If you configure an LTI tool in sakai.properties (or typically security.properties)
then you get this error logging:
2018-06-25 19:57:11,333 ERROR ajp-nio-8009-exec-2 org.sakaiproject.basiclti.util.SakaiBLTIUtil - Exception when decrypting secret - this is normal if the secret is unencrypted
which I think is inappropriate, because (as the erorr says) it's normal.
I believe this should be a DEBUG log entry.