InnoDB creates two types of temporary files during online alter operation: merge sort files and online alter logs which contain changes made to the table for the duration of online DDL. I see that merge sort files are encrypted in row0merge.cc, but I see no encryption in row0log.cc for online alter logs.
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This is not true anymore. Starting from 10.1.27, 10.2.9 encryption is supported also for online alter logs if innodb-encrypt-log=ON.