All the years since OsmoSGSN came first into existance, it never had gained GPRS encryption support. While the original code had been written with encryption in mind, and libosmocore even contained a plugin infrastructure for GPRS encryption plugins, nobody had so far connected the dots, figured out the bugs in the existing code and made it fully work.
Thanks to analysis by Dieter Spaar and Max Suraev, we now have a functional implementation of GPRS encryption in OsmoSGSN. The SGSN contains the core infrastructure for it, while encyption is handled via libosmocore. A GEA3 implementation has just been merged to libosmocore - we also have experimentally verified operation with GEA1 + GEA2, but unfortunately no public documentation / implementation of those security by obscurity algorithms is available yet.
In terms of the SGSN changes required: Most have been merged, while some are still in the gerrit review process, see https://gerrit.osmocom.org/#/q/topic:gea