GPGME: Invalid crypto engine when using slapt-get
Again with the errors about using slapt-get or the gui
If you get the error GPGME: Invalid crypto engine this indicates that you dont have the proper or no gpg keys so you have to add them if necessary.
also from the slapt-get faq : Recent versions of slapt-get support GPG key verification via the GPGME library. GPGME requires gnupg to support the OpenPGP protocol. At this time gnupg2 is not compatible with GPGME and so gnupg must also be installed. gnupg 1.x can be installed side by side with gnupg2.
So if it is still not working try to use the mirror provided by Dr. GrEen in this thread. you may still have to use the --no-md5sum to get the downloads to work.