GnuPG is a free and comprehensive implementation of the OpenPGP standard. It enables encryption and signing of data and communications, featuring a key management system and support for public key directories. While primarily a command-line tool, GnuPG is designed for integration with other applications. It also supports S/MIME and SSH.
Here are some excellent frontend tools compatible with GnuPG:
GpgFrontend
GpgFrontend intuitively streamlines encryption, decryption, and digital signing. Rooted in the trusted gpg4usb project, GpgFrontend enhances simplicity while incorporating OpenPGP’s advanced functionalities. Its mission is to make GnuPG’s encryption tools accessible to everyone, regardless of their technical background.
KGpg
KGpg provides a streamlined interface for GnuPG. With KGpg, you can set up and manage encryption keys, import and export them, and access detailed information such as key signatures, trust levels, and expiration dates, all through an intuitive and user-friendly design.
Kleopatra
Kleopatra is a versatile, open-source certificate manager and graphical front-end for cryptographic services, tailored for OpenPGP and S/MIME (X.509) certificates. As part of the KDE ecosystem, it offers an intuitive interface for managing encryption keys, signing and verifying data, and securely encrypting or decrypting files and emails.