We are using Mysql 5.7.18 with keyring plugin. We use the xtrabackup version 2.4.7 the parameters for backup is as follows
xtrabackup --backup --user=root --password=x xxxxxxxxxxxx --keyring-file-data=/opt/mysql/admin/CARD/cfg/keyring --server_id=1 --compress --parallel=4 --compress-threads=4 --no-timestamp --target-dir=/opt/mysqlbackup/CARD/2017-04-24-09-54-full
Even when using incremental backups the delta is a full backup of the encrypted table.
This is a huge problem since the database is 665GB big and we take daily backups