https://stackoverflow.com/questions/6481627/java-security-illegal-key-size-or-default-parameters
replace files local_policy.jar and US_export_policy.jar under e.g:
C:\Program Files\Java\jre1.8.0_141\lib\security
Also install BouncyCastle provider in java lib.
When using GCM, if the other party sends back a message, the gcm id unregisters. You can still send messages, but no longer receive them. This seems to be some kind of bug with GCM or signal server.