Problem with package signing in Debian
I don't know if this has already came to your attention but with the recent changes in APT tolerance to SHA1 algorithms, the geogebra repository is now non-compatible with current Debian Testing and (I think) with future Ubuntu 16.04.
W: http://www.geogebra.net/lin...: Signature by key 98272894F6478AA4434B41D3C072A32983A736CF uses weak digest algorithm (SHA1)
Some links about the issue:
Hope this helps