Kleopatra - Kleopatra is a certificate manager and GUI for GnuPG.
YAATA - YAATA app offers features that let you send multimedia files such as images, videos, audio, and GIFs, etc.
GnuPG - GnuPG is a complete and free implementation of the OpenPGP standard as defined by RFC4880 (also known as PGP).
Enigmail - Security extension to Mozilla Thunderbird and SeaMonkey
GPG Suite - Use GPG Suite to encrypt, decrypt, sign and verify files or messages.
Easy Message - Easy Message app comes up with features to help users in effortlessly sending text messages to a phone number, which is not in the contact list.