I've been using it for what seems like years and it works great.
https://f-droid.org/repository/browse/?fdid=org.smssecure.smssecure
Fdroid, install, launch, click yes use for default sms client, done.
When I change phones: export local backup, copy file to new phone, import,
copy backup in a secure location, delete other copies, done.
I'm surprised mjg59 did not mention it.