Note: some modifications have been done to the UE too, but for syntactic
reasons. The UE may fail to work, not tested.

For the UE, we have the problem of handling properly SecurityModeCommand.
We don't verify its integrity. This is a bit complicated to handle properly
and may require some API changes in the PDCP layer. So, as of today, the
integrity of this message is not verified.

Also, NSA mode has been touched, but it should not be a problem. But not
tested, so not sure.