Package: x2goclient Version: 4.0.5.1
The Repository for Debian Jessie on <http://packages.x2go.org/> is signed with a key F4A7678C9C6B0B2B. But documentation on <http://wiki.x2go.org/doku.php/wiki:repositories:debian> says to install key E1F958385BFE2B6E.
It seems, the package "x2go-keyring" also has only the older key; which is no surprise:
root@faepop69 ~ # apt-cache policy x2go-keyring x2go-keyring: Installed: 2012.07.23+wheezy~main~17~build1 Candidate: 2012.07.23+wheezy~main~17~build1 Version table: *** 2012.07.23+wheezy~main~17~build1 0 100 /var/lib/dpkg/status 2012.07.23~jessie~main~17~build1 0 500 http://packages.x2go.org/debian/ jessie/main amd64 Packages
Even if the package carried a recent key it would not update, since the version number on Jessie is smaller then on wheezy.
Best regards Andreas
Andreas Hirczy <ahi@itp.tugraz.at> https://itp.tugraz.at/~ahi/ Graz University of Technology phone: +43/316/873- 8190 Institute of Theoretical and Computational Physics fax: +43/316/873-10 8190 Petersgasse 16, A-8010 Graz mobile: +43/664/859 23 57