I installed (and use daily) GPG4USB, it's a GUI for GPG, encrypt, unencrypt, sign, verify and others tasks. (manage keys)
https://www.gpg4usb.org/
The issue started when I updated Xubuntu from 16.04 to 18.04
I tested
./start_linux_32bits
./start_linux_64bits
with latest version of gpg4usb (0.3.3.2) and with 0.3.3.1
./start_linux_64bit
[Error] Source: GPGME String: "Invalid crypto engine" [Error] Source: GPGME String: "Invalid crypto engine" [Error] Source: GPGME String: "Invalid crypto engine"
could be related to GPGME, and how gpg4usb interact with it.
Xubuntu is updated
ii gpg 2.2.4-1ubuntu1 amd64 GNU Privacy Guard -- minimalist public key operations
ii gpg-agent 2.2.4-1ubuntu1 amd64 GNU privacy guard - cryptographic agent
ii gpg-wks-client 2.2.4-1ubuntu1 amd64 GNU privacy guard - Web Key Service client
ii gpg-wks-server 2.2.4-1ubuntu1 amd64 GNU privacy guard - Web Key Service server
ii gpgconf 2.2.4-1ubuntu1 amd64 GNU privacy guard - core configuration utilities
ii gpgsm 2.2.4-1ubuntu1 amd64 GNU privacy guard - S/MIME version
ii gpgv 2.2.4-1ubuntu1 amd64 GNU privacy guard - signature verification tool
ii libgpg-error0:amd64 1.27-6 amd64 library for common error values and messages in GnuPG components
ii libgpg-error0:i386 1.27-6 i386 library for common error values and messages in GnuPG components
ii libgpgme11:amd64 1.10.0-1ubuntu1 amd64 GPGME - GnuPG Made Easy (library)
ii libgpgmepp6:amd64 1.10.0-1ubuntu1 amd64 C++ wrapper library for GPGME