The packagename changed from former single zulucrypt to the two packages zulucrypt-gui and zulucrypt-cli, and you need the additional extra/next repo:
deb http://packages.siduction.org/extra next main
LANG=C apt-cache policy zulucrypt-gui
zulucrypt-gui:
Installed: 4.7.1-1.siduction.2
Candidate: 4.7.1-1.siduction.2
Version table:
4.7.1-1.siduction.2 0
500 http://packages.siduction.org/extra/ next/main amd64 Packages