatile. It can be used for encryption and decryption purposes as well as signing and verification purposes.

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.”